Structural Basis for Potent Neutralization of Betacoronaviruses by Single-Domain Camelid Antibodies.

Coronaviruses make use of a large envelope protein called spike (S) to engage host cell receptors and catalyze membrane fusion. Because of the vital role that these S proteins play, they represent a vulnerable target for the development of therapeutics. Here, we describe the isolation of single-domain antibodies (VHHs) from a llama immunized with prefusion-stabilized coronavirus spikes. These VHHs neutralize MERS-CoV or SARS-CoV-1 S pseudotyped viruses, respectively. Crystal structures of these VHHs bound to their respective viral targets reveal two distinct epitopes, but both VHHs interfere with receptor binding. We also show cross-reactivity between the SARS-CoV-1 S-directed VHH and SARS-CoV-2 S and demonstrate that this cross-reactive VHH neutralizes SARS-CoV-2 S pseudotyped viruses as a bivalent human IgG Fc-fusion. These data provide a molecular basis for the neutralization of pathogenic betacoronaviruses by VHHs and suggest that these molecules may serve as useful therapeutics during coronavirus outbreaks.

Chemical-controlled Activation of Antiviral Myxovirus Resistance Protein 1.

The antiviral myxovirus resistance protein 1 (MX1) is an interferon-induced GTPase that plays an important role in the defense of mammalian cells against influenza A viruses. Mouse MX1 interacts with the influenza ribonucleoprotein complexes (vRNPs) and can prevent the interaction between polymerase basic 2 (PB2) and the nucleoprotein (NP) of influenza A viruses. However, it is unclear whether mouse MX1 disrupts the PB2-NP interaction in the context of pre-existing vRNPs or prevents the assembly of new vRNP components. Here, we describe a conditionally active mouse MX1 variant that only exerts antiviral activity in the presence of a small molecule drug. Once activated, this MX1 construct phenocopies the antiviral and NP binding activity of wild type MX1. The interaction between PB2 and NP is disrupted within minutes after the addition of the small molecule activator. These findings support a model in which mouse MX1 interacts with the incoming influenza A vRNPs and inhibits their activity by disrupting the PB2-NP interaction.

Functional Comparison of Mx1 from Two Different Mouse Species Reveals the Involvement of Loop L4 in the Antiviral Activity against Influenza A Viruses.

UNLABELLED: The interferon-induced Mx1 gene is an important part of the mammalian defense against influenza viruses. Mus musculus Mx1 inhibits influenza A virus replication and transcription by suppressing the polymerase activity of viral ribonucleoproteins (vRNPs). Here, we compared the anti-influenza virus activity of Mx1 from Mus musculus A2G with that of its ortholog from Mus spretus. We found that the antiviral activity of M. spretus Mx1 was less potent than that of M. musculus Mx1. Comparison of the M. musculus Mx1 sequence with the M. spretus Mx1 sequence revealed 25 amino acid differences, over half of which were present in the GTPase domain and 2 of which were present in loop L4. However, the in vitro GTPase activity of Mx1 from the two mouse species was similar. Replacement of one of the residues in loop L4 in M. spretus Mx1 by the corresponding residue of A2G Mx1 increased its antiviral activity. We also show that deletion of loop L4 prevented the binding of Mx1 to influenza A virus nucleoprotein and, hence, abolished the antiviral activity of mouse Mx1. These results indicate that loop L4 of mouse Mx1 is a determinant of antiviral activity. Our findings suggest that Mx proteins from different mammals use a common mechanism to inhibit influenza A viruses. IMPORTANCE: Mx proteins are evolutionarily conserved in vertebrates and inhibit a wide range of viruses. Still, the exact details of their antiviral mechanisms remain largely unknown. Functional comparison of the Mx genes from two species that diverged relatively recently in evolution can provide novel insights into these mechanisms. We show that both Mus musculus A2G Mx1 and Mus spretus Mx1 target the influenza virus nucleoprotein. We also found that loop L4 in mouse Mx1 is crucial for its antiviral activity, as was recently reported for primate MxA. This indicates that human and mouse Mx proteins, which have diverged by 75 million years of evolution, recognize and inhibit influenza A viruses by a common mechanism.

An affinity-enhanced, broadly neutralizing heavy chain-only antibody protects against SARS-CoV-2 infection in animal models.

