Mesenchymal stem cells prevent H7N9 virus infection via rejuvenating immune environment to inhibit immune-overactivity.

BACKGROUND: Influenza is a clinically important infectious disease with a high fatality rate, which always results in severe pneumonia. Mesenchymal stem cells (MSCs) exhibit promising therapeutic effects on severe viral pneumonia, but whether MSCs prevent virus infection and contribute to the prevention of influenza remains unknown. METHODS: ICR mice were pretreated with human umbilical cord (hUC) MSCs and then infected with the influenza H7N9 virus. Weight, survival days, and lung index of mice were recorded. Serum antibody against influenza H7N9 virus was detected according to the hemagglutination inhibition method. Before and after virus infection, T cell and B cell subtypes in the peripheral blood of mice were evaluated by flow cytometry. Cytokines in the supernatants of MSCs, innate immune cells, and mouse broncho alveolar lavage fluid (BALF) were determined by enzyme-linked immunosorbent assay (ELISA) or Luminex Assay. RESULTS: Pretreatment with MSCs protected mice against influenza H7N9 virus infection. Weight loss, survival rate, and structural and functional damage to the lungs of infected mice were significantly improved. Mechanistically, MSCs modulated T lymphocyte response in virus-infected mice and inhibited the cGAS/STING pathway. Importantly, the protective effect of MSCs was mediated by cell-to-cell communications and attenuation of cytokine storm caused by immune overactivation.

H1N1 exposure during the convalescent stage of SARS-CoV-2 infection results in enhanced lung pathologic damage in hACE2 transgenic mice.

ABSTRACTThe risk of secondary infection with SARS-CoV-2 and influenza A virus is becoming a practical problem that must be addressed as the flu season merges with the COVID-19 pandemic. As SARS-CoV-2 and influenza A virus have been found in patients, understanding the in vivo characteristics of the secondary infection between these two viruses is a high priority. Here, hACE2 transgenic mice were challenged with the H1N1 virus at a nonlethal dose during the convalescent stage on 7 and 14 days post SARS-CoV-2 infection, and importantly, subsequent H1N1 infection showed enhanced viral shedding and virus tissue distribution. Histopathological observation revealed an extensive pathological change in the lungs related to H1N1 infection in mice recovered from SARS-CoV-2 infection, with severe inflammation infiltration and bronchiole disruption. Moreover, upon H1N1 exposure on 7 and 14 dpi of SARS-CoV-2 infection, the lymphocyte population activated at a lower level with T cell suppressed in both PBMC and lung. These findings will be valuable for evaluating antiviral therapeutics and vaccines as well as guiding public health work.

Concentrated Secretome of Adipose Stromal Cells Limits Influenza A Virus-Induced Lung Injury in Mice.

Despite vaccination and antivirals, influenza remains a communicable disease of high burden, with limited therapeutic options available to patients that develop complications. Here, we report the development and preclinical characterization of Adipose Stromal Cell (ASC) concentrated secretome (CS), generated by process adaptable to current Good Manufacturing Practices (cGMP) standards. We demonstrate that ASC-CS limits pulmonary histopathological changes, infiltration of inflammatory cells, protein leak, water accumulation, and arterial oxygen saturation (spO2) reduction in murine model of lung infection with influenza A virus (IAV) when first administered six days post-infection. The ability to limit lung injury is sustained in ASC-CS preparations stored at -80 °C for three years. Priming of the ASC with inflammatory factors TNFα and IFNγ enhances ASC-CS ability to suppress lung injury. IAV infection is associated with dramatic increases in programmed cell death ligand (PDL1) and angiopoietin 2 (Angpt2) levels. ASC-CS application significantly reduces both PDL1 and Angpt2 levels. Neutralization of PDL1 with anti-mouse PDL1 antibody starting Day6 onward effectively ablates lung PDL1, but only non-significantly reduces Angpt2 release. Most importantly, late-phase PDL1 neutralization results in negligible suppression of protein leakage and inflammatory cell infiltration, suggesting that suppression of PDL1 does not play a critical role in ASC-CS therapeutic effects.

