Detection of dengue virus RNA by reverse transcription-polymerase chain reaction in the liver and lymphoid organs but not in the brain in fatal human infection

New biomarkers for liver involvement by dengue infection in adult Vietnamese patients: a case-control study

Liver injury with marked elevation of aspartate aminotransferase enzyme (AST) is commonly observed in dengue infection. To understand the pathogenesis of this liver damage, we compared the plasma levels of hepatic specific, centrilobular predominant enzymes (glutamate dehydrogenase, GLDH; glutathione S transferase-α, αGST), periportal enriched 4-hydroxyphenylpyruvate dioxygenase (HPPD), periportal predominant arginase-1 (ARG-1), and other non-specific biomarkers (paraoxonase-1, PON-1) in patients with different outcomes of dengue infection. This hospital-based study enrolled 87 adult dengue patients, stratified into three groups based on plasma AST levels (< 80, 80-400, > 400 U/L) in a 1:1:1 ratio (n = 40, n = 40, n = 40, respectively. The new liver enzymes in the blood samples from the 4th to 6th days of their illness were measured by commercial enzyme-linked immunosorbent assay (ELISA) or colorimetric kits. Based on the diagnosis at discharge days, our patients were classified as 40 (46%) dengue without warning signs (D), 35 (40.2%) dengue with warning signs (DWS), and 11 (12.6%) severe dengue (SD) with either shock (two patients) or AST level over 1000 U/L (nine patients), using the 2009 WHO classification. The group of high AST (> 400 U/L) also had higher ALT, GLDH, ARG-1, and HPPD than the other groups, while the high (> 400 U/L) and moderate (80-400 U/L) AST groups had higher ALT, αGST, ARG-1, and HPPD than the low AST group (< 80 U/L). There was a good correlation between AST, alanine aminotransferase enzyme (ALT), and the new liver biomarkers such as GLDH, αGST, ARG-1, and HPPD. Our findings suggest that dengue-induced liver damage initiates predominantly in the centrilobular area toward the portal area during the dengue progression. Moreover, these new biomarkers should be investigated further to explain the pathogenesis of dengue and to validate their prognostic utility.

Dengue virus infection induces selective expansion of Vγ4 and Vγ6TCR γδ T cells in the small intestine and a cytokine storm driving vascular leakage in mice

Dengue is a major health problem in tropical and subtropical regions. Some patients develop a severe form of dengue, called dengue hemorrhagic fever, which can be fatal. Severe dengue is associated with a transient increase in vascular permeability. A cytokine storm is thought to be the cause of the vascular leakage. Although there are various research reports on the pathogenic mechanism, the complete pathological process remains poorly understood. We previously reported that dengue virus (DENV) type 3 P12/08 strain caused a lethal systemic infection and severe vascular leakage in interferon (IFN)-α/β and γ receptor knockout mice (IFN-α/β/γRKO mice), and that blockade of TNF-α signaling protected mice. Here, we performed transcriptome analysis of liver and small intestine samples collected chronologically from P12/08-infected IFN-α/β/γRKO mice in the presence/absence of blockade of TNF-α signaling and evaluated the cytokine and effector-level events. Blockade of TNF-α signaling mainly protected the small intestine but not the liver. Infection induced the selective expansion of IL-17A-producing Vγ4 and Vγ6 T cell receptor (TCR) γδ T cells in the small intestine, and IL-17A, together with TNF-α, played a critical role in the transition to severe disease via the induction of inflammatory cytokines such as TNF-α, IL-1β, and particularly the excess production of IL-6. Infection also induced the infiltration of neutrophils, as well as neutrophil collagenase/matrix metalloprotease 8 production. Blockade of IL-17A signaling reduced mortality and suppressed the expression of most of these cytokines, including TNF-α, indicating that IL-17A and TNF-α synergistically enhance cytokine expression. Blockade of IL-17A prevented nuclear translocation of NF-κB p65 in stroma-like cells and epithelial cells in the small intestine but only partially prevented recruitment of immune cells to the small intestine. This study provides an overall picture of the pathogenesis of infection in individual mice at the cytokine and effector levels.

Angiotensin II and dengue

Dengue is a disease caused by a flavivirus that is transmitted principally by the bite of an Aedes aegypti mosquito and represents a major public-health problem. Many studies have been carried out to identify soluble factors that are involved in the pathogenesis of this infection. Cytokines, soluble factors, and oxidative stress have been reported to be involved in the development of severe disease. Angiotensin II (Ang II) is a hormone with the ability to induce the production of cytokines and soluble factors related to the inflammatory processes and coagulation disorders observed in dengue. However, a direct involvement of Ang II in this disease has not been demonstrated. This review primarily summarizes the pathophysiology of dengue, the role of Ang II in various diseases, and reports that are highly suggestive of the involvement of this hormone in dengue.

Morphological Aspects and Viremia Analysis of BALB/c Murine Model Experimentally Infected with Dengue Virus Serotype 4

Ever since its brief introduction in the Brazilian territory in 1981, dengue virus serotype 4 (DENV-4) remained absent from the national epidemiological scenario for almost 25 years. The emergence of DENV-4 in 2010 resulted in epidemics in most Brazilian states. DENV-4, however, remains one of the least studied among the four DENV serotypes. Despite being known as a mild serotype, DENV-4 is associated with severe cases and deaths and deserves to be investigated; however, the lack of suitable experimental animal models is a limiting factor for pathogenesis studies. Here, we aimed to investigate the susceptibility and potential tropism of DENV-4 for liver, lung and heart of an immunocompetent mice model, and to evaluate and investigate the resulting morphological and ultrastructural alterations upon viral infection. BALB/c mice were inoculated intravenously with non-neuroadapted doses of DENV-4 isolated from a human case. The histopathological analysis of liver revealed typical alterations of DENV, such as microsteatosis, edema and vascular congestion, while in lung, widespread areas of hemorrhage and interstitial pneumonia were observed. While milder alterations were present in heart, characterized by limited hemorrhage and discrete presence of inflammatory infiltrate, the disorganization of the structure of the intercalated disc is of particular interest. DENV-4 RNA was detected in liver, lung, heart and serum of BALB/c mice through qRT-PCR, while the NS3 viral protein was observed in all of the aforementioned organs through immunohistochemistry. These findings indicate the susceptibility of the model to the serotype and further reinforce the usefulness of BALB/c mice in studying the many alterations caused by DENV.

