γ/δ T cells and α/β T cells differ in their developmental patterns of receptor expression and modulation requirements

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Abstract
Measurement of the cell surface levels of γ/δ (TcR1) and α/β (TcR2) T cell receptors in the chicken revealed that thymocyte subpopulations that express these receptor isotypes differ remarkably in their rates of receptor acquisition. Whereas TcR 1 expression was relatively high (> 104 sites per cell) beginning on day 12 of embryonic life, the initial levels of TcR2 expression on embryonic thymocytes were relatively low (approximately 103 sites per cell) when first measurable on day 15, and increased gradually as a function of T cell maturation. In peripheral tissues, the TcR 1 cells also expressed higher receptor levels than did the TcR 2 cells, but the difference was only 2–3‐fold. The TcR 2 receptors on immature T cells could be easily modulated by receptor cross‐linkage, very much like immunoglobulin receptors on immature B cells. While the TcR 2 receptor modulation occurred within minutes, TcR 1 receptor modulation required several hours for completion, even in the embryonic thymus. The data indicate very different developmental programs for TcR 1 and TcR 2 expression, and suggest fundamental differences in clonal selection modes for the two T cell subpopulations.