Killing of Edwardsiella ictaluri by macrophages from channel catfish immune and susceptible to enteric septicemia of catfish

The role of peritoneal macrophages in immunity to enteric septicemia of catfish (ESC) after infection with live Edwardsiella ictaluri was investigated. Channel catfish macrophage-mediated bacteriocidal activity was dependent on the macrophage:bacteria ratio. Ratios of 1:1 to 1:12 exhibited significant differences (P ≤ 0.05) in killing between macrophages from immune fish when compared to killing by macrophages from susceptible fish at 2.5 h. At 5 h, macrophages from immune fish were capable of effective killing (83.3%) at a 1:24 effector:target ratio, whereas macrophages from susceptible fish killed significantly (P ≤ 0.05) less (56.9%). Macrophage bacteriocidal activity was significantly greater (P ≤ 0.05) in macrophages from individual immune fish (93.4%) compared to macrophages from individual susceptible fish (85.4%). The kinetics of macrophage killing showed a linear increase in bacteriocidal activity from 1 to 3 h. Opsonization with immune serum enabled macrophages from immune fish to kill bacteria more effectively (93.8 vs. 75.9%) at 2.5 h. Opsonization of E. ictaluri with immune serum significantly suppressed the killing ability of macrophages from susceptible fish (46.2%) at 2.5 h. The results suggest that macrophages from fish immune to ESC had a greater capacity to kill E. ictaluri than macrophages from susceptible fish especially when E. ictaluri were opsonized with anti-E. ictaluri antibody.