Dengue viremia titer, antibody response pattern, and virus serotype correlate with disease severity

Dengue virus surveillance: the co-circulation of DENV-1, DENV-2 and DENV-3 in the State of Rio de Janeiro, Brazil

This study presents the results obtained in the monitoring of dengue virus (DENV) transmission in the Greater Metropolitan Region of the State of Rio de Janeiro, in the period 2000-2001. A total of 5324 serum samples from suspected cases of dengue were analysed in order to confirm dengue infection. The introduction of DENV-3 to the region in December 2000 resulted in the co-circulation of three serotypes: DENV-1, DENV-2 and DENV-3. In this study, virus isolation and/or reverse transcriptase PCR (RT-PCR) confirmed 52.3% (42/79) of DENV-3 cases, showing the importance of acute serum samples in the virological surveillance of the disease. Despite the introduction of a new serotype, an outbreak due to DENV-1 was observed in the municipality of Niteroi. The restriction site-specific PCR (RSS-PCR) patterns obtained for DENV-1 and DENV-2 isolated in that period showed that those strains belonged to the subtypes previously circulating in the state. DENV-3 RSS-PCR patterns confirmed that these viruses belonged to subtype C (Sri Lanka/India strains), represented by the strain circulating on the American continent. These data showed the importance of an active surveillance programme in countries where dengue is endemic.

Recombinant, live-attenuated tetravalent dengue virus vaccine formulations induce a balanced, broad, and protective neutralizing antibody response against each of the four serotypes in rhesus monkeys

Three tetravalent vaccine (TV) formulations of previously described monovalent dengue (DEN) virus vaccine candidates were compared to a tetravalent formulation of wild-type DEN viruses (T-wt) for replication in SCID mice transplanted with human liver cells (SCID-HuH-7) or for replication and immunogenicity in rhesus monkeys. TV-1 consists of recombinant DEN1, -2, -3, and -4, each with a 30-nucleotide deletion in the 3' untranslated region (Delta30). TV-2 consists of rDEN1Delta30, rDEN4Delta30, and two antigenic chimeric viruses, rDEN2/4Delta30 and rDEN3/4Delta30, both also bearing the Delta30 mutation. TV-3 consists of rDEN1Delta30, rDEN2Delta30, rDEN4Delta30, and a 10-fold higher dose of rDEN3/4Delta30. TV-1 and TV-2 were attenuated in SCID-HuH-7 mice with minimal interference in replication among the virus components. TV-1, -2, and -3 were attenuated in rhesus monkeys as measured by duration and peak of viremia. Each monkey immunized with TV-1 and TV-3 seroconverted to the four DEN components by day 28 with neutralization titers ranging from 1:52 to 1:273 and 1:59 to 1:144 for TV-1 and TV-3, respectively. TV-2 induced low antibody titers to DEN2 and DEN3, but a booster immunization after 4 months increased the neutralizing antibody titers to greater than 1:100 against each serotype and elicited broad neutralizing activity against 19 of 20 DEN subtypes. A single dose of TV-2 induced protection against wild-type DEN1, DEN3, and DEN4 challenge, but not DEN2. However, two doses of TV-2 or TV-3 induced protection against DEN2 challenge. Two tetravalent formulations, TV-2 and TV-3, possess properties of a successful DEN vaccine and can be considered for evaluation in clinical trials.

Dengue virus inhibits alpha interferon signaling by reducing STAT2 expression

Alpha/beta interferon (IFN-alpha/beta) is a key mediator of innate antiviral responses but has little effect on the established replication of dengue viruses, which are mosquito-borne flaviviruses of immense global health importance. Understanding how the IFN system is inhibited in dengue virus-infected cells would provide critical insights into disease pathogenesis. In a recent study analyzing the ability of individual dengue virus-encoded proteins to antagonize the IFN response, nonstructural (NS) protein 4B and possibly NS2A and NS4A were identified as candidate IFN antagonists. In monkey cells, NS4B appeared to inhibit both the IFN-alpha/beta and IFN-gamma signal transduction pathways, which are distinct but overlapping (J. L. Munoz-Jordan, G. G. Sanchez-Burgos, M. Laurent-Rolle, and A. Garcia-Sastre, Proc. Natl. Acad. Sci. USA 100:14333-14338, 2003). For this study, we examined the effects of dengue virus on the human IFN system, using cell lines that were stably transfected with self-replicating subgenomic dengue virus RNA (replicons) and that expressed all of the dengue virus nonstructural proteins together. We show here that in replicon-containing cells dengue virus RNA replication and the replication of encephalomyocarditis virus, an IFN-sensitive virus, are resistant to the antiviral effects of IFN-alpha. The presence of dengue virus replicons reduces global IFN-alpha-stimulated gene expression and specifically inhibits IFN-alpha but not IFN-gamma signal transduction. In cells containing replicons or infected with dengue virus, we found reduced levels of signal transducer and activator of transcription 2 (STAT2), which is a key component of IFN-alpha but not IFN-gamma signaling. Collectively, these data show that dengue virus is capable of subverting the human IFN response by down-regulating STAT2 expression.

Early T-cell responses to dengue virus epitopes in Vietnamese adults with secondary dengue virus infections

T-cell responses to dengue viruses may be important in both protective immunity and pathogenesis. This study of 48 Vietnamese adults with secondary dengue virus infections defined the breadth and magnitude of peripheral T-cell responses to 260 overlapping peptide antigens derived from a dengue virus serotype 2 (DV2) isolate. Forty-seven different peptides evoked significant gamma interferon enzyme-linked immunospot (ELISPOT) assay responses in 39 patients; of these, 34 peptides contained potentially novel T-cell epitopes. NS3 and particularly NS3200-324 were important T-cell targets. The breadth and magnitude of ELISPOT responses to DV2 peptides were independent of the infecting dengue virus serotype, suggesting that cross-reactive T cells dominate the acute response during secondary infection. Acute ELISPOT responses were weakly correlated with the extent of hemoconcentration in individual patients but not with the nadir of thrombocytopenia or overall clinical disease grade. NS3556-564 and Env414-422 were identified as novel HLA-A*24 and B*07-restricted CD8+ T-cell epitopes, respectively. Acute T-cell responses to natural variants of Env414-422 and NS3556-564 were largely cross-reactive and peaked during disease convalescence. The results highlight the importance of NS3 and cross-reactive T cells during acute secondary infection but suggest that the overall breadth and magnitude of the T-cell response is not significantly related to clinical disease grade.

