Immunity and immunoregulation in helminth infections

Immunity

and immunoregulation

in helminth

infections

R. Alan Wilson University

Parasitic

helminths

populations, effective

vaccines

pathology,

continue

particularly that

is evident.

minimize With

to characterize

the context

of recent

tropics worm

immune

realization

burdens,

responses

Opinion

in human

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thus reducing

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to helminths,

in our understanding

Introduction

effort is

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of the cytokine subvert

makes the task of vaccine harder.

1993, 5:53&547

tive candidate as a mediator of immunity as it regulates the class switching of I& the epithelial attachment and feeding site of T. muris may make it particularly vulnerable to attack by this class of antibody [ 3.1.

The division of T helper (Th) cells into subsets on the basis of their cytokine secretion prohles, together with the still growing list of cytokines (and the reagents to as say and manipulate them) continues to exert a profound influence on our understanding of immunity to infectious agents. Murine responses to the protozoan parasite Leisb mania major have provided the most clear-cut example of the functional role of Th-cell subsets in vivo [ 1] and the detailed characterization of this host-parasite interaction has stimulated the search for similar decisive effects in helminth infections. This review of immunity to helminths is necessarily selective, focusing on parasitic diseases of medical importance.

Animal

subtropics.

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development

Intestinal

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responses to their own advantage

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of York, York,

The three hallmarks of nematode infection, eosinophilia, intestinal mastocytosis and IgE production, are clearly controlled by cytokines of the Th2-cell subset [5**]. A primary inoculum of Heligosomoides po&pw.s establishes a chronic infection in the BALB/c mouse gut, but the immune system prevents subsequent doses of larvae from developing; the mice are also resistant to challenge after curative chemotherapy. Administration of monoclonal antibodies (mAbs) to IL-4 or IL-4 receptor abolished anti-larval immunity and, in chronically infected mice, allowed the accumulation of successive waves of challenge parasites [6]. However, mice depleted of IgE by administration of neutralizing mAb before H. PO& gyru.s infection showed little or no decrease in their ability to inhibit the development of adult worms, or adult fecundity in a challenge infection [5**]. Similarly, W/?P’ mice that are deficient in mast cells developed the same level of protective immunity as normal littermates. The fundamental role of IL-4 in this infection system thus awaits clarification. Surprisingly, in view of the emphasis given to eosinophils as effector cells in immunity to helminths, in vivo neutralization of IL-5, which prevents eosinophilia, had no effect on H. pol’gyrus infection and must call into question the presumed role of these cells [ 61.

helminthiases

models

Most work on intestinal helminthiases has centred on the various well-developed animal models. To date, the intestinal nematode Tricburis muris appears to be unique in that reciprocal activation of Th-cell subsets is associated with short or long-lived Infections [ 21. A clear polarization of response has been observed in mice: interferon (IFN)-)I secretion was predominant in two susceptible strains, whereas interleukin (IL)-4, IL-5 and IL-9 were the main secretory products in a resistant strain [3*]. The obvious conclusion is that protective immunity is associated with activation of the Th2-cell subset. Successful manipulations of the immune response have been performed to increase worm survival in resistant mice and curtail infections in susceptible animals [4]. The mechanism that establishes the dominant Th-cell subset in this infection remains unclear. However, IL-5 is considered an attrac-

Analysis of Na&bost?-o0ngylu.s braziliensis infection in mice has produced even more puzzling results. This nematode is a classical stimulator of IgE production and intestinal mastocytosis. Administration of mAb to IL-3 and IL-4 suppressed the intestinal mast cell response by 85-90%. IgE production was also abrogated but the treatments had no effect on the expulsion of worms [7]. Conversely,

Abbreviations CST-giutathione

S-transferase; IFl%nterferon;

IL-interleukin; Th-T

538

@

mAlwmonoclonal

helper; TNF-tumour

Current

Biology

antibody;

necrosis factor.

Ltd ISSN 0952-7915

SCID--severe

combined

immunodeficient;

Immunity

daily treatment of mice with recombinant IFN-)I or IFN-cl permitted a more severe and long-lasting infection of N. 6ruziZienszIsto develop [5**]. The IFN-mediated inhibition of protection was associated with the downregu lation of ‘H-&cell responses, but its effect was eventually overcome, and the worms expelled. Infections with Trichinellu spiralis induce a more complex set of events and there is even less consensus about the underlying mechanisms. In rats, a primary exposure is short-lived and confers a long-lasting immunity in which 99% of a larval challenge is expelled in a matter of hours [8]. The mechanism of rapid expulsion has two components, a requirement for antibody, and a local cell-dependent intestinal step which may be relatively non-specific as H. p~lygyruscan substitute for 7: spiralis IgE is a particularly efficient antibody component but mAbs of IgGr, IgGZa and IgGzc isotypes can confer ‘rapid expulsion’ to rats previously exposed to H. polygyrus. In mice, which have been extensively used as experimental hosts for T. spiralis, a primary infection is expelled af ter approximately 2 weeks, the exact timing being straindependent. A secondary challenge is rejected rapidly but this immunity is expressed only during and immediately after the primary infection and has been termed ‘associative expulsion’ [9]. This rejection is inhibited by administration of mAb to CD4, confirming its immunological specificity. In two mouse strains, which differed in the timing of primary expulsion, antigen-stimulated mesenteric lymph node cultures secreted increased amounts of Th2 cytokines compared with controls but negligible amounts of IFN-y; no relationship was observed between cytokine levels in culture and the speed of expulsion [ 101. It was concluded that a Th2-cell response mediates expulsion of T. spirulis, as well as the associated inllam mation and pathology. In this context, administration of mAbs against the receptor for stem cell factor (c-kit) to mice infected with 7: spiralis both reduced the intestinal mast cell population to background levels and elevated worm burdens [ 111. A different conclusion was reached with another strain of mouse, by enumerating cytokine-secreting cells [ 121. A striking compartmentalization of responses was observed: cells that produced IFN-?/ predominated in the spleen, whereas those that produced IL-5 predominated in the mesenteric lymph nodes. Furthermore, early activation of cells that secrete IFN-)I and IL-2 in mesenteric lymph nodes, with little subsequent activation of cells that secrete IL-4 and IL-5, distinguished resistant from susceptible strains of mice [ 13.1. Variations in the ability to elicit IFN-y production may have resulted from differences in antigen-presenting function, particularly IL-I production. These findings support the hypothesis that a primary 7: spiralis infection has a tendency to induce Thl-type responses in resistant mouse strains and Th2type responses in susceptible strains. The potential role of eosinophils as effector cells in immunity to ‘I: spiralis has been evaluated [ 141. Marked variations in the kinetics and intensity of responses were apparent over a range of seven mouse strains. However,

and immunoregulation

in helminth

infections

Wilson

no significant differences were observed in the levels of intestinal eosinophilia in resistant and susceptible strains over the period of rapid worm expulsion. This suggests that eosinophils have no direct effector function against the intestinal stages of 7: spiralis Furthermore, administration of anti-IL-5 mAb to mice caused a depletion in the number eosinophils, yet had no detectable effect on primary or secondary T. spiralis burdens [ 15.1. Thus, mouse eosinophils may not be the anti-parasite effector cells they were once thought to be.

