Effects of two commercially available immunostimulants on leukocyte function of foals following ex vivo exposure to Rhodococcus equi

The objective of this study was to determine the effect of immunostimulants on neutrophil, macrophage, and lymphocyte function following ex vivo exposure to Rhodococcus equi. Eighteen foals were randomly assigned to one of 3 treatment groups. Treatment consisted of inactivated Propionibacterium acnes (PA), inactivated parapoxvirus ovis (PPVO), or saline (control) administered on days 0 (7 days of age), 2, and 8. Bronchoalveolar lavage (BAL) fluid and blood were collected on days 0 (baseline), 12, 24 and 36. Intracellular replication of R. equi in macrophages, cytokine induction by R. equi-infected macrophages, phagocytic and oxidative burst activity of neutrophils, lymphoproliferative responses, and cytokine induction of proliferating lymphocytes were measured. Neutrophils from foals treated with PPVO had significantly greater ability to phagocytize R. equi and undergo oxidative burst on day 12 and day 24 compared to baseline values. On day 24, foals treated with PPVO had significantly greater phagocytosis and oxidative burst than foals treated with PA. Treatment with PA resulted in significantly less intracellular proliferation of R. equi within monocyte-derived macrophages on day 12 compared to control foals. The ability of R. equi to replicate in BAL macrophages decreased significantly (P = 0.005) with time with lower replication in BAL macrophages of older foals compared to younger foals, regardless of treatment. On day 12, TNF-α induction in monocyte-derived macrophages and IL-12 p40 induction in BAL macrophages infected with R. equi was significantly higher in foals treated with PPVO than in controls. Lymphoproliferative responses and IFN-γ induction were not significantly different between groups.