A custom-designed recombinant multiepitope protein as a dengue diagnostic reagent

Currently, dengue fever is the most important re-emerging mosquito-borne viral disease, with the major proportion of the target population residing in the developing countries of the world. In endemic areas, potentially fatal secondary dengue infections, characterized by high anti-dengue IgG antibody titers, are most common. Most currently available commercial dengue diagnostic kits rely on the use of whole virus antigens and are consequently associated with false positives due to serologic cross-reactivity, high cost of antigen production, and biohazard risk. This has prompted the need to develop an alternate antigen to replace the whole virus antigen in diagnostic tests. We have designed and expressed a novel recombinant protein antigen by assembling key immunodominant linear IgG-specific dengue virus epitopes, chosen on the basis of pepscan analysis, phage display, and computer predictions. The recombinant dengue multiepitope protein was expressed to high levels in Escherichia coli, purified in a single step, yielding >25 mg pure protein per liter culture. We developed an in-house enzyme-linked immunosorbent assay (ELISA) to detect anti-dengue antibodies in a panel of 20 patient sera using the purified recombinant dengue multiepitope protein as the capture antigen. The ELISA results were in excellent agreement with those obtained using a commercially available diagnostic test, Dengue Duo rapid strip test from PanBio, Australia. The high epitope density, careful choice of epitopes, and the use of E. coli system for expression, coupled to simple purification, jointly have the potential to lead to the development of an inexpensive diagnostic test with a high degree of sensitivity and specificity.

A variant in the CD209 promoter is associated with severity of dengue disease

Dengue fever and dengue hemorrhagic fever are mosquito-borne viral diseases. Dendritic cell–specific ICAM-3 grabbing nonintegrin (DC-SIGN1, encoded by CD209), an attachment receptor of dengue virus, is essential for productive infection of dendritic cells^1,2. Here, we report strong association between a promoter variant of CD209, DCSIGN1-336, and risk of dengue fever compared with dengue hemorrhagic fever or population controls. The G allele of the variant DCSIGN1-336 was associated with strong protection against dengue fever in three independent cohorts from Thailand, with a carrier frequency of 4.7% in individuals with dengue fever compared with 22.4% in individuals with dengue hemorrhagic fever (odds ratio for risk of dengue hemorrhagic fever versus dengue fever: 5.84, P = 1.4 × 10⁻⁷) and 19.5% in controls (odds ratio for protection: 4.90, P = 2 × 10⁻⁶). This variant affects an Sp1-like binding site and transcriptional activity in vitro. These results indicate that CD209 has a crucial role in dengue pathogenesis, which discriminates between severe dengue fever and dengue hemorrhagic fever. This may have consequences for therapeutic and preventive strategies.

Eco-epidemiological analysis of dengue infection during an outbreak of dengue fever, India

Background

This study was designed to find out a relationship of dengue infection with climatic factors such as rainfall, temperature and relative humidity during the dengue fever epidemic in the year 2003. Blood samples were collected from 1550 patients experiencing a febrile illness clinically consistent with dengue infection. Serological confirmation of Dengue Infection was done using Dengue Duo IgM and IgG Rapid Strip test (Pan Bio, Australia), which detected dengue-specific antibodies. Monthly data of total rainfall, temperature and relative humidity for the year 2003 was obtained from Meteorological Department of Delhi, New Delhi and retrospectively analyzed.

Results

Out of 1550 suspected cases, 893 cases (57.36%) were confirmed as serologically positive. The difference between numbers of serologically positive cases during different months was significant (p < 0.05). Larger proportions of serologically positive cases were observed among adults. Outbreak coincided mainly with the post monsoon period of subnormal rainfall. The difference between serologically positive cases as compared to serologically negative ones in post monsoon period was significantly higher (p < 0.001). The difference in the rainfall and temperature between three seasonal periods was significant (p < 0.05).

Conclusion

This prospective study highlighted rain, temperature and relative humidity as the major and important climatic factors, which could alone or collectively be responsible for an outbreak. More studies in this regard could further reveal the correlation between the climatic changes and dengue outbreaks, which would help in making the strategies and plans to forecast any outbreak in future well in advance.

Circulating human antibodies against dengue NS1 protein: potential of recombinant D2V-NS1 proteins in diagnostic tests

The dengue virus (DV) causes one of the most important arthropod-borne human viral diseases throughout the tropical and subtropical countries. However, the morbidity and mortality of DV infections could be reduced with an early hospitalization care and a rapid risk identification of developing the dengue haemorrhagic fever (DHF). The nonstructural glycoprotein 1 (NS1) has been pointed as a reagent for immune-assay diagnostic test optimization. To evaluate this potential, recombinant DV2-NS1 proteins (rNS1) were produced from Escherichia coli (NS1EC) and insect cells (NS1IC) expression. The tests were performed by analysis of a human serum panel reacted against different rNS1 forms. The results demonstrated high correspondence between the DV positive sera and the assay results using native or refolded forms of either NS1IC or NS1EC. Also, the IgG and IgM anti-rNS1 level profiles showed distinct distribution, depending on protein form and disease status. However, the IgM anti-rNS1 reactions did not show sensibility to detect the DV in primary infections. The data obtained from the paired serum samples reactivity comparison suggested a heterogeneous human immune response and absence of correspondence between the IgG and IgM profile levels. Moreover, a patient with negative reference test could be detected by specific IgG anti-rNS1 assays presented here. Therefore, these results sustain the usefulness of dengue nonstructural proteins, in particular the NS1, in diagnostic tests as a complementary reagent.

