African Swine Fever Dampens Type I IFN
The mechanism by which the highly pathogenic African swine fever virus (ASFV) dampens the type I IFN response is investigated by Huang and Xu et al. (p. 2754) in this Top Read. The authors inspected 102 viral proteins and identified the viral E2 ubiquitin–conjugating enzyme pI251L as the most significant inhibitor of IFN levels. Eliminating the ubiquitin conjugation activity of pI251L through mutagenesis did not inhibit the normal IFN response. Rather, it was the physical binding of pI251L to the host E3 ubiquitin ligase RNF138 that correlated with reduced type I IFN transcription. The physical association of RNF138 and pI251L led to degradation of the host E3 ligase RNF128, which in turn led to a reduction in K63-linked ubiquitination of the host TBK1 kinase. The aberrant ubiquitination of TBK1 lowered its ability to phosphorylate the transcriptional regulator IRF3, thus lowering IFN production. Together, these data demonstrated how the viral protein pI251L short-circuits innate viral alert signals from the cytoplasm to the nucleus.
IFN-λ: An Unappreciated Driver of Lupus?
Type I IFNs are antiviral cytokines that play a significant role in systemic lupus erythematosus (SLE). However, the role of type III IFN (IFN-λ) in SLE pathogenesis is not well characterized. In this Top Read, Barnas et al. (p. 2660) uncover how IFN-λ may drive pathogenic B cell activity. Since murine B cells do not respond to IFN-λ, the authors used primary human B cells to conduct their studies. In addition to finding an IFN-λ blood signature in SLE patients, the authors also showed that these levels positively correlate with pathogenic, autoimmune plasma cell development (CD11c+T-bet+CD21−). Furthermore, they found that the expression level of the IFN-λ receptor on B cells in patients with SLE strongly correlated with the expansion and differentiation of the pathogenic, autoimmune B cell subset. These data suggest that, in addition to type I IFN, IFN-λ may be another driver of SLE and its associated B cell abnormalities.
Regulating Cytokines under Stress
Hypoxia-inducible factor-1α (HIF-1α) is a transcription factor with roles in cellular metabolism, stress response, and cytokine regulation. In this Top Read, Ahmed et al. (p. 2813) demonstrate that HIF-1α amplifies inflammatory cytokines in macrophages following TLR3 stimulation. Both low m.w. and high m.w. polyinosinic-polycytidylic acid (Poly(I:C)) stimulation resulted in the induction of HIF-1α expression, which was blocked by inhibiting TLR3 signaling. Blocking polyinosinic:polycytidylic acid–induced HIF-1α dimerization in a low glucose environment decreased inflammatory cytokine production and reduced the levels of both IκBα and p-IκBα, suggesting HIF-1α may modulate the NF-κB pathway. Dimeric pyruvate kinase M2 (PKM2) was shown previously to stabilize nuclear HIF-1α. Here, PKM2 accumulated in the nucleus, coincident with HIF-1α under stressful conditions, and the stabilized tetrameric PKM2 form resulted in decreased HIF-1α upon TLR3 engagement. Collectively, these data suggest that HIF-1α may fine-tune TLR3 driven inflammatory responses under stressful conditions.
β-Glucan Trains Innate Immunity
Innate immunological memory, or trained immunity, is an emerging concept with parallels to vaccine-induced immunity, but significantly differs in two ways: innate trained immunity persists for shorter lengths of time; however, it induces broader protection against a variety of pathogens. In this Top Read, Stothers et al. (p. 2785) showed that β-glucan, a pathogen-associated molecular pattern derived from the fungal cell wall, induced trained immunity in macrophages that persisted for more than seven days, independent of canonical β-glucan receptors, such as Dectin-1 and TLR2. The β-glucan–induced trained immunity was effective at protecting mice from Pseudomonas aeruginosa infection via augmented recruitment of innate immune cells to the site of infection, facilitating bacterial clearance. In addition, the trained macrophages adopted a unique antimicrobial phenotype, including enhanced cytokine and chemokine production, phagocytosis, and reactive oxygen species production. Bacterial infections are a global health burden that can be deadly to immunocompromised individuals. Immunomodulatory compounds, such as β-glucan, which induce trained immunity are an exciting tool, which may be used to enhance current antimicrobial therapies.
Hemocyanin Regulates the Shrimp Microbiome
While the role of extracellular hemocyanin in shrimp has been well characterized as a crucial component of antimicrobial immunity, the function of intracellular hemocyanin in this process remains less understood. In this Top Read, Zheng et al. (p. 2733) show how intracellular hemocyanin maintains an appropriate balance of microbes that colonize the shrimp gastrointestinal tract (GIT). The authors found that diseased shrimp had gastrointestinal abnormalities, which correlated with low hemocyanin, greater bacterial burden but lower bacterial diversity among several components of the GIT, particularly in the hepatopancreas. Diseased hepatopancreas were enriched with pathogenic Vibrio bacteria and had significantly reduced proportion of Photobacterium microbes. When intracellular hemocyanin was experimentally depleted in the hepatopancreas, the overall bacterial population initially dropped, followed by a stage of recovery wherein Vibrio became significantly enriched over Photobacterium. The skewed proportion of microbes in hemocyanin depleted shrimp persisted despite fecal microbial transplant from control shrimp, suggesting that hemocyanin is required to inhibit the emergence of pathogenic bacteria. Hemocyanin depletion also caused epigenetic changes to several metabolic pathways, which correlated with mitochondrial damage and increased production of reactive oxygen species (ROS). ROS scavenging applied to hemocyanin depleted shrimp reversed the redistribution of bacterial populations. Thus, unlike the antioxidant role of extracellular hemocyanin, intracellular hemocyanin provides an oxidizing environment needed to maintain the microbial balance important to shrimp gut health.