Single-domain antibodies and their formatting to combat viral infections

Since their discovery in the 1990s, single-domain antibodies (VHHs), also known as NanobodiesA (R), 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

Potent Single-Domain Antibodies that Arrest Respiratory Syncytial Virus Fusion Protein in its Prefusion State

Human respiratory syncytial virus (RSV) is the main cause of lower respiratory tract infections in young children. The RSV fusion protein (F) is highly conserved and is the only viral membrane protein that is essential for infection. The prefusion conformation of RSV F is considered the most relevant target for antiviral strategies because it is the fusion-competent form of the protein and the primary target of neutralizing activity present in human serum. Here, we describe two llama-derived single-domain antibodies (VHHs) that have potent RSV-neutralizing activity and bind selectively to prefusion RSV F with picomolar affinity. Crystal structures of these VHHs in complex with prefusion F show that they recognize a conserved cavity formed by two F protomers. In addition, the VHHs prevent RSV replication and lung infiltration of inflammatory monocytes and T cells in RSV-challenged mice. These prefusion F-specific VHHs represent promising antiviral agents against RSV

Potent single-domain antibodies that arrest respiratory syncytial virus fusion protein in its prefusion state

Human respiratory syncytial virus (RSV) is the main cause of lower respiratory tract infections in young children. The RSV fusion protein (F) is highly conserved and is the only viral membrane protein that is essential for infection. The prefusion conformation of RSV F is considered the most relevant target for antiviral strategies because it is the fusion-competent form of the protein and the primary target of neutralizing activity present in human serum. Here, we describe two llama-derived single-domain antibodies (VHHs) that have potent RSV-neutralizing activity and bind selectively to prefusion RSV F with picomolar affinity. Crystal structures of these VHHs in complex with prefusion F show that they recognize a conserved cavity formed by two F protomers. In addition, the VHHs prevent RSV replication and lung infiltration of inflammatory monocytes and T cells in RSV-challenged mice. These prefusion F-specific VHHs represent promising antiviral agents against RSV

Vaccines against human respiratory syncytial virus in clinical trials, where are we now?

Introduction: Human respiratory syncytial virus (RSV) is a major health threat both for the very young and the elderly. With yearly 3.2 million hospital admissions and approximately 118,000 deaths due to RSV in children across the globe, the impact of this infectious disease is very high. Development of a safe RSV vaccine is of utmost importance but has proven to be challenging for several reasons. Researchers are faced with the history of a failed RSV vaccine trial, difficult target populations, a virus that naturally does not induce a long-lasting immune response and ambiguity concerning the optimal correlate of protection. Many different vaccine formats are being tested in preclinical models and about 30 candidate RSV vaccines are being evaluated in clinical trials. Areas covered: In this review we focus on the difficulties concerning the development of an effective RSV vaccine and discuss vaccines that are currently in clinical trials and how they have dealt with these challenges. We review live-attenuated vaccines, vectored vaccines, subunit vaccines and particle-based vaccines. Expert opinion: It is clear that this field is progressing rapidly with several promising RSV vaccine candidates. A safe and effective RSV vaccine might be on the brink of clinical implementation soon

Clinical potential of prefusion RSV F-specific antibodies

Human respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infections in the very young. The RSV fusion protein (F) is essential for virus entry because it mediates viral and host membrane fusion. During this fusion process F is converted from a metastable prefusion conformation into an energetically favored postfusion state. Antibodies that target F can prevent viral entry and reduce disease caused by RSV. During recent years, many prefusion F-specific antibodies have been described. These antibodies typically have stronger RSV-neutralizing activity compared to those that also bind F in the postfusion conformation. Here, we describe how F-specific antibodies protect against RSV and why specifically targeting prefusion F could have great clinical potential

How RSV proteins join forces to overcome the host innate immune response

Respiratory syncytial virus (RSV) is the leading cause of severe acute lower respiratory tract infections in infants worldwide. Although several pattern recognition receptors (PRRs) can sense RSV-derived pathogen-associated molecular patterns (PAMPs), infection with RSV is typically associated with low to undetectable levels of type I interferons (IFNs). Multiple RSV proteins can hinder the host's innate immune response. The main players are NS1 and NS2 which suppress type I IFN production and signalling in multiple ways. The recruitment of innate immune cells and the production of several cytokines are reduced by RSV G. Next, RSV N can sequester immunostimulatory proteins to inclusion bodies (IBs). N might also facilitate the assembly of a multiprotein complex that is responsible for the negative regulation of innate immune pathways. Furthermore, RSV M modulates the host's innate immune response. The nuclear accumulation of RSV M has been linked to an impaired host gene transcription, in particular for nuclear-encoded mitochondrial proteins. In addition, RSV M might also directly target mitochondrial proteins which results in a reduced mitochondrion-mediated innate immune recognition of RSV. Lastly, RSV SH might prolong the viral replication in infected cells and influence cytokine production

A vulnerable, membrane-proximal site in human respiratory syncytial virus F revealed by a prefusion-specific single-domain antibody

Human respiratory syncytial virus (RSV) is a major cause of lower respiratory tract disease, especially in young children and the elderly. The fusion protein (F) exists in a pre- and postfusion conformation and is the main target of RSV-neutralizing antibodies. Highly potent RSV-neutralizing antibodies typically bind sites that are unique to the prefusion conformation of F. In this study, we screened a single-domain antibody (VHH) library derived from a llama immunized with prefusion-stabilized F and identified a prefusion F-specific VHH that can neutralize RSV A at subnanomolar concentrations. Structural analysis revealed that this VHH primarily binds to antigenic site I while also making contacts with residues in antigenic sites III and IV. This new VHH reveals a previously underappreciated membrane-proximal region sensitive to neutralization. IMPORTANCE RSV is an important respiratory pathogen. This study describes a prefusion F-specific VHH that primarily binds to antigenic site I of RSV F. This is the first time that a prefusion F-specific antibody that binds this site has been reported. In general, antibodies that bind to site I are poorly neutralizing, whereas the VHH described here neutralizes RSV A at subnanomolar concentrations. Our findings contribute to insights into the RSV F antigenic map

GlycoVHH : optimal sites for introducing N-glycans on the camelid VHH antibody scaffold and use for macrophage delivery

As small and stable high-affinity antigen binders, VHHs boast attractive characteristics both for therapeutic use in various disease indications, and as versatile reagents in research and diagnostics. To further increase the versatility of VHHs, we explored the VHH scaffold in a structure-guided approach to select regions where the introduction of an N-glycosylation N-X-T sequon and its associated glycan should not interfere with protein folding or epitope recognition. We expressed variants of such glycoengineered VHHs in the Pichia pastoris GlycoSwitchM5 strain, allowing us to pinpoint preferred sites at which Man(5)GlcNAc(2)-glycans can be introduced at high site occupancy without affecting antigen binding. A VHH carrying predominantly a Man(5)GlcNAc(2) N-glycan at one of these preferred sites showed highly efficient, glycan-dependent uptake by Mf4/4 macrophages in vitro and by alveolar lung macrophages in vivo, illustrating one potential application of glyco-engineered VHHs: a glycan-based targeting approach for lung macrophage endolysosomal system delivery. The set of optimal artificial VHH N-glycosylation sites identified in this study can serve as a blueprint for targeted glyco-engineering of other VHHs, enabling site-specific functionalization through the rapidly expanding toolbox of synthetic glycobiology