Seasonal human coronavirus antibodies are boosted upon SARS-CoV-2 infection but not associated with protection.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has rapidly spread within the human population. Although SARS-CoV-2 is a novel coronavirus, most humans had been previously exposed to other antigenically distinct common seasonal human coronaviruses (hCoVs) before the coronavirus disease 2019 (COVID-19) pandemic. Here, we quantified levels of SARS-CoV-2-reactive antibodies and hCoV-reactive antibodies in serum samples collected from 431 humans before the COVID-19 pandemic. We then quantified pre-pandemic antibody levels in serum from a separate cohort of 251 individuals who became PCR-confirmed infected with SARS-CoV-2. Finally, we longitudinally measured hCoV and SARS-CoV-2 antibodies in the serum of hospitalized COVID-19 patients. Our studies indicate that most individuals possessed hCoV-reactive antibodies before the COVID-19 pandemic. We determined that ∼20% of these individuals possessed non-neutralizing antibodies that cross-reacted with SARS-CoV-2 spike and nucleocapsid proteins. These antibodies were not associated with protection against SARS-CoV-2 infections or hospitalizations, but they were boosted upon SARS-CoV-2 infection.

SARS-CoV-2 reservoir in post-acute sequelae of COVID-19 (PASC).

Millions of people are suffering from Long COVID or post-acute sequelae of COVID-19 (PASC). Several biological factors have emerged as potential drivers of PASC pathology. Some individuals with PASC may not fully clear the coronavirus SARS-CoV-2 after acute infection. Instead, replicating virus and/or viral RNA-potentially capable of being translated to produce viral proteins-persist in tissue as a 'reservoir'. This reservoir could modulate host immune responses or release viral proteins into the circulation. Here we review studies that have identified SARS-CoV-2 RNA/protein or immune responses indicative of a SARS-CoV-2 reservoir in PASC samples. Mechanisms by which a SARS-CoV-2 reservoir may contribute to PASC pathology, including coagulation, microbiome and neuroimmune abnormalities, are delineated. We identify research priorities to guide the further study of a SARS-CoV-2 reservoir in PASC, with the goal that clinical trials of antivirals or other therapeutics with potential to clear a SARS-CoV-2 reservoir are accelerated.

Comparative Flavivirus-Host Protein Interaction Mapping Reveals Mechanisms of Dengue and Zika Virus Pathogenesis.

Mosquito-borne flaviviruses, including dengue virus (DENV) and Zika virus (ZIKV), are a growing public health concern. Systems-level analysis of how flaviviruses hijack cellular processes through virus-host protein-protein interactions (PPIs) provides information about their replication and pathogenic mechanisms. We used affinity purification-mass spectrometry (AP-MS) to compare flavivirus-host interactions for two viruses (DENV and ZIKV) in two hosts (human and mosquito). Conserved virus-host PPIs revealed that the flavivirus NS5 protein suppresses interferon stimulated genes by inhibiting recruitment of the transcription complex PAF1C and that chemical modulation of SEC61 inhibits DENV and ZIKV replication in human and mosquito cells. Finally, we identified a ZIKV-specific interaction between NS4A and ANKLE2, a gene linked to hereditary microcephaly, and showed that ZIKV NS4A causes microcephaly in Drosophila in an ANKLE2-dependent manner. Thus, comparative flavivirus-host PPI mapping provides biological insights and, when coupled with in vivo models, can be used to unravel pathogenic mechanisms.

Gutcha: The microbiota drives tumor-induced mortality.

Tumors can cause wasting and mortality, but the connection between these outcomes is unclear. In this issue of Immunity, Chen and colleagues find the outcomes are separable as the tumor-altered gut microbiota activates renal immunity and alters metabolism, leading to mortality independently of wasting.

The RNA helicase DDX39A binds a conserved structure in chikungunya virus RNA to control infection.

Alphaviruses are a large group of re-emerging arthropod-borne RNA viruses. The compact viral RNA genomes harbor diverse structures that facilitate replication. These structures can be recognized by antiviral cellular RNA-binding proteins, including DExD-box (DDX) helicases, that bind viral RNAs to control infection. The full spectrum of antiviral DDXs and the structures that are recognized remain unclear. Genetic screening identified DDX39A as antiviral against the alphavirus chikungunya virus (CHIKV) and other medically relevant alphaviruses. Upon infection, the predominantly nuclear DDX39A accumulates in the cytoplasm inhibiting alphavirus replication, independent of the canonical interferon pathway. Biochemically, DDX39A binds to CHIKV genomic RNA, interacting with the 5' conserved sequence element (5'CSE), which is essential for the antiviral activity of DDX39A. Altogether, DDX39A relocalization and binding to a conserved structural element in the alphavirus genomic RNA attenuates infection, revealing a previously unknown layer to the cellular control of infection.

