Thymic reticulum in mice II. Culture and characterization of nonepithelial phagocytic cells of the thymic reticulum: Their role in the syngeneic stimulation of thymic medullary lymphocytes

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Abstract
In the present report the characteristics of nonepithelial phagocytic cells of the murine thymic reticulum are described. Primary cultures were established from thymic fragments. Nonadherent cel s with hairy membranes proliferated on the surface of established primary monolayers. These cells were recovered and replated in secondary cultures were they appeared as large adherent cells with dendritic shape. At the electron microscopic level, phagocytic cells of the thymic reticulum in culture (P‐TR‐C) appear as clear vacuolated cells with an indented nucleus and few lysosomes; this morphological aspect makes them different from the common macrophage, despite their phagocytic capacity. P‐TR‐C are positive for nonspecific esterase, acid phosphatase which is found in the few lysosomes present, 5′‐nucleotidase and α‐D‐mannosidase, but negative for peroxidase. A high proportion of α‐mannosidase‐positive cells is inconsistent with the common macrophage, but in common with other cells with dendritic shape such as Langerhans cells. They are Thy‐1−, Ig− and nearly half of them are IA+. P‐TR‐C can be defined as the stimulator cells for syngeneic stimulation; they are able to induce the proliferation of lymphocytes enriched in mature syngeneic medullary thymocytes, but not in immature cortical ones. Characteristics of P‐TR‐C make them very similar to the interdigitating cells described in the peripheral lymphoid organs and in the thymus in situ.