Thl-

and ThZ-cell

responses:

protection

versus

pathology?

The disparate observations on murine responses to in testinal nematode parasites have provoked two very different interpretations. Finkelman and Urban [ 5**,16] consider that the predominant Th2-cell responses are important for protection, albeit for reasons that are still elusive, and view production of Thl cytokines as the appropriate response for intracellular infections such as Leishaania. They propose an ingenious hypothesis to account for the bias in favour of Th2 cytokines, rea soning that the host must have a means of recognizing characteristics common to members of a class of infectious agents. The favoured candidates are the proteolytic enzymes released by nematode larvae (and by schistosome eggs; see below). They further speculate that the common factor acts by triggering cytokine release either from non-T cells (e.g. basophils) or a specialized T-cell subset and it is these ‘early’ cytokines that guide the differentiation of antigen-specific T cells in the direction of either Thl or Th2 cells. The incorporation of a host component in the hypothesis provides a potential explanation of why a particular pathogen triggers a Thl -cell response in one mouse strain and a Th2-cell response in another. The alternative interpretation has been proposed by Sher and Coffman [I?‘**] who conclude that in certain helminth infections it is the Thl-cell response that is protective. They question why parasites causing chronic infections would induce responses potentially lethal to themselves, suggesting that the elevated IgE levels and eosinophilia are simply immunopathological manifestations of the Th2-cell response which the worms induce to downregulate protective Thl -cell responses. Arguably, if the Th2-cell response were protective then it must involve as yet undiscovered cytokine-induced effector mechanisms not requiring IgE or eosinophils. Furthermore, Pritchard [18-l has suggested that the disproportionately high plasma level of non-parasite specific IgE induced in helm&h infections is a parasite-protective strategy. This antibody would compete for Fc, receptors on various cells thereby disabling immune effector mechanisms. Thus an IgE response would be most beneficial to the host when it was directed solely against parasite antigens. It is notable that in human hookworm infections, IgGi autoantibodies develop with specificity for IgE and may provide protection against immune-complex and hypersensitivity reactions mediated by IgE [19].

539

540

Immunity

to infection

Filariasis Human

helminth-infected individuals with elevated serum IgE and eosinophilia has revealed a correlation between IL-4 and IL-5, both with respect to the frequency of responder cells, and the total cytokine production of the cell population [24]. Additionally, serum IgE levels can be correlated directly with antigen-driven IL-4 production, and inversely with IFN-y production [25]. Indeed, mAb neutralization of IFN-y in vitro significantly augmented IL-4 production suggesting that in viva, IFN-), and IL-4 reciprocally regulate IKE production. The immunological responsiveness of filarial patients with either asymptomatic microfilaraemia or chronic lymphatic obstruction has recently been compared [26-l. Lymphocyte proliferation to mitogens and to streptolysin-0 (a non-parasite antigen) was similar in both groups, but the frequency of parasite-specific T cells was significantly lower in microlilaraemic patients than in those with chronic pathology. The proportion of lymphocytes producing parasite-specific IgE and IgG was also lower in the filaraemic group. These findings support the concept that tolerance by clonal anergy is a critical mechanism for maintaining the microfilaraemic state. A greater hyporesponsiveness of the Thl- compared with the Th2-cell subset [20] could explain the elevated IgE production in the filariasis patients. Alternatively, parasite-derived factors may account for the abundance of non-parasite specific antibody.

responses

Lymphatic filariasis (Fig. 1), caused by a related group of nematodes, is characterized by a protracted, stable association between parasite and host with elevated IgE and IgG4 levels, and peripheral eosinophilia [20,21*], Within an endemic area the human population exhibits a spectrum of disease. All individuals are likely to be seropositive but one group, the endemic normals who may be resistant to reinfection, have neither symptoms nor circulating microfilariae [ 21.1. The largest group comprises asymptomatic individuals, hyporesponsive (i.e. tolerant) to parasite antigens, with microIilariae in the peripheral blood. Finally, some hyper-responsive people show signs of chronic pathology in one or more forms but may or may not have circulating larvae. Classically this disease spectrum has been bterpreted as a distinct set of alternative states following infection. More recently, Day [22] has suggested that it represents a temporal sequence with induction of tolerance preceding the microfilaremic state, which in turn progresses either to the endemic normal situation or chronic lymphatic pathology. It is clear that filarial infections promote a dominant Th2cell response. Thus, a higher proportion of mitogendriven T-cell clones derived from filariasis patients produced IL-4, and a lower propo.tion IFN-)I, in comparison with clones from control individuals [23]. Characterization of the peripheral .blood T-cell responses of

The mechanism for initiating and maintaining the parasite-specific hyporesponsiveness associated with mi-

Mosquito vector Larvae develop

I uIn A

Fig.1. The life cycle of Wechereria

L3 infects

through

in thoracic

mosquito

muscles

bite

bancrofti the causative agent of bancroftian filariasis. Man becomes infected when bitten by the mosquito vector which carries third stage CL31 larvae. These larvae leave the skin via lymphatic vessels and lodge at some point in the lymphatic system where males and females grow slowly to maturity and mate. The female releases embryonic larvae, termed microfilariae, over a long peroid of time. These larvae travel down the lymphatics to enter the bloodstream where they circulate, often with a marked diurnal peroidicity in the numbers present in the peripheral blood. The life cycle is continued when a mosquito takes up the microfilariae during a blood meal. The chronic pathology associated with filarial infection is the result of immune responses to the mature worms and their products in the lymphatic system.

