The origin of T cell colonies from T depleted human lymphocyte populations: analysis by limiting dilution

We have examined the identity of the T colony progenitor cell (TCPC) in T cell depleted peripheral blood mononuclear cells (PBM-E-). PBM were cultured in a limiting dilution assay utilizing PHA, T cell growth factor (TCGF) and irradiated feeder cells in semi-solid medium. The TCPC frequency was 0.10 in PBM-E+ and 0.0006 in PBM-E-. The observed TCPC frequency in PBM-E- was directly proportional to the number of contaminating PBM-E+; all of the T colonies which arose from PBM-E- could be accounted for by contaminating T cells. In other experiments, E+ cells were generated from PBM-E-, incubated in the presence of PHA and TCGF. This appearance of E+ cells could be totally abrogated by incubation with hydroxyurea. Similarly, the TCPC frequency in PBM-E- was increased by pre-incubation with PHA and TCGF, and this increase was blocked by hydroxyurea. These experiments suggest that a small number of contaminating T cells can rapidly expand in culture and that these residual T cells, not a population of pre-T cells, are the source of TCPC within the PBM-E- population.