Ia-like antigens on human T lymphocytes: Relationship to other surface markers, role in mixed lymphocyte reactions, and structural profile

Less than 1% of nylon wool purified human T lymphocytes react with monoclonal antibodies to Ia-like antigens in an indirect rosette microassay. After PHA stimulation, about 70% of T lymphocytes including both Fcγ and Fcμ receptor bearing subpopulations acquire Ia-like antigens having structural properties similar to those of B lymphoid cell-derived Ia-like antigens. The expression of Ia-like antigens on T cells cultured with PHA occurs either by induction or increased synthesis rather than by clonal expansion of an Ia-like antigen-bearing subpopulation present in unstimulated T cell preparations or by T cell uptake of Ia-like antigens shed from Ia-like antigen-bearing cells. The increase in Ia-like antigen-bearing T cells during a 4-day culture with PHA correlated closely with the synthesis of DNA but was contrasted by a decrease in the percentage of Fcμ receptor T cells to undetectable levels by 48 hr in culture. During this time the percentage of cells bearing Fcγ receptors, HLA-A,B,C antigens, and membrane bound Ig did not show any significant change. Blocking studies showed no relationship between newly expressed Ia-like antigens on PHA-activated T cells and receptors for either sheep erythrocytes or the Fc portion of IgG. PHA-activated T cells bearing Ia-like antigens stimulated a unidirectional allogenic MLR that can be blocked by the addition of monoclonal antibodies to Ia-like antigens but not to HLA-A,B,C antigens. These data indicate that T cell Ia-like antigens are structurally and functionally similar to B cell Ia-like antigens and suggest the importance of these antigens to T cell-activated function.