Characterization of a T-lymphocyte membrane protein involved in T-cell function: its contribution to T-cell recognition or cellular interaction.
A monoclonal antibody (Ab188) specific for guinea-pig T lymphocytes recognizes a membrane heterodimer protein (alpha-chain, MW 43,000; beta-chain, 39,000) and inhibits efficiently the antigen-, mitogen- or alloantigen-induced T-cell proliferation. The role of this protein in T-cell activation was analysed in more detail with emphasis on the recognition or activation event of T cells. Quin 2, an intracellularly trapped calcium indicator, was used to measure calcium influx into T cells. The addition of the mitogen concanavalin A to T cells loaded previously with quin 2 induced an increase in fluorescence, revealing an increase in intracellular free calcium. This calcium increase is considered as one of the primary events in the initiation of T-cell activation and was blocked by Ab188. In contrast, other T-cell specific antibodies that react to a comparable extent with T cells had no effect on calcium increase. This indicates that Ab188 is directed to a protein involved in a very early step of T-cell activation. Alloreactive T-cell lines were established from a secondary mixed leucocyte culture by soft agar cloning. Single T-cell colonies were picked and were propagated by repeated restimulation with allogeneic macrophages differing in MHC-class II antigens. In a chromium release assay, the cytotoxic activity of several strain 13 T-cell lines directed against strain 2 Ia antigens was inhibited by Ab188 to about 50%. Similarly, Ab188 inhibited the cytotoxic activity of a MHC-class I-restricted TNP-specific T-cell line of strain 2 guinea-pigs to about 50%. In contrast, lectin-mediated cytotoxicity of the T-cell lines against murine P815 mastocytoma target cells remained unaffected. These results indicate that Ab188 interferes with the function of a protein contributing to the recognition event in the process of T-cell activation.