Immunity

crofilamemia remains unclear. The role of IL-10 has yet to be explored but it is notable that addition of neutralizing rt-,~b to in vitro cultures of peripheral blood mononuclear cells from microfilaraemic patients restored their ability to proliferate, but had no such effect on those from individuals with chronic pathology [27**]. The ~t,atus of IL-10 as a candidate for the regulator of hyporesponsiveness has been strengthened by the recent demonstration that it inhibits antigen-induced proliferation of both Thl and Th2 human clones via its action on antigen-presenting cells [ 281. Conversely, the greater frequency of lymphocytes that secrete IFN-y in patients with chronic pathology, relative to those with microfilaraemia [27-l, indicates a shift in the balance towards Thl-cell responses when clonal anergy breaks down. This may be a double-edged immunological weapon, promoting protection if directed against the infective L3 stage, but also causing chronic lymphatic disease if the responses are triggered by adult or microfilarial antigens. A factor which complicates interpretation of immunity to lilariasis is the existence of blocking antibodies in the serum of lilarial patients [29-l. The inhibition of allergic responsiveness is mediated by antibodies of the IgG* subclass, with a lesser contribution from IgGr. A correlation between the functional level of blocking activity and clinical status has been observed, with microlilaraemic patients having lo-fold higher levels than those with chronic pathology. As IgE and IgG4 are co-regulated, this could be a further manifestation of diminished Th2-cell reactivity in the latter group.

Animal

models and filarial

antigens

Brugiu pabungi infections in cats are probably the best model for lymphatic ftlariasis [ 21*]. Immunity to infective larvae develops before immunity to adult worms and the latter is manifest as a precipitous drop in the numbers of circulating microfilariae. A small proportion of cats given a single inoculation of L3 larvae never develop microfllaraemia [30]. These animals show the highest antibody levels against somatic extracts of the parasite, compared with those either persistently positive for microlilariae or becoming spontaneously negative. Studies with immunocompromised rats suggest that regulation of microftlaraemia involves a T cell dependent mechanism associated with a specific IgM response, and separate from those controlling larval establishment or adult worm death [31]. Semi-permissive BALB/c mice have been used to assess immune responses induced by irradiated L3 larvae [32]. These appeared to be strongly biased in the direction of Th2 cells with high levels of IL-5 and IL-9 secreted by spleen cells in vitro upon stimulation with antigen or mitogen; production of IFN-), was, in contrast, negligible and levels of IL-4 were low. The mongolian jird (Meriones unguicuhtus) is susceptible to infection with Brugia malayi and has been used to investigate acquired immunity. Exposure to radiationattenuated L3 larvae induced a significant level of protection, which appeared to operate against challenge larvae, and also caused stunting of the growth of adults [33], Sera from vaccinated, but not infected animals were

and immunoregulation

in helminth

infections

Wilson

strongly reactive with L3 surface antigens; whether antibodies play a role in protection is uncertain. However, responses to paramyosin, a 97 kDa somatic antigen were stronger in vaccinated than infected jirds; two immunizations with the recombinant molecule induced significant protection as judged by adult worm recoveries, worm lengths and blood microiilaria counts [34-l. All the laboratory models of lilariasis suffer shortcomings and this has hampered identification of antigens which might mediate protection against L3 larvae, adults or microfilariae. The major soluble glycoprotein of adult Brugia has been identified as a secreted glutathione peroxidase which may protect the worm against oxidative damage [35] A recombinant antigen of molecular weight 62kDa from B. malayi reduced the microiilaraemia in a mouse model of immunity [ 2 1.1. The same antigen is highly immunogenic in infected children and adults but the isotype and magnitude of antibody reactivity do not correlate with the microfilaraemic status of asymptomatic individuals [ 361.

Schistosomiasis Human

responses

Schistosomiasis (Fig. 2) is a water-borne infection of man caused by members of the parasitic trematode genus Schistosoma. The convex age-prevalence curve of schistosomiasis in human populations may be in part the result of exposure patterns, but age-dependent acquired resistance to infection is now considered an important contributor. Several recent studies have revealed the production of specific IgE antibody, and Hagan [37*] has put the case for its role as mediator of protective immunity in man. In patients susceptible to reinfection, levels of IgM and IgG specific for Schistosoma mansoni were equal to or greater than those in the resistant group, whereas IgE levels were 6%fold higher in the most resistant individuals [38**]. As already noted in iilarial infections [29-a], competing antibody of the IgG4 subclass was also produced, levels being particularly high in the least resistant individuals [39-l A similar reciprocal relationship between IgE and IgG4 levels has been reported for Schistosoma haematobium infection [40], The specific function of IgG4 may be to control recognition of antigen by IgE, due to its similar specificity spectrum, and consequently to regulate ana phylactic reactions and IgE-mediated immunity [39-l. In a Kenyan study, IgE reactivity against an adult worm antigen preparation correlated positively with age and negatively with reinfection by S. mansoni [41-l. Amongst the antigens recognized by IgE, a molecule of 22 kDa was particularly prominent. The parasite target of the human IgE response is a source of speculation. The observation that several indicators of immediate hypersensitivity increase with patient age, following the same time course as the acquisition of resistance, has been taken to imply that newly penetrated larvae are the target [ 38**]. However, in another study it has been assumed that neither the newly transformed schistosomulum nor the adult are vulnerable

541

542

Immunity

to infection

Freshwater

snail

intermediate

host

??Site

of parasitization by adult worms

3 1993 Current

Opinion

in

Immunology

Fig. 2. The life cycle of S. mansoni. Thk mammalian host is infected by Ere,e-swimming cercaria larvae which penetrate the skin, metamorphosing into schistosomula as they do so. After some days these larvae leave the skin via venules, and to a lesser extent lymphatics, and travel to the lungs where they spend 3-4 days adapting for intravascular migration. They pass from the lungs via the left side of the heart to be distributed to all systemic organs in proportion to cardiac output. Those schistosomula that pass down splanchnic arteries, traverse the intestinal capillaries to reach the site of parasitization in the hepatic portal tract; most are trapped in the liver, begin to feed on blood, and grow to maturity. In contrast with events in the liver, schistosomula reaching other organs negotiate the capillary beds and are carried back via the right heart to the lungs to continue the migratory process. Mature male and female schistosomes mate in the portal tract and the male carries the female upstream to the intestinal tissues where she deposits eggs. A proportion of these pass through the intestinal tissues to reach the lumen of the organ, and are voided in the faeces to continue the life cycle. Other eggs break free and travel down the portal tract to lodge in the liver where they provoke the granulomatous response which initiates the sequence of pathology associated with schistosomiasis.

