Dengue virus NS1 triggers endothelial permeability and vascular leak that is prevented by NS1 vaccination

Dengue virus NS1 triggers endothelial permeability and vascular leak, and vaccination with NS1 induces a protective antibody response. A leak in the dike Everyone knows how mosquitos can wreck an end-of-summer picnic. But in some climates, these pesky intruders persist and carry a variety of detrimental diseases—some with no preventative vaccines or targeted therapies. One such passenger is dengue virus (DENV), which infects up to 400 million people each year and comes in several serotypes (1 to 4) and disease presentations—from mild infection to severe disease and sometimes death. But to treat or prevent dengue requires that we have a more complete picture of the disease pathology. Now, Modhiran et al. and Beatty et al. describe the results of in vitro and in vivo experiments that point to circulating dengue virus non-structural protein 1 (NS1) and the innate immune Toll-like receptor 4 (TLR4) as a focus for basic scientists as well as vaccine and drug developers. DENV infection protects a patient from future reinfection with the same DENV serotype as well as producing temporary immune protection from severe dengue disease caused by a different DENV serotype. But unlike diamonds, this immune protection doesn’t last forever, and when the protected period passes, the patient becomes at increased risk of enhanced infection and progression to severe disease if he or she is infected with a second DENV serotype. This severe form of dengue infection is believed to result from immunopathogenic processes that induce cytokine storm and cause vascular leakage that leads to shock. Until now, no dengue viral proteins have been linked to vascular endothelium permeability (that is, vascular leakage). Beatty et al. show that inoculation of mice with DENV NS1 protein alone induces both vascular leak and secretion of inflammatory cytokines and that administration of NS1 with a sublethal dose of DENV2 leads to lethal vascular leak syndrome. In human endothelial cell monolayers in culture, NS1 from any of the four DENV serotypes triggered endothelial barrier permeability. NS1’s pathogenic effects were blocked by NS1-immune polyclonal mouse serum or monoclonal antibodies to NS1 (in vivo and in vitro), and immunization of mice with NS1 protected against lethal DENV2 challenge. In an independent study, Mondrian et al. explore the underlying of NS1’s effects. They show that highly purified NS1 acts as a pathogen-associated molecular pattern (PAMP) that activates mouse macrophages and human peripheral blood mononuclear cells (PBMCs) in culture via TLR4, resulting in release of inflammatory cytokines—an effect that was blocked by either a TLR4 antagonist or an anti-TLR4 antibody. Then, in an in vitro model of vascular leak, the authors found that NS1 fractured the integrity of endothelial cell monolayers through a TLR4-dependent pathway, a finding that was supported by the observation that a TLR4 antagonist quelled capillary leak in a mouse model of dengue virus infection. Together, these new findings highlight NS1 as an instigator of dengue-associated vascular leak and thus pinpoint a potential target for dengue drugs and component for dengue vaccines. The four dengue virus serotypes (DENV1 to DENV4) are mosquito-borne flaviviruses that cause up to ~100 million cases of dengue annually worldwide. Severe disease is thought to result from immunopathogenic processes involving serotype cross-reactive antibodies and T cells that together induce vasoactive cytokines, causing vascular leakage that leads to shock. However, no viral proteins have been directly implicated in triggering endothelial permeability, which results in vascular leakage. DENV nonstructural protein 1 (NS1) is secreted and circulates in patients’ blood during acute infection; high levels of NS1 are associated with severe disease. We show that inoculation of mice with DENV NS1 alone induces both vascular leakage and production of key inflammatory cytokines. Furthermore, simultaneous administration of NS1 with a sublethal dose of DENV2 results in a lethal vascular leak syndrome. We also demonstrate that NS1 from DENV1, DENV2, DENV3, and DENV4 triggers endothelial barrier dysfunction, causing increased permeability of human endothelial cell monolayers in vitro. These pathogenic effects of physiologically relevant amounts of NS1 in vivo and in vitro were blocked by NS1-immune polyclonal mouse serum or monoclonal antibodies to NS1, and immunization of mice with NS1 from DENV1 to DENV4 protected against lethal DENV2 challenge. These findings add an important and previously overlooked component to the causes of dengue vascular leak, identify a new potential target for dengue therapeutics, and support inclusion of NS1 in dengue vaccines.