Probing the stability of class I major histocompatibility complex (MHC) molecules on the surface of human cells

We used binding of a fluorescent adduct of beta(2)-microglobulin, fluorescein beta(2)m, to probe the stability of class I HLA molecules on the surface of human cells. The weight of the literature suggests that this ligand binds to heavy chains that have lost beta(2)m and possibly peptide as well. Hence F1-beta(2)m reports on the stability of the class I HLA trimer. A small fraction of HLA molecules, similar to 5%, binds F1-beta(2)m on both resting and activated T cells. A larger fraction of all HLA molecules binds F1-beta(2)m in FO-1 cells, beta(2)m-deficient cells, transfected with various B2m genes. HLA molecules of FO-1 cells are more stable when expressed with human beta(2)m, than when expressed with mouse beta(2)m. The non-covalent association of HLA heavy chains, beta(2) and peptide implies that eventually every molecule of HLA trimer ought to dissociate and bind F1-beta(2)m. In fact, the extent of exchange is limited by the lifetime of a given molecule at the cell surface. beta(2)m exchange decreases as cell concentration increases, suggesting that some density-dependent process acts to enhance degradation or denaturation of beta(2)m-free HLA heavy chains.