The innate immune system acts as the first line of defense against viral infections which is frequently targeted by DENV for immune evasion (Morrison et al., 2012). DENV has been shown to interfere with innate immune signaling via different mechanisms, thus inhibiting antiviral response. Therefore, to mount a successful infection, DENV must inhibit the main innate immune response which is based on the type I IFN system. DENV possesses several intrinsic characteristics that prevent it from being detected. Vesicle packets (VPs) are microenvironments that are localized within the endoplasmic reticulum (ER) which allow the concentration of viral proteins and metabolites that are required for viral replication. These isolated spaces also act as a mechanism of evasion to exclude themselves from potential inhibitory factors in the host (Arakawa and Morita, 2019). This passive evasion mechanism allows the delayed interaction between pathogen recognition receptors (PRRs) and pathogen-associated molecular patterns (PAMPs).

Upon viral replication in the isolated spaces of VPs, the RNA molecules possess the ability to interfere with the RLR-dependent signaling. RLR-signalling is the recognition pathway involving RIG-I-like receptors (RLR) for recognizing PAMPs of DENV and a disruption of this pathway leads to blocking PRR response. 2’-O methylation and partial degradation of viral RNA (vRNA) by host factors are two modification strategies that could occur to the vRNA, allowing it to prevent detection by the innate immune system (Tremblay et al., 2019). NS5 contains a domain that has methyltransferase (MTase) activity which is responsible for catalyzing the methylation that occurs on the 2’-OH position of the first nucleotide which allows the vRNA to camouflage itself as cellular mRNA (Potisopon et al., 2014). The 2’-O methylation is a common evasion strategy found in viruses and is pivotal in affecting coronavirus detection in the host (Paramasivam, 2020). Partial degradation of the vRNAs by host nucleases also acts as an evasion mechanism as incomplete degradation results in subgenomic flavivirus RNA (sfRNA) which are likely to be resistant to further degradation. The sfRNA is derived from the stalling of 5’-3’exonuclease 1 (XRN1), causing it to malfunction and prematurely cut the vRNA (Clarke et al., 2015). With reference to other flaviviruses, the accumulation of sfRNA can contribute to help the virus to circumvent the antiviral signaling pathways (Schuessler et al., 2012; Donald et al., 2016). A previous study has reported the accumulation of sfRNAs in DENV that could inhibit tripartite motif containing 25 (TRIM25) deubiquitylation by ubiquitin carboxyl-terminal hydrolase 15 (USP15) (Manokaran et al., 2015). This prevented the activation of RIG-I receptors and subsequent interactions with the mitochondrial antiviral signaling (MAVS) adaptor which led to the hijacking of IFN signaling (Okamoto et al., 2018). The 5′ and 3′ untranslated regions (UTRs) are critical for viral genome replication and translation (Ng et al., 2017). The 3’UTR induced pathogenicity by contributing to innate immune evasion of host cells. Secondary structures in the 3′ UTR have been shown to stabilize viral RNA and provide resistance to degradation by cellular RNase and promote the accumulation of sub-genomic RNAs (Funk et al., 2010).

DENV is known as a weak inducer of type I IFN response as it has evolved multiple strategies to antagonize the host IFN system (Wu et al., 2013). The NS3 protein of DENV inhibited the translocation of RIG-I to the adaptor protein MAVS, which is found in the inner membrane of mitochondria by interacting with the chaperone protein 14-3-3ϵ (Chan and Gack, 2016). NS4a inhibited the interaction of RIG-I with the adaptor protein MAVS by binding to the N-terminal CARD-like domain and C-terminal transmembrane domain of MAVS, resulting in the suppression of IRF3 activation and IFN production (He et al., 2016). NS2a and NS4b from DENV1, 2, 4, and NS4a from DENV1 were shown to abrogate IFN-β production by blocking the RIG-I/MAVS signaling pathway and preventing phosphorylation of TBK1/IRF (Dalrymple et al., 2015). NS2b inhibited type I IFN production by targeting cyclic GMP-AMP synthase (cGAS) for lysosomal degradation and preventing mitochondrial DNA sensing (Aguirre et al., 2017). NS2b/3 protease blocked IFN production and subverted the host’s innate immunity by cleaving STING (Yu et al., 2012). NS2A and NS3 have been shown to impair RIG-I and TLR-3 signaling pathways by degrading STING and IRF3 (Castillo Ramirez and Urcuqui-Inchima, 2015). By inactivating the mitochondrial fission factor dynamin-related protein 1, NS4b induced mitochondrial elongation, resulting in altered mitochondria-associated membranes (MAMs), increased DENV replication, and decreased IFN production (Barbier et al., 2017). The involvement of DENV non-structural proteins in the evasion of innate immune response is illustrated in Figure 1 .

