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Cytomegalovirus-specific T-cell therapies: current status and future prospects

Adoptive transfer of T cells specific for viral pathogens offers an attractive method for hastening immune reconstitution and protective immunity in patients following stem cell transplantation. The largest experience to date has been in the context of treatment or prevention of cytomegalovirus or Epstein–Barr virus. A number of technical hurdles have now been overcome allowing consideration of more widespread application of products compliant with Good Manufacturing Practice regulations, and of the development of commercialization pathways for these products. This review summarizes progress to date and highlights some of the areas that remain problematic and that require further innovation and evaluation before more widespread adoption is considered. Keywords:  cellular therapy • cytokine capture • cytomegalovirus • MHC multimer • T cell

The combination of immunoablative conditioning therapy and post-transplant immuno­ suppression leads to profound qualitative and quantitative deficiencies in T-cell immunity following allogeneic hematopoietic stem cell transplantation, which are further exacerbated when T-cell depletion is employed to reduce the risk of graft-versus-host disease. A delay in reconstitution of cytomegalo­virus (CMV)specific T-cell responses lead­s to a high incidence of CMV infection that can manifest as end-organ disease, whilst early recovery of CD8 + CMV-specific T cells reduces the risk of development of CMV infection [1–3] . In individuals expressing the HLA A*0201 allele, immunity to the CMV pp65 antigen correlates inversely with risk of infection [4,5] . The risk of CMV infection is related to both recipient and donor CMV serostatus. CMVseropositive hematopoietic stem cell transplantation recipients have the highest risk of CMV infection (60–90%) compared with 30–50% of CMV-sero­ negative recipients with a seropositive donor [6] . Sensitive molecular surveillance techniques, such as the quantitative polymerase chain reaction, allows very early detection of

10.2217/IMT.14.99 © 2015 Future Medicine Ltd

Emma Nicholson1 & Karl S Peggs*,1,2 Department of Haematology, University College London Hospital, London, NW1 2BU, UK 2 University College London Cancer Institute, Paul O’Gorman Building, 72 Huntley Street, London, WC1E 6BT, UK *Author for correspondence: Tel.: +44 207 679 6236 Fax: +44 207 679 6222 karl.peggs@ ucl.ac.uk 1

CMV infection (viral replication) by detection of viral DNA in blood, allowing preemptive antiviral therapies to be administered at the onset of CMV viremia, and reducing progression to CMV disease (defined by the presence of end-organ damage). This is important because CMV disease, particularly pneumonitis, is still associated with considerable morbidity and mortality. The use of sensitive surveillance monitoring has reduced the incidence of early CMV disease to 4–6% from historical levels approaching 30%. Modern antiviral therapies have good activity against the virus, but can lead to myelosuppression (ganciclovir, valganciclovir, cidofovir) and/or renal impairment (foscarnet, cidofovir) leading to increased morbidity. Numerous small Phase I–II studies demonstrate the potential to enhance CMV-specific immune reconstitution by adoptive transfer of donor-derived CMV-specific T cells in cases where the donor is CMV seropositive [6–11] . Healthy CMV-seropositive individuals have high frequencies of circulating CMVspecific T cells [12] and various strategies have been developed for isolating and expanding

