Studies on pneumolysin, the thiol-activated toxin of Streptococcus pneumoniae.

In addition to being haemolytic, pneumolysin, the thiol- activated toxin of Streptococcus pneumoniae, modulates a number of cellular functions at sublytic concentration. The effect of sublytic concentrations of pneumolysin on neutrophil respiratory burst and degranulation was studied, as was the effect of the toxin on the viability of neutrophils at higher toxin concentrations, and pneumolysin -neutrophil binding at both 4 and 37°C. It was demonstrated that neither inhibition of the respiratory burst, nor degranulation, was due to the cytolytic activity of pneumolysin. The use of the modified pneumolysins Cys428>Ala, Cys428>Ser, and Cys428>Gly in these assays revealed that the activity of pneumolysin at cytotoxic concentrations correlated with its haemolytic activity. It was proposed was that the cytotoxic activity of the toxin was due the formation of small pores in the plasma membrane which allowed an influx of calcium into the cell. Parallels were drawn with the subcytocidal effects of other bacterial toxins. Despite inhibiting the respiratory burst of neutrophils, pneumolysin was not found to inhibit their bactericidal activity towards pneumococci. In a murine model of pneumococcal pneumonia, immunisation with pneumolysin was found to extend survival and slow the increase in pneumococcal numbers in the lungs, liver, spleen, brain and blood. Incubation of monocytes with pneumolysin triggered the release of interleukin-1 but not tumour necrosis factor. The role of pneumolysin in the pathogenesis of pneumococcal disease was discussed. Of the 2 regions of homology between pneumolysin and the acute phase protein C-reactive protein, one is known to be important for complement activation by the toxin. Site-directed mutagenesis of the pneumolysin gene showed that the other region of homology was not important for complement activation or haemolysis.