Evidence for dimers of MHC class II molecules in B lymphocytes and their role in low affinity T cell responses

The crystallographic structure of the MHC class II molecule showed that the alpha beta heterodimer can itself dimerize to form a four chain (alpha beta)2 complex of 120 kDa. Here we provide evidence for the existence of a 120 kDa (alpha beta)2 complex of the class II I-Ek molecules in mouse B cells. Both a 60 kDa and a 120 kDa form of I-Ek are detected by Western blotting and by immunoprecipitation under conditions in which class II alpha beta heterodimers are stable. The 120 kDa I-Ek complex does not contain Ii and, upon warming, dissociates into free alpha and beta chains. The 120 kDa I-Ek complex is expressed at the cell surface, is active in antigen presentation, and appears to play a significant role in T cell responses to low affinity but not to high affinity antigens, possibly by facilitating cross-linking of the T cell receptors.