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Review  Nicholson & Peggs them. All of the earlier strategies employed culturebased techniques to generate CMV-specific T cells. Examples include those utilizing repetitive stimulation with dendritic cells (DCs) pulsed with CMV peptides or lysates [8,13] and those utilizing artificial APCs [14,15] . While such therapeutic products had demonstrable efficacy and safety in Phase I and II trials, their clinical applicability is constrained by the time and labor involved in their generation to the required GMP standards. More recent strategies take advantage of the high frequencies of circulating CMV-specific T cells in normal individuals, employing direct isolation using cytokine capture techniques or HLA-multimer technology. All of these techniques, however, are applicable only in the context of a CMV-seropositive donor. Third party CMV-specific T cells potentially offer an ‘off-the-shelf’ treatment strategy, which can be utilized in the setting of both CMV-negative and -positive donors and also cord blood transplants. The major questions regarding these strategies relate to the durability of persistence of adoptively transferred cells and associated immunity, as early rejection is commonly documented. Generation of CMV-specific T cells by in vitro stimulation techniques CMV-specific T cells can be generated by the ex vivo stimulation of donor-derived T cells with CMV antigens. In vitro culture techniques allow a selective increase in the proportion of donor-derived CMVspecific T cells, and transfer of relatively small numbers of T cells can lead to impressive expansion and apparent resolution of CMV infection. This risk of graft-versus-host disease (GVHD) appears to be low. The initial studies demonstrating the feasibility of adoptively transferring CMV-specific immunity by Riddell [16] and Walter [17] et al. employed mono­ clonal CMV-specific CD8 + cytotoxic T lymphocytes (CTLs) generated by limiting dilution approaches and repeated ex vivo stimulation with autologous CMVinfected fibroblasts. CMV-specific CD8 + CTLs were adoptively transferred into HSCT recipients, resulting in reconstitution of CMV-specific cytotoxic responses with no increase in GVHD. CMV-specific CTL responses were sustained for 8 weeks following adoptive transfer. Patients lacking endogenous recovery of CD4 + T helper cells had a decrease in magnitude of the CD8 + T-cell response, while those with good recovery of CMV-specific CD4 + responses had more sustained CD8 + responses. These initial studies were performed in T-replete HLA-matched sibling transplants and the majority of patients were CMV seronegative, i.e., this was a relatively low risk population for CMV infection and disease. In addition the production of this product required extensive (8–10 weeks) in vitro cul-

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ture in order to generate the high number of T cells for adoptive transfer. The use of infectious viral particles within the culture also carried the potential for transfer of infection following T-cell transfer. Use of polyclonal or oligoclonal CMV-specific T-cell products enables shorter in vitro culture times. Polyclonal CMV T cells can be expanded by culturing donor T cells with monocyte-derived DCs pulsed with CMV lysate or with HLA restricted CMV peptides. The use of the HLA-A2 restricted NLV peptide (derived from pp65) efficiently generates CD8 + CTLs, but restricts application to donors who express HLAA2. Overlapping peptide pools derived from pp65 can be used to expand T cells with multiple antigenic specificities across a broader array of HLA restricting elements. Alternatively, adenoviral vectors encoding the whole of pp65 antigen can be used to transfect DCs. This leads to intracellular processing of the pp65 protein and can generate both CD4 + and CD8 + T-cell responses to a variety of epitopes. One potential advantage of such strategies is the co-expansion of adeno­virus-specific T-cell populations. Studies evaluating the transfer of polyclonal CMV T cells demonstrated that much smaller doses of transferred T cells could provide effective anti-CMV immunity. Transferring relatively low doses of T cells is desirable as it reduces the requirement for prolonged ex vivo culture to expand the product. Peggs et al. generated CMV-specific cell ‘lines’ from donor-derived peripheral blood mononuclear cells (PBMCs) co-cultured with DCs pulsed with CMV lysate. CMV-specific T cells were administered to hematopoietic stem cell transplant recipients in a pre-emptive fashion following the first detection in peripheral blood of CMV DNA. All of the recipients were CMV-positive and >90% had undergone a T-cell depleted transplant. 50% of the patients in the study cleared CMV without the need for antiviral treatment and there were no episodes of CMV disease. The cell doses transferred were low (grade I GVHD (2 of these had undergone T replete transplants). None of the patients treated prophylactically required antiviral treatment, while in the pre-emptive group 2 required

