The Role of Macrophages in the Activation of T-Lymphocytes by Concanavalin A I. Macrophages Support Proliferation after Commitment of Lymphocytes

Abstract

T lymphocytes require the presence of small numbers of macrophages to enter mitosis when activated by mitogens such as concanavalin A. The aim of this study was to decide when macrophages may operate in the course of activation. Lymphocytes from the mesenteric lymph nodes of Lewis rats were depleted of macrophages by adherence on 0.1 mm glass bead columns. Lymphocytes in the column effluent, between 40 and 50 percent of the cells originally placed on the column, contained less than 0.2 percent macrophages as determined by uptake of neutral red. Macrophage-depleted lymphocytes were completely unresponsive to stimulation with con A or PHA to enter DNA synthesis. The proliferative response to con A was completely restored by adding macrophages elicited intraperitoneally by various means. Addition of an optimal number of macrophages after 24 hrs of con A stimulation of macrophage-depleted lymphocytes also supported DNA synthesis, although to only 40-60% when compared to DNA synthesis in the continuous presence of macrophages, with a peak response temporarily identical to cultures where macrophages were present from the beginning. After 24 hrs of con A stimulation all lymphocytes were committed for mitogenesis, as removal of surface bound con A by a-methyl mannoside had no effect when added at this time. Substitution of macrophages after 24 hrs of con A stimulation in the presence of α-methyl mannoside resulted in an identical induction of DNA synthesis as in the absence of this hapten sugar. The data fit a hypothesis that initiation of lymphocyte activation with con A does not require the presence of macrophages. Macrophages may deliver a second signal, which is independent of the binding of the mitogen to these helper cells, and which supports induction of DNA replication after lymphocytes are committed for mitogenesis.