The lncRNA ALPHA specifically targets chikungunya virus to control infection.

Arthropod-borne viruses, including the alphavirus chikungunya virus (CHIKV), cause acute disease in millions of people and utilize potent mechanisms to antagonize and circumvent innate immune pathways including the type I interferon (IFN) pathway. In response, hosts have evolved antiviral counterdefense strategies that remain incompletely understood. Recent studies have found that long noncoding RNAs (lncRNAs) regulate classical innate immune pathways; how lncRNAs contribute to additional antiviral counterdefenses remains unclear. Using high-throughput genetic screening, we identified a cytoplasmic antiviral lncRNA that we named antiviral lncRNA prohibiting human alphaviruses (ALPHA), which is transcriptionally induced by alphaviruses and functions independently of IFN to inhibit the replication of CHIKV and its closest relative, O'nyong'nyong virus (ONNV), but not other viruses. Furthermore, we showed that ALPHA interacts with CHIKV genomic RNA and restrains viral RNA replication. Together, our findings reveal that ALPHA and potentially other lncRNAs can mediate non-canonical antiviral immune responses against specific viruses.

Stem-loop recognition by DDX17 facilitates miRNA processing and antiviral defense.

DEAD-box helicases play essential roles in RNA metabolism across species, but emerging data suggest that they have additional functions in immunity. Through RNAi screening, we identify an evolutionarily conserved and interferon-independent role for the DEAD-box helicase DDX17 in restricting Rift Valley fever virus (RVFV), a mosquito-transmitted virus in the bunyavirus family that causes severe morbidity and mortality in humans and livestock. Loss of Drosophila DDX17 (Rm62) in cells and flies enhanced RVFV infection. Similarly, depletion of DDX17 but not the related helicase DDX5 increased RVFV replication in human cells. Using crosslinking immunoprecipitation high-throughput sequencing (CLIP-seq), we show that DDX17 binds the stem loops of host pri-miRNA to facilitate their processing and also an essential stem loop in bunyaviral RNA to restrict infection. Thus, DDX17 has dual roles in the recognition of stem loops: in the nucleus for endogenous microRNA (miRNA) biogenesis and in the cytoplasm for surveillance against structured non-self-elements.

Rising to the challenge of COVID-19: Working on SARS-CoV-2 during the pandemic.

COVID-19 altered our lives and pushed scientific research to operate at breakneck speed, leading to significant breakthroughs in record time. We asked experts in the field about the challenges they faced in transitioning, rapidly but safely, to working on the virus while navigating the shutdown. Their voices converge on the importance of teamwork, forging new collaborations, and working toward a shared goal.

Pyrimidine inhibitors synergize with nucleoside analogues to block SARS-CoV-2.

The SARS-CoV-2 virus has infected more than 261 million people and has led to more than 5 million deaths in the past year and a half1 ( https://www.who.org/ ). Individuals with SARS-CoV-2 infection typically develop mild-to-severe flu-like symptoms, whereas infection of a subset of individuals leads to severe-to-fatal clinical outcomes2. Although vaccines have been rapidly developed to combat SARS-CoV-2, there has been a dearth of antiviral therapeutics. There is an urgent need for therapeutics, which has been amplified by the emerging threats of variants that may evade vaccines. Large-scale efforts are underway to identify antiviral drugs. Here we screened approximately 18,000 drugs for antiviral activity using live virus infection in human respiratory cells and validated 122 drugs with antiviral activity and selectivity against SARS-CoV-2. Among these candidates are 16 nucleoside analogues, the largest category of clinically used antivirals. This included the antivirals remdesivir and molnupiravir, which have been approved for use in COVID-19. RNA viruses rely on a high supply of nucleoside triphosphates from the host to efficiently replicate, and we identified a panel of host nucleoside biosynthesis inhibitors as antiviral. Moreover, we found that combining pyrimidine biosynthesis inhibitors with antiviral nucleoside analogues synergistically inhibits SARS-CoV-2 infection in vitro and in vivo against emerging strains of SARS-CoV-2, suggesting a clinical path forward.