Use of Animal Models in Studying Roles of Antibodies and Their Secretion Cells in Dengue Vaccine Development

The cardinal feature of adaptive immunity is its ability to form memory responses that can be rapidly recalled to contain pathogens upon reencountering. Conferring a robust memory immune response to an infection is a key feature for a successful vaccination program. The plasmablasts are cells that not only can secret non-neutralizing antibodies but also can secrete the specific antibodies essential to neutralize and inactivate the invading pathogens. Dengue has been recognized as one of the most important vector-borne human viral diseases globally. Currently, supportive care with vigilant monitoring is the standard practice since there is as yet no approved therapeutic modality to treat dengue. Even though the approved vaccine has become available, its low efficacy with the potential to cause harm is the major hurdle to promote the widespread usage of the vaccine. Despite the decades of research on dengue, the major challenge in dengue vaccine development is the absence of suitable experimental animal models that reflect the pathological features and clinical symptoms, as seen in humans. Dengue is transmitted by the bite of mosquitoes carrying infectious dengue virus (DENV), which has four distinct serotypes. Recently, cases resulting from unconventional transmission routes, such as blood transfusion, organs as well as stem cells and bone marrow transplantations, and mother-to-infant vertical transmission, have been reported, suggesting an alternate route of DENV transmission exists in nature. This review discusses issues and challenges needing to be resolved to develop an effective dengue vaccine. Development of a robust and reliable dengue animal model that can reflect not only dynamic human clinical symptoms but also can answer around why preexisting neutralizing antibodies do not confer protection upon re-infection and immune protection marker for dengue vaccine efficacy evaluation.

Dengue hepatitis with acute liver failure: Clinical, biochemical, histopathological characteristics and predictors of outcome

BACKGROUND

Hepatitis infection from non-hepatotropic viruses such as dengue virus (DENV) is increasing worldwide. There is increasing recognition of the changing epidemiology and atypical presentations of DENV infection including acute liver failure (ALF). There is paucity of data regarding incidence, disease characteristics, and markers of prognosis in patients who develop DENV-related ALF.

METHODS

We aimed to study the incidence, clinical features, laboratory characteristics, and determinants of outcome in patients of DENV presenting with ALF. We reviewed all patients with DENV infection and focused on DENV-related ALF from 2014 to 2017. Diagnosis of DENV and ALF was confirmed by serological tests and standard criteria, respectively.

RESULTS

Thirty-six patients (20 men, mean age 32.3) developed ALF among 10 108 patients with DENV infection (0.35%). Twenty-one patients died (58.3%). Although bilirubin, aspartate and alanine aminotransferase, and international normalized ratio were markedly elevated in all patients with DENV ALF, there was no statistically significant difference between survivors and non-survivors. Lactate levels, pH at admission, and model for end-stage liver disease (MELD) score were the only predictors of mortality. Lactate levels were significantly higher in non-survivors (11.5 ± 4.2 mmol/L) than survivors (6.3 ± 3.6 mmol/L) (P < 0.001). MELD score in non-survivors (26.7 ± 10.2) was significantly higher than in survivors (20 ± 7.2) (P = 0.039). Receiver operator characteristic curve showed lactate or pH to be a superior prognostic marker than MELD with an area under the curve of 0.80, 0.79, and 0.70, respectively.

CONCLUSION

Dengue hepatitis progressed to ALF in 0.35%. Development of ALF was associated with a high mortality (> 50%). Lactate level, pH, and MELD score at admission were significant determinants of outcome.

Zika Virus Induced More Severe Inflammatory Response Than Dengue Virus in Chicken Embryonic Livers

Dengue (DENV) and Zika virus (ZIKV) are important flaviviruses in tropical and subtropical regions, causing severe Dengue Hemorrhagic Fever (DHF)/Dengue Shock Syndrome (DSS) and microcephaly, respectively. The infection of both viruses during pregnancy were reported with adverse fetal outcomes. To investigate the effects of ZIKV and DENV infections on fetal development, we established an infection model in chicken embryos. Compared with DENV-2, the infection of ZIKV significantly retarded the development of chicken embryos. High viral loads of both DENV-2 and ZIKV was detected in brain, eye and heart 7 and 11 days post-infection, respectively. Interestingly, only ZIKV but not DENV-2 was detected in the liver. Even both of them induced apparent liver inflammation, ZIKV infection showed a more severe inflammatory response than DENV-2 infection based on the inflammation scores and the gene expression levels of IL-1β, TNF, IL-6, and TGFβ-2 in liver. Our results demonstrated that ZIKV induced more severe inflammatory response in chicken embryo liver compared to DENV-2, which might partially attribute to viral replication in liver cells. Clinicians should be aware of the potential liver injury associated with ZIKV infection in patients, especially in perinatal fetuses.

Heart and liver are infected in fatal cases of dengue: three PCR based case studies

Background

Dengue is a global problem mainly in the tropics. Meticulous clinical management of cases has reduced the death rate significantly, but large numbers of people still succumb to severe complications of the infection. Presence of myocarditis is often overlooked leading to a poor outcome. Clinical management guidelines of dengue do not stress the importance of myocarditis as a manifestation in dengue infection. Severe hepatic dysfunction also needs emphasis.

Case presentation

We present three patients who had come to hospital on the 3rd day of fever. Two of them (case 1 and 3) were in shock on admission and case 2, who was stable on the3^rd day, went into the critical phase and developed shock while in the hospital on the 4^thday. All three had tachycardia on admission that got worse with time. The clinical course was unstable with fluctuations in urine output and deterioration of organ function. Despite frequent monitoring and life support they survived only 2–3 days in hospital. All three patients had myocarditis during the critical phase. In the first case, myocarditis was confirmed by troponin estimation and echocardiogram. In the second and third cases, histopathology confirmed myocarditis. Haemorrhagic necrosis of the liver was found in case 2 and 3 with exponential rise of transaminases. In all three cases, viral RNA was detected in both heart and liver tissues by PCR amplification.

Conclusions

We stress that detection of myocarditis and liver involvement in any dengue patient is important from the onset of the illness where treatment should be tailored to prevent development of hypotension. Our findings are novel as PCR and histology are rarely done on tissues of deceased dengue patients in the world. Studies are needed to find therapeutic interventions to reverse cardiac and hepatic dysfunction in dengue infection.

