Dissection of macrophage differentiation pathways in cutaneous macrophage disorders and in vitro

Macrophages play important roles in immunity and inflammation, and in allergic, granulomatous and neoplastic diseases. Here, we present the indepth results of an ongoing study of macrophage differentiation pathways in cutaneous macrophage disorders and in vitro. Up to now, a total of 40 cases of cutaneous macrophage disorders (histiocytoses and granulomas) and related diseases were examined using a panel of monoclonal and polyclonal antibodies to macrophage differentiation antigens (mAb MS‐1, mAb αCDla, mAb αCD34, mAb RM 3/1, mAb αCD11c, mAb αCD36, mAb MAC 387, mAb 27E10, polyclonal antibodies ccMRP‐8 and ‐14, mAb aCD68. mAb 25F9, mAb DRC1‐R4/23, and mAb 1F10). Of these, MS‐1 high molecular weight protein, synthesized by non‐continuous sinusoidal endothelial cells and highly dendritic perivascular macrophages in normal human organs, is the most specific macrophage differentiation marker. MS‐1 high molecular weight protein is selectively expressed by cutaneous non‐Langerhans cell histiocytoses, and proves to be a valuable diagnostic tool for these diseases. MS‐1 high molecular weight protein is not found in Langerhans cell histiocytosis cells, epithelioid cells in sarcoidosis, and palisading histio‐cytes in granuloma annulare. MS‐1+ macrophages may be found intermingled in cellular type dermatofibroma and in foreign body granulomas; they differ from MS‐1+ non‐Langerhans cell histiocytosis cells by their highly dendritic morphology, and thus rather resemble the MS‐1+ macrophages in normal skin. RM 3/1 antigen shows a similar, but broader expression pattern including non‐Langerhans cell histiocytoses, xanthel‐asmata palpebrarum, foreign body granulomas, granuloma annulare, and cellular type dermatofibroma. Moreover, xanthelasmata palpebrarum para‐digmatically represent a class of macrophage lesions with strong RM 3/1, but little MS‐1 antigen expression. In sarcoidosis, RM 3/1 + macrophages are only found at the very periphery of epithelioid cell granulomas. In contrast, 25F9 antigen is strongly and consistently expressed in epithelioid cells of sarcoidosis, and in foreign body granulomas. In cultured human monocytes/macrophages, RM 3/1 antigen is expressed early on, while MS‐1 high molecular weight protein and 25F9 antigen are late and very late macrophage differentiation antigens, respectively. Expression of RM 3/1 antigen and MS‐1 high molecular weight protein is inducible by glucocorticoid and interleukin‐4, and less so by interleukin‐13 and interlcukin‐10, and combinations thereof, while 25F9 antigen seems to be less influenced by these agents. Interferon‐y (and less so tumor necrosis factor‐ex) inhibit expression of all three antigens in cultured human monocytes/macrophages. In monocytic leukemia cell line THP‐1. RM 3/1 antigen is only induced by a combined treatment of phorbolester and glucocorticoid and peaks at 3‐7 days. In different subclones of the cell line, 25F9 antigen is either inducible by phorbolester alone or by combined treatment with glucocorticoid or it is constitutively expressed and hardly modulated. In contrast, MS‐1 high molecular weight protein cannot be induced in THP‐1 cells by the agents tested. Inhibition of MS‐1 high molecular weight protein, and RM 3/1 and 25F9 antigens by interferon‐y suggests that macrophages characterized by these phenotypic traits must by counted among the alternatively activated macrophage populations. As a result of our study, alternative macrophage activation should be viewed as a multifacetted process resulting in various, partially overlapping, partially complementary macrophage phenolypes both in vivo and in vitro.