C4b-Binding Protein and Factor H Attenuate NLRP3 Inflammasome-Mediated Signalling Response during Group A Streptococci Infection in Human Cells.

Introduction: Streptococcus pyogenes (group A streptococcus; GAS) is a pathogen causing over half a million deaths annually worldwide. Human immune cells respond to GAS infection by activating the NLRP3 inflammasome leading to the release of pro-inflammatory cytokines that 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: The inflammasome response was investigated using primary human cells and the strain GAS-AP1. Cytokine responses were evaluated by ELISA. C4BP internalisation was investigated using confocal microscopy. Activation of the NLRP3 inflammasome components was assessed by Western blotting.

Results: Interleukin-1β (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 without affecting 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 gasdermin D N-terminal fragments by interfering with caspase-1 enzymatic activity.

Conclusion: Given that the amount of IL-1β modulates the severity of GAS infection, our results provide new insights into the effect of FH and internalised C4BP to control GAS sensing by inflammasomes.