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

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.