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

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.