Generation of cytotoxic T-lymphocyte precursor cells in T-cell colonies grown in vitro

The role of the thymus in T-lymphocyte differentiation remains unclear. The demonstration that the thymus can restrict the T-lymphocyte specificity repertoire suggests that T cells acquire specificity within the thymus. However, the demonstrations of immunocompetent helper T cells and cytotoxic T-lymphocyte precursor cells (CLPs) in athymic nude mice suggest that the acquisition of some T-cell reactivity may occur without the thymus. We have been using T-cell colonies grown in vitro as a model system for studying various aspects of T-cell differentiation in both mouse and man. In one study we showed that CLPs can be found in T-cell colonies grown from spleen cells of normal mice, each colony containing CLPs of several different specificities. The colonies containing CLPs are not clonal, appearing to have a colony-forming unit (CFU-T) of two (perhaps three) cells. Here we provide direct evidence that the CLPs are spontaneously produced in the colonies. In addition, the cells of the CFU-T were characterized with antisera directed against the cell-surface marker Thy-1, which is present on all murine T cells, and the cell-surface markers Lyt-1 and Lyt-2, which are differentially distributed on different T-cell subclasses. We found that the CFU-T contains both a Thy-1+ and a Thy-1- cell, neither of which seems to carry either Lyt-1 or Lyt-2 surface markers.