Cell Ontogeny : Organ Location of Maturing

Cytotoxic depletion studies with anti-Lyt-l , anti-Lyt-2, and anti-Lyt-3 antisera demonstrated that functional mouse T cell subpopulations differ in their expression of these surface antigens. Helper T cells were sensitive to depletion only with antiLyt-1, whereas suppressor and cytotoxic T cells were depleted only with anti-Lyt-2 or anti-Lyt-3 (1-3). Thus, helper T cells were classified as Lyt-l+2-3 and suppressor and cytotoxic T cells as Lyt-l-2+3 +. Supposedly immature T cells, in contrast, were sensitive to depletion by all three antisera and were therefore classified as Lyt-I+2+3 +. These latter cells constituted -50% of T cells in the adult spleen, but almost 100% in spleens from 7-d-old mice (I). With the advent of monoclonal antibodies directed against each of these three Lyt antigens, it became possible to take full advantage of the analytic capabilities of the fluorescence-activated cell sorter (FACS) x to examine the quantitative expression of these antigens on lymphocyte subpopulations by immunofluorescence. These studies, conducted with adult mice, confirmed that Lyt-1 ispresent on all thymocytes and, surprisingly, showed that Lyt-1 is also found on essentially all peripheral T cells, albeit to a lesser extent on Lyt-2 + than on Lyt-2cells (4, 5). This corroborated earlier reports demonstrating that cytotoxic T cells can be depleted under optimal conditions with either conventional or monoclonal anti-Lyt-I antibodies (6, 7). It therefore established Lyt-1 as a quantitative rather than a qualitative marker for distinguishing functional and maturational T cell subpopulations. Lyt-2, in contrast, was shown to be present on fewer peripheral T cells than originally reported but, as before, to be present on most thymocytes (4, 5). Thus it remains as a qualitative marker for a T cell subpopulation in the periphery and, because of its higher frequency in the thymus, as a marker for distinguishing immature from mature T cell populations during ontogeny. Lyt-3, which is located on the same