Helminth Immunity and Vaccines in Sheep

Special Article on Parasitology

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Helminth Immunity and Vaccines in Sheep John T. Neilson, Ph.D.*

Ruminants infected with helminth parasites can, in general, develop immune responses that influence the host-parasite relationship. The degree of this influence varies among host-parasite systems. For example, sheep seem incapable of diminishing Fasciola hepatica infections by immunologically mediated mechanisms but can exercise effective control over Nematodirus spathiger populations by an adaptive immune response. Thus, as one would expect with varied biological interactions, not all host-parasite systems are created equal. Functional immune responses can, in many of these relationships, tip the scales in favor of the host, thereby lessening biological damage to the host. In others, this is not achieved. In some infections (for example, Oesophagostomum columbianum in sheep), the immunologically mediated mechanisms stimulated by the parasite can cause host tissue damage, compromising the health of the host. Where immunologic intervention by vaccination is contemplated in an effort to benefit the host at the expense of the parasite, the preceding considerations must be taken into account. Any consideration of potential vaccination against parasitic helminths should first include an understanding of the host's immunologic system and how the parasite interfaces with it. Parasites Immunoregulation of parasites in ruminants through vaccination will be limited to those parasite species that have an economic impact on the production of these animals. This article will be limited to a consideration of such parasites. When considering host immune responses to these pathogens, it is useful to separate them into two groups: those species restricted to the gastrointestinal tract, and those with migratory phases in other body tissues. A parasite's migratory behavior and predilection site inNote: Owing to the constraints of size, this article could not be included in the July 1986 issue of Veterinary Clinics of North America: Food Animal Practice on Parasites: Epidemiology and Control (Vol. 2, No.2).

* Professor, Department of Infectious Diseases, University of Florida College of Veterinary Medicine, Gainesville, Florida.

Veterinary Clinics of North America: Food Animal Practice-Vol. 2, No.3, November 1986

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fluence the nature of the host immune response and, consequently, immunoprophylactic strategies. Economically important nematodes infecting the abomasum of sheep and goats are Haemonchus contortus and Ostertagia circumcincta. The former is a serious problem in locations of high rainfall and warm climate, and the latter is more prevalent in colder climates. The small intestine is the predilection site of N. filicollis and N. spathiger, and infections of these parasites are debilitating in young lambs. Trichostrongylus colubriformis also parasitizes the small intestine of sheep and goats. The major, economically important nematode infecting the large bowel or colon is O. columbianum. The tapeworm Moniezia expansa is geographically widespread, but economic loss is only encountered when the small intestine of lambs harbors heavy infections. Normally, adult animals easily tolerate low infections of this cestode. Parasites that migrate and are found in tissues other than the gastrointestinal tract and cause economic losses in goats and sheep are Fasciola hepatica, the cysticercus stage of Taenia ovis, Echinococcus granulosus, and Dictyocaulus filaria. Gastrointestinal Immune Responses

Given the importance of gastrointestinal parasite infection as a constraint on small ruminant production, control through vaccination is an attractive goal. A logical approach to the development of vaccine strategies requires an understanding of host immune mechanisms along this organ system. The induction of a specific immune response, in vivo, involves union between the foreign antigen and macrophages. Presentation of antigen by the macrophage to small Band T lymphocytes with receptors that specifically recognize an antigenic determinant on the foreign molecule causes the lymphocyte to undergo blast transformation and cell division. Clonal expansion of B lymphocytes yields a population of plasma cells secreting antibody specific for the stimulating antigen, while expansion ofT-lymphocyte popluations mediates cellular immune mechanisms. Some or all of the stages of the parasites discussed in this article traumatize, to varying degrees, the intestinal tissues at the predilection site of the worm. Therefore, there is intimate association of parasite antigens and elements of the immune system, the lymphocytes, macrophages, and granulocytes and the secreted or liberated products of these cells. Potential parasite antigen sources include secretions associated with feeding, migration, and ecdysis, and parasite tissue macromolecules should death of the parasite occur in the interstitial compartment of the host. The parasite-induced trauma of the gut tissue will facilitate parasite antigen presentation to the immune elements of the gut interstitial fluid and, in cases in which blood sucking occurs, directly to the vascular system. The tissue fluid of the gut, being formed by excess capillary filtration over vascular reabsorption, contains serum immu-

