IAI Accepts, published online ahead of print on 4 November 2013 Infect. Immun. doi:10.1128/IAI.01104-13 Copyright © 2013, American Society for Microbiology. All Rights Reserved.
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Antibodies mediate the formation of neutrophil extracellular traps
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(NETs) in the middle ear and facilitate secondary pneumococcal
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otitis media
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Kirsty R. Short1+, Maren von Köckritz-Blickwede2, Jeroen D. Langereis3, Keng Yih Chew4,
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Emma R. Job1, Charles W. Armitage5, Brandon Hatcher6, Kohtaro Fujihashi6, Patrick C.
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Reading1,7, Peter W. Hermans3, Odilia L. Wijburg1*§ and Dimitri A Diavatopoulos3*§
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Department of Microbiology and Immunology, The University of Melbourne, Melbourne, Australia 2 Department of Physiological Chemistry, University of Veterinary Medicine, Hannover, Germany 3 Laboratory of Pediatric Infectious Diseases, Department of Pediatrics, Radboud University Medical Centre, Nijmegen, The Netherlands 4 Department of Zoology, The University of Melbourne, Melbourne, Australia 5 Institute of Health and Biomedical Innovation, Queensland University of Technology, Brisbane, Queensland, Australia 6 Department of Paediatric Dentistry and Microbiology, The University of Alabama at Birmingham, Birmingham, U.S.A 7 WHO Collaborating Centre for Reference and Research on Influenza, Parkville, Victoria, Australia
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Current address: Department of Viroscience, Rotterdam, The Netherlands * These two authors contributed equally. § Corresponding author: [email protected] and [email protected].
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Running title: Antibodies, NETs and otitis media 1
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ABSTRACT: Otitis media (OM; a middle ear infection) is a common childhood illness which
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can leave some children with permanent hearing loss. OM can arise following infection with a
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variety of different pathogens, including a co-infection with as influenza A virus (IAV) and
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Streptococcus pneumoniae (the pneumococcus). We and others have demonstrated that co-
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infection with IAV facilitates the replication of pneumococci in the middle ear. Specifically, we
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have used a mouse-model of OM to show that IAV facilitates the outgrowth of S. pneumoniae in
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the middle ear by inducing middle ear inflammation. Here, we seek to understand how the host
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inflammatory response facilitates bacterial outgrowth in the middle ear. Using B-cell deficient
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infant mice, we show that antibodies play a crucial role in facilitating pneumococcal replication.
44
We subsequently show that this is due to antibody-dependent neutrophil extracellular traps
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(NETs) formation in the middle ear, which instead of clearing the infection allow the bacteria to
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replicate. We further demonstrate the importance of these NETs as a potential therapeutic target
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through the transtympanic administration of a DNase, which effectively reduces the bacterial
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load in the middle ear. Taken together, these data provide a novel insight into how pneumococci
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are able to replicate in the middle ear cavity and induce disease.
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Key Words: Otitis media, Streptococcus pneumoniae, influenza virus, viral-bacterial co-
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infection, NETs, antibodies, neutrophils
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2
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INTRODUCTION
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Otitis media (OM) is one of the most common paediatric diseases worldwide. It can affect up to
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80% of children before the age of three years, and can lead to permanent hearing loss [1]. Up to
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70% of cases of acute OM are caused by viral-bacterial co-infections [2]. Of particular relevance
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are co-infections with influenza A virus (IAV) and the bacterium Streptococcus pneumoniae. In
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clinical cases and experimental models, infection with IAV facilitates the replication of
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S. pneumoniae in the middle ear [3-8]. Using an infant mouse model of OM (designed to mimic
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the underdeveloped immune system of children) we have previously demonstrated that the
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development of pneumococcal OM in co-infected mice was due to the inflammation induced by
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IAV in the middle ear [3,8]. However, the mechanisms by which the host inflammatory response
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mediates secondary pneumococcal OM remain undefined.
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The middle ear has few residential leukocytes and an infection in this organ results in an influx
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of neutrophils, macrophages and lymphocytes [9-11]. Neutrophils have traditionally been
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considered to play a protective role in OM [12,13]. However, recent studies have speculated that
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neutrophils may contribute to bacterial persistence in the middle ear via the formation of
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neutrophil extracellular traps (NETs) [14-16]. NETs refer to the extracellular DNA produced by
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neutrophils to ‘trap’ bacterial pathogens. This extracellular DNA is studded with histones and
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antimicrobial compounds to kill the trapped bacteria [17]. Interestingly, the pneumococcal
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capsule and D-alanine residues on pneumococcal lipoteichoic acids can inhibit NET killing [18],
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potentially enabling the pneumococcus to survive and persist within biofilm-like NET-structures
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in the middle ear.
3
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Pneumococcal OM predominately develops in the absence of pre-existing immunity, with
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incidence peaking between 6 months (when maternal antibodies have waned) and 2 years, when
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specific immunity develops [19]. In these immunologically naïve individuals, natural antibodies
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may represent an important defence mechanism against influenza-virus mediated pneumococcal
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disease, as is seen in pneumococcal sepsis [20]. Conversely, the formation of immune complexes
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in the middle ear may facilitate, rather than clear, bacterial OM [21], suggesting that organ-
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specific differences may exist with regards to the role of antibodies during pneumococcal
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disease. Moreover, the ability of antibodies to interact with neutrophils in the middle ear [19],
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and the suggestion that neutrophils may facilitate bacterial OM [14,15], may indicate that the
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role of antibodies and neutrophils in pneumococcal-influenza OM is more complex than simply
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protecting against disease development.
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Here, we use B6.uMT-/- mice (which lack B lymphocytes) [22] to investigate the role of
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antibodies in pneumococcal-influenza OM. Our data suggests that antibodies facilitate the
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development of secondary bacterial OM by inducing NETs in the middle ear. These NETs,
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instead of clearing the pneumococci, may then provide scaffolding for bacterial outgrowth.
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Accordingly, DNAse treatment reduced pneumococcal OM. These data provide a new
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mechanistic insight into pneumococcal-IAV co-infections and identify NETs as an important
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target for treating and preventing pneumococcal OM.
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4
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MATERIALS AND METHODS:
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Viral and bacterial strains: The bioluminescent S. pneumoniae strain EF3030lux (type 19F) [23]
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was used in all experiments. Influenza virus strain A/Udorn/307/72 (H3N2) was used to model
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infection with IAV. Virus stocks were prepared in embryonated eggs and quantified as described
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previously [24].
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Mice: Animal experiments were approved by the Animal Ethics Committee of the University of
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Melbourne and were conducted in accordance with the relevant Australian legislation. B6,
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B6.uMT-/- and B6.pIgR-/- mice were bred and housed under SPF conditions at the Department of
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Microbiology and Immunology, the University of Melbourne. B6.uMT-/- mice lack B
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lymphocytes and antibodies (although these mice can selectively produce some antibodies)
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[22,25,26]. In contrast, B6.pIgR-/- mice are deficient in the polymeric Ig receptor (pIgR) [27,28].
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Accordingly, these mice are unable to secrete polymeric antibodies and the serum of these mice
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contains significantly more IgA and IgG than sera from C57BL/6 (B6) mice [27,28].
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Infection of mice: Five-day old B6 and B6.µMT-/- mice were colonised intranasally with 2×103
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colony forming units (CFU) of S. pneumoniae EF3030Lux or PBS in 3μLs. At 14-days of age,
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mice were infected intranasally with 102.5 plaque forming units (PFU) of egg-grown IAV in
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3μLs. Six days post-IAV infection, mice were euthanized and organs were collected for analysis.
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Enumeration of bacterial and viral load: Tissues used to quantify bacterial and viral load were
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collected and processed as described previously [3,23]. Viral load was determined using plaque
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assays on MDCK cells, as described elsewhere [24].
