C4b-Binding Protein and Factor H Inhibit Inflammasome Activation during Group A Streptococci Infection in Human Cells.

Abstract

Introduction. Streptococcus pyogenes (Group A Streptococcus; GAS) is a pathogen that causes over half a million deaths annually worldwide. Human immune cells respond to GAS infection by activating the NLRP3 inflammasome that leads to pro-inflammatory cytokines release which acts to control infection. We investigated the role of C4b-binding protein (C4BP) and Factor H (FH) in the inflammasome response to GAS, as they are recruited by GAS to prevent complement deposition and limit phagocytosis. Methods. Inflammasome response was investigated using isolated primary human cells and the GAS-AP1 strain. Cytokine responses were evaluated by ELISA. C4BP internalisation was investigated using confocal microscopy. Western blotting was used to evaluate the activation of NLRP3 inflammasome components. Results. IL-1β release, induced by GAS-AP1, was inhibited by FH which interferes with priming of human cells. In contrast, C4BP restricted the IL-1β response with no effect on cell priming. C4BP was engulfed by cells together with bacteria and excluded from low-pH vesicles, but localised within the cytosol and near the ASC speck inflammasome complex. C4BP did not inhibit either the inflammasome complex assembly or caspase-1 activation. However, C4BP limited the cleavage of gasderminD N-terminal fragments by interfering with caspase-1 enzymatic activity. Conclusion. Our results provide new insights on the effect of FH and internalised C4BP to control GAS sensing by inflammasomes.