[41**]. Paradoxically, cytotoxic mechanisms involving IKE are ineffective against older larvae in vitro.

Although the foregoing studies all point to IgE as the principal mediator of protective immunity, other mechanisms could be involved. An age-dependent evolution of human IgA responses to schistosome glutathione Stransferase (Sm28GST), a candidate vaccine antigen, has recently been identified [42*-l. The antibodies were capable of neutralizing the activity of the enzyme and impaired both the fecundity of egg-laying adult worms, and the ability of schistosome eggs to hatch. Two further reports describe human responses to schistosomes reminiscent of filarial infections. In one, a majority of patients who frequently came into contact with water showed an absence of lymphoproliferative responses to schistosome antigens despite reacting to unrelated antigens [43]. The unresponsiveness appeared to be correlated with the degree of infection and there was also an impairment of IFN-)I production to schistosome antigen in vitro but not to mitogen. In the second study a group of patients analogous to the endemic normals of filariasis was identified [44]. These individuals were seropositive, with cellular reactivity to schistosome antigen, but no recent history of chemotherapy. The level of IFN-y production

by peripheral blood mononuclear cells stimulated with a soluble antigen preparation was low in the majority of infected patients, independent of clinical status, but elevated in the endemic normal group. The observations are consistent with the idea that IFN-)I, and by inference delayed-type hypersensitivity, contribute to protective immunity in man.

lmmunoregulation

and granuloma

formation

Although schistosome cercariae utilize protease secretions to penetrate host skin [16] they nevertheless induce a dominant Thl-type response in murine hosts (see below) [45]. However, the onset of egg deposition after worm maturation results in a dramatic shift to a Th2 profile, and the switch can be triggered by injection of schistosome eggs alone [ 17**,45]. Thereafter, murine (and human?) responses to a schistosome infection are dominated by Th2 characteristics, such as eosinophilia and IgE production. The recognition that such a dramatic change can occur during the course of a parasitic infection has focused interest on the immunoregulatory mechanisms involved, and on the events by which the schistosome egg elicits the hepatic granulo-

Immunity

matous response which is the major cause of morbidity in schistosomiasis. The primary immune response to schistosome eggs has been explored in a model system involving their subcuta neous injection into mice. Initially, this elicits IFN-y production by cells from draining lymph nodes followed by a ThO (mixed) prolile of cytokines [46]. By 7-10 days IFN-y production is severely depressed, whereas that of IL-2, and of Th2 qtokines, continues. The downregula tion of Thl-cell responses is effected by IL-10 released from the Th2 cells [48]. The expanding lymphocyte population appears to require IL-4, rather than IL-2 as the major growth factor and shows altered expression of memory markers 1471. The sequential pattern of qtokine release during granuloma formation has been characterized in a model system involving embolization of eggs in the lungs of schistosome-infected mice [ 49’91, IL-I was produced first, followed in order by IL-4, IFN-), and tumour necrosis factor (TNF). The dominant role of IL-4 as an inIlammatory cytokine was underlined by the reduction in size of both pulmonary and hepatic granulomas which followed administration of neutralizing mAb to IL-4 [49”,50], and an increase in size produced by recombinant IL-4 [ 501. Antibodies to IL-2 also reduced hepatic granuloma size as well as blood and tissue eosinophilia [51]. IL-2 may thus be a prerequisite for IL-5 production by Th2 cells

I521. Granuloma formation may not be totally dominated by Th2 cytokines and the immune modulation that occurs in chronically infected mice has been attributed to continued IFN-y production [49=-l. However, another study has concluded that granuloma modulation was the result of a generalized suppression of cytokine synthesis [ 531. Thl-cell clones that are reactive with egg antigens have been derived and these are capable of stimulating gran uloma formation in viva [54]. Other panels of Th-cell clones, derived from mice immunized with egg antigen, produced IL-2, IL-4 or both cytokines [55]. Additionally, the elicitation of granulomas by injection of egg-antigen fractions coupled to Sephadex beads has revealed a differential reactivity of T cells with at least one fraction stimulating IFN-)I production [ 561. The cause of the intense fibrosis that accompanies resolution of the egg-induced granuloma has also been clarified with the identification of a novel cytokine [57-l. This cytokine, fibroblast stimulating factor-l, is present on the surface of and is secreted by CD4+ T cells recovered from granulomas, and has a powerful growth-stimulating activity on fibroblasts. It has recently been demonstrated that severe combined immunodeficient (SCID) mice can be reconstituted to produce a granulomatous response to schistosome eggs by administration of recombinant TNFa [58**]. Addi tionally, TNF-a appears to act as a reproductive or developmental stimulus for adult female worms. The fact that a single cytokine can facilitate granuloma formation has been attributed to its pleiotropic effects on inflammatory cells such as macrophages, in the absence of B or T lymphocytes [59]. These data have been linked to an earlier suggestion that the promotion of a granu-

and immunoregulation

in helminth

infections

Wilson

lomatous inflammation by the schistosome egg is essential for its export from the host intestinal tissues [60]. Thus, it seems possible that the parasite has subverted the defense mechanisms of the host to facilitate its own reproduction and transmission.

The irradiated

vaccine

model

of immunity

Progress has been made in understanding the mechanisms which underlie the protective response elicited in laboratory hosts, particularly mice, by exposure to radiation-attenuated cercariae. In contrast with normal larvae, irradiated parasites that persist in the lymph nodes draining the skin exposure site, induce an intense proliferation of T cells [6I]. Antigenic stimulation of these cells in z&-o has revealed an initial ThO profile of secretion, with Thl qtokines such as IFN-)I becoming dominant by 21 days [62]. Heavily irradiated parasites, which fail to reach the skin-draining nodes, do not rem inforce the later Thl cell dominated response, and do not induce protection [63]. A proportion of optimally irradiated parasites travel to the lungs where they stimulate an inflammation which recruits a population of lymphocytes, largely CD4+ T cells, to this organ [64]. These cells also secrete IFN--y upon antigenic stimulation in vitro, and may represent a memory/effecter population which arms the lungs against the arrival of challenge larvae. The immune-mediated elimination of challenge parasites takes place predominantly in the lungs of vaccinated mice. Newly arrived larvae trigger the formation of a focal inflammatory response, rich in CD4+ T cells, The crucial role of IFN-)I as the cytokine orchestrating this pulmonary effector response was demonstrated when neutralizing mAbs to IFN-)I, administered to vaccinated mice between 4 and 16 days post-challenge, produced a 90% abrogation of immunity [65**]. Previous studies suggest that the pulmonary foci, at least in their early stages, acted to block onward migration of the parasites through the vasculature, rather than by direct cytotoxic killing [66], The primary role of IFN-)I, and other cytokines produced within an inflammatory focus, may therefore be to upregulate intercellular adhesion molecules, thereby contributing to the blocking capacity of the cell mass.

