MHC I of the Great Reed Warbler Promotes a Flat Peptide Binding Mode

The major histocompatibility complex (MHC) plays a key role in pathogen recognition as part of the adaptive immune system. MHC I gene copy numbers in birds of the order Passeriformes (songbirds) are substantially larger compared to other birds. MHC I diversity and antigen presentation have been carefully characterised in chicken Gallus gallus of the order Galliformes; chickens express few MHC I genes and often present antigens that bulge out of the peptide binding cleft. This observation raises the question of whether MHC I presents antigens in a similar way in species with many MHC genes? Here, we present the X-ray structure of MHC I from the great reed warbler Acrocephalus arundinaceus (Acar3) a long-distance migratory songbird. Structural analysis shows that MHC I binds the antigen in a flat conformation due to a sequentially well-conserved restriction point, acting like a pair of tweezers, within the peptide binding grove, created by Arg97 and Arg155. This more stringent antigen presentation by Acar MHC I molecules may partly explain the high MHC gene copy numbers seen in the great reed warbler.