5
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Histology and immunofluorescence: Middle ears were collected and processed for histological
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analysis essentially as described previously [3]. Antigen retrieval was performed by microwave
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heating slides in citrate buffer (Antigen retrieval solution, Dako). Slides were treated with
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Image-iT™ FX Signal Enhancer (Invitrogen), as recommended by the manufacturer and stained
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with rabbit anti-CRAMP, a marker for mouse NETs (provided by Richard Gallo, UCSD,
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California [29], 7.75µg/mL) or the respective isotype controls (rabbit IgG whole molecule,
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Jackson ImmunoResearch). Slides were subsequently stained with an Alexa 647-conjugated goat
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anti-rabbit antibody (Invitrogen) and mounted on glass slides using Prolong Gold with 4’,6’-
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diamidino-2-phenylindole (DAPI, Invitrogen). For each sample (individual ear), a minimum of
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three randomly selected images were acquired (Leica DMI6000CS confocal microscope) and
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used for quantification of NET-producing cells. Data were expressed as percentages of NET-
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forming cells in relation to the total number of cells. The mean value derived from random
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images per sample was used for statistical analysis.
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Antibody transfer: 100µL of sera was transferred to B6.µMT-/- mice daily (from 14-19 days
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old) via intraperitoneal (i.p.) injection. Sera used for transfer experiments was collected and
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pooled from naive B6.pIgR-/- mice (male and female) that were > 6 weeks of age.
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Transtympanic injection: IgA was enriched from naive B6.pIgR-/- sera using ammonium
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sulfate immunoprecipitation. IgG was then depleted with Protein G (Genscript, USA), and IgA
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was further purified using mouse IgA purification resin (Affiland SA, Belgium) as per the
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manufacturer’s instructions. IgA purity was confirmed by SDS-PAGE, Western immunoblot and
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ELISA (data not shown). Purified IgA (2µg) or PBS was then injected transtympanically into the
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middle ear of S. pneumoniae EF3030lux colonised B6. µMT mice four and five days post-IAV
6
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infection. Alternatively, 10µg of DNase (Sigma, U.S.A.) or PBS was injected transtympanically
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into the middle ear of co-infected B6 mice four and five days post-IAV infection.
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Enzyme linked Immunosorbance Assay (ELISA): Total IgA, IgM and IgG in serum samples
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and clarified middle ear homogenates were measured by a standard ELISA [22]. Briefly, 96-well
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Maxisorp immunoplates (Nunc, U.S.A.) were coated with goat anti-mouse IgA, goat anti-mouse
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IgM or goat anti mouse IgG (Sigma). Wells were blocked with BSA, washed and then serum or
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middle ear homogenates were added. To generate a standard curve, serial dilutions of known
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amounts of purified mouse IgA (BD Pharmingen, USA), mouse IgM (Millipore, USA.) or mouse
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IgG (AbD Serotech, UK) were included on each plate. Following incubation and washing, goat
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anti-mouse IgA/IgM/IgG horse-radish peroxidise conjugated antibodies (Southern Biotech,
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USA) were added. Bound antibody was visualized using OPD substrate and absorbance was
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measured at 492nm in a Multiskan plate reader (Labsystems, Finland) and antibody
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concentrations were determined using the relevant standard curve.
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7
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RESULTS:
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B6.µMT-/- mice display reduced pneumococcal outgrowth in the middle ear
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To assess the role of antibodies in pneumococcal-influenza OM, B6.µMT-/- mice and B6 mice
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were colonised with S. pneumoniae. Nine days later mice were infected with IAV and six days
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later OM was monitored. Six-days post IAV infection was selected as the time-point of interest
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as we have previously assessed the kinetics of the infection and shown that day six represents the
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peak of pneumococcal OM in infant B6 mice [3]. Consistent with our previous findings [3], B6
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mice had high bacterial titres in the middle ear following IAV infection (Fig. 1A), which we
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have previously shown to result in a 20dB hearing loss [8]. Strikingly, B6.µMT-/- mice displayed
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significantly lower pneumococcal titres in the middle ear compared to B6 mice (Fig. 1A). The
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titres of pneumococci in the middle ears of B6.µMT-/- mice (approximately 103 CFU) is
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consistent with bacterial titres in the middle ears of mice infected with S. pneumoniae alone
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[3,8], and we have demonstrated that this amount of bacteria in the ears does not result in hearing
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loss [8]. The difference in bacterial outgrowth between B6 and B6.µMT-/- mice was not due to
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impaired replication of IAV in B6.µMT-/- mice or a reduced inflammatory infiltrate in the ears of
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B6.µMT-/- mice (Fig. S1). Importantly, the observed decrease in pneumococcal replication was
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restricted to the middle ear, as no differences in pneumococcal titres were observed between B6
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and B6.µMT-/- mice in the nasopharynx (Fig. 1B).
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Antibodies facilitates pneumococcal outgrowth in the middle ear
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B6.µMT-/- mice lack B lymphocytes and therefore, for the most part, do not produce antibodies
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[22]. However, these mice can produce at least some level of IgA in response to Salmonella [25]
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as well as IgE/IgG in the lung in response to Aspergillus fumigatus [26]. Therefore, to determine 8
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if antibodies could restore pneumococcal growth, we adoptively transferred serum into B6.µMT/-
recipient mice from naive B6.pIgR-/- mice (Fig. 2A). B6.pIgR-/- mice were used instead of wild-
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type B6 mice, as these mice have higher levels of serum IgG and IgA than wild-type mice
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[27,28], therefore reducing the number of intraperitoneal injections required. Sera was derived
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from naïve mice, rather than from mice with specific anti-pneumococcal antibodies, as we found
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that 20-day old co-infected mice have not yet developed significant levels of specific anti-
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pneumococcal antibodies in the middle ear (Fig S2). Therefore, any role putative role of
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antibodies in pneumococcal OM is not likely to be driven by specific anti-pneumococcal
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antibodies. The transfer of naïve serum to co-infected B6.µMT-/- mice significantly increased
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bacterial titres in the middle ear relative to mice treated with PBS (Fig. 2A). Serum transfer also
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resulted in middle ear antibody titres that were equivalent to co-infected B6 mice (Fig. 2C-E). To
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confirm that antibodies facilitated pneumococcal outgrowth in the middle ear, we injected IgA
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purified from naïve sera directly into the middle ears of co-infected B6.uMT-/- mice. Although
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we were unable to use a large number of mice for these experiments, due to the difficulties
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associated with administering transtympanic injections to infant mice, the administration of IgA
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resulted in higher bacterial and IgA titres compared to the injection of PBS (Fig 2F and 2G). No
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reactivity was observed between the IgA used for these transtympanic injections and
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pneumococcal or IAV antigens (Fig. S3A & B), suggesting that OM can occur in the absence of
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specific anti-pneumococcal/IAV antibodies.
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NETs facilitate pneumococcal outgrowth in the ears of co-infected B6 mice
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We have previously demonstrated that infection with IAV induces an influx of neutrophils into
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the middle ear cavity of infant mice [3]. Moreover, pneumococci in the ear co-localise with this
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influx of neutrophils [3] and neutrophils are thought to interact with antibodies in the middle ear 9
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[19]. In light of the suggested role of NETs in OM [14,15], we reasoned that antibodies in the
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middle ear may facilitate bacterial outgrowth by inducing NETs. To test this hypothesis, B6 and
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B6.µMT-/- mice were co-infected with S. pneumoniae and IAV and NET formation in the middle
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ear was assessed. Immunofluorescence showed that B6.μMT-/- mice formed significantly less
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NETs in the middle ear compared to co-infected wild-type B6 mice (Fig. 3A and B). We
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confirmed with immunofluorescence that pneumococci localised to the neutrophilic infiltrate in
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the middle ears of NET positive B6 mice (see Fig. S4). The production of NETs in the middle
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ear was independent of S. pneumoniae, as NETs were also detected in the middle ears of B6
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mice infected with IAV alone (Fig. 3A and B).
