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African swine fever virus (ASFV) is a giant and complex DNA virus that causes a highly contagious and often lethal swine disease for which no vaccine is available. Using an optimized image reconstruction strategy, we solved the ASFV capsid structure up to 4.1 angstroms, which is built from 17,280 proteins, including one major (p72) and four minor (M1249L, p17, p49, and H240R) capsid proteins organized into pentasymmetrons and trisymmetrons. The atomic structure of the p72 protein informs putative conformational epitopes, distinguishing ASFV from other nucleocytoplasmic large DNA viruses. The minor capsid proteins form a complicated network below the outer capsid shell, stabilizing the capsid by holding adjacent capsomers together. Acting as core organizers, 100-nanometer-long M1249L proteins run along each edge of the trisymmetrons that bridge two neighboring pentasymmetrons and form extensive intermolecular networks with other capsid proteins, driving the formation of the capsid framework. These structural details unveil the basis of capsid stability and assembly, opening up new avenues for African swine fever vaccine development.

Bacteria have evolved diverse immunity mechanisms to protect themselves against the constant onslaught of bacteriophages 1 – 3 . Similar to how eukaryotic innate immune systems sense foreign invaders through pathogen-associated molecular patterns 4 (PAMPs), many bacterial immune systems that respond to bacteriophage infection require phage-specific triggers to be activated. However, the identities of such triggers and the sensing mechanisms remain largely unknown. Here we identify and investigate the anti-phage function of CapRel SJ46 , a fused toxin–antitoxin system that protects Escherichia coli against diverse phages. Using genetic, biochemical and structural analyses, we demonstrate that the C-terminal domain of CapRel SJ46 regulates the toxic N-terminal region, serving as both antitoxin and phage infection sensor. Following infection by certain phages, newly synthesized major capsid protein binds directly to the C-terminal domain of CapRel SJ46 to relieve autoinhibition, enabling the toxin domain to pyrophosphorylate tRNAs, which blocks translation to restrict viral infection. Collectively, our results reveal the molecular mechanism by which a bacterial immune system directly senses a conserved, essential component of phages, suggesting a PAMP-like sensing model for toxin–antitoxin-mediated innate immunity in bacteria. We provide evidence that CapRels and their phage-encoded triggers are engaged in a ‘Red Queen conflict’ 5 , revealing a new front in the intense coevolutionary battle between phages and bacteria. Given that capsid proteins of some eukaryotic viruses are known to stimulate innate immune signalling in mammalian hosts 6 – 10 , our results reveal a deeply conserved facet of immunity. Genetic, biochemical and structural studies provide insights into the function of Escherichia coli CapRel SJ46 as a fused anti-phage toxin–antitoxin system that binds SECΦ27 Gp57 capsid protein.

Virus-like particles (VLPs) are non-infectious self-assembling nanoparticles, useful in medicine and nanotechnology. Their repetitive molecularly-defined architecture is attractive for engineering multivalency, notably for vaccination. However, decorating VLPs with target-antigens by genetic fusion or chemical modification is time-consuming and often leads to capsid misassembly or antigen misfolding, hindering generation of protective immunity. Here we establish a platform for irreversibly decorating VLPs simply by mixing with protein antigen. SpyCatcher is a genetically-encoded protein designed to spontaneously form a covalent bond to its peptide-partner SpyTag. We expressed in E. coli VLPs from the bacteriophage AP205 genetically fused to SpyCatcher. We demonstrated quantitative covalent coupling to SpyCatcher-VLPs after mixing with SpyTag-linked to malaria antigens, including CIDR and Pfs25. In addition, we showed coupling to the VLPs for peptides relevant to cancer from epidermal growth factor receptor and telomerase. Injecting SpyCatcher-VLPs decorated with a malarial antigen efficiently induced antibody responses after only a single immunization. This simple, efficient and modular decoration of nanoparticles should accelerate vaccine development, as well as other applications of nanoparticle devices.