Candidate

vaccine

antigens

A number of antigens for inclusion in a vaccine against S. mansoni have been characterized, and new candidates are being evaluated. Amongst these, an integral membrane protein Sm23, first identified by sera from mice multiply vaccinated with irradiated cercariae, is unusual in being expressed on the surface of lung schistosomula and shows homology with a superfamily of unknown function [67]. It has been speculated that Sm23 may mimic one or more host members of the family, perhaps playing a role in parasite adhesion or motility; its protective potential remains to be defined. A 62 kDa fragment of a 200kDa myosin molecule has also been identified using vaccine serum [Gw]. The recombinant

543

544

Immunity to infection

molecule conferred high levels of protective immunity on mice when formulated in proteosome complexes with the outer membrane of meningococcus, or in micelles, and administered on at least three occasions to generate high antibody titres. The native protein is expressed on the surface of newly transformed schistosomula. Studies on Sm28GST have continued apace; a single administration of the recombinant antigen to rats was shown to produce up to 59% reduction in worm burden [69]. High levels of IgE and IgA antibodies were elicited, both isotypes being effective in cytotoxicity assays against newly transformed schistosomula, using eosinophils as the effector cell. This suggests the existence of a novel parasite-killing mechanism involving IgA and eosinophils. Epitope mapping of the Sm28GST molecule has revealed that specific peptides can elicit antibodies in rats which partially inhibit enzyme activity, and when passively transferred to mice, reduce tissue egg deposition and the ability of eggs to hatch [ 70.1. If these attributes were apparent in human vaccine trials they would confer the dual benefits of protection against infection and egg-induced pathology.

Concluding

ELSE KJ, HULTNEXL, GRENCISRK: Cellular Immune Responses to the Murine Nematode Parasite Trichuris muris. II. DIfferential Induction of Th-cell Subsets in Resistant Versus Susceptible Mice. Immunology 1992, 75~232-237. Reports the mitogen-stimulated production of Th2 cytokines by mesenteric lymph node cells from mice that are resistant to infection with 7Y mur& and the production of Thl cytokines by cells from susceptible mice.

3. .

4.

ELSE KJ, HIJLTNERL, GRENCISRK: Modulation of Cytokine Production and Response Phenotypes In Murine Trichuriasis. Parasite Immunol l-2, 14:441449.

URBAN JF, MADDENKB, SVE~C A, CHEEVERA, TRO~A PP, GAUSE WC, KATONA IM, FINKELMAN FD: The Importance of TH2 Cytokines in Protective Immunity to Nematodes. Immunol Rev 1992, 127205220. A raiew, which includes hitherto unpublished information, summarizing the evidence that Th2 cytokines are the agents that control protective immunity to nematodes. The potential role of proteases as inducers of a Th2-cell responses is outlined. 5. ..

6.

URBANJF, KATONAIM, PAUL WE, FINKEU~ANFD: lnterleukin 4 is Important in Protective Immunity to a Gastrointestinal Nematode Infection in Mice. Proc Natl Acad Sci USA 1991, 88:551%5517.

7.

MADDENKE%,URBANJF, ZILTENERHJ, SCHRADER JW, FINKELMAN FD, KATONA 1~: Antibodies to IL-3 and IL-4 Suppress

Helminth-induced Intestinal Mastocytosis. J Immunol 147:1387-1391.

remarks

Initial events in lymphoid tissues after helminth infection may be crucial in determining whether the immune response is driven in a Thl- or Th2-cell direction. In this context, both the secretions of infective larvae and interaction with particular types of antigen-presenting cell may be decisive. The occurrence of later parasite-driven switches has been highlighted by research on schistosomes, and microfilariae could operate in a similar way. Responses driven by Thl cells deal very successfully with migrating schistosome larvae and may turn out to be equally effective against the L3 larvae of filarial nematodes which have a similar tissue migration phase. We seem to be a long way from explaining how the effector mechanisms driven by Th2 cells might operate in protective immunity against nematodes or schistosomes. The debate about the utility of Th2-cell responses seems set to continue, particularly for chronic infections where their exacerbation may protect the parasite and dam age the host. Work in progress should delineate more clearly the strategies available to deal with helminths and will enhance our ability to manipulate host responses for our benefit.

References and recommended

reading

1991,

8.

BELLRG, APPLETONJA, NEGRAO-COFXEADA, ADAMSIS: Rapid Expulsion of Trichinella spiralis in Adult Rats Mediated by MonoclonaI Antibodies of Distinct IgG lsotypes. Immunoloc3/ 1992, 75:520-527.

9.

BELLRG: Tricbinella spiralis Evidence that Mice Do Not Express Rapid Expulsion. Exp Parasitol 1992, 74:417-430.

10.

GR&VCISRK, HULTNERL, EISE KJ: Host Protective Immunity to

Tricbenella spiralis in Mice: Activation of Th CeII Subsets and Lymphokine Secretion in Mice Expressing Different Response Phenotypes. Immunolog)l 1991, 74~329332. 11.

GRENCISRK, ELSE KJ, HUNTLEYJF, NISHIKAWA Sl: The

12.

KELLY EAB, CRUZ ES, HAUDAKM, WASSOMDL: IFN-y- and IL-5-

in Vivo Role of Stem CeII Factor (okif Ligand) on Mastocytosis and Host Protective 1mmunIty to the Intestinal Nematode Tricbinella spiralis in Mice. Parasite Immunol 1993, 1555-59.

producing Cells Compartmentalize to Different Lymphoid Organs in Tricbinella spiral&infected Mice. J Immunol 1991,

147:30&311.

POND L, WA~SOM DL, HAYES CE: InfIuence of Resistant and Susceptible Genotype, IL-l, and Lymphoid Organ on Tricbinella spiralisinduced Cytokine Secretion. J Immunol 1992, 149957-965. An analysis of the cytokine responses to primary 7: piralis infection. The compartmentalization of cytokine-producing cells is analyzed by filter immunoplaque assay and a role for Thl cytokines in protection is

13. .

proposed.