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To confirm that reduced NET formation in B6.μMT-/- mice was due to the absence of antibodies
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in these mice, NET formation was assessed in co-infected B6.μMT-/- mice following
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transtympanic injection of IgA or PBS. The administration of IgA resulted in a significant
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increase in NET production in the middle ear compared to injections with PBS (Fig. 3C). PBS
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treated B6.µMT-/- mice displayed reduced NET formation compared to B6.µMT-/- mice that were
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not treated transtympanically (Fig. 3B & C) suggesting that the transtympanic injection itself
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may limit the detection of NETs by immunofluorescence. Nevertheless, the significant increase
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in NETs observed in IgA treated mice relative to PBS treated mice suggests that antibodies can
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facilitate NET production in the middle ears.
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Due to the sequestered nature of the middle ear, depletion of neutrophils using the 1A8
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monoclonal antibody is ineffectual [8]. Instead, to investigate whether or not the antibody-
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dependent difference in NET production observed between B6 and B6.µMT-/- mice affected
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bacterial titres in the middle ear, B6 mice were co-infected with S. pneumoniae EF3030lux and
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IAV. DNase was then injected transtympanically in order to destroy the NETs present in the 10
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middle ear. Alternatively, control mice were treated with PBS. Bacterial titres in the middle ear
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were then determined six days post-IAV infection. Treatment with DNAse resulted in
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significantly less pneumococci in the middle ear compared to PBS treatment (Fig. 3D). These
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data thus suggest that the decreased NET production in B6.µMT-/- mice may help reduce
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pneumococcal outgrowth in the middle ear.
228
DISCUSSION:
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OM represents a major healthcare burden worldwide and can arise from co-infection with IAV
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and S. pneumoniae. We have previously shown that IAV facilitates pneumococcal outgrowth in
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the middle ear by triggering middle ear inflammation [3,8]. The present study suggests that the
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ability of antibodies to trigger NET production in the middle ear may be a key component of the
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host inflammatory response that facilitates the development of bacterial middle ear disease.
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It has previously been suggested that NETs may be an important mechanism by which
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pneumococci persist in the middle ear [14,16], as they are able to reside in NETs and resist NET-
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mediated killing [18,30]. Consistent with this notion, bacterial biofilms entangled with DNA
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strands from NETs can be found in the in the middle ear effusions of children with recurrent
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acute OM [16]. Accordingly, it has been suggested that DNases may be a useful adjunct
239
treatment in children with recurrent or chronic otitis media [16]. However, the therapeutic
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benefit of DNase treatment in an animal model of OM has yet to be demonstrated. Here, we
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show that not only do co-infected mice develop NETs in the middle ear, but that the
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transtympanic administration of a DNAse reduces pneumococcal outgrowth in the middle ear of
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IAV infected mice. DNases have previously been used very successfully to treat patients with
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cystic fibrosis [31-33]. Similarly, a study from the early 1960s showed that a DNase derived
11
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from beef pancreas had a therapeutic effect on patients with OM [34]. Our data may thus suggest
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that DNAse treatment could be an efficacious treatment option for children with OM. Indeed, the
247
ability of a DNase (Dornase-alpha) to resolve OM in children is currently the subject of a clinical
248
trial in Australia, whereby Dornase-alpha is administered to the middle ears of children
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undergoing surgery for grommet insertion [35]. In the present study the transtympanic
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administration of DNAse to the middle ear was designed to mimic this surgical administration
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DNAse. Of course, an animal model can never fully recapitulate the complexity of human
252
disease and it remains possible that DNAse treatment would not reduce bacterial titres in
253
children, or that this would not result in a subsequent improvement in hearing. Nevertheless, our
254
data suggest that the therapeutic benefits of DNAse treatment in OM is an interesting area of
255
further study.
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Surprisingly, we identified antibodies as an important trigger for NET production in the middle
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ear. Previous studies have suggested that B6.uMT-/- mice can at least produce IgE and IgG upon
258
challenge with a fungal pathogen [26]. However, the potential production of IgE/IgG in these
259
mice following microbial challenge [26] does not detract from our findings that antibodies
260
mediate NET formation and pneumococcal OM. We showed that the transfer of purified IgA to
261
B6.uMT-/- mice induced NET production in the middle ear, which clearly indicates that
262
antibodies contribute to disease development. One possible explanation to reconcile our findings
263
with the suggestion that B6.uMT-/- mice still produce some antibodies upon infection [26] may
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be that a certain threshold level of antibodies are required for the development of secondary
265
pneumococcal OM. Alternatively, it is possible that the CD19(+)CD9(+)IgD(+) B-1 cells found
266
by Ghosh et al [26] in the lungs of μMT-/- mice are not present in the middle ear.
267 12
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A variety of different stimuli have been identified as triggering NET formation either in vivo or
269
in vitro [36]. These include IL-8, components of the complement cascade and select bacterial and
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fungal pathogens [36]. It is therefore interesting to consider the ways in which antibodies may
271
trigger NET formation in the middle ear. Antibodies could induce NET formation in an indirect
272
manner by activating the complement cascade [36,37]. The chemotactic complement-derived
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peptide complement factor 5a (C5a) can then induce NET formation by neutrophils that have
274
been primed with granulocyte macrophage colony-stimulating factor or interferons [36]. Whilst
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IAV can trigger the expression of interferon regulated genes in the middle ear [8], a role for
276
complement in middle ear NET production and pneumococcal OM is inconsistent with previous
277
reports that complement protects against the development of pneumococcal OM [38]. Moreover,
278
in vitro we have found that IgA is able to induce both NET formation and pneumococcal
279
outgrowth in the absence of complement (data not shown). Thus, clearly elucidating the pathway
280
by which antibodies induce NET formation in the middle ear remains an important area for
281
future studies.
282
In this study we used purified IgA to confirm the role of antibodies in NETs and pneumococcal
283
outgrowth in the middle ear. However, other antibody isotypes may also be able to induce NET
284
formation in vivo. Indeed, our preliminary data suggests that the presence of IgG (in the absence
285
of IgA) in the middle ears of co-infected B6.μMT-/- mice is also sufficient to facilitate
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pneumococcal outgrowth (data not shown). Thus, the interactions between antibodies and NET-
287
producing neutrophils may not be restricted to one antibody isotype.
288
The antibodies involved in the development of OM in the present study were unlikely to be
289
specific for S. pneumoniae or IAV, as middle ear disease could be induced in B6.μMT-/- mice by
290
the transfer of IgA which did not bind to pneumococci or IAV antigens. Moreover, co-infected 13
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B6 mice (which develop pneumococcal OM) did not possess pneumococcal specific antibodies
292
in the middle ear. It currently remains unclear if the ‘OM-inducing’ antibodies observed in this
293
study were natural antibodies (i.e polyspecific, low affinity antibodies that can be produced in
294
the absence of apparent antigenic stimuli) [39] or high-affinity antibodies that were produced in
295
response to non-pneumococcal antigens. The possibility also exists that these antibodies actually
296
bind to self-antigens, as autoantibodies are known inducers of NET production [40].
