The overlooked "nonclassical" functions of major histocompatibility complex (MHC) class II antigens in immune and nonimmune cells

Besides their "classical" antigenic peptide-presenting activity, major hist ocompatibility complex (MHC) class II antigens can activate different cellu lar functions in immune and nonimmune cells. However, this "nonclassical" r ole and its functional consequences are still substantially overlooked. In this review, we will focus on these alternative functional properties of MH C class II antigens, to reawaken attention to their present and foreseeable immunobiologic and pathogenetic implications. The main issues that will be addressed concern 1) the role of MHC class II molecules as basic component s of exchangeable oligomeric protein complexes with intracellular signaling ability; 2) the nonclassical functions of MHC class II antigens in immune cells; 3) the pathogenetic role of MHC class II antigens in inflammatory/au toimmune and infectious disease; and 4) the functional role of MHC class II antigens in solid malignancies. (C) 1999 Wiley-Liss, Inc.