No abstract

Recombinant, two-domain MHC Class II proteins (α1β1MHCII) that lack sequence from the membrane proximal α2 and β2 domains of MHC Class II adhere preferentially to B cells in vivo and ex vivo; whereas, full length (four-domain) constructs do not bind. Because α1β1MHCII molecules that contain only native MHC Class II sequence have preferential affinity for B cells, we hypothesize that a natural, cell-free form of MHC Class II (similar to recombinant α1β1MHCII) exists and has affinity for a receptor expressed by B cells. Evidence that endogenous, natural MHC Class II molecules transfer to B cells and that α1β1MHCII has inter- and intra-species affinity for B cells suggest that this is a natural and evolutionarily conserved property. And evidence that α1β1MHCII can stimulate CD4+ T cells suggests that there is an immune function for partial, cell-free MHC Class II. Transfer of immunoreactive, cell-free α1β1MHCII/Ag complexes to B cells suggests a here-to-fore unrecognized pathway for T cell activation.