Papers of particular interest, published within the annual period of raiew, have been h&h&&d as: . of special Interest .. of outstanding interest 1.

LOCKSLEY RM, Louts JA: Immunology of Leishmaniasis. Cur?’ @in Immurwl 1992, 4:41%418.

2.

Subsets in Mouse Trichuriasis. Parasi~ol Today 1991, 7:31>316. ELSE KJ,

GRENCIS RK:

Helper

T-cell

14.

IAMMA.S DA, WAKEUND, MITCHELL IA, TUOHY M, ELSE KJ: Genetic InlIuences upon EosinophIlIa and Resistance in Mice Infected with Tricbinella spiralis. Parasitology l!B2, 105:117-124.

15. .

HERND~NFJ, KAYES SG: Depletion of EosinophiIs by Anti-IL-5 MonoclonaI Antibody Treatment of Mice Infected with Trlcbinella spiralis does not Alter Parasite Burden or

Immunity and immunoregulation

Immunologic Resistance to Reinfection. J Immunol 1992, 1493642-3647. A clear demonstration that administration of mAb fo IL-5 depletes mice infected with 7: spiral& of eosinophils but has no effect on the outcome of primary or secondary infections. 16.

FINKEU~ANFD, URBAN JF: Cytokines: Making Choice. Par&to1 Toaizy 1992, 8:311-314.

the

in extension of earlier work on blocking antibodies (IgG,) in the serum of filariasis patients. Blocking activity is higher in individuals with microfilaraemia than chronic pathology. See also [39-l. 30.

FLETCHER C, BIRCHDW, DENHAMDA: Cats with Single Brugiu pahangi Infections: Relationship Between Parasitological Status and Humoral Responses to Somatic and Surface Parasite Antigens. Parasite Immunol 1992, 14:33+350.

31.

LAWRENCE R, DENHAMDA: Brugiu pahangi Infections in Immune-compromised Rats Demonstrate that Separate Mechanisms Control Adult Worm and MicroIiIariaI Numbers. Parasite Immunol 1992, 14:371-384.

32.

BANCRO~ AJ, GRENCIS RK, ELSE,KJ, DEVANEYE: Cytokine Pro-

Right

SHERA, COFFMANRL: Regulation of Immunity to Parasites by T Cells and T Cell-derived Cytokines. Annu Rev Immunol 1992, 10:385-409. A comprehensive review which places T-cell and cytokine responses to parasitic helminths in the broader context of protozoan and other infectious agents. 17. ..

PRITCHARDDI: Immunity to Helminths: is Too Much IgE Parasite- Rather than Host-protective? Parasite Immunol 1993, 15:%9. A thought-provoking commentaty on the ability of helminth infections to stimulate IgE production. It is argued that high levels of non-parasitespecific IgE may benefit the parasite by blocking specific release of mediators from mast cells. See [37*]. 18. .

19.

SHAKIBF, PmCHARDDI, WALSHEA, SMITHSJ, POWELL-RICHARDS A, KUMAR S, EDMONDS P: The Detection of Autoantibodies to IgE in Plasma of Individuals Infected with Hookworm (Necutor americanus) and the Demonstration of a Predominant IgGl anti-IgE Autoantibody Response. Parasite Immunol 1993, 15~47-53.

20.

MAIZELSRM, LAWRENCE RA Immunological

Key Feature 1271-276.

SEWRK ME, MWELS RM, YAZDANBAKHSH M: Immunity and the Prospects for Vaccination against FiIariasis. Immunobiology 1992, l&1:263-281. Various strategies for the development of an anti-!iIariaI vaccine are considered The limitations of animal models and the sparse information on filarial antigens are reviewed.

22.

DAY KP: ‘Ihe Endemic Normal in Lymphatic Filariasis: A Static Concept. Parasitol Today 1991, 7341-343.

23.

ROMAGNANI S: Regulation and Deregulation

of Human IgE

Synthesis. Immunol Today 1990, 11:316321. 24.

MAHANn S, ABRAMSJS, KING CL, LulAvE AP, NUTMANTB: ParalIel Regulation of IL-4 and IL-5 in Human Helminth Infections. J Immunol 1992, l&:3567-3571.

25.

KING Cl Low CC, NUTMANTB: IgE Production in Human Helminth Infection. Reciprocal Interrelationship between IL-4 and IFNy. J Immunol 1993, 150:187%1880.

KING CL, Ku-MI V, POWDEXTER RW, KUMARI S, JAYARAMAN K, ALLINGDW, OTIESENEA, NUTMANTB: Immunologic Tolerance in Lymphatic Fiiasis. J Clin Invest 1992, 89:1403-1410. The most detailed analysis of responses to filarial infection in human patients with microlilaraemia or chronic pathology Factors influencing the development and maintenance of hyporesponsiveness are explored. 26. ..

KING CL, NUTMANTB: Biological Role of Helper T-cell Subsets in HeIminth Infections. Cbem Immunol 1992, 54:136-165. A comprehensive review, including much hitherto unpublished information, on the role of Th-cell subsets in hehninth infection. Its content very much reflects the authors’ research on human fiIarial infections. 27. ..

28.

29. ..

33.

F, GIUDLZIMG, BIAGIO~ R, ROMAGNAMS: Human IL-10 is Produced by Both Type 1 Helper (Thl) and Type 2 Helper (Th2) T Cell Clones and Inhibits Their Antigen-speciIic Proliferation and Cytokine Production. J Immunol 1993, 150:353-360.

WEIL GJ, BEN-WEN L, Lrmrs F, CHANDRA~HEKAR R: Brugia

BEN-WENL, CHANDR_&HEKAR R, WEILGJ: Vaccination with Recombinant FiIarial Paramyosin Induces Partial Immunity Infections in Jirds. J Immunol 1993, to Brugia malayi 150:188-1885. Notable for the successful induction of immunity to a filarial parasite using a recombinant protein.

34. .

Identification of the of Lymphatic FiIxSecretory Homo@ Acad Sci USA 1992,

35.

COOKSON E, BL~.XTERML, SELKIRK ME: Major Soluble Cuticular Glycoprotein ial Nematode Parasites (gp29) as a of Glutathione Peroxidase. Proc Natl 89:5837-5841.

36.

E, DAY K, EL-ZEINYA, KAZlJR4JW, HAzLE7T FE, PEARLMAN NILSONTW, ALPERS MP: Antigenicity of a Protective Recom-

binant Filarial Protein in Human Dis 1992, 1661453-1457.

