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

Levels of soluble ST2 in serum associated with severity of dengue due to tumour necrosis factor alpha stimulation

The interleukin-1 receptor-like-1 protein (IL1RL1), also known as ST2, has been shown previously to regulate T-cell function and is produced by T cells and endothelial cells. It was reported recently to be elevated in mild dengue patients during acute disease. The ST2 gene encodes several splice products: L (long), V (short) and s (soluble). A cohort of 38 patients with dengue haemorrhagic fever (DHF) and mild dengue fever (DF) were evaluated using a secreted soluble ST2 (sST2) ELISA. The RNA expression of ST2 was evaluated by real-time quantitative RT-PCR using patients' peripheral blood mononuclear cells (PBMCs) and in vitro using human umbilical vein endothelial cells (HUVECs) exposed to sera from dengue patients. DHF patients had higher levels of serum sST2, tumour necrosis factor alpha (TNF-alpha), interleukin (IL)-8 and IL-10 compared with DF patients and normal healthy control individuals. However, viraemia was indistinguishable between mild and severe cases. No changes in ST2 mRNA expression were found in PBMCs from these two groups of dengue patients. In vitro, sST2 was elevated in HUVECs treated with patient sera. Neutralization of TNF-alpha in patient sera by pre-treatment with a TNF-alpha antibody inhibited the upregulation of sST2 expression in HUVECs. These results implicate serum TNF-alpha in the modulation of expression of sST2 in an in vitro system, and indicate that sST2 could be associated with the severity of disease. Further studies to determine whether sST2 levels are predictive of the severe form of the disease and the role of sST2 in immune regulation are warranted.

Inhibition of intrahepatic gamma interferon production by hepatitis C virus nonstructural protein 5A in transgenic mice

Hepatitis C virus (HCV) utilizes strategies to suppress or evade the host immune response for establishment of persistent infection. We have shown previously that HCV nonstructural protein 5A (NS5A) impairs tumor necrosis factor alpha (TNF-alpha)-mediated apoptosis. In this study, we have examined the immunomodulatory role of HCV NS5A protein in transgenic mouse (NS5A-Tg) liver when mice were challenged with an unrelated hepatotropic adenovirus as a nonspecific stimulus. Hepatotropic adenovirus was introduced intravenously into NS5A-Tg mice and control mice, and virus clearance from liver was compared over a time course of 3 weeks. The differential mRNA expression levels of 84 cytokine-related genes, signal pathway molecules, transcription factors, and cell surface molecules were determined using real-time reverse transcription-PCR array. NS5A-Tg mice failed to clear adenovirus from liver up to 3 weeks postinfection while control mice cleared virus within 1 to 2 weeks. Subsequent study revealed that gamma interferon (IFN-gamma) expression is inhibited at both the mRNA and protein levels in NS5A-Tg mice, and an inverse expression of transcription factors Gata-3 and Tbx21 is observed. However, TNF-alpha mRNA and protein expression were elevated in both NS5A-Tg and control mice. Together, our results suggested that HCV NS5A acts as an immunomodulator by inhibiting IFN-gamma production and may play an important role toward establishment of chronic HCV infection.

Homeostatic regulation of T effector to Treg ratios in an area of seasonal malaria transmission

An important aspect of clinical immunity to malaria is the ability to down-regulate inflammatory responses, once parasitaemia is under control, in order to avoid immune-mediated pathology. The role of classical (CD4(+)CD25(+)CD127(lo/-)FOXP3(+)) Treg in this process, however, remains controversial. Thus, we have characterized the frequency, phenotype and function of Treg populations, over time, in healthy individuals in The Gambia. We observed that both the percentage and the absolute number of CD4(+)FOXP3(+)CD127(lo/-) T cells were higher among individuals living in a rural village with highly seasonal malaria transmission than among individuals living in an urban area where malaria rarely occurs. These CD4(+)FOXP3(+)CD127(lo/-) T cells exhibited an effector memory and apoptosis-prone phenotype and suppressed cytokine production in response to malaria antigen. Cells from individuals exposed to malaria expressed significantly higher levels of mRNA for forkhead box P3 and T-box 21 (T-BET) at the end of the malaria transmission season than at the end of the non-transmission season. Importantly, the ratio of T-BET to forkhead box P3 was remarkably consistent between populations and over time, indicating that in healthy individuals, a transient increase in Th1 responses during the malaria transmission season is balanced by a commensurate Treg response, ensuring that immune homeostasis is maintained.

