Background: Group A Streptococcus (GAS, Streptococcus pyogenes) commonly infects the skin and can cause severe infections such as necrotizing fasciitis. Keratinocytes are one of the first cells to contact GAS and are poised to initiate early inflammatory responses. The proinflammatory cytokine IL-18 is typically activated intracellularly by the inflammasome protease caspase-1, but keratinocytes constitutively secrete the inert form (pro-IL-18) which lacks activity unless processed by an extracellular protease.
Methods: Cytokine arrays were used to compare the secretion of IL-18 and other proinflammatory cytokines by common cell lines and primary human keratinocytes. Activation of IL-18 was examined during infection with wild type and knockout strains of GAS and other pathogenic and commensal skin microbiota using reporter cells and western blots. Activation was visualized in-vitro with recombinant forms of pro-IL-18 and activating proteases. Whole human blood and keratinocyte/PBMC co-cultures were used to model the ability of IL-18 to promote IFN-γ production by T cells.
Results: Extracellular pro-IL-18 generated by keratinocytes is directly processed into a mature active form by the GAS protease SpeB. This mechanism contributes to the proinflammatory response against GAS, resulting in T cell activation and the secretion of IFN-γ that restricts GAS growth. Most other major bacterial pathogens and microbiota of this skin did not have significant IL-18-maturing ability.
Conclusions: These results suggest keratinocyte-secreted pro-IL-18 is a sentinel that sounds an early alarm to foreign proteases. It is highly sensitive to GAS, yet tolerant to species typically resident on the healthy skin, suggesting a mechanism for pathogen discrimination.