Pathogenic viral infections represent a major challenge to human health. Host immune responses to respiratory viruses are closely associated with microbiome and metabolism via the gut-lung axis. It has been known that host defense against influenza A virus (IAV) involves activation of the NLRP3 inflammasome, however, mechanisms behind the protective function of NLRP3 are not fully known. Here we show that an isolated bacterial strain, Bifidobacterium pseudolongum NjM1, enriched in the gut microbiota of Nlrp3^−/− mice, protects wild-type but not Nlrp3 deficient mice against IAV infection. This effect depends on the enhanced production of type I interferon (IFN-I) mediated by NjM1-derived acetate. Application of exogenous acetate reproduces the protective effect of NjM1. Mechanistically, NLRP3 bridges GPR43 and MAVS, and promotes the oligomerization and signalling of MAVS; while acetate enhances MAVS aggregation upon GPR43 engagement, leading to elevated IFN-I production. Thus, our data support a model of NLRP3 mediating enhanced induction of IFN-I via acetate-producing bacterium and suggest that the acetate-GPR43-NLRP3-MAVS-IFN-I signalling axis is a potential therapeutic target against respiratory viral infections.

The Zika virus (ZIKV) has two lineages, Asian and African, and their impact on developing brains has not been compared. Dengue virus (DENV) is a close family member of ZIKV and co-circulates with ZIKV. Here we performed intracerebral inoculation of embryonic mouse brains with dengue virus 2 (DENV2), and found that DENV2 is sufficient to cause smaller brain size due to increased cell death in neural progenitor cells (NPCs) and neurons. Compared to the currently circulating Asian lineage of ZIKV (MEX1-44), DENV2 grows slower, causes less neuronal death, and fails to cause postnatal animal death. Surprisingly, our side-by-side comparison uncovered that African ZIKV isolate (MR-766) is more potent in causing brain damage and postnatal lethality than MEX1-44. In comparison to MEX1-44, MR-766 grows faster in NPCs and in the developing brain, and causes more pronounced cell death in NPCs and neurons, resulting in more severe neuronal loss. Together, these results reveal that DENV2 is sufficient to cause smaller brain sizes, and suggest that the ZIKV African lineage is more virulent and causes more severe brain damage than the Asian lineage.

The mechanism by which inflammasome activation is modulated remains unclear. In this study, we identified an AIM2-interacting protein, the E3 ubiquitin ligase HUWE1, which was also found to interact with NLRP3 and NLRC4 through the HIN domain of AIM2 and the NACHT domains of NLRP3 and NLRC4. The BH3 domain of HUWE1 was important for its interaction with NLRP3, AIM2, and NLRC4. Caspase-1 maturation, IL-1β release, and pyroptosis were reduced in Huwe1-deficient bone marrow–derived macrophages (BMDMs) compared with WT BMDMs in response to stimuli to induce NLRP3, NLRC4, and AIM2 inflammasome activation. Furthermore, the activation of NLRP3, NLRC4, and AIM2 inflammasomes in both mouse and human cells was remarkably reduced by treatment with the HUWE1 inhibitor BI8622. HUWE1 mediated the K27-linked polyubiquitination of AIM2, NLRP3, and NLRC4, which led to inflammasome assembly, ASC speck formation, and sustained caspase-1 activation. Huwe1-deficient mice had an increased bacterial burden and decreased caspase-1 activation and IL-1β production upon Salmonella, Francisella, or Acinetobacter baumannii infection. Our study provides insights into the mechanisms of inflammasome activation as well as a potential therapeutic target against bacterial infection.

Lysine-63–linked (K63-linked) polyubiquitination of TRAF3 coordinates the engagement of pattern-recognition receptors with recruited adaptor proteins and downstream activator TBK1 in pathways that induce type I IFN. Whether autoubiquitination or other E3 ligases mediate K63-linked TRAF3 polyubiquitination remains unclear. We demonstrated that mice deficient in the E3 ligase gene Hectd3 remarkably increased host defense against infection by intracellular bacteria Francisella novicida, Mycobacterium, and Listeria by limiting bacterial dissemination. In the absence of HECTD3, type I IFN response was impaired during bacterial infection both in vivo and in vitro. HECTD3 regulated type I IFN production by mediating K63-linked polyubiquitination of TRAF3 at residue K138. The catalytic domain of HECTD3 regulated TRAF3 K63 polyubiquitination, which enabled TRAF3-TBK1 complex formation. Our study offers insights into mechanisms of TRAF3 modulation and provides potential therapeutic targets against infections by intracellular bacteria and inflammatory diseases.