CANINE CUTANEOUS EPITHELIOTROPIC LYMPHOMA (MYCOSIS-FUNGOIDES) IS A PROLIFERATIVE DISORDER OF CD8(-CELLS() T)

Canine epitheliotropic lymphoma (mycosis fungoides [MF]) is a spontane ous neoplasm of skin and mucous membranes that occurs in old dogs (mea n age 11 years) and has no breed predilection. The lesions evolve from a patch-plaque stage with prominent epitheliotropism into a tumor sta ge in which distant metastasis is observed Unlike human MI;, epithelio tropism of the lymphoid infiltrate is still prominent in tumor stage l esions. Tropism of the lymphoid infiltrate for adnexal structures, esp ecially hair follicles and aprocrine sweat glands, was marked in all c linical stages of canine MF. Twenty-three cases of MF were subjected t o extensive immunophenotypic analysis in which reagents specific for c anine leukocyte antigens and fresh frozen tissue sections of the canin e lesions were used. Canine MF proved to be a T cell lymphoma in which the epitheliotropic lymphocytes consistently expressed CD3 (22 cases) and CD8 (13 cases); CD3(+)CD4(-)CD8(-) lymphocytes predominated in th e remaining 4 cases. In this regard, canine MF clearly differed from h uman MF in which a CD4 immunophenotype predominates in the T cell infi ltrate. Lack of expression of CD45RA by epitheliotropic T cells and in tense expression of a beta 1 integrin (VLA-4-like) suggested that T ce lls in canine MF belonged to the memory subpopulation, as has been sug gested for T cells in human MF. Pan-T cell antigen loss or discordant expression also proved useful as phenotypic indicators of neoplasia in canine ML. Loss of CD5 was observed in epitheliotropic T cells in 63% of cases. Discordance of neoplastic T cell Thy-1 expression was frequ ently observed between epithelial and dermal or submucosal compartment s. We conclude that canine MI;still represents a useful spontaneous an imal disease model of human cutaneous T cell lymphoma, despite the imm unophenotypic differences, which may reflect operational differences b etween human and canine skin-associated lymphoid tissue.