Brief DeBnitive Report

A number of cell surface markers are expressed on resting peripheral blood (PB) T cells (1). However, there are a number of surface markers that are expressed only in low proportions or not at all on resting T cells, but that are clearly expressed in varying proportions on activated T cells. Among these markers of activated T cells, the receptors for the Fc portion of IgG (FcyR) are on small fractions of PB sheep erythrocyte (E)-rosetting cells, usually referred to as Ty cells (2, 3). The proportions of FcTR-positive cells increase markedly upon activation of the cell (4). In addition, Ia-like antigens are expressed on a small proportion of freshly isolated circulating T cells; these are included within the Ty fraction (5). Again, upon activation of T cells, variable proportions will express Ia-like antigens (6-8). Recently, a monoclonal antibody (4F2) has been described which binds to a non-Ia, 120,000-mol wt cell surface antigen present on resting PB monocytes, but which is present in negligible amounts on resting PB lymphocytes (9, 10). However, the 4Fz antigen is expressed on a large proportion of lymphocytes which have been activated by mitogen and/or alloantigen stimulation (9). The precise identity of the mixed lymphocyte reaction (MLR)-activated specific cytotoxic T cell (CTL) in humans has not been characterized by currently available cell surface markers. In this study we have analyzed MLR-act ivated T cells to determine the relationship between specific cytotoxicity and the phenotypic expression of these activation markers. The results indicate that C T L are found exclusively in a population of cells lacking detectable FcyR (FcyR-) but expressing 4F2 antigen. By contrast, C T L were detectable in both the Ia-positive and Ia-negative population of activated T cells.