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

Abstract The crystallographic structure of the MHC class II molecule showed that the αβ heterodimer can itself dimerize to form a four chain ( αβ ) 2 complex of 120 kDa. Here we provide evidence for the existence of a 120 kDa ( αβ ) 2 complex of the class II I-E k molecules in mouse B cells. Both a 60 kDa and a 120 kDa form of I-E k are detected by Western blotting and by immunoprecipitation under conditions in which class II αβ heterodimers are stable. The 120 kDa I-E k complex does not contain li and, upon warming, dissociates into free α and β chains. The 120 kDa I-E k 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.