Legionella pneumophila and related organisms induce the generation of procoagulant activity by peripheral mononuclear cells in vitro.

We investigated the in vitro effect of various members of the genus Legionella on human peripheral mononuclear cells. All the strains tested induced the generation of strong procoagulant activity (tissue factor) when incubated for a prolonged period of time with pure mononuclear cell suspensions. This effect was dependent on the number of bacteria. The production of mononuclear cell procoagulant activity was also observed after the addition of bacteria to citrated whole blood. Escherichia coli 0111:B4 showed similar activity, but Staphylococcus aureus was much less effective. These findings suggest that the presence of endotoxin-like substance(s) in the outer cell wall of Legionellae could contribute to the stimulation of mononuclear cells. The production of tissue factor, a potent trigger of blood clotting, by these cells could help us to understand the mechanism(s) responsible for the activation of intravascular coagulation associated with severe legionellosis.