Recent studies have shown that maturation of CD4-8- double negative (DN) thymocytes to the CD4+8+ double positive (DP) stage is dependent on expression of the T cell receptor (TCR)-β polypeptide. The exact mechanism by which the TCR-β chain regulates this maturation step remains unknown. Previous experiments had suggested that in the presence of some TCR+ thymocytes, additional DN thymocytes not expressing a TCR-β chain may be recruited to mature to the DP stage. The recent demonstration of an immature TCR-β-CD3 complex on early thymocytes lead to the alternative hypothesis that signal transduction through an immature TCR-CD3 complex may induce maturation to the DP stage. In the latter case, maturation to the DP stage would depend on the expression of TCR-β-CD3 in the same cell. We examined these two hypotheses by studying the expression of the intra- and extracellular CD3ε, CD3ζ, and TCR-β polypeptides in intrathymic subpopulations during embryogenesis. CD3ε and CD3ζ were expressed intracellularly 2 and 1 d, respectively, before intracellular expression of the TCR-β chain, potentially allowing immediate surface expression of an immature TCR-β-CD3 complex as soon as functional rearrangement of a TCR-β gene locus has been accomplished. Calcium mobilization could be induced by stimulation with anti-CD3ε mAb as soon as intracellular TCR-β was detectable, suggesting that a functional TCR-β-CD3 complex is indeed expressed on the surface of early thymocytes. From day 17 on, most cells were in the DP stage, and over 95% of the DP cells expressed on the TCR-β chain intracellularly. At day 19 of gestation, extremely low concentrations of TCR-β chain and CD3ε were detectable on the cell surface of nearly all thymocytes previously thought to be TCR-CD3 negative. These findings strongly support the hypothesis that maturation to the DP stage depends on surface expression of and subsequent signal transduction through an immature TCR-β-CD3 complex and suggest that maturation to the DP stage by recruitment, if it occurs at all, is of minor relevance.
|Number of pages||9|
|Journal||Journal of Experimental Medicine|
|Publication status||Published - Dec 1 1993|
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