Neuroinflammation (inflammation in brain) has been known to play an important role in the development of dengue virus disease. Recently, studies from both clinical and experimental models suggest the involvement of neuroinflammation in dengue viral disease. Studies in clinical setup demonstrated that, microglial cells are actively involved in the patients having dengue virus infection, showing involvement of innate immune response in neuroinflammation. It was further proved that, clinical isolates of dengue-2 virus were able to initiate the pathologic response when injected in the mice brain. Natural killer cells were also found to play a crucial role to activate adaptive immune response. Notably, CXCL10/IFN-inducible protein 10 and CXCR3 are involved in dengue virus-mediated pathogenesis and play an important role in the development of dengue virus-mediated paralysis. In a latest report, it was seen that intracranial injection of dengue virus increases the CD8+ T-cell infiltration in brain, showing an important mechanism of neuroinflammation during the dengue virus infection. A similar study has described that, when DENV-3 is injected into the mice, it enhances the infiltration of CD8+ and CD4+ T cells as well as neutrophils. Cells immune-reactive against NS3 antigen were found throughout the brain. In conclusion, we focus on the various molecular mechanisms which contribute to the basic understanding about the role of neuroinflammation in dengue fever. These mechanisms will help in better understanding dengue pathophysiology and thus help in the development of possible therapeutics.
In the recent decades, dengue virus infection has become a major health problem globally. There are generally four main types of dengue virus serotypes [1, 2]. Dengue infections in general are caused to the host by Aedes aegypti and Aedes albopictus mosquitoes [3, 4]. Dengue virus comes under the flaviviridae family and is very similar to yellow fever, Japanese encephalitis virus, and St. Louis and West Nile viruses . In fact, infection from one kind of serotype allowing patients a lifelong protection from the same or homologous serotype is no longer true . It is important to note that this protection is not a protection from other serotypes . The second-time infection with the other serotype of the primary infection results in very severe complications [8, 9]. In fact, antibodies generated in the primary infection lead towards severe complication in the secondary infections . The progress in the development of the vaccine has been unsuccessful only due to the limited knowledge of the immune response against dengue virus . Dengue virus envelop is basically made up of lipid and contains a single-positive stranded RNA as a genome [11, 12]. The dengue gene encodes a big protein precursor of approximately 3,400 amino acids long. This big precursor protein is subsequently post-translationally modified by virus-encoded protease NS2B/NS3 and host cell signalases to produce seven non-structural and three structural proteins . These three structural proteins, core, pre-membrane, and envelope are involved in the virus particle assembly, while seven non-structural proteins, i.e., NS1 to NS5, are believed to play an important role in the viral RNA replication and host immune responses . It is now accepted that immune system is activated by these nonstructural virus particles that play an important role in the development of neuroinflammation in dengue fever .
The traditional classification of the World Health Organization (WHO) of dengue fever is still practiced, which includes two major parts, the dengue fever and dengue hemorrhagic fever/dengue shock syndrome . Recent studies have suggested that these play a huge role in inflammation-associated neurodegeneration . It was evident that pro-inflammatory cytokines and chemokines were elevated in dengue fever patients . The mediators of inflammation were found upregulated with severe dengue disease . Therefore, it can be speculated that peripheral inflammation might be initiating the inflammatory cascade in the brain and be responsible for inflammation-associated responses.
Dengue virus is generally considered a non-neurotrophic virus pathogen; however, some scientists have found it in brain fluids and dengue antigen in brain tissue . It is now considered that dengue infection should be responsible for the initiation and progression of the inflammation in brain . Recently, dengue virus is associated with encephalitis and various kinds of neuropathies . Therefore, nowadays the dengue infection has been considered as one of the causes of encephalitis . It is further interesting to note that patients showing dengue mediated inflammation exhibited various signs like intermittent tremors, convulsions stiffness in neck, altered consciousness, and sometimes coma. It is now considered that different kinds of virus antigens are involved in the dengue pathogenesis, which initiate and progress the disease . The key component involved in the mechanism of neuroinflammation in dengue patients therefore need a systemic review .
