Chimeric camel/human heavy-chain antibodies protect against MERS-CoV infection

Antibodies and vaccines against Middle East respiratory syndrome coronavirus

The Middle East respiratory syndrome coronavirus (MERS-CoV) has spread through 27 countries and infected more than 2,200 people since its first outbreak in Saudi Arabia in 2012. The high fatality rate (35.4%) of this novel coronavirus and its persistent wide spread infectiousness in animal reservoirs have generated tremendous global public health concern. However, no licensed therapeutic agents or vaccines against MERS-CoV are currently available and only a limited few have entered clinical trials. Among all the potential targets of MERS-CoV, the spike glycoprotein (S) has been the most well-studied due to its critical role in mediating viral entry and in inducing a protective antibody response in infected individuals. The most notable studies include the recent discoveries of monoclonal antibodies and development of candidate vaccines against the S glycoprotein. Structural characterization of MERS-CoV S protein bound with these monoclonal antibodies has provided insights into the mechanisms of humoral immune responses against MERS-CoV infection. The current review aims to highlight these developments and discuss possible hurdles and strategies to translate these discoveries into ultimate medical interventions against MERS-CoV infection.

Camelid VHHs Fused to Human Fc Fragments Provide Long Term Protection Against Botulinum Neurotoxin A in Mice

The bacterium Clostridium botulinum is the causative agent of botulism-a severe intoxication caused by botulinum neurotoxin (BoNT) and characterized by damage to the nervous system. In an effort to develop novel C. botulinum immunotherapeutics, camelid single-domain antibodies (sdAbs, VHHs, or nanobodies) could be used due to their unique structure and characteristics. In this study, VHHs were produced using phage display technology. A total of 15 different monoclonal VHHs were selected based on their comlementarity-determining region 3 (CDR3) sequences. Different toxin lethal dose (LD50) challenges with each selected phage clone were conducted in vivo to check their neutralizing potency. We demonstrated that modification of neutralizing VHHs with a human immunoglobulin G (IgG)1 Fc (fragment crystallizable) fragment (fusionbody, VHH-Fc) significantly increased the circulation time in the blood (up to 14 days). At the same time, VHH-Fc showed the protective activity 1000 times higher than monomeric form when challenged with 5 LD50. Moreover, VHH-Fcs remained protective even 14 days after antibody administration. These results indicate that this VHH-Fc could be used as an effective long term antitoxin protection against botulinum type A.

Species-Specific Colocalization of Middle East Respiratory Syndrome Coronavirus Attachment and Entry Receptors

MERS-CoV uses the S1^B domain of its spike protein to attach to its host receptor, dipeptidyl peptidase 4 (DPP4). The tissue localization of DPP4 has been mapped in different susceptible species. On the other hand, the S1^A domain, the N-terminal domain of this spike protein, preferentially binds to several glycotopes of α2,3-sialic acids, the attachment factor of MERS-CoV. Here we show, using a novel method, that the S1^A domain specifically binds to the nasal epithelium of dromedary camels, alveolar epithelium of humans, and intestinal epithelium of common pipistrelle bats. In contrast, it does not bind to the nasal epithelium of pigs or rabbits, nor does it bind to the intestinal epithelium of serotine bats and frugivorous bat species. This finding supports the importance of the S1^A domain in MERS-CoV infection and tropism, suggests its role in transmission, and highlights its potential use as a component of novel vaccine candidates.

Middle East respiratory syndrome coronavirus (MERS-CoV) uses the S1^B domain of its spike protein to bind to dipeptidyl peptidase 4 (DPP4), its functional receptor, and its S1^A domain to bind to sialic acids. The tissue localization of DPP4 in humans, bats, camelids, pigs, and rabbits generally correlates with MERS-CoV tropism, highlighting the role of DPP4 in virus pathogenesis and transmission. However, MERS-CoV S1^A does not indiscriminately bind to all α2,3-sialic acids, and the species-specific binding and tissue distribution of these sialic acids in different MERS-CoV-susceptible species have not been investigated. We established a novel method to detect these sialic acids on tissue sections of various organs of different susceptible species by using nanoparticles displaying multivalent MERS-CoV S1^A. We found that the nanoparticles specifically bound to the nasal epithelial cells of dromedary camels, type II pneumocytes in human lungs, and the intestinal epithelial cells of common pipistrelle bats. Desialylation by neuraminidase abolished nanoparticle binding and significantly reduced MERS-CoV infection in primary susceptible cells. In contrast, S1^A nanoparticles did not bind to the intestinal epithelium of serotine bats and frugivorous bat species, nor did they bind to the nasal epithelium of pigs and rabbits. Both pigs and rabbits have been shown to shed less infectious virus than dromedary camels and do not transmit the virus via either contact or airborne routes. Our results depict species-specific colocalization of MERS-CoV entry and attachment receptors, which may be relevant in the transmission and pathogenesis of MERS-CoV.

IMPORTANCE MERS-CoV uses the S1^B domain of its spike protein to attach to its host receptor, dipeptidyl peptidase 4 (DPP4). The tissue localization of DPP4 has been mapped in different susceptible species. On the other hand, the S1^A domain, the N-terminal domain of this spike protein, preferentially binds to several glycotopes of α2,3-sialic acids, the attachment factor of MERS-CoV. Here we show, using a novel method, that the S1^A domain specifically binds to the nasal epithelium of dromedary camels, alveolar epithelium of humans, and intestinal epithelium of common pipistrelle bats. In contrast, it does not bind to the nasal epithelium of pigs or rabbits, nor does it bind to the intestinal epithelium of serotine bats and frugivorous bat species. This finding supports the importance of the S1^A domain in MERS-CoV infection and tropism, suggests its role in transmission, and highlights its potential use as a component of novel vaccine candidates.

