A major histocompatibility complex (MHC) class II molecule that binds the same viral pathogen peptide with both nonamer and decamer core sequences for presentation to T cells

Classical molecules encoded by the major histocompatibility complex (MHC) are central to immune responses. Compared to typical mammals, the chicken MHC is small and simple, determining life or death from economically-important pathogens like avian influenza virus and Marek’s disease virus (MDV). Several genes within the tightly-linked chicken MHC have been suggested to determine resistance and susceptibility to MDV, but it was a surprise to find that the dominantly-expressed class II molecule from the resistant B2 haplotype employed a novel peptide-binding mode with a decamer core sequence compared to the susceptible B19 haplotype with a typical nonamer core. We examined the crystal structure of the dominantly-expressed class II molecule from another resistant haplotype, B21, which is extremely frequent in commercial chicken flocks, to find that it bound the same MDV peptide with both nonamer and decamer cores, revealing an unexpected plasticity of binding that potentially increases the immune response to this devastating pathogen.