Structure and immune recognition of HLA-B27 antigens: implications for disease association

HLA-B27 is strongly associated with susceptibility to ankylosing spondylitis and other spondyloarthropathies. Structural analysis of this antigen has revealed the existence of multiple variants, or subtypes, in human populations. The structural microheterogeneity of these subtypes deeply affects allospecific T cell recognition and most of it occurs at an spatial cluster within the peptide binding groove of the molecule. Many polymorphic residues whose combination is unique to HLA-B27 but is conserved among subtypes are clustered in a spatially separated site of the groove from that where most subtype polymorphism occurs. Site-directed mutagenesis and DNA-mediated gene transfer has been used to show that the positions that are polymorphic among subtypes are highly relevant for modulating T cell recognition, so that immunologically silent changes do not occur. These studies have also revealed an extremely high clonotypic diversity in the alloreactive response against HLA-B27. The structural basis for this diversity has been examined by sequencing the clonotypic T cell receptors. The analysis shows a certain bias in V beta gene segment usage, as well as other recurrent structural motives, among T cell receptor beta chains from HLA-B27-specific cytotoxic T cell clones.