Macrophage-lymphocyte clusters in the immune response to soluble protein antigen in vitro. IV. Synergy of lymphocytes in the formation of clusters.

T-lymphocyte-enriched lymph node lymphocytes from guinea pigs immunized with Mycobacterium tuberculosis produce clusters with macrophages when cultivated on monolayers of syngeneic purified protein derivative of tuberculin (PPD)-pulsed peritoneal macrophages. The clusters consist of a macrophage with a central lymphocyte attached to it, and several peripheral lymphocytes attached to the central one. By mechanical manipulation immune lymphocytes incubated on monolayers of PPD-pulsed macrophages were separated into those which adhered firmly to the macrophages after 4 hr of culture and those which did not adhere. While neither of the two populations was able to produce significant numbers of clusters alone, they did so in combination. The number of macrophage-lymphocyte clusters which are produced in a culture depends not only on the absolute number of immune lymphocytes in the culture, but also on the concentration of lymphocytes per area of the macrophage monolayer, with high concentrations resulting in high numbers of clusters. Autoradiographic studies showed that the DNA-synthesizing lymphocytes physically associated with macrophages were located mainly inside the clusters in cultures with high concentrations of lymphocytes but mainly outside the clusters in cultures with low concentrations of lymphocytes.