Pneumolysin-Dependent Calpain Activation and Interleukin-1α Secretion in Macrophages Infected with Streptococcus pneumoniae.

Pneumolysin (PLY), a major virulence factor of , is a pore-forming cytolysin that modulates host innate responses contributing to host defense against and pathogenesis of pneumococcal infections. Interleukin-1α (IL-1α) has been shown to be involved in tissue damage in a pneumococcal pneumonia model; however, the mechanism by which this cytokine is produced during infection remains unclear. In this study, we examined the role of PLY in IL-1α production. Although the strains induced similar levels of pro-IL-1α expression, wild-type D39, but not a deletion mutant of the gene (Δ), induced the secretion of mature IL-1α from host macrophages, suggesting that PLY is critical for the maturation and secretion of IL-1α during infection. Further experiments with calcium chelators and calpain inhibitors indicated that extracellular calcium ions and calpains (calcium-dependent proteases) facilitated the maturation and secretion of IL-1α from D39-infected macrophages. Moreover, we found that PLY plays a critical role in calcium influx and calpain activation, as elevated intracellular calcium levels and the degradation of the calpain substrate α-fodrin were detected in macrophages infected with D39 but not the Δ strain. These results suggested that PLY induces the influx of calcium in -infected macrophages, followed by calpain activation and subsequent IL-1α maturation and secretion.