Vaccination against intestinal parasites

VACCINATION

AGAINST INTESTINAL

PARASITES

H.R.P. MILLER Moredun Research Institute 408 Gilmerton Road, Edinburgh, EH 17 7JH

INTRODUCTION

The success achieved in providing safe and effective vaccines against albeit relatively few viral and bacterial infections in man and domestic animals contrasts with the much more limited progress made in developing vaccines against helminths. Only one vaccine, against the lung-worm Dictyocaulus viviparus in cattle, is commercially available (Clegg & Smith, 1978) and, although vaccination of dogs against the intestinal nematode Ancylostoma caninum was effective, it was not successful commercially (Miller, 1978). Both of the helminth vaccines cited above employed irradiation-attenuated larvae; these, like attenuated viral vaccines, apparently being more effective than soluble antigens. However, for a variety of commercial and safety reasons it is unlikely that attenuated larvae can be used to provide protection against intestinal nematodiasis in man. The more desirable approach is to identify those antigens (protective antigens) which induce resistance and to devise an appropriate mode of delivery which will provide lasting protection to the population at risk. The practicality of such an approach has, in the past, seemed something of a pipe-dream because of the difficulties of obtaining sufficient, highly purified nematode antigens and of delivering them in the appropriately immunogenic form. With the advent of gene cloning techniques, there is now a real possibility of generating large amounts of a relevant protective antigen and of introducing it into the host systemically, orally or in a viral or bacterial vector. Nevertheless the initial problems remain: which antigens and by what route? HOST/PARASITE

INTERACTIONS

Several of the constraints on the host and parasite which affect the course of infection are listed in table I, and perhaps the most important of these is the genotype of the host. For example, it has long been known that very considerable individual variations in worm burdens occur amongst domestic animals and, recent epidemiological surveys of human infections have, similarly, revealed that a relatively small proportion of a parasitized population can harbour the majority of worms (Schad & Anderson, 1985). An obvious question is whether such apparently unresponsive individuals could be protected by vaccination or whether they would remain as a reservoir of infection. The young host is generally more susceptible to infection than adults and this is particularly true of ruminant livestock, although the cause of this age-related susceptibility has yet to be clearly identified. Similarly, lactating and malnourished animals are more susceptible than nulliparous, well-nourished animals (Dargie, 1980). Resistance to infection is developed in the face of continuous challenge, but it is not known how long the host might remain resistant if it were placed in a worm-free environment. This point is relevant in defining vaccine strategies and raises the question as to whether the protective mechanism in the gut, once built up, persists for months or years or whether it wanes rapidly as the enterocytes and other cells of the mucosa are renewed.

43

H.R. P. Miller

44

DURATION The responsive

OF INFECTION

host

Acquired resistance against a nematode parasite by its natural host can be expressed at a number of different levels. For example, a hyperimmune animal, or a host which has recently experienced and eliminated a primary infection, may reject nematode larvae as soon as they enter the gut (Russell & Castro, 1979). This phenomenon has been described in a number of host-parasite interactions and is referred to as rapid expulsion (RE) (Bell & McGregor, 1980; Miller, 1984). Elimination of intestinal nematodes after they have established in the gut, but before they achieve patency also occurs (Dineen & Wagland, 1966) and may be associated with a stage-specific event in the life-cycle of the parasite. Expulsion of adult worms which have achieved patency may occur over the course of 2-3 days in rodents (Jarrett, Jarrett & Urquhart, 1968) or over several months in ruminants (Miller, 1984). Alternatively, the response may merely cause stunting of parasites, a reduction in worm fecundity or, under certain circumstances, inhibition of infective larvae after they have reached their niche in the mucosa (Miller, 1984). Non-responsive

host

The longevity of GI nematodes is probably a function of the ability of the parasite to evade or suppress the host response, and of the host’s inability to mount an effective immunity (Table 1). Blood-sucking nematodes such as Necator or Ancylostomu in man or Haemonchus in sheep may survive many months without engendering a protective response from the host (Ogilvie & de Savigny, 1982; Dargie, 1980). The longevity of these parasite species in adult hosts is well-established and, for Haemonchus contortus, is related to the breed and genotype of the sheep (Miller, 1984). Furthermore, parasites against which the adult host is capable of mounting effective immunity may survive for much longer periods in the young host. This is well-described for nematode infections both in the laboratory and the field (Dargie 1980). TABLE I-CONSTRAIKTS

