Help and suppression by lymphoid cells as a function of cellular concentration.

The phytohemagglutinin-stimulated uptake of [3H]thymidine in mixtures of human lymphocytes from the same source was shown to depend on the cell concentration in vitro as well as on the period of cultivation. "Helper" and "suppressor" effects were obtained by varying the concentration of cells and the periods of cultivation. The possibility that helper and suppressor subpopulations were responsible was avoided by mixing lymphoid cell line cells with others of the same monoclonal origin. Even under these conditions, both the direction and the extent of activity depended on the same two variables. This weakens the case for postulating the existence of distinct subpopulations of lymphocytes with helper or suppressor properties. This case was based on the use of damaging treatments believed to separate cell populations which were then found to differ in their helper and suppressor properties. We propose instead that the effect of such treatments is mediated through changes in the concentrations of interacting cells. Our data make it clear that the function of lymphoid cells ascertained in one set of conditions need not apply within a different cellular environment.