Modified Human Beta 2‐Microglobulin (desLys58) Displays Decreased Affinity for the Heavy Chain of MHC Class I and Induces Nitric Oxide Production and Apoptosis

Beta2-microglobulin (β2m) 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. β2m 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 desLys58β2m (dβ2m). This work aims to study the effect of dβ2m on peptide binding to MHC-I, the influence of dβ2m on the binding of β2m to the MHC-I heavy chain and the biological activity of dβ2m. Both β2m and dβ2m are able to support the generation of MHC-I/peptide complexes at 18 °C, but complexes formed in the presence of dβ2m destabilize at 37 °C. Moreover, a 250 times higher concentration of dβ2m than of β2m is needed to displace MHC-I associated β2m from the cell surface. In addition, only β2m is able to restore MHC-I/peptide complex formation on acid-treated cells whereas dβ2m appears to bind preferentially to denatured MHC-I heavy chains. In cell cultures, exogenously added dβ2m, but not β2m, induces apoptotic cell death in monocytic leukaemic cell lines but spares other kinds of leukaemic cells. Additionally, the presence of dβ2m, and to a lesser extent β2m, enhances IFN-γ-induced NO production by monocytic leukaemic cells. In conclusion, these data show that dβ2m is not able to support the formation of a stable tri-molecular MHC-I complex at physiological temperature and that dβ2m exerts other biological functions compared to β2m when bound to cells.