Indolent cutaneous T-cell lymphoma presenting as cutaneous lymphocytosis in dogs

Cutaneous lymphoproliferative diseasesencompass a spectrum of lesions, ranging from self-limiting, reactive infiltrates to high-grade lymphomas. In humans, cutaneous lymphocytosis(CL) refers to self-limiting or slowly progressive monomorphic lymphocytic infiltratesof mostly unknown cause. It morphologically mimics cutaneous lymphoma. CL in cats also is a slowly progressive disease. Immunophenotypingand clonality testing of feline CL support an indolent lymphoma for the majority of cases studied. This study reports CL in dogs. Erythematous, scaly and alopecic macules, patches or plaques were present in eight dogs. Breedpredilection was not observed; six of eight dogs were females; and ages ranged from 5 to 14 years. Diffuse monomorphicnon-epitheliotropic infiltrates of CD3+ (eight of eight), CD45− (four of eight) or CD45+/− (four of eight) and CD45RA− (seven of eight) T lymphocytes were present in the superficial and mid-dermis. Further immunophenotypingof five cases revealed TCR-γδ+ T cells (one of five) or TCR-αβ+ (four of five) T cells. TCR-αβ+ populations were either CD8+ (two of four) or CD4−CD8− (2/4). Clonality testing found clonal (seven of eight) or pseudoclonal (one of eight) rearrangement of the TCR-gamma locus of the lesional T cells. Prednisone, prednisolone and methylprednisolone acetatewere the most commonly administered drugs. The lesions remained stable for long periods up to 6 years. Five dogs were euthanized due to progression of the skin lesions (three of five), peripheral lymphadenopathy of unknown origin (one of five) or high-grade lymphoma(one of five). One dog was lostfor follow-upand two dogs are still alive (17 and 9 months after diagnosis). Canine CL is best considered an initially indolent lymphoma, with slow progression and a potential for progression to high-grade lymphoma.