Kingella kingae cytotoxin RtxA induces an innate immune response in oral epithelial cells

The genus <i>Kingella</i> comprises four common species: the commensals <i>K. oralis</i> and <i>K. denitrificans</i>, the emerging paediatric pathogen <i>K. kingae</i> and the novel species <i>K. negevensis</i>. Improved diagnostic methods have led to the recognition of <i>K. kingae</i> as a major cause of septic arthritis in young children. The key virulence factor responsible for the pathogenesis of <i>K. kingae</i> is its cytotoxin RtxA, which is thought to facilitate host invasion. After binding to target cells, RtxA inserts into the host cell membrane and forms ion-conducting membrane pores that disrupt normal cell physiology and ultimately lead to cell death. In this study, we analysed the pro-inflammatory response of oral epithelial cells to a clinical isolate of <i>K. kingae</i>, its isogenic RtxA-deficient mutant and the commensals <i>K. oralis</i> and <i>K. denitrificans</i>, which do not produce RtxA. The results show that infection of cells with <i>K. kingae</i>, but not with the RtxA-deficient mutant and the commensal species, leads to increased expression and secretion of certain pro-inflammatory cytokines and chemokines. Furthermore, the RtxA-producing <i>K. kingae</i>, but not the RtxA-deficient mutant, upregulated the expression of <i>DEFB4A</i> and <i>SLPI</i> genes encoding antimicrobial peptides. These findings demonstrate that the RtxA toxin induces an innate immune response in oral epithelial cells.