Type I IFN signaling induces transcription of IFN-signaling genes (ISGs) through the activation of the JAK-STAT pathway. This pathway involves type I IFN receptors (IFNARs) that are commonly present on nucleated cells, the activation of the signaling pathway is initiated through the binding of IFN α/β. This activates Janus kinase 1 (JAK1) and tyrosine kinase (Tyk2), which phosphorylate signal transducer and activator of transcription 1 (STAT1) and signal transducer and activator of transcription 2 (STAT2). Through these cascades of events, the phosphorylated STAT1 and STAT2 will associate with interferon regulatory factor 9 (IRF9) to form a transcription factor (IFN-stimulated gene factor 3) (ISGF3) (Fink and Grandvaux, 2013) which binds to an IFN-stimulated response element (ISRE) to turn on the transcription of ISGs ( Figure 2 ).

During ADE, engagement of FcR by DENV–antibody immune complexes and entry of immune complexes into target cells resulted in downregulation of expression of TLR-3, -4 and -7 (Kulkarni, 2020). Suppression of retinoic acid-inducible gene 1 (RIG-1) and melanoma differentiation-associated protein 5 (MDA-5) signaling were also observed in DENV-ADE-infected K562 cells, resulting in the inhibition of type I IFN production and IFN-mediated antiviral responses (Huang et al., 2016). Hence, type I IFN signaling would occur in the absence of enhancing antibodies.

The inhibition of the IFN signalling pathway has been reported in several publications involving human or non-human primate cells infected with DENVs and led to decreased type I IFN signalling through blocking signal transduction for phosphorylation of STAT1 or degradation of STAT2 proteins (Muñoz-Jordán et al., 2003; Jones et al., 2005; Ashour et al., 2009). Studies involving human K562 cells (Jones et al., 2005) and A549 cells (Muñoz-Jordán et al., 2003) expressing dengue replicons had shown an overall decrease of STAT 1 phosphorylation with lower STAT2 levels. On the other hand, a study using non-human primate cells (VERO cells) had shown a decrease in STAT2 levels when infected with either DENV-1 or 2 (Ashour et al., 2009).

The products of ISGs have been reported to be involved in antiviral immune response upon infection of viruses and are also pivotal in establishing the adaptive immune response which is responsible for the removal of the infection (Boo and Yang, 2010). The importance of this pathway can be further illustrated through an experiment to quantify the amount of viral RNA present in mice when comparing STAT1, STAT2 knockout mice to the wild-type mice. The results presented showed that mice with single-deficient STAT1 or 2 possessed up to 100 folds more viral RNA after 72 hours of infection (Perry et al., 2011). This indicates the importance of type I IFN response in preventing DENV RNA replication.

NS2A, NS4A and NS4B can interfere with the type I IFN response by inhibiting the upstream phosphorylation of STAT1. These non-structural proteins, NS2A, NS4A and NS4B are associated with the cellular membrane and each has its own respective role in viral replication and maturation. NS2A has been proven to have a vital role in viral assembly (Leung et al., 2008) and for NS4A and NS4B, they have been shown to co-localize with double-stranded RNA (dsRNA) and the structural protein E, suggesting that these proteins are associated with replication complex of the virus (Miller et al., 2007).

It is interesting to know that even though these non-structural proteins inhibit ISG production in the JAK-STAT pathway, only NS4B has an effect that is potent enough to have an antagonistic effect on the pathway. Otherwise, the effect of the STAT1 inhibition can be recapitulated and points to the presence of an interplay between the three viral proteins in inhibiting IFN type I response (Muñoz-Jordán et al., 2003; Munoz-Jordán et al., 2005).

A dysfunctional innate immune response can lead to the manifestation of severe dengue symptoms due to the inhibition of both IFN production and IFN signaling pathways, as well as increased secretion of inflammatory cytokines, lipid mediators and chemokines (Malavige et al., 2020).

Evasion of the complement response by DENV is modulated by secretion of the NS1 protein. NS1 inhibits the activation of the classical and lectin pathways of the complement system by binding to the complement protein C4 and activating the protease C1s, leading to the cleavage of C4 to C4b and reduces the deposition of C4b on the surface (Avirutnan et al., 2010). Avirutnan et al. (2011) demonstrated that NS1 recruited C4b binding protein, which could suppress complement pathways by inhibiting C4b activity (Avirutnan et al., 2011). Other than that, NS1 prevented the formation of membrane attack complex by forming the NS1-VN complex with the complement regulator vitronectin (VN). This interaction has been observed both in vitro and in DENV-infected patients (Conde et al., 2016). Lastly, NS1 has shown ability to evade from complement response by competitively binding to mannose-binding lectin (MBL) and preventing DENV recognition by MBL, thereby protecting the virus from neutralization (Thiemmeca et al., 2016).

