Canine cutaneous epitheliotropic lymphoma (mycosis fungoides) is a proliferative disorder of CD8+ T cells.

Canine epitheliotropic lymphoma (mycosis fungoides [MF]) is a spontaneous neoplasm of skin and mucous membranes that occurs in old dogs (mean age 11 years) and has no breed predilection. The lesions evolve from a patch-plaque stage with prominent epitheliotropism into a tumor stage in which distant metastasis is observed. Unlike human MF, epitheliotropism of the lymphoid infiltrate is still prominent in tumor stage lesions. Tropism of the lymphoid infiltrate for adnexal structures, especially hair follicles and apocrine sweat glands, was marked in all clinical stages of canine MF. Twenty-three cases of MF were subjected to extensive immunophenotypic analysis in which reagents specific for canine leukocyte antigens and fresh frozen tissue sections of the canine lesions were used. Canine MF proved to be a T cell lymphoma in which the epitheliotropic lymphocytes consistently expressed CD3 (22 cases) and CD8 (19 cases); CD3+CD4-CD8- lymphocytes predominated in the remaining 4 cases. In this regard, canine MF clearly differed from human MF in which a CD4 immunophenotype predominates in the T cell infiltrate. Lack of expression of CD45RA by epitheliotropic T cells and intense expression of a beta 1 integrin (VLA-4-like) suggested that T cells in canine MF belonged to the memory subpopulation, as has been suggested for T cells in human MF. Pan-T cell antigen loss or discordant expression also proved useful as phenotypic indicators of neoplasia in canine MF. Loss of CD5 was observed in epitheliotropic T cells in 63% of cases. Discordance of neoplastic T cell Thy-1 expression was frequently observed between epithelial and dermal or submucosal compartments. We conclude that canine MF still represents a useful spontaneous animal disease model of human cutaneous T cell lymphoma, despite the immunophenotypic differences, which may reflect operational differences between human and canine skin-associated lymphoid tissue.