Distinct roles for FOXP3 and FOXP3 CD4 T cells in regulating cellular immunity to uncomplicated and severe Plasmodium falciparum malaria

Failure to establish an appropriate balance between pro- and anti-inflammatory immune responses is believed to contribute to pathogenesis of severe malaria. To determine whether this balance is maintained by classical regulatory T cells (CD4(+) FOXP3(+) CD127(-/low); Tregs) we compared cellular responses between Gambian children (n = 124) with severe Plasmodium falciparum malaria or uncomplicated malaria infections. Although no significant differences in Treg numbers or function were observed between the groups, Treg activity during acute disease was inversely correlated with malaria-specific memory responses detectable 28 days later. Thus, while Tregs may not regulate acute malarial inflammation, they may limit memory responses to levels that subsequently facilitate parasite clearance without causing immunopathology. Importantly, we identified a population of FOXP3(-), CD45RO(+) CD4(+) T cells which coproduce IL-10 and IFN-gamma. These cells are more prevalent in children with uncomplicated malaria than in those with severe disease, suggesting that they may be the regulators of acute malarial inflammation.

Combination of CTLA-4 and TGFbeta1 gene polymorphisms associated with dengue hemorrhagic fever and virus load in a dengue-2 outbreak

The pathogenesis of dengue hemorrhagic fever (DHF) has been considered to be massive immune activation of T cells. Abnormal expression of the immune regulatory molecules, CTLA-4 and TGFbeta1, leads to disturbances of regulatory T cell immune response. We investigate the contribution of CTLA-4 and TGFbeta1 in DHF by analyzing them for association with virus load in blood and polymorphisms of CTLA-4 +49A/G, and TGFbeta1 -509C/T in a DEN-2 outbreak. The increased frequency of the TGFbeta1 -509 CC genotype in patients with DHF was compared to those with dengue fever (OR=1.9, p=0.034). Moreover, the presence of the CTLA-4 +49 G allele and TGFbeta1 -509 CC genotype increased the susceptibility to risk of DHF (OR=2.1, p=0.028) and significantly higher virus load (p=0.013). This finding suggests that a combination of CTLA-4 and TGFbeta1 polymorphisms is associated with the susceptibility of DHF and higher virus load.

The history and evolution of human dengue emergence

Dengue viruses (DENV) are the most important human arboviral pathogens. Transmission in tropical and subtropical regions of the world includes a sylvatic, enzootic cycle between nonhuman primates and arboreal mosquitoes of the genus Aedes, and an urban, endemic/epidemic cycle principally between Aedes aegypti, a mosquito that exploits peridomestic water containers as its larval habitats, and human reservoir hosts that are preferred for blood feeding. Genetic studies suggest that all four serotypes of endemic/epidemic DENV evolved independently from ancestral, sylvatic viruses and subsequently became both ecologically and evolutionarily distinct. The independent evolution of these four serotypes was accompanied by the expansion of the sylvatic progenitors' host range in Asia to new vectors and hosts, which probably occurred gradually over a period of several hundred years. Although many emerging viral pathogens adapt to human replication and transmission, the available evidence indicates that adaptation to humans is probably not a necessary component of sylvatic DENV emergence. These findings imply that the sylvatic DENV cycles in Asia and West Africa will remain a potential source of re-emergence. Sustained urban vector control programs and/or human vaccination will be required to control DEN because the enzootic vectors and primate reservoir hosts are not amenable to interventions.

Elevated levels of soluble ST2 protein in dengue virus infected patients

Levels of the soluble form of the interleukin-1 receptor-like 1 protein (IL-1RL-1/ST2) are elevated in the serum of patients with diseases characterized by an inflammatory response. The objective of this study was to determine the concentration of soluble ST2 (sST2) in dengue infected patients during the course of the disease. Twenty-four patients with confirmed dengue infection, classified as dengue fever, and 11 patients with other febrile illness (OFI) were evaluated. Levels of sST2 in serum and laboratory variables usually altered during dengue infections were measured. Dengue infected patients had higher serum sST2 levels than OFI at the end of the febrile stage and at defervescence (p=0.0088 and p=0.0004, respectively). Patients with secondary dengue infections had higher serum sST2 levels compared with patients with primary dengue infections (p=0.047 at last day of fever and p=0.030 at defervescence). Furthermore, in dengue infected patients, we found a significant negative correlation of sST2 with platelet and WBC counts, and positive correlation with thrombin time and transaminases activity. We suggest that sST2 could be a potential marker of dengue infection, could be associated with severity or could play a role in the immune response in secondary dengue virus infection.

