SUMMARY The cationic polysaccharide chitosan is an attractive candidate adjuvant capable of driving potent cell-mediated immunity, but the mechanism by which it acts is not clear. We show that chitosan promotes dendritic cell maturation by inducing type I interferons (IFNs) and enhances antigen-specific T helper 1 (Th1) responses in a type I IFN receptor-dependent manner. The induction of type I IFNs, IFN-stimulated genes and dendritic cell maturation by chitosan required the cytoplasmic DNA sensor cGAS and STING, implicating this pathway in dendritic cell activation. Additionally, this process was dependent on mitochondrial reactive oxygen species and the presence of cytoplasmic DNA. Chitosan-mediated enhancement of antigen specific Th1 and immunoglobulin G2c responses following vaccination was dependent on both cGAS and STING. These findings demonstrate that a cationic polymer can engage the STING-cGAS pathway to trigger innate and adaptive immune responses.

Increased glycolytic metabolism recently emerged as an essential process driving host defense against Mycobacterium tuberculosis (Mtb), but little is known about how this process is regulated during infection. Here, we observe repression of host glycolysis in Mtb-infected macrophages, which is dependent on sustained upregulation of anti-inflammatory microRNA-21 (miR-21) by proliferating mycobacteria. The dampening of glycolysis by miR-21 is mediated through targeting of phosphofructokinase muscle (PFK-M) isoform at the committed step of glycolysis, which facilitates bacterial growth by limiting pro-inflammatory mediators, chiefly interleukin-1b (IL-1b). Unlike other glycolytic genes, PFK-M expression and activity is repressed during Mtb infection through miR-21-mediated regulation, while other less-active isoenzymes dominate. Notably, interferon-g (IFN-g), which drives Mtb host defense, inhibits miR-21 expression, forcing an isoenzyme switch in the PFK complex, augmenting PFK-M expression and macrophage glycolysis. These findings place the targeting of PFK-M by miR-21 as a key node controlling macrophage immunometabolic function.

Assembly of inflammasomes after infection or injury leads to the release of interleukin-1b (IL-1b) and to pyroptosis. After inflammasome activation, cells either pyroptose or enter a hyperactivated state defined by IL-1b secretion without cell death, but what controls these different outcomes is unknown. Here, we show that removal of the Toll-IL-1R protein SARM from macrophages uncouples inflammasome-dependent cytokine release and pyroptosis, whereby cells displayed increased IL-1b production but reduced pyroptosis. Correspondingly, increasing SARM in cells caused less IL-1b release and more pyroptosis. SARM suppressed IL-1b by directly restraining the NLRP3 inflammasome and, hence, caspase-1 activation. Consistent with a role for SARM in pyroptosis, Sarm1 À/À mice were protected from lipopolysaccharide (LPS)-stimulated sepsis. Pyroptosisinducing, but not hyperactivating, NLRP3 stimulants caused SARM-dependent mitochondrial depolarization. Thus, SARM-dependent mitochondrial depolarization distinguishes NLRP3 activators that cause pyroptosis from those that do not, and SARM modulation represents a cell-intrinsic mechanism to regulate cell fate after inflammasome activation.

These data imply a new function for pulmonary CD103(+) DCs in mucosal activation of iNKT cells and establish a critical role for both IFN-γ and IL-17 signalling pathways in mediating the innate immune response to S. pneumoniae.