Critical Involvement Of Pneumolysin In Production Of Interleukin-1 Alpha And Caspase-1-Dependent Cytokines In Infection With Streptococcus Pneumoniae In Vitro: A Novel Function Of Pneumolysin In Caspase-1 Activation

moniae. This protein toxin has also been shown to activate the Toll-like receptor 4 (TLR4) signaling pathway. In this study, a mutant S. pneumoniae strain deficient in pneumolysin (Delta ply) and a recombinant pneumolysin protein (rPLY) were constructed. Upon infection of macrophages in vitro, the ability to induce the production of interieukin-1 alpha (IL-1 alpha), IL-1 beta, and IL-18 was severely impaired in the Delta ply mutant, whereas there was no marked difference in the induction of tumor necrosis factor alpha (TNF-alpha) and IL-12p40 between the wild type and the Delta ply mutant of S. pneumoniae. When macrophages were stimulated with rPLY, the production of IL-1a, IL-1 beta, and IL-18 was strongly induced in a TLR4-dependent manner, whereas lipopolysaccharide, a canonical TLR4 agonist, hardly induced these cytokines. In contrast, lipopolysaccharide was more potent than rPLY in inducing the production of TNF-alpha, IL-6, and IL-12p40, the cytokines requiring no caspase activation. Activation of caspase-1 was observed in macrophages stimulated with rPLY but not in those stimulated with lipopolysaccharide, and the level of activation was higher in macrophages infected with wild-type S. pneumoniae than in those infected with the Delta ply mutant. These results clearly indicate that pneumolysin plays a key role in the host response to S. pneumoniae, particularly in the induction of caspase-1-dependent cytokines.