Immunology

and Cell Biology

(\99\)

69. 2 8 5 - 2 9 4

Review Modulation of the immune response by tachykinins ANTHONY EGLEZOS,* PAUL V. ANDREWS,t RICHARD L. BOYD* AND ROBERT D. HELMEt ^University of Melbourne, Department of Medicine, Royal Melbourne Hospital, Parkville ^National Research Institute of Gerontology and Geriatric Medicine, North West Hospital, Parkville tMonash University, Department of Pathology and Immunology, Alfred Hospital, Prahran, Victoria, Australia (Subtnilted 27 March 1991. Accepted for publication 6 August 1991.)

INTRODUCTtON The potential exists for neuropeptides to act at a number of different levels of the immune response. The widespread innervation of lymphoid organs, and the close association of nerves with lymphocytes and structural components of lymphoid organs could be a mechanism for the transfer of messages between the nervous and immune systems. These interactions provide the basic building blocks of'neuroimmunomodulation'. Neuro-immunomodulation 'deals with the control and regulation of immune function by the central and peripheral nervous systems... and the peptides they contain and control' (1). The immunomodulatory properties of substance P (SP) have been the subject of a number of reviews in the past (2-4). They have discussed the potential of SP, somatostatin and vasoactive intestinal peptide to alter In vitro immune reactivity. The previous in vitro studies can be viewed as the building blocks on which recent in vivo studies are based. The present review focuses on the immunomodulatory properties of the tachykinins (TK) and calcitonin gene-related peptide (CGRP) which co-exists with SP in primary afferent neurons. The studies described

include both in vitro and more recent in vivo observations.

THE TACHYKtNINS The TK are a family of neuropeptides contained in sensory neurons that share a conserved carboxyl terminal pentapeptide sequence of: -Phe-X-Phe-Gly-Leu-Met-NH2 where X is an aromatic or branched aliphatic amino acid. This group of peptides, as inferred by their name, is characterized by a rapid stimulatory effect on smooth muscle preparations (5). The TK are present in both mammalian and non-mammalian organisms. Tachykinin receptors The modulatory effects ofthe TK are mediated by actions on different receptor types, collectively termed neurokinin (NK) receptors^. To date there are three different NK receptors characterized by the agonist potencies ofthe mammalian TK, SP, neurokinin A (NKA), neurokinin B (NKB) and, the non-mammalian TK, physal-

Corrcspondcncc: A. Eglczos. NRIGGM. North West Hospital. Poplar Rd. Parkville. Vic. 3052. Australia. .•Ibbrc'viation.s u.sed in ihi.spaper: CGRP. calcitonin gene-related peptide; CGRP-IR. CGRP immunoreactivity; Con-A. concanavalin-A; ELE. eledoisin; IFN. interferon; IFN-7. gamma-interferon; Ig. immunoglobulin; IL-I. interleukin I; lL-2. interleukin 2; IL-6. interlcukin 6; i.p.. intraperitoneally; KD. receptor binding constant; KAS. kassinin; MLN. mesenteric lymph node; NK. neurokinin; NKA. neurokinin A; NKB. neurokinin B; PFC. plaque forming cell; PHA. phytohaemagglutinin; PHY. physalaemin; PLN. popliteal lymph node; PP. Peyer"s patch; PWM. pokcwccd mitogen; SP-IR. SP immunoreactivity; SP. substance P; SP-R. SP receptor; SRBC. sheep red blood cells; TK. tachykinins; TNF-a. tumour necrosis factor-alpha.

