Immunohistochemical Profiling of Canine Intestinal T-Cell Lymphomas.

Human enteropathy-associated T-cell lymphoma (EATL) is classified into 2 distinct subgroups based on their phenotypes (type I and type II). Canine intestinal T-cell lymphoma can be morphologically classified into large and small cell lymphomas (LCL and SCL, respectively). Their association with human EATL or immunohistochemical and biological features has not been well characterized. In this study, the immunohistochemical profiles of 17 cases of LCL and 33 cases of SCL were evaluated with markers used for human EATL classification. Morphologically, LCL was characterized by sheet-like proliferation of large to moderately sized neoplastic lymphocytes, with scant clear cytoplasm and pleomorphic, irregularly shaped nuclei containing distinctive nucleoli and scattered chromatin. In contrast, SCL was characterized by the proliferation of monomorphic small neoplastic lymphocytes, accompanied by infiltration of nonneoplastic plasma cells. Interestingly, 8 cases demonstrated mixed LCL and SCL morphologies. Granular cytoplasmic expression of granzyme B was observed in most LCL and SCL cases. Membranous expression of CD56 was demonstrated in only 2 of 17 LCL and 0 of 33 SCL cases. Coexpression of CD20 by neoplastic T cells was observed in more SCL cases (16/33; 48%) than LCL cases (1/17; 6%). The CD56-positive cells in 2 cases were negative for CD20. Although canine LCL shares common features with human EATL type I, canine SCL cells and human EATL type II differ in their immunophenotype. Canine intestinal T-cell lymphoma had a homogeneous immunophenotype regardless of cell morphology. The findings of this study may indicate large cell transformation of SCL rather than 2 distinct entities.