Aspects of the Immunobiology of Dictyocaulus viviparus Infection

The irradiated larval vaccine for the control of bovine parasitic bronchitis is the only helminth vaccine widely used commercially and the continued success of this vaccine has limited the amount of research on the immunobiology of the Dictyocaulus viviparus host/parasite relationship. By examining some of the properties of the surface or released products of D. viviparus, it was hoped that the studies reported here would provide some insight into the immune mechanisms directed against this nematode. The first experiments (Chapter 3) were designed to examine the surface of several developmental stages of D. viviparus using the IFAT. Most of the stages were found to express stage-specific surface antigens and the relevance of these in immune avoidance in the host is discussed. When adult parasites were maintained in mammalian tissue culture conditions, they were observed to shed surface-bound antibody, which was considered to be of relevance in survival of adult worms in the lungs. Another interesting finding was the apparent immunogenicity of the L3 sheath, a structure previously assumed to be lost in the rumen of the host. Further studies indicated that the L3 stage may, in fact, penetrate the host mucosa with its sheath intact and the possibilty that the sheath may act to divert immune effector mechanisms away from the immunogenic surface of migrating stages during a primary infection was proposed. In addition, a strong heterophile IgM reactivity was observed against the cuticle of the exsheathed L3 and it was suggested that this antibody may block, by steric hindrance, the binding of more effective isotypes to L3 surface epitopes whose recognition might be essential to the development of protective immunity. In Chapter 4, the results of studies on parasite surface recognition by antibodies in sera from infected and vaccinated calves are presented. It was found that L3 surface-specific antibody did not increase in calves vaccinated twice with D. viviparus larvae irradiated at 400Gy or 1000Gy. These results confirmed previous studies, using the complement fixation and indirect haemagglutination tests, and indicated that immunity induced by vaccination with 400Gy-irradiated larvae was not necessarily accompanied by a measurable rise in specific antibody. On challenge of all vaccinated calves, a typical anamnestic response was observed which indicated that the calves had been primed sufficiently against the surface of the L3 stages. Antibody responses directed against the surface of the adult stage in normal L3-infected calves followed classical antibody kinetics in that there was an increase in IgM by Day 52 of infection, followed by a higher and more sustained increase in IgG1 and IgG2 subclasses. This response to normal L3 infection perhaps reflected the longevity of the adult surface antigens presented to the host in the respiratory tract. Interestingly, the IgG isotypes appeared to increase at the presumed time of adult parasite expulsion and the relevance of these antibody increases are discussed. In Chapter 5, immunisation experiments against D. viviparus in guinea pigs are presented. The results showed that a high degree of protection (86% mean reduction in worm burden) was obtained with adult ES products in the context of Freund's Complete adjuvant. The efficacy of these in vitro released products was compared with somatic extracts of adult and L3 stages, both of which failed to stimulate significant levels of protection. In the final chapter of this thesis (Chapter 6), D. viviparus was examined for the presence of an enzyme of possible immunomodulatory activity, acetylcholinesterase. The objective of this approach was to search for molecules which may be essential to parasite survival within the host. Five isoforms of AChE were found to be present in both larval and adult stages and all isoforms were released into culture by L4/5 and adult stages, but not by L3. Evidence for in vivo release was provided by the fact that antibodies in sera from infected calves bound the AChE isoenzymes. Antibodies specific for these parasite enzymes were also found in the serum of adult ES-immunised guinea pigs and this response appeared to be under genetic control. A preliminary investigation of the protective capacity of D. viviparus AChE was performed in guinea pigs using an adult ES fraction enriched for this enzyme. This preparation, in the context of FCA, stimulated a significant degree of protective immunity when compared with the adjuvant control group, but not when compared with the challenge controls. Again, the native adult ES products failed to confer a significant degree of protective immunity when administered with either FCA or a plurionic block copolymer adjuvant and the possible reasons for this failure are discussed. The fact that the AChE enzymes were recognised by infected hosts and could confer some level of protection when administered in an enriched form argues for an important role of AChE in the immunobiology of this host/parasite relationship