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aemin (PHY), kassinin (KAS) and eledoisin (ELE) (7). NK-1: SP>PHY>NKA^NKBS= ELE 3= KAS NK-2: NK-3: SP NKB >>ELE>KAS>NKA = Development of TK agonists, antagonists and isomeric analogues has aided in the characterization ofthe NK receptors. The SP analogues septide (8) and senktide (9) are highly selective agonists of NK-1 and NK-3 receptors respectively, while [P-Ala^]-NKA4-io is a specific agonist of NK-2 receptors (10). The D-Pro and L-Pro analogues of [Glp^, Pro^]-SP6-ii have been used to differentiate all three NK receptor types (8). Using the ratio of potencies of D-Pro/L-Pro, Lee et al. showed that the ratio is 100, I and 0.001 for NK-1, NK-2 and NK-3 receptors respectively (7). The use of TK antagonists, however, has proven less useful. The SP analogues which were used by many groups showed antagonistic activity for apparently more than one NK receptor, including combined NK-1 and NK-2 receptor antagonism (11,12). Interest in multiple NK receptors has arisen from studies showing that peripheral tissues contain predominantly NK-1 and NK-2 receptors (13-15). In the immune system, there has not been a systematic investigation of TK-receptor type specificities, although NK receptors are known to exist both in lymphoid tissues and on lymphocytes. Indirect evidence has shown the existence of NK-1 type receptors on human lymphocytes (16) and guinea-pig macrophages (17). NK-2 receptors have been implicated in mediation ofthe modulatory effects ofthe TK on in vitro and in vivo lymphocyte function (18,19). Using plasma clearance rate and conventional library screening techniques, Hershey and Krause (20) isolated a cDNA encoding the rat SP-receptor (SP-R). The SP-R is similar to the NKA-receptor cDNA cloned using the oocyte expression system (21) and belongs to the same superfamily of rhodopsin-like G proteincoupled receptors. Activation of the SP-R involves the hydrolysis of inositol phospholipids as seeond messengers, possibly through the stimulation of phosphoinositide-specific phospholipase C (22). The SP-R polypeptide consists of 407 amino aeid residues, with a molecular mass of 46 385 Da and a mean isoelectric point of 6.1 (20). The distribution ofthe mRNA for the SP-R is con-

sistent with that of the known distribution of SP.

ROLE OF TACHYKtNtNS tN THE tMMUNE RESPONSE The following review will discuss the relevant roles of SP, NKA and NKB. Calcitonin generelated peptide will also be discussed due to its widespread anatomie co-existence with the TK (23). Innervation of lymphoid organs

The thymus is innervated by afferent nerves containing the tachykinins SP and CGRP. These nerves have fibres in close contact with all components ofthe thymus (24-26). They have been associated with the capsule and the lymphocyte populations. Some fibres also appear to be in close contact with mast eells (24-26) in both rats and mice, whereas in the bovine spleen. Fried et al. (27) showed the presence of sensory nerves containing SP-immunoreactivity (SP-IR) around the small blood vessels. SP-IR and GCRP-immunoreactivity (CGRPIR) co-exist in the same fibres innervating lymph nodes (28). Fibres are associated with small blood vessels that branch once they enter the node and end in the parenchyma (29). These nerves are present in both perivascular and paravascular areas. They are found on the capsule, in the medulla, medullary cords, deep cortex and internodular cortex in close association with lymphoid cells (28,29). The skin, while not normally considered an immune organ, contains a variety of cell types, including mast cells, Langerhan's cells, keratinocytes, fibroblasts as well as lymphocytes (30). For a variety of antigens, the skin is the site of entry into the body and is consequently the first site of immune recognition. Peptidergic nerve fibres, eontaining SP-IR (31) or CGRP-IR (32,33) exist in skin from many species including rat (31) and humans (34). These nerves lie in the epidermis as free nerve endings (32,34) and around blood vessels, sweat glands and hair follicles (34) in the dermis. They are found in both hairy and hairless skin (33,35,36) from many anatomic sites. Neuropeptide receptors in lymphoid organs

Receptors for the TK have also been demonstrated in thymus, spleen and lymph nodes (29,37,38).

