Expression of CD21 is developmentally regulated during thymic maturation of human T lymphocytes

CD21, the C3d/CD23/Epstein-Barr virus (EBV), receptor is expressed at low d ensity on cells of the T lineage. Immature thymocytes express CD21 with hig h density. In the present study, we have analyzed the expression of CD21 du ring intrathymic maturation of T cells, An intense staining for CD21 was ob served at the double-negative stage and at the stage of early acquisition o f CD4, CD21 expression was decreased at the double-positive and single-posi tive stages, to then reach levels similar to those of peripheral brood T ce lls. Staining of thymus sections showed a bright fluorescent signal on thym ocytes entering the thymus in the cortical region. Taking advantage of the immature phenotype of cells expressing high amounts of CD21 (CD21(+ +)), we depleted thymocyte suspensions in CD3(+) and CD8(+) cells to study the pro perties of CD21 on this cell subset. Triggering of CD21 with its ligands iC 3b, CD23 and anti-CD21 mAb did not alter the proliferative response of thym ocytes to IL-7, and did not induce the differentiation of early cells into CD4(+)CD8(+) thymocytes. Immunoprecipitation did not reveal any molecule as sociated with CD21 that could play a signaling role in thymocytes. Finally, EBV induced a down-regulation of CD21 and an up-regulation of CD1 in CD21( + +) thymocytes. Taken together, our observations demonstrate a regulated e xpression of CD21 on human thymocytes and suggest that the CD21(+ +) subset may be a target for EBV. We further suggest that CD21 on early thymocytes acts as a ligand for CD23-expressing cells in the thymus.