Altered T helper 1 reaction but not increase of virus load in patients with dengue hemorrhagic fever

To investigate whether dengue-2 patients with and without dengue hemorrhagic fever had different virus load, immune mediators, or T helper (Th) reaction, we simultaneously measured virus load, immune mediators and the Th1/Th2 transcription factors T-bet/GATA-3 mRNA expression in a large outbreak of dengue-2 infections in Southern Taiwan. Results showed that virus load was not significantly different between patients with and without dengue hemorrhagic fever. Patients with dengue fever had higher IFN-gamma levels, but patients with dengue hemorrhagic fever had significantly higher IL-10 levels. Further studies showed that patients with dengue hemorrhagic fever had a significantly lower T-bet than those with dengue fever, but GATA-3 mRNA expression in peripheral blood leukocytes was not significant difference between both groups. In conclusion, altered Th1 reaction as reflected by lower T-bet mRNA expression associated with higher IL-10 levels might be involved in the pathogenesis of dengue hemorrhagic fever.

Dengue fever: new paradigms for a changing epidemiology

Dengue is the most important arthropod-borne viral disease of public health significance. Compared with nine reporting countries in the 1950s, today the geographic distribution includes more than 100 countries worldwide. Many of these had not reported dengue for 20 or more years and several have no known history of the disease. The World Health Organization estimates that more than 2.5 billion people are at risk of dengue infection. First recognised in the 1950s, it has become a leading cause of child mortality in several Asian and South American countries.

This paper reviews the changing epidemiology of the disease, focusing on host and societal factors and drawing on national and regional journals as well as international publications. It does not include vaccine and vector issues. We have selected areas where the literature raises challenges to prevailing views and those that are key for improved service delivery in poor countries.

Shifts in modal age, rural spread, and social and biological determinants of race- and sex-related susceptibility have major implications for health services. Behavioural risk factors, individual determinants of outcome and leading indicators of severe illness are poorly understood, compromising effectiveness of control programmes. Early detection and case management practices were noted as a critical factor for survival. Inadequacy of sound statistical methods compromised conclusions on case fatality or disease-specific mortality rates, especially since the data were often based on hospitalised patients who actively sought care in tertiary centres.

Well-targeted operational research, such as population-based epidemiological studies with clear operational objectives, is urgently needed to make progress in control and prevention.

Expression of cytokine, chemokine, and adhesion molecules during endothelial cell activation induced by antibodies against dengue virus nonstructural protein 1

Vascular dysfunction is a hallmark associated with disease onset in dengue hemorrhagic fever and dengue shock syndrome. In addition to direct viral damage, immune responses to dengue virus (DV) infection may also underlie the pathogenesis of disease. We have proposed a mechanism of molecular mimicry in which Abs directed against DV nonstructural protein 1 (NS1) cross-react with endothelial cells and induce damage. In this study, we demonstrated the inflammatory endothelial cell activation induced by anti-DV NS1 via the transcription factor NF-kappaB-regulated pathway. Protein phosphorylation and NF-kappaB activation were observed after anti-DV NS1 stimulation in a human microvascular endothelial cell line-1. The cytokine and chemokine production, including IL-6, IL-8, and MCP-1, but not RANTES, in endothelial cells increased after treatment with anti-DV NS1 Abs. The expression of IL-6, IL-8, and MCP-1 was blocked by the preabsorption of anti-DV NS1 with DV NS1 or by the inhibition of NF-kappaB activation. Furthermore, the increases in both ICAM-1 expression and the ability of human PBMC to adhere to endothelial cells were also observed, and these effects were inhibited by pretreatment with anti-ICAM-1 or anti-MCP-1 Abs. Therefore, in addition to endothelial cell apoptosis, as previously reported, inflammatory activation occurs in endothelial cells after stimulation by anti-DV NS1 Abs. These results suggest the involvement of anti-DV NS1 Abs in the vasculopathy of DV infection.

Selection for virulent dengue viruses occurs in humans and mosquitoes

Dengue is the most common mosquito-borne viral disease in humans. The spread of both mosquito vectors and viruses has led to the resurgence of epidemic dengue fever (a self-limited flu-like syndrome) and the emergence of dengue hemorrhagic fever (severe dengue with bleeding abnormalities) in urban centers of the tropics. There are no animal or laboratory models of dengue disease; indirect evidence suggests that dengue viruses differ in virulence, including their pathogenicities for humans and epidemic potential. We developed two assay systems (using human dendritic cells and Aedes aegypti mosquitoes) for measuring differences in virus replication that correlate with the potential to cause hemorrhagic dengue and increased virus transmission. Infection and growth experiments showed that dengue serotype 2 viruses causing dengue hemorrhagic fever epidemics (Southeast Asian genotype) can outcompete viruses that cause dengue fever only (American genotype). This fact implies that Southeast Asian genotype viruses will continue to displace other viruses, causing more hemorrhagic dengue epidemics.

Serological markers during dengue 3 primary and secondary infections

BACKGROUND

The detection of the IgM antibody for the dengue virus in serum by ELISA has become one of the most important and useful methods for diagnosis of dengue using a single acute-phase serum sample. Currently, this system is an invaluable tool for the surveillance of dengue fever (DF) and dengue hemorrhagic fever (DHF). The usefulness of other serological markers such as IgA and IgE have been less studied.

OBJECTIVE

To study the IgM, IgA and IgE specific antibody response in dengue 3 infected patients with different clinical picture and type of infection.

STUDY DESIGN

One hundred and twenty-seven serum samples collected on days 5-7 at the onset of fever from clinically and serologically confirmed dengue cases were studied. Forty-two were classified as primary dengue fever cases, 48 as secondary dengue fever cases and 37 as secondary dengue hemorrhagic fever cases. All samples were tested by capture ELISA in order to detect dengue IgM, IgA and IgE antibodies.

RESULTS AND CONCLUSIONS

In this study, significant differences were observed in the IgM, IgA and IgE response between the study groups. High IgA and IgE OD ratios in secondary dengue cases were found. The usefulness of serotype specific IgM antibody detection is also analyzed and discussed. A priority for future dengue research in terms of protection, recovery of infection and immunopathogenesis is to elucidate the role of these immunoglobulins. The cross reactivity response to IgM between dengue virus serotypes in primary and secondary cases should also be more studied.