MODULATION OF IMMUNE RESPONSE BY TK

In the rat thymus, SP binding sites were found to be localized in the medulla, associated with the thymic vasculature. Using the classification scheme for NK receptors of Lee et al. (39), the receptors appear to behave as NK-l receptors (37). In the rat spleen, SP binding sites have been shown to be localized in areas of specialized splenic macrophages in the marginal zone (37, 38), Binding sites for SP but not NKA or NKB have been found in germinal centres, blood vessels, trabeculae, medulla and paracortex of the canine mesenteric lymph node (MLN), CGRP

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receptor binding sites appear to be localized in the same area (29), Receptor expression on celts of immune system

Lymphocyte and mononuclear leucocyte receptors have been characterized using both radioiodinated and fluoresceinated neuropeptides (Table I; 2.3,4), Receptors for SP (SP-R) have been the most widely studied and are the best characterized, and tlie following discussion of immunocyte neuropeptide receptor expression

I'able I Role of TK in the immune svsteni Receptor expression

SP

NKA

NKB

CGRP

Tcell Human Murine Bcell Human Murine Macrophage Guinea-pig Monocyte Human PMN

Rabbit Cell-lines lM-9

Molt-4b Hut 78 Jurkat P388 Dl Cellular functions Proliferation

Tcell Bcell PBMC

Fibroblast Sm, muscle Keratinoeyte NK eell activity Ig production Phagocytosis H2O2 production IL-I production lL-6 production TNF-a production !FN-y production Chemotaxis PMN Monocyte Lymphocyte recireulation Mast cell degranulation Thvmic involution PBMC = peripheral blood mononuclear cell, PMN = polymorphonuclear neutrophil, + = receptor present; - = no receptor (receptor expression), + = stimulation; - = inhibition (cellular functions).

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will be limited to this peptide. Receptors for NKA and NKB have not yet been characterized on immune eells, SP binds to a specific cell surface receptor on human circulating lymphoctyes. It binds 21% of the total T cells (16) and, following activation of these cells by the T cell mitogen phytohaemagglutinin (PHA), this binding increased to 35%, SP-R are expressed on both Th/i and Tc/s subsets, with binding to Th/i predominating (16), The human eirculating T cell SP-R is of a single class of receptor with a receptor binding constant (KD) of 180 nmol/L, and approximately 7000 receptor sites per T cell. The Ijinding of SP to this receptor is dependent on the C-terminal end of the peptide as SP4-11 inhibits and SP1-7 has no efleet on the undeeapeptide SPi-i 1 binding (16), B cells and several T cell lines such as Hut 78 and Jurkat cells have no detectable SP-R, but the lM-9 human T lymphoblastoid cell line expresses SP-R on more than 80% of its cells. This cell line was therefore used to further examine the SP-R (40), The surface membrane SP-R on the IM-9 cells has a KD of 0,65 nmol/L, and there are over 22 000 receptor sites/cell. The approximately 300-fold higher KD for the SP-R on the lM-9 compared to that of the circulating T lymphocyte went unremarked by the authors of the two articles, but it is reasonable to conclude that lymphocytes purified from peripheral blood are of quite different cell type compared to the IM-9 T lymphoblastoid cell line. Once again, the C-terminal fragment of SP (SP4_ii) but not the A'-terminal fragment (SP1-7), competes for binding of the whole SP molecule to this receptor (40), Analysis of affinity labelled and solubilized receptor showed membrane proteins of 116, 72, 64, 58 and 33 kDa (2). The ligand binding fragment of the lymphoblast receptor was later characterized by cross-linking techniques to be a 33 kDa protein (41). The monoclonal antibody 5G10, directed against the lymphoblast SP-R, bound with highest affinity to the 33 kDa protein, confirming its importance (42), Pulse-chase analysis and treatment of the receptor with tunicamycin (an inhibitor of protein glyeosylation by A'-linked sugars) showed that the lymphoblast SP-R was a glycosylated protein (43), Organist et al. (43) also showed that the 33 kDa and another 36kDa form of the reeeptor were precursors of the mature glycosylated 38 kDa form of the SP-R. Murine spleen and Peyer's patch (PP) T and B cells have also been shown to possess a high affinity SP-R (44), This receptor has a similar KD

