Abstract
Thymic repertoire selection requires the expression of the alpha beta CD3 T-cell receptor (TCR) together with the coreceptors CD4 and CD8. The appearance of CD4 and CD8 on thymocytes is the hallmark of a complex maturation step, accompanied by downregulation of the interleukin 2 receptor (IL-2R) alpha chain, arrest of rearrangement (i.e., allelic exclusion) of the TCR beta-chain locus, a burst of cell divisions, and reduction in cell size. This maturation step is inhibited in TCR beta-chain-deficient mouse strains and may depend on surface expression of an immature TCR complex containing CD3 and TCR beta chains but no TCR alpha chain. Here we show that the CD4+8+ double-positive (DP) stage can be induced by treatment of fetal thymic organ cultures with anti-CD3 epsilon monoclonal antibodies in several TCR beta-chain-deficient mouse strains: severe combined immunodeficient (scid) mice, mice carrying a mutation in the recombination activating gene 1 (Rag-1), or mice carrying a deletion in the TCR beta-chain locus itself. These findings suggest that CD3 epsilon is expressed on the thymocyte surface independent of and prior to the TCR beta chain. The data are consistent with the notion that in wild-type mice the DP stage is induced by transmembrane signaling through an immature CD3-TCR beta-chain complex, which can be bypassed by crosslinking of CD3 epsilon alone.