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CD8+ T cell-mediated cancer clearance is often suppressed by the interaction between inhibitory molecules like PD-1 and PD-L1, an interaction acts like brakes to prevent T cell overreaction under normal conditions but is exploited by tumor cells to escape the immune surveillance. Immune checkpoint inhibitors have revolutionized cancer therapeutics by removing such brakes. Unfortunately, only a minority of cancer patients respond to immunotherapies presumably due to inadequate immunity. Antitumor immunity depends on the activation of the cGAS-STING pathway, as STING-deficient mice fail to stimulate tumor-infiltrating dendritic cells (DCs) to activate CD8+ T cells. STING agonists also enhance natural killer (NK) cells to mediate the clearance of CD8+ T cell-resistant tumors. Therefore STING agonists have been intensively sought after. We previously discovered that manganese (Mn) is indispensable for the host defense against cytosolic dsDNA by activating cGAS-STING. Here we report that Mn is also essential in innate immune sensing of tumors and enhances adaptive immune responses against tumors. Mn-insufficient mice had significantly enhanced tumor growth and metastasis, with greatly reduced tumor-infiltrating CD8+ T cells. Mechanically, Mn2+ promoted DC and macrophage maturation and tumor-specific antigen presentation, augmented CD8+ T cell differentiation, activation and NK cell activation, and increased memory CD8+ T cells. Combining Mn2+ with immune checkpoint inhibition synergistically boosted antitumor efficacies and reduced the anti-PD-1 antibody dosage required in mice. Importantly, a completed phase 1 clinical trial with the combined regimen of Mn2+ and anti-PD-1 antibody showed promising efficacy, exhibiting type I IFN induction, manageable safety and revived responses to immunotherapy in most patients with advanced metastatic solid tumors. We propose that this combination strategy warrants further clinical translation.

Aluminum adjuvants have been used for a century in various vaccines due to its ability to potentiate humoral immunity and safety records since 1920s. Manganese is an essential micronutrient required for diverse biological activities in cells. We previously found that Mn2+ is a strong type I-interferon stimulator activating the cGAS-STING pathway. Herein we report that a colloidal manganese salt (MnJ) is a potent adjuvant to induce both humoral and cellular immune responses, particularly CTL activation. When administrated intranasally, MnJ was also a strong mucosal adjuvant, inducing high levels of IgA antibodies. MnJ strongly promoted dendritic cell maturation and antigen-specific T cell activation. Interestingly, IL-1/-18 induction and release by Mn2+-activated ASC-mediated inflammasomes were not observed. MnJ showed great adjuvant effects to all tested antigens including inactivated viruses, recombinant proteins and peptides by either intramuscular or intranasal immunization. These findings may have implications in developing potent but safe Mn2+-containing vaccines.

Viral infection triggers host defenses through pattern-recognition receptor-mediated cytokine production, inflammasome activation, and apoptosis of the infected cells. Inflammasome-activated caspases are known to cleave cyclic GMP-AMP synthase (cGAS). Here, we found that apoptotic caspases are critically involved in regulating both DNA and RNA virus-triggered host defenses, in which activated caspase-3 cleaved cGAS, MAVS, and IRF3 to prevent cytokine overproduction. Caspase-3 was exclusively required in human cells, whereas caspase-7 was involved only in murine cells to inactivate cGAS, reflecting distinct regulatory mechanisms in different species. Caspase-mediated cGAS cleavage was enhanced in the presence of dsDNA. Alternative MAVS cleavage sites were used to ensure the inactivation of this critical protein. Elevated type I IFNs were detected in caspase-3-deficient cells without any infection. Casp3-/- mice consistently showed increased resistance to viral infection and experimental autoimmune encephalomyelitis. Our results demonstrate that apoptotic caspases control innate immunity and maintain immune homeostasis against viral infection.

Cytosolic dsDNA activates the cyclic GMP-AMP synthase (cGAS)–stimulator of IFN genes (STING) pathway to produce cytokines, including type I IFNs. The roles of many critical proteins, including NEMO, IKKβ, and TBK1, in this pathway are unclear because of the lack of an appropriate system to study. In this article, we report that lower FBS concentrations in culture medium conferred high sensitivities to dsDNA in otherwise unresponsive cells, whereas higher FBS levels abrogated this sensitivity. Based on this finding, we demonstrated genetically that NEMO was critically involved in the cGAS–STING pathway. Cytosolic DNA activated TRIM32 and TRIM56 to synthesize ubiquitin chains that bound NEMO and subsequently activated IKKβ. Activated IKKβ, but not IKKα, was required for TBK1 and NF-κB activation. In contrast, TBK1 was reciprocally required for NF-κB activation, probably by directly phosphorylating IKKβ. Thus, our findings identified a unique innate immune activation cascade in which TBK1–IKKβ formed a positive feedback loop to assure robust cytokine production during cGAS–STING activation.

Summary Viral infection triggers host innate immune responses that result in the production of various cytokines including type I interferons (IFN), activation of inflammasomes, and programmed cell death of the infected cells. Tight control of inflammatory cytokine production is crucial for the triggering of an effective immune response that can resolve the infection without causing host pathology. In examining the inflammatory response of Asc −/− and Casp1 −/− macrophages, we found that deficiency in these molecules resulted in increased IFN production upon DNA virus infection, but not RNA virus challenge. Investigation of the underlying mechanism revealed that upon canonical and non-canonical inflammasome activation, caspase-1 interacted with cyclic GMP-AMP (cGAMP) synthase (cGAS), cleaving it and dampening cGAS-STING-mediated IFN production. Deficiency in inflammasome signaling enhanced host resistance to DNA virus in vitro and in vivo, and this regulatory role extended to other inflammatory caspases. Thus, inflammasome activation dampens cGAS-dependent signaling, suggesting cross-regulation between intracellular DNA-sensing pathways.