Nature and mechanisms of action of co-operating cells controlling human T-colony formation.

The mechanisms of action and the nature of the co-operating cells (CC) controlling human T-lymphocyte-colony formation were investigated. Media conditioned by PHA-stimulated blood mononuclear cells (MC) were tested for their capacity to induce T-colony formation in the effluent cell population, obtained after anti (Fab')2 cell affinity chromatography of MC. This cell population has been previously shown to still contain T-colony-forming cells (TCFC), but to be devoid of a co-operating cell population essential for T-cell-colony growth, thus requiring a feeder layer containing media conditioned by PHA-stimulated MC in order to generate T colonies. Further evidence is also presented that: phytohaemagglutinin (PHA) was necessary to two steps of T-colony formation: (i) for the production of colony promoting activity (CPA) by PHA-stimulated MC; (ii) for the induction of the TCFC to generate a colony, in the presence of CPA. There were high producers and low producers of CPA. The low CPA production observed with some donors could be explained by a suppressive effect mediated by phagocytic cells, presumably monocytes, whereas the cells retained on the anti-(Fab')2 immunoadsorbent (mainly B cells) were able to produce very high CPA levels.