ON

Hosr

AND PARASITE INTERACTIONS WHICH AFFECT THE COURSE OF INFECTIOV.

HOST

PARASITE Host Resistance . . .

Hormonal

I

Nutritional Duration of protection

Status

INTERACTION . . . . . Host Susceptibility

W;e$t{ht~quency

of

Life cycle -

Immunogenicity Immunosuppression Concurrent

infection

Experimental studies have shown that for many nematode infections it is the adult worms which persist and survive and this is true of T. rnuris and N. dubius in mice (Wakelin, 1985: Behnke & Robinson, 1985) and of Ostertugia sp. and Haer)lonchus contortus in ruminants, even though, in the latter instance the host is highly resistant to incoming infective larvae (Miller, 1984). Persistent nematodiasis, with the possibility that adult worm secretes antiphlogistic (Castro, 1980) or immunomodulatory factors (Behnke & Robinson, 1985) which promote chronicity, has obvious implications in deciding vaccine strategies. EFFECTOR Rapid expulsion

MECHANISMS

(RE)

This effector response should be considered separately from expulsion of established larvae or adults because it occurs within minutes rather than days or weeks of challenge (Russell & Castro, 1979) and, therefore. before the majority of larvae attach to or enter the mucosa itself. Invariably, these early infective stages come into contact with the superficial mucus and it is at this interface where early rejection may occur (Lee & Ogilvie, 1981: Miller, 1984). Several mechanisms, supported by experimental data. (reviewed in Castro, 1982; Miller. 1984) have been proposed to explain RE:-1) biochemical changes in the properties of enteric epithelial brush borders

Intestinal parasites-vaccination

45

disorient incoming larvae 2) co-operative interaction between mucus glycoprotein and serum-derived IgG promotes mucus-trapping 3) changes in the quantity and physical properties of the superficial mucus occur as a consequence of local hypersensitivity mechanisms and of the translocation of serum proteins into superficial mucus 4) SRS-A-like inflammatory mediators present in the mucus cause paralysis of nematode larvae 5) monoamines released from mast cells and basophils reduce worm survival. Because these events occur so rapidly after challenge, the effector functions must be immediately available for activation. Clearly, therefore, the relevant inflammatory cells will already be in sits; the mucosa may be bathed in parasite-specific IgA and mast cells are likely to be coated with parasite-specific IgE. Triggering of the response may be mediated by the 10% of larvae that invariably reach their niche in the mucosa (Russell & Castro, 1979) and the local hypersensitivity reaction which ensues is followed by boosting of the local production of IgA-immunoblasts and of IgA in lymph (Smith, Jackson, Jackson & Williams, 1985). Expulsion

of estublished

worms

In the immune, or partially immune host, larvae may establish and moult before they are prematurely expelled as, for example, the early expulsion of T.colubriformis from guinea pigs (Dineen & Wagland, 1966). Elimination of fourth stage larvae was associated with increased levels of monoamine in the primed gut and could be induced experimentally in non-immune guinea pigs by perfusion of the intestine with these monoamines (Miller, 1984). Primary infections with gastrointestinal nematodes in their natural hosts invariably progress to patency and the eventual immune elimination of adult worms occurs over a variable period of time depending on both host and parasite. Careful analysis of the kinetics of worm expulsion has revealed that exponential loss of worms occurs (Jarrett et al., 1968) and that the time of onset of expulsion and the rate of worm loss vary according to the host genotype (Wakelin, 1985). Manipulation of the host by thymectomy, irradiation, adoptive or passive immunization. or a combination of these treatments, has established that the thymus and T-cells are crucial in the development of resistance during primary infections (Miller, 1984). Immunopathological changes associated with protection are listed in Table II and it is likely that different effector functions operate at different levels in the mucosa depending whether the nematode resides on the surface or within the mucosal tissues (Miller. 1984). TABLE II-I.%IWJNOPATHOLOGIC~L EVENTS ASSOCIAI ED WI’IH

