Clinical, immunophenotypic and functional characterisation of T-cell leukaemia in six horses

REASON FOR PERFORMING STUDY: Lymphoid leukaemia (LL) is rare in equids. In man, immunophenotypic classification identifies distinct leukaemic types with different treatment strategies. Improved understanding and classification of equine LL may allow similar advances. OBJECTIVES: To document the clinical, immunophenotypic and functional characteristics in 6 cases of equine LL of T-cell origin. METHODS: The clinical records and pathological findings from 6 cases of equine LL were analysed. Immunohistochemistry to identify T or B lymphocytes was performed on paraffin embedded tissues in 4 cases. Peripheral blood mononuclear cells (PBMC) were phenotyped for expression of CD4, CD8, MHC class I and II and B-cell antigens in 4 cases using monoclonal antibodies (mAbs) and flow cytometry. Neoplastic lymphocytes from 4 horses were stimulated with mitogens. RESULTS AND CONCLUSIONS: Six horses of various breeds were identified with LL of T-cell origin. The clinical course and presenting signs varied. Neoplastic lymphocytes were identified in peripheral blood samples from all horses and tissue invasion was confirmed at examination post mortem in 4 horses. Immunophenotyping identified a predominance of CD3+ T-cells in lymphoid tissues and CD4+ T-cells in circulating peripheral blood mononuclear cells (PBMC) in the affected horses. Neoplastic lymphocytes from the 4 cases that were tested failed to proliferate in response to mitogens. POTENTIAL RELEVANCE: Characterisation of the clinical, pathological and immunological findings in 6 horses with LL has added to reports of this rare condition, characterised it in greater detail and therefore provides a starting point for further investigations.