Towards a solution to MERS: protective human monoclonal antibodies targeting different domains and functions of the MERS-coronavirus spike glycoprotein

The Middle-East respiratory syndrome coronavirus (MERS-CoV) is a zoonotic virus that causes severe and often fatal respiratory disease in humans. Efforts to develop antibody-based therapies have focused on neutralizing antibodies that target the receptor binding domain of the viral spike protein thereby blocking receptor binding. Here, we developed a set of human monoclonal antibodies that target functionally distinct domains of the MERS-CoV spike protein. These antibodies belong to six distinct epitope groups and interfere with the three critical entry functions of the MERS-CoV spike protein: sialic acid binding, receptor binding and membrane fusion. Passive immunization with potently as well as with poorly neutralizing antibodies protected mice from lethal MERS-CoV challenge. Collectively, these antibodies offer new ways to gain humoral protection in humans against the emerging MERS-CoV by targeting different spike protein epitopes and functions.

Enhanced Ability of Oligomeric Nanobodies Targeting MERS Coronavirus Receptor-Binding Domain

Middle East respiratory syndrome (MERS) coronavirus (MERS-CoV), an infectious coronavirus first reported in 2012, has a mortality rate greater than 35%. Therapeutic antibodies are key tools for preventing and treating MERS-CoV infection, but to date no such agents have been approved for treatment of this virus. Nanobodies (Nbs) are camelid heavy chain variable domains with properties distinct from those of conventional antibodies and antibody fragments. We generated two oligomeric Nbs by linking two or three monomeric Nbs (Mono-Nbs) targeting the MERS-CoV receptor-binding domain (RBD), and compared their RBD-binding affinity, RBD⁻receptor binding inhibition, stability, and neutralizing and cross-neutralizing activity against MERS-CoV. Relative to Mono-Nb, dimeric Nb (Di-Nb) and trimeric Nb (Tri-Nb) had significantly greater ability to bind MERS-CoV RBD proteins with or without mutations in the RBD, thereby potently blocking RBD⁻MERS-CoV receptor binding. The engineered oligomeric Nbs were very stable under extreme conditions, including low or high pH, protease (pepsin), chaotropic denaturant (urea), and high temperature. Importantly, Di-Nb and Tri-Nb exerted significantly elevated broad-spectrum neutralizing activity against at least 19 human and camel MERS-CoV strains isolated in different countries and years. Overall, the engineered Nbs could be developed into effective therapeutic agents for prevention and treatment of MERS-CoV infection.

Advances in MERS-CoV Vaccines and Therapeutics Based on the Receptor-Binding Domain

Middle East respiratory syndrome (MERS) coronavirus (MERS-CoV) is an infectious virus that was first reported in 2012. The MERS-CoV genome encodes four major structural proteins, among which the spike (S) protein has a key role in viral infection and pathogenesis. The receptor-binding domain (RBD) of the S protein contains a critical neutralizing domain and is an important target for development of MERS vaccines and therapeutics. In this review, we describe the relevant features of the MERS-CoV S-protein RBD, summarize recent advances in the development of MERS-CoV RBD-based vaccines and therapeutic antibodies, and illustrate potential challenges and strategies to further improve their efficacy.

Single intranasal immunization with chimpanzee adenovirus-based vaccine induces sustained and protective immunity against MERS-CoV infection

The recently identified Middle East Respiratory Syndrome Coronavirus (MERS-CoV) causes severe and fatal acute respiratory illness in humans. However, no approved prophylactic and therapeutic interventions are currently available. The MERS-CoV envelope spike protein serves as a crucial target for neutralizing antibodies and vaccine development, as it plays a critical role in mediating viral entry through interactions with the cellular receptor, dipeptidyl peptidase 4 (DPP4). Here, we constructed a recombinant rare serotype of the chimpanzee adenovirus 68 (AdC68) that expresses full-length MERS-CoV S protein (AdC68-S). Single intranasal immunization with AdC68-S induced robust and sustained neutralizing antibody and T cell responses in BALB/c mice. In a human DPP4 knock-in (hDPP4-KI) mouse model, it completely protected against lethal challenge with a mouse-adapted MERS-CoV (MERS-CoV-MA). Passive transfer of immune sera to naïve hDPP4-KI mice also provided survival advantages from lethal MERS-CoV-MA challenge. Analysis of sera absorption and isolated monoclonal antibodies from immunized mice demonstrated that the potent and broad neutralizing activity was largely attributed to antibodies targeting the receptor binding domain (RBD) of the S protein. These results show that AdC68-S can induce protective immune responses in mice and represent a promising candidate for further development against MERS-CoV infection in both dromedaries and humans.

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.

Neutralizing Monoclonal Antibodies as Promising Therapeutics against Middle East Respiratory Syndrome Coronavirus Infection

Since emerging in 2012, Middle East Respiratory Syndrome Coronavirus (MERS-CoV) has been a global public health threat with a high fatality rate and worldwide distribution. There are no approved vaccines or therapies for MERS until now. Passive immunotherapy with neutralizing monoclonal antibodies (mAbs) is an effective prophylactic and therapeutic reagent against emerging viruses. In this article, we review current advances in neutralizing mAbs against MERS-CoV. The receptor-binding domain (RBD) in the spike protein of MERS-CoV is a major target, and mouse, camel, or human-derived neutralizing mAbs targeting RBD have been developed. A major problem with neutralizing mAb therapy is mutant escape under selective pressure, which can be solved by combination of neutralizing mAbs targeting different epitopes. Neutralizing mAbs are currently under preclinical evaluation, and they are promising candidate therapeutic agents against MERS-CoV infection.