A meta-analysis reveals the effectiveness of probiotics and prebiotics against respiratory viral infection.

Experimental experience suggests that microbial agents including probiotics and prebiotics (representative microbial agents) play a critical role in defending against respiratory virus infection. We aim to systematically examine these agents' effect on respiratory viral infection and encourage research into clinical applications. An electronic literature search was conducted from published data with a combination of a microbial agents search component containing synonyms for microbial agents-related terms and a customized search component for respiratory virus infection. Hazard ratio (HR), risk ratio (RR) and standard deviation (SD) were employed as effect estimates. In 45 preclinical studies, the mortality rates decreased in the respiratory viral infection models that included prebiotics or prebiotics as interventions (HR: 0.70; 95% confidence interval (CI): 0.56-0.87; P=0.002). There was a significant decrease in viral load due to improved gut microbiota (SD: -1.22; 95% CI: -1.50 to -0.94; P<0.001). Concentrations of interferon (IFN)-α (SD: 1.05; 95% CI: 0.33-1.77; P=0.004), IFN-γ (SD: 0.83; 95% CI: 0.01-1.65; P=0.05) and interleukin (IL)-12 (SD: 2.42; 95% CI: 0.32-4.52; P=0.02), IL-1ß (SD: 0.01; 95% CI: -0.37 to 0.40; P=0.94) increased, whereas those of TNF-α (SD: -0.58; 95% CI: -1.59 to 0.43; P=0.26) and IL-6 (SD: -0.59; 95% CI: -1.24 to 0.07; P=0.08) decreased. Six clinical studies had lower symptom scores (SD: -0.09; 95% CI: -0.44 to 0.26; P=0.61) and less incidence of infection (RR: 0.80; 95% CI: 0.64-1.01; P=0.06). Our research indicates that probiotics and prebiotics pose a defensive possibility on respiratory viral infection and may encourage the clinical application.

Role of γδ T cells in controlling viral infections with a focus on influenza virus: implications for designing novel therapeutic approaches.

BACKGROUND: Influenza virus infection is among the most detrimental threats to the health of humans and some animals, infecting millions of people annually all around the world and in many thousands of cases giving rise to pneumonia and death. All those health crises happen despite previous and recent developments in anti-influenza vaccination, suggesting the need for employing more sophisticated methods to control this malign infection. Main body The innate immunity modules are at the forefront of combating against influenza infection in the respiratory tract, among which, innate T cells, particularly gamma-delta (γδ) T cells, play a critical role in filling the gap needed for adaptive immune cells maturation, linking the innate and adaptive immunity together. Upon infection with influenza virus, production of cytokines and chemokines including CCL3, CCL4, and CCL5 from respiratory epithelium recruits γδ T cells at the site of infection in a CCR5 receptor-dependent fashion. Next, γδ T cells become activated in response to influenza virus infection and produce large amounts of proinflammatory cytokines, especially IL-17A. Regardless of γδ T cells' roles in triggering the adaptive arm of the immune system, they also protect the respiratory epithelium by cytolytic and non-cytolytic antiviral mechanisms, as well as by enhancing neutrophils and natural killer cells recruitment to the infection site. CONCLUSION: In this review, we explored varied strategies of γδ T cells in defense to influenza virus infection and how they can potentially provide balanced protective immune responses against infected cells. The results may provide a potential window for the incorporation of intact or engineered γδ T cells for developing novel antiviral approaches or for immunotherapeutic purposes.

Structure-Based Modification of an Anti-neuraminidase Human Antibody Restores Protection Efficacy against the Drifted Influenza Virus.