Serotype-specific entry of dengue virus into liver cells: identification of the 37-kilodalton/67-kilodalton high-affinity laminin receptor as a dengue virus serotype 1 receptor

Dengue virus, the causative agent of dengue fever, dengue shock syndrome, and dengue hemorrhagic fever, infects susceptible cells by initially binding to a receptor(s) located on the host cell surface. Evidence to date suggests that receptor usage may be cell and serotype specific, and this study sought to identify dengue virus serotype 1 binding proteins on the surface of liver cells, a known target organ. By using a virus overlay protein binding assay (VOPBA), in both nondenaturing and denaturing gel systems, a putative dengue virus serotype 1 binding protein of approximately 37 kDa expressed on the surface of liver (HepG2) cells was identified. Mass spectrometry analysis identified a candidate protein, the 37/67-kDa high-affinity laminin receptor. Entry of the dengue virus serotype 1 was significantly inhibited in a dose-dependent manner by both antibodies directed against the 37/67-kDa high-affinity laminin receptor and soluble laminin. No inhibition of virus entry was seen with dengue virus serotypes 2, 3, or 4, demonstrating that the 37/67-kDa high-affinity laminin receptor is a serotype-specific receptor for dengue virus entry into liver cells.

Can hematocrit and platelet determination on admission predict shock in hospitalized children with dengue hemorrhagic fever? A clinical observation from a small outbreak

Dengue infection is a major public health problem, affecting children in the Southeast Asia region. In Thailand, the dengue hemorrhagic fever is still a major infectious disease among the children with up to two to three epidemics per year. Hemoconcentration accompanied by platelet depletion are the predominant laboratory signs of dengue hemorrhagic fever. Findings from 23 hospitalized patients with dengue hemorrhafic fever in a small outbreak in a provincial hospital in Thailand are reported. The question if hematocrit and platelet determination on admission can predict shock in hospitalized children with dengue hemorrhagic fever was studied. The data from the discharge summary of these patients were studied, focusing on the admission hematology laboratory data. Regression analysis was used to test the correlation between the admission hematology laboratory data (hematocrit, white blood cell count, and platelet) and the focused outcome (shock or no shock). Of these 23 patients, shock did not develop in 19, and shock developed in four. There was no significant difference in admission hematology laboratory data between both groups (p < 0.05). The regression analysis revealed no significant correlation between the studied hematology laboratory data and the focused outcome (p > 0.05). It means that the three studied investigations on admission cannot predict shock in our hospitalized dengue hemorrhagic cases. From this study, it can imply that closed monitoring of dengue hemorrhagic patients is necessary. The general practitioner cannot rely on the admission hematology laboratory data to predict shock in these patients. Additionally, it might confirm that, although dengue infection can be fatal, with proper supportive treatment, especially hospitalization and hydration for severe cases, the outcome is very good.

Vertical dengue infection: case reports and review

Two vertical dengue infection cases are presented, and 15 others are reviewed. Twelve mothers had fever 0-9 (median, 2) days antepartum. The fevers of 17 neonates occurred at 1-11 (median, 4) days of life and lasted for 1-5 (median, 3) days. Neonatal thrombocytopenia was detected at 1-11 (median, 6) days of life and lasted for 3-18 (median, 6) days; the lowest platelet counts were 5-75 x 10(3) (median, 19 x 10 (3))/mm3. One neonate died.

Introduction of mutations into the non-structural genes or 3' untranslated region of an attenuated dengue virus type 4 vaccine candidate further decreases replication in rhesus monkeys while retaining protective immunity

A dengue virus vaccine candidate, rDEN4Delta30, has been previously reported to be safe and immunogenic in humans, but a subset of vaccinees developed asymptomatic rash, elevation of liver enzymes and/or mild neutropenia. In the current study, mutations that had previously been shown to reduce replication of DEN4 virus in suckling mice and/or in SCID mice engrafted with human liver cells (SCID-HuH-7 mice) were introduced into rDEN4Delta30 in an attempt to further attenuate this virus. Three of the five resulting modified rDEN4Delta30 viruses showed decreased replication in SCID-HuH-7 mice relative to rDEN4Delta30. Moreover, in rhesus monkeys, two of the modified rDEN4Delta30 viruses showed a decrease in replication relative to rDEN4Delta30 while generating levels of neutralizing antibody similar to rDEN4Delta30 virus. All of the modified rDEN4Delta30 viruses completely protected immunized rhesus monkeys from challenge with wild-type DEN4 virus. Based on their attenuation for both human liver cells and rhesus monkeys, two of the modified rDEN4Delta30 vaccine candidates are currently being prepared for use in clinical trials. The application of these attenuating mutations to flavivirus vaccine development is discussed.

Dengue-specific CD8+ T cells have both protective and pathogenic roles in dengue virus infection

To analyze roles of memory T cells in the pathogenesis of dengue (DEN) virus infection, a DEN virus-specific CD8+ cell clone (2D42 cell) was employed to investigate its in vivo function after DEN virus infection using an animal model. HepG2 grafted severe combined immunodeficient (HepG2-grafted SCID) mice were divided into three groups--group A: HepG2-grafted SCID mice were inoculated intraperitoneally (ip) with 2D42 cells and then ip-infected with DEN virus type 2 (DEN-2); group B: HepG2-grafted SCID mice were inoculated with naive mouse thymocytes (NMT) and then ip-infected with DEN-2; group C: HepG2-grafted SCID mice were ip-infected with DEN-2 alone. Eighty percentage of group A mice died at average day 12.8 post-infection (p.i.) and 20% of them recovered from the disease after showing clinical signs and survived more than 3 months. They showed severe manifestations including dramatically decreased platelet count, decreased hematocrit, anemia, viremia and high frequency of histopathological changes in several organs. All of group B mice also showed the above severe clinical signs. One hundred percentage mortality rate was noted in these mice and death occurred at average day 10.8 p.i., which was the earliest among three groups. Although the mice from group C showed 100% mortality rate and similar clinical signs, death observed in these mice occurred at average day 17.4 p.i. and the manifestations were slight and developed slowly. Our results suggested both protective and pathogenic roles for DEN-specific CD8+ T cell in DEN virus infection, whereas NMT did not provided any protection.

