Alterations in gene expression that orchestrate eukaryotic cellular differentiation often require appropriate interactions between differentiating cells and a specialized microenvironment. During T-lymphocyte differentiation, immature thymocytes undergo a stringent intrathymic selection process that requires intimate contact with thymic stromal elements. Since this selection process generates T cells that are self-tolerant and recognize nominal antigen only within the context of self-major histocompatibility antigen complex molecules, it is possible that thymocyte/stromal cell interactions are mediated, in part, by antigen-specific receptors expressed on differentiating thymocytes. However, the developmental stage at which alpha/beta antigen-specific receptors are expressed during T-cell maturation has been a matter of debate. To address this issue, we have studied alpha/beta T-cell antigen receptor gene and protein expression on normal thymocyte subsets of AKR/J mice, as well as on a panel of AKR/J primary thymic lymphomas characterized for CD4 (L3T4) and CD8 (Lyt-2) differentiation antigen expression. The data unequivocally demonstrate that alpha/beta heterodimers are expressed not only on phenotypically mature thymocytes but also on the majority of CD4+8+ double-positive cells that comprise the predominant nonmature thymocyte subset. Furthermore, a fraction of thymocytes in the CD4-8- double-negative compartment, known to contain progenitor cells, also expresses readily detectable cell-surface alpha/beta receptors. Therefore, during the process of intrathymic selection, interactions between nonmature thymocytes and stromal cells via the antigen-receptor complex may play a pivotal role in T-cell differentiation and should be considered in formulating schemes for functional T-cell selection
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