Evaluation of functional heterogeneity in the CD8 subset with T cells from T cell receptor-transgenic mice

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Abstract
The question of functional differentiation within the CD8 subset has been addressed in a model of TcR‐transgenic (TcR‐tg) mice expressing a TcR specific for H‐2Kb (Ti). CD8+ Ti+ T cells present in the periphery of these mice have no cytotoxic T lymphocyte (CTL) activity unless they are stimulated with H‐2Kb‐expressing cells. In contrast to T cells from normal H‐2k littermates, alloantigen induction of CTL from TcR‐tg mice is independent of CD4+ T helper (Th) cells and is accompanied by high level secretion of interleukin‐(IL)‐2 by Ti+ CD8+ T cells. Precursor frequency analysis performed on CD8+ cells from TcR‐tg mice revealed a high frequency of Th as compared to CTL precursors. This raised the possibility of the existence of distinct subpopulations within CD8+ precursors with different requirements for differentiation to functional CTL. FACS analyses (performed on resting and on in vitro stimulated T cells from normal and TcR‐tg mice) demonstrated a heterogeneous expression of Ly‐6C on CD8+ cells with a large enrichment of Ly‐6C− cells among the Ti+ cells which persisted after stimulation with H‐2b cells in conditions that led to a homogeneous expression of the activation markers pgp‐1 and CD69. The possibility that Ly‐6C expression could mark functionally different subpopulations in CD8+ T cells was investigated. Stimulation of sorted populations of Ly‐6C− and Ly‐6C+ cells allowed detection of CTL precursors in both these subsets and the majority of limiting dilution wells containing one pCTL also scored positive for IL‐2 secretion. Thus, for CD8+ T cells expressing the same TcR, differentiation led to acquisition of both IL‐2 secretion and CTL function and there was no evidence for the existence of a distinct population of helper‐dependent CTL precursors.