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on both cell types from each tissue, with a mean of 0,6 nmol/L, T and B cells from PP have more receptor sites per cell than those from spleen (647, 975 i',v 195, 190 respectively). Therefore, eontrary to the results in the human (16), murine B cells do express SP-R, Other cells that possess reeeptor binding sites for SP include rabbit polymorphonuclear leueocytes (45), human monocytes (46) and guinea-pig macrophages (17), In all of these cells, the C-terminal end of SP is important for binding to the receptor. In guineapig maerophages, the SP-R is a single high affinity receptor, similar to the lymphoblastoid SP-R, with a KDofO.l nmol/L (17), Receptors for CGRP have reeently been described on cells involved in immune responses. Umeda and Arisawa (47) characterized CGRP receptors on mouse T cells, and Abello el al. described specific CGRP receptors on the plasma membrane of the interleukin-1 (IL-1) secreting murine macrophage eell-like cell line P338 DI (48), These receptors are coupled functionally to adenylate cyclase with the lymphocytic receptor having a higher affinity. Two classes of binding site were described for the murine T lymphocytes, a high affinity (KD 0,35 nmol/L and a low affinity (KD 48 nmol/L) speeies. The single receptor type found on the P338 Dl cell line has a KD of 1,76 nmol/L, These KD values correspond approximately to the values observed in studies of the effects of CGRP on these cell types, Neuropeptide regulation of cellular responses

Studies both in Wi'oand in vitro have shown that various neuropeptides influence T and B cell proliferation, immunoglobulin secretion, cellular chemotaxis and lymphocytic migration (Table 1). In vitro The stimulatory effects of SP on lymphocytic proliferation were first described by Payan et al. (49), who showed that SP and SP(4-11) were able to stimulate proliferation of human T lymphocytes in the presence and absenee of prior mitogenic stimulation. In the absence of mitogen, SP was 10 times more potent that the C-terminal fragment, stimulating proliferation by 60-70% at 10 nmol/L, However, following mitogenic stimulation with PHA or Coneanavalin-A (Con-A), the C -terminal fragment was more potent (49), Using a SP-antagonist, they were able to inhibit the response of the peptides in the absence of mitogen, but not the effects on mitogen-stimulated

MODULATION OF IMMUNE RESPONSE BY TK

cells. This suggests the presence of two different receptor types, or the involvement of a non-SPR mediated effect on mitogen-stimulated cells. Although Liping et al. were unable to reproduce these results (50), it has become widely accepted that SP has immunomodulatory activity, but the mechanism of action is not known, SP is stimulatory for murine PP, MLN and spleen lymphocytes at concentrations between I nmol/L and 0,1 |imol/L, SP is maximally stimulatory in this murine system at 10 nmol/L, increasing the response by over 60% (51), SP has been shown to have both stimulatory and inhibitory effects on human lymphocytic responses, Scicchitano et al. showed that time of incubation with peptide was the important factor rather than the length of preincubation (52), Using suboptimal Con-A (1 |j.g/mL) levels, they observed an inhibition of the Con-A response if cells were incubated with SP for 24 h, SP had no effect at 48 h, but it enhanced the mitogen response following a 72 h incubation period (52). Although these results were for murine lymphocytes, the basic principles may hold true in other species. It appears that varying the time of culturing cells in vitro with neuropeptide may answer some of the contradictions observed by different investigators. However, another possible reason for variation when using human cells may be the clinical status of the people used as subjects. Human peripheral blood lymphocytes from birch-pollen allergic patients had a significantly different Con-A response in the presence of SP to those from normal controls (53), These studies show that subject selection criteria are important and need to be controlled to prevent confusing and contradictory data being reported, SP enhances the pokeweed mitogen (PWM)induced release of gamma-interferon (IFN-y) from human mononuclear cells, an effect not observed in PHA-stimulated cells (54), It also stimulates the release of both immunoglobulin and IFN-y from human duodenal mucosal cells (55), This effect of SP was related to dose of peptide and concentration of mitogen. At 1 pmol/L, SP only stimulates interferon (IFN) release induced by high concentrations of (0,125 ng/mL) PWM and not by lower concentrations (0,125 ng/mL), At 0,1 nmol/L the significant stimulatory effect of SP was observed at both mitogen concentrations; however, altering the incubation times from 24 to 48 h confined the stimulatory effect of 0.1 nmol/L SP to only the high PWM concentration. Results of this kind reflect the complexity of the finely tuned