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no antiviral treatment and 7 required only a single treatment episode. All of those treated prophylactically had evidence of in vivo expansion. CMV infection, as opposed to the dose of cells infused was associated with higher peak levels of expansion. Within 4 weeks post transfer the predominant T cells were effector memory and terminally differentiated effector cells. Third party viral-specific T cells The use of third party viral-specific T cells could circumvent the need for bespoke patient-specific therapies, potentially reducing costs, speeding delivery and facilitating more widespread application. However, they face the dual hurdles of a potentially higher risk of rejection or of GVHD if successfully engrafted than in the case of second party cells due to nonshared HLA-antigens. Initial studies using partially HLA matched third party virus-specific T cells were performed in the context of Epstein–Barr virus (EBV)-associated post-transplant lymphoproliferative disorders (PTLD) [22–24] . The EBV-specific T cells were matched at a minimum of 3/6 HLA alleles (HLA-A, -B and -DR) with the recipient. They were adoptively transferred to solid organ transplant recipients with PTLD resulting in a 42% complete response rate. The strategy was extended to PTLD occurring post hematopoietic stem cell transplantation, leading to a complete or partial remission in 68% of patients [25,26] . Leen et al. [27,28] generated a bank of 32 virus-specific T cell lines from individuals with common HLA molecules who had preexisting immunity to EBV, adenovirus and CMV. From this cell bank, a cell line matching at least 1 HLA-allele to which virus-specific immunity was restricted could be identified for 90% of the screened patients within 24 h Eighteen different cell lines were administered to 50 patients who had viral infections deemed refractory to pharmacotherapy. A 74% complete or partial response rate was demonstrated by 6 weeks post infusion (73% for CMV, 77% for adenovirus and 66% for EBV infections) with 89% of responses being durable. Two patients developed de novo GVHD but there were no direct immediate adverse effects secondary to T-cell infusion. Demonstration of in vivo expansion of infused populations was difficult. Nevertheless, infused cells could be detected at very low levels by molecular techniques for prolonged periods in a minority of cases, and reconstitution of antiviral immunity of presumed endogenous second party derivation was demonstrated. Multivirus-specific T-cell products CTL lines have been generated that have specificity for more than one virus allowing targeting of

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Cytomegalovirus-specific T-cell therapies: current status & future prospects 

the most common viruses occurring post-transplant with one single cellular product. CMV and adenovirus-specific T cells were generated by co-culture of donor-derived T cells with APCs transduced with a recombinant adenoviral vector encoding CMV pp65. EBV transformed B cells transduced with the same vector were then used to restimulate the CMV and adenovirus-specific T cells, thus generating EBV, CMV and adenovirus-specific CTLs within the one culture. These trispecific T cells were administered to 14 allograft recipients in a prophylactic setting leading to predominantly an increase in CMV and EBVspecific immunity in recipients and leading to protective antiviral responses [28] . Alternatively, overlapping peptide pools from the more prevalent immunological target antigens of each virus can be used to restimulate and expand pre-existing immunity. This strategy can be extended to include other viruses of interest. Gerdemann et al. [29] produced a single preparation of polyclonal CD4 + and CD8 + CTLs specific for 15 immunodominant antigens derived from EBV, CMV, Adenovirus, BK virus, HHV6, RSV and Influenza. CTLs were generated from donor-derived PBMCs following stimulation with a pepmix (peptide library of overlapping 15mers) in the presence of IL-4 and IL-7 to promote long-term survival and expansion. The donor PBMCs acted as both a source of APCs and responding T cells. IL-4 and IL-7 led to selective expansion of polyclonal Th1 polarized T cells that produced multiple effector cytokines upon stimulation. These products had demonstrable antiviral activity against CMV, EBV and adenovirus in vitro. 1 × 108 CTLs were produced within 10 days with a >10 fold enrichment in virus-specific cells and a corresponding reduction in alloreactive T cells. This technique increases the range of viral antigens that can be recognized by a single CTL product and reduces the impact of antigenic competition to try to retain both high and low frequency T cells. The production of CTLs that recognize multiple differing viral epitopes also minimizes potential for virus escape due to epitope loss. These multispecific T cells were adoptively transferred to allogeneic hematopoietic stem cell recipients with complete clearance of virus in 80%, including patients that had greater than one viral infection [30] . In each patient the decrease in viral load was associated with an increase in the frequency of antigen-specific T cells without any increase in rates of GVHD. This technique has been further expanded to generate a viral-specific T-cell product that recognizes 12 different antigens from 5 viruses (EBV, adenovirus, CMV, BK virus and HHV-6). The transfer of these multivirus-specific T cells to allograft recipients led to an overall response rate of 94% [31] .

