Interactions of virulent and avirulent Legionella pneumophila with human polymorphonuclear leukocytes

Isogenic virulent/avirulent variants of Legionella pneumophila (Lp) were examined in vitro for their effect on PMN function as determined by measurement of peak chemiluminescence and superoxide anion production. When exposed to virulent/avirulent strains of Lp, PMN function was less than that seen following exposure to E. coli, the reduction of the oxidative burst being more pronounced with virulent Lp strains in both assays. To determine whether differences in reduced PMN function correlate with differences in complement component C3 binding by virulent and avirulent Lp, four pairs of organisms were incubated in normal human serum and examined by quantitative immunofluorescence. All strains of Lp bound less C3 than E. coli, although each to different degrees. Avirulent isogenic strains bound more C3 than their virulent counterparts with one exception. Depressed PMN function following exposure to Lp may be due to reduced C3 binding, although other virulence-associated factors may be involved.