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.