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neuro-immunomodulation which exists in vivo. SP also modulates lymphocytic proliferation and immunoglobulin secretion in an organ- and antibody isotype-specific manner. In murine spleen, MLN and PP, immunoglobulin A (IgA) secretion is significantly increased by SP between 1 nmol/L and 0,1 |imol/L; this is a result similar to that seen for lymphocytic proliferation (49,51) and in the duodenal mucosa (55), The effect on IgA secretion was greatest and most sensitive in PP where it was increased by more than 300% (51), while splenic lymphocytic IgM secretion was increased, but only by a small amount (51). IgG secretion by human B cells is also increased, an effect not as obvious when phagocytic or adherent cells are removed from culture (56), This study not only suggests the possible importance of phagocytic cells for the immunomodulatory effects of SP on immunoglobulin secretion, but also showed that the results for SP-mediated effects in human studies vary from stimulatory to inhibitory effects, depending on the concentration used, NKA and CGRP do not have mitogenic properties for lymphocytes but, similar to SP, their effects on mitogen-induced proliferation vary depending on the concentration of mitogen, NKA and NKA4_io have a dose-related stimulatory effect on PHA-induced proliferation of murine mononuclear leucocytes (18), This was observed at stimulatory PHA levels of 1 ng. where NKA4_io was 16 times more potent. When PHA levels were increased to 5 and 25 |ig, both of these peptides had no effect. The effects of the two peptides were evident at concentrations as low as 1 fmol/L; EC50 (concentration producing a 50% of maximal stimulatory response) values were 0,6 pmol/L and 0,04 pmol/L for NKA and NKA 4_io respectively. Due to the high potency of NKA 4_io, it was concluded that the receptor involved may be a receptor of the NK-2 type (18), NKA has also been shown to modulate human B cell IgG production and is slightly more effective than SP (56), CGRP stimulates Con-A induced lymphocytic proliferation, maximal at 1 )imol/L with an EC50 of 4,6 nmol/L, The effect of CGRP on peripheral blood mononuclear cells was also variable, depending on mitogen concentration. At high PHA (25 ng) levels, CGRP had an inhibitory effect; at 5 ng PHA, CGRP had no effect at all, whereas at 1 ng PHA, CGRP had a nett stimulatory effect (18), In another study, CGRP dose-dependently (0,1-1000 nmol/L) increased the cAMP concentration in lymphocytes and inhibited mitogen-induced stimulation