Alternative splicing and cancer: insights, opportunities, and challenges from an expanding view of the transcriptome.

Over the past decade there has been increased awareness of the potential role of alternative splicing in the etiology of cancer. In particular, advances in RNA-Sequencing technology and analysis has led to a wave of discoveries in the last few years regarding the causes and functional relevance of alternative splicing in cancer. Here we discuss the current understanding of the connections between splicing and cancer, with a focus on the most recent findings. We also discuss remaining questions and challenges that must be addressed in order to use our knowledge of splicing to guide the diagnosis and treatment of cancer.

Going in Circles: The Black Box of Circular RNA Immunogenicity.

Two recent papers in Molecular Cell (Chen et al., 2019; Wesselhoeft et al., 2019) have probed the putative immunogenicity of circular RNAs (circRNAs). These studies indicate that the stimulatory capacity of circRNAs depends on factors including the specific RNA, the mode of biogenesis, RNA modifications, cell type, and the means of delivery.

The Integrator complex cleaves nascent mRNAs to attenuate transcription.

Cellular homeostasis requires transcriptional outputs to be coordinated, and many events post-transcription initiation can dictate the levels and functions of mature transcripts. To systematically identify regulators of inducible gene expression, we performed high-throughput RNAi screening of the Drosophila Metallothionein A (MtnA) promoter. This revealed that the Integrator complex, which has a well-established role in 3' end processing of small nuclear RNAs (snRNAs), attenuates MtnA transcription during copper stress. Integrator complex subunit 11 (IntS11) endonucleolytically cleaves MtnA transcripts, resulting in premature transcription termination and degradation of the nascent RNAs by the RNA exosome, a complex also identified in the screen. Using RNA-seq, we then identified >400 additional Drosophila protein-coding genes whose expression increases upon Integrator depletion. We focused on a subset of these genes and confirmed that Integrator is bound to their 5' ends and negatively regulates their transcription via IntS11 endonuclease activity. Many noncatalytic Integrator subunits, which are largely dispensable for snRNA processing, also have regulatory roles at these protein-coding genes, possibly by controlling Integrator recruitment or RNA polymerase II dynamics. Altogether, our results suggest that attenuation via Integrator cleavage limits production of many full-length mRNAs, allowing precise control of transcription outputs.

The lipopeptide Pam3CSK4 inhibits Rift Valley fever virus infection and protects from encephalitis.

Rift Valley fever virus (RVFV) is an encephalitic bunyavirus that can infect neurons in the brain. There are no approved therapeutics that can protect from RVFV encephalitis. Innate immunity, the first line of defense against infection, canonically antagonizes viruses through interferon signaling. We found that interferons did not efficiently protect primary cortical neurons from RVFV, unlike other cell types. To identify alternative neuronal antiviral pathways, we screened innate immune ligands and discovered that the TLR2 ligand Pam3CSK4 inhibited RVFV infection, and other bunyaviruses. Mechanistically, we found that Pam3CSK4 blocks viral fusion, independent of TLR2. In a mouse model of RVFV encephalitis, Pam3CSK4 treatment protected animals from infection and mortality. Overall, Pam3CSK4 is a bunyavirus fusion inhibitor active in primary neurons and the brain, representing a new approach toward the development of treatments for encephalitic bunyavirus infections.

A RHIM with a View: FLYing with Functional Amyloids.

Recognition of bacterial peptidoglycan by the Drosophila IMD pathway triggers NF-κB activation and an associated immune response. In this issue of Immunity, Kleino et al. (2017) show that proteins in the IMD pathway form functional amyloids via a cryptic motif resembling the RHIM motif found in mammalian RIPK proteins. Amyloid formation can be negatively regulated, suggesting that it presents a regulatory point in multiple biological processes.

DDX RNA helicases: key players in cellular homeostasis and innate antiviral immunity.