Here, we investigate a monoclonal antibody, Z2B3, isolated from an H7N9-infected patient, that exhibited cross-reactivity to both N9 (group 2) and a broad range of seasonal and avian N1 (group 1) proteins but lost activity to the N1 with the substitution K432E. This substitution exists in 99.25% of seasonal influenza strains after 2013. The NA-Z2B3 complex structures indicated that Z2B3 binds within the conserved active site of the neuraminidase (NA) protein. A salt bridge between D102 in Z2B3 and K432 in NA plays an important role in binding. Structure-based modification of Z2B3 with D102R in heavy chain reversed the salt bridge and restored the binding and inhibition of N1 with E432. Furthermore, Z2B3-D102R can protect mice from A/Serbia/NS-601/2014 H1N1 virus (NA contains E432) infection while the wild-type Z2B3 antibody shows no protection. This study demonstrates that a broadly reactive and protective antibody to NA can be in principle edited to restore binding and inhibition to recently drifted N1 NA and regain protection against the variant influenza strain.IMPORTANCE The immune system produces antibodies to protect the human body from harmful invaders. The monoclonal antibody (MAb) is one kind of effective antivirals. In this study, we isolated an antibody (Z2B3) from an H7N9 influenza virus-infected child. It shows cross-reactivity to both group 1 (N1) and group 2 (N9) neuraminidases (NAs) but is sensitive to N1 NA with a K432E substitution. Structural analysis of the NA-antibody fragment antigen-binding (Fab) complex provides a clue for antibody modification, and the modified antibody restored binding and inhibition to recently drifted N1 NA and regained protection against the variant influenza strain. This finding suggests that antibodies to NA may be a useful therapy and can be in principle edited to defeat drifted influenza virus.

Immunotherapy targeting the Streptococcus pyogenes M protein or streptolysin O to treat or prevent influenza A superinfection.

Viral infections complicated by a bacterial infection are typically referred to as coinfections or superinfections. Streptococcus pyogenes, the group A streptococcus (GAS), is not the most common bacteria associated with influenza A virus (IAV) superinfections but did cause significant mortality during the 2009 influenza pandemic even though all isolates are susceptible to penicillin. One approach to improve the outcome of these infections is to use passive immunization targeting GAS. To test this idea, we assessed the efficacy of passive immunotherapy using antisera against either the streptococcal M protein or streptolysin O (SLO) in a murine model of IAV-GAS superinfection. Prophylactic treatment of mice with antiserum to either SLO or the M protein decreased morbidity compared to mice treated with non-immune sera; however, neither significantly decreased mortality. Therapeutic use of antisera to SLO decreased morbidity compared to mice treated with non-immune sera but neither antisera significantly reduced mortality. Overall, the results suggest that further development of antibodies targeting the M protein or SLO may be a useful adjunct in the treatment of invasive GAS diseases, including IAV-GAS superinfections, which may be particularly important during influenza pandemics.

Influenza virus glycoprotein-reactive human monoclonal antibodies.

Influenza continues to be a significant public health challenge. Two glycoproteins on the surface of influenza virus, hemagglutinin and neuraminidase, play a prominent role in the process of influenza virus infection and release. Monoclonal antibodies targeting glycoproteins can effectively prevent the spread of the virus. In this review, we summarized currently reported human monoclonal antibodies targeting glycoproteins of influenza A and B viruses.

Current status of cell-based therapies for respiratory virus infections: applicability to COVID-19.

The severe respiratory consequences of the coronavirus disease 2019 (COVID-19) pandemic have prompted urgent need for novel therapies. Cell-based approaches, primarily using mesenchymal stem (stromal) cells (MSCs), have demonstrated safety and possible efficacy in patients with acute respiratory distress syndrome (ARDS), although they are not yet well studied in respiratory virus-induced ARDS. Limited pre-clinical data suggest that systemic MSC administration can significantly reduce respiratory virus (influenza strains H5N1 and H9N2)-induced lung injury; however, there are no available data in models of coronavirus respiratory infection.There is a rapidly increasing number of clinical investigations of cell-based therapy approaches for COVID-19. These utilise a range of different cell sources, doses, dosing strategies and targeted patient populations. To provide a rational strategy to maximise potential therapeutic use, it is critically important to understand the relevant pre-clinical studies and postulated mechanisms of MSC actions in respiratory virus-induced lung injuries. This review presents these, along with consideration of current clinical investigations.

