Immunohistology of thymic nurse cells.

The demonstration of thymic nurse cells (TNC), complexes between stromal cells and thymocytes, in cell suspensions of murine thymuses, prompted us to investigate (1) the relationship of TNC to other thymic stromal cell types defined in situ, and (2) the maturation stage of the enclosed thymocytes. To this purpose we incubated frozen sections of TNC suspensions with various monoclonal antisera directed to T cells and stromal cell types, using immunohistology. This approach enabled us to study antigen expression on the "nursing" cell itself and to analyze the phenotype of the enclosed lymphocytes in cross sections of TNC. The results show that lymphocytes enveloped by TNC express high levels of Thy-1, moderate levels of T200, and variable amounts of Lyt-1. Due to enzymatic degradation Lyt-2 expression could not be studied. The enveloped cells also bear PNA receptors, but no detectable I-A/E antigens. Expression of H-2K antigens on enclosed thymocytes varied from weak to absent. The "nursing" cells react with ER-TR4, a monoclonal antibody which detects cortical epithelial-reticular cells. In addition TNC express I-A/E and H-2K antigens. In contrast, TNC do not react with ER-TR 5 and 7, monoclonal antibodies, which detect medullary epithelial cells and reticular fibroblasts, respectively. TNC do not express the macrophage antigens Mac-1 and Mac-2. We conclude that TNC in vitro represent the in vivo association of epithelial-reticular cells with cortical thymocytes. However, the enclosed thymocytes do not constitute a phenotypically distinct subset of subcapsular or outer cortical cells.