Signaling Cross-Talk between MHC Class II Molecular Conformers in Resting Murine B Cells

In addition to functioning as a ligand to engage TCRs and drive TCR signaling, MHC class II molecules are signaling molecules that generate a number of signals within APCs, such as B lymphocytes. Moreover, MHC class II signaling is critical for B cell activation and development of a robust humoral immune response. Murine class II molecules exist in two distinct conformational states, based primarily on the differential pairing of transmembrane domain GxxxG dimerization motifs (i.e., M1- and M2-paired class II). This laboratory has previously reported that the binding of a multimerized form of an anti-class II mAb that selectively recognizes M1-paired I-A^k class II drives intracellular calcium signaling in resting murine B cells and that this signaling is dependent on both src and Syk protein tyrosine kinase activity. In contrast, multimerized forms of two different anti-I-A^k mAbs that bind both M1- and M2-paired class II fail to elicit a response. In this report, a flow cytometry-based calcium flux assay is used to demonstrate that coligation of M1- and M2-paired I-A^k class II results in the active and selective inhibition of M1-paired I-A^k class II B cell calcium signaling by M2-paired class II molecules. Because M1- and M2-paired class II can be loaded with different sets of peptides derived from Ags acquired by distinct pathways of endocytosis, these findings suggest an MHC class II signaling-based mechanism by which CD4 T cells of differing specificities can either enhance or suppress B cell activation.