The role of MHC class I heterodimer expression in mouse ankylosing enthesopathy.

Ankylosing enthesopathy (ANKENT) is a spontaneous mouse joint disease with strikingly similar pathology to human HLA-B27-associated enthesopathies such as ankylosing spondylitis. In C57Bl/10 mice, transgenic HLA-B2702 as well as H2 genes have been shown to be relative risk factors for ANKENT. To investigate the role of major histocompatibility complex (MHC) class I expression in disease pathogenesis, ANKENT occurrence was compared among beta2-microglobulin (beta2m) knockout littermates with or without transgenes for HLA-B2702 and human beta2m. In the knockout phenotype lacking beta2m, ANKENT occurrence is significantly reduced (P = 0.016). In the absence of beta2m, B2702 is not detected on the cell membrane, nor does it increase the risk for ANKENT. This means that the previous finding that HLA-B2702 increases susceptibility to ANKENT in C57Bl/10 mice cannot be ascribed to a transgene insertion effect. Rather, in order to increase disease susceptibility, B2702 must be coexpressed with mouse beta2m (mo-beta2m). In contrast, when HLA-B2702 is expressed with beta2m of human origin, disease susceptibility is not affected. Thus, both H2(b)-derived class I heterodimers and HLA-B*2702/mo-beta2m heterodimers contribute to ANKENT susceptibility.