Generation of neutralizing and non-neutralizing monoclonal antibodies against H7N9 influenza virus.

The H7N9 viruses have been circulating for six years. The insertion of a polybasic cleavage site in the haemagglutinin (HA) protein of H7N9 has resulted in the emergence of a highly pathogenic (HP) avian influenza virus. Currently, there are limited studies on neutralizing monoclonal antibodies(mAbs) against HP H7N9 AIVs. In this study, mice were immunized with inactivated H7N9 vaccine of A/ZJU01/PR8/2013 to produce murine mAbs. Finally, two murine mAbs against the HA of low pathogenic (LP) virus were produced and characterized. Characterization included determining mAbs binding breadth and affinity, in vitro neutralization capacity, and potential in vivo protection. Two of these mAbs, 1H10 and 2D1, have been identified to have therapeutic and prophylactic efficacy against the HP strain in mouse passive transfer-viral challenge experiments. The mAb 1H10 was most efficacious, even if the treatment-time was as late as 72 h post-infection, or the therapeutic dose was as low as 1 mg/kg; and it was confirmed to have haemagglutination inhibition and neutralizing activity on both LP-and HP-H7N9 strains. Further study indicated that the protection provided by 2D1 was mediated by antibody-dependent cellular cytotoxicity. The mAbs described here provide promising results and merit further development into potential antiviral therapeutics for H7N9 infection.

Efficacy of anti-influenza immunoglobulin (FLU-IGIV) in ferrets and mice infected with 2009 pandemic influenza virus.

Seasonal influenza causes significant morbidity and mortality around the world each year, even with the use of vaccines and antivirals. There is a need for more effective treatments for severe and hospitalized cases of influenza. In this study, we have tested the efficacy of a human plasma-derived IgG product (FLU-IGIV) against seasonal influenza in mouse and ferret models of influenza infection. FLU-IGIV successfully protected mice (100% survival) against lethal influenza infection. Also, the survival rate observed with FLU-IGIV treatment was better than the survival rate observed with oseltamivir (60% survival). FLU-IGIV significantly reduced the viral load in the lungs compared to placebo (PBS) in ferrets infected with influenza A/California/07/2009 (H1N1pdm09) virus. Overall, these studies demonstrate the efficacy of human plasma-derived FLU-IGIV in relevant animal models of influenza virus infection.

A Broad and Potent H1-Specific Human Monoclonal Antibody Produced in Plants Prevents Influenza Virus Infection and Transmission in Guinea Pigs.

Although seasonal influenza vaccines block most predominant influenza types and subtypes, humans still remain vulnerable to waves of seasonal and new potential pandemic influenza viruses for which no immunity may exist because of viral antigenic drift and/or shift. Previously, we described a human monoclonal antibody (hMAb), KPF1, which was produced in human embryonic kidney 293T cells (KPF1-HEK) with broad and potent neutralizing activity against H1N1 influenza A viruses (IAV) in vitro, and prophylactic and therapeutic activities in vivo. In this study, we produced hMAb KPF1 in tobacco plants (KPF1-Antx) and demonstrated how the plant-produced KPF1-Antx hMAb possesses similar biological activity compared with the mammalian-produced KPF1-HEK hMAb. KPF1-Antx hMAb showed broad binding to recombinant HA proteins and H1N1 IAV, including A/California/04/2009 (pH1N1) in vitro, which was comparable to that observed with KPF1-HEK hMAb. Importantly, prophylactic administration of KPF1-Antx hMAb to guinea pigs prevented pH1N1 infection and transmission in both prophylactic and therapeutic experiments, substantiating its clinical potential to prevent and treat H1N1 infections. Collectively, this study demonstrated, for the first time, a plant-produced influenza hMAb with in vitro and in vivo activity against influenza virus. Because of the many advantages of plant-produced hMAbs, such as rapid batch production, low cost, and the absence of mammalian cell products, they represent an alternative strategy for the production of immunotherapeutics for the treatment of influenza viral infections, including emerging seasonal and/or pandemic strains.

