Antibodies against immature virions are not a discriminating factor for dengue disease severity.

Humoral immunity plays an important role in controlling dengue virus (DENV) infection. Antibodies (Abs) developed during primary infection protect against subsequent infection with the same dengue serotype, but can enhance disease following secondary infection with a heterologous serotype. A DENV virion has two surface proteins, envelope protein E and (pre)-membrane protein (pr)M, and inefficient cleavage of the prM protein during maturation of progeny virions leads to the secretion of immature and partially immature particles. Interestingly, we and others found that historically regarded non-infectious prM-containing DENV particles can become highly infectious in the presence of E- and prM-Abs. Accordingly, we hypothesized that these virions contribute to the exacerbation of disease during secondary infection. Here, we tested this hypothesis and investigated the ability of acute sera of 30 DENV2-infected patients with different grades of disease severity, to bind, neutralize and/or enhance immature DENV2. We found that a significant fraction of serum Abs bind to the prM protein and to immature virions, but we observed no significant difference between the disease severity groups. Furthermore, functional analysis of the Abs did not underscore any specific correlation between the neutralizing/enhancing activity towards immature DENV2 and the development of more severe disease. Based on our analysis of acute sera, we conclude that Abs binding to immature virions are not a discriminating factor in dengue pathogenesis.

[A possible fifth dengue virus serotype]. / Een mogelijk vijfde serotype van het denguevirus.

Sylvatic dengue viruses are both evolutionarily and ecologically distinguishable from the human dengue virus (DENV). Sporadic episodes of sylvatic human infections in West Africa and Southeast Asia suggest that sylvatic DENV regularly come into contact with human beings. Following a study on the sylvatic transmission cycle in Malaysia in 2007, researchers announced that a new DENV serotype, DENV-5, had been discovered. Scientists are still sceptical about these new findings, and indicate that more data is necessary to determine whether this 'new' virus really is a different serotype or whether it is a variant of one of the four DENV serotypes already known. The good news is that this new variant has not yet established itself in the human transmission cycle. However, if it really is a new serotype this will have implications for the long-term control of dengue using vaccines currently under development.

Immature dengue virus is infectious in human immature dendritic cells via interaction with the receptor molecule DC-SIGN.

BACKGROUND: Dengue Virus (DENV) is the most common mosquito-borne viral infection worldwide. Important target cells during DENV infection are macrophages, monocytes, and immature dendritic cells (imDCs). DENV-infected cells are known to secrete a large number of partially immature and fully immature particles alongside mature virions. Fully immature DENV particles are considered non-infectious, but antibodies have been shown to rescue their infectious properties. This suggests that immature DENV particles only contribute to the viral load observed in patients with a heterologous DENV re-infection. METHODOLOGY/PRINCIPAL FINDINGS: In this study, we re-evaluated the infectious properties of fully immature particles in absence and presence of anti-DENV human serum. We show that immature DENV is infectious in cells expressing DC-SIGN. Furthermore, we demonstrate that immature dendritic cells, in contrast to macrophage-like cells, do not support antibody-dependent enhancement of immature DENV. CONCLUSIONS/SIGNIFICANCE: Our data shows that immature DENV can infect imDCs through interaction with DC-SIGN, suggesting that immature and partially immature DENV particles may contribute to dengue pathogenesis during primary infection. Furthermore, since antibodies do not further stimulate DENV infectivity on imDCs we propose that macrophages/monocytes rather than imDCs contribute to the increased viral load observed during severe heterotypic DENV re-infections.

Antibodies against the envelope glycoprotein promote infectivity of immature dengue virus serotype 2.

