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SARS-CoV-2, the virus responsible for the COVID-19 pandemic, has been found capable of inducing long term effects commonly referred to as post-acute sequelae of SARS-CoV-2 (PASC) or long COVID. To define the molecular basis of this condition, we compared the short- and long-term responses to influenza A virus and SARSCoV-2 in the golden hamster model. These data demonstrated that SARS-CoV-2 resulted in sustained changes to lung, kidney, and brain. The most significant change in response to SARS-CoV-2 was observed in the olfactory bulb, where persistent inflammation was visible beyond one month post infection. This was characterized by microglial activation, pro-inflammatory cytokine production, and a Type I interferon (IFN-I) response in the absence of detectable virus. Given the connection between olfactory bulb injury and neurological disorders, we postulate that this prolonged inflammation is an underlying cause of long COVID. Funding Information: This work was funded by generous support from the Marc Haas Foundation, the National Institutes of Health (NCI (R01CA234614) and NIAID (2R01AI107301) and NIDDK (R01DK121072 and 1RO3DK117252) to Department of Medicine, Weill Cornell Medicine (R.E.S.)), and DARPA’s PREPARE Program (HR0011-20-2-0040). The work was further funded by NINDS (NS111251, NSO86444, NSO86444S1)(V.Z., R.A.S.). Ethics Approval Statement: The Tissue Procurement Facility operates under Institutional Review Board (IRB) approved protocol and follows guidelines set by Health Insurance Portability and Accountability Act (HIPAA). Experiments using samples from human subjects were conducted in accordance with local regulations and with the approval of the IRB at the Weill Cornell Medicine. The autopsy samples are considered human tissue research and were collected under IRB protocols 20-04021814 and 19-11021069. All autopsies have consent for research use from next of kin, and these studies were determined as exempt by IRB at Weill Cornell Medicine under those protocol numbers.

SARS-CoV-2 has caused significant morbidity and mortality across the globe. As the virus spreads, new variants are arising that show enhanced capacity to bypass pre-existing immunity. To understand the memory response to SARS-CoV-2, here we monitored SARS-CoV-2-specific T and B cells in a longitudinal study of infected and recovered golden hamsters. We demonstrate that engagement of the innate immune system following SARS-CoV-2 infection was delayed but was followed by a pronounced adaptive response. Moreover, T cell adoptive transfer conferred a reduction in virus levels and rapid induction of SARS-CoV-2-specific B cells demonstrating both lymphocyte populations contributed to the overall response. Reinfection of recovered animals with the SARS-CoV-2 beta variant showed SARS-CoV-2-specific memory T and B cells could effectively control the infection through neutralizing antibodies. These data suggest that the adaptive immune response to SARS-CoV-2 is sufficient to provide protection to the host, independent of the emergence of novel variants.

The emergence and spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has resulted in significant global morbidity, mortality, and societal disruption. A better understanding of virus-host interactions may potentiate therapeutic insights toward limiting this infection. Here we investigated the dynamics of the systemic response to SARS-CoV-2 in hamsters by histological analysis and transcriptional profiling. Infection resulted in consistently high levels of virus in the upper and lower respiratory tracts and sporadic occurrence in other distal tissues. A longitudinal cohort revealed a wave of inflammation, including a type I interferon (IFN-I) response, that was evident in all tissues regardless of viral presence but was insufficient to prevent disease progression. Bolstering the antiviral response with intranasal administration of recombinant IFN-I reduced viral disease, prevented transmission, and lowered inflammation in vivo. This study defines the systemic host response to SARS-CoV-2 infection and supports use of intranasal IFN-I as an effective means of early treatment., Graphical abstract, The host response to SARS-CoV-2 results in significant inflammation. To understand this biology, Hoagland et al. utilize infected hamsters to elucidate transcriptional footprints across tissues longitudinally, showing an inflammatory response beyond the site of acute replication. Local administration of IFN-I reduces virus load and improves immune infiltrate.