Tyrosine kinase/phosphatase inhibitors decrease dengue virus production in HepG2 cells

Dengue virus is the causative agent of dengue fever, dengue hemorrhagic fever, and dengue shock syndrome. High rates of dengue virus replication and virion production are related to disease severity. To identify anti-DENV compounds, we performed cell-based ELISA testing to detect the level of DENV E protein expression. Among a total of 83 inhibitors, eight were identified as inhibitors with antiviral activity. Epidermal growth factor receptor inhibitor II (EGFR/ErbB-2/ErbB-4 inhibitor II) and protein tyrosine phosphatase inhibitor IV (PTP inhibitor IV) significantly inhibited dengue virus production and demonstrated low toxicity in hepatocyte cell lines. Our results suggest the efficacy of tyrosine kinase/phosphatase inhibitors in decreasing dengue virus production in HepG2 cells.

Impact of dengue virus (serotype DENV-2) infection on liver of BALB/c mice: A histopathological analysis

In this research, we characterized the histopathological impact of dengue virus (serotype DENV-2) infection in livers of BALB/c mice. The mice were infected with different doses of DENV-2 via intraperitoneal injection and liver tissues were processed for histological analyses and variation was documented. In the BALB/c mouse model, typical liver tissues showed regular hepatocyte architecture, with normal endothelial cells surrounding sinusoid capillary. Based on histopathological observations, the liver sections of BALB/c mice infected by DENV-2 exhibited a loss of cell integrity, with a widening of the sinusoidal spaces. There were marked increases in the infiltration of mononuclear cells. The areas of hemorrhage and micro- and macrovesicular steatosis were noted. Necrosis and apoptosis were abundantly present. The hallmark of viral infection, i.e., cytopathic effects, included intracellular edema and vacuole formation, cumulatively led to sinusoidal and lobular collapse in the liver. The histopathological studies on autopsy specimens of fatal human DENV cases are important to shed light on tissue damage for preventive and treatment modalities, in order to manage future DENV infections. In this framework, the method present here on BALB/c mouse model may be used to study not only the effects of infections by other DENV serotypes, but also to investigate the effects of novel drugs, such as recently developed nano-formulations, and the relative recovery ability with intact immune functions of host.

Dengue infection

Dengue is widespread throughout the tropics and local spatial variation in dengue virus transmission is strongly influenced by rainfall, temperature, urbanization and distribution of the principal mosquito vector Aedes aegypti. Currently, endemic dengue virus transmission is reported in the Eastern Mediterranean, American, South-East Asian, Western Pacific and African regions, whereas sporadic local transmission has been reported in Europe and the United States as the result of virus introduction to areas where Ae. aegypti and Aedes albopictus, a secondary vector, occur. The global burden of the disease is not well known, but its epidemiological patterns are alarming for both human health and the global economy. Dengue has been identified as a disease of the future owing to trends toward increased urbanization, scarce water supplies and, possibly, environmental change. According to the WHO, dengue control is technically feasible with coordinated international technical and financial support for national programmes. This Primer provides a general overview on dengue, covering epidemiology, control, disease mechanisms, diagnosis, treatment and research priorities.

Transcriptome profiling reveals differential expression of interferon family induced by dengue virus 2 in human endothelial cells on tissue culture plastic and polyacrylamide hydrogel

A cell model is critical for studying the molecular mechanisms of dengue virus 2 (DENV-2) invasions and cell bioactivity can be easily affected by the substrate matrix. Tissue culture plastic (TCP) and polyacrylamide hydrogel (PAMH) are two kinds of matrices widely used for cells. The effects of different matrices on the cultured cells with DENV-2 invasion remain unknown. To address the issue, the effects of TCP and PAMH were explored in primary human umbilical vein endothelial cells (HUVECs) with DENV-2 invasion. HUVECs were assigned into four groups: group A (cultured on TCP), group B (cultured on PAMH), group C (cultured on TCP with DENV-2 invasion), and group D (cultured on PAMH with DENV-2 invasion). Flow cytometry was performed on HUVECs after 48-hr culture. Gene expression patterns were analyzed by gene microarray. The levels of interleukin-29 (IL-29) were measured by real-time qRT-PCR and ELISA. There were no cell apoptosis induced by DENV-2 in HUVECs cultured on TCP and PAMH (P > 0.05). After DENV-2 invasion, the up-regulated genes involve in the activities of oligoadenylate synthetase (OAS), interferon-related cytokine, and growth factors so on. The up-regulated pathways involve in the responses to DENV-2 and innate immunity. IL-29 was induced in the HUVECs on PAMH when compared with the cells on TCP (P < 0.05). Thus, different matrices cause different immune responses, which should be considered in the cell models for exploring the molecular mechanisms of DENV-induced diseases.

Mouse models of dengue virus infection for vaccine testing

Dengue is a mosquito-borne disease caused by four serologically and genetically related viruses termed DENV-1 to DENV-4. With an annual global burden of approximately 390 million infections occurring in the tropics and subtropics worldwide, an effective vaccine to combat dengue is urgently needed. Historically, a major impediment to dengue research has been development of a suitable small animal infection model that mimics the features of human illness in the absence of neurologic disease that was the hallmark of earlier mouse models. Recent advances in immunocompromised murine infection models have resulted in development of lethal DENV-2, DENV-3 and DENV-4 models in AG129 mice that are deficient in both the interferon-α/β receptor (IFN-α/β R) and the interferon-γ receptor (IFN-γR). These models mimic many hallmark features of dengue disease in humans, such as viremia, thrombocytopenia, vascular leakage, and cytokine storm. Importantly AG129 mice develop lethal, acute, disseminated infection with systemic viral loads, which is characteristic of typical dengue illness. Infected AG129 mice generate an antibody response to DENV, and antibody-dependent enhancement (ADE) models have been established by both passive and maternal transfer of DENV-immune sera. Several steps have been taken to refine DENV mouse models. Viruses generated by peripheral in vivo passages incur substitutions that provide a virulent phenotype using smaller inocula. Because IFN signaling has a major role in immunity to DENV, mice that generate a cellular immune response are desired, but striking the balance between susceptibility to DENV and intact immunity is complicated. Great strides have been made using single-deficient IFN-α/βR mice for DENV-2 infection, and conditional knockdowns may offer additional approaches to provide a panoramic view that includes viral virulence and host immunity. Ultimately, the DENV AG129 mouse models result in reproducible lethality and offer multiple disease parameters to evaluate protection by candidate vaccines.

