Substantial gaps remain in the basic understanding of the pathogenesis of dengue disease. In large part, this limitation is related to the lack of a suitable animal model . Rhesus monkeys develop viremia similar in pattern to humans after dengue virus challenge but do not develop clinical disease. Careful epidemiologic and experimental challenge studies in humans have provided valuable information on dengue virus infection, but detailed data on virus distribution in vivo are available only from small numbers of patients with more severe disease, unusual manifestations, or the later stages of infection. Little pathogenetic information is available concerning milder infections, which constitute the vast majority of cases.
THE DENGUE VIRAL REPLICATION CYCLE
Dengue viruses are members of the family Flaviviridae genus Flavivirus. They are small enveloped viruses containing a single-strand RNA genome of positive polarity . Dengue viruses infect a wide range of human and nonhuman cell types in vitro. Viral replication involves the following steps:
- Attachment to the cell surface
- Entry into the cytoplasm
- Translation of viral proteins
- Replication of the viral RNA genome
- Formation of virions (encapsidation)
- Release from the cell
Binding of dengue virions to cells, which is mediated by the major viral envelope (E) glycoprotein, is critical for infectivity . The determination of the three-dimensional structures of the dengue E glycoprotein and the intact virion has facilitated the understanding of this process [4-6]. Dengue viruses bind via the E glycoprotein to viral receptors on the cell surface, which may include heparan sulfate or the lectin DC-SIGN [7,8]; they can also bind to cell surface immunoglobulin receptors in the presence of antibodies to the E glycoprotein or membrane precursor (pre-M) protein, as described further below .
Following fusion of viral and cell membranes in acidified endocytic vesicles, the viral RNA enters the cytoplasm. The viral proteins are then translated directly from the viral RNA as a single polyprotein, which is cleaved to yield the three structural and seven nonstructural proteins . Cleavage of several of the viral proteins requires a functional viral protease encoded in the nonstructural protein NS3. The nonstructural protein NS5 is the viral RNA-dependent RNA polymerase, which assembles with several other viral proteins and several host proteins to form the replication complex. This complex transcribes the viral RNA to produce negative-strand viral RNA, which serves as the template for the production of the viral genomic RNA.