PRIMARY

SITE

IMMUNOPATHOLOGY

Epithelium

Villous Atrophy Crypt Hyperplasia BBM Biochemistrv* Goblet Cell Hyperplasia Mastocytosis Ia Expression

Lamina

* + ND (T)

propria

INFECTION

THE DEVELOPMENT OF RESISIANCE EFFECTS OF ADOPTIVE OR PASSIVE IMMUNIZATION IMMUNE IMMUNE LYMPHOCYTES SERUM

+ + +

+ 0) + (T) ND

ND ND ND

+ + +

+ CT)

+

ND

N+D

+

+ CT)

+ + + + +

f(T) +tT) +tT) + (‘?) ND ND ND

ND ND + ND ND ND ND ND

.

Eosinophils Basophils Mastocytosis T. Lymphocyte Blasts lgA 1 IgG } Immunocytes IgE i Macrophages

ND

Altered biochemistry of brush-border membranes. Increased levels at time of, or shortly before, worm expulsion. Not done. Augmented by adoptive immunization with T cell-enriched immune thymus-deprivation. Data from reviews by Castro (1982). Miller (1984), Wakelin t 1985), and Smith

+(T)

lymphocytes et A/.

(1985)

or

reduced

by

H.R.P.

46

Miller

Primary nematode expulsion is associated with the gradual, predominantly T cell-mediated, recruitment of immunocytes, granulocytes, mast cells and mucus-secreting cells; alterations in the kinetics of mucosal epithelial enterocytes and in the biochemistry of columnar epithelium also occur. Once recruited, these inflammatory cells can be triggered by an IgE-mediated hypersensitivity reaction to release inflammatory mediators within minutes of challenge and thereby promote RE of incoming larvae (Reviewed in Miller, 1984). HOST SENSITISATION,

THE PRESENTATION

OF NEMATODE

ANTIGENS

Analysis of the role of the gut in antigen presentation is, perhaps one of the least understood but most important aspects of intestinal nematodiasis. Studies have shown that specialized lymphoid tissues in Peyer’s patches, have a special facility for antigen-sampling and for activating T helper cells which, in turn, promote IgA B cell activation and the latter enter lymph and home to the mucosal tissues via the blood (Reynolds, 1985). What is not clear is whether Peyer’s patches play a major role in responding to nematode antigens, or whether other localities in the mucosa have a comparable capacity to sample, process and present antigens. The respective roles of antigen-presenting cells in the gut lamina propria, and of enterocytes, in the processing and presentation of nematode antigens to the immune system have not been defined. The lamina propria is particularly rich in MHC class II (la)-bearing macrophages, B cells and dendritic cells (Mayrhofer, Pugh & Barclay, 1983). Enterocytes also express Ia, the degree of expression in rats being both age and strain-dependent (Mayrhofer et al., 1983) and is modulated during nematode infection (Barclay & Mason, 1982). Interestingly, the DA strain of rat which expresses Ia most strongly in its enterocytes (Mayrhofer, et al., 1983) is the strain which responds most rapidly in the rejection of a primary N. brusiliensis infection (Miller, 1984). It is, perhaps, significant that young rats (less than 6 weeks old) express little or no epithelial Ia (Mayrhofer et ul., 1983) and are unable to eliminate primary N. brusiliensis infections (Miller, 1984). The nature of the class II elements on the surface of antigen-presenting cells may limit the host in its ability to respond to nematode antigens and this has already been described in vitro for T. spiralis antigens in mice (Krco, Wassom, Abramson & David, 1983). Furthermore, primed blast cells from a T cell subset (L3T4 +ve Lyt 2-ve phenotype) will, when adoptively transferred to naive mice, confer resistance to T. spiralis (Grencis, Riedlinger & Wakelin, 1985), a result which also implicates class II MHC restriction in the recognition of T. spirulis antigen, but in this case in vim (Grencis et al., 1985). STAGE-SPECIFICITY Surface