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TCR gene therapy for production of CMV-specific T cells For patients with a CMV-seronegative donor, including cord blood donors, CMV-specific T cells cannot be selected or easily cultured from the donor peripheral blood. Naive CMV-specific T cells are difficult to isolate from the peripheral blood of CMV-negative donors [32,33] . As a result, CMV-seropositive recipients of HSCT from CMV-seronegative donors are considered to be at particular risk of developing CMV related disease because the reconstitution of CMV-specific T-cell responses is absent. Culture techniques allowing expansion of CMV-specific T cells from CMV naive donors have been described [34] and results of clinical application keenly awaited. An alternative strategy to generate CMV-specific T cells is to utilize retroviral transduction to introduce a TCR with specificity for CMV into polyclonal T cells, thereby redirecting their specificity. TCR gene transfer generates functional T cells that have the same specificity as the parental clone and can secrete cytokines and proliferate in response to specific antigen, and lyse antigen-­ positive targets in vitro [35,36] . High affinity T-cell clones can be isolated using MHC-peptide tetramer staining and/or their ability to recognize and lyse target cells that have been pulsed with low concentrations of cognate peptide/MHC. Schub et al. [37] selected 4 different CMV-specific CD8+ T-cell clones, isolated the alpha and beta chains of their TCRs and then inserted these into retroviral vectors to transduce primary T cells. The transgenic T cells could be specifically expanded and enriched in vivo in response to endogenously processed antigen and had demonstrable increase in antigen-specific cytotoxicity and cytokine secretion. One possible drawback of TCR gene therapy is mispairing of the introduced and endogenous TCR chains, leading to production of novel TCRs of un­defined specificity. There are now a number of strategies to limit mispairing such as murinization of the human TCR constant region or engineering the α and β chains to express an additional disulfide bond [38,39] . Expansion of T cells co-expressing potentially allo­ reactive TCRs also poses a potential risk in terms of induction of GVHD. TCR gene modified T cells can, however, be modified to express suicide genes which allows deletion should toxicities develop post transfer [40,41] . A Phase I study of CMV TCR transduced T cells for treatment of CMV infection post HSCT is ongoing in the UK. Enhancing persistence of adoptively transferred CMV-specific T cells In order to deliver effective anti-viral responses, adoptively transferred cells must have the ability to persist

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9

Sibling and MUD

Sibling, MUD and MMUD

Sibling, MUD and MMUD

Sibling, MUD and MMUD

Donor type

5/9

24/50

2/9

24/30

T-cell depletion

 

36 +/+ 14 -/+

6 +/+ 3 -/+

24 +/+ 6 -/+

CMV R/D

8.6 × 103 /kg

2 × 107 CMV CTLs/m2

2 × 107 CMV CTLs/m2

1 × 105 total cultured cells/kg

Median cell dose

CMV specific T cells detected in all patients 10 days post infusion; two had long-term detectable responses. All had reduction in CMV viremia, 8/9 complete viral clearance 3/9 required additional antiviral therapy

26/50 reactivated CMV (14 of these had reactivated prior to infusion of CTL). 9/26 required antiviral therapy. Compared to a nontreated cohort, no reduction in rate of CMV reactivation but reduction in number of patients requiring antiviral therapy and reduced treatment days

6/9 had a detectable rise in NLV specific T cells with a 1.85-fold increase in cell number detected post transfer. 2/9 developed CMV viremia; 0/9 developed CMV disease

Prophylactic group – three patients required antiviral therapy, one had CMV viremia without needing treatment. Pre-emptive group – ten patients required antiviral therapy

Outcome

CMV: Cytomegalovirus; CTL: Cytotoxic T lymphocyte; HSCT: Hematopoietic stem cell transplantation; DC: Dendritic cell; MMUD: Mismatched unrelated donor; MUD: Matched unrelated donor; PBMC: Peripheral blood mononuclear cell; R/D: Recipient/donor.