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(57). The variance in the effects of these peptides may be due to the fact that mononuclear cell preparations are a mixture of maerophages, T and B eells, all of which may have funetionally distinct receptors that vary in neuropeptide selectivity and affinity. The neuropeptides might also affect different regulatory cytokines, depending upon the concentrations used, and therefore alter the overall balance. It can be appreciated therefore that there eould be many reasons for the discrepancies which have been documented for the lymphoproliferative effects ofTK. SP, as well as NKA and CGRP (and to a lesser extent NKB), have been shown to modulate macrophage function. Guinea-pig macrophages primed with Corynebacterium parvuin have increased chemiluminescence responses and arachidonic aeid metabolism in the presence of SP (17). This effect is mediated by the C-terminal end ofthe peptide and is maximal at 0.6 (imol/L (58). However, the stimulatory effects of SP on the phagocytosis of yeast is maximal at 40 nmol/L and is mediated by the A'-terminal end. It appears that the effects of SP may be mediated by both ends of the peptide. Human alveolar macrophages can be induced by SP to synthesize and release both IL-1 and tumour necrosis factor-alpha (TNF-a) (59). In fact SP, SP4-11 and NKA are capable of inducing human macrophages togenerate IL-1, interleukin-6 (IL6) and TNF-a (60), whereas NKB is only able to stimulate macrophages to generate IL-I (61). SIVii is most potent in these effeets with a maximal response at 7.4 pmol/L, SP at levels below 22 pmol/L is ineffective. NKA and NKB were more potent than SP but less so than NKA4-10 (61). A more recent study used selective TK agonists to study guinea-pig alveolar macrophages (62). An NK-2 selective agonist ([P-Ala'^]-NKA4-io) maximally stimulated 0 - " production at 100 nmol/L whereas (MePhe^)NKB(NK —3 selective agonist) was almost ineffective and (Pro')-SP (NK— 1 selective agonist) was stimulatory at 0.1-1 nmol/L. The weak activity of (Pro'*)-SP suggests the presence of a minor population of NK-1 sites but the major stimulatory function of the TK appears to be mediated by NK-2 receptors. CGRP inhibits the ability of maerophages to produce H2O2 in response to IFN-7 (63). One interesting study has shown that the P388 Dl murine macrophage cell line is able to produce SP (64). This raises the possibility that macrophages could modulate their own function

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in an autocrine manner as well as the functions of other eells through the release of SP. Cellular chemotaxis of rabbit neutrophils (45). human monocytes (46) and rat neutrophils (65.66) is modulated by SP. Other investigators have suggested SP is involved in repair mechanisms (67.68) and hypersensitivity reactions (69,70). An //; vitro protective role tor SP has been observed in wound healing. SP stimulation of proliferative responses of smooth muscle cells, skin fibroblasts (67) and keratinoeytes (68) implieate SP in the wound healing response during tissue damage and inflammation. In addition. Nilsson et al. provided evidence to show that NKA was more potent than SP. suggesting another possible role for NK-2 receptor mediated activity (67). Pharmacological (71,72) and histochemical (73) studies provide a role for SP in hypersensitivity. SP was found to be capable of degranulating mast cells, leading to histamine release. The contribution ofthis cell type of hypersensitivity is either via the direct effects of histamine on the vasculature, or indirectly through the release of other mediators from a variety of cell types (74). SP release histamine from mast cells in a dose-dependent manner, with an EC50 of 10 nmol/L (71.72). This effeet is mediated by the A'-terminal end ofthe peptide and is maximal at approximately 1 |.miol/L(71). On the other hand, human broncho-alveolar maerophages are not affeeted by SP (75) and Croitoru cl al. found that SP stimulated NK cell aetivity in intestinal intra-epithelial leueocytes but did not affeet splenie cells (76). These studies suggest there may be tissue or species specificity for the actions ofthe TK. In vivo There are few studies involving the in vivo immunomodulatory effects ofthe TK. One of the first studies to show an effect of SP on immunological function following in vivo infusion was that of Moore (77) showing an effect on lymphocytie reeirculation. SP (50 ng/mL) increased lymphatic drainage and the output of both small cells and blast cells from sheep popliteal lymph nodes (PLN). These increases (2080%) occur within the first 2-4 h following peptide infusion into the node (77.78). The studies used only small sample sizes, and although they are interesting, require further validation and better control. SP has also been ascribed an //) vivo protective role in reversal of stress-induced thymic involution (79). SP injected intraperitoneally (i.p.) at 125 ng/kg 1 h before immobilization stress decreased the stress-induced in-

