Intrathymic T cell receptor (TcR) targeting in mice lacking CD4 or major histocompatibility complex (MHC) class II: Rescue of CD4 T cell lineage without co‐engagement of TcR/CD4 by MHC class II

A critical step during intrathymic T cell development, termed positive selection, is associated with rescue of short‐lived, immature thymocytes from programmed cell death, T cell lineage commitment, and induction of lineage‐specific differentiation programs. T cell receptor (TcR)‐major histocompatibility complex (MHC) interactions during positive selection can be closely mimicked by targeting TcR on immature thymocytes to cortical epithelial cells in situ via hybrid antibodies. Here, we show that antibody‐mediated TcR signaling in mice deficient for CD4 or MHC class II expression induces polyclonal differentiation of the CD4 T cell lineage. Following a single TcR signal pulse in situ, a temporal sequence of phenotype changes can be discerned: CD69 up‐regulation (< 1 day), CD8 down‐regulation, TcR up‐regulation (1–1.5 days) and down‐regulation of the heat‐stable antigen (1.5–2 days). Differentiation of phenotypically and functionally mature CD4 T cells in situ is attained within 3 days. Rescue of CD4 lineage T cells in the absence of TcR/CD4 co‐engagement by MHC class II in this experimental system supports the stochastic/selective model of T cell lineage commitment.