MURAMIDASE - A USEFUL MONOCYTE/MACROPHAGE IMMUNOCYTOCHEMICAL MARKER IN SWINE, OF SPECIAL INTEREST IN EXPERIMENTAL CARDIOVASCULAR-DISEASE

The reliability of a rabbit polyclonal antibody against muramidase to identify monocytes/macrophages in swine was evaluated by immunostaining of cell smears and formaldehyde-fixed, paraffin-embedded tissue sections. Blood in tissue sections, cell smears (peripheral blood, buffy coat, and isolated mononuclear cells), and cultured mononuclear cells (adherent monocytes) contained positively stained cells with a morphology and in a number corresponding to that expected for a monocyte marker. Polymorphonuclear leukocytes (PMN), lymphocytes, and platelets were negative. In normal organs and tissues, mesenchymal cells with a distribution similar to that expected for macrophages were found to stain positively for muramidase. In pathologic tissues, positively stained inflammatory cells were identified in wounds, infected lungs, recently infarcted myocardium, and acute (variable numbers), organizing (often many), and healed (usually few) arterial thrombi. Enzymatic unmasking of antigenic determinants by trypsinization was necessary to achieve strong and consistent staining of monocytes/macrophages in tissue sections. A variety of epithelial cells of no differential diagnostic significance for monocyte/macrophage identification (e.g., renal proximal tubular cells) also stained positive for muramidase. The staining pattern of muramidase in swine corresponds to that described in humans, in whom muramidase has been shown to be a valuable marker of monocytes/macrophages. Swine PMN were, however, not stained or only weakly stained, whereas human PMN reportedly are strongly positive. As in humans, swine cardiac myocytes, smooth muscle cells, endothelial cells, lymphocytes, and platelets were consistently negative. This antibody against muramidase is a useful immunohistochemical marker for swine monocytes/macrophages in formaldehyde-fixed, paraffin-embedded tissues.