Broadly neutralizing antibodies are an important treatment for individuals with coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Antibody-based therapeutics are also essential for pandemic preparedness against future Sarbecovirus outbreaks. Camelid-derived single domain antibodies (VHHs) exhibit potent antimicrobial activity and are being developed as SARS-CoV-2­neutralizing antibody-like therapeutics. Here, we identified VHHs that neutralize both SARS-CoV-1 and SARS-CoV-2, including now circulating variants. We observed that the VHHs bound to a highly conserved epitope in the receptor binding domain of the viral spike protein that is difficult to access for human antibodies. Structure-guided molecular modeling, combined with rapid yeast-based prototyping, resulted in an affinity enhanced VHH-human immunoglobulin G1 Fc fusion molecule with subnanomolar neutralizing activity. This VHH-Fc fusion protein, produced in and purified from cultured Chinese hamster ovary cells, controlled SARS-CoV-2 replication in prophylactic and therapeutic settings in mice expressing human angiotensin converting enzyme 2 and in hamsters infected with SARS-CoV-2. These data led to affinity-enhanced selection of the VHH, XVR011, a stable anti­COVID-19 biologic that is now being evaluated in the clinic.

mRNA Encoding a Bispecific Single Domain Antibody Construct Protects against Influenza A Virus Infection in Mice.

To date, mRNA-based biologics have mainly been developed for prophylactic and therapeutic vaccination to combat infectious diseases or cancer. In the past years, optimization of the characteristics of in vitro transcribed mRNA has led to significant reduction of the inflammatory responses. Thanks to this, mRNA therapeutics have entered the field of passive immunization. Here, we established an mRNA treatment that is based on mRNA that codes for a bispecific single-domain antibody construct that can selectively recruit innate immune cells to cells infected with influenza A virus. The constructs consist of a single-domain antibody that binds to the ectodomain of the conserved influenza A matrix protein 2, while the other single-domain antibody binds to the activating mouse Fcγ receptor IV. Formulating the mRNA into DOTAP (1,2-dioleoyl-3-trimethylammonium-propane)/cholesterol nanoparticles and delivering these intratracheally to mice allowed the production of the bispecific single-domain antibody in the lungs, and administration of these mRNA-particles prior to influenza A virus infection was associated with a significant reduction in viral titers and a reduced morbidity in mice. Overall, our data provide evidence that the local delivery of mRNA encoding a bispecific single-domain antibody format in the lungs could be a promising pulmonary antiviral prophylactic treatment.

STAT2 signaling restricts viral dissemination but drives severe pneumonia in SARS-CoV-2 infected hamsters.

Emergence of SARS-CoV-2 causing COVID-19 has resulted in hundreds of thousands of deaths. In search for key targets of effective therapeutics, robust animal models mimicking COVID-19 in humans are urgently needed. Here, we show that Syrian hamsters, in contrast to mice, are highly permissive to SARS-CoV-2 and develop bronchopneumonia and strong inflammatory responses in the lungs with neutrophil infiltration and edema, further confirmed as consolidations visualized by micro-CT alike in clinical practice. Moreover, we identify an exuberant innate immune response as key player in pathogenesis, in which STAT2 signaling plays a dual role, driving severe lung injury on the one hand, yet restricting systemic virus dissemination on the other. Our results reveal the importance of STAT2-dependent interferon responses in the pathogenesis and virus control during SARS-CoV-2 infection and may help rationalizing new strategies for the treatment of COVID-19 patients.

Selective Engagement of FcγRIV by a M2e-Specific Single Domain Antibody Construct Protects Against Influenza A Virus Infection.

