312 Homologous Mouse and Human

The mouse Lyt-1, Lyt-2, and Lyt-3 lymphocyte surface antigens have been used for some years now as immunogenet ic markers whose selective expression distinguishes maturat ional and functional T cell subpopulations (1, 2). Lyt-2 and Lyt-3, for example, are selectively expressed on cytotoxic and suppressor T cells but are not found on helper T cells (3, 4). Similarly, cytotoxic depletion studies originally indicated that Lyt-1 is selectively expressed on helper T cells but not on suppressor or cytotoxic T cells (5, 6). More sensitive immunofluorescence techniques, however, have not shown that Lyt-1 is found on all T cells (7-10) but is present in higher levels on helper T cells than on suppressor or cytotoxic T cells (10). In addition, each of the Lyt-1, Lyt-2, and Lyt-3 antigens undergoes characteristic changes in surface-density expression as T cells mature in the thymus and peripheral lymphoid tissues (9, I0). Functional subpopulat ions of h u m a n T cells have also been defined by selective expression of certain surface antigens. Two subpopulat ions of T cells were originally defined by a heteroantiserum termed aTH2 (11). TH2 ÷ cells were shown to mediate suppressor effects in vitro and to be responsible for most of the killing in cell-mediated lympholysis (CML) 1 (12). In contrast, TH2cells had markedly less cytotoxic activity in C M L but were found to amplify the functions of other cells and to proliferate in response to specific antigens (mumps and tetanus toxoid) (12). Recently, both of these subsets have been redefined by monoclonal antibodies (1315) 2 and analogies drawn between the h u m a n antigens and mouse Lyt antigens (15). The analogy is based primarily on the view that Lyt-1 is expressed on the helper/ inducer but not the suppressor/cytotoxic subset in the mouse. As this view needs to be