Targeting the IL-22/IL-22BP axis enhances tight junctions and reduces inflammation during influenza infection.

The seasonal burden of influenza coupled with the pandemic outbreaks of more pathogenic strains underscore a critical need to understand the pathophysiology of influenza injury in the lung. Interleukin-22 (IL-22) is a promising cytokine that is critical in protecting the lung during infection. This cytokine is strongly regulated by the soluble receptor IL-22-binding protein (IL-22BP), which is constitutively expressed in the lungs where it inhibits IL-22 activity. The IL-22/IL-22BP axis is thought to prevent chronic exposure of epithelial cells to IL-22. However, the importance of this axis is not understood during an infection such as influenza. Here we demonstrate through the use of IL-22BP-knockout mice (il-22ra2-/-) that a pro-IL-22 environment reduces pulmonary inflammation during H1N1 (PR8/34 H1N1) infection and protects the lung by promoting tight junction formation. We confirmed these results in normal human bronchial epithelial cells in vitro demonstrating improved membrane resistance and induction of the tight junction proteins Cldn4, Tjp1, and Tjp2. Importantly, we show that administering recombinant IL-22 in vivo reduces inflammation and fluid leak into the lung. Taken together, our results demonstrate the IL-22/IL-22BP axis is a potential targetable pathway for reducing influenza-induced pneumonia.

Generation of Stable Influenza Virus Hemagglutinin through Structure-Guided Recombination.

Hemagglutinin (HA) is the major surface antigen of influenza virus and the most promising influenza vaccine immunogen. In 2013, the devastating H7N9 influenza virus was identified in China, which induced high mortality. The HA of this virus (H7) is relatively unstable, making it challenging to produce an effective vaccine. To improve the stability of HA protein from H7N9 influenza virus for better vaccine antigens without impairing immunogenicity, we recombined the HA from H7N9 (H7) with a more stable HA from H3N2 (H3) by structure-guided recombination, resulting in six chimeric HAs, FrA-FrF. Two of these chimeric HAs, FrB and FrC, exhibited proper hemagglutination activity and presented improved thermal stability compared to the original H7. Mice immunized with FrB and FrC elicited H7-specific antibodies comparable to those induced by parental H7, and the antisera collected from these immunized mice successfully inhibited H7N9 infection in a microneutralization assay. These results suggest that our structural-recombination approach can create stabilizing chimeric antigens while maintaining proper immunogenicity, which may not only benefit the construction of more stable HA vaccines to fight against H7N9 infection, but also facilitate effective vaccine improvements for other influenza viruses or infectious pathogens. In addition, this study also demonstrates the potential for better engineering of multimeric protein complexes like HA to achieve improved function, which are often immunologically or pharmaceutically important but difficult to modify.

New antibody-based prevention and treatment options for influenza.

The antigenic diversity of human influenza viruses represents a challenge to the development of vaccines with durable immune protection. In addition, small molecule anti-influenza viral drugs can bring clinical relief to influenza patients but the emergence of drug resistant viruses can rapidly limit the effectiveness of such drugs. In the past decade, a number of human monoclonal antibodies have been described that can bind to and neutralize a broad range of influenza A and B viruses. Most of these monoclonal antibodies are directed against the viral hemagglutinin (HA) stalk and some have now been evaluated in early to mid-stage clinical trials. An important conclusion from these clinical studies is that hemagglutinin stalk-specific antibodies are safe and can reduce influenza symptoms. In addition, examples of bi- and multi-specific anti-influenza antibodies are discussed, although such antibodies have not yet progressed into clinical testing. In the future, antibody-based therapies might become part of our arsenal to prevent and treat influenza.

