Discrepancy in expression ability of Tac antigen and Ia determinants defined by monoclonal antibodies on activated or cultured cord blood T lymphocytes.

T lymphocytes from cord blood and adults stimulated with mitogens and alloantigen were analyzed using monoclonal antibodies that detect Ia and Tac antigens associated with lymphocyte activation. Less than 5% of T cells freshly isolated or cultured without stimuli expressed Ia and Tac antigens assessed by the indirect immunofluorescence method. When both T cells from cord blood and adults were cultured with phytohemagglutinin (PHA), concanavalin A (Con A), pokeweed mitogen (PWM), or allogeneic cells, variable proportions of them expressed Tac antigen. The expression of Tac antigen on stimulated T cells increased in parallel with DNA synthesis, and was significantly correlated with the proliferative response of stimulated T cells. In contrast to Tac antigen, the expression of Ia antigens appeared to show some time lag behind the proliferative response. Although expression of Ia antigens was too weak to assess in PHA- or Con A-stimulated cultures, adult T cells were stimulated by PWM or allogeneic cells to express strongly Ia antigens in culture. However, the percentage of Ia+ cells in cord blood T cells stimulated with PWM or allogeneic cells was markedly lower as compared with that of adults. In addition, cord blood T cells grown in the presence of T cell growth factor showed the same degree of Tac antigen as adult ones did, whereas their expression of Ia antigens was negligible as compared with adult controls. These observations suggested that T cells in cord blood might be deficient by nature in their ability to express Ia antigens on activation. The inability to develop Ia+ T cells seemed to be characteristic of functional immaturity of T cells in early human ontogeny.