Immunohistochemical and histochemical stains are useful adjunct techniques in the diagnosis of canine cutaneous round cell
tumors, which can appear histologically similar. We applied a panel of
monoclonal antibodies (recognizing
tryptase,
chymase,
serotonin for mast cells; CD1a, CD18, MHC class II for histiocytes; CD3 for T lymphocytes; CD79a for B lymphocytes and plasma cells) and one histochemical
stain (
naphthol AS-D chloroacetate for
chymase activity) to
formalin-fixed,
paraffin-embedded sections of canine cutaneous mast cell
tumors,
histiocytomas,
lymphosarcomas,
plasmacytomas, and unidentified round cell
tumors. Of 21
tumors with a histologic diagnosis of mast cell
tumor, 7/7 (100%) grade I, 6/7 (85.7%) grade II, and 3/7 (42.9%) grade III
tumors were diagnosed as mast cell
tumors based on positive staining for
tryptase antigen and
chymase activity. Mast cells were positive for both
tryptase antigen and
chymase activity, indicating equal efficacy of
tryptase immunohistochemistry and
chymase histochemistry.
Chymase was detected immunohistochemically in both
tumor and nontumor cells, while
serotonin was not detected in most mast cell
tumors, and thus, neither was useful in the diagnosis of mast cell
tumors. Immunohistochemistry to detect CD18 and MHC class II was equally effective in staining
histiocytomas, although
lymphosarcoma must be ruled out through the use of CD3 and CD79a immunohistochemistry. Immunohistochemistry using three different
monoclonal antibodies to human CD1a showed no cross-reactivity in canine
histiocytomas and was not useful. A final diagnosis was obtained for 4/5 (80%) of the unidentified
tumors, indicating the usefulness of multiple stains in poorly differentiated round cell
tumors.