The search for nucleoside/nucleotide analog inhibitors of dengue virus

Nucleoside analogs represent the largest class of antiviral agents and have been actively pursued for potential therapy of dengue virus (DENV) infection. Early success in the treatment of human immunodeficiency virus (HIV) infection and the recent approval of sofosbuvir for chronic hepatitis C have provided proof of concept for this class of compounds in clinics. Here we review (i) nucleoside analogs with known anti-DENV activity; (ii) challenges of the nucleoside antiviral approach for dengue; and (iii) potential strategies to overcome these challenges. This article forms part of a symposium in Antiviral Research on flavivirus drug discovery.

Distribution of fitness in populations of dengue viruses

Genetically diverse RNA viruses like dengue viruses (DENVs) segregate into multiple, genetically distinct, lineages that temporally arise and disappear on a regular basis. Lineage turnover may occur through multiple processes such as, stochastic or due to variations in fitness. To determine the variation of fitness, we measured the distribution of fitness within DENV populations and correlated it with lineage extinction and replacement. The fitness of most members within a population proved lower than the aggregate fitness of populations from which they were drawn, but lineage replacement events were not associated with changes in the distribution of fitness. These data provide insights into variations in fitness of DENV populations, extending our understanding of the complexity between members of individual populations.

Modulation of α-enolase post-translational modifications by dengue virus: increased secretion of the basic isoforms in infected hepatic cells

Hepatic cells are major sites of dengue virus (DENV) replication and liver injury constitutes a characteristic of severe forms of dengue. The role of hepatic cells in dengue pathogenesis is not well established, but since hepatocytes are the major source of plasma proteins, changes in protein secretion by these cells during infection might contribute to disease progression. Previously, we showed that DENV infection alters the secretion pattern of hepatic HepG2 cells, with α-enolase appearing as one of the major proteins secreted in higher levels by infected cells. ELISA analysis demonstrated that DENV infection modulates α-enolase secretion in HepG2 cells in a dose-dependent manner, but has no effect on its gene expression and on the intracellular content of the protein as assessed by PCR and western blot analyses, respectively. Two-dimensional western blots showed that both intracellular and secreted forms of α-enolase appear as five spots, revealing α-enolase isoforms with similar molecular weights but distinct isoeletric points. Remarkably, quantification of each spot content revealed that DENV infection shifts the isoform distribution pattern of secreted α-enolase towards the basic isoforms, whereas the intracellular protein remains unaltered, suggesting that post-translational modifications might be involved in α-enolase secretion by infected cells. These findings provide new insights into the mechanisms underlying α-enolase secretion by hepatic cells and its relationship with the role of liver in dengue pathogenesis. In addition, preliminary results obtained with plasma samples from DENV-infected patients suggest an association between plasma levels of α-enolase and disease severity. Since α-enolase binds plasminogen and modulates its activation, it is plausible to speculate the association of the increase in α-enolase secretion by infected hepatic cells with the haemostatic dysfunction observed in dengue patients including the promotion of fibrinolysis and vascular permeability alterations.

Kinome siRNA screen identifies novel cell-type specific dengue host target genes

Dengue is a global emerging infectious disease, with no specific treatment available. To identify novel human host cell targets important for dengue virus infection and replication, an image-based high-throughput siRNA assay screening of a human kinome siRNA library was conducted using human hepatocyte cell line Huh7 infected with a recent dengue serotype 2 virus isolate BR DEN2 01-01. In the primary siRNA screening of 779 kinase-related genes, knockdown of 22 genes showed a reduction in DENV-2 infection. Conversely, knockdown of 8 genes enhanced viral infection. To assess host cell specificity, the confirmed hits were tested in the DENV-infected monocytic cell line U937. While the expression of EIF2AK3, ETNK2 and SMAD7 was regulated in both cell lines after infection, most kinases were hepatocyte-specific. Monocytic cells represent initial targets of infection and an antiviral treatment targeting these cells is probably most effective to reduce initial viral load. In turn, infection of the liver could contribute to pathogenesis, and the novel hepatocyte-specific human targets identified here could be important for dengue infection and pathogenesis.

Mouse models to study dengue virus immunology and pathogenesis

The development of a compelling murine model of dengue virus (DENV) infection has been challenging, because DENV clinical isolates do not readily replicate or cause pathology in immunocompetent mice. However, research using immunocompromised mice and/or mouse-adapted viruses allows investigation of questions that may be impossible to address in human studies. In this review, we discuss the potential strengths and limitations of existing mouse models of dengue disease. Human studies are descriptive by nature; moreover, the strain, time, and sequence of infection are often unknown. In contrast, in mice, the conditions of infection are well defined and a large number of experimental parameters can be varied at will. Therefore, mouse models offer an opportunity to experimentally test hypotheses that are based on epidemiological observations. In particular, gain-of-function or loss-of-function models can be established to assess how different components of the immune system (either alone or in combination) contribute to protection or pathogenesis during secondary infections or after vaccination. In addition, mouse models have been used for pre-clinical testing of anti-viral drugs or for vaccine development studies. Conclusions based on mouse experiments must be extrapolated to DENV-infection in humans with caution due to the inherent limitations of animal models. However, research in mouse models is a useful complement to in vitro and epidemiological data, and may delineate new areas that deserve attention during future human studies.

Activation of peripheral blood mononuclear cells by dengue virus infection depotentiates balapiravir

In a recent clinical trial, balapiravir, a prodrug of a cytidine analog (R1479), failed to achieve efficacy (reducing viremia after treatment) in dengue patients, although the plasma trough concentration of R1479 remained above the 50% effective concentration (EC(50)). Here, we report experimental evidence to explain the discrepancy between the in vitro and in vivo results and its implication for drug development. R1479 lost its potency by 125-fold when balapiravir was used to treat primary human peripheral blood mononuclear cells (PBMCs; one of the major cells targeted for viral replication) that were preinfected with dengue virus. The elevated EC(50) was greater than the plasma trough concentration of R1479 observed in dengue patients treated with balapiravir and could possibly explain the efficacy failure. Mechanistically, dengue virus infection triggered PBMCs to generate cytokines, which decreased their efficiency of conversion of R1479 to its triphosphate form (the active antiviral ingredient), resulting in decreased antiviral potency. In contrast to the cytidine-based compound R1479, the potency of an adenosine-based inhibitor of dengue virus (NITD008) was much less affected. Taken together, our results demonstrate that viral infection in patients before treatment could significantly affect the conversion of the prodrug to its active form; such an effect should be calculated when estimating the dose efficacious for humans.

