Sequential proliferation induced in human peripheral blood lymphocytes by mitogen. II. Suppression by PHA-activated cells.

Human peripheral blood cells from cultures stimulated by phytohemagglutinin (PHA) for 3 days suppressed continued proliferation when added to feeder layer cultures of autologous lymphocytes previously stimulated with mitogen. Proliferation 1 day after the addition of inhibitory cells was reduced 50 to 60% as measured by tritiated thymidine (3HTdR) uptake and mitotic index. It is suggested that a similar suppressive phenomenon may be acting in most standard mitogen-stimulated cultures to limit cell division after only a few days of culture. Suppression of continued lymphocyte proliferation in the feeder layer cultures by cells from PHA-stimulated cultures was not due to cell crowding, carry-over of mitogen, or nutrient depletion; nor was it an artifact of thymidine incorporation. Apparent suppression of proliferation by an extract of these cells, however, was artifactual; the extract inhibited 3HTdR uptake but did not reduce mitotic activity. It is not known whether the inhibition exerted by the PHA-activated cells is mediate through a discrete subpopulation of "suppressor cells" or represents a regulatory function of any mitotic or post-mitotic cell, or both.