Cobbold et al. CMV specific CD8 + T cells purified using HLA-peptide tetramers (derived from pp65 or CMV-IE-1

7/9 pre-emptive; 2/9 following persistent viremia

Prophylactic

Blyth et al.

PBMC co50 culture with DCs transfected with adenoviral vector encoding pp65

Prophylactic

20/30 pre-emptive 10/30 prophylactic

Treatment strategy

Micklethwaite PBMC co-culture 9 et al. with DCs pulsed with pp65

PBMC co-culture 30 with DCs pulsed with CMV lysate

Peggs et al.

No. of HSCT recipients

Production technique

Study 

Table 1. Summary of Phase I/II studies of cytomegalovirus-specific T-cell therapy.

[20]

[10]

[18]

[6]

Ref.

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IFN-γ capture

IFN-γ capture

Feuchtinger et al.

Peggs et al.

18

18

No. of HSCT recipients

Donor type

T-cell depletion

11 pre-emptive; 7 prophylactic

Sibling

13/18

Refractory infections MUD, 16/18 (all on concurrent haploidentical antiviral therapy) and cord

Treatment strategy

13 +/+ 5 -/+

15 +/+ 2 -/+ 1 +/-

CMV R/D

2840 CD4 + cells/kg 630 CD8 + cells/kg (CMV specific)

21 × 103 /kg pp65 specific T cells

Median cell dose

2/9 pre-emptive group required no antiviral therapy; 7/9 pre-emptive group required a single treatment episode. None of the prophylactic group required antiviral treatment.

15/18 had clearance of CMV viremia or a >1 log reduction in CMV viremia associated with detectable in vivo expansion of CMV specific T cells. 3 non responders.

Outcome

CMV: Cytomegalovirus; CTL: Cytotoxic T lymphocyte; HSCT: Hematopoietic stem cell transplantation; DC: Dendritic cell; MMUD: Mismatched unrelated donor; MUD: Matched unrelated donor; PBMC: Peripheral blood mononuclear cell; R/D: Recipient/donor.

Production technique

Study 

Table 1. Summary of Phase I/II studies of CMV-specific T-cell therapy (cont.).

[9]

[7]

Ref.