The Evidence for the Involvement of Neuroinflammation in Dengue Virus Disease
Neuroinflammation is an inflammatory mechanism of brain and is involved in many vector-borne diseases [22, 23]. A vast variety of clinical, experimental studies demonstrate that there is a clear-cut role of neuroinflammation in dengue virus infections. Many clinical observations suggest the involvement of neuroinflammation in dengue viral disease (Fig. 1). Some manifestations in central nervous systems were also found in the dengue patients . In a report of young patients diagnosed with dengue encephalitis, it was observed that there is involvement of central nervous system . There were also more cases of neurological symptoms which were reported in the severe dengue cases . The disseminated encephalomyelitis in acute form has also been reported in the dengue hemorrhagic fever conditions [27, 28]. In a report, it was also observed that there is opsoclonus-myoclonus-ataxia syndrome, which was believed to be in dengue patients . Acute polyneuritis has been associated with the dengue infection . A study on the pediatric cases reported that there is linkage in the persistent infection in the brain and neuroinflammation as a key neurological manifestation . The activation of brain microglial cells is an important mechanism seen during the neuroinflammatory process. It is important to note that microglial activation was seen in the case of dengue infection of central nervous system . Interestingly, there has been a presence of effector-memory CD8+ T cells inside the central nervous system of dengue patients who were diagnosed with dengue virus encephalitis . There have been many reported cases of encephalitis in the dengue cases . In a recent study, “cerebral venous thrombosis” presentation was observed, which is considered to be an unusual presentation of pathogenesis . When dengue disease is enhanced by the antibody, it generates a profound neuroinflammatory response . On day one of dengue infection, it can lead to the development of encephalitis [37, 38]. A recent case report from Pakistan has also published an acute encephalopathy due to dengue infection . There have also been cases with cross-reactive IgM responses in dengue patients with other diseases. Therefore, a particular diagnosis is important in the diagnosis of dengue . In case of dual infection with malaria, dengue patients show some special kind of clinical manifestations .
A significant amount of evidence from experimental models confirmed the involvement of neuroinflammation in dengue viral disease. Viral infections can rupture the blood brain barrier and allows immune cells to pass to the brain [42, 43]. The fact that systemic inflammation is involved in the dengue virus encephalopathy comes from intratracheal infection of dengue virus in mice . The dengue virus-3 infected mice model shows higher levels of anxiety in comparison with controls . It was demonstrated that CXCL10/IFN-inducible protein 10 and CXCR3 are involved in the dengue virus-mediated pathogenesis and play an important role in the development of dengue virus-mediated paralysis . Combined immune-deficient mice also showed the symptoms of paralysis due to dengue virus, showing the involvement of immune system . It has now been clearly indicated that there is a huge role of microglia-mediated neuroinflammation in the dengue cases . Vaccination study in mice showed that a tetravalent vaccine candidate protects from dengue virus-mediated virus encephalitis . It was further proved that clinical isolates of dengue-2 virus were able to initiate the pathologic response in brain . Microglia were found to be protective in dengue virus-induced encephalitis in a mice model . It was further confirmed using dengue virus isolates in BALB/c mice that dengue hemorrhagic fever can be clearly distinguished from dengue shock syndrome . Recently, a tetravalent vaccine study in mice showed some promising results and suggested its use as a potential drug candidate in future against dengue virus infection . It was also seen that a dengue virus isolate and its proteins cause death of human microvascular endothelial cells, which indicates its relevance to the pathogenesis . Some inhibitory strategies also indirectly support the involvement of neuroinflammation in dengue viral disease. In line of this, many strategies have been established like heat shock factor-1, which can be a good antiviral tool to combat the dengue virus infection . Human heme oxygenase-1 was associated to be an important host cell factor against dengue virus replication . Another dual miRNA targeting was also found to be suitable for the restriction of host range that attenuates dengue virus virulence in brain .
The Role of Innate Immune Response in the Development of Neuroinflammation in Dengue Viral Disease
Numerous lines of evidence support the involvement of innate immune response in dengue pathogenesis [56, 57]. It was long back speculated that innate immune cells of the body keep a check and neutralize dengue virus before it reaches the different parts of the body [58, 59]. In this area, mainly macrophages and monocytes were considered, which is also supported by the latest evidence . Nowadays, other cells and mediators of the immune system are found to be responsible in the development or activation of innate immune response and thus subsequently neuroinflammation [32, 61]. Recently, natural killer cells are gaining much more importance in dengue viral disease due to their vast variety of functions and advancement in their importance in the regulation of innate and adaptive immune system. The natural killer cells are tremendously involved in the pathogenesis of dengue virus disease by showing their early activation . Moreover, natural killer cells also keep check on the dengue-infected cells of the body and destroy them at their earliest infection . The endothelial cells, when becoming infected with the dengue infection, significantly stimulate the activation of innate immune response [64, 65]. The altered level of toll-like receptor expression was also observed during the dengue virus infections . The invasion of innate immune response was found to be accomplished by the methylation [67, 68]. However, it is now confirmed that nonstructural proteins of the dengue virus are responsible for invading the innate immune response . In line of this, mast cells and neutrophils also seem to be responsible for the innate immune reactions and development of devastating pathogenesis [70, 71]. Reports also point out that there is clear-cut role of mast cells in the development of dengue pathogenesis . A recent study from mice shown that the effect of NS1 is mediated by the mast cell response and thus the severity of the dengue infection . It was also understood that dengue infection causes severe complications in the brain, as observed in the twin case .
