Modified Human Beta 2-Microglobulin (desLys(58)) Displays Decreased Affinity for the Heavy Chain of MHC Class I and Induces Nitric Oxide Production and Apoptosis

Abstract

Beta2-microglobulin (beta 2m) is the light chain of major histocompatibility complex class I (MHC-I) molecules, and is a prerequisite for the binding of peptides to the heavy chain and their presentation to CD8(+) T cells. beta 2m can be modified in vivo and in vitro by proteolytic cleavage by complement C1 and subsequent carboxypeptidase B-like activity - processes that lead to the generation of desLys(58)beta 2m (d beta 2m). This work aims to study the effect of d beta 2m on peptide binding to MHC-I, the influence of d beta 2m on the binding of beta 2m to the MHC-I heavy chain and the biological activity of d beta 2m. Both beta 2m and d beta 2m are able to support the generation of MHC-I/peptide complexes at 18 degrees C, but complexes formed in the presence of d beta 2m destabilize at 37 degrees C. Moreover, a 250 times higher concentration of d beta 2m than of beta 2m is needed to displace MHC-I associated beta 2m from the cell surface. In addition, only beta 2m is able to restore MHC-I/peptide complex formation on acid-treated cells whereas d beta 2m appears to bind preferentially to denatured MHC-I heavy chains. In cell cultures, exogenously added d beta 2m, but not beta 2m, induces apoptotic cell death in monocytic leukaemic cell lines but spares other kinds of leukaemic cells. Additionally, the presence of d beta 2m, and to a lesser extent beta 2m, enhances IFN-gamma-induced NO production by monocytic leukaemic cells. In conclusion, these data show that d beta 2m is not able to support the formation of a stable tri-molecular MHC-I complex at physiological temperature and that d beta 2m exerts other biological functions compared to beta 2m when bound to cells.