S-Carboxymethylcysteine inhibits adherence of Streptococcus pneumoniae to human alveolar epithelial cells

Streptococcus pneumoniae is a major pathogen of respiratory infections that utilizes platelet-activating factor receptor (PAFR) for firm adherence to host cells. The mucolytic agent S-carboxymethylcysteine (S-CMC) has been shown to exert inhibitory effects against infection by several respiratory pathogens including S. pneumoniae in vitro and in vivo. Moreover, clinical studies have implicated the benefits of S-CMC in preventing exacerbation of chronic obstructive pulmonary disease, which is considered to be related to respiratory infections. In this study, to assess whether the potency of S-CMC is attributable to inhibition of pneumococcal adherence to host cells, an alveolar epithelial cell line stimulated with interleukin-1α was used as a model of inflamed epithelial cells. Despite upregulation of PAFR by inflammatory activation, treatment with S-CMC efficiently inhibited pneumococcal adherence to host epithelial cells. In order to gain insight into the inhibitory mechanism, the effects of S-CMC on PAFR expression were also investigated. Following treatment with S-CMC, PAFR expression was reduced at both mRNA and post-transcriptional levels. Interestingly, S-CMC was also effective in inhibiting pneumococcal adherence to cells transfected with PAFR small interfering RNAs. These results indicate S-CMC as a probable inhibitor targeting numerous epithelial receptors that interact with S. pneumoniae.