Abstract

Bacillus Calmette-Guerin (BCG) stimulated splenic and thymic lymphocytes in vitro as measured by uptake of 3H-thymidine. This activation of lymphocytes by BCG required the presence of a critical concentration of macrophages. Thymus cells containing no more than 0.25% macrophages were stimulated by BCG, but reduction of macrophages below this level by adherence to plastic abolished the response. Reconstitution with purified macrophages completely restored the response. A high concentration of adherent cells (“macrophages”) depressed the response of splenic lymphocytes, as judged by the improvement in DNA synthesis after reduction of the proportion of adherent cells in the spleen cell population. Bacillus Calmette-Guerin augmented the production of lymphocyte-activating factor (LAF) from purified splenic adherent cells, but the presence of lymphocytes made that augmentation considerably greater. These data reaffirm the bidirectional nature of the relationship between lymphocytes and macrophages. They further show that BCG can create highly activated populations of each type of cell, in part by enhancing their interaction.

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