OF NEMATODE

ANTIGENS

antigens

With the advent of surface iodination techniques, and their application to nematodes, several of the immunogenic surface antigens have been defined (Philipp & Rumjaneck, 1984). The identification of stage-specific antigens of limited heterogeneity on the surface of T. spirulis has been followed by comparable observations on other intestinal nematodes notably N. brusihensis and T. canis (Philipp & Rumjaneck, 1984). Several of these parasites shed surface antigens in vitro (Philipp & Rumjaneck, 1984) but the physiological significance of this has yet to be discovered. Sera from Trichine/lu-infected rats immunoprecipitate cuticular antigens in a strictly stage-specific fashion which coincides with the development of each stage (Philipp & Rumjaneck, 1984). Similarly, for N. brusiliensis, single stage infections result in antibody reactive only to antigen from the homologous stage (Philipp & Rumjaneck, 1984). However, the correlation between these serum antibodies and protection has not been studied, nor have the immunoglobulin isotypes reacting with each surface antigen been characterized, either from serum, or from the site of infection itself. An IgGl monoclonal antibody, raised against a 64,000 Dalton surface antigen of newborn T. spirdis mediated in vitro killing of larvae by eosinophils. When newborn larvae were opsonized with this monoclonal IgG, their infectivity was significantly reduced (Ortega-Pierres, MacKenzie & Parkhouse, 1984). This result suggests that in the tissues, at least, IgGl directed against a single polypeptide can induce helminthotoxic effector granulocytes and presumably a similar mechanism would operate within the gut mucosa. It will be important to determine whether monoclonal immunoglobulins of the IgA and IgE isotypes, directed against surface cuticular antigens. can reduce the infectivity of nematodes in the gut, especially those which dwell in the gut lumen. Despite the stage-specificity of systemic responses to T. spiralis, the major protective antigens are associated with muscle larvae and with pre-adult worms (Bell and McGregor 1980). Interestingly rats exposed to an abbreviated infection regime with T. spiralis can mount a short-lived RE response but none of the challenge infection becomes trapped in mucus (Bell, Adams & Ogden, 1984). This contrasts with the

Intestinal parasites-vaccination

47

substantial mucus-trapping which occurs during RE if the immunizing infection achieves patency and newborn larvae are deposited in the tissues (Bell et al., 1984). Co-operative interaction between mucus glycoprotein and serum-derived IgG may mediate mucus-trapping (Lee & Ogilvie, 1981), and it is possible that the relevant IgG is not generated until muscle larvae have reached a certain stage of maturation. Hitherto, it has not been possible to elute antigens from the cuticle in sufficient quantity and purity to test their efficacy as immunogens. By using cetyltrimethylammonium bromide (CTAB) Pritchard et al., (1986) have obtained significant quantities of surface antigen from N. dubius without affecting the viability of the worm. Similar techniques applied to T. spiralis muscle larvae have again yielded significant quantities of cuticlar antigen without reducing the infectivity of CTAB-treated larvae (Grencis et al., 1986). As little as 5Op,g of CTAB-eluted antigen in complete Freund’s adjuvant, when injected into mice, conferred significant protection within 8 days of challenge with muscle larvae (Grencis et al., 1986). Secreted antigens