Lower respiratory tract infections, such as infections caused by influenza A viruses, are a constant threat for public health. Antivirals are indispensable to control disease caused by epidemic as well as pandemic influenza A. We developed a novel anti-influenza A virus approach based on an engineered single-domain antibody (VHH) construct that can selectively recruit innate immune cells to the sites of virus replication. This protective construct comprises two VHHs. One VHH binds with nanomolar affinity to the conserved influenza A matrix protein 2 (M2) ectodomain (M2e). Co-crystal structure analysis revealed that the complementarity determining regions 2 and 3 of this VHH embrace M2e. The second selected VHH specifically binds to the mouse Fcγ Receptor IV (FcγRIV) and was genetically fused to the M2e-specific VHH, which resulted in a bi-specific VHH-based construct that could be efficiently expressed in Pichia pastoris. In the presence of M2 expressing or influenza A virus-infected target cells, this single domain antibody construct selectively activated the mouse FcγRIV. Moreover, intranasal delivery of this bispecific FcγRIV-engaging VHH construct protected wild type but not FcγRIV-/- mice against challenge with an H3N2 influenza virus. These results provide proof of concept that VHHs directed against a surface exposed viral antigen can be readily armed with effector functions that trigger protective antiviral activity beyond direct virus neutralization.

Vaccine options for influenza: thinking small.

Vaccines that direct the immune response towards conserved B cell epitopes of influenza viruses can provide broad protection. In many instances, this requires the design of vaccine antigens that stimulate the immune system to levels that far exceed the natural responses towards such antigens. Here we focus on the matrix protein 2 ectodomain (M2e) as a 'universal' influenza A vaccine candidate. Thanks to its small size and high solubility, M2e can be expressed and delivered in almost any format. Protection against experimental influenza A virus challenge by M2e-based vaccines has been demonstrated in natural host of influenza and clinical studies demonstrated that such vaccines are safe and immunogenic. M2e-specific antibodies protect mainly by Fc receptor-mediated antibody-dependent cellular phagocytosis activity, which is reminiscent to how antibodies directed against the hemagglutinin stalk protect in vivo. Fighting influenza with a broadly protective influenza vaccine will likely require a blend of conserved antigens. M2e deserves its place in such a blend.

Single-Domain Antibodies and Their Formatting to Combat Viral Infections.

Since their discovery in the 1990s, single-domain antibodies (VHHs), also known as Nanobodies®, have changed the landscape of affinity reagents. The outstanding solubility, stability, and specificity of VHHs, as well as their small size, ease of production and formatting flexibility favor VHHs over conventional antibody formats for many applications. The exceptional ease by which it is possible to fuse VHHs with different molecular modules has been particularly explored in the context of viral infections. In this review, we focus on VHH formats that have been developed to combat viruses including influenza viruses, human immunodeficiency virus-1 (HIV-1), and human respiratory syncytial virus (RSV). Such formats may significantly increase the affinity, half-life, breadth of protection of an antiviral VHH and reduce the risk of viral escape. In addition, VHHs can be equipped with effector functions, for example to guide components of the immune system with high precision to sites of viral infection.

Co-immunoprecipitation of the Mouse Mx1 Protein with the Influenza A Virus Nucleoprotein.

Studying the interaction between proteins is key in understanding their function(s). A very powerful method that is frequently used to study interactions of proteins with other macromolecules in a complex sample is called co-immunoprecipitation. The described co-immunoprecipitation protocol allows to demonstrate and further investigate the interaction between the antiviral myxovirus resistance protein 1 (Mx1) and one of its viral targets, the influenza A virus nucleoprotein (NP). The protocol starts with transfected mammalian cells, but it is also possible to use influenza A virus infected cells as starting material. After cell lysis, the viral NP protein is pulled-down with a specific antibody and the resulting immune-complexes are precipitated with protein G beads. The successful pull-down of NP and the co-immunoprecipitation of the antiviral Mx1 protein are subsequently revealed by western blotting. A prerequisite for successful co-immunoprecipitation of Mx1 with NP is the presence of N-ethylmaleimide (NEM) in the cell lysis buffer. NEM alkylates free thiol groups. Presumably this reaction stabilizes the weak and/or transient NP-Mx1 interaction by preserving a specific conformation of Mx1, its viral target or an unknown third component. An important limitation of co-immunoprecipitation experiments is the inadvertent pull-down of contaminating proteins, caused by nonspecific binding of proteins to the protein G beads or antibodies. Therefore, it is very important to include control settings to exclude false positive results. The described co-immunoprecipitation protocol can be used to study the interaction of Mx proteins from different vertebrate species with viral proteins, any pair of proteins, or of a protein with other macromolecules. The beneficial role of NEM to stabilize weak and/or transient interactions needs to be tested for each interaction pair individually.