Peptide-conformed beta(2)m-free class I heavy chains are intermediates in generation of soluble HLA by the membrane-bound metalloproteinase

Molecular mechanisms of soluble HLA-release by a membrane-bound metalloprot einase (MPase) are nor defined, We have investigated the possibility that c ertain beta(2),-microglobulin (beta(2)m)-free heavy chains (HC) retain pept ide-induced conformations before and after the cleavage by using mutant HLA -A2.242K HC with reduced affinity for beta(2)m We show that dissociation of HC/beta(2)m complexes on the surface of C1R lymphoblastoid cells generates both conformed and non-conformed beta(2)m-free HC recognized by conformati on-dependent antibodies. Conformed HC, having bound the HLA-AZ-specific pep tide HTLV-1 tax 11-19, can retain their proper conformations after dissocia tion of beta(2)m Further, conformed and non-conformed surface beta(2)m-free HC are cleaved by the MPase, and some released HC preserve their conformat ions, Exogenous beta(2)m binds only to conformed HC, and protects them from cleavage as effectively as the MPase inhibitor BB-2116. We propose that so luble HLA-release requires generation of peptide-conformed beta(2)m-free HC intermediates on the cell surface, which are then cleaved by the MPase and in solution map reassociate with beta(2)m. Given the role of soluble HLA i n the indirect allorecognition, the activity of this MPase may be important in transplant rejection. (C) American Society for Histocompatibility and I mmunogenetics, 1999. Published by Elsevier Science Inc.