Cross-reactive dengue virus (DENV) antibodies directed against the envelope (E) and precursor membrane (prM) proteins are believed to contribute to the development of severe dengue disease by facilitating antibody-dependent enhancement of infection. We and others recently demonstrated that anti-prM antibodies render essentially non-infectious immature DENV infectious in Fcγ-receptor-expressing cells. Immature DENV particles are abundantly present in standard (st) virus preparations due to inefficient processing of prM to M during virus maturation. Structural analysis has revealed that the E protein is exposed in immature particles and this prompted us to investigate whether antibodies to E render immature particles infectious. To this end, we analyzed the enhancing properties of 27 anti-E antibodies directed against distinct structural domains. Of these, 23 bound to immature particles, and 15 enhanced infectivity of immature DENV in a furin-dependent manner. The significance of these findings was subsequently tested in vivo using the well-established West Nile virus (WNV) mouse model. Remarkably, mice injected with immature WNV opsonized with anti-E mAbs or immune serum produced a lethal infection in a dose-dependent manner, whereas in the absence of antibody immature WNV virions caused no morbidity or mortality. Furthermore, enhancement infection studies with standard (st) DENV preparations opsonized with anti-E mAbs in the presence or absence of furin inhibitor revealed that prM-containing particles present within st virus preparations contribute to antibody-dependent enhancement of infection. Taken together, our results support the notion that antibodies against the structural proteins prM and E both can promote pathogenesis by enhancing infectivity of prM-containing immature and partially mature flavivirus particles.

A fusion-loop antibody enhances the infectious properties of immature flavivirus particles.

Flavivirus-infected cells secrete a mixture of mature, partially immature, and fully immature particles into the extracellular space. Although mature virions are highly infectious, prM-containing fully immature virions are noninfectious largely because the prM protein inhibits the cell attachment and fusogenic properties of the virus. If, however, cell attachment and entry are facilitated by anti-prM antibodies, immature flavivirus becomes infectious after efficient processing of the prM protein by the endosomal protease furin. A recent study demonstrated that E53, a cross-reactive monoclonal antibody (MAb) that engages the highly conserved fusion-loop peptide within the flavivirus envelope glycoprotein, preferentially binds to immature flavivirus particles. We investigated here the infectious potential of fully immature West Nile virus (WNV) and dengue virus (DENV) particles opsonized with E53 MAb and observed that, like anti-prM antibodies, this anti-E antibody also has the capacity to render fully immature flaviviruses infectious. E53-mediated enhancement of both immature WNV and DENV depended on efficient cell entry and the enzymatic activity of the endosomal furin. Furthermore, we also observed that E53-opsonized immature DENV particles but not WNV particles required a more acidic pH for efficient cleavage of prM by furin, adding greater complexity to the dynamics of antibody-mediated infection of immature flavivirus virions.

Immature dengue virus: a veiled pathogen?

Cells infected with dengue virus release a high proportion of immature prM-containing virions. In accordance, substantial levels of prM antibodies are found in sera of infected humans. Furthermore, it has been recently described that the rates of prM antibody responses are significantly higher in patients with secondary infection compared to those with primary infection. This suggests that immature dengue virus may play a role in disease pathogenesis. Interestingly, however, numerous functional studies have revealed that immature particles lack the ability to infect cells. In this report, we show that fully immature dengue particles become highly infectious upon interaction with prM antibodies. We demonstrate that prM antibodies facilitate efficient binding and cell entry of immature particles into Fc-receptor-expressing cells. In addition, enzymatic activity of furin is critical to render the internalized immature virus infectious. Together, these data suggest that during a secondary infection or primary infection of infants born to dengue-immune mothers, immature particles have the potential to be highly infectious and hence may contribute to the development of severe disease.

[Dengue: a growing risk to travellers to tropical and sub-tropical regions]. / Dengue: een toenemend risico voor reizigers naar tropische en subtropische landen.

Dengue is currently the most common arboviral infection worldwide. Due to global climate change and other factors, the vector of the virus - the Aedes mosquito - has spread considerably over the past decades. Dengue is endemic in almost all tropical and sub-tropical regions of the world; meaning that approximately 40% of the world population is at risk of acquiring a dengue infection. The clinical features of dengue vary from a non-specific febrile illness (dengue fever) to at times fatal serious conditions such as dengue haemorrhagic fever (DHF) and dengue shock syndrome (DSS). Considering the limited possibilities of prevention it is anticipated that the incidence of dengue will increase in the future. It is expected that health-care providers in non-endemic regions will encounter dengue-infected patients with increasing frequency in their practices.