Bancroftian

Filariasis. J I?$

HAGAN P: IgE and Protective Immunity to Helminth Infec37. . tions. Parasite Immunol 1993, 15:1-4. Reviews current evidence for the role of parasite~specific IgE as a mediator of protective immunity, particularly in the context of schistosom&is. See [18-l. 38. ..

DESSEINAJ, COUISSINIER P, DEMEURE C, RIHETP, KOHISTAEDT S, CARNEIRO-CARVALHO D, OUATARA M, GOUDOT-CROZEL V, DESSEINH, BOURC~IS A, ABEL L, CARVWO EM, PRATAA:

Environmental, Genetic and Immunological Factors in Humansoni. Imm Invest 1992, man Resistance to Scbistosoma 21:42-53. A comprehensive review of studies on human responses to S. mansoni in Brazil. The analysis points to IgE as the effector molecule, and the newly penetrated larva as the target. RIHET P, DEMEURE CE, DESSEIN AJ, BOURGOISA: Strong Serum Inhibition of SpeciIic IgE Correlated to Competing IgG4, Revealed by a New Methodology in Subjects from a S. mansoni Endemic Area. Eur J Immunoll992, 22:2063%2070. A demonstration of IgGq antibodies blocking IgE responses in human schistosomiasis. See also [29**].

39. .

40.

HAGAN P, BLUMENTHAL UJ, DUNNE DW, SIMPSONAJG, WILKINS HA: Human IgE, IgG4 and Resistance to Reinfection with Schistosoma haematobium. Nature 1991, 349243-245.

41. ..

DUNNE DW, BUTTERWORTH AE, FULFORDAJC, KARlw HC, L&%LF( JG, OUMA JH, CAPRON A, PIERCE RJ, STIJRR~CK RF: Immunity After Treatment of Human Schistosomiasis:

DEL PRETE G, DE CARU M, ALMERIGOGNA

Husw~ R, POINDEXTER RW, OTTESENEA: Control of AlIergic Reactivity in Human Filariasis. Predominant Localization of Blocking Antibody to the IgG4 Subclass. J Immunol 1992, l&:2731-2737.

duction in BALB/c Mice Immunized with Radiation Attenuated Third Stage Larvae of the FiIariaI Nematode, Brugia puhangi. J Immunol 1993, 150:1-8. maluyt Antibody Responses to Larval Antigens in Infected and Immunized Jirds. Exp Parasitoi 1992, 74:315-323.

Tolerance: The in Human Fikuiasis. Par&to1 To&y 1991,

21. .

in helminth infections Wilson

Association Between IgE Antibodies to Adult Worm Antigens and Resistance to Reinfection. Eur J Immunol 1992, 22:1483-1494. The most recent results from the prolonged study of human responses to S. munsoni infection in Keny. IgE responses to adult worm antigens are negativelycorrelated with reinfection.

42. ..

GRZ~CHJ-M, GREZEL D, Xu CB, NEVR~NCK J-L CAPRONM, OUMA JH, BUTTERWORTH AJZ,CAPRONA: IgA Antibodies to a Protec-

545

546

Immunity to infection isation of a Panel of Egg Antigen-specific Murine T Cell Clones. Eur J Immunol 1992, 22:917-922.

tive Antigen in Human schistosomiasis mansoni. J Immunol 1993, 150:527-535. Human IgA responses to candidate vaccine antigen of S. mansoni Sm28GST in a Kenyan population, are reported. The IgA antibodies neutralize the enzymlc acirvity of GST, and impair fecundicity via reduced egg laying by female worms and the production of viable miracida. See [70*].

56.

LUKACSNW, BOROS DL: Utilization of Fractionated Soluble Egg Antigens Reveals Selectively Modulated Granulomatous and Lymphokine Responses during Murine schistosomiasis mansoni. Infect Immun 1992, 60:32W3216.

RIBERODE JESUSAM, ALMEIDARP, BACELLU 0, ARAUJOMI, DEMEURE C, BINAJC, DE?~EINAJ, CARVAIHOEM: Correlation

57. .

PRAKASHS, WYLER DJ: Fibroblast Stimulation in Schistoso-

43.

Between Cell-mediated Immunity and Degree of Infection ln Subjects Living in an Endemic Area of Schistosomiasis. Eur J Immunol 1993, 23~152-158. 44.

BAHLA-OUVEIRA LMG, GAZINELLI G, ELOI-SANTOS SM, CUNHAMELTJR, ALOES-OLIVEIRA LF, SILVEIRA AMS, VLANAIRC, Cmo J, SOUZAA, CORK%OUVEIRAR: Differential Cellular Reactivity to Adult Worm Antigens of Patients with Diierent Clinical Forms of schistosomiasis mansoni. Tram R Sot Trop Med Hz 1992, 86~57-61.

45.

SHERA, GAZZINELLI RT, OWALD IP, CLER~CI M, KULLBERG M, PEARCE EJ, BERZOFSKY JA, MOSMANNTR, JAMES SL, MORSEHC, SHEARER GM: Role of T-cell Derived Cytokines in the Down-

m&is. XII. Identification of Cm+ Lymphocytes within Schistosomal Egg Granulomas as a Source of an Apparently Novel Fibroblast Growth Factor (FsF-1). J Immunol 1992, 148:35833587. Repor& a novel cytokine responsible for granuloma fibrosis. M, RECTORE, RITIER D, MCKERROWJH: Tumour Necrosis Factor a Restores Granulomas and Induces Parasite Egg-laying in Schistosome-infected SCID Mice. Nature 1992, 356hO4-607. This paper documents the unexpected finding that TNI-a is both capable of reconstituting granulomas in SCID mice, and stimulating egg laying by adult female schistosomes. 58. ..

AMIRI P, L~~KSLEZY RM, PARSLOWTG, ~ADICK

59.

SHER A: Parasitizing the

Cytokine System. Nature

1992,

356~565-566

regulation of Immune Responses in Parasitic and Retroviral Infection. Immunol Rev 1992, 127:185204

60.

46.

EJ:Cm+ Th2 Response Induced by S&is VEUAAT, PEARCE tosoma mansoni Eggs Develops Rapidly, Through an Early, Transient, ThO-like Stage. J Immunol 1992, 148:228%229O.

