Characterization of the T lymphocyte colony-forming cells and evidence for the acquisition of T cell markers in the absence of the thymic microenvironment in man.

The nature of the T colony-forming cell (T-CFC) in agar is still controversial. We present evidence that blood mononuclear cells depleted of T cells by E-rosetting or lysis with OKT 3 and complement can give rise to E+ OKT 3+ colonies in the presence of supernatants of PHA-stimulated lymphocytes (P-SUP) containing a cell growth factor. Both OKT 4+ (67%) and OKT 8+ (32%) cells were found in the colonies, and less than 10% of these cells were Ia+. We consider that colonies in agar can arise from the proliferation and maturation of a circulating immature progenitor (T colony-forming cell, or T-CFC) that does not need the thymic environment to acquire the OKT 3, 4, and 8 antigens or E receptor. In contrast, Ia+ OKT 3+ cells that are maintained by P-SUP in long-term liquid culture are reportedly derived from mature T cells, and this proliferation might represent the counterpart of the in vivo response to any mitogenic stimulation of the peripheral compartment of mature T cells.