Ia-positive macrophages bind and internalize viable lymphocytes in murine thymus

Macrophages have been shown to be present in thymus throughout its development. In the present study monoclonal and polyclonal antimacrophage reagents were used to identify, quantitate, and determine the distribution of thymic macrophages. Those studies demonstrated that significant numbers of macrophages were evenly distributed throughout the cortex and medulla, and that macrophages account for most, if not all, Ia positivity in murine thymus. Suspensions of thymic cells prepared by enzyme digestion contained 2-4% macrophage antigen-positive cells, over 95% of which were I-Ak positive in double-labeling studies. Removal of lymphocytes and macrophages left only epithelial cells and those failed to label for Ia. Subsequent to mild enzymatic digestion, up to 80% of the thymic macrophages were in the form of lymphocyte/macrophage rosettes. Morphologic evaluation of the thymocyte rosettes revealed that some of the macrophages contained internalized lymphocytes. The proportion of macrophages with internalized lymphocytes generally was less than 10%, but during the first 4 weeks of life values often approached 30%. Nurse cells, which were shown through double labeling to express both Ia and macrophage-associated antigen, were included in the population of rosetted cells which had internalized lymphocytes. The results demonstrated that there is a high level of interaction between lymphocytes and Ia-positive macrophages in the thymus which is greatest during the immediate postnatal period.