Metabolic products, released by worms in vitro, and sometimes described as functional antigens, conferred varying degrees of protection when used in early vaccination trials (Clegg & Smith, 1978). Similarly, protective antigens have been identified in the excretory glands of Oe. rudiatum and in the stichosome cells of T. spiru/is and T. muris (Reviewed in Clegg & Smith, 1978). For T. spiralis, radiolabelling experiments have confirmed the stage-specificity of secreted products (Parkhouse & Clark, 1983). Purified products from stichosome cells of T. spiralis muscle larvae are highly immunogenic and as little as 1Okg of a highly purified antigen will, when injected with complete Freund’s adjuvant into mice, substantially reduce the deposition of newborn larvae following challenge with T. spiralis (Silberstein & Despommier, 1984). These antigens are secreted by worms in vitro, and other stichosome-derived glycoproteins are known to be partially effective in inducing RE in rats (Despommier, 1981). One such antigen has recently been purified to homogeneity and shown to be a glycoprotein of high allergenicity which, when injected with adjuvant into rats, induced specific IgE antibodies and, importantly, significant worm expulsion 24 hours after challenge (Durham, Murrell & Lee, 1984). However, RE after vaccination (40-63%) was less complete than that achieved by natural infection (90%) (Russell & Castro, 1979). The stichosome-derived T. spiralis protective antigens were derived from infective muscle larvae but for T. colubriformis, metabolites from fourth stage larvae, when given in a single subcutaneous dose (lOO~g/lOOg body weight) to guinea pigs 21 days before challenge conferred >90% protection 14 days later (Rothwell & Love, 1974). These results are consistent with the fact that fourth stage larvae are themselves highly immunogenic and a major target of the protective response (Dineen & Wagland, 1966). Functional antigens contain a variety of proteinases, hydrolytic enzymes, and, for certain parasites, acetylcholinesterases (Clegg & Smith, 1978). The physiological functions of these enzymes are poorly understood, and attempts to use them as immunogens have met with relatively little success (eg. Rothwell & Merritt, 1975). By contrast, secretions released during moulting from third to fourth stage Ascaris su~4m larvae are immunogenic and a glycoprotein isolated from this moulting fluid was protective in guinea pigs. Similarly, antigens released from moulting third stage H. contortus larvae have conferred some immunity in sheep (Lloyd, 1981). Worm homogenates

Soluble extracts from whole worms have been extensively tested as potential immunogens (Reviewed in Clegg & Smith, 1978). Stage specificity of protection with some of these homogenates have been described (Lloyd, 1981). Until now it has proved difficult to identify the relevant protective component but by two dimensional electrophoresis, electroblotting, and probing with antisera from immune animals, major protective antigens can now be partially identified (O’Donnell, Dineen, Rothwell & Marshall, 1985). A more precise characterization of these antigens may be achieved by raising monoclonal antibodies which could then be used either for affinity purification of the antigens themselves, or as probes in gene-cloning procedures. VACCINATION Stage specificity

A major feature of both protective response and of the vaccine trials described in this review, is the tendency for different stages in the life-cycle of the parasite to express unique antigens and to elicit distinctive responses in the host. Because, in many systems, larval antigens confer significant protection C

48

H. R. P. Miller

(Clegg & Smith, 1978; Lloyd, 1981) there is a prospect of immunizing RE and thus greatly reduce the likelihood of the parasite establishing

the host in such a way as to promote in the mucosa.

