The immune control of chronic equine infectious anaemia (EIA) lentiviral infection was investigated by specifically depleting CD5 T lymphocytes with monoclonal antibody (MAb) or by immunosuppression with corticosteroids. MAb was given at 25 to 50 mg/day intravenously for 11 days. Murine IgG1 anti-equine CD2 MAb ( = 2 horses) or IgG1 ( = 2) and IgG2a control MAb ( = 2 normal; 2 EIA-infected) did not deplete CD2 T lymphocytes in horses. Horses given murine IgG2a anti-CD5 MAb HB19A ( = 4 normal; 5 EIA-infected) had depletion of peripheral blood CD5 T lymphocytes during treatment. These horses, however, maintained a residual population of CD2 T lymphocytes [15 (±3)% of pretreatment numbers] that did not express CD5 but expressed either CD4 or CD8. These antigenically modulated CD5 T lymphocytes responded normally to intradermal inoculation with phytohaemagglutinin and to allogeneic leukocyte stimulation in one-way mixed lymphocyte reactions. EIA virus-infected horses ( = 5) did not develop recrudescent viraemia or disease following CD5 T lymphocyte depletion. Immunosuppression of EIA virus-infected horses with corticosteroids (1 mg/kg body weight/day, intravenously for 9 days) resulted in detectable recrudescent EIA viraemia in 6/11 horses (55%) and recrudescent disease in 9/11 horses (82%). Normal horses ( = 3) treated with corticosteroids developed no clinical disease. These results demonstrate that the use of murine IgG2a MAbs to appropriate equine lymphocyte antigens will facilitate investigation of the role of T lymphocyte subpopulations in the control of EIA or other important equine diseases.


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