miRNA is a small single-stranded non-coding RNA molecule with approximately 22 nucleotides that functions in post-transcriptional regulation of gene expression (Ambros, 2004). miRNA plays a pivotal role in the regulation of viral replication (Umbach and Cullen, 2009). Upregulation of miRNA-30e* in HeLa and U937 cells upon DENV infection restored type I IFN production and could suppress DENV replication through targeting Iκβα and subsequent activation of NF-κB signaling (Zhu et al., 2014). Overexpression of Let-7c was shown to inhibit the replication of DENV-2 and DENV-4 in human hepatoma Huh-7 cells through the modulation of host factors such as heme oxygenase-1 (HO-1) and BTB domain and CNC homolog 1 (BACH1). HO-1 has been found to reduce DENV replication by suppressing NS2B/NS3 protease activity, resulting in the stimulation of antiviral IFN responses (Tseng et al., 2016). Additionally, miRNA-155 has been shown to block DENV replication by specifically targeting BACH1, which induced HO-1-mediated suppression of NS2B/NS3 and enhanced antiviral IFN responses (Su et al., 2020). miRNA-34 increased the production of type I IFN and ISG expression by inhibiting the Wnt signaling, resulting in the inhibition of viral replication (Smith et al., 2017). The expression of the SIAH E3 ubiquitin protein ligase 1 (SIAH1) was found to be downregulated by miRNA-424, which was then shown to impede DENV replication. By binding to and ubiquitinating the adaptor protein MyD88, inhibition of SIAH1 expression resulted in promoting MyD88-mediated NF-κB signaling (Murphy Schafer et al., 2020).

The humoral immune response plays an important role in controlling DENV infection and dissemination. Neutralizing antibodies (nAbs) play a central role in the humoral immune system by preventing and clearing an infection. DENV infection induces the production of high titres of nAbs which provide long-term immunity to the DENV serotype during a primary infection and short-term immunity to a heterologous DENV serotype during a secondary infection. nAbs function to prevent DENV entry into cells by recognizing conformational epitopes, glycans, and specific regions of the envelope or capsid proteins. However, DENV-induced humoral immune responses are also associated with the development of severe disease manifestations such as DHF and DSS (Chiappelli et al., 2014). Therefore, this suggests that dengue viruses have evolved various strategies to modulate the host humoral immune responses.

The rapid evolution of RNA viruses occurred due to the RNA polymerase which is prone to errors during viral RNA replication. The build-up of mutations in the viral genome and selection pressure of the host immunity might lead to changes in the viral proteins (Ye et al., 2013). Apart from that, RNA viruses also contain low fidelity RNA-dependent RNA polymerases, which generate viral quasispecies that contain random mutations throughout its viral genome. These viral quasispecies contain distinct antigenic epitopes which enable evasions of recognition by host neutralizing antibodies (Diamond, 2003; Ye et al., 2013). Mutations that caused changes to the amino acid residues in domain III of the DENV E protein have been reported to assist in viral evasion from host neutralizing antibodies during DENV infections (Lin et al., 1994; Lok et al., 2001). A previous study reported that antibody-escape DENV-1-related variants isolated from the sera of DENV-infected patients replicated to higher titres than the circulating DENV-1 wild type (Chua et al., 2006). Genetic variations between the E proteins of DENV-4 genotypic variants were also reported to cause a difference in their maturation status, glycosylation pattern, growth kinetics, and foci morphology. In addition, this study also reported that infection or vaccination with a particular DENV-4 variant induced variable levels of neutralizing antibodies towards other DENV-4 variants (Gallichotte et al., 2018). Another study investigated the impact of genetic variations between the prM and E proteins of DENV-2 genotypic variants on the responses of human neutralizing antibodies. It was found that certain DENV-2 strains were more susceptible to neutralizing antibodies as compared to other DENV-2 strains (Martinez et al., 2020). Therefore, these data suggested that antigenic variations might be an immune evasion strategy employed by DENV which led to higher levels of DENV infections in hosts.

The humoral immune system may also be directly subverted by DENV to facilitate infections. DENV may use immune complexes that contain infectious viruses to promote the spread of infections via the lymphatic system through a mechanism known as antibody-dependent enhancement (ADE) (Diamond, 2003). ADE plays a significant role in the pathogenesis of secondary DENV infections (Halstead, 2003). Studies have found that neutralizing antibodies elicited from the first infection bind to DENV and promote its entry into Fc receptor-bearing cells, resulting in ADE and caused more severe dengue. In addition, it also exploits these enhancing antibodies for intracellular immune evasion by carrying out two mechanisms (Chareonsirisuthigul et al., 2007; Modhiran et al., 2010; Ubol et al., 2010). First, entry of DENV-antibody complexes into human monocytic cells activate the negative signalling regulators: authophagy-related 5-authophagy-related 12 (Atg5-Atg12), selective androgen-receptor modulator (SARM), dihydroxyacetone kinase (DAK), TANK. SARM and TANK function