HETEROGENEITY OF MOUSE THYMIC MACROPHAGES .1. IMMUNOHISTOCHEMICAL ANALYSIS

As the first step toward understanding the identity and functions of t hymic macrophages in situ, we examined the phenotypic heterogeneity of mouse thymic macrophages in tissue sections by the immunohistochemica l double staining method with four monoclonal antibodies (F4/80, Mac-2 , anti-CD32/16 and anti-I-A antibodies) as macrophage markers. Morphol ogically, three types of macrophages were identified: dendritic, round and hat-shaped. Dendritic macrophages were scattered throughout the t hymus, and most of them were stained by all four markers. Among these macrophages, those at the cortico-medullary region (CMR) expressed a h igh intensity of CD32/16 antigen. Round macrophages were also distribu ted throughout the thymus; most of them, however, were localized in th e cortico-medullary region to the medulla. These cells were F4/80-nega tive, Mac-2-positive, CD32/ 16-negative and I-A-positive. In contrast, round macrophages located at the cortex expressed F4/80. Flat-shaped macrophages were localized at the subcapsular region of the cortex whe re active lymphopoiesis was observed. This type was positive for F4/80 and CD32/16, but negative for Mac-2. Furthermore, most of the three t ypes of thymic macrophages showed intense reactions of the I-A, antige n within the cytoplasm in addition to the expression of I-A antigen on the cell membrane. These results indicate that morphological characte ristics of thymic macrophages at different locations reflect phenotypi c variations detected in immunohistochemistry, and suggest that these different type macrophages may play distinct roles at various location s in thymocyte development in the thymus.