prM-reactive antibodies reveal a role for partially mature virions in dengue virus pathogenesis.

Cleavage of the flavivirus premembrane (prM) structural protein during maturation can be inefficient. The contribution of partially mature flavivirus virions that retain uncleaved prM to pathogenesis during primary infection is unknown. To investigate this question, we characterized the functional properties of newly-generated dengue virus (DENV) prM-reactive monoclonal antibodies (mAbs) in vitro and using a mouse model of DENV disease. Anti-prM mAbs neutralized DENV infection in a virion maturation state-dependent manner. Alanine scanning mutagenesis and cryoelectron microscopy of anti-prM mAbs in complex with immature DENV defined two modes of attachment to a single antigenic site. In vivo, passive transfer of intact anti-prM mAbs resulted in an antibody-dependent enhancement of disease. However, protection against DENV-induced lethality was observed when the transferred mAbs were genetically modified to inhibit their ability to interact with Fcγ receptors. These data establish that in addition to mature forms of the virus, partially mature infectious prM+ virions can also contribute to pathogenesis during primary DENV infections.

A protective human monoclonal antibody targeting the West Nile virus E protein preferentially recognizes mature virions.

West Nile virus (WNV), a member of the Flavivirus genus, is a leading cause of viral encephalitis in the United States1. The development of neutralizing antibodies against the flavivirus envelope (E) protein is critical for immunity and vaccine protection2. Previously identified candidate therapeutic mouse and human neutralizing monoclonal antibodies (mAbs) target epitopes within the E domain III lateral ridge and the domain I-II hinge region, respectively3. To explore the neutralizing antibody repertoire elicited by WNV infection for potential therapeutic application, we isolated ten mAbs from WNV-infected individuals. mAb WNV-86 neutralized WNV with a 50% inhibitory concentration of 2 ng ml-1, one of the most potently neutralizing flavivirus-specific antibodies ever isolated. WNV-86 targets an epitope in E domain II, and preferentially recognizes mature virions lacking an uncleaved form of the chaperone protein prM, unlike most flavivirus-specific antibodies4. In vitro selection experiments revealed a neutralization escape mechanism involving a glycan addition to E domain II. Finally, a single dose of WNV-86 administered two days post-infection protected mice from lethal WNV challenge. This study identifies a highly potent human neutralizing mAb with therapeutic potential that targets an epitope preferentially displayed on mature virions.

Packaged and Free Satellite Tobacco Mosaic Virus (STMV) RNA Genomes Adopt Distinct Conformational States.

The RNA genomes of viruses likely undergo multiple functionally important conformational changes during their replication cycles, changes that are poorly understood at present. We used two complementary in-solution RNA structure probing strategies (SHAPE-MaP and RING-MaP) to examine the structure of the RNA genome of satellite tobacco mosaic virus inside authentic virions and in a capsid-free state. Both RNA states feature similar three-domain architectures in which each major replicative function-translation, capsid coding, and genome synthesis-fall into distinct domains. There are, however, large conformational differences between the in-virion and capsid-free states, primarily in one arm of the central T domain. These data support a model in which the packaged capsid-bound RNA is constrained in a local high-energy conformation by the native capsid shell. The removal of the viral capsid then allows the RNA genome to relax into a more thermodynamically stable conformation. The RNA architecture of the central T domain thus likely changes during capsid assembly and disassembly and may play a role in genome packaging.

De Novo Sphingolipid Biosynthesis Is Required for Adipocyte Survival and Metabolic Homeostasis.

Sphingolipids are a diverse class of essential cellular lipids that function as structural membrane components and as signaling molecules. Cells acquire sphingolipids by both de novo biosynthesis and recycling of exogenous sphingolipids. The individual importance of these pathways for the generation of essential sphingolipids in differentiated cells is not well understood. To investigate the requirement for de novo sphingolipid biosynthesis in adipocytes, a cell type with highly regulated lipid metabolism, we generated mice with an adipocyte-specific deletion of Sptlc1 Sptlc1 is an obligate subunit of serine palmitoyltransferase, the enzyme responsible for the first and rate-limiting step of de novo sphingolipid biosynthesis. These mice, which initially developed adipose tissue, exhibited a striking age-dependent loss of adipose tissue accompanied by evidence of adipocyte death, increased macrophage infiltration, and tissue fibrosis. Adipocyte differentiation was not affected by the Sptlc1 deletion. The mice also had elevated fasting blood glucose, fatty liver, and insulin resistance. Collectively, these data indicate that de novo sphingolipid biosynthesis is required for adipocyte cell viability and normal metabolic function and that reduced de novo sphingolipid biosynthesis within adipocytes is associated with adipocyte death, adipose tissue remodeling, and metabolic dysfunction.