Immunohistologic analysis of a human pulmonary alveolar macrophage antigen

PAM1 is a 200-kDa polypeptide antigen present on lavaged human alveolar macrophages but not on monocytes, peritoneal macrophages, breast milk macrophages, or other normal hematopoietic cells studied by flow cytometry. We have characterized the distribution of expression of this antigen by cells in tissues by using immunohistologic techniques. Normal and diseased lung as well as lymph nodes, spleen, kidney, liver, GI tract, and skin were studied. PAM1 was expressed strongly on the surface and weakly in the cytoplasm of most alveolar macrophages in all 15 of the lung specimens. Occasional interstitial macrophages had weak to moderate staining for this antigen but the majority did not stain. The distribution, pattern, and intensity of staining for PAM1 was the same in normal lung specimens and those with interstitial pneumonitis, despite the increase in mononuclear cells in the latter. Dermal histiocytes and Kuppfer cells expressed PAM1 weakly. Sinus histiocytes in lymph nodes were moderately to strongly positive. Although lymphoid cell suspensions (tonsil) were negative by flow cytometry, five of six lymph nodes had positive cells by immunohistology. PAM1 was also detected on endothelial cells of splenic sinusoids in all 6 specimens but not on any other endothelium. Hence, while PAM1 is expressed most strongly on alveolar macrophages, it can also be demonstrated in other locations using sensitive immunohistologic techniques. Since circulating monocytes are antigen negative and some lung interstitial macrophages bear antigen, PAM1 may be a useful marker for studies of the differentiation of mononuclear cells in the lung.