Allotype-Dependent Responses to the Vaccine Candidate Thrombospondin-Like Protein of Dictyocaulus viviparus in Calves

Abstract

The nematode Dictyocaulus viviparus causes parasitic bronchitis in cattle. There is a vaccine based on irradiated larvae against this parasite. However, no memory response is induced, and donor calves are needed for culturing the larvae. Therefore, a well-defined subunit vaccine would be welcomed. Because thrombospondin-like protein (TLP) is an immunodominant protein from the brush border of adult parasites, it was tested as a vaccine candidate. Calves (n = 7) were vaccinated twice with TLP in Quil-A, and antibody responses, IgG2 allotypes and protection were determined. Protection is defined here as decreased worm counts from challenge infection compared with age-matched control calves (N = 7). Only 27% protection (not significant) was found in the vaccinated calves. However, strong IgG and IgE booster responses occurred after challenge infection, mostly directed against the glycan-phosphorylcholine moiety of the protein. Most interesting was the difference in protection in calves of the different IgG2 allotypes. The two best protected calves from the vaccinated group were the only two calves of the homozygote IgG2b genotype. Because the IgG2b has a more rigid hinge region than the IgG2a allotype, it is more resistant to parasite proteases or parasite Ig binding proteins, and it is a better complement activator. Therefore, even with the small number of calves from this study, results suggest calves of the homozygous IgG2b genotype might be better protected against D. viviparus than calves of other genotypes.