Norovirus gastroenteritis is a major public health burden worldwide. Although fecal shedding is important for transmission of enteric viruses, little is known about the immune factors that restrict persistent enteric infection. We report here that although the cytokines interferon-α (IFN-α) and IFN-β prevented the systemic spread of murine norovirus (MNoV), only IFN-λ controlled persistent enteric infection. Infection-dependent induction of IFN-λ was governed by the MNoV capsid protein and correlated with diminished enteric persistence. Treatment of established infection with IFN-λ cured mice in a manner requiring nonhematopoietic cell expression of the IFN-λ receptor, Ifnlr1, and independent of adaptive immunity. These results suggest the therapeutic potential of IFN-λ for curing virus infection in the gastrointestinal tract.

Foot-and-mouth disease virus (FMDV) causes an economically devastating disease of livestock that is controlled in endemic areas by vaccines containing intact inactivated FMDV particles. In this study, a novel monoclonal antibody named 5B6 has been identified and characterised, that permits the detection of all serotypes of FMDV via a conserved epitope near the N-terminus of the VP4 capsid protein. The antibody recognises intact virus particles known as 146S (the protective antigen) which contain VP4 and not dissociated capsids known as 12S (poorly protective antigen) which lack VP4. This allowed the development of a universal assay to specifically detect the protective antigen in vaccine samples using a simple ELISA. Such a test could be used to assess the quality of formulated vaccine following manufacture or prior to administration, or to assess unformulated vaccine antigen, and would be of great utility to enhance the effectiveness of FMD vaccination programmes.

The type I interferon (IFN) system plays an important role in controlling herpesvirus infections, but it is unclear which IFN-mediated effectors interfere with herpesvirus replication. Here we report that human myxovirus resistance protein B (MxB, also designated Mx2) is a potent human herpesvirus restriction factor in the context of IFN. We demonstrate that ectopic MxB expression restricts a range of herpesviruses from the Alphaherpesvirinae and Gammaherpesvirinae , including herpes simplex virus 1 and 2 (HSV-1 and HSV-2), and Kaposi’s sarcoma-associated herpesvirus (KSHV). MxB restriction of HSV-1 and HSV-2 requires GTPase function, in contrast to restriction of lentiviruses. MxB inhibits the delivery of incoming HSV-1 DNA to the nucleus and the appearance of empty capsids, but not the capsid delivery to the cytoplasm or tegument dissociation from the capsid. Our study identifies MxB as a potent pan-herpesvirus restriction factor which blocks the uncoating of viral DNA from the incoming viral capsid. MxB is an interferon-induced GTPase that inhibits HIV replication. Here, Crameri et al. show that MxB restricts replication of herpesviruses by inhibiting delivery of incoming viral DNA into the nucleus, and this antiviral activity depends on MxB’s GTPase activity.

The coordinated activities of innate and adaptive immunity are critical for effective protection against viruses. To counter this, some viruses have evolved sophisticated strategies to circumvent immune cell recognition. In particular, cytomegaloviruses encode large arsenals of molecules that seek to subvert T cell and natural killer cell function via a remarkable array of mechanisms. Consequently, these ‘immunoevasins’ play a fundamental role in shaping the nature of the immune system by driving the evolution of new immune receptors and recognition mechanisms. Here, we review the diverse strategies adopted by cytomegaloviruses to target immune pathways and outline the host’s response.This Review focuses on the cytomegaloviruses and the sophisticated strategies they have evolved to evade immune recognition. The authors suggest a better appreciation of these pathways could have clinical implications beyond antiviral immunity, for instance in understanding immune evasion in cancer.

