The spike protein (S) of SARS-CoV-2 mediates entry into human cells by interacting with human angiotensin-converting enzyme 2 (ACE2) through its receptor-binding domain (RBD). Here, we report identification of CD209L/L-SIGN and a related protein, CD209/DSIGN as alternative receptors capable of mediating SARS-CoV-2 entry into human cells. Immunofluorescence staining of human tissues revealed a prominent expression of CD209L in the lung and kidney epithelial and endothelial cells of small and medium-sized vessels, whereas CD209 was detected only in a limited number of cell types. Biochemical assays revealed that ectopically expressed CD209L and CD209 bind to S-RBD and mediate SARS-CoV-2 Spseudotyped virus entry. Furthermore, we demonstrate that human endothelial cells endogenously express CD209L and are permissive to SARS-CoV-2 infection. Soluble CD209L-Fc neutralized virus entry.Our observations show that CD209L and CD209 serve as alternative receptors for SARS-CoV-2 in diseaserelevant cell types, including the vascular system. This may have implications for antiviral drug development.

As the COVID-19 pandemic continues to spread, investigating the processes underlying the interactions between SARS-CoV-2 and its hosts is of high importance. Here, we report the identification of CD209L/L-SIGN and the related protein CD209/DC-SIGN as receptors capable of mediating SARS-CoV-2 entry into human cells. Immunofluorescence staining of human tissues revealed prominent expression of CD209L in the lung and kidney epithelia and endothelia. Multiple biochemical assays using a purified recombinant SARS-CoV-2 spike receptor-binding domain (S-RBD) or S1 encompassing both N termal domain and RBD and ectopically expressed CD209L and CD209 revealed that CD209L and CD209 interact with S-RBD. CD209L contains two N -glycosylation sequons, at sites N92 and N361, but we determined that only site N92 is occupied. Removal of the N -glycosylation at this site enhances the binding of S-RBD with CD209L. CD209L also interacts with ACE2, suggesting a role for heterodimerization of CD209L and ACE2 in SARS-CoV-2 entry and infection in cell types where both are present. Furthermore, we demonstrate that human endothelial cells are permissive to SARS-CoV-2 infection, and interference with CD209L activity by a knockdown strategy or with soluble CD209L inhibits virus entry. Our observations demonstrate that CD209L and CD209 serve as alternative receptors for SARS-CoV-2 in disease-relevant cell types, including the vascular system. This property is particularly important in tissues where ACE2 has low expression or is absent and may have implications for antiviral drug development.

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The COVID-19 pandemic has drastically impacted work, economy, and way of life. Sensitive measurement of SARS-CoV-2 specific antibodies would provide new insight into pre-existing immunity, virus transmission dynamics, and the nuances of SARS-CoV-2 pathogenesis. To date, existing SARS-CoV-2 serology tests have limited utility due to insufficient reliable detection of antibody levels lower than what is typically present after several days of symptoms. To measure lower quantities of SARS-CoV-2 IgM, IgG, and IgA with higher resolution than existing assays, we developed a new ELISA protocol with a distinct plate washing procedure and timed plate development via use of a standard curve. Very low optical densities from samples added to buffer coated wells at as low as a 1:5 dilution are reported using this ‘BU ELISA’ method. Use of this method revealed circulating SARS-CoV-2 receptor binding domain (RBD) and nucleocapsid protein (N) reactive antibodies (IgG, IgM, and/or IgA) in 44 and 100 percent of pre-pandemic subjects, respectively, and the magnitude of these antibodies tracked with antibody levels of analogous viral proteins from endemic coronavirus (eCoV) strains. The disease status (HIV, SLE) of unexposed subjects was not linked with SARS-CoV-2 reactive antibody levels; however, quantities were significantly lower in subjects over 70 years of age compared with younger counterparts. Also, we measured SARS-CoV-2 RBD- and N- specific IgM, IgG, and IgA antibodies from 29 SARS-CoV-2 infected individuals at varying disease states, including 10 acute COVID-19 hospitalized subjects with negative serology results by the EUA approved Abbott IgG chemiluminescent microparticle immunoassay. Measurements of SARS-CoV-2 RBD- and N- specific IgM, IgG, IgA levels measured by the BU ELISA revealed higher signal from 9 of the 10 Abbott test negative COVID-19 subjects than all pre-pandemic samples for at least one antibody specificity/isotype, implicating improved serologic identification of SARS-CoV-2 infection via multi-parameter, high sensitive antibody detection. We propose that this improved ELISA protocol, which is straightforward to perform, low cost, and uses readily available commercial reagents, is a useful tool to elucidate new information about SARS-CoV-2 infection and immunity and has promising implications for improved detection of all analytes measurable by this platform.

Group B streptococci (GBS) are encapsulated, β-hemolytic bacteria that are a common cause of infections in human newborns and certain adults. Two factors important for GBS virulence are the sialic acid capsular polysaccharide that promotes immune evasion and the hemolytic pigment that induces host cell cytotoxcity. These virulence factors are often oppositely regulated by the CovR/CovS two-component system. Clinical GBS strains exhibiting hyperhemolysis and low capsule due to pathoadaptive covR/S mutations have been isolated from patients. Given the importance of capsule to GBS virulence, we predicted that a decrease or loss of capsule would attenuate the virulence of covR/S mutants. Surprisingly, hyperhemolytic GBS with low or no capsule exhibit increased virulence, intracellular persistence, and blood-brain barrier penetration, which was independent of a Trojan horse mechanism of barrier penetration. Additionally, intracellular persistence enabled both hemolytic and hyperhemolytic GBS to evade antibiotics routinely used to treat these infections. The finding that diminished capsule expression promotes GBS virulence, intracellular persistence, and antibiotic evasion has important implications for sustained antibiotic therapy and efficacy of capsule-based vaccines.

