MHC class II genotype-by-pathogen genotype interaction for infection prevalence in a natural rodent-Borrelia system

MHC genes are extraordinarily polymorphic in most taxa. Host-pathogen
coevolution driven by negative frequency-dependent selection (NFDS) is one
of the main hypotheses for the maintenance of such immunogenetic
variation. Here we test a critical but rarely tested assumption of this
hypothesis—that MHC alleles affect resistance/susceptibility to a pathogen
in a strain-specific way, i.e. that there is a host genotype-by-pathogen
genotype interaction. In a field study of bank voles naturally infected
with the tick-transmitted bacterium Borrelia afzelii, we tested for MHC
class II (DQB) genotype-by-B. afzelii strain interactions for infection
prevalence between ten DQB alleles and seven strains. One allele (DQB*37)
showed an interaction, such that voles carrying DQB*37 had higher
prevalence of two strains and lower prevalence of one strain than
individuals without the allele. These findings were corroborated by
analyses of strain composition of infections, which revealed an effect of
DQB*37 in the form of lower b diversity among infections in voles carrying
the allele. Taken together, these results provide rare support at the
molecular genetic level for a key assumption of models of antagonistic
coevolution through NFDS.