297
It is important to note that despite the findings of this study, specific anti-pneumococcal
298
antibodies are still likely to protect against pneumococcal OM, as has been observed in clinical
299
trials of anti-pneumococcal conjugate vaccines [41,42]. This is due, in part, to the ability of anti-
300
capsular antibodies to reduce nasopharyngeal colonisation with the corresponding serotype
301
strains [43,44]. Colonisation is the essential first step in the development of pneumococcal OM
302
and an increased number of pneumococci in the nose is associated with an increased risk of OM
303
[45,46]. Therefore, regardless of whether or not anti-pneumococcal antibodies are able to induce
304
NETS in the middle ear, anti-pneumococcal antibodies are still likely to decrease the risk of OM
305
by reducing pneumococcal colonisation in the nasopharynx.
306
Finally, this study found that the role of ‘OM-inducing antibodies’ in pneumococcal outgrowth
307
was at least to some extent tissue-specific, as B6.µMT-/- mice did not display reduced
308
pneumococcal titres in the nasal cavity relative to wild-type B6 mice. However, the role of
309
NETS in secondary pneumococcal disease may not be restricted to the middle ear. It has recently
310
been demonstrated that mice co-infected with S. pneumoniae and influenza virus display
311
increased pulmonary lesions and NET formation in the lung compared to mice infected with
312
either pathogen alone, and these NETs were unable to kill S. pneumoniae [47]. Therefore,
313
determining the role of NETs, and the stimuli required for their production, in the pathogenesis 14
314
of invasive pneumococcal disease (i.e. pneumococcal pneumonia, sepsis and meningitis) remains
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a key area of further study.
316 317 318 319 320 321 322 323 324
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ACKNOWLEDGEMENTS
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KRS is supported by a GlaxoSmithKline post-graduate support grant and the Elizabeth and
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Vernon Puzey post-graduate research scholarship. DAD is supported by the 7th Framework
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Programme of the European Commission (ETB grant 08010). OLW is supported by a Career
329
Development Fellowship (RD Wright Fellowships) from the Australian National Health and
330
Medical Research Council. The Melbourne World Health Organisation Collaborating Centre for
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Reference and Research on Influenza is supported by the Australian Government Department of
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Health and Ageing. Part of this work was supported by the Australia National Health and
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Medical Research Council (NHMRC) grant number 1044976.
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19. Faden HS. 1997. Immunology of the middle ear: role of local and systemic antibodies in clearance of viruses and bacteria. Ann. N. Y. Acad. Sci. 830: 49‐60. 20. Briles DE, Claflin JL, Schroer K, Forman C. 1981. Mouse IgG3 antibodies are highly protective against infection with Streptococcus pneumoniae. Nature. 294:88‐90. 21. Palva T, Lehtinen T, Rinne J. 1983. Immune complexes in middle ear fluid in chronic secretory otitis media. Ann. Otol. Rhinol. Laryngol. 92: 42‐44. 22. Kitamura D, Roes J, Kühn R, Rajewsky K. 1991. AB cell‐deficient mouse by targeted disruption of the membrane exon of the immunoglobulin μ chain gene. Nature 350: 423‐426. 23. Diavatopoulos D, Short K, Price J, Wilksch J, Brown L, Briles D, Strugnell R, Wijburg O. 2010. Influenza A virus facilitates Streptococcus pneumoniae transmission and disease. FASEB. J. 24: 1789‐1798. 24. Short KR, Diavatopoulos DA, Reading PC, Brown LE, Rogers KL, Strugnell RA, Wijburg OLC. 2011. Using Bioluminescent Imaging to Investigate Synergism Between Streptococcus pneumoniae and Influenza A Virus in Infant Mice. J. Vis. Exp.: e2357. 25. Macpherson AJ, Lamarre A, McCoy K, Harriman GR, Odermatt B, Dougan G, Hengartner H, Zinkernagel RM. 2001. IgA production without mu or delta chain expression in developing B cells. Nat. Immunol. 2: 625‐631. 26. Ghosh S, Hoselton SA, Schuh JM. 2012. u‐chain‐deficient mice possess B‐1 cells and produce IgG and IgE, but not IgA, following systemic sensitization and inhalational challenge in a fungal asthma model. J Immunol 189: 1322‐1329.
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27. Uren TK, Johansen FE, Wijburg OLC, Koentgen F, Brandtzaeg P, Strugnell RA. 2003. Role of the polymeric Ig receptor in mucosal B cell homeostasis. J. Immunol. 170: 2531‐2539.
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28. Sait LC, Galic M, Price JD, Simpfendorfer KR, Diavatopoulos DA, Uren TK, Janssen PH, Wijburg OLC, Strugnell RA. 2007. Secretory antibodies reduce systemic antibody responses against the gastrointestinal commensal flora. Int. Immunol. 19: 257‐265. 29. Dorschner RA, Pestonjamasp VK, Tamakuwala S, Ohtake T, Rudisill J, Nizet V, Agerberth B, Gudmundsson GH, Gallo RL. 2001 Cutaneous injury induces the release of cathelicidin anti‐ microbial peptides active against group A Streptococcus. J Invest Dematol 117: 91‐97. 30. Moorthy AN, Narasaraju T, Rai P, Perumalsamy R, Tan K, Wang S, Engelward B, Chow VT. 2013. In vivo and in vitro studies on the roles of neutrophil extracellular traps during secondary pneumococcal pneumonia after primary pulmonary influenza infection. Front. Immunol. 4:56. 31. Aitken ML, Burke W, McDonald G, Shak S, Montgomery AB, Smith A. 1992. Recombinant human DNase inhalation in normal subjects and patients with cystic fibrosis. JAMA 267: 1947‐1951. 32. Shah PL, Scott SF, Knight RA, Marriott C, Ranasinha C, Hodson ME. 1996. In vivo effects of recombinant human DNase I on sputum in patients with cystic fibrosis. Thorax 51: 119‐125. 33. Shak S, Capon DJ, Hellmiss R, Marsters SA, Baker CL. 1990. Recombinant human DNase I reduces the viscosity of cystic fibrosis sputum. Proc. Natl. Acad. Sci. U S A 87: 9188‐9192. 34. Loch WE, Loch MH. 1962. Enzyme treatment of ear infections. Local use of pancreatic dornase. Laryngoscope 72: 598‐607. 35. Research, T. I. f. C. H. 2013. "Dornase Alfa Study." Retrieved 28/6/2013, 2013, from http://vaccine.childhealthresearch.org.au/clinical‐trials/dornase‐alfa.aspx. 36. von Köckritz‐Blickwede M, Nizet V. 2009. Innate immunity turned inside‐out: antimicrobial defense by phagocyte extracellular traps. J. Mol. Med. 87: 775‐783. 37. Nascimento MTC, Wardini AB, Pinto‐da‐Silva LH, Saraiva EM. 2012. ETosis: A Microbicidal Mechanism beyond Cell Death. J. Parasitol. Res. 2012.