Cytomegalovirus-specific T-cell therapies: current status & future prospects 

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Review  Nicholson & Peggs and mount recall responses. Multiple rounds of in vitro stimulation of antigen-specific T cells typically lead to a population of T cells that predominantly have an end stage effector phenotype. This may adversely affect their ability to persist post transfer, leading to reduction in antigen-specific responses [42,43] . Scheinberg et al. characterized the T-cell responses in donor-recipient pairs pre and post hematopoietic stem cell transplantation and demonstrated that less differentiated CD27+ CD57- CD8 + CMV-specific donor T cells had superior persistence in vivo than the terminal effector cells (CD27+ CD57-) and that higher numbers of these memory CD8 + T cells led to superior protection against viral reactivation in the recipient [44] . The transfer of naive or memory T-cell populations may be more efficacious at providing long-term immunity as demonstrated by adoptive immunotherapy in tumor models. Central memory CD8 + T cells maintain high levels of CD62L and CCR7 and can efficiently recirculate through secondary lymphoid organs where they encounter APCs while effector memory CD8 + T cells downregulate the expression of CD62L and thus preferentially recirculate in peripheral tissues. Effector memory T cells have lower levels of expansion on antigen rechallenge compared with central memory T cells [43,45] . Klebanoff et al. showed that adoptive transfer of tumor-specific central memory T cells (CD62L + CCR7+) led to more effective antitumor responses than the transfer of effector memory T cells (CD62L-) [43] . Following adoptive transfer into unconditioned primates, CD8 + CMV-specific effector T cells derived from central memory T cells had enhanced persistence in vivo compared with effector T cells derived from effector memory precursors. These central memory-derived effector T cells could migrate to secondary lymphoid organs and they had the ability to revert back to either a CD62L- or CD62L + memory cell phenotype [46] . The most effective T-cell population for adoptive immunotherapy may be naive T cells. Using a pmel-1 TCR transgenic mouse model (where T cells were specific for a melanoma antigen, gp100, also expressed by self-tissues) the adoptive transfer of CD62L + CD44low CD8 + naive T cells into mice resulted in faster clearance of established melanoma compared with the transfer of central memory CD62L + CD44high CD8 + T cells. This superior tumor protection was associated with greater in vivo expansion and production of greater levels of IFN-γ and IL-2 post transfer [47] . Lymphodepletion is very effective at promoting the expansion and persistence of adoptively transferred T-cell populations [48,49] . Lymphodepletion drives homeostatic proliferation of residual peripheral T cells that reconstitute the peripheral T-cell pool [50,51] . The

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same lymphopenia driven homeostatic proliferation also drives expansion of adoptively transferred T cells [52] . The homeostatic cytokines, IL-7 and IL-15 and also interactions between TCR and self-peptide-MHC are thought to promote this proliferation [53–56] . The amount of proliferation of the adoptively transferred T cells is limited by the amount of IL-7 and IL-15 and the amount of self-peptide-MHC that is available. In the lymphodepleted environment post hematopoietic stem cell transplantation, the competition for access to homeostatic cytokines is lessened, thus promoting the expansion of adoptively transferred CMV-specific T cells. Conclusion A number of techniques now exist for production of CMV-specific T-cell products for adoptive transfer to promote CMV-specific immunity post hemato­poietic stem cell transplantation (Table 1) . Studies using donor-derived CMV-specific T cells produced either by stimulation and expansion in vitro or by direct selection techniques have demonstrated their potential, effected by transfer of relatively low numbers of CMVspecific T cells that undergo subsequent expansion in vivo. CMV-specific T cells have been shown to persist in vivo post transfer and thus potentially provide memory responses to prevent late CMV reactivation and provide long lasting anti-CMV immunity. To date the adoptive transfer of CMV-specific T-cell products has not led to an excess of GVHD. The use of trispecific T cells offers the potential to reconstitute anti­ viral immunity directed against multiple viruses using a single T-cell product while use of banked third party T cells offers more immediate access to CMV-specific T cells, bypassing the need for individualized production of CTLs for each patient. Randomized Phase IIIII studies are required to compare outcomes of adoptive CMV-specific T-cell transfer versus current best antiviral treatments and to determine whether these T cells are best administered prophylactically prior to detection of CMV viremia or in a pre-emptive fashion. Comparison of transfer of different phenotypic T-cell populations, for example, memory versus effector versus naive may elucidate which population of T cells provides the most persistent and effective anti CMV immunity post adoptive transfer. Future perspective Once definitive evidence for efficacy is available from randomized confirmatory studies, more widespread application will be possible through either local production within GMP-compliant stem cell facilities, or through commercial partner organizations. These should gain rapid acceptance if they result in signifi-

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Cytomegalovirus-specific T-cell therapies: current status & future prospects 

cant reductions in the requirement for conventional antiviral drugs, though the relative difficulty in delivering bespoke patient-specific products might limit initial application to those with more resistant infections. Further evidence of efficacy of third party products would be transformative in this regard, but these will require careful evaluation in prospective randomized studies that should be delivered within the next

Review

5 years. After many years in which we have seen no new novel antiviral drugs emerging, we now see the prospect of evaluation of a number of new agents as prophylactics in randomized Phase III studies (e.g., brincidofovir, letermovir). How these will fare, and how they could be integrated with adoptive cell therapy approaches should become a major focus of clinical research activity in coming years.

