Viruses must evade the host innate defenses for replication and dengue is no exception. During secondary infection with a heterologous dengue virus (DENV) serotype, DENV is opsonized with sub-or nonneutralizing antibodies that enhance infection of monocytes, macrophages, and dendritic cells via the Fc-gamma receptor (FcγR), a process termed antibody-dependent enhancement of DENV infection. However, this enhancement of DENV infection is curious as cross-linking of activating FcγRs signals an early antiviral response by inducing the type-I IFN-stimulated genes (ISGs). Entry through activating FcγR would thus place DENV in an intracellular environment unfavorable for enhanced replication. Here we demonstrate that, to escape this antiviral response, antibody-opsonized DENV coligates leukocyte Ig-like receptor-B1 (LILRB1) to inhibit FcγR signaling for ISG expression. This immunoreceptor tyrosine-based inhibition motif-bearing receptor recruits Src homology phosphatase-1 to dephosphorylate spleen tyrosine kinase (Syk). As Syk is a key intermediate of FcγR signaling, LILRB1 coligation resulted in reduced ISG expression for enhanced DENV replication. Our findings suggest a unique mechanism for DENV to evade an early antiviral response for enhanced infection.early innate immune response | innate immune signaling | immune evasion D espite long-lived serotype-specific immunity upon initial infection, predicted global prevalence of dengue now surpasses World Health Organization estimates by more than threefold with 390 million cases annually (1). Furthermore, the risk of severe disease is augmented by cross-reactive or subneutralizing levels of antibody (2, 3), which opsonize dengue virus (DENV) to ligate Fc-gamma receptor (FcγR) for entry into monocytes, macrophages, and dendritic cells, a phenomenon known as antibody-dependent enhancement (ADE) of DENV infection (4, 5). The resultant greater viral burden leads to increased systemic inflammation that precipitates plasma leakage, a hallmark of dengue hemorrhagic fever (6). However, ligation of the activating FcγRs by immune complexes has been shown to induce type-I IFN stimulated genes (ISGs), independent of autocrine or paracrine IFN activity, unless the inhibitory FcγRIIB is coligated (7). We and others reported recently that coligation of FcγRIIB by DENV immune complexes requires high antibody concentration, and such coligation inhibited the entry of DENV immune complexes into monocytes (8, 9). At low antibody concentrations where ADE occurs, the inhibitory FcγRIIB is not coligated (9). Ligation of the activating FcγRs by DENV opsonized with subneutralizing levels of antibody would thus induce the expression of ISGs and hinder DENV replication (10). Here, we demonstrate that DENV employs a unique evasive mechanism by coligating LILRB1 to down-regulate the early antiviral responses triggered by activating FcγRs for ADE.Results ADE Differs in THP-1 Subclones. Our work was enabled by the isolation of subclones of THP-1 cells with different phenotypes to ADE. The low rate of...

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A characteristic clinical feature of dengue virus infection is thrombocytopenia, though its underlying mechanism is not definitively determined. By adoptive transfer of human CD34؉ fetal liver cells into immunodeficient mice, we have constructed humanized mice with significant levels of human platelets, monocytes/macrophages, and hepatocytes. Infection of these mice with both lab-adapted and clinical strains of dengue virus induces characteristic human hematological changes, including transient leukopenia and thrombocytopenia. We show that the specific depletion of human platelets is not mediated by antibodies in the periphery or reduced production of human thrombopoietin in the liver but reduction of human megakaryocytes and megakaryocyte progenitors in the bone marrow of the infected mice. These findings identify inhibition of platelet production in the bone marrow as a key mechanism underlying dengue-induced thrombocytopenia and suggest the utility of the improved humanized mouse model in studying dengue virus infection and pathogenesis in a human cell context. Dengue is an acute febrile illness caused by dengue virus (DENV), which is spread through mosquito vectors. Dengue manifests in a wide range of clinical symptoms and is usually accompanied by hematological changes, such as leukopenia and thrombocytopenia in mild cases and plasma leakage, hemorrhage, or organ impairment, such as liver damage, in severe cases (1). Even after decades of research, the cause of thrombocytopenia or platelet drop during dengue disease is still unclear.Platelets are small (2 to 3 m), anucleated cells which play a major role in homeostasis and coagulation. They are derived from megakaryocytes, which are large (30-to 100-m), nucleated, polyploid cells. Megakaryocytes differentiate from hematopoietic stem cells in the bone marrow (BM) (2, 3) through progenitor cells known as megakaryocytic CFU (CFU-MK) that express CD34 and CD41 (4). Thrombopoietin (TPO) is the principal cytokine that drives the expansion and differentiation of the progenitors to megakaryocytes (3, 5) and is produced mainly by hepatocytes in the liver (6).Two hypotheses have been proposed to explain thrombocytopenia during DENV infection: clearance of platelets from periphery and loss of platelet production in the BM. The peripheral mechanism is thought to involve antibody-mediated depletion where the antibody-opsonized DENV binds to platelets, which are then cleared by activated immune cells (7,8). Another possibility is that antibodies to the viral nonstructural protein (NS1) crossreact with autoantigens expressed on platelets, which binds and tags them for clearance (9, 10). Alternatively, platelet production in the BM could be suppressed, although the evidence supporting this hypothesis is lacking due to difficulties in obtaining BM biopsy specimens from acute dengue patients. One report shows that the BM was hypocellular early during dengue disease but later became hypercellular, as if recovered from acute suppression (11). There is also evidence that DENV i...

Background: The SARS coronavirus is the etiologic agent for the epidemic of the Severe Acute Respiratory Syndrome. The recent emergence of this new pathogen, the careful tracing of its transmission patterns, and the ability to propagate in culture allows the exploration of the mutational dynamics of the SARS-CoV in human populations.

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