The nature of the physical interaction between guinea pig non-glass-adherent lymphoid cells and syngeneic macrophages in vitro was investigated. This cellular interaction was found to require the presence of metabolically intact macrophages but neither serum nor antigen. Peritoneal, splenic, or alveolar macrophages were significantly more capable of interacting with thymocytes than either polymorphonuclear leukocytes or fibroblasts. The role of the non-glass-adherent cell was passive in that heat-killed or metabolically poisoned thymocytes were bound by normal macrophages. Two normal lymphoid cell populations, thymocytes, and lymph node lymphocytes, were bound to macrophages in significantly larger numbers than either L2C leukemia cells or erythrocytes. Thus, specificity for each of the participant cell types was demonstrated. These data indicate that macrophages possess a unique ability to recognize and bind lymphocytes and thymocytes by a mechanism which is distinguishable from other known macrophage receptors.

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