Interaction of polymorphonuclear neutrophils with Escherichia coli. Effect of enterotoxin on phagocytosis, killing, chemotaxis, and cyclic AMP

Abstract

Enterotoxigenic Escherichia coli are associated with noninflammatory diarrhea and stimulate adenylate cyclase activity of mammalian cells, thereby increasing intracellular cyclic adenosine 3',5'-monophosphate (cyclic AMP). Increased concentrations of cyclic AMP in polymorphonuclear neutrophils (PMN) inhibit phagocytosis, candidacidal activity, granule discharge, and chemotactic responsiveness. We examined the effect of enterotoxin on the interaction of human PMN with E. coli. Enterotoxigenic and nonenterotoxigenic strains, including serotypes of E. coli identical except for the presence or absence of the plasmid coding for enterotoxin production, were utilized. Enterotoxigenic and nonenterotoxigenic E. coli, tumbled with PMN, were phagocytized and killed (>97%) equally well, and these strains stimulated PMN hexose monophosphate shunt activity equivalently.However, a chemotaxis assay under agarose demonstrated that filtrates of 10 enterotoxigenic strains were less chemotactic for PMN by 15+/-2% total migration or 46+/-1% directed migration, when compared with 6 non-enterotoxigenic strains (P < 0.001). Inactivation of the enterotoxin by heat (65 degrees C for 30 min) or antibodies formed to E. coli enterotoxin eliminated the inhibitory effect of the enterotoxic filtrates for PMN chemotaxis. Addition of purified E. coli enterotoxin directly to the PMN decreased chemotaxis to E. coli filtrates by 32+/-2% (P < 0.001). These data suggest that the effect was due to the heat-labile enterotoxin. The phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine (0.1 mM), which potentiates effects due to an increase in intracellular cyclic AMP, further decreased total PMN migration (random plus directed) toward enterotoxic filtrates to 46% of that to nonenterotoxic filtrates (P < 0.001). Addition of cholera toxin (1 mug/ml), which is similar to E. coli enterotoxin, to the PMN inhibited total migration toward nonenterotoxic filtrates by 16+/-2% (P < 0.001). Exogenous dibutyryl cyclic AMP (2 mM) inhibited total PMN migration toward E. coli filtrates by 32% (P < 0.001). PMN intracellular cyclic AMP levels increased by 220% after 2 h of incubation with purified E. coli enterotoxin. The decreased chemotactic attractiveness of enterotoxic E. coli filtrates appears to be related to the ability of enterotoxin to increase cyclic AMP in PMN. Enterotoxin production by E. coli may be advantageous to the microbe by decreasing its chemotactic appeal for PMN.