Phenotypic changes that TCR V gamma 3+ fetal thymocytes undergo during their maturation into dendritic epidermal T cells.

Murine Thy-1+, TCR V gamma 3/V delta 1+ dendritic epidermal T cells (DETC) express CD2 antigens, but differ from most other T-cell subsets in their absence of CD4, CD5, and CD8 antigens. To determine whether negativity for those antigens is an intrinsic feature of a given T-cell population or whether such triple-negative T cells go through a maturational stage during which they express these antigens, we determined the phenotype of TCR V gamma 3+ fetal thymocytes, which are the precursor cells of DETC. We found that TCR V gamma 3+ fetal thymocytes at day 17 of gestation are CD2+, CD5+, mostly CD8+, and partly CD4+. The expression of CD5 is highest on early TCR V gamma 3+ thymocytes; these cells express intermediate levels of CD5 when they leave the thymus and lose CD5 expression until or shortly after arrival in the epidermis. A similar loss of CD5 expression by TCR V gamma 3+ cells was observed in vitro under various culture conditions. To determine whether expression of CD5 is important for the maturation of DETC, we searched for these cells in the epidermis of CD5-deficient mice. There was no alteration in the number of Thy-1+/TCR V gamma 3+ dendritic cells in the epidermis of CD5-/- mice. Even though the latter finding speaks against a pivotal role of CD5 during the maturation of DETC, the described cell system may serve as a useful tool in further experiments aimed to clarify the function of the CD5 glycoprotein as well as the mechanism(s) regulating its expression.