Differences in Ly 2- L3T4- thymocytes from foetal and adult murine thymus.

The thymus is the major site of mammalian T cell production. The exact steps which occur in the thymus and give rise to mature T cells have not been defined, but there is general agreement that the earliest T cells are included in the group of thymic lymphoid cells lacking Ly 2 (CD8) and L3T4 (CD4). This population represents 2-6% of adult thymocytes and the vast majority of thymocytes in the mouse embryo until about day 16 of gestation. It has often been assumed that the foetal and adult CD4- CD8- thymocytes are equivalent. This paper shows that there are significant differences between the CD4- CD8- cells from these sources, in that the adult includes at least two subsets which are undetectable in the embryo. These two subsets of CD4- CD8- cells are both Ly 1high, B2A2- and M1/69-; one is Thy 1+ and one is Thy 1-. Each represents 20-25% of adult CBA double negative thymocytes. Both these populations are excluded from analyses of CD4- CD8- thymocytes which have been further selected as Ly 1low, a procedure adopted in several studies of early thymocytes. Even those subpopulations of CD4- CD8- cells which appear to express similar markers in adult and embryo thymus are quite different when analysed for cell size (forward light scatter), with the embryonic forms being much larger.