COLLEYDG, NIX NA: Do Schistosomes Exploit the Host ProInliammatory Cytokine TNF-a for their Own Survival? Parasitol Today 1992, 8:355357.

61.

CONSTANT SL, WILSONRA In vivo Responses in the Draining

47.

VELLAAT, HULSEBOSCH MD, PEARCEEJ: Schistosoma man-

Lymph Nodes of Mice Exposed to Scbistosoma mansoni: Preferential Proliferation of T Cells is Central to the Induction of Protective Immunity. Cell Immunol 1992, 139~145-161.

soni Eggs Induce Antigen-responsive CD44-hi T Helper 2 Cells and IL-4-secreting CD&-lo Cells. J Immunol 1992, 14917141722. 48.

SHERA, FIORENTINO D, CASPARP, ‘PEARCE EJ, MOSMANN TR: Pro-

62.

Patterns of Cytokine Production and Proliferation by T Lymphocytes Differ in Mice Vaccinated or Infected with Schistosoma mansoni. Immunology 1991, 73~327-333.

duction of IL-10 By CD4+ T Lymphocytes Correlates with Down-regulation of Thl Cytokine Synthesis in Helm&h Infection. J Immunol 1991, 147:2713_2716. 49. ..

CHENSUESW, TEREBUHPD, WARMINGTONKS, HERSHEYSD, EVANOFFHL, KUNKELSL, HIGASHI GI: Role of IL-4 and

IFNr in Schisrosoma mansoni Egg-induced Hypersensitivity Granuloma Formation. Orchestration, Relative Contribution, and Relationship to Macrophage Function. J Immunol 1992, 148:90&906. Describes the sequential production of cytokines by cells present in granulomas following intravenous injection of eggs into previously infected mice. 50.

51.

CHEEVER AW, FINKELMAN FD, CASPAR P, HEINYS, MACEDONIA JG, SHERA: Treatment with Anti-IL-2 Antibodies Reduces Hep-

atic Pathology and Eosinophilia in Schislosoma mansoniinfected Mice While Selectively Inhibiting T Cell IL-5 Production. J Immunol 1992, 148:3244-3248. 52.

MEWAU A, ELLIOTD, MATHEWR, BLUMA, WEINST~CKJV: IL2 Contributes to the IL-5 Response in Granulomas from Mice Infected with Schistosoma mansoni. J Immunoll993, 150:536542.

53.

HENDERX)NGS, Lu X, MCCURLEYTL, COLIEY DG: In

CHIKUNG~~OSM, KANAZAWA T, DAYALY, STADECKER MJ: The

Cell-mediated Response to Schistosomal Antigens at the Clonal Level. In Viva Functions of Cloned Murine Egg Antigen-speciIic CD4+ T Helper Type 1 Lymphocytes. J Immunol 1991, 147~3921-3925. 55.

CHIKUNGUWOSM, Harris TS, BRODEUR PH, HARN DA, STADECKER MJ: The Cell-mediated Response to Schistosomal

Antigens at the Clonal Level: Development and Character-

MOUNTFORD AP, COUL~~N PS, PEMBERTON RM, SPIES LE, WILSON RA: The Generation of Interferon-y-producing T

Lymphocytes in Skin-draining Lymph Nodes, and Their Recruitment to the Lungs, is Associated with Protectbre Immunity to Schistosoma mansoni Immunology 1992, 75:250-256. 64.

SMY~HIESLE, COUI~~N PS, WILSON RA: T Cell-derived Cy-

tokines Associated with Pulmonary Immune Mechanisms in Mice Vaccinated with Irradiated Cercariae of S&is& soma mansoni. J Immunol 1992, 148:1512-1518. 65. ..

SMYTHIES LE, COULSONPS, Wrlso~ RA: Monoclonal Antibody

66.

COLJWN PS, WIISON RA Examination of the Mechanisms of Pulmonary Phase Resistance to Scbistosoma mansoni in Vaccinated Mice. Am J Trc@ Med Hyg 1988, 38:529-539.

67.

REYNOLDSSR, SHOEMAKER CB, HARN DA: T and B Epitope Mapping of SM23, an Integral Membrane Protein of Schistosoma mansoni. J Immunol 1992, 149:399F4001.

68. ..

AMORY S~ISSONLM, MA~TER~~NCP, TOM TD, MCNALLYMT,

69.

GREZELD, CAPRONM, GRZYCH J-M, FONTAINEJ, LECOCQ J-P, CAPRONA: Protective Immunity Induced in Rat Schistoso-

to IFN-y Modifies Pulmonary Inlkunmatory Responses and Abrogates Immunity to Schistosoma mansoni in Mice Vaccinated with Attenuated Cercariae. J Immunol 1992, 149~3654-3658. A clear demonstration of the central role ot IFN-y in orchestrating the pulmonary effector responses to challenge in mice vaccinated with irradiated cercariae.

Viva

Molecular Analysis of Lymphokines Involved in the Murine Immune Response during Scbistosoma mansoni Infection. J Immunol 1992, 148:2261-2269. 54.

63.

YAMASHITA T, BOROS DL: IL-4 Influences IL-2 Production

and Granulomatous Inflammation in Murine schistosomiasis mansoni. J Immunol 1992, 14936593664.

PEMBERTON RM, SMITHIESLE, MOUNTFORDAP, WIIS~N RA:

LOWELLGH, STRANDM: Induction of Protective Immunity in Mice using a 62-kDa Recombinant Fragment of a Schistosoma mansoni Surface Antigen. J Immunol 1992, 149:3612-3620. The characterizationof a new vaccine candidate antigen and its formulation in proteosomes to induce up to 75% protection against challenge in mice.

Immunitv and immunoreeulation

70. .

in helminth infections Wilson

miasis by a Single Dose of tbe Sm28GST Recombinant Antigen: Effector Mechanisms Involving IgE and IgA Antibodies. Eur J Immunol 1993, 23:454-460.

Explores the roles of IgE, and particularly of &A, in the protective responses of man and laboratory hosts to Sm28GST, a candidate vaccine antigen. See [49**].

Xu CB, VERWAERDEC, GRAS-MASSEH, FONTAINEJ, Bossus M, TRO~IN F, WOLQWCZUKI, TARTARA, CAPRONA: S&is tosoma munsoni 2%kDa Glutatbione S-transferase and Immunity against Parasite Fecundity and Egg Viability. J Immunol 1993, 150:94&949.

RA Wilson, Department UK.

of Biology, University of York, York YOl SDD,

547