Different clinical and laboratory manifestations between dengue haemorrhagic fever and dengue fever with bleeding tendency

The incidence of dengue fever (DF) is estimated to have increased 30-fold in the past 50 years. The incidence of dengue haemorrhagic fever (DHF), a life-threatening complication of DF, is also increasing. The need for better classification of the severity of dengue infections has been proposed in order to clarify different entities of dengue infections. We defined a class of patients with DF with bleeding tendency (DF w/B) to differentiate further the varying pathogenesis among DF, DF w/B and DHF. In a hospital-based study in Taiwan, we compared clinical features, biochemistry and immune mediators among patients with DHF, DF w/B and DF. Results showed that DF w/B patients, similar to DHF patients, had a higher rate of secondary dengue infection (P<0.001) as well as higher IL-10 (P=0.023) and lower IFNgamma (P=0.009) levels than DF patients. In contrast, DHF patients had significantly higher soluble vascular cell adhesion molecule 1 levels than DF w/B patients (P=0.038) and DF patients (P<0.001). This study provides new insight into the different immune mechanisms of DF, DF w/B and DHF. DF involves a Th1 reaction and DF w/B involves an altered Th2 reaction, whereas DHF involves an altered Th2 reaction and augmented vascular insult.

Implications of previous subclinical dengue infection but not virus load in dengue hemorrhagic fever

In a study comparing the virus load and immune reaction between patients with primary and secondary dengue-2 (DEN-2) infections in a hospital-based analysis, we found that 40.7% (55/135) of the 135 patients had secondary DEN-2 infection following a DEN-2 outbreak in southern Taiwan. Most of the secondary infections had subclinical primary dengue infections (78.2%; 43/55). Patients with secondary DEN-2 infections had lower platelet counts, and blood interferon-alpha and virus load, but significantly higher interleukin-10 (P=0.030) and anti-DEN-1 neutralization titers (P=0.013) than those with primary infection. Patients with secondary DEN-2 infection also had a higher rate of dengue hemorrhagic fever (DHF) (61.7% vs. 36.3%). A previous subclinical dengue infection is involved in the secondary DEN-2 infection associated with altered immune reaction and higher DHF rate, but lower blood virus load.

Immunopathological mechanisms in dengue and dengue hemorrhagic fever

PURPOSE OF REVIEW

The continued emergence of dengue virus infection and its severe disease manifestation, dengue hemorrhagic fever, is a growing public health problem. The majority of severe infections occur upon secondary encounters with heterologous dengue virus serotypes, suggesting an immune-mediated process.

RECENT FINDINGS

Significant findings in the past year include a greater understanding of dengue virus interactions with target cells such as dendritic cells, hepatocytes and endothelial cells. Infection of these cells results in the production of immune mediators that then shape the adaptive humoral and cellular immune response. The circulation of high levels of secreted NS1 in the presence of pre-existing heterologous non-neutralizing antibody may mediate complement activation and trigger plasma leakage. The role of enhancing antibodies in disease pathogenesis remains unclear. Recent studies demonstrate low avidity crossreactive T cells, which may produce an altered profile of cytokines leading to plasma leakage. Ongoing prospective studies that include epidemiological, virological and immunological risk factors are crucial to our understanding of the mechanisms of immunopathogenesis of dengue hemorrhagic fever.

SUMMARY

The immune mechanisms that lead to dengue hemorrhagic fever are complex and need to be elucidated further for the development of therapeutics as well as safe and efficacious dengue vaccines.

Kinetics of antibodies in sera, saliva, and urine samples from adult patients with primary or secondary dengue 3 virus infections

OBJECTIVES

The kinetics of three serological markers (IgM, IgA, and IgG) in serum, saliva, and urine samples from adult patients with primary or secondary dengue infection were studied.

DESIGN

Serum, saliva, and urine samples were collected from 22 patients with clinical and confirmed dengue 3 virus infection during the outbreak in Havana City in 2001. They were tested by capture IgM (MAC-ELISA), IgA (AAC-ELISA), and IgE (EAC-ELISA) and IgG ELISA inhibition method (EIM) to detect specific dengue antibodies.

RESULTS

Similar kinetics were observed in IgM, IgA, and IgG antibodies in saliva and IgA and IgG in urine samples from secondary cases compared with kinetics in serum samples, although the values were lower. No IgG antibody was detected in saliva and urine samples in primary cases and IgM antibody was not detected in urine samples from either primary or secondary infection. All secondary cases were positive for IgG in saliva and urine samples at day 7. The kinetics of specific IgE antibodies in primary and secondary cases were different.

CONCLUSIONS

The kinetics of three serological markers (IgM, IgA, and IgG) in serum, saliva, and urine samples from adult patients with primary or secondary dengue 3 virus infection were studied for the first time, showing its behavior and usefulness in dengue virus diagnosis. The specific IgE could play a role as a serological marker in secondary infections.