MODULATION OF IMMUNE RESPONSE BY TK

volution ofthe thymus from 40 to 12%. When SP4_ii was used (52.8 mg/kg, i.p.), thymic weight was not decreased but increased by 50% following stress (79), suggesting the possible involvement of NK-1 receptors. The only control used in this study was the uninjected stressed rat. Rats treated with either vehicle, or an unrelated peptide, would have been better controls and given greater credibility to the stress-protective role of SP. The reasons for the protective effects of SP are unknown, but possibilities include SP-mediated inhibition of adrenal function and SP protection of the thymus and/or lymphocytes from the steroid hormones. SP has been shown to have an effect on immunoglobulin production following in vivo infusion. This has been examined most extensively using the neonatally capsaiein-treated rat (80,81). Following neonatal treatment of rats with a single subcutaneous injection of capsaicin (50 mg/kg), there is a decrease of more than 70% in antigen-specific immunoglobulin production by PLN lymphocytes following sheep red blood cell (SRBC) stimulation as measured by the Cunningham plaque forming cell (PFC) assay. This effect of capsaicin was reversed by an infusion of SP at concentrations of 1-10 |imol/L (80), increasing the response from 30% of normal to almost 80% of normal at 10 nmol/L. The effects of neonatal capsaicin treatment were not due to an effeet on the time-course ofthe PFC response (82), but were most likely mediated by an effect on T cell production of interleukin-2 (IL-2) (A. Eglezos, unpubl. data). A similar immunostimulatory effect was also observed following infusion of NKA (19). NKA was 10 times more potent than SP, increasing the response to over 100% ofthe normal response at 10 vmol/L. pD2 values (negative log ofthe EC50) were 6.65 ± 0.19 compared with 5.98 ±0.14 for NKA and SP respectively. Unlike SP, NKA significantly stimulated the PFC response in capsaicin-treated rats at concentrations as low as 10 nmol/L, suggesting mediation of this response by NK-2 receptors. Mediation by NK-2 reeeptors was further validated by examination of the potency ratio of the D- and L-Pro analogues of [Glp^, Pro'']-SP6-ii(7), which was 1:1 (19). This result is in accordance with the in vitro results of Casini el al. (18). Similar effects of SP were also observed in the murine system. Following infusion of 77 nmol/L SP for 7 days at 24 nL/day, murine spleen and PP lymphoeytic proliferative responses to in vitro stimulation with Con-A were increased 100-300% respec-

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tively. Immunoglobulin secretion measured by radio-immunoassay of supernatants was also increased, with a 270 and 188% increase in IgA secretion in PP and spleen respeetively. It was also observed that splenic lgA+ PFC were increased by almost 400% (83). SUMMARY Neuro-immunology is' becoming an increasingly important discipline of immunology. This review has examined the immunomodulatory function of one group of neuropeptides, theTK, particularly SP and NKA. These peptides are localized in primary afferent nerves which have been shown to innervate several immune organs. In addition, binding sites for the TK have been demonstrated in thymus, spleen and lymph node. Several immune cell types also express neurokinin reeeptors including human circulating lymphocytes with binding to the Th/i class predominating, murine T and B cells, a human T lymphoblastoid cell line, human monocytes, rabbit polymorphonuclear leueoeytes and guinea-pig macrophages. The apposition of nerves with immune eells and reeeptors for neuropeptides thus produces an environment for interaction between the nervous and immune systems. Studies in vitro and, more recently, in vivo have examined how the TK regulate immune cell responses. The TK stimulate proliferation of T cells, enhance mitogen-induced release of eytokines including IFN-y, TNF-a, IL-1 and IL6 from mononuclear eells and macrophages, enhance immunoglobulin secretion and affeet cellular chemotaxis and phagocytosis. Studies //) vivo have shown a role for TK in lymphocyte reeirculation of sheep lymph nodes, reversal of stress-induced thymic involution and Ig production in both rat and mouse. Many of these effects appear to be mediated via NK-2 type reeeptors. To date, most ofthe work has involved studies in vitro, but the results from these are now being validated by studies in vivo where both the immune system and neuropeptides are able to interact at many anatomical sites. The complexities ofthe immune and the nervous systems mean that only a small number of potential interactions has been examined. Future studies can be expected to amplify these observations, especially with respect to the understanding of inflammatory and immune diseases in humans.

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