Evaluation by flow cytometry of antibody-dependent enhancement (ADE) of dengue infection by sera from Thai children immunized with a live-attenuated tetravalent dengue vaccine

Sera from Thai children immunized with a live-attenuated tetravalent dengue virus vaccine or from naturally infected age-matched site-control subjects were examined for immune enhancement capacity by a highly reproducible flow cytometric assay in Fc receptor-bearing K562 human cells. None of the sera under study corresponded to cases of severe dengue disease. In parallel assays employing each dengue virus serotype, we found no or only minimal antibody-dependent enhancement (ADE) when sera from vaccinated or control subjects were used at a low serum dilution [1/12] that approximated the in vivo condition. Among sera that exhibited homotypic neutralizing antibody activity against DV1-3, the level correlated with absence of ADE or infection with the respective serotype. Similarly, a broad heterotypic neutralizing antibody response that included all four serotypes was linked to complete absence of K562 cell infection. In contrast, at higher serum dilutions a correlation between breadth of antibody response and heightened immune enhancement emerged, a pattern identical to that observed among control subjects. These findings support the use of live dengue vaccines and protocols that induce broad serotype-specific neutralizing antibody responses, but they also suggest that clinically relevant immune enhancement may not be likely if this is not uniformly achieved after the first immunization.

Phylogeography and molecular evolution of dengue 2 in the Caribbean basin, 1981-2000

We sequenced the envelope (E) genes of 59 DEN-2 isolates collected from ten Caribbean islands, six South American countries, and two Central American countries between 1981 and 2000, a period characterized by hyperendemicity and increased incidence of severe dengue. Fifty-two isolates belonged to "American/Asian" subtype IIIb, possessing a characteristic polar residue at envelope aa position 390 (N [n = 48] or S [n = 4]) common to that group. Six isolates from Trinidad (1981), Honduras (1991 [4]), and El Salvador (1987) fell into the "Native American" subtype V (D at aa 390), and one from Honduras (1986) belonged to "Asian" subtype I. The data suggest that after its first isolation in the Caribbean in 1981, genotype IIIb spread throughout the Americas and effectively replaced subtype V throughout the Caribbean basin. The strain also evolved into several distinct lineages, based on substitutions in the E glycoprotein (amino acids 91 and 131), two of which were still in circulation in 2000. Interestingly, a molecular clock did not fit the data well, suggesting that other sources of rate variation, such as differential selection or differences in effective population sizes, may exist among lineages. Our results indicate the importance of large temporal- and geographical-scale phylogenetic studies in understanding disease dynamics, particularly where replacements between regions can occur.

Myanmar dengue outbreak associated with displacement of serotypes 2, 3, and 4 by dengue 1

In 2001, Myanmar (Burma) had its largest outbreak of dengue—15,361 reported cases of dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS), including 192 deaths. That year, 95% of dengue viruses isolated from patients were serotype 1 viruses belonging to two lineages that had diverged from an earlier, now extinct, lineage sometime before 1998. The ratio of DHF to DSS cases in 2001 was not significantly different from that in 2000, when 1,816 cases of DHF/DSS were reported and dengue 1 also was the most frequently isolated serotype. However, the 2001 ratio was significantly higher than that in 1998 (also an outbreak year) and in 1999, when all four serotypes were detected and serotypes 1, 2, and 3 were recovered in similar numbers. The large number of clinical cases in 2001 may have been due, in part, to a preponderance of infections with dengue 1 viruses.

Dengue: an arthropod-borne disease of global importance

Dengue viruses cause a variable spectrum of disease that ranges from an undifferentiated fever to dengue fever to the potentially fatal dengue shock syndrome. Due to the increased incidence and geographical distribution of dengue in the last 50 years, dengue is becoming increasingly recognised as one of the world's major infectious diseases. This article will review clinical and diagnostic aspects of dengue virus infections. It also presents our current knowledge of the pathophysiology of severe dengue and addresses the importance of dengue virus infections in those travelling to parts of the world where dengue is endemic.

Prior infection and passive transfer of neutralizing antibody prevent replication of severe acute respiratory syndrome coronavirus in the respiratory tract of mice

Following intranasal administration, the severe acute respiratory syndrome (SARS) coronavirus replicated to high titers in the respiratory tracts of BALB/c mice. Peak replication was seen in the absence of disease on day 1 or 2, depending on the dose administered, and the virus was cleared within a week. Viral antigen and nucleic acid were detected in bronchiolar epithelial cells during peak viral replication. Mice developed a neutralizing antibody response and were protected from reinfection 28 days following primary infection. Passive transfer of immune serum to naïve mice prevented virus replication in the lower respiratory tract following intranasal challenge. Thus, antibodies, acting alone, can prevent replication of the SARS coronavirus in the lung, a promising observation for the development of vaccines, immunotherapy, and immunoprophylaxis regimens.

Microevolution and virulence of dengue viruses

The evolution of dengue viruses has had a major impact on their virulence for humans and on the epidemiology of dengue disease around the world. Although antigenic and genetic differences in virus strains had become evident, it is mainly due to the lack of animal models of disease that has made it difficult to detect differences in virulence of dengue viruses. However, phylogenetic studies of many different dengue virus samples have led to the association between specific genotypes (within serotypes) and the presentation of more or less severe disease. Currently, dengue viruses can be classified as being of epidemiologically low, medium, or high impact; i.e., some viruses may remain in sylvatic cycles of little or low transmissibility to humans, others produce dengue fever (DF) only, and some genotypes have been associated with the potential to cause the more severe dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) in addition to DF. Although the factors that contribute to dengue virus epidemiology are complex, studies have suggested that specific viral structures may contribute to increased replication in human target cells and to increased transmission by the mosquito vector; however, the immune status and possibly the genetic background of the host are also determinants of virulence or disease presentation. As to the question of whether dengue viruses are evolving toward virulence as they continue to spread throughout the world, phylogenetic and epidemiological analyses suggest that the more virulent genotypes are now displacing those that have lower epidemiological impact; there is no evidence for the transmission of antigenically aberrant, new strains.

