Morphology, immunohistochemistry and TUNEL staining of peritoneal inflammatory cells of cats with experimentally-induced wet FIP. A Omentum of an experimentally infected cat with FIPV is markedly expanded by inflammatory cells and covered with a thick sheet of peritoneal inflammation (rectangle area), ×40, hematoxylin and eosin stain. B ×400 magnification of the area outlined in A demonstrates sheets of inflammatory cells comprised predominantly of large vacuolated round cells (macrophages) and neutrophils. There are lesser numbers of scattered small lymphocytes, hematoxylin and eosin stain. C Most of the mononuclear cells stained positive for the CD18 cell surface marker, ×400, hematoxylin counterstain. D Staining for the CD3 T cell surface marker was limited to small numbers of small mononuclear cells, ×400, hematoxylin counterstain. E Staining for the CD79α B cell surface marker was limited to a low percentage of small mononuclear cells, ×400, hematoxylin counterstain. F Section of formalin-fixed, paraffin embedded and TUNEL stained inflamed omentum (*), ×40 magnification. Positive staining cells were concentrated in the inflamed surface. G High magnification of region outlined in the rectangle in F. TUNEL staining was pronounced in both cytoplasm and nuclei of large multi-vacuolated macrophages, while only a small proportion of more normal appearing monocyte/macrophages, lymphocytes and neutrophils stained positive in nuclei, ×400.