Too much or too little—better than some: Dengue fever is caused by a mosquito-transmitted flavivirus resembling Zika virus. Both viruses can cause severe diseases in humans with catastrophic sequelae. It has been suspected in humans, and shown in animal models, that the host's immune responses can make disease worse. Katzelnicket al.examined data from a long-term study of Nicaraguan children exposed to dengue virus (see the Perspective by Feinberg and Ahmed). They confirmed that antibody-dependent enhancement of disease occurs at a specific range of antibody concentrations. Low levels of antibody did not enhance disease, intermediate levels exacerbated disease, and high antibody titers protected against severe disease. These findings have major implications for vaccines against flaviviruses. Indeed, recent vaccine trials have shown evidence of severe disease in some recipients who were previously exposed to virus.Science, this issue p.929; see also p.865

Dengue virus NS1 triggers endothelial permeability and vascular leak, and vaccination with NS1 induces a protective antibody response.

Immunity to one of the four dengue virus (DV) serotypes can increase disease severity in humans upon subsequent infection with another DV serotype. Serotype cross-reactive antibodies facilitate DV infection of myeloid cells in vitro by promoting virus entry via Fcγ receptors (FcγR), a process known as antibody-dependent enhancement (ADE). However, despite decades of investigation, no in vivo model for antibody enhancement of dengue disease severity has been described. Analogous to human infants who receive anti-DV antibodies by transplacental transfer and develop severe dengue disease during primary infection, we show here that passive administration of anti-DV antibodies is sufficient to enhance DV infection and disease in mice using both mouse-adapted and clinical DV isolates. Antibody-enhanced lethal disease featured many of the hallmarks of severe dengue disease in humans, including thrombocytopenia, vascular leakage, elevated serum cytokine levels, and increased systemic viral burden in serum and tissue phagocytes. Passive transfer of a high dose of serotype-specific antibodies eliminated viremia, but lower doses of these antibodies or cross-reactive polyclonal or monoclonal antibodies all enhanced disease in vivo even when antibody levels were neutralizing in vitro. In contrast, a genetically engineered antibody variant (E60-N297Q) that cannot bind FcγR exhibited prophylactic and therapeutic efficacy against ADE-induced lethal challenge. These observations provide insight into the pathogenesis of antibody-enhanced dengue disease and identify a novel strategy for the design of therapeutic antibodies against dengue. Dengue is the most common vector-borne viral disease of humans, with over 3 billion people at risk for infection and 50–100 million infections in tropical and subtropical regions each year. Dengue virus (DV) causes a spectrum of clinical disease ranging from an acute debilitating, self-limited febrile illness (DF) to a life-threatening vascular leakage syndrome, referred to as dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS). There are four serotypes of DENV; infection with one serotype is thought to protect against re-infection with the same serotype, but may either protect against or enhance infection with one of the other three serotypes. Epidemiological and in vitro data has implicated anti-DENV antibodies in mediating pathogenesis of a second DENV infection. However, it is unclear which antibody conditions are protective and which exacerbate disease in vivo, in part because no animal model of antibody-enhanced dengue disease has been available. Here, we present the first animal model of antibody-enhanced severe DENV infection. Importantly, this model recapitulates many aspects of human disease, including vascular leakage, elevated serum cytokine levels, reduced platelet count, and disseminated infection of tissue phagocytes. Furthermore, we demonstrate the utility of this model by showing that a genetically modified anti-DENV antibody that fails to bind the Fcγ receptor has prophylactic and therapeutic efficacy against lethal DENV challenge in vivo.

Dengue is the most prevalent arboviral disease in humans and a major public health problem worldwide. Systemic plasma leakage, leading to hypovolemic shock and potentially fatal complications, is a critical determinant of dengue severity. Recently, we and others described a novel pathogenic effect of secreted dengue virus (DENV) non-structural protein 1 (NS1) in triggering hyperpermeability of human endothelial cells in vitro and systemic vascular leakage in vivo. NS1 was shown to activate toll-like receptor 4 signaling in primary human myeloid cells, leading to secretion of pro-inflammatory cytokines and vascular leakage. However, distinct endothelial cell-intrinsic mechanisms of NS1-induced hyperpermeability remained to be defined. The endothelial glycocalyx layer (EGL) is a network of membrane-bound proteoglycans and glycoproteins lining the vascular endothelium that plays a key role in regulating endothelial barrier function. Here, we demonstrate that DENV NS1 disrupts the EGL on human pulmonary microvascular endothelial cells, inducing degradation of sialic acid and shedding of heparan sulfate proteoglycans. This effect is mediated by NS1-induced expression of sialidases and heparanase, respectively. NS1 also activates cathepsin L, a lysosomal cysteine proteinase, in endothelial cells, which activates heparanase via enzymatic cleavage. Specific inhibitors of sialidases, heparanase, and cathepsin L prevent DENV NS1-induced EGL disruption and endothelial hyperpermeability. All of these effects are specific to NS1 from DENV1-4 and are not induced by NS1 from West Nile virus, a related flavivirus. Together, our data suggest an important role for EGL disruption in DENV NS1-mediated endothelial dysfunction during severe dengue disease. Dengue is the most prevalent mosquito-borne disease in humans and represents a major public health problem worldwide. Leakage of fluids and molecules from the bloodstream into tissues can lead to shock and potentially death and is a critical determinant of dengue disease severity. Recently, we showed that a secreted protein from dengue virus (DENV)-infected cells, non-structural protein 1 (NS1), can trigger increased leakage both in human cell culture and mouse models. It has been shown that NS1 can activate toll-like receptor 4 on peripheral blood mononuclear cells, leading to secretion of pro-inflammatory cytokines that can result in vascular leak. However, the mechanism by which NS1 triggers hyperpermeability directly in human endothelial cells remained undefined. The endothelial glycocalyx layer (EGL) is a network of membrane-bound molecules that lines endothelial cells on the inside of blood vessels, helping to regulate proper vascular function. Here, we show that DENV NS1 can disrupt the integrity of the EGL, inducing breakdown and shedding of key components. This is mediated by NS1 induction of cellular enzymes (e.g., sialidases, heparanase, and cathepsin L) that contribute to EGL alterations. Inhibitors that block these enzymes prevent both EGL disruption and endothelial permeability. These effects were all demonstrated to be specific to NS1 from DENV serotypes 1–4, as NS1 from the related West Nile Virus did not produce EGL alterations or increased leakage. Our study suggests a novel role for DENV NS1 in inducing EGL disruption to increase fluid leakage during severe dengue disease.