Dengue diagnosis, advances and challenges

Dengue diagnosis was one of the topics discussed at the symposium 'The Global Threat of Dengue - Desperately Seeking Solutions' organized during the 10th International Congress of Infectious Diseases held in Singapore in 2002. In this paper, a review is presented focusing on the main advances, problems and challenges of dengue diagnosis.IgM capture ELISA, virus isolation in mosquito cell lines and live mosquitoes, dengue specific monoclonal antibodies and PCR have all represented major advances in dengue diagnosis. However, an appropriate rapid, early and accessible diagnostic method useful both for epidemiological surveillance and clinical diagnosis is still needed. Also, tools that suggest a prognosis allowing for better management are also needed. Finally, laboratory infrastructure, technical expertise and research capacity must be improved in endemic countries in order to positively influence dengue surveillance, clinical case management and the development of new approaches to dengue control.

Role of Specific CD8+ T Cells in the Severity of a Fulminant Zoonotic Viral Hemorrhagic Fever, Hantavirus Pulmonary Syndrome

We report on the role of specific CD8(+) T cells in the pathogenesis of a highly lethal human viral disease, hantavirus pulmonary syndrome (HPS). HPS is a zoonotic disease caused by transmission of Sin Nombre virus (SNV) from chronically infected deer mice. In humans, this fulminant infection is characterized by lung capillary leakage, respiratory failure, and cardiogenic shock. Individuals with HLA-B*3501 have an increased risk of developing severe HPS, suggesting that CD8(+) T cell responses to SNV contribute to pathogenesis. We identified three CD8(+) T cell epitopes in SNV presented by HLA-B*3501 and quantitated circulating SNV-specific CD8(+) T cells in 11 acute HPS patients using HLA/peptide tetramers. We found significantly higher frequencies of SNV-specific T cells in patients with severe HPS requiring mechanical ventilation (up to 44.2% of CD8(+) T cells) than in moderately ill HPS patients hospitalized but not requiring mechanical ventilation (up to 9.8% of CD8(+) T cells). These results imply that virus-specific CD8(+) T cells contribute to HPS disease outcome. Intense CD8(+) T cell responses to SNV may be induced by the encounter of the unnatural human host to this zoonotic virus without coevolution. This may also be the immunopathologic basis of other life-threatening human virus infections.

Why are dengue virus serotypes so distantly related? Enhancement and limiting serotype similarity between dengue virus strains

Dengue virus, the causative agent of dengue fever, has four major serotypes characterized by large genetic and immunological distances. We propose that the unusually large distances between the serotypes can be explained in the light of a process of antibody-dependent enhancement (ADE) leading to increased mortality. Antibody-dependent enhancement results from a new infection with a particular serotype in an individual with acquired immunity to a different serotype. Classical dengue fever causes negligible mortality, but ADE leads to the risk of developing the significantly more dangerous dengue haemorrhagic fever (DHF) and dengue shock syndrome (DSS). A mathematical model is presented that describes the epidemiological dynamics of two serotypes of a pathogen where there is the possibility of co-infection and reinfection by a different serotype, along with increased mortality as a result of enhancement. We show that if there is no or slightly increased mortality after reinfection (enhancement), serotypes with a small immunological distance can stably coexist. This suggests that a cloud of serotypes with minor serological differences will constitute the viral population. By contrast, if enhancement is sufficiently great, a substantial immunological distance is necessary for two serotypes to stably coexist in the population. Therefore, high mortality owing to enhancement leads to an evolutionarily stable viral community comprising a set of distantly separated serotypes.

Policymakers’ views on dengue fever/dengue haemorrhagic fever and the need for dengue vaccines in four southeast Asian countries

A survey of policymakers and other influential professionals in four southeast Asian countries (Cambodia, Indonesia, Philippines and Vietnam) was conducted to determine policymakers' views on the public health importance of dengue fever and dengue haemorrhagic fever (DHF), the need for a vaccine and the determinants influencing its potential introduction. The survey, which involved face-to-face interviews with policymakers, health programme managers, researchers, opinion leaders and other key informants, revealed an almost uniformly high level of concern about dengue fever/DHF and a high perceived need for a dengue vaccine. Several characteristics of the disease contribute to this high sense of priority, including its geographic spread, occurrence in outbreaks, the recurrent risk of infection each dengue season, its severity and the difficulty in diagnosis and management, its urban predominance, its burden on hospitals, and its economic toll on governments and families. Research felt to be key to future decision-making regarding dengue vaccine introduction include: disease surveillance studies, in-country vaccine trials or pilot projects, and studies on the economic burden of dengue and the cost-effectiveness of dengue vaccines. The results suggest favourable conditions for public and private sector markets for dengue vaccines and the need for creative financing strategies to ensure their accessibility to poor children in dengue-endemic countries.

