Inhibition of human monocyte respiratory burst, degranulation, phospholipid methylation and bactericidal activity by pneumolysin.

The interaction between the pneumococcal toxin pneumolysin and human monocytes was examined. At non-cytotoxic concentrations (0.5-2.5 HU/10(6) cells) pneumolysin depressed the oxygen-dependent respiratory burst in monocytes, induced by opsonized zymosan or phorbol myristate acetate (PMA). This included depressed hexose-monophosphate shunt activity and hydrogen peroxide production. The toxin also depressed the ability of monocytes to degranulate (measured by release of lysozyme) in response to the above stimuli. Phospholipid transmethylation was also markedly decreased by pretreating monocytes with pneumolysin. These effects on monocyte functions were accompanied by a decreased ability of pneumolysin-treated monocytes to kill Streptococcus pneumoniae, the organism that produces the toxin. Cholesterol, which inhibits the haemolytic activity of the toxin, was shown to abrogate the effects of pneumolysin on monocytes.