Different antigens may stimulate different effector functions Having chosen the appropriate stage from which to isolate antigens, there remains the question as to whether one or several antigens are required. For example, there is abundant evidence that allergens induce parasite-specific IgE responses but it is not known if they will also promote T cell-mediated generation of mast cells or recruitment of basophils (Miller, 1984). Most of the published literature, even today, analyses the specificities of serum IgG against nematode antigens but do these same antigens provoke mucosal IgA responses? Developments over the last 5 years which may allow systemic testing of parasite antigens include cloning of parasite-specific T cells (Krco et al., 1983), generation of mucosal mast cells in I,itro (Jarrett & Haig, 1984), and construction of T-cell hybrids releasing a variety of lymphokines, such as an eosinophil differentiating factor (Warren & Sanderson, 198.5). As assay systems improve, especially with the use of gene cloning technology to identify lymphokines, it should be possible to test purified nematode antigens for their capacity to induce the release of interleukins 1, 2, and 3 in vitro. Similarly, in vitro measurement of helper T, or Tt cell function in the differentiation of parasite specific IgA or IgE B cells can now be developed. These in ritro systems will help to define the capacity of a parasite antigen to stimulate the range of host protective mechanisms listed in Table II. Purification of nematode antigens There has been some progress in developing axenic cultures of medically important parasites, especially for preparation of immunodiagnostic antigens (Ogilvie & de Savigny, 1982) but it is unlikely that this complex technology could be applied on an industrial scale, especially since protective antigens would still need to be recovered and purified. The alternative approach, alluded to in the introduction, of applying recombinant DNA techniques holds more promise. Assuming that important protective antigens are polypeptides, then expression of these in bacterial or viral vectors may eventually prove an effective method of delivering them to the gut. However, many of the antigens are glycoproteins (Parkhouse & Clark, 1983) and, if the glycosylated moieties are themselves immunogenic, the gene products may then need to be expressed in a system which synthesises glycosylated antigen. Similarly, if tertiary structure and, therefore, disulphide bonds are essential for the immunogenicity of the cloned antigen, bacterial expression products may differ from native proteins in their three-dimensional structure and consequently turn out to be poor immunogens and even poorer vaccines. Several groups are currently utilizing recombinant DNA techniques to obtain protective nematode antigens and hopefully their experience will be recounted at this or subsequent ICOPA meetings. Anti-idiotype vaccines. Although an experimental system at this stage, the development of monoclonal antibodies against a 38Kd glycoprotein on the surface of Schistosoma mansoni schistosomula and their use as immunogens to raise anti-idiotype antibodies has shown considerable potential (Grzych. Capron, Lambert, Dissous, Torres & Capron, 1985). Because the anti-idiotype antibody combining sites mimic three-dimensional structures of immunodominant antigens, they by-pass the requirement to purify the antigen itself. Additional advantages of this approach are:--(i) that an antigen which may be carbohydrate or even glycolipid is presented in a different molecular form and this may be more immunogenic than the native antigen (ii) different subsets of lymphocytes may be stimulated and (iii) synthetic peptides can be constructed which could eventually provide a cheap, safe vaccine. Mode of delivery Although in many vaccine trials using inert nematode antigens there has been a measure of protection, relatively few attempts have been made to monitor the local enteric response in the vaccinated host. However, it is clear that with the exception of the study by Durham rt al., (1984), nematode larvae do establish in vaccinated animals before they are expelled prematurely. For example, systemic vaccination with whole worm extracts of N. hrasiliensis was followed by significant worm expulsion 10 days after challenge (Murray, Robinson, Grierson & Crawford, 1979) and was associated with mucosal mastocytosis. Since it is unlikely that systemic immunization induces enteric mucosal mastocytosis or, for that matter, enteric IgA responses or goblet cell hyperplasia, the presence of the worms in the gut was probably necessary to generate mucosal mastocytosis. Systemic priming may, however, hasten the onset of development of this response. Elegant studies by Bell & McGregor (1980) tend to confirm this viewpoint, since they showed that RE

Intestinal parasites-vaccination

49

did not occur in rats vaccinated systemically with extracts of T. spiralis muscle larvae unless they had experienced enteric infection with the unrelated parasite N. dubius. It seems likely that this enteral stimulus recruited non-specific effector mechanisms which, in concert with the specific response to systemic priming, effected RE. Theoretically, therefore, systemic priming is unlikely to produce a sterile immunity in the host nor, apparently, does oral immunization (Poulain, Pery & Lufau, 1976). Systemic priming may be appropriate for nematodes with a migratory infective phase or for parasites, like T. spiralis, which develop a systemic phase of infection. For parasites of veterinary importance which inhabit the gut throughout their cycle in the host, other ways must be sought of inducing gastricienteric sensitization. One approach which has shown promise in man, and which could also be applied to young ruminants in the pre- ruminant phase of their life, is oral administration of antigen conjugated to bacterial enterotoxin subunits; these bind to epithelium but do not induce secretory changes (Klipstein, Engert & Houghton, 1985). Good enteral IgA and systemic IgG responses to a synthetic peptide of E. co/i heat-stable enterotoxin were detected within 5 weeks of oral administration (Klipstein et al., 1985). Attenuated vaccines and selective breeding