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38. Tong HH, Li YX, Stahl GL, Thurman JM. 2010. Enhanced susceptibility to acute pneumococcal otitis media in mice deficient in complement C1qa, factor B, and factor B/C2. Infect. Immun. 78: 976‐ 983. 39. Casali P, Schettino E. 1996 Structure and function of natural antibodies. Immunology of Silicones: Springer. pp. 167‐179. 40. Kessenbrock K, Krumbholz M, Schönermarck U, Back W, Gross WL, Werb Z, Gröne HJ, Brinkmann V, Jenne DE. 2009. Netting neutrophils in autoimmune small‐vessel vasculitis. Nat Med 15: 623‐ 625. 41. Eskola J, Kilpi T, Palmu A, Jokinen J, Eerola M, Haapakoski J, Herva E, Takala A, Käyhty H, Karma P. 2001. Efficacy of a pneumococcal conjugate vaccine against acute otitis media. New Eng. J. Med. 344: 403‐409. 42. Fireman B, Black SB, Shinefield HR, Lee J, Lewis E, Ray P. 2003. Impact of the pneumococcal conjugate vaccine on otitis media. Pediat. Infect. Dis. J. 22: 10‐16. 43. Cohen R, Levy C, Bingen E, Koskas M, Nave I, Varon E. 2012. Impact of 13‐valent pneumococcal conjugate vaccine on pneumococcal nasopharyngeal carriage in children with acute otitis media. Pediat. Infect. Dis. J. 31: 297‐301. 44. Cohen R, Levy C, Bonnet E, Grondin S, Desvignes V, Lecuyer A, Fritzell B, Varon E. 2010. Dynamic of pneumococcal nasopharyngeal carriage in children with acute otitis media following PCV7 introduction in France. Vaccine 28: 6114‐6121. 45. Kadioglu A, Weiser JN, Paton JC, Andrew PW. 2008. The role of Streptococcus pneumoniae virulence factors in host respiratory colonization and disease. Nat. Rev. Microbiol. 6: 288‐301. 46. Smith‐Vaughan H, Byun R, Nadkarni M, Jacques NA, Hunter N, Halpin S, Morris PS, Leach AJ. 2006. Measuring nasal bacterial load and its association with otitis media. BMC Ear Nose Throat Disord. 6: 10. 47. Moorthy AN, Narasaraju T, Rai P, Perumalsamy R, Tan K, Wang S, Engelward B, Chow VT. 2013. In vivo and in vitro studies on the roles of neutrophil extracellular traps during secondary pneumococcal pneumonia after primary pulmonary influenza infection. Front. Immunol. 4: 56.
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460 461 462
FIGURE LEGENDS:
463
Figure 1: Co-infected B6.µMT-/- mice do not display pneumococcal outgrowth in the middle
464
ear. A) Titers of S. pneumoniae EF3030lux in the middle ears of mice six days after i.n. infection
465
with IAV. Each data point represents an individual ear from one mouse. Statistical significance
466
was determined using a Mann-Whitney U-Test and statistical significance is denoted by three
467
asterisks (p < 0.001). Data are pooled from a minimum of two independent experiments. A
468
dashed line indicates the detection limit of the assay. B) Titers of S. pneumoniae EF3030lux in the
469
nasal cavity of mice six days after i.n. infection with IAV. Statistical significance was
470
determined using a Mann-Whitney U-Test. Data are pooled from two independent experiments.
471
A dashed line indicates the detection limit of the assay.
472 473
Figure 2: The transfer of sera induces pneumococcal OM in B6.µMT-/- mice. A) Titers of S.
474
pneumoniae EF3030lux in the middle ears of B6.µMT-/- mice six days after i.n. infection with
475
IAV. Mice were treated by daily i.p. injection from 14-days old to 19-days old with sera from
476
adult B6.pIgR-/- mice or PBS. Each data point represents an individual ear from one mouse.
477
Mann-Whitney U-Test and statistical significance is denoted by one asterisk (p < 0.05). Data are
478
pooled from a minimum of two independent experiments. A dashed line indicates the detection
479
limit of the assay. B-D) Middle ear antibody titres of IgM (B), IgG (C) and IgA (D) in co-
480
infected 20-day old B6.µMT-/- mice following transfer of sera/PBS or co-infected 20-day old
481
wild-type B6 mice (in grey bars). Statistical significance was determined using a Kruskal-Wallis 20
482
test with Dunn’s Multiple Comparison test and is denoted by one asterisk (p
1
Antibodies mediate the formation of neutrophil extracellular traps
2
(NETs) in the middle ear and facilitate secondary pneumococcal
3
otitis media
4 5 6
Kirsty R. Short1+, Maren von Köckritz-Blickwede2, Jeroen D. Langereis3, Keng Yih Chew4,
7
Emma R. Job1, Charles W. Armitage5, Brandon Hatcher6, Kohtaro Fujihashi6, Patrick C.
8
Reading1,7, Peter W. Hermans3, Odilia L. Wijburg1*§ and Dimitri A Diavatopoulos3*§
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
1
Department of Microbiology and Immunology, The University of Melbourne, Melbourne, Australia 2 Department of Physiological Chemistry, University of Veterinary Medicine, Hannover, Germany 3 Laboratory of Pediatric Infectious Diseases, Department of Pediatrics, Radboud University Medical Centre, Nijmegen, The Netherlands 4 Department of Zoology, The University of Melbourne, Melbourne, Australia 5 Institute of Health and Biomedical Innovation, Queensland University of Technology, Brisbane, Queensland, Australia 6 Department of Paediatric Dentistry and Microbiology, The University of Alabama at Birmingham, Birmingham, U.S.A 7 WHO Collaborating Centre for Reference and Research on Influenza, Parkville, Victoria, Australia
+
Current address: Department of Viroscience, Rotterdam, The Netherlands * These two authors contributed equally. § Corresponding author: [email protected] and [email protected].
30 31 32 33 34
Running title: Antibodies, NETs and otitis media 1
35
ABSTRACT: Otitis media (OM; a middle ear infection) is a common childhood illness which
36
can leave some children with permanent hearing loss. OM can arise following infection with a
37
variety of different pathogens, including a co-infection with as influenza A virus (IAV) and
38
Streptococcus pneumoniae (the pneumococcus). We and others have demonstrated that co-
39
infection with IAV facilitates the replication of pneumococci in the middle ear. Specifically, we
40
have used a mouse-model of OM to show that IAV facilitates the outgrowth of S. pneumoniae in
41
the middle ear by inducing middle ear inflammation. Here, we seek to understand how the host
42
inflammatory response facilitates bacterial outgrowth in the middle ear. Using B-cell deficient
43
infant mice, we show that antibodies play a crucial role in facilitating pneumococcal replication.
44
We subsequently show that this is due to antibody-dependent neutrophil extracellular traps
45
(NETs) formation in the middle ear, which instead of clearing the infection allow the bacteria to
46
replicate. We further demonstrate the importance of these NETs as a potential therapeutic target
47
through the transtympanic administration of a DNase, which effectively reduces the bacterial
48
load in the middle ear. Taken together, these data provide a novel insight into how pneumococci
49
are able to replicate in the middle ear cavity and induce disease.
50 51
Key Words: Otitis media, Streptococcus pneumoniae, influenza virus, viral-bacterial co-
52
infection, NETs, antibodies, neutrophils
53 54 55 56 57
2
58
INTRODUCTION
59
Otitis media (OM) is one of the most common paediatric diseases worldwide. It can affect up to
60
80% of children before the age of three years, and can lead to permanent hearing loss [1]. Up to
61
70% of cases of acute OM are caused by viral-bacterial co-infections [2]. Of particular relevance
62
are co-infections with influenza A virus (IAV) and the bacterium Streptococcus pneumoniae. In
63
clinical cases and experimental models, infection with IAV facilitates the replication of
64
S. pneumoniae in the middle ear [3-8]. Using an infant mouse model of OM (designed to mimic
65
the underdeveloped immune system of children) we have previously demonstrated that the
66
development of pneumococcal OM in co-infected mice was due to the inflammation induced by
67
IAV in the middle ear [3,8]. However, the mechanisms by which the host inflammatory response
68
mediates secondary pneumococcal OM remain undefined.
69
The middle ear has few residential leukocytes and an infection in this organ results in an influx
70
of neutrophils, macrophages and lymphocytes [9-11]. Neutrophils have traditionally been
71
considered to play a protective role in OM [12,13]. However, recent studies have speculated that
72
neutrophils may contribute to bacterial persistence in the middle ear via the formation of
73
neutrophil extracellular traps (NETs) [14-16]. NETs refer to the extracellular DNA produced by
74
neutrophils to ‘trap’ bacterial pathogens. This extracellular DNA is studded with histones and
75
antimicrobial compounds to kill the trapped bacteria [17]. Interestingly, the pneumococcal
76
capsule and D-alanine residues on pneumococcal lipoteichoic acids can inhibit NET killing [18],
77
potentially enabling the pneumococcus to survive and persist within biofilm-like NET-structures
78
in the middle ear.