Ten years of dengue drug discovery: progress and prospects

To combat neglected diseases, the Novartis Institute of Tropical Diseases (NITD) was founded in 2002 through private-public funding from Novartis and the Singapore Economic Development Board. One of NITD's missions is to develop antivirals for dengue virus (DENV), the most prevalent mosquito-borne viral pathogen. Neither vaccine nor antiviral is currently available for DENV. Here we review the progress in dengue drug discovery made at NITD as well as the major discoveries made by academia and other companies. Four strategies have been pursued to identify inhibitors of DENV through targeting both viral and host proteins: (i) HTS (high-throughput screening) using virus replication assays; (ii) HTS using viral enzyme assays; (iii) structure-based in silico docking and rational design; (iv) repurposing hepatitis C virus inhibitors for DENV. Along the developmental process from hit finding to clinical candidate, many inhibitors did not advance beyond the stage of hit-to-lead optimization, due to their poor selectivity, physiochemical or pharmacokinetic properties. Only a few compounds showed efficacy in the AG129 DENV mouse model. Two nucleoside analogs, NITD-008 and Balapiravir, entered preclinical animal safety study and clinic trial, but both were terminated due to toxicity and lack of potency, respectively. Celgosivir, a host alpha-glucosidase inhibitor, is currently under clinical trial; its clinical efficacy remains to be determined. The knowledge accumulated during the past decade has provided a better rationale for ongoing dengue drug discovery. Though challenging, we are optimistic that this continuous, concerted effort will lead to an effective dengue therapy.

Recent advances in DENV receptors

Dengue is an old disease caused by the mosquito-borne dengue viruses (DENVs), which have four antigenically distinct serotypes (DENV1-4). Infection by any of them can cause dengue fever (DF) and/or a more serious disease, that is, dengue hemorrhagic fever (DHF) or dengue shock syndrome (DSS). In recent decades, incidence of dengue disease has increased 30-fold, putting a third to half of the world's population living in dengue-endemic areas at high infection risk. However, the pathogenesis of the disease is still poorly understood. The virus binding with its host cell is not only a first and critical step in their replication cycle but also a key factor for the pathogenicity. In recent years, there have been significant advances in understanding interactions of DENVs with their target cells such as dendritic cells (DC), macrophages, endothelial cells, and hepatocytes. Although DENVs reportedly attach to a variety of receptors on these cells, consensus DENV receptors have not been defined. In this review, we summarize receptors for DENVs on different cells identified in recent years.

Mapping the interactions of dengue virus NS1 protein with human liver proteins using a yeast two-hybrid system: identification of C1q as an interacting partner

Dengue constitutes a global health concern. The clinical manifestation of this disease varies from mild febrile illness to severe hemorrhage and/or fatal hypovolemic shock. Flavivirus nonstructural protein 1 (NS1) is a secreted glycoprotein that is displayed on the surface of infected cells but is absent in viral particles. NS1 accumulates at high levels in the plasma of dengue virus (DENV)-infected patients, and previous reports highlight its involvement in immune evasion, dengue severity, liver dysfunction and pathogenesis. In the present study, we performed a yeast two-hybrid screen to search for DENV2 NS1-interacting partners using a human liver cDNA library. We identified fifty genes, including human complement component 1 (C1q), which was confirmed by coimmunoprecipitation, ELISA and immunofluorescence assays, revealing for the first time the direct binding of this protein to NS1. Furthermore, the majority of the identified genes encode proteins that are secreted into the plasma of patients, and most of these proteins are classified as acute-phase proteins (APPs), such as plasminogen, haptoglobin, hemopexin, α-2-HS-glycoprotein, retinol binding protein 4, transferrin, and C4. The results presented here confirm the direct interaction of DENV NS1 with a key protein of the complement system and suggest a role for this complement protein in the pathogenesis of DENV infection.

Roles for endothelial cells in dengue virus infection

Dengue viruses cause two severe diseases that alter vascular fluid barrier functions, dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). The endothelium is the primary fluid barrier of the vasculature and ultimately the effects of dengue virus infection that cause capillary leakage impact endothelial cell (EC) barrier functions. The ability of dengue virus to infect the endothelium provides a direct means for dengue to alter capillary permeability, permit virus replication, and induce responses that recruit immune cells to the endothelium. Recent studies focused on dengue virus infection of primary ECs have demonstrated that ECs are efficiently infected, rapidly produce viral progeny, and elicit immune enhancing cytokine responses that may contribute to pathogenesis. Furthermore, infected ECs have also been implicated in enhancing viremia and immunopathogenesis within murine dengue disease models. Thus dengue-infected ECs have the potential to directly contribute to immune enhancement, capillary permeability, viremia, and immune targeting of the endothelium. These effects implicate responses of the infected endothelium in dengue pathogenesis and rationalize therapeutic targeting of the endothelium and EC responses as a means of reducing the severity of dengue virus disease.

A model of DENV-3 infection that recapitulates severe disease and highlights the importance of IFN-γ in host resistance to infection

There are few animal models of dengue infection, especially in immunocompetent mice. Here, we describe alterations found in adult immunocompetent mice inoculated with an adapted Dengue virus (DENV-3) strain. Infection of mice with the adapted DENV-3 caused inoculum-dependent lethality that was preceded by several hematological and biochemical changes and increased virus dissemination, features consistent with severe disease manifestation in humans. IFN-γ expression increased after DENV-3 infection of WT mice and this was preceded by increase in expression of IL-12 and IL-18. In DENV-3-inoculated IFN-γ(-/-) mice, there was enhanced lethality, which was preceded by severe disease manifestation and virus replication. Lack of IFN-γ production was associated with diminished NO-synthase 2 (NOS2) expression and higher susceptibility of NOS2(-/-) mice to DENV-3 infection. Therefore, mechanisms of protection to DENV-3 infection rely on IFN-γ-NOS2-NO-dependent control of viral replication and of disease severity, a pathway showed to be relevant for resistance to DENV infection in other experimental and clinical settings. Thus, the model of DENV-3 infection in immunocompetent mice described here represents a significant advance in animal models of severe dengue disease and may provide an important tool to the elucidation of immunopathogenesis of disease and of protective mechanisms associated with infection.

