In the mouse the maturation stage of the peripheral CD4+ CD45RA+ subset is different from that of the CD8+ CD45RA+ subset.

In vitro studies have suggested that the presence of CD45RA on subsets of CD4 and CD8 cells defines naive T cells and that, in response to antigen, CD45RA+ cells become CD45RA- along a differentiation pathway. To test the hypothesis that CD45RA+ cells are naive cells which have just left the thymus, young mice were thymectomized. This would be predicted to lead to a fall in the size of the peripheral pool of CD45RA+ T cells. However, the changes in the size of this pool would also be dependent on the life-span and self renewal capacity of the CD45RA+ T cells in the periphery. Therefore, to test the contribution of the thymus to the peripheral CD45RA+ pool, the percentage of CD45RA+ cells among spleen lymphocyte subsets was studied from 10 days up to 2 years of age in thymectomized and control mice. We also studied the expression of the memory marker CD44 on the CD45RA subsets of CD4 and CD8 cells, as well as the effect of in vitro activation on expression of CD45RA. Our results show that CD8+ CD45RA+ cells are mainly CD44- and their maintenance is dependent on the presence of the thymus. In contrast, the majority of CD4+ CD45RA+ are CD44+ and are not affected by thymectomy. This indicates that the maturation stage of CD8+ CD45RA+ cells is different from that of CD4+ CD45RA+ cells.