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noglobulins. In addition, immunoglobulins produced by plasma cells in the lamina propria are discharged at the local site. The origin of macrophages in the gut region is not fully known; however, the small lymphocytes come from a recirculating pool. The small lymphocytes that extravasate in the gut wall and mesenteric lymph nodes of sheep are not selected at random but are a special subpopulation of T and B lymphocytes that home specifically to the gut tissues. This phenomenon gives rise to a gut-associated pool of circulating lymphocytes distinct from the somatic-splenic pool, a fact that has implications for gut versus systemic immunity. A proportion of the immunoblasts, lymphocytes that have responded to parasite immunogens presented in the gut, will be plasmablasts, and unlike other species, in which such plasma cells will primarily secrete IgA, at least half the plasma cells in the sheep will produce IgG. In sheep and other ruminants, IgG 1 is perhaps quantitatively more important as a secretory antibody than IgA. Regardless of the Ig isotype secreted, the B lymphocytes, having arrived in their final destination in the wall of the gut will differentiate to plasma cells, and during a life span of a few days, provide much of the immunoglobulin component of mucous secretions. In addition to the specific immune responses occurring at the gut level through antigenic stimulation of T and B lymphocytes, the resulting inflammatory response results in effector mechanisms that, although deleterious to survival of the parasite, are nonspecific in nature. These include vasoactive amines, leukotrienes, and mucous secretion from increased numbers of goblet cells. The involvement of biologically active amines and leukotrienes in immunologically mediated, anti-parasite mechanisms leads to a consideration of the granulocytes of gut tissue. Mucosal mast cells are smaller, possess granules with lower sulfation or mucopolysaccharides, little heparin, and smaller amounts of histamine and serotonin than mast cells of other sites. Mucosal mast cells are functionally different from their peritoneal or connective tissue counterparts, for they respond quite differently to a variety of secretagogues. Factors released from T lymphocytes may be necessary for mucosal mast cell differentiation and proliferation. Hence, a specific antigenic stimulation is a prerequisite for development of this nonspecific response. Eosinophils have long been implicated in responses to parasitic worms, and granuolated intraepithelial lymphocytes may well function as natural killer lymphocytes. Thus successful immunoprophylaxis for gut-dwelling parasites begs an understanding of the common mucosal immunologic system encompassing the mucosal localization of B-cells of several isotypes, T -cells, the secreted products of mucosal granulocytes, and mucousproducing goblet cells. Immune Responses at Sites Other Than the Gut Parasites whose life cycles involve penetration of the skin or the epithelium of the gut followed by migration through the host tissues

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to various predilection sites (lung or liver, for example) are obviously in intimate contact with the host's systemic immune system. Consequently, the potential exists for immune mechanisms to block the successful development and establishment of the metazoan parasite. Such mechanisms include those mediated by all isotypes of antibodies. Antibodies can neutralize or inactivate to varying degrees enzymes associated with numerous biological functions essential to the parasite. Surface macromolecules of the parasite essential for site recognition and attachment can be compromised by specific antibody. Antibodydependent mechanisms can involve the activation of complement, resulting in parasite membrane damage. Once specific antibodies interact with their parasite epitopes, granulocytes can be focused in close proximity where the products of degranulation harm the parasite. Similarly, phagocytosis can be enhanced through this antibody interaction. Cellular immune mechanisms have also been implicated in protective host responses. Cytotoxic T -cells and natural killer cells, both lymphoid cells, may have the ability to damage parasites through direct contact, but this is equivocal. Certainly inflammatory responses as a result of hypersensitivity reactions mediated by sensitized T-Iymphocytes can have deleterious effects upon parasites. Hypersensitivity reactions and inflammatory secretions in body tissues where parasites are found can lead to fibrotic events that encapsulate parasites or, in concert with other immunologic effector mechanisms, damage and destroy the parasite. Vaccination

Successful vaccination of cattle against lung worm infection using irradiation-attenuated Dictyocaulus viviparus infective larvae has been achieved and is an economic management tool in the cattle industry of Europe. Other irradiated parasite vaccines have been developed, but none has matched the commercially exploitable value of the bovine lung worm vaccine. Vaccines composed of killed parasite antigen or their metabolic products have given partial protection against a number of parasites of domestic livestock. Under experimental conditions, significant levels of protection have been induced by immunization of the appropriate domestic animal hosts (cattle, sheep, and pigs) against the following nematode parasites: D. v'tv'tparus, Oesophagostomum radiatum, Trichostrongylus colubriformis, Haemonchus contortus, Ostertagia circumcincta, Strongyloides papillosus, and Trichinella spiralis. Killed worm antigen and metabolic products of cestode parasites have been successful in protecting sheep against Cysticercus tenuicollis, C. ovis, and Moniezia expansa, and cattle against C. bovis. Attempts to immunize sheep and cattle against the trematode Fasciola hepatica with vaccines composed of killed worm antigen have met with limited success. Invariably, the experimental anti-parasite vaccines used in most