A Blunted Blood Plasmacytoid Dendritic Cell Response to an Acute Systemic Viral Infection Is Associated with Increased Disease Severity

At least two distinct human dendritic cell (DC) subsets are produced in the bone marrow and circulate in the peripheral blood-precursor myeloid DCs (pre-mDCs) and plasmacytoid DCs (PDCs). Both lineages of DCs are instrumental in antiviral innate immunity and shaping Th1 adaptive immune responses. PDCs are the most potent IFN-alpha-producing cells to viral pathogens. Dengue, an acute flavivirus disease, provides a model to study DC responses to a self-limited human viral infection. We analyzed circulating DC subsets in a prospective study of children with dengue across a broad range of illness severities: healthy controls; mild, nondengue, presumed viral infections; moderately ill dengue fever; and, the most severe form of illness, dengue hemorrhagic fever. We also examined PDC responses in monkeys with asymptomatic dengue viremia and to dengue virus exposure in vitro. The absolute number and frequency of circulating pre-mDCs early in acute viral illness decreased as illness severity increased. Depressed pre-mDC blood levels appeared to be part of the typical innate immune response to acute viral infection. The frequency of circulating PDCs trended upward and the absolute number of circulating PDCs remained stable early in moderately ill children with dengue fever, mild other, nondengue, febrile illness, and monkeys with asymptomatic dengue viremia. However, there was an early decrease in circulating PDC levels in children who subsequently developed dengue hemorrhagic fever. A blunted blood PDC response to dengue virus infection was associated with higher viremia levels, and was part of an altered innate immune response and pathogenetic cascade leading to severe disease.

Preventive transfusion in Dengue shock syndrome-is it necessary?

We compared 53 patients with Dengue shock syndrome (DSS) who received preventive transfusions with 53 who did not. Significant differences in the development of pulmonary edema and length of hospitalization (P<.05) and none in hemorrhage (P=.136) were observed. Preventive transfusions did not produce sustained improvements in the coagulation status in DSS.

Improved molecular detection of dengue virus serotype 1 variants

The dengue virus molecular typing method described by Lanciotti and coworkers (R. S. Lanciotti, C. H. Calisher, D. J. Gubler, G. J. Chang, and A. Vance-Vorndam, J. Clin. Microbiol. 30:545-551, 1992) is used worldwide for diagnosis and surveillance. However, it failed to detect DENV-1 variants in Cambodia due to a point mutation. We describe an improvement of the method that allows the detection of additional DENV-1 strains, including potential variants.

A trade-off in replication in mosquito versus mammalian systems conferred by a point mutation in the NS4B protein of dengue virus type 4

An acceptable live-attenuated dengue virus vaccine candidate should have low potential for transmission by mosquitoes. We have identified and characterized a mutation in dengue virus type 4 (DEN4) that decreases the ability of the virus to infect mosquitoes. A panel of 1248 mutagenized virus clones generated previously by chemical mutagenesis was screened for decreased replication in mosquito C6/36 cells but efficient replication in simian Vero cells. One virus met these criteria and contained a single coding mutation: a C-to-U mutation at nucleotide 7129 resulting in a Pro-to-Leu change in amino acid 101 of the nonstructural 4B gene (NS4B P101L). This mutation results in decreased replication in C6/36 cells relative to wild-type DEN4, decreased infectivity for mosquitoes, enhanced replication in Vero and human HuH-7 cells, and enhanced replication in SCID mice implanted with HuH-7 cells (SCID-HuH-7 mice). A recombinant DEN4 virus (rDEN4) bearing this mutation exhibited the same set of phenotypes. Addition of the NS4B P101L mutation to rDEN4 bearing a 30 nucleotide deletion (Delta30) decreased the ability of the double-mutant virus to infect mosquitoes but increased its ability to replicate in SCID-HuH-7 mice. Although the NS4B P101L mutation decreases infectivity of DEN4 for mosquitoes, its ability to enhance replication in SCID-HuH-7 mice suggests that it might not be advantageous to include this specific mutation in an rDEN4 vaccine. The opposing effects of the NS4B P101L mutation in mosquito and vertebrate systems suggest that the NS4B protein is involved in maintaining the balance between efficient replication in the mosquito vector and the human host.

Original antigenic sin and apoptosis in the pathogenesis of dengue hemorrhagic fever

Dengue virus presents a growing threat to public health in the developing world. Four major serotypes of dengue virus have been characterized, and epidemiological evidence shows that dengue hemorrhagic fever (DHF), the more serious manifestation of the disease, occurs more frequently upon reinfection with a second serotype. We have studied dengue virus-specific T-cell responses in Thai children. During acute infection, few dengue-responsive CD8+ T cells were recovered; most of those present showed an activated phenotype and were undergoing programmed cell death. Many dengue-specific T cells were of low affinity for the infecting virus and showed higher affinity for other, probably previously encountered strains. Profound T-cell activation and death may contribute to the systemic disturbances leading to DHF, and original antigenic sin in the T-cell responses may suppress or delay viral elimination, leading to higher viral loads and increased immunopathology.

Evasion of innate and adaptive immunity by flaviviruses

After a virus infects an animal, antiviral responses are generated that attempt to prevent dissemination. Interferons, antibody, complement, T and natural killer cells all contribute to the control and eradication of viral infections. Most flaviviruses, with the exception of some of the encephalitic viruses, cause acute disease and do not establish persistent infection. The outcome of flavivirus infection in an animal is determined by a balance between the speed of viral replication and spread, and the immune system response. Although many of the mechanistic details require further elucidation, flaviviruses have evolved specific tactics to evade the innate and adaptive immune response. A more thorough understanding of these principles could lead to improved models for viral pathogenesis and to strategies for the development of novel antiviral agents.

Public versus personal serotypes of a viral quasispecies

Noncytopathic RNA viruses persist in their natural hosts at various levels as highly mutating quasispecies. They exhibit only one known serotype. In most inbred DBA2 mice infected with 2 x 10(4) or 2 x 10(6) plaque-forming units (pfu) of lymphocytic choriomeningitis virus (LCMV), the virus is transiently controlled below detectable levels measured with conventional assays (<1.7 pfu), but reemerges despite a common neutralizing Ab (nAb) response. Wild-type virus and cloned mutant viruses that had escaped polyclonal nAb responses in vivo induced nAb titers in new hosts that were usually cross-reactive; some sera were highly specific for certain mutants. The few mice that controlled LCMV infection for >170 days produced not only nAb against wild-type but also variably against many other mutants isolated from other mice with reemerging viremia. When DBA2 mice were immunized and boosted with 200 pfu of a LCMV mutant, the neutralizing Ab response was limited to the immunizing "personal" clone. Thus, in contrast to classical serotype-defined cytopathic viruses (e.g., polio viruses) that induce strictly non-cross-reactive nAb titers, LCMV, a noncytopathic RNA virus, represents a dynamic multiplicity of personal serological submutants. Together, these mutants form a generally recognized "public" serotype. These findings may help to explain aspects of human infections and Ab responses against hepatitis B virus, hepatitis C virus, and HIV.

