Transgenic nice were produced in which human HLA-Cw4 is stably integrated, behaves as a single Mendelian trait, and, being under the transcriptional control of human CD2, is selectively and efficiently expressed in T lymphocytes. These mice were used as a model system to dtermine whether HLA-type C molecules can be exposed on the surface of activated lymphocytes as free heavy chains, non-associated with β2-microglobulin (β2m). In our transgenic mice we could identify HLA-Cw4 molecules either as free chains or as β2m-associated molecules by the use of monoclonal antibodies specific for either conformation of HLA class I and nonreactive to mouse H2 molecules. Resting mouse lymphocytes were shown by western transfer analysis to contain sizeable amounts of HLA-Cw4 free chains, but they exposed on thier surface HLA-Cw4 only in association with β2m,a s indicated by flow cytometric measurements. Conversely, where the content of total HLA-Cw4 was increased, lectin-activated mouse lymphocytes exposed on thier outer cell membrane HLA-Cw4 molecules in both conformation, namely, also as free heavy chains, Isoelectrofocusing analysis confirmed the presence of both HLA-Cw4 molecular conformations in activated T cells and indicated that HLA-Cw4 heavy chains can bind to mouse β2m with the same low affinity displayed for human β2m. The results of our experiments led us to conclude that (1) association with β2m is not necessary for the exposure of HLA-C on the surface of activated T lymphocytes and (2)o cell activation affects the balance between the two conformation forms of HLA-C.
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