Rapid isolation of a potent human antibody against H7N9 influenza virus from an infected patient.

Influenza virus A H7N9 remains a serious threat to public health due to the lack of effective vaccines and drugs. In this study, a neutralizing human antibody named 3L11 was rapidly isolated from the switched memory B cells of a patient infected with H7N9. The antibody 3L11 was encoded by the heavy-chain VH1-8 gene and the light-chain VL2-13 gene that had undergone somatic mutations, and conferred high affinity binding to H7N9 hemagglutinins (HAs). It promoted killing of infected cells by antibody-dependent cell-mediated cytotoxicity (ADCC). Epitope mapping by mass spectroscopy (MS) indicated that 3L11 bound to the peptide 149-175 of HAs that contained the 150-loop of the receptor-binding site (RBS). Additionally, the 3L11 escape strains had G151R (Gly151→Arg151) and S152P (Ser152→Pro152) mutations within a conserved antigenic site A near the RBS that were not observed in field strains. Importantly, 3L11 fully protected mice against a lethal H7N9 virus challenge, in both pre- and postexposure administration regimens. Altogether, this work demonstrates the feasibility of rapid isolation of neutralizing H7N9 antibodies from infected patients and provides a potential prophylactic and therapeutic agent against H7N9 viruses.

A guinea pig IFNA1 gene with antiviral activity against human influenza virus infection.

We previously reported a natural antisense (AS) RNA as an important modulator of human interferon-Alpha1 (IFNA1) mRNA levels. Here, we identified the guinea pig (Cavia porcellus) IFNA1 gene to enable a proof-of-concept experiment to be performed to confirm that the AS-mRNA regulatory axis exerts in vivo control over innate immunity. We selected a guinea pig model system for influenza virus infection because guinea pigs encode a functional Mx1 gene, an important anti-viral effector in the type I interferon pathway. We identified 15 guinea pig IFNA1 gene candidates upon bioinformatic analysis and selected the three candidates with the highest sequence homology to Homo sapiens, Mus musculus and Marmota himalayana IFNA1. The anti-viral activity of guinea pig IFN-Alpha1 protein against influenza virus A/Puerto Rico/8/34- or endomyocarditis virus-infection was then determined for the three gene candidates. We identified cpIFNA1 as the candidate with the highest sequence homologies and best anti-viral effects. cpIFNA1 will enable us to perform a proof-of-concept experiment to verify that IFN-Alpha1 AS increases cpIFNA1 mRNA levels, resulting in inhibition of influenza virus proliferation in vivo.

Broad and Protective Influenza B Virus Neuraminidase Antibodies in Humans after Vaccination and their Clonal Persistence as Plasma Cells.

Although most seasonal inactivated influenza vaccines (IIV) contain neuraminidase (NA), the extent and mechanisms of action of protective human NA-specific humoral responses induced by vaccination are poorly resolved. Due to the propensity of influenza virus for antigenic drift and shift and its tendency to elicit predominantly strain-specific antibodies, humanity remains susceptible to waves of new strains of seasonal viruses and is at risk from viruses with pandemic potential for which limited or no immunity may exist. Here we demonstrate that the use of IIV results in increased levels of influenza B virus (IBV) NA-specific serum antibodies. Detailed analysis of the IBV NA B cell response indicates concurrent expansion of IBV NA-specific peripheral blood plasmablasts 7 days after IIV immunization which express monoclonal antibodies with broad and potent antiviral activity against both IBV Victoria and Yamagata lineages and prophylactic and therapeutic activity in mice. These IBV NA-specific B cell clonal lineages persisted in CD138+ long-lived bone marrow plasma cells. These results represent the first demonstration that IIV-induced NA human antibodies can protect and treat influenza virus infection in vivo and suggest that IIV can induce a subset of IBV NA-specific B cells with broad protective potential, a feature that warrants further study for universal influenza vaccine development.IMPORTANCE Influenza virus infections continue to cause substantial morbidity and mortality despite the availability of seasonal vaccines. The extensive genetic variability in seasonal and potentially pandemic influenza strains necessitates new vaccine strategies that can induce universal protection by focusing the immune response on generating protective antibodies against conserved targets such as regions within the influenza neuraminidase protein. We have demonstrated that seasonal immunization stimulates neuraminidase-specific antibodies in humans that are broad and potent in their protection from influenza B virus when tested in mice. These antibodies further persist in the bone marrow, where they are expressed by long-lived antibody-producing cells, referred to here as plasma cells. The significance in our research is the demonstration that seasonal influenza immunization can induce a subset of neuraminidase-specific B cells with broad protective potential, a process that if further studied and enhanced could aid in the development of a universal influenza vaccine.