Molecular detection and typing of dengue viruses from archived tissues of fatal cases by rt-PCR and sequencing: diagnostic and epidemiologic implications

Diagnosis of dengue virus (DENV) infection in fatal cases is challenging because of the frequent unavailability of blood or fresh tissues. For formalin-fixed, paraffin-embedded (FFPE) tissues immunohistochemistry (IHC) can be used; however, it may not be as sensitive and serotyping is not possible. The application of reverse transcription-polymerase chain reaction (RT-PCR) for the detection of DENV in FFPE tissues has been very limited. We evaluated FFPE autopsy tissues of 122 patients with suspected DENV infection by flavivirus and DENV RT-PCR, sequencing, and DENV IHC. The DENV was detected in 61 (50%) cases by RT-PCR or IHC. The RT-PCR and sequencing detected DENV in 60 (49%) cases (DENV-1 in 16, DENV-2 in 27, DENV-3 in 8, and DENV-4 in 6 cases). No serotype could be identified in three cases. The IHC detected DENV antigens in 50 (40%) cases. The RT-PCR using FFPE tissue improves detection of DENV in fatal cases and provides sequence information useful for typing and epidemiologic studies.

RNA interference mediated inhibition of dengue virus multiplication and entry in HepG2 cells

BACKGROUND

Dengue virus-host cell interaction initiates when the virus binds to the attachment receptors followed by endocytic internalization of the virus particle. Successful entry into the cell is necessary for infection initiation. Currently, there is no protective vaccine or antiviral treatment for dengue infection. Targeting the viral entry pathway has become an attractive therapeutic strategy to block infection. This study aimed to investigate the effect of silencing the GRP78 and clathrin-mediated endocytosis on dengue virus entry and multiplication into HepG2 cells.

METHODOLOGY/PRINCIPAL FINDINGS

HepG2 cells were transfected using specific siRNAs to silence the cellular surface receptor (GRP78) and clathrin-mediated endocytosis pathway. Gene expression analysis showed a marked down-regulation of the targeted genes (87.2%, 90.3%, and 87.8% for GRP78, CLTC, and DNM2 respectively) in transfected HepG2 cells when measured by RT-qPCR. Intracellular and extracellular viral RNA loads were quantified by RT-qPCR to investigate the effect of silencing the attachment receptor and clathrin-mediated endocytosis on dengue virus entry. Silenced cells showed a significant reduction of intracellular (92.4%) and extracellular viral RNA load (71.4%) compared to non-silenced cells. Flow cytometry analysis showed a marked reduction of infected cells (89.7%) in silenced HepG2 cells compared to non-silenced cells. Furthermore, the ability to generate infectious virions using the plaque assay was reduced 1.07 log in silenced HepG2 cells.

CONCLUSIONS/SIGNIFICANCE

Silencing the attachment receptor and clathrin-mediated endocytosis using siRNA could inhibit dengue virus entry and multiplication into HepG2 cells. This leads to reduction of infected cells as well as the viral load, which might function as a unique and promising therapeutic agent for attenuating dengue infection and prevent the development of dengue fever to the severe life-threatening DHF or DSS. Furthermore, a decrease of viremia in humans can result in the reduction of infected vectors and thus, halt of the transmission cycle.

The pathogenesis of dengue

Dengue is an important cause of childhood and adult morbidity in Asian and Latin American countries and its geographic footprint is growing. The clinical manifestations of dengue are the expression of a constellation of host and viral factors, some acquired, others intrinsic to the individual. The virulence of the virus plus the flavivirus infection history, age, gender and genotype of the host all appear to help shape the severity of infection. Similarly, the characteristics of the innate and acquired host immune response subsequent to infection are also likely determinants of outcome. This review summarises recent developments in the understanding of dengue pathogenesis and their relevance to dengue vaccine development.

Detection of the dengue non-structural 1 antigen in cerebral spinal fluid samples using a commercially available enzyme-linked immunosorbent assay

The involvement of the central nervous system in dengue infections has been reported in countries where the disease in endemic. The purpose of this study was to determine whether an enzyme-linked immunosorbent assay kit designed to detect the dengue NS1 antigen in serum was able to detect this antigen in cerebral spinal fluid (CSF) samples from patients with fatal outcomes. To evaluate the sensitivity of the kit, 26 dengue-positive CSF samples were used. The Pan-E Dengue Early kit was able to detect the NS1 antigen in 13 of 26 dengue-positive CSF samples, resulting in a sensitivity of 50% (95% confidence interval, 29.9-70.1%) and specificity of 100% (95% confidence interval, 75.3-100%). The kit was able to detect the NS1 antigen in CSF of individuals who had died of dengue. When used in combination with IgM, the detection rate rose to 92.3%. This study reports a method for rapidly detecting the dengue virus in CSF, thereby increasing the diagnosis of dengue fever cases with unusual neurological manifestations.

Common marmoset (Callithrix jacchus) as a primate model of dengue virus infection: development of high levels of viraemia and demonstration of protective immunity

Dengue virus (DENV) causes a wide range of illnesses in humans: dengue fever (DF), dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). Animal models that constantly develop high levels of viraemia are required for the development of protective and preventive measures. Common marmosets (Callithrix jacchus) demonstrated high levels of viraemia after inoculation with clinical isolates of four serotypes of DENV; in particular, over 10(6) genome copies ml(-1) after inoculation with DENV-2. Non-structural protein 1 and DENV-specific IgM and IgG antibodies were consistently detected. The DENV-2 genome was detected in lymphoid organs including the lymph nodes, spleen and thymus, and also in non-lymphoid organs. DENV antigen was detected by immunohistochemistry in the liver and spleen from inoculated marmosets. Four marmosets were reinoculated with DENV-2 at 33 weeks after primary inoculation with DENV-2. The DENV-2 genome was not detected in any of these marmosets, indicating protection from a secondary infection. The results indicate that common marmosets are highly sensitive to DENV infection, and suggest that marmosets could be a reliable primate model for the evaluation of candidate vaccines.

Dengue: a continuing global threat

Dengue fever and dengue haemorrhagic fever are important arthropod-borne viral diseases. Each year, there are ∼50 million dengue infections and ∼500,000 individuals are hospitalized with dengue haemorrhagic fever, mainly in Southeast Asia, the Pacific and the Americas. Illness is produced by any of the four dengue virus serotypes. A global strategy aimed at increasing the capacity for surveillance and outbreak response, changing behaviours and reducing the disease burden using integrated vector management in conjunction with early and accurate diagnosis has been advocated. Antiviral drugs and vaccines that are currently under development could also make an important contribution to dengue control in the future.