RNA helicases are integral in RNA metabolism, performing important roles in cellular homeostasis and stress responses. In particular, the DExD/H-box (DDX) helicase family possesses a conserved catalytic core that binds structural features rather than specific sequences in RNA targets. DDXs have critical roles in all aspects of RNA metabolism including ribosome biogenesis, translation, RNA export, and RNA stability. Importantly, functional specialization within this family arises from divergent N and C termini and is driven at least in part by gene duplications with 18 of the 42 human helicases having paralogs. In addition to their key roles in the homeostatic control of cellular RNA, these factors have critical roles in RNA virus infection. The canonical RIG-I-like receptors (RLRs) play pivotal roles in cytoplasmic sensing of viral RNA structures, inducing antiviral gene expression. Additional RNA helicases function as viral sensors or regulators, further diversifying the innate immune defense arsenal. Moreover, some of these helicases have been coopted by viruses to facilitate their replication. Altogether, DDX helicases exhibit functional specificity, playing intricate roles in RNA metabolism and host defense. This review will discuss the mechanisms by which these RNA helicases recognize diverse RNA structures in cellular and viral RNAs, and how this impacts RNA processing and innate immune responses.

The Output of Protein-Coding Genes Shifts to Circular RNAs When the Pre-mRNA Processing Machinery Is Limiting.

Many eukaryotic genes generate linear mRNAs and circular RNAs, but it is largely unknown how the ratio of linear to circular RNA is controlled or modulated. Using RNAi screening in Drosophila cells, we identify many core spliceosome and transcription termination factors that control the RNA outputs of reporter and endogenous genes. When spliceosome components were depleted or inhibited pharmacologically, the steady-state levels of circular RNAs increased while expression of their associated linear mRNAs concomitantly decreased. Upon inhibiting RNA polymerase II termination via depletion of the cleavage/polyadenylation machinery, circular RNA levels were similarly increased. This is because readthrough transcripts now extend into downstream genes and are subjected to backsplicing. In total, these results demonstrate that inhibition or slowing of canonical pre-mRNA processing events shifts the steady-state output of protein-coding genes toward circular RNAs. This is in part because nascent RNAs become directed into alternative pathways that lead to circular RNA production.

Ars2 regulates both miRNA- and siRNA- dependent silencing and suppresses RNA virus infection in Drosophila.

Intrinsic immune responses autonomously inhibit viral replication and spread. One pathway that restricts viral infection in plants and insects is RNA interference (RNAi), which targets and degrades viral RNA to limit infection. To identify additional genes involved in intrinsic antiviral immunity, we screened Drosophila cells for modulators of viral infection using an RNAi library. We identified Ars2 as a key component of Drosophila antiviral immunity. Loss of Ars2 in cells, or in flies, increases susceptibility to RNA viruses. Consistent with its antiviral properties, we found that Ars2 physically interacts with Dcr-2, modulates its activity in vitro, and is required for siRNA-mediated silencing. Furthermore, we show that Ars2 plays an essential role in miRNA-mediated silencing, interacting with the Microprocessor and stabilizing pri-miRNAs. The identification of Ars2 as a player in these small RNA pathways provides new insight into the biogenesis of small RNAs that may be extended to other systems.

Ars2 links the nuclear cap-binding complex to RNA interference and cell proliferation.

Here we identify a component of the nuclear RNA cap-binding complex (CBC), Ars2, that is important for miRNA biogenesis and critical for cell proliferation. Unlike other components of the CBC, Ars2 expression is linked to the proliferative state of the cell. Deletion of Ars2 is developmentally lethal, and deletion in adult mice led to bone marrow failure whereas parenchymal organs composed of nonproliferating cells were unaffected. Depletion of Ars2 or CBP80 from proliferating cells impaired miRNA-mediated repression and led to alterations in primary miRNA processing in the nucleus. Ars2 depletion also reduced the levels of several miRNAs, including miR-21, let-7, and miR-155, that are implicated in cellular transformation. These findings provide evidence for a role for Ars2 in RNA interference regulation during cell proliferation.

Alternative splicing redefines landscape of commonly mutated genes in acute myeloid leukemia.

Most genes associated with acute myeloid leukemia (AML) are mutated in less than 10% of patients, suggesting that alternative mechanisms of gene disruption contribute to this disease. Here, we find a set of splicing events that alter the expression of a subset of AML-associated genes independent of known somatic mutations. In particular, aberrant splicing triples the number of patients with reduced functional EZH2 compared with that predicted by somatic mutation alone. In addition, we unexpectedly find that the nonsense-mediated decay factor DHX34 exhibits widespread alternative splicing in sporadic AML, resulting in a premature stop codon that phenocopies the loss-of-function germline mutations observed in familial AML. Together, these results demonstrate that classical mutation analysis underestimates the burden of functional gene disruption in AML and highlight the importance of assessing the contribution of alternative splicing to gene dysregulation in human disease.