3
79
Pneumococcal OM predominately develops in the absence of pre-existing immunity, with
80
incidence peaking between 6 months (when maternal antibodies have waned) and 2 years, when
81
specific immunity develops [19]. In these immunologically naïve individuals, natural antibodies
82
may represent an important defence mechanism against influenza-virus mediated pneumococcal
83
disease, as is seen in pneumococcal sepsis [20]. Conversely, the formation of immune complexes
84
in the middle ear may facilitate, rather than clear, bacterial OM [21], suggesting that organ-
85
specific differences may exist with regards to the role of antibodies during pneumococcal
86
disease. Moreover, the ability of antibodies to interact with neutrophils in the middle ear [19],
87
and the suggestion that neutrophils may facilitate bacterial OM [14,15], may indicate that the
88
role of antibodies and neutrophils in pneumococcal-influenza OM is more complex than simply
89
protecting against disease development.
90
Here, we use B6.uMT-/- mice (which lack B lymphocytes) [22] to investigate the role of
91
antibodies in pneumococcal-influenza OM. Our data suggests that antibodies facilitate the
92
development of secondary bacterial OM by inducing NETs in the middle ear. These NETs,
93
instead of clearing the pneumococci, may then provide scaffolding for bacterial outgrowth.
94
Accordingly, DNAse treatment reduced pneumococcal OM. These data provide a new
95
mechanistic insight into pneumococcal-IAV co-infections and identify NETs as an important
96
target for treating and preventing pneumococcal OM.
97
4
98
MATERIALS AND METHODS:
99
Viral and bacterial strains: The bioluminescent S. pneumoniae strain EF3030lux (type 19F) [23]
100
was used in all experiments. Influenza virus strain A/Udorn/307/72 (H3N2) was used to model
101
infection with IAV. Virus stocks were prepared in embryonated eggs and quantified as described
102
previously [24].
103 104
Mice: Animal experiments were approved by the Animal Ethics Committee of the University of
105
Melbourne and were conducted in accordance with the relevant Australian legislation. B6,
106
B6.uMT-/- and B6.pIgR-/- mice were bred and housed under SPF conditions at the Department of
107
Microbiology and Immunology, the University of Melbourne. B6.uMT-/- mice lack B
108
lymphocytes and antibodies (although these mice can selectively produce some antibodies)
109
[22,25,26]. In contrast, B6.pIgR-/- mice are deficient in the polymeric Ig receptor (pIgR) [27,28].
110
Accordingly, these mice are unable to secrete polymeric antibodies and the serum of these mice
111
contains significantly more IgA and IgG than sera from C57BL/6 (B6) mice [27,28].
112
Infection of mice: Five-day old B6 and B6.µMT-/- mice were colonised intranasally with 2×103
113
colony forming units (CFU) of S. pneumoniae EF3030Lux or PBS in 3μLs. At 14-days of age,
114
mice were infected intranasally with 102.5 plaque forming units (PFU) of egg-grown IAV in
115
3μLs. Six days post-IAV infection, mice were euthanized and organs were collected for analysis.
116
Enumeration of bacterial and viral load: Tissues used to quantify bacterial and viral load were
117
collected and processed as described previously [3,23]. Viral load was determined using plaque
118
assays on MDCK cells, as described elsewhere [24].
5
119
Histology and immunofluorescence: Middle ears were collected and processed for histological
120
analysis essentially as described previously [3]. Antigen retrieval was performed by microwave
121
heating slides in citrate buffer (Antigen retrieval solution, Dako). Slides were treated with
122
Image-iT™ FX Signal Enhancer (Invitrogen), as recommended by the manufacturer and stained
123
with rabbit anti-CRAMP, a marker for mouse NETs (provided by Richard Gallo, UCSD,
124
California [29], 7.75µg/mL) or the respective isotype controls (rabbit IgG whole molecule,
125
Jackson ImmunoResearch). Slides were subsequently stained with an Alexa 647-conjugated goat
126
anti-rabbit antibody (Invitrogen) and mounted on glass slides using Prolong Gold with 4’,6’-
127
diamidino-2-phenylindole (DAPI, Invitrogen). For each sample (individual ear), a minimum of
128
three randomly selected images were acquired (Leica DMI6000CS confocal microscope) and
129
used for quantification of NET-producing cells. Data were expressed as percentages of NET-
130
forming cells in relation to the total number of cells. The mean value derived from random
131
images per sample was used for statistical analysis.
132
Antibody transfer: 100µL of sera was transferred to B6.µMT-/- mice daily (from 14-19 days
133
old) via intraperitoneal (i.p.) injection. Sera used for transfer experiments was collected and
134
pooled from naive B6.pIgR-/- mice (male and female) that were > 6 weeks of age.
135
Transtympanic injection: IgA was enriched from naive B6.pIgR-/- sera using ammonium
136
sulfate immunoprecipitation. IgG was then depleted with Protein G (Genscript, USA), and IgA
137
was further purified using mouse IgA purification resin (Affiland SA, Belgium) as per the
138
manufacturer’s instructions. IgA purity was confirmed by SDS-PAGE, Western immunoblot and
139
ELISA (data not shown). Purified IgA (2µg) or PBS was then injected transtympanically into the
140
middle ear of S. pneumoniae EF3030lux colonised B6. µMT mice four and five days post-IAV
6
141
infection. Alternatively, 10µg of DNase (Sigma, U.S.A.) or PBS was injected transtympanically
142
into the middle ear of co-infected B6 mice four and five days post-IAV infection.
143
Enzyme linked Immunosorbance Assay (ELISA): Total IgA, IgM and IgG in serum samples
144
and clarified middle ear homogenates were measured by a standard ELISA [22]. Briefly, 96-well
145
Maxisorp immunoplates (Nunc, U.S.A.) were coated with goat anti-mouse IgA, goat anti-mouse
146
IgM or goat anti mouse IgG (Sigma). Wells were blocked with BSA, washed and then serum or
147
middle ear homogenates were added. To generate a standard curve, serial dilutions of known
148
amounts of purified mouse IgA (BD Pharmingen, USA), mouse IgM (Millipore, USA.) or mouse
149
IgG (AbD Serotech, UK) were included on each plate. Following incubation and washing, goat
150
anti-mouse IgA/IgM/IgG horse-radish peroxidise conjugated antibodies (Southern Biotech,
151
USA) were added. Bound antibody was visualized using OPD substrate and absorbance was
152
measured at 492nm in a Multiskan plate reader (Labsystems, Finland) and antibody
153
concentrations were determined using the relevant standard curve.
154
7
155
RESULTS:
156
B6.µMT-/- mice display reduced pneumococcal outgrowth in the middle ear
157
To assess the role of antibodies in pneumococcal-influenza OM, B6.µMT-/- mice and B6 mice
158
were colonised with S. pneumoniae. Nine days later mice were infected with IAV and six days
159
later OM was monitored. Six-days post IAV infection was selected as the time-point of interest
160
as we have previously assessed the kinetics of the infection and shown that day six represents the
161
peak of pneumococcal OM in infant B6 mice [3]. Consistent with our previous findings [3], B6
162
mice had high bacterial titres in the middle ear following IAV infection (Fig. 1A), which we
163
have previously shown to result in a 20dB hearing loss [8]. Strikingly, B6.µMT-/- mice displayed
164
significantly lower pneumococcal titres in the middle ear compared to B6 mice (Fig. 1A). The
165
titres of pneumococci in the middle ears of B6.µMT-/- mice (approximately 103 CFU) is
166
consistent with bacterial titres in the middle ears of mice infected with S. pneumoniae alone
167
[3,8], and we have demonstrated that this amount of bacteria in the ears does not result in hearing
168
loss [8]. The difference in bacterial outgrowth between B6 and B6.µMT-/- mice was not due to
169
impaired replication of IAV in B6.µMT-/- mice or a reduced inflammatory infiltrate in the ears of
170
B6.µMT-/- mice (Fig. S1). Importantly, the observed decrease in pneumococcal replication was
171
restricted to the middle ear, as no differences in pneumococcal titres were observed between B6
172
and B6.µMT-/- mice in the nasopharynx (Fig. 1B).
