Neutralization of zoonotic retroviruses by human antibodies: Genotype-specific epitopes within the receptor-binding domain from simian foamy virus.

Infection with viruses of animal origin pose a significant threat to human populations. Simian foamy viruses (SFVs) are frequently transmitted to humans, in which they establish a life-long infection, with the persistence of replication-competent virus. However, zoonotic SFVs do not induce severe disease nor are they transmitted between humans. Thus, SFVs represent a model of zoonotic retroviruses that lead to a chronic infection successfully controlled by the human immune system. We previously showed that infected humans develop potent neutralizing antibodies (nAbs). Within the viral envelope (Env), the surface protein (SU) carries a variable region that defines two genotypes, overlaps with the receptor binding domain (RBD), and is the exclusive target of nAbs. However, its antigenic determinants are not understood. Here, we characterized nAbs present in plasma samples from SFV-infected individuals living in Central Africa. Neutralization assays were carried out in the presence of recombinant SU that compete with SU at the surface of viral vector particles. We defined the regions targeted by the nAbs using mutant SU proteins modified at the glycosylation sites, RBD functional subregions, and genotype-specific sequences that present properties of B-cell epitopes. We observed that nAbs target conformational epitopes. We identified three major epitopic regions: the loops at the apex of the RBD, which likely mediate interactions between Env protomers to form Env trimers, a loop located in the vicinity of the heparan binding site, and a region proximal to the highly conserved glycosylation site N8. We provide information on how nAbs specific for each of the two viral genotypes target different epitopes. Two common immune escape mechanisms, sequence variation and glycan shielding, were not observed. We propose a model according to which the neutralization mechanisms rely on the nAbs to block the Env conformational change and/or interfere with binding to susceptible cells. As the SFV RBD is structurally different from known retroviral RBDs, our data provide fundamental knowledge on the structural basis for the inhibition of viruses by nAbs. Trial registration: The study was registered at www.clinicaltrials.gov: https://clinicaltrials.gov/ct2/show/NCT03225794/.

The crystal structure of a simian Foamy Virus receptor binding domain provides clues about entry into host cells.

The surface envelope glycoprotein (Env) of all retroviruses mediates virus binding to cells and fusion of the viral and cellular membranes. A structure-function relationship for the HIV Env that belongs to the Orthoretrovirus subfamily has been well established. Structural information is however largely missing for the Env of Foamy viruses (FVs), the second retroviral subfamily. In this work we present the X-ray structure of the receptor binding domain (RBD) of a simian FV Env at 2.57 Å resolution, revealing two subdomains and an unprecedented fold. We have generated a model for the organization of the RBDs within the trimeric Env, which indicates that the upper subdomains form a cage-like structure at the apex of the Env, and identified residues K342, R343, R359 and R369 in the lower subdomain as key players for the interaction of the RBD and viral particles with heparan sulfate.

A partial form of inherited human USP18 deficiency underlies infection and inflammation.

Human USP18 is an interferon (IFN)-stimulated gene product and a negative regulator of type I IFN (IFN-I) signaling. It also removes covalently linked ISG15 from proteins, in a process called deISGylation. In turn, ISG15 prevents USP18 from being degraded by the proteasome. Autosomal recessive complete USP18 deficiency is life-threatening in infancy owing to uncontrolled IFN-I-mediated autoinflammation. We report three Moroccan siblings with autoinflammation and mycobacterial disease who are homozygous for a new USP18 variant. We demonstrate that the mutant USP18 (p.I60N) is normally stabilized by ISG15 and efficient for deISGylation but interacts poorly with the receptor-anchoring STAT2 and is impaired in negative regulation of IFN-I signaling. We also show that IFN-γ-dependent induction of IL-12 and IL-23 is reduced owing to IFN-I-mediated impairment of myeloid cells to produce both cytokines. Thus, insufficient negative regulation of IFN-I signaling by USP18-I60N underlies a specific type I interferonopathy, which impairs IL-12 and IL-23 production by myeloid cells, thereby explaining predisposition to mycobacterial disease.

USP18 and ISG15 coordinately impact on SKP2 and cell cycle progression.

USP18 is an isopeptidase that cleaves the ubiquitin-like ISG15 from conjugates and is also an essential negative feedback regulator of type I interferon signaling. We and others reported that USP18 protein is stabilized by ISG15 and targeted for degradation by SKP2 (S-phase kinase associated protein 2), the substrate-recognition subunit of the SCFSKP2 ubiquitin E3 ligase complex, which operates in cell cycle progression. Here, we have analyzed how, under non stimulated conditions, USP18, ISG15 and SKP2 communicate with each other, by enforcing or silencing their expression. We found that USP18 and SKP2 interact and that free ISG15 abrogates the complex, liberating USP18 from degradation and concomitantly driving SKP2 to degradation and/or ISGylation. These data reveal a dynamic interplay where the substrate USP18 stabilizes SKP2, both exogenous and endogenous. Consistent with this we show that silencing of baseline USP18 slows down progression of HeLa S3 cells towards S phase. Our findings point to USP18 and ISG15 as unexpected new SKP2 regulators, which aid in cell cycle progression at homeostasis.