[T cell leukemia antigens, FT-1 and FT-2].
The understanding of the differentiation process of lymphocytes in the embryonic thymus has been based on extrapolation from adult thymic lymphocytes. The study presented here was mainly concerned with the new cell surface antigens designated FT (FT-1, FT-2), which are expressed on fetal thymocytes of all mouse strains examined, but not on thymocytes or any other lymphoid cells of adult mice. One of the interesting features of FT antigens is the timing of their expression in connection with the ontogeny of thymic lymphocytes. A majority of fetal thymocytes at the 13th day of gestation express FT antigens, whereas no positive cells are found in fetal liver. Therefore, FT antigens seem to appear as soon as the stem cells have reached the thymus. The proportion of FT+ cells then declines sharply with increase in the time of gestation, while Thy-1+ cells increase in inverse proportion. All of these results seem to support the idea that the thymocytes expressing FT antigens are replaced with Thy-1+ thymocytes, although the possibility of the transition from FT+ to Thy-1+ cells cannot be excluded. It was also demonstrated that fetal thymocytes are heterogeneous with respect to the expression of FT-1 and FT-2 antigens. Especially, thymic lymphocytes at the earlier stages of ontogeny can be divided into at least three subpopulations, FT-1+2+, FT-1+2- and FT-1-2-. With the emergence of Lyt antigens, these subpopulations seem to differentiate and acquire far more complicated features. Such cellular heterogeneity probably reflects the complexed events like selection, deletion or amplification occurring in the embryonic thymus. Another interesting aspect of FT antigens is their reappearance on the surface of thymic leukemia cells. Biochemical studies have indicated that the molecular weight of FT-1 antigen on leukemia cells is about 100,000 by means of biosynthetic labeling with either [3H]-galactose or [35S]-methionine. FT-1 antigen on fetal thymocytes also appeared as a major band with m.w. 100,000. Two-dimensional gel electrophoresis revealed that FT-1 antigenic determinants appear to reside on a family of glycoproteins with extensive charge heterogeneity. All of these results suggest that FT antigens, especially FT-1 antigen, will be useful markers for understanding the mechanism underlying the activation of normally silent genetic information in leukemia cells. In summary, further biochemical and genetic analysis of FT antigens will contribute to an understanding of the ontogenic development of T cells as well as the leukemogenesis of T cell leukemias.