173
Antibodies facilitates pneumococcal outgrowth in the middle ear
174
B6.µMT-/- mice lack B lymphocytes and therefore, for the most part, do not produce antibodies
175
[22]. However, these mice can produce at least some level of IgA in response to Salmonella [25]
176
as well as IgE/IgG in the lung in response to Aspergillus fumigatus [26]. Therefore, to determine 8
177 178
if antibodies could restore pneumococcal growth, we adoptively transferred serum into B6.µMT/-
recipient mice from naive B6.pIgR-/- mice (Fig. 2A). B6.pIgR-/- mice were used instead of wild-
179
type B6 mice, as these mice have higher levels of serum IgG and IgA than wild-type mice
180
[27,28], therefore reducing the number of intraperitoneal injections required. Sera was derived
181
from naïve mice, rather than from mice with specific anti-pneumococcal antibodies, as we found
182
that 20-day old co-infected mice have not yet developed significant levels of specific anti-
183
pneumococcal antibodies in the middle ear (Fig S2). Therefore, any role putative role of
184
antibodies in pneumococcal OM is not likely to be driven by specific anti-pneumococcal
185
antibodies. The transfer of naïve serum to co-infected B6.µMT-/- mice significantly increased
186
bacterial titres in the middle ear relative to mice treated with PBS (Fig. 2A). Serum transfer also
187
resulted in middle ear antibody titres that were equivalent to co-infected B6 mice (Fig. 2C-E). To
188
confirm that antibodies facilitated pneumococcal outgrowth in the middle ear, we injected IgA
189
purified from naïve sera directly into the middle ears of co-infected B6.uMT-/- mice. Although
190
we were unable to use a large number of mice for these experiments, due to the difficulties
191
associated with administering transtympanic injections to infant mice, the administration of IgA
192
resulted in higher bacterial and IgA titres compared to the injection of PBS (Fig 2F and 2G). No
193
reactivity was observed between the IgA used for these transtympanic injections and
194
pneumococcal or IAV antigens (Fig. S3A & B), suggesting that OM can occur in the absence of
195
specific anti-pneumococcal/IAV antibodies.
196
NETs facilitate pneumococcal outgrowth in the ears of co-infected B6 mice
197
We have previously demonstrated that infection with IAV induces an influx of neutrophils into
198
the middle ear cavity of infant mice [3]. Moreover, pneumococci in the ear co-localise with this
199
influx of neutrophils [3] and neutrophils are thought to interact with antibodies in the middle ear 9
200
[19]. In light of the suggested role of NETs in OM [14,15], we reasoned that antibodies in the
201
middle ear may facilitate bacterial outgrowth by inducing NETs. To test this hypothesis, B6 and
202
B6.µMT-/- mice were co-infected with S. pneumoniae and IAV and NET formation in the middle
203
ear was assessed. Immunofluorescence showed that B6.μMT-/- mice formed significantly less
204
NETs in the middle ear compared to co-infected wild-type B6 mice (Fig. 3A and B). We
205
confirmed with immunofluorescence that pneumococci localised to the neutrophilic infiltrate in
206
the middle ears of NET positive B6 mice (see Fig. S4). The production of NETs in the middle
207
ear was independent of S. pneumoniae, as NETs were also detected in the middle ears of B6
208
mice infected with IAV alone (Fig. 3A and B).
209
To confirm that reduced NET formation in B6.μMT-/- mice was due to the absence of antibodies
210
in these mice, NET formation was assessed in co-infected B6.μMT-/- mice following
211
transtympanic injection of IgA or PBS. The administration of IgA resulted in a significant
212
increase in NET production in the middle ear compared to injections with PBS (Fig. 3C). PBS
213
treated B6.µMT-/- mice displayed reduced NET formation compared to B6.µMT-/- mice that were
214
not treated transtympanically (Fig. 3B & C) suggesting that the transtympanic injection itself
215
may limit the detection of NETs by immunofluorescence. Nevertheless, the significant increase
216
in NETs observed in IgA treated mice relative to PBS treated mice suggests that antibodies can
217
facilitate NET production in the middle ears.
218
Due to the sequestered nature of the middle ear, depletion of neutrophils using the 1A8
219
monoclonal antibody is ineffectual [8]. Instead, to investigate whether or not the antibody-
220
dependent difference in NET production observed between B6 and B6.µMT-/- mice affected
221
bacterial titres in the middle ear, B6 mice were co-infected with S. pneumoniae EF3030lux and
222
IAV. DNase was then injected transtympanically in order to destroy the NETs present in the 10
223
middle ear. Alternatively, control mice were treated with PBS. Bacterial titres in the middle ear
224
were then determined six days post-IAV infection. Treatment with DNAse resulted in
225
significantly less pneumococci in the middle ear compared to PBS treatment (Fig. 3D). These
226
data thus suggest that the decreased NET production in B6.µMT-/- mice may help reduce
227
pneumococcal outgrowth in the middle ear.
228
DISCUSSION:
229
OM represents a major healthcare burden worldwide and can arise from co-infection with IAV
230
and S. pneumoniae. We have previously shown that IAV facilitates pneumococcal outgrowth in
231
the middle ear by triggering middle ear inflammation [3,8]. The present study suggests that the
232
ability of antibodies to trigger NET production in the middle ear may be a key component of the
233
host inflammatory response that facilitates the development of bacterial middle ear disease.
234
It has previously been suggested that NETs may be an important mechanism by which
235
pneumococci persist in the middle ear [14,16], as they are able to reside in NETs and resist NET-
236
mediated killing [18,30]. Consistent with this notion, bacterial biofilms entangled with DNA
237
strands from NETs can be found in the in the middle ear effusions of children with recurrent
238
acute OM [16]. Accordingly, it has been suggested that DNases may be a useful adjunct
239
treatment in children with recurrent or chronic otitis media [16]. However, the therapeutic
240
benefit of DNase treatment in an animal model of OM has yet to be demonstrated. Here, we
241
show that not only do co-infected mice develop NETs in the middle ear, but that the
242
transtympanic administration of a DNAse reduces pneumococcal outgrowth in the middle ear of
243
IAV infected mice. DNases have previously been used very successfully to treat patients with
244
cystic fibrosis [31-33]. Similarly, a study from the early 1960s showed that a DNase derived
11
245
from beef pancreas had a therapeutic effect on patients with OM [34]. Our data may thus suggest
246
that DNAse treatment could be an efficacious treatment option for children with OM. Indeed, the
247
ability of a DNase (Dornase-alpha) to resolve OM in children is currently the subject of a clinical
248
trial in Australia, whereby Dornase-alpha is administered to the middle ears of children
249
undergoing surgery for grommet insertion [35]. In the present study the transtympanic
250
administration of DNAse to the middle ear was designed to mimic this surgical administration
251
DNAse. Of course, an animal model can never fully recapitulate the complexity of human
252
disease and it remains possible that DNAse treatment would not reduce bacterial titres in
253
children, or that this would not result in a subsequent improvement in hearing. Nevertheless, our
254
data suggest that the therapeutic benefits of DNAse treatment in OM is an interesting area of
255
further study.