As briefly mentioned in the introduction, irradiated larval vaccines have provided the only commercially successful nematode vaccines to date. Experimentally, irradiated larvae have also been shown to protect against both gastric and enteric nematode infections (Lloyd, 1981). However, young ruminants are generally less responsive than adults, both to natural infection and to vaccination with radiationattenuated larvae, although by a programme of assortative mating Dineen and colleagues (Windon, Dineen & Kelly, 1980) have identified lambs with highly heritable resistance or susceptibility traits to vaccination with Trichostrongylus colubriformis. Furthermore, an ovine lymphocyte antigen (SYl) was found to be present in high frequency in responder sheep and in lower frequency in non-responder animals (Outteridge, Windon & Dineen, 1985). It was concluded that this antigen was likely to be part of the ovine major histocompatibility complex (Outteridge et al., 1985). These experiments are fundamental to the understanding of the host parasite interaction, and complement more detailed genetic studies that are in progress with mice (Wakelin, 1985). In the latter, H2 linked and background genes control responsiveness to T. spiralis at at least 3 levels (Wakelin, 1985) and include most of the responses listed in Table II. For sheep also, the resistant lambs mounted a globule leukocyte response (Windon et al., 1978), globule leukocytes being partially discharged mucosal mast cells (Miller, 1984). However studies of the serum IgG response in responder and non-responder guinea pigs and in vaccinated or hyperimmune sheep given larval infections failed to reveal any differences by electroblotting and it may be more informative to electroblot intestinal 1gA antibodies from animals of different immune responsiveness (O’Donnell et al., 1985). Such an approach is feasible with gastric or intestinal lymph from adult sheep and from lambs where both the specificity and titres of 1gA response can be analysed and quantified (Adams, Merritt & Cripps, 1980; Smith et al., 1985). SUMMARY The capacity of mammalian hosts to respond to gastrointestinal nematodiasis is a function of the age, nutritional and reproductive status, and genotype of the host and the ability of the parasite to evade, suppress, or modify, the host response. Infective nematode larvae may be rapidly expelled from the immune host before they can establish in the mucosa or, in a naive or partially immune host they may be expelled at a later stage of their life cycle. The presence of immunoinflammatory cells in the mucosa is apparently a pre-requisite for full expression of the rapid expulsion response. A remarkable degree of stage-specificity is evident in the expression of resistance but the manner in which the stage-specific antigens are presented to the lymphoid apparatus in vivo is poorly understood. In vitro, Major Histocompatibility Complex Class I1 elements restrict T cell responses to nematode antigens and, in viva, class I1 MHC restriction to nematode infection may also occur. New techniques to identify and elute nematode cuticular antigens are now available, and vaccination studies suggest that some of them induce protective responses. Secreted antigens, as well as antigens isolated from secretory organs, are protective, as are antigens isolated from whole worm homogenates. Again, a substantial degree of stage-specificity is demonstrable with these purified nematode antigens. This latter observation must be taken into account when deciding vaccine strategies and so should the relative capacities of different antigens to stimulate the complex immunoinflammatory response in the mucosa. The merits of adopting newer technologies to obtain appropriately immunogenic peptides are discussed as is the use of attenuated larvae.

50

H.R. P. Miller

Acknowledgments

I am grateful to Mrs. A. Baird for typing the manuscript help in its preparation.

and to Mr. G. Newlands for his

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