Antigen-independent maturation of CD2, CD11a/CD18, CD44, and CD58 expression on thymic emigrants in fetal and postnatal sheep

We have compared the expression of CD2, CD11a/CD18, CD44, and CD58 on alpha beta and gamma delta T cells emigrating from the fetal and postnatal thymus. We report that both gamma delta and the CD4(+)CD8(-) and CD4(-)CD8(+) subsets of alpha beta T cells express mature levels of the adhesion molecules CD11a/CD18, CD44, and CD58 upon emigration from the thymus. Whereas CD44 is up-regulated on gamma delta(+) thymocytes prior to export, down-regulation of both CD11a/CD18 and CD58 occurs prior to emigration from the thymus, suggesting that down-regulation of these molecules may be a final maturational step taken by developing gamma delta T cells before their export from the thymus. In contrast, there is continued up-regulation of CD2 on gamma delta and alpha beta T cells upon emigration from the thymus and as they move into the mature peripheral T-cell pool. There was also a marked reduction in the number of CD2(+) gamma delta T cells exported during fetal development that was associated with a marked reduction in the percentage of CD2+ gamma delta thymocytes exported. The postthymic maturation of CD2 and the other changes in adhesion-molecule expression appear to be independent of extrinsic antigen, as the same changes were observed in the antigen-free environment of the fetus as in the postnatal lamb, which has been exposed to extrinsic antigen. It thus appears that these changes in adhesion-molecule expression are as a result of the normal maturation pathway from a developing thymocyte to a mature peripheral T cell.