MEMBRANE EXPRESSION OF HLA-CW4 FREE CHAINS IN ACTIVATED T-CELLS OF TRANSGENIC MICE

Transgenic mice were produced in which human HLA-Cw4 is stably integra ted, behaves as a single Mendelian trait, and, being under the transcr iptional control of human CD2, is selectively and efficiently expresse d in T lymphocytes. These mice were used as a model system to determin e whether HLA-type C molecules can be exposed on the surface of activa ted lymphocytes as free heavy chains, non-associated with beta 2-micro globulin (beta(2)m) In our transgenic mice we could identify HLA-Cw4 m olecules either as free chains or as beta(2)m-associated molecules by the use of monoclonal antibodies specific for either conformation of H LA class I and nonreactive to mouse H2 molecules. Resting mouse lympho cytes were shown by western transfer analysis to contain sizeable amou nts of HLA-Cw4 free chains, but they exposed on their surface HLA-Cw4 only in association with beta(2)m, as indicated by flow cytometric mea surements, Conversely, where the content of total HLA-Cw4 was increase d, lectin-activated mouse lymphocytes exposed on their outer cell memb rane HLA-Cw4 molecules in both conformations, namely, also as free hea vy chains. Isoelectrofocusing analysis confirmed the presence of both HLA-Cw4 molecular conformations in activated T cells and indicated tha t HLA-Cw4 heavy chains can bind to mouse beta(2)m with the same low af finity displayed for human beta(2)m. The results of our experiments le d us to conclude that (1) association with beta(2)m is not necessary f or the exposure of HLA-C on the surface of activated T lymphocytes and (2) cell activation affects the balance between the two conformationa l forms of HLA-C.