CD4+ TSCMs in the Bone Marrow Assist in Maturation of Antibodies against Influenza in Mice.

The bone marrow (BM) is not only a reservoir of hematopoietic stem cells but a repository of immunological memory cells. Further characterizing BM-resident memory T cells would be helpful to reveal the complicated relationship between the BM and immunological memory. In this study, we identified CD122high stem cell antigen-1 (Sca-1) high B cell lymphoma 2 (Bcl-2) high CD4+ stem cell-like memory T cells (TSCMs) as a distinct memory T cell subset, which preferentially reside in the BM, where they respond vigorously to blood-borne antigens. Interestingly, the natural CD4+ TSCMs homing to the BM colocalized with VCAM-1+ IL-15+ IL-7+ CXCL-12+ stromal cells. Furthermore, compared to spleen-resident CD4+ TSCMs, BM-resident TSCMs induced the production of high-affinity antibodies against influenza by B lymphocytes more efficiently. Taken together, these observations indicate that the BM provides an appropriate microenvironment for the survival of CD4+ TSCMs, which broadens our knowledge regarding the memory maintenance of antigen-specific CD4+ T lymphocytes.

An IgM antibody targeting the receptor binding site of influenza B blocks viral infection with great breadth and potency.

Broadly neutralizing antibodies (bnAbs) targeting the receptor binding site (RBS) of hemagglutinin (HA) have potential for developing into powerful anti-influenza agents. Several previously reported influenza B bnAbs are nevertheless unable to neutralize a portion of influenza B virus variants. HA-specific bnAbs with hemagglutination inhibition (HI) activity may possess the ability to block virus entry directly. Polymeric IgM antibodies are expected to more effectively inhibit virus attachment and entry into target cells due to their higher avidity and/or steric hindrance. We therefore hypothesized that certain RBS-targeted IgM antibodies with strong cross-lineage HI activity might display broader and more potent antiviral activity against rapidly evolving influenza B viruses. Methods: In this study, we generated IgM and IgG bnAbs targeting the RBS of influenza B virus using the murine hybridoma technique. IgM and IgG versions of the same antibodies were then developed by isotype switching and characterized in subsequent in vitro and in vivo experiments. Results: Two IgM and two IgG bnAbs against influenza B virus HA were identified. Of these, one IgM subtype antibody, C7G6-IgM, showed strong HI and neutralization activities against all 20 representative influenza B strains tested, with higher potency and broader breadth of anti-influenza activity in vitro than the IgG subtype variant of itself, or other previously-reported influenza B bnAbs. Furthermore, C7G6-IgM conferred excellent cross-protection against distinct lineages of influenza B viruses in mice and ferrets, performing better than the anti-influenza drug oseltamivir, and showed an additive antiviral effect when administered in combination with oseltamivir. Mechanistically, C7G6-IgM potently inhibits infection with influenza B virus strains from different lineages by blocking viral entry. Conclusion: In summary, our study highlights the potential of IgM subtype antibodies in combatting pathogenic microbes. Moreover, C7G6-IgM is a promising candidate for the development of prophylactics or therapeutics against influenza B infection.