Immunologic function in horses after non-specific immunostimulant administration

Inactivated Propionibacterium acnes is a biologic response modifier for treatment of non-specific respiratory disease in horses. The objectives of this investigation were to determine alterations in phagocytic activity, phenotypic expression of lymphocyte subpopulations and lymphokine-activated killing cell response in healthy young horses. Samples were collected on day 0, 7 and 14 of the investigation. Blood samples were obtained via jugular venipuncture and pulmonary leukocytes were recovered via bronchoalveolar lavage (BAL). Commercially available P. acnes (Eqstim) was administered intravenously on days 7, 9 and 11 of the investigation. Fever was observed on days 8 and 10, indicating immune reaction. Total peripheral blood white cell count was increased (P < 0.05) on day 14 after P. acnes administration compared to values on days 0 and 7. Total BAL fluid cell count decreased (P < 0.01) on day 14 compared to values on days 0 and 7, which was characterized by a decrease in total lymphocyte (P < 0.01) and macrophage (P < 0.01) counts. The proportion of lymphocytes in BAL fluid decreased (P < 0.005) on day 14 compared to values on days 0 and 7, and the proportion of macrophages increased (P < 0.005) on day 14 compared to values on days 0 and 7. P. acnes administration increased the total (P < 0.05) and proportional (P < 0.05) counts of CD4+ T lymphocytes in peripheral blood. Bronchoalveolar lavage fluid proportion of CD4+ (P < 0.05), CD5+ (P < 0.001) and MHC II (P < 0.05) lymphocytes increased on day 14 after P. acnes administration compared to values on days 0 and 7. Nonopsonized phagocytic activity in peripheral blood increased (P < 0.0005) on day 14 after P. acnes administration compared to values on days 0 and 7. Lymphokine-activated killing cell activity in peripheral blood and BAL fluid leukocytes was enhanced (P < 0.005) on day 14 after P. acnes administration compared to values on days 0 and 7. Serum IgG and IgM concentrations were within laboratory reference values and were not altered by administration of P. acnes. This investigation demonstrated immunostimulant and immunomodulatory properties of P. acnes, characterized by increased CD4+ T lymphocyte expression and LAK activity in peripheral blood and BAL fluid, increased nonopsonized phagocytosis in peripheral blood leukocytes and decreased pulmonary cellularity.