Porcine circovirus type 3 (PCV3) is an emerging swine pathogen associated with acute porcine dermatitis and nephropathy syndrome (PDNS)-like clinical signs, reproductive failure, and multisystemic inflammation. Current evidence shows that PCV3 is spread worldwide, and its high incidence may pose a threat to the global pig industry. Capsid (Cap) protein is the sole structural protein which plays an important role in inducing protective immunity against PCV3 infection. In this study, monoclonal antibodies (mAbs) against Cap protein of PCV3 were produced by the hybridoma technique. Subsequently, 12 serial overlapping peptides (P1 to P12) spanning the entire region of Cap were synthesized to determine the B cell epitope regions using the mAbs. Results from dot-blot and peptide ELISA identified that P3, P9, and P10 were the major B cell antigenic regions. Fine mapping by shorter N- and C-terminal truncated peptides confirmed that the motifs 57NKPWH61, 140KHSRYFT146, and 161QSLFFF166 were linear B cell epitopes, which were highly conserved among different PCV3 strains. Interestingly, we found that the motif 140KHSRYFT146 was highly conserved in all reported types of PCVs (i.e., PCV1, PCV2, PCV3, and PCV4), except for the substitution (Y → K → R) of the first residue. This is the first research to identify B cell epitopes of PCV3 Cap, and these findings may lead to a better understanding of the antibody–antigen interaction and provide some guidance for PCV3 vaccine design.Key points• The recombinant Cap protein of PCV3 was expressed and purified in soluble form.• PCV3 Cap-specific mAbs prepared in this study had no cross-reactivity with PCV1/PCV2 Cap.• This is the first report of three conserved linear B cell epitopes on PCV3 Cap.• The minimal residues of the epitopes were 57–61 aa, 140–146 aa, and 161–166 aa.

Anelloviruses are nonpathogenic viruses that comprise a major portion of the human virome. Despite being ubiquitous in the human population, anelloviruses (ANVs) remain poorly understood. Basic features of the virus, such as the identity of its capsid protein and the structure of the viral particle, have been unclear until now. Here, we use cryogenic electron microscopy to describe the first structure of an ANV-like particle. The particle, formed by 60 jelly roll domain-containing ANV capsid proteins, forms an icosahedral particle core from which spike domains extend to form a salient part of the particle surface. The spike domains come together around the 5-fold symmetry axis to form crown-like features. The base of the spike domain, the P1 subdomain, shares some sequence conservation between ANV strains while a hypervariable region, forming the P2 subdomain, is at the spike domain apex. We propose that this structure renders the particle less susceptible to antibody neutralization by hiding vulnerable conserved domains while exposing highly diverse epitopes as immunological decoys, thereby contributing to the immune evasion properties of anelloviruses. These results shed light on the structure of anelloviruses and provide a framework to understand their interactions with the immune system. The authors provide the first anellovirus-like particle structure determined by CryoEM. The authors propose hypervariable regions on the spike domains extending from the particle surface contribute to the immune evasion properties of anelloviruses.

Unlike most other picornaviruses, foot-and-mouth disease (FMD) intact virions (146S) dissociate easily into small pentameric subunits (12S). This causes a dramatically decreased immunogenicity by a mechanism that remains elusive. Here, we present the high-resolution structures of 12S (3.2 Å) and its immune complex of a single-domain antibody (VHH) targeting the particle interior (3.2 Å), as well as two 146S-specific VHHs complexed to distinct sites on the 146S capsid surface (3.6 Å and 2.9 Å). The antigenic landscape of 146S is depicted using 13 known FMD virus-antibody complexes. Comparison of the immunogenicity of 146S and 12S in pigs, focusing on the resulting antigenic sites and incorporating structural analysis, reveals that dissociation of 146S leads to structural alteration and destruction of multiple epitopes, resulting in significant differences in antibody profiles/lineages induced by 12S and 146S. Furthermore, 146S generates higher synergistic neutralizing antibody titers compared to 12S, whereas both particles induce similar total FMD virus specific antibody titers. This study can guide the structure-based rational design of novel multivalent and broad-spectrum recombinant vaccines for protection against FMD. Foot-and-mouth disease vaccine efficacy is reduced when full virion 146S dissociates into pentameric 12S subunit. Here, the authors elucidate the molecular basis of this compromised immunogenicity by comparing antigenicity, humoral immunogenicity and structures of 146S and 12S particles.