We have studied the physical interaction between macrophages and lymphocytes during the immune response to purified protein derivative of tuberculin (PPD) in vitro. Mixtures of peritoneal macrophages and lymph node lymphocytes from guinea pigs immunized with tubercle bacilli formed cell clusters during 20 h of culture with PPD. The number of clusters produced was correlated to the number of immune lymphocytes in the cultures. Peritoneal macrophages which had been pulsed with PPD and untreated lymph node lymphocytes produced cell clusters in the absence of free PPD in numbers equivalent to those produced by the same cells in the presence of free PPD. In cultures containing a mixture of PPD-pulsed macrophages, not-pulsed macrophages, and immune lymphocytes with no free PPD, cell clusters developed mainly between the antigen-pulsed macrophages and lymphocytes. Cluster formation was antigen-specific with the specificity residing in the lymphocytes, mainly or exclusively in the T lymphocytes. These data indicate that in the process of cell cluster formation macrophages serve as antigen-binding (or -processing) cells, while a subpopulation of lymphocytes interact physically and specifically with the macrophages.

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