Liver involvement associated with dengue infection in adults in Vietnam

Globally, the number of adults hospitalized with dengue has increased markedly in recent years. It has been suggested that hepatic dysfunction is more significant in this group than among children. We describe the spectrum and evolution of disease manifestations among 644 adults with dengue who were prospectively recruited on admission to a major infectious disease hospital in southern Vietnam and compare them with a group of patients with similar illnesses not caused by dengue. Transaminase levels increased in virtually all dengue patients and correlated with other markers of disease severity. However, peak enzyme values usually occurred later than other complications. Clinically severe liver involvement was infrequent and idiosyncratic, but usually resulted in severe bleeding. Chronic co-infection with hepatitis B was associated with modestly but significantly increased levels of alanine aminotransferase, but did not otherwise impact the clinical picture.

A BALB/c mouse model shows that liver involvement in dengue disease is immune-mediated

In the present study, BALB/c mice were used to develop a model for the hepatic injury associated to dengue infection. Histological analysis after subcutaneous inoculation with a low viral dose of dengue-2 virus showed Kupffer cell hyperplasia and an increased inflammatory cellular infiltrate next to the bile ducts on days 5, 7 and 14 post-inoculation, mainly characterized by the presence of mononuclear cells. The liver mRNA transcription level of IL-1β was highest on the 5th day post-infection (p.i.) and decreased by the 21st day, TNF-α showed a peak of mRNA transcription after 14 days p.i. coinciding with the regression of cellular infiltrates and elevated expression of TGF-β mRNA. Serum AST and ALT levels were slightly elevated at 7 and 14 days post-infection. Dengue-2 RNA levels were undetectable in the liver on any of the days following inoculation. Our observations suggest that, as it is true for humans, the animals undergo a transient and slight liver inflammation, probably due to local cytokine production and cellular infiltration in the liver.

Enhanced infection of liver sinusoidal endothelial cells in a mouse model of antibody-induced severe dengue disease

Dengue virus (DENV) causes disease ranging from dengue fever (DF), a self-limited febrile illness, to the potentially lethal dengue hemorrhagic fever and dengue shock syndrome (DHF/DSS). DHF/DSS usually occurs in patients who have acquired DENV-reactive antibodies prior to infection, either from a previous infection with a heterologous DENV serotype or from an immune mother. Hence, it has been hypothesized that subneutralizing levels of antibodies exacerbate disease, a phenomenon termed antibody-dependent enhancement (ADE). However, given the lack of suitable animal models for DENV infection, the mechanism of ADE and its contribution to pathology remain elusive. Here we demonstrate in mice that DENV-specific antibodies can sufficiently increase severity of disease so that a mostly nonlethal illness becomes a fatal disease resembling human DHF/DSS. Antibodies promote massive infection of liver sinusoidal endothelial cells (LSECs), resulting in increased systemic levels of virus. Thus, a subprotective humoral response may, under some circumstances, have pathological consequences.

Dengue virus pathogenesis: an integrated view

Much remains to be learned about the pathogenesis of the different manifestations of dengue virus (DENV) infections in humans. They may range from subclinical infection to dengue fever, dengue hemorrhagic fever (DHF), and eventually dengue shock syndrome (DSS). As both cell tropism and tissue tropism of DENV are considered major determinants in the pathogenesis of dengue, there is a critical need for adequate tropism assays, animal models, and human autopsy data. More than 50 years of research on dengue has resulted in a host of literature, which strongly suggests that the pathogenesis of DHF and DSS involves viral virulence factors and detrimental host responses, collectively resulting in abnormal hemostasis and increased vascular permeability. Differential targeting of specific vascular beds is likely to trigger the localized vascular hyperpermeability underlying DSS. A personalized approach to the study of pathogenesis will elucidate the basis of individual risk for development of DHF and DSS as well as identify the genetic and environmental bases for differences in risk for development of severe disease.

Hepatic damage associated with dengue-2 virus replication in liver cells of BALB/c mice

One difficulty in studying dengue virus (DENV) is the lack of an experimental model that reproduces the human disease. In a previous work, we have shown that BALB/c mice intraperitoneally inoculated with a DENV-2 isolate presented viremia and mild focal areas of liver injuries. In this study, mice were inoculated by the intravenous route and presented extensive damage areas in the liver tissue, which were evaluated by histopathological and ultrastructural analysis. Hepatic injury was noted mainly around the central vein and portal tracts. Damages consist of hepatocyte injury, including steatosis, swelling and necrosis. Further, erythrophagocytosis, intercellular edema and vascular damages were evident, including hemorrhage, which is characteristic of the dengue-induced hepatitis in human liver. Hepatic lesions were already noted 2 days post infection (p.i.), although effects were more extensive after the seventh day p.i. An increase in alanine aminotransferase and aspartate aminotransferase serum levels was detected 7 and 14 days p.i., respectively, and had correlation to hepatic lesions. Alterations caused by the DENV infection were self-limiting, with a remarkable reduction of all liver damages 49 days p.i. Virus antigens were detected in hepatocytes, Kupffer cells and vascular endothelium, suggesting virus replication in these cells. In situ hybridization, using a probe that anneals in the virus negative RNA strand, showed positive reaction in hepatocytes and vascular endothelium cells of infected mice, thus confirming virus replication in such cells. In general, results revealed that this mouse model reproduces some histopathological effects observed in humans and supports previous findings indicating virus replication in the hepatic tissue.

Cardif-mediated signaling controls the initial innate response to dengue virus in vivo

The role of Cardif-dependent signaling in controlling dengue virus (DENV) infection and regulating type I interferon (IFN) production in vivo was examined in Cardif-deficient mice. DENV RNA levels were significantly elevated in both the serum and lymphoid tissues of Cardif(-/-) mice at early times compared to those in wild-type animals. Type I IFN production was delayed in these locales of Cardif(-/-) mice until 18 h postinfection, indicating that Cardif regulates the initial type I IFN response in lymphoid tissues. In contrast, DENV viral loads in nonlymphoid tissues were similar between Cardif(-/-) and wild-type mice. These results reveal that RNA helicase-mediated sensing acts as a first line of innate defense against DENV infection in vivo and functions in a tissue-dependent manner.