Efficiency of dengue serotype 2 virus strains to infect and disseminate in Aedes aegypti

Dengue serotype 2 (DEN-2) viruses with the potential to cause dengue hemorrhagic fever have been shown to belong to the Southeast (SE) Asian genotype. These viruses appear to be rapidly displacing the American genotype of DEN-2 in the Western Hemisphere. To determine whether distinct genotypes of DEN-2 virus are better adapted to mosquito transmission, we classified 15 viral strains of DEN-2 phylogenetically and compared their ability to infect and disseminate in different populations of Aedes aegypti mosquitoes. Envelope gene nucleotide sequence analysis confirmed that six strains belonged to the American genotype and nine strains were of the SE Asian genotype. The overall rate of disseminated infection in mosquitoes from Texas was 27% for the SE Asian genotype versus 9% for the American genotype. This pattern of infection was similar in another population of mosquitoes sampled from southern Mexico (30% versus 13%). Together, these findings suggest that Ae. aegypti tends to be more susceptible to infection by DEN-2 viruses of the SE Asian genotype than to those of the American genotype, and this may have epidemiologic implications.

Analysis of saturation binding and saturation infection for dengue serotypes 1 and 2 in liver cells

OBJECTIVE

The liver has been increasingly recognized as a significant target organ in the pathogenesis of dengue virus infection. However, only two contradictory studies have examined the binding of the dengue virus to liver cells. This study therefore sought to investigate the binding of the dengue virus to HepG2 cells.

METHODS

Radiolabeled dengue virus serotypes 1 and 2 were prepared through viral propagation in Vero cells. Increasing amounts of virus were then incubated with HepG2 cells to determine the ability of the virus to achieve saturation of binding on HepG2 cells.

RESULTS

Results indicated that it was not possible to reach saturation of binding under experimentally achievable conditions. We then sought to determine whether it was possible to reach a state of saturation of infection, by using increasingly high titers of virus on a constant number of cells. Dengue serotype 1 showed no evidence of saturation of infection, even at titers of 5,000 viruses per cell. In contrast, dengue serotype 2 became saturated at levels of approximately 3,000 viruses per cell.

CONCLUSIONS

These results are consistent with proposals that dengue virus binding to cells is mediated initially through a low-affinity interaction with an abundant molecule on the surface of the cell and secondly through interaction with a less commonly expressed molecule, which is required for viral internalization.

American genotype structures decrease dengue virus output from human monocytes and dendritic cells

The dengue virus type 2 structures probably involved in human virulence were previously defined by sequencing the complete genome of both American and Southeast (SE) Asian genotype templates in patient serum (K. C. Leitmeyer et al., J. Virol. 73:4738-4747, 1999). We have now evaluated the effects of introducing a mutation in the envelope glycoprotein (E) gene and/or replacement of 5'- and 3'-nontranslated regions on dengue virus replication in human primary cell cultures. A series of chimeric infectious clones were generated containing different combinations of American and SE Asian genotype sequences. Some of the chimeric viruses had altered plaque morphology in mammalian cells; however, they replicated at similar rates in mosquito cells as measured by quantitative reverse transcription-PCR and plaque assay. Although susceptibility to virus infection varied from donor to donor in experiments using human macrophage and dendritic cells, we were able to measure consistent differences in viral RNA output per infected cell. Using this measurement, we demonstrated that the chimeric virus containing the E mutation had a lower virus output compared to the parental infectious clone. A larger reduction in virus output was observed for the triple mutant and the wild-type, American genotype virus from which chimeric inserts were derived. It appears that the three changes function synergistically, although the E mutation alone gives a lower output compared to the 5'- and 3'-terminal mutations. The data suggest that these changes may be responsible for decreased dengue virus replication in human target cells and for virulence characteristics during infection.

Molecular evolution and phylogeny of dengue type 4 virus in the Caribbean

We sequenced the E gene and adjacent prM/M and NS1 junctions (1940 bp) of 48 Dengue-4 (DEN-4) isolates collected between 1981 and 1999 from 8 Caribbean islands and from 7 South and Central American countries. Phylogenetic analysis confirms a single introduction in the early 1980s and a high degree of gene flow resulting in a pattern of evolution defined more by time period than geographic origin, especially within the Caribbean basin. A modern Caribbean clade consisting of four distinct lineages has arisen, comprised of isolates from Caribbean islands and nearby regions of South America. This clade is defined by three amino acid substitutions in the E (aa 163 and 351) and NS1 (aa 52) proteins. These findings highlight the importance of migration and gene flow in dengue viral change and suggest that efforts to understand disease dynamics in the Caribbean basin need to focus at regional, rather than local scales.

High levels of plasma dengue viral load during defervescence in patients with dengue hemorrhagic fever: implications for pathogenesis

Studies of the pathogenesis of dengue hemorrhagic fever (DHF), a potentially life-threatening disease, have revealed the importance of initial high levels of virus replication. However, the possible involvement of virus during the transition from fever to defervescence, a critical stage in determining the severity of disease, has not been appreciated. Using quantitative reverse transcription-polymerase chain reaction, we examined the levels of plasma dengue viral load during both fever and defervescence periods in patients from a DEN-3 outbreak in southern Taiwan in 1998. Higher levels of plasma dengue viral RNA were found in DHF patients than in DF patients. During defervescence, while the level of plasma dengue viral RNA was undetectable in most DF patients, it remains high in all DHF patients. Using a modified immunoprecipitation assay, we demonstrated for the first time that the plasma dengue viruses persisting during defervescence were in the immune complexes for most DHF patients. These findings suggest that continued active viral replication or delay in the clearance of viremia contributes to the pathogenesis of DHF. Moreover, high levels of plasma dengue viral RNA during defervescence may serve as a disease marker for DHF.