Summary Viral pandemics, such as the one caused by SARS-CoV-2, pose an imminent threat to humanity. Because of its recent emergence, there is a paucity of information regarding viral behavior and host response following SARS-CoV-2 infection. Here we offer an in-depth analysis of the transcriptional response to SARS-CoV-2 compared with other respiratory viruses. Cell and animal models of SARS-CoV-2 infection, in addition to transcriptional and serum profiling of COVID-19 patients, consistently revealed a unique and inappropriate inflammatory response. This response is defined by low levels of type I and III interferons juxtaposed to elevated chemokines and high expression of IL-6. We propose that reduced innate antiviral defenses coupled with exuberant inflammatory cytokine production are the defining and driving features of COVID-19., Graphical Abstract, Highlights • SARS-CoV-2 infection induces low IFN-I and -III levels with a moderate ISG response • Strong chemokine expression is consistent across in vitro, ex vivo, and in vivo models • Low innate antiviral defenses and high pro-inflammatory cues contribute to COVID-19, In comparison to other respiratory viruses, SARS-CoV-2 infection drives a lower antiviral transcriptional response that is marked by low IFN-I and IFN-III levels and elevated chemokine expression, which could explain the pro-inflammatory disease state associated with COVID-19.

SUMMARY Influenza A viruses (IAVs) have a remarkable tropism in their ability to circulate in both mammalian and avian species. The IAV NS1 protein is a multifunctional virulence factor that inhibits the type I interferon host response through a myriad of mechanisms. How NS1 has evolved to enable this remarkable property across species and its specific impact in the overall replication, pathogenicity, and host preference remain unknown. Here we analyze the NS1 evolutionary landscape and host tropism using a barcoded library of recombinant IAVs. Results show a surprisingly great variety of NS1 phenotypes according to their ability to replicate in different hosts. The IAV NS1 genes appear to have taken diverse and random evolutionary pathways within their multiple phylogenetic lineages. In summary, the high evolutionary plasticity of this viral protein underscores the ability of IAVs to adapt to multiple hosts and aids in our understanding of its global prevalence., Graphical Abstract, In Brief Muñoz-Moreno et al. report that influenza A virus NS1 undergoes diverse and unpredictable evolutionary pathways based on its different phylogenetic lineages. A high-throughput approach using a barcoded library is used to test the interactions between NS1-recombinant viruses and to study their preference for specific or multiple hosts.

SummaryResponding to an influenza A virus (IAV) infection demands an effective intrinsic cellular defense strategy to slow replication. To identify contributing host factors to this defense, we exploited the host microRNA pathway to perform an in vivo RNAi screen. To this end, IAV, lacking a functional NS1 antagonist, was engineered to encode individual siRNAs against antiviral host genes in an effort to rescue attenuation. This screening platform resulted in the enrichment of strains targeting virus-activated transcription factors, specific antiviral effectors, and intracellular pattern recognition receptors (PRRs). Interestingly, in addition to RIG-I, the PRR for IAV, a virus with the capacity to silence MDA5 also emerged as a dominant strain in wild-type, but not in MDA5-deficient mice. Transcriptional profiling of infected knockout cells confirmed RIG-I to be the primary PRR for IAV but implicated MDA5 as a significant contributor to the cellular defense against influenza A virus.

SummaryRNA interference (RNAi) has been extensively used to identify host factors affecting virus infection but requires exogenous delivery of short interfering RNAs (siRNAs), thus limiting the technique to nonphysiological infection models and a single defined cell type. We report an alternative screening approach using siRNA delivery via infection with a replication-competent RNA virus. In this system, natural selection, defined by siRNA production, permits the identification of host restriction factors through virus enrichment during a physiological infection. We validate this approach with a large-scale siRNA screen in the context of an in vivo alphavirus infection. Monitoring virus evolution across four independent screens identified two categories of enriched siRNAs: specific effectors of the direct antiviral arsenal and host factors that indirectly dampened the overall antiviral response. These results suggest that pathogenicity may be defined by the ability of the virus to antagonize broad cellular responses and specific antiviral factors.