The rodent-borne hantavirus Puumala virus (PUUV) and related agents cause hemorrhagic fever with renal syndrome (HFRS) in humans. Other hantaviruses, including Andes virus (ANDV) and Sin Nombre virus, cause a distinct zoonotic disease, hantavirus cardiopulmonary syndrome (HCPS). Although these infections are severe and have substantial case fatality rates, no FDA-approved hantavirus countermeasures are available. Recent work suggests that monoclonal antibodies may have therapeutic utility. We describe here the isolation of human neutralizing antibodies (nAbs) against tetrameric Gn/Gc glycoprotein spikes from PUUV-experienced donors. We define a dominant class of nAbs recognizing the “capping loop” of Gn that masks the hydrophobic fusion loops in Gc. A subset of nAbs in this class, including ADI-42898, bound Gn/Gc complexes but not Gn alone, strongly suggesting that they recognize a quaternary epitope encompassing both Gn and Gc. ADI-42898 blocked the cell entry of seven HCPS- and HFRS-associated hantaviruses, and single doses of this nAb could protect Syrian hamsters and bank voles challenged with the highly virulent HCPS-causing ANDV and HFRS-causing PUUV, respectively. ADI-42898 is a promising candidate for clinical development as a countermeasure for both HCPS and HFRS, and its mode of Gn/Gc recognition informs the development of broadly protective hantavirus vaccines.

Emerging rodent-borne hantaviruses cause severe diseases in humans with no approved vaccines or therapeutics. We recently isolated a monoclonal broadly neutralizing antibody (nAb) from a Puumala virus–experienced human donor. Here, we report its structure bound to its target, the Gn/Gc glycoprotein heterodimer comprising the viral fusion complex. The structure explains the broad activity of the nAb: It recognizes conserved Gc fusion loop sequences and the main chain of variable Gn sequences, thereby straddling the Gn/Gc heterodimer and locking it in its prefusion conformation. We show that the nAb’s accelerated dissociation from the divergent Andes virus Gn/Gc at endosomal acidic pH limits its potency against this highly lethal virus and correct this liability by engineering an optimized variant that sets a benchmark as a candidate pan-hantavirus therapeutic.

Tick-borne encephalitis virus (TBEV) causes tick-borne encephalitis (TBE), a severe and sometimes life-threatening disease characterized by viral invasion of the central nervous system with symptoms of neuroinflammation^1,2. As with other orthoflaviviruses—enveloped, arthropod-borne RNA viruses—host factors required for TBEV entry remain poorly defined. Here we used a genome-scale CRISPR–Cas9-based screen to identify LRP8, an apolipoprotein E and reelin receptor with high expression in the brain, as a TBEV receptor. LRP8 downregulation reduced TBEV infection in human cells, and its overexpression enhanced infection. LRP8 bound directly to the TBEV E glycoprotein and mediated viral attachment and internalization into cells. An LRP8-based soluble decoy blocked infection of human cell lines and neuronal cells and protected mice from lethal TBEV challenge. LRP8’s role as a TBEV receptor has implications for TBEV neuropathogenesis and the development of antiviral countermeasures.

Crimean-Congo hemorrhagic fever virus (CCHFV) is a priority pathogen transmitted by tick bites, with no vaccines or specific therapeutics approved to date. Severe disease manifestations include hemorrhage, endothelial dysfunction, and multiorgan failure. Infected cells release the viral glycoprotein GP38, whose extracellular function is presently unknown. GP38 is considered an important target for vaccine and therapeutic design because GP38-specific antibodies can protect against severe disease in animal models, albeit through an unknown mechanism of action. Here, we showed that GP38 induces endothelial barrier dysfunction in vitro by disrupting the endothelial glycocalyx layer and triggering hyperpermeability. We also demonstrated that GP38 alone can cause vascular leak in a mouse model. We found that CCHFV infection leads to vascular leak in vivo, which was exacerbated by exogenous administration of GP38, facilitating dissemination of CCHFV into target tissues such as the liver. Protective antibodies that recognized specific antigenic sites on GP38, but not a protective neutralizing antibody binding the structural protein Gc, potently inhibited endothelial hyperpermeability in vitro and vascular leak in vivo during CCHFV infection. This work uncovers a function of the circulating viral protein GP38 as a viral toxin in CCHFV pathogenesis and elucidates a potential mode of action of nonneutralizing yet protective GP38-specific antibodies.

Crimean-Congo hemorrhagic fever virus can cause lethal disease in humans yet there are no approved medical countermeasures. Viral glycoprotein GP38, exclusive to Nairoviridae, is a target of protective antibodies and is a key antigen in preclinical vaccine candidates. Here, we isolate 188 GP38-specific antibodies from human survivors of infection. Competition experiments show that these antibodies bind across 5 distinct antigenic sites, encompassing 11 overlapping regions. Additionally, we show structures of GP38 bound with 9 of these antibodies targeting different antigenic sites. Although these GP38-specific antibodies are non-neutralizing, several display protective efficacy equal to or better than murine antibody 13G8 in two highly stringent rodent models of infection. Together, these data expand our understanding regarding this important viral protein and may inform the development of broadly effective CCHFV antibody therapeutics.

Evaluating the adaptive immune responses to natural infection with Crimean-Congo hemorrhagic fever (CCHF) virus (CCHFV) in human survivors is critical to the development of medical countermeasures. However, the correlates of protection are unknown. As the most prevalent tick-borne human hemorrhagic fever virus with case fatality rates of 5%–30% and worldwide distribution, there is an urgent need to fill these knowledge gaps. Here, we describe adaptive immune responses in a cohort of Ugandan CCHF survivors via serial sampling over 6 years. We demonstrate persistent antibodies after infection and cross-neutralization against various clades of authentic CCHFV, as well as potent effector function. Moreover, we show for the first time persistent, polyfunctional antigen-specific memory T-cell responses to multiple CCHFV proteins up to 9 years after infection. Together, this data provides immunological benchmarks for evaluating CCHFV medical countermeasures and information that can be leveraged toward vaccine immunogen design and viral target identification for monoclonal antibody therapies.