Exacerbated and persistent innate immune response marked by pro-inflammatory cytokine expression is thought to be a major driver of chronic COVID-19 pathology. Although macrophages are not the primary target cells of SARS-CoV-2 infection in humans, viral RNA and antigens in activated monocytes and macrophages have been detected in post-mortem samples, and dysfunctional monocytes and macrophages have been hypothesized to contribute to a protracted hyper-inflammatory state in COVID-19 patients. In this study, we demonstrate that CD169, a myeloid cell specific I-type lectin, facilitated ACE2-independent SARS-CoV-2 fusion and entry in macrophages. CD169- mediated SARS-CoV-2 entry in macrophages resulted in expression of viral genomic and sub-genomic (sg) RNAs with minimal viral protein expression and no infectious viral particle release, suggesting a post-entry restriction of the SARS-CoV-2 replication cycle. Intriguingly this post-entry replication block was alleviated by exogenous ACE2 expression in macrophages. Restricted expression of viral gRNA and sgRNA in CD169+ macrophages elicited a pro-inflammatory cytokine expression (TNFα, IL-6 and IL-1β) in a RIG-I, MDA-5 and MAVS-dependent manner, which was suppressed by remdesivir pre- treatment. These findings suggest that de novo expression of SARS-CoV-2 RNA in macrophages contributes to the pro-inflammatory cytokine signature and that blocking CD169-mediated ACE2 independent infection and subsequent activation of macrophages by viral RNA might alleviate COVID-19-associated hyperinflammatory response.Author SummaryOver-exuberant production of pro-inflammatory cytokine expression by macrophages has been hypothesized to contribute to severity of COVID-19 disease. Molecular mechanisms that contribute to macrophage-intrinsic immune activation during SARS- CoV-2 infection are not fully understood. Here we show that CD169, a macrophage- specific sialic-acid binding lectin, facilitates abortive SARS-CoV-2 infection of macrophages that results in innate immune sensing of viral replication intermediates and production of proinflammatory responses. We identify an ACE2-independent, CD169- mediated endosomal viral entry mechanism that results in cytoplasmic delivery of viral capsids and initiation of virus replication, but absence of infectious viral production. Restricted viral replication in CD169+ macrophages and detection of viral genomic and sub-genomic RNAs by cytoplasmic RIG-I-like receptor family members, RIG-I and MDA5, and initiation of downstream signaling via the adaptor protein MAVS, was required for innate immune activation. These studies uncover mechanisms important for initiation of innate immune sensing of SARS-CoV-2 infection in macrophages, persistent activation of which might contribute to severe COVID-19 pathophysiology.

The SARS-CoV-2 pandemic has caused widespread illness, loss of life, and socioeconomic disruption that is unlikely to resolve until vaccines are widely adopted, and effective therapeutic treatments become established. Here, a well curated and annotated library of 6710 clinical and preclinical molecules, covering diverse chemical scaffolds and known host targets was evaluated for inhibition of SARS-CoV-2 infection in multiple infection models. Multi-concentration, high-content immunocytofluorescence-based screening identified 172 strongly active small molecules, including 52 with submicromolar potencies. The active molecules were extensively triaged by in vitro mechanistic assays, including human primary cell models of infection and the most promising, obatoclax, was tested for in vivo efficacy. Structural and mechanistic classification of compounds revealed known and novel chemotypes and potential host targets involved in each step of the virus replication cycle including BET proteins, microtubule function, mTOR, ER kinases, protein synthesis and ion channel function. In the mouse disease model obatoclax effectively reduced lung virus load by 10-fold. Overall, this work provides an important, publicly accessible, foundation for development of novel treatments for COVID-19, establishes human primary cell-based pharmacological models for evaluation of therapeutics and identifies new insights into SARS-CoV-2 infection mechanisms.

Exacerbated and persistent innate immune response marked by pro-inflammatory cytokine expression is thought to be a major driver of chronic COVID-19 pathology. Although macrophages are not the primary target cells of SARS-CoV-2 infection in humans, viral RNA and antigens in activated monocytes and macrophages have been detected in post-mortem samples, and dysfunctional monocytes and macrophages have been hypothesized to contribute to a protracted hyper-inflammatory state in COVID-19 patients. In this study, we demonstrate that CD169, a myeloid cell specific I-type lectin, facilitated ACE2-independent SARS-CoV-2 fusion and entry in macrophages. CD169-mediated SARS-CoV-2 entry in macrophages resulted in expression of viral genomic and subgenomic RNAs with minimal viral protein expression and no infectious viral particle release, suggesting a post-entry restriction of the SARS-CoV-2 replication cycle. Intriguingly this post-entry replication block was alleviated by exogenous ACE2 expression in macrophages. Restricted expression of viral genomic and subgenomic RNA in CD169+ macrophages elicited a pro-inflammatory cytokine expression (TNFα, IL-6 and IL-1β) in a RIG-I, MDA-5 and MAVS-dependent manner, which was suppressed by remdesivir treatment. These findings suggest that de novo expression of SARS-CoV-2 RNA in macrophages contributes to the pro-inflammatory cytokine signature and that blocking CD169-mediated ACE2 independent infection and subsequent activation of macrophages by viral RNA might alleviate COVID-19-associated hyperinflammatory response.