The Role of Adaptive Immune Response in the Development of Neuroinflammation in Dengue Viral Disease
Apart from innate immune response, dengue virus infection causes adaptive immune response [75, 76]. Evidence from different clinical and experimental models now support that, there is clear-cut involvement of adaptive immune response in the development of neuroinflammation-associated pathophysiology in dengue viral disease [75, 77]. It was observed that, levels of IL-4 and -10 in patient’s serum were significantly increased in severe dengue cases. However, serum IL-2 levels were found unaffected along with the severity of cases [78, 79]. Recently, Ab-Rahman et al.  suggested that sCD163 can be used as a biomarker to identify the severe dengue cases from the normal dengue cases . A vital information came from dengue dead patients where hemophagocytic syndrome inside bone marrow was the key component responsible for death . When DENV-2 strain 66985 was injected in BALB/c mice by intravenous route, a severe form of insult is observed in the central nervous system . In a latest study, it was seen that intracranial injection of dengue virus increases CD8+ T-cell Infiltration in brain, showing an important mechanism of neuroinflammation during the dengue virus infection . Another similar study described that, when DENV-3 is injected into the mice, it enhances infiltration of CD8+ and CD4+ T cells as well as neutrophils. Cells immune-reactive against NS3 antigen were found throughout the brain . Immune-reactive cells for anti-dengue were visualized throughout the brain and also altered the behavioral changes that occur before the death of mice . Similarly, in a study, it was found that DENV2 induces encephalitis in mice . Interestingly, it was observed that dengue-specific CD8+ T cells play beneficial as well as destructive roles . A detailed study in brain and spinal cord of rhesus monkey (Macaca mulatta) has shown the neuro-virulence mechanisms of dengue-2 virus . It is now widely believed that Th-17 cells strongly participate in the evasion of blood brain barrier in the severe cases of dengue viral infection and development of neuroinflammation [42, 86]. While it is important to see that, there is effect of peripheral immune cells to the brain, there is a recent study in mice which shows that brain cells also effect peripheral immune system . Taken together, it can be concluded that there is significant involvement of both innate and adaptive immune system in the neuroinflammation, and brain inflammation is profoundly connected to the peripheral immune system of the body (Fig. 2).
Some Important Aspects That Are Needed to Be Explored in Dengue Virus Disease
From the above-mentioned studies, it is now believed that, there is a clear-cut role of immune system in the initiation and progression of inflammation in brain [88, 89]. In this line, various different kinds of immune cells and their mediators need to be explored to understand the pathophysiology of disease . For example, studies suggest that there is a profound involvement of monocytes in dengue pathogenesis, but their exact role is not understood [90, 91]. Monocytes are cells which are the most infected cells of the body during the dengue virus infections. Here, we highlight the role of monocytes in this disease, but how they target the neuroinflammation is not yet understood . As we know, the secondary infection in dengue is much more severe than the primary and involves some cells of allergic responses . Therefore, the role of mast cells in this dengue infection has been postulated, but the exact role still needs to be figured out in details . Further, the role of eosinophils needs to be explored in the dengue disease conditions . Moreover, the roles of matrix metalloproteases need to be known in the severe cases of dengue disease . Matrix metalloproteinases are very critical in the development of severe form of dengue pathology specially hemorrhagic conditions and dengue shock syndrome (DSS). We emphasize on exploration of innate immune cells mediated role of matrix metalloproteinases in the plasma leakage associated to Dengue Shock Syndrome as well as in the neuroinflammation.
Conclusion and Future Perspectives
In this review, we have shown that there is clear involvement of innate immune system and subsequent inflammatory response in the initiation of neuroinflammatory process in the brain [57, 96]. It is also further assessed that, both kinds of immune response (innate and adaptive) take part in the initiation and progression of encephalitic pathophysiology of disease. It was seen that intracranial injection of dengue virus increases CD8+ T-cell infiltration in brain, showing an important mechanism of neuroinflammation during the dengue virus infection. A similar study described that when DENV-3 is injected into the mice, it enhances the infiltration of CD8+ and CD4+ T cells as well as neutrophils. The immune cells reactive against NS3 antigen of dengue virus were found throughout the brain. Taking all these together, it is now considered that dengue infection involves lots of immunologic mediators which induce the neuroinflammation in brain and may be responsible for the encephalitic death of dengue infected patients. Therefore, future strategies should be developed to understand and manage neuroinflammation in the brain.
The young scientist grant No. SB/YS/LS-198/2014 to Dr. Rituraj Niranjan by SERB (DST, India.) is gratefully acknowledged. Technical help from Vaishnavi Srinivasan is also acknowledged.
The authors declare that they have no conflict of interests.