Early expression of human CD4 delays thymic differentiation in transgenic mice

CD4 is a cell surface molecule expressed mostly on cells of the T-cell lineage. Studies have shown that this molecule plays an important role in positive and negative selection of T cells in the thymus. It is not surprising therefore, that in T-cell ontogeny, CD4 starts to be expressed on thymocyte subpopulations about to undergo these selection processes. The human CD4 molecule was expressed in mouse thymus ontogeny using a promoter, MMTVD, which targets expression as early as day 14 of ontogeny, prior to expression of endogenous TCR, CD4 and CD8. Thymic ontogeny is delayed in foetal MMTVD-CD4 mice. Human CD4-expressing thymuses show a twofold reduction in cellularity at days 17 and 18 of ontogeny compared with non-transgenic control littermate thymuses, and paradoxically, MMTVD-CD4 thymuses contain more cells in the S and G2/M stages of the cell cycle than control thymuses do. At the cell surface marker level, MMTVD-CD4 thymocytes show a delay in surface expression of CD3, murine CD4 and murine CD8, along with persistent expression of IL2R alpha compared with foetal non-transgenic littermates. Biochemical studies show that, although MMTVD-CD4 thymocytes do not express surface CD3, cytoplasmic CD3 epsilon proteins as well as TCR beta incomplete and complete transcripts are present in foetal day-17 thymocytes. Low levels of surface CD3/TCR expression, however, could partly be due to the low levels of zeta mRNA and proteins detected in these cells. These results suggest that CD4 is not expressed until a certain stage of differentiation not only because it is not yet required for selection processes, but because it can lead to a reversible deregulation of thymocyte development.