Executive summary Background • Cytomegalovirus (CMV) infection is still a major cause of morbidity and mortality post allogeneic hematopoietic stem cell transplantation. • The use of sensitive surveillance monitoring and pre-emptive antiviral treatment has reduced the incidence of early CMV disease but leads to a high rates of treatment-related side effects and potentially antiviral resistance. • CMV specific immune reconstitution can be enhanced by the adoptive transfer of CMV specific T cells post transplant.

In vitro stimulation techniques • Polyclonal CMV specific T cells can be expanded in vitro by culturing donor-derived T cells with dendritic cells pulsed with CMV lysate or HLA restricted CMV peptides. • Adoptive transfer to hematopoietic stem cell transplant recipients in both prophylactic or pre-emptive settings can lead to effective reconstitution of anti-CMV immunity and clearance of CMV viremia.

Direct selection techniques • The majority of CMV-seropositive donors have high frequencies of circulating CMV reactive T cells. • These CMV specific T cells can be directly selected from donor peripheral blood mononuclear cells using HLA multimer technology or by IFN-γ capture techniques. • This requires less ex vivo manipulation, removing the need for prolonged in vitro culture thus leading to more efficient generation of the cellular product.

Third party viral specific T cells • Partially HLA matched third party virus specific T cells have been used for treatment of Epstein–Barr virus (EBV) associated post-transplant lymphoproliferative disorders and also for treatment of patients with adenovirus or CMV virus reactivation post-transplant. • This allows a more ‘off the shelf’ approach to generating CMV specific T cells but carries potentially a higher risk of rejection or graft-versus-host disease due to nonshared HLA antigens.

Multivirus specific T cells • Trispecific T cells have been generated targeting EBV, adenovirus and CMV within a single cytotoxic T lymphocyte culture by transduction of APCs with an adenoviral vector encoding CMV peptides followed by restimulation with donor-derived EBV transformed B cells transduced with the same adenoviral vector. • Multivirus specific T cells specific for 15 immunodominant antigens derived from EBV, CMV, adenovirus, BK virus, HHV6, RSV and influenza can be produced following stimulation of donor-derived peripheral blood mononuclear cells with a pepmix (peptide library of overlapping 15mers). This technique increases the range of viral antigens that can be targeted using a single T-cell product.

TCR gene therapy for production of CMV specific T cells • CMV-seropositive recipients of HSCT from CMV-seronegative donors are at particular risk of developing CMV related disease because reconstitution of CMV specific T-cell responses is absent and CMV specific T cells cannot be isolated from CMV-negative donors. • Retroviral transduction can introduce a T-cell receptor with specificity for CMV into polyclonal T cells thus redirecting their specificity. • A Phase I study of CMV T-cell receptor transduced T cells for treatment of CMV infection post HSCT is ongoing in the UK.

Summary • A number of techniques now exist for production of CMV-specific T-cell products for adoptive transfer to promote CMV-specific immunity post hematopoietic stem cell transplantation. • Their efficacy compared with conventional antiviral treatment needs to be demonstrated in randomized controlled trials, which may lead to more widespread adoption of these treatment strategies following CMV reactivation.

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Review  Nicholson & Peggs Financial & competing interests disclosure The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employ-

ment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties. No writing assistance was utilized in the production of this manuscript.

pharmacotherapy after allogeneic stem cell transplantation. Blood 121(18), 3745–3758 (2013).

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••

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••

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Immunotherapy (2015) 7(2)

future science group

Cytomegalovirus-specific T-cell therapies: current status and future prospects.

Adoptive transfer of T cells specific for viral pathogens offers an attractive method for hastening immune reconstitution and protective immunity in p...
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