Soluble HLA proteins with bound peptides are released from the cell surface by the membrane metalloproteinase

The metalloproteinase (MPase)-mediated pathway of MHC class I processing is a distinct cellular mechanism that generates soluble HLA proteins. It has been implicated in modulation of immune responses induced during transplant ation events. It: is, therefore, important to define the characteristics of soluble HLA species produced by the MPase pathway. We have previously show n that some mutant peptide-conformed beta (2)-microglobulin (Pm) free heavy chains (HC) with lower affinity for beta (2)m can be released into superna tants by the MPase. These soluble conformed beta (2)m-free HC intermediates can re-associate with beta (2)m in solution giving rise to beta (2)m-assoc iated HC. We now demonstrate that also nonmutant soluble conformed beta (2) m-free HC can be detected in supernatants of activated cells. These soluble HC intermediated appear to have bound peptides and readily reassociate wit h exogenous beta (2)m producing beta (2)m-associated HC char are stable at physiologic temperature. Thus, generation of peptide-conformed beta (2)m-fr ee HC intermediates is an important step which precedes generation of both soluble beta (2)m-free and beta (2)m-associated HC by thc MPase pathway ope rating in activated cells. (C) American Society for Histocompatibility and Immunogenetics, 2000. Published by Elsevier Science Inc.