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past studies have been complex mixtures of parasite somatic or metabolic components. An oft-repeated reason for the less-than-adequate protection induced by such vaccines is their molecular complexity and inclusion of nonfunctional antigens. Vaccines composed primarily of functional antigens (that is, parasite molecules bearing epitopes that trigger immune responses deleterious to the survival of the parasite in the host) have long been advocated as those most likely to succeed. Adjuvantation and the nonspecific stimulation of host efIector mechanisms are important considerations in vaccination protocols. Likewise, the dose of antigen and adjuvant, number of doses given, the route of administration, and the use of antigen vehicles such as liposomes will all impact upon the success of a vaccination regimen. A further factor mitigating against successful immunization to parasites, stems from an evolutionary adaption of parasite to host, thus minimizing host damage and enabling reproduction of the parasite. Host immune responses to well-adapted parasites can be exceedingly weak, and in the absence of strong immunoresponsiveness to infections, artificial stimulation of an effective immune response may be biologically impossible. A possible strategy to combat this is to stimulate host responses to parasite antigens not normally encountered by the host. When the neutralization of a macromolecule bearing antigenic epitopes is detrimental to a biological function essential to the parasite, damage to the latter could occur. An example of this strategy is the sensitization of hosts to tick gut antigens, which results in harmful"effects to the ticks when they consume a blood meal and, hence, anti-tick gut antibody, from an immunized host. A consequence of the close evolutionary experience of host and parasite has led to mechanisms whereby parasites evade the host's potentially lethal immune responses. Some parasites can incorporate host antigens, others can shed soluble antigens and even specifically immunosuppress the host. The various ways in which parasites evade or avoid the consequences of host immunization need to be understood if successful immunoprophylaxis of parasitic disease is to be accomplished. Successful vaccination of domestic animals may be enhanced by the selective breeding of those individual animals within a species that exhibit a greater resistance to the parasite. The selection of responders and rejection of the more susceptible nonresponders from a sheep flock has increased overall resistance to T. colubriformis. Any genetic linkage between this responsiveness and the Ir gene locus of sheep has yet to be demonstrated.

Summary Vaccination of domestic animal species against various parasitic helminths using attenuated parasites or nonliving parasitic material is possible. Improved prospects for vaccines composed of somatic and metabolic parasite components hinge on the isolation and characterization of helminth protective antigens and their synthesis by modern bioengineering techniques. Vaccination strategies beg an understand-

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ing of the host's immune effector mechanisms for their most efficient prolonged stimulation. Parameters of importance are antigen dose, frequency of and interval between doses, use of liposomes or other antigen delivery vehicles, and the use and choice of adjuvants.

SUGGESTED READINGS 1. Capron, A.: Immunoparasitology. Clin. Immunol. ABerg., 2:487, 1982. 2. Lied, W. R, and Williams J. F.: Helminth parasites and the host inflammatory system. In Florkin, M., and Scheer, B. (eds.): Chemical Zoology. Volume 11. New York, Academic Press, 1979, p. 229. 3. Miller, H. R P.: The protective mucosal response against gastrointestinal nematodes in ruminants and laboratory animals. Vet. Immunol. Immunopathol., 6:167,1984. 4. Mitchell, G. F., and Anders, R F.: Parasite antigens and their immunogenicity in infected hosts. In Sela, M. (ed.): The Antigens. Volume 6. New York, Academic Press, 1982, p. 69. 5. Outteridge, P. M.: Veterinary Immunology. New York, Academic Press, 1985. 6. Urquhart, G. M.: Application of immunity in the control of parasitic disease. Vet. Parasitol., 6:217, 1980. 7. Wakelin, D.: Immunity to Parasites. London, England, Edward Arnold, 1984. Department of Infectious Diseases College of Veterinary Medicine University of Florida Gainesville, Florida 32601