DISSOCIATION OF CYTOLYSIS AND MONOKINE RELEASE BY BOVINE MONONUCLEAR PHAGOCYTES INCUBATED WITH PASTEURELLA-HAEMOLYTICA PARTIALLY PURIFIED LEUKOTOXIN AND LIPOPOLYSACCHARIDE

The bovine respiratory pathogen Pasteurella haemolytica secretes an ex otoxin that is specific for ruminant leukocytes (leukotoxin). Previous studies have shown that subcytolytic concentrations of the leukotoxin stimulate bovine neutrophils to undergo a respiratory burst and degra nulate. Relatively little is known about the stimulatory effects of th e leukotoxin on bovine mononuclear phagocytes. In this study, we compa red the relative cytolytic effects of partially purified leukotoxin on bovine peripheral blood monocytes and alveolar macrophages, We found monocytes to be approximately 8- to 10-fold more sensitive than alveol ar macrophages to the cytolytic effect of leukotoxin. In addition, inc ubation of monocytes and alveolar macrophages with sublethal doses of leukotoxin stimulated release of IL-1 and TNF activities in a dose-dep endent manner. Addition of an antileukotoxin MAb neutralized the cytol ytic effects of leukotoxin, but potentiated TNF release. Heat inactiva tion also blocked the cytolytic activity of LKT, but only slightly red uced its ability to stimulate TNF release. Although the leukotoxin pre parations were estimated to have only small amounts of lipopolysacchar ide (LPS) contamination, as determined by a standard Limulus amebocyte lysate coagulation assay, a chromogenic Limulus assay indicated much greater amounts of LPS were present. Adding equivalent doses of P. hae molytica LPS largely duplicated the monokine release stimulated by leu kotoxin. These results suggest that the stimulatory effects of the P. haemolytica leukotoxin on bovine mononuclear phagocytes may principall y involve LPS, perhaps complexed with leukotoxin.