Relative ability to provide help: an explanation for Con A-induced suppression.

Concanavalin A (Con A) induced suppression of human peripheral lymphocytes has been studied in vitro at different cell concentrations using a 20 microliters inverted Terasaki plate culture system in which the uptake of 3H-thymidine was measured. Conventional assessment of suppression relates the response of cells to Con A in the presence of Con A-pretreated autologous cells, with the response of cells preincubated without Con A (control cells). The level of suppression so calculated was affected both by serum-related high background counts and by autologous stimulation of responding cells by mitomycin-treated Con A-pretreated cells. The high background was largely removed by the use of serum-free Iscove's medium. The data involving different cell concentrations and periods of cultivation indicate that the assessment of suppression is inadequate. Both Con A-pretreated and control cells are shown to 'help' the response of autologous responder cells to added soluble Con A. When the help by Con A-treated cells is less marked than that by control cells, apparent suppression is seen since the control cell provide the conventional baseline from which suppression is judged. After longer periods of cultivation the Con A-pretreated cells increased responses more than did the control cells leading to apparent enhancement.