Interaction between Salmonella typhimurium and phagocytic cells in pigs - Phagocytosis, oxidative burst and killing in polymorphonuclear leukocytes and monocytes

Interactions between Salmonella typhimurium and peripheral blood leucocytes from healthy, Salmonella-free pigs were investigated in vitro. Both granul ocytes and monocytes phagocytized FITC-labelled heat-killed Salmonella bact eria as shown by flow cytometry. Phagocytosis in whole blood and isolated l eucocytes was measured as acquired fluorescence in the leukocytes and was b oth time and dose related. Living, serum-opsonized Salmonella bacteria indu ced a dose-dependent oxidative burst in PMNs and monocytes as measured by l uminol-enhanced chemiluminescence (LC). When opsonized in normal serum the Salmonella bacteria, in the range of 2-5 x 10(7) cfu, induced a LC response in monocytes comparable to the level of responses induced by phorbol myris tate acetate (PMA) and opsonized zymosan, and the Salmonella-induced respon se was only marginally reduced by superoxide dismutase (SOD). Intracellular killing of Salmonella by monocytes was assessed from plate colony counts o f lysed monocytes and showed that Salmonella typhimurium was able to surviv e and proliferate in adherent monocytes in vitro despite a reduction in int racellular cfu during the first hour's incubation in cells from some pigs. Experiments with the exhaustion of oxidative burst in non-adherent monocyte s were performed by prestimulation with PMA, heat-killed Salmonella or buff er. Prestimulation with PMA led to a strong reduction in oxidative burst in duced by living opsonized Salmonella bacteria, whereas prestimulation with heat-killed bacteria gave rise to an enhanced response. In these experiment s intracellular killing of the added living Salmonella gave variable result s, in that monocytes from two out of three pigs showed no essential change in intracellular bactericidal activity, but with cells from one pig a less pronounced bactericidal activity was found after prestimulation with PMA. ( C) 1999 Elsevier Science B.V. All rights reserved.