Efficient dengue virus (DENV) infection of human muscle satellite cells upregulates type I interferon response genes and differentially modulates MHC I expression on bystander and DENV-infected cells

Dengue virus (DENV) is a mosquito-borne flavivirus that causes an acute febrile disease in humans, characterized by musculoskeletal pain, headache, rash and leukopenia. The cause of myalgia during DENV infection is still unknown. To determine whether DENV can infect primary muscle cells, human muscle satellite cells were exposed to DENV in vitro. The results demonstrated for the first time high-efficiency infection and replication of DENV in human primary muscle satellite cells. Changes in global gene expression were also examined in these cells following DENV infection using Affymetrix GeneChip analysis. The differentially regulated genes belonged to two main functional categories: cell growth and development, and antiviral type I interferon (IFN) response genes. Increased expression of the type I IFN response genes for tumour necrosis factor-related apoptosis-inducing ligand (TRAIL), melanoma-derived antigen 5 (MDA-5), IFN-gamma-inducible protein 10 (IP-10), galectin 3 soluble binding protein (LGals3BP) and IFN response factor 7 (IRF7) was confirmed by quantitative RT-PCR. Furthermore, higher levels of cell-surface-bound intracellular adhesion molecule-1 (ICAM-1) and soluble ICAM-1 in the cell-culture medium were detected following DENV infection. However, DENV infection impaired the ability of the infected cells in the culture medium to upregulate cell-surface expression of MHC I molecules, suggesting a possible mechanism of immune evasion by DENV. The findings of this study warrant further clinical research to identify whether muscle cells are targets for DENV infection during the acute stage of the disease in vivo.

A mouse-passaged dengue virus strain with reduced affinity for heparan sulfate causes severe disease in mice by establishing increased systemic viral loads

The four serotypes of dengue virus (DENV1 to DENV4) cause extensive morbidity and mortality. A major obstacle to studying disease pathogenesis and developing therapies has been the lack of a small-animal model. We previously reported isolation of a DENV2 strain, obtained by passaging a clinical isolate between mosquito cells and mice, that caused severe DENV disease in mice and contained multiple mutations, including many in domain II of the envelope (E) protein. Here, we describe a recombinant virus, differing from the non-mouse-passaged virus by two mutations in the E protein, that induces vascular leakage and tumor necrosis factor alpha (TNF-alpha)-mediated lethality, while the non-mouse-passaged virus causes paralysis. This recombinant virus has a weaker affinity for heparan sulfate, resulting in an increased serum half-life, higher systemic viral loads, and high levels of TNF-alpha in the serum of infected mice. These results exemplify the role of the E protein in modulating virion clearance and connect the effect of clearance on the systemic viral loads responsible for severe disease manifestations.

Identification of dengue virus binding proteins using affinity chromatography

Several studies have identified putative dengue virus receptors using virus overlay protein binding assays (VOPBA) with some apparent success. Given that this technique relies upon the use of electrophoresis of proteins through polyacrylamide gels with varying amounts of protein denaturation, the physiological relevance of the proteins isolated is open to question. To address this issue a Sepharose 4B-dengue virus serotype 2-affinity column was constructed to selectively bind dengue virus binding proteins from HepG2 (liver) cell membrane preparations. Results show that GRP78, but not the 37/67 kDa high affinity laminin receptor, was specifically bound by the column. This result is consistent with earlier work and shows that while affinity chromatography may provide a useful adjunct to VOPBA based studies particularly in cases where proteins maybe sensitive to denaturation, proteins isolated by VOPBA can be physiologically relevant.

Infection of human primary hepatocytes with dengue virus serotype 2

While the impact of the dengue viruses on liver function is prominent as shown by hepatomegaly, liver enzyme abnormality, occasional fulminant hepatic failure and histological changes including hepatocellular necrosis, significant debate exists as to the possible involvement of the predominant cell type in the liver, hepatocytes, in the disease process. To address this issue purified human primary hepatocytes were exposed to dengue virus serotype 2 and the production of de novo viral progeny was established by standard plaque assay, RT-PCR and immunocytochemistry. To investigate the response of the primary hepatocytes to infection, the expression of a panel of 9 cytokine genes (IFN-beta, TRAIL, MCP-1, IL-6, IL-1beta, IL-8, MIP-1alpha, MIP-1beta, and RANTES) was semi-quantitatively investigated by RT-PCR and up-regulation of TRAIL, MIP-1alpha, IFN-beta, MIP-1beta, IL-8, and RANTES was observed in response to infection. The induction of IL-8 in response to infection was accompanied by the secretion of IL-8 as verified by ELISA assay. The ability of hepatocytes to be infected with dengue virus serotype 2 in vitro support evidence implicating human hepatocytes as a target cell in cases of dengue virus infection, and provide the first experimental evidence to support the large number of clinical studies that implicate the liver as a critical target organ in severe cases of dengue infection.

cDNA-AFLP analysis of differential gene expression in human hepatoma cells (HepG2) upon dengue virus infection

In infectious diseases, the disease pathogenesis is the outcome of the interaction between the genome of the host and the genome of the pathogen. Despite the wide distribution of dengue infections in the world, and the large number of annual infections, few studies have investigated how the dengue genome alters the global transcriptional profile of the host cell. To investigate alterations in the liver cell transcriptome in response to dengue virus infection, liver cells (HepG2) were infected with dengue serotype 2 at MOI 5 and at 3 days post-infection RNA extracted and analyzed by cDNA-AFLP in parallel with mock-infected cells. From 73 primer combinations over 5,000 transcription-derived fragments (TDFs) were observed, of which approximately 10% were regulated differentially in response to infection. Sixty-five TDFs were subsequently cloned and sequenced and 27 unique gene transcripts identified. Semi-quantitative reverse transcription (RT)-PCR was used to validate the expression of 12 of these genes and 10 transcripts (CK2, KIAA509, HSP70, AK3L, NIPA, PHIP, RiboS4, JEM-1, MALT1, and HSI12044) were confirmed to be differentially regulated, with four transcripts (HSP70, NIPA, RiboS4, and JEM-1) showing a greater than twofold regulation. These results suggest that the expression of a large number of genes is altered in response to dengue virus infection of liver cells, and that cDNA-AFLP is a useful tool for obtaining information on both characterized and as yet uncharacterized transcripts whose expression is altered during the infection process.