Interplay between vertebrate adaptive immunity and bacterial infectivity genes: Bank vole MHC versus Borrelia afzelii OspC

Coevolution of parasites with their hosts may lead to balancing selection on genesinvolved in determining the specificity of host–parasite interactions, but examplesof such specific interactions in wild vertebrates are scarce. Here, we investigatedwhether the polymorphic outer surface protein C (OspC), used by the Lyme diseaseagent, Borrelia afzelii, to manipulate vertebrate host innate immunity, interacts withpolymorphic major histocompatibility genes (MHC), while concurrently eliciting astrong antibody response, in one of its main hosts in Europe, the bank vole. We foundsignals of balancing selection acting on OspC, resulting in little differentiation in OspCvariant frequencies between years. Neither MHC alleles nor their inferred functionalgroupings (supertypes) significantly predicted the specificity of infection with strainscarrying different OspC variants. However, we found that MHC alleles, but not super-types, significantly predicted the level of IgG antibodies against two common OspCvariants among seropositive individuals. Our results thus indicate that MHC allelesdiffer in their ability to induce antibody responses against specific OspC variants,which may contribute to selection of OspC polymorphism by the vole immune system.