AbstractUsing a new Ly‐6C‐specific antibody (Monts‐1) we show that this class of antigens are differentially expressed on monocytes/macrophages and endothelial cells. Recently elicited peritoneal exudate Mac‐1+mononuclear cells, as well as Mac‐1+mononuclear cells in the bone marrow and in the peripheral blood, express high levels of Ly‐6C. Ly‐6C+mononuclear Mac‐1+cells are absent in normal uninflamed skin, but are present in high numbers in skin lesions 3 days after the s.c. injection of lipopolysaccharide, concanavalin A or complete Freunďs adjuvant. In addition, large Ly‐6C+mononuclear cells are predominant in chronic granulomas induced by complete Freunďs adjuvant. Resident macrophages in a variety of tissues express low levels or in many cases do not express Ly‐6C. Two out of three monocyte‐like cell lines are Ly‐6C+, whereas macrophage‐like cell lines are negative. Ly‐6C+monocytes/macrophages lose the Ly‐6C antigen within 24 h afterin vitroculture. Ly‐6C‐ cultured monocytes and Ly‐6C‐ monocyte‐like cell lines, but not fully differentiated macrophages and macrophage‐like cell lines, can be induced to express the Ly‐6C antigen by interferon‐γ. A population of small vessel endothelial cells in diverse tissues also express high levels of Ly‐6C. The present findings suggest that the Ly‐6C antigen family, shown by others to be involved in T cell activation, may have more general importance in immune responses and
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