256
Surprisingly, we identified antibodies as an important trigger for NET production in the middle
257
ear. Previous studies have suggested that B6.uMT-/- mice can at least produce IgE and IgG upon
258
challenge with a fungal pathogen [26]. However, the potential production of IgE/IgG in these
259
mice following microbial challenge [26] does not detract from our findings that antibodies
260
mediate NET formation and pneumococcal OM. We showed that the transfer of purified IgA to
261
B6.uMT-/- mice induced NET production in the middle ear, which clearly indicates that
262
antibodies contribute to disease development. One possible explanation to reconcile our findings
263
with the suggestion that B6.uMT-/- mice still produce some antibodies upon infection [26] may
264
be that a certain threshold level of antibodies are required for the development of secondary
265
pneumococcal OM. Alternatively, it is possible that the CD19(+)CD9(+)IgD(+) B-1 cells found
266
by Ghosh et al [26] in the lungs of μMT-/- mice are not present in the middle ear.
267 12
268
A variety of different stimuli have been identified as triggering NET formation either in vivo or
269
in vitro [36]. These include IL-8, components of the complement cascade and select bacterial and
270
fungal pathogens [36]. It is therefore interesting to consider the ways in which antibodies may
271
trigger NET formation in the middle ear. Antibodies could induce NET formation in an indirect
272
manner by activating the complement cascade [36,37]. The chemotactic complement-derived
273
peptide complement factor 5a (C5a) can then induce NET formation by neutrophils that have
274
been primed with granulocyte macrophage colony-stimulating factor or interferons [36]. Whilst
275
IAV can trigger the expression of interferon regulated genes in the middle ear [8], a role for
276
complement in middle ear NET production and pneumococcal OM is inconsistent with previous
277
reports that complement protects against the development of pneumococcal OM [38]. Moreover,
278
in vitro we have found that IgA is able to induce both NET formation and pneumococcal
279
outgrowth in the absence of complement (data not shown). Thus, clearly elucidating the pathway
280
by which antibodies induce NET formation in the middle ear remains an important area for
281
future studies.
282
In this study we used purified IgA to confirm the role of antibodies in NETs and pneumococcal
283
outgrowth in the middle ear. However, other antibody isotypes may also be able to induce NET
284
formation in vivo. Indeed, our preliminary data suggests that the presence of IgG (in the absence
285
of IgA) in the middle ears of co-infected B6.μMT-/- mice is also sufficient to facilitate
286
pneumococcal outgrowth (data not shown). Thus, the interactions between antibodies and NET-
287
producing neutrophils may not be restricted to one antibody isotype.
288
The antibodies involved in the development of OM in the present study were unlikely to be
289
specific for S. pneumoniae or IAV, as middle ear disease could be induced in B6.μMT-/- mice by
290
the transfer of IgA which did not bind to pneumococci or IAV antigens. Moreover, co-infected 13
291
B6 mice (which develop pneumococcal OM) did not possess pneumococcal specific antibodies
292
in the middle ear. It currently remains unclear if the ‘OM-inducing’ antibodies observed in this
293
study were natural antibodies (i.e polyspecific, low affinity antibodies that can be produced in
294
the absence of apparent antigenic stimuli) [39] or high-affinity antibodies that were produced in
295
response to non-pneumococcal antigens. The possibility also exists that these antibodies actually
296
bind to self-antigens, as autoantibodies are known inducers of NET production [40].
297
It is important to note that despite the findings of this study, specific anti-pneumococcal
298
antibodies are still likely to protect against pneumococcal OM, as has been observed in clinical
299
trials of anti-pneumococcal conjugate vaccines [41,42]. This is due, in part, to the ability of anti-
300
capsular antibodies to reduce nasopharyngeal colonisation with the corresponding serotype
301
strains [43,44]. Colonisation is the essential first step in the development of pneumococcal OM
302
and an increased number of pneumococci in the nose is associated with an increased risk of OM
303
[45,46]. Therefore, regardless of whether or not anti-pneumococcal antibodies are able to induce
304
NETS in the middle ear, anti-pneumococcal antibodies are still likely to decrease the risk of OM
305
by reducing pneumococcal colonisation in the nasopharynx.
306
Finally, this study found that the role of ‘OM-inducing antibodies’ in pneumococcal outgrowth
307
was at least to some extent tissue-specific, as B6.µMT-/- mice did not display reduced
308
pneumococcal titres in the nasal cavity relative to wild-type B6 mice. However, the role of
309
NETS in secondary pneumococcal disease may not be restricted to the middle ear. It has recently
310
been demonstrated that mice co-infected with S. pneumoniae and influenza virus display
311
increased pulmonary lesions and NET formation in the lung compared to mice infected with
312
either pathogen alone, and these NETs were unable to kill S. pneumoniae [47]. Therefore,
313
determining the role of NETs, and the stimuli required for their production, in the pathogenesis 14
314
of invasive pneumococcal disease (i.e. pneumococcal pneumonia, sepsis and meningitis) remains
315
a key area of further study.
316 317 318 319 320 321 322 323 324
15
325
ACKNOWLEDGEMENTS
326
KRS is supported by a GlaxoSmithKline post-graduate support grant and the Elizabeth and
327
Vernon Puzey post-graduate research scholarship. DAD is supported by the 7th Framework
328
Programme of the European Commission (ETB grant 08010). OLW is supported by a Career
329
Development Fellowship (RD Wright Fellowships) from the Australian National Health and
330
Medical Research Council. The Melbourne World Health Organisation Collaborating Centre for
331
Reference and Research on Influenza is supported by the Australian Government Department of
332
Health and Ageing. Part of this work was supported by the Australia National Health and
333
Medical Research Council (NHMRC) grant number 1044976.
334
16
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FIGURE LEGENDS:
463
Figure 1: Co-infected B6.µMT-/- mice do not display pneumococcal outgrowth in the middle
464
ear. A) Titers of S. pneumoniae EF3030lux in the middle ears of mice six days after i.n. infection
465
with IAV. Each data point represents an individual ear from one mouse. Statistical significance
466
was determined using a Mann-Whitney U-Test and statistical significance is denoted by three
467
asterisks (p < 0.001). Data are pooled from a minimum of two independent experiments. A
468
dashed line indicates the detection limit of the assay. B) Titers of S. pneumoniae EF3030lux in the
469
nasal cavity of mice six days after i.n. infection with IAV. Statistical significance was
470
determined using a Mann-Whitney U-Test. Data are pooled from two independent experiments.
471
A dashed line indicates the detection limit of the assay.
472 473
Figure 2: The transfer of sera induces pneumococcal OM in B6.µMT-/- mice. A) Titers of S.
474
pneumoniae EF3030lux in the middle ears of B6.µMT-/- mice six days after i.n. infection with
475
IAV. Mice were treated by daily i.p. injection from 14-days old to 19-days old with sera from
476
adult B6.pIgR-/- mice or PBS. Each data point represents an individual ear from one mouse.
477
Mann-Whitney U-Test and statistical significance is denoted by one asterisk (p < 0.05). Data are
478
pooled from a minimum of two independent experiments. A dashed line indicates the detection
479
limit of the assay. B-D) Middle ear antibody titres of IgM (B), IgG (C) and IgA (D) in co-
480
infected 20-day old B6.µMT-/- mice following transfer of sera/PBS or co-infected 20-day old
481
wild-type B6 mice (in grey bars). Statistical significance was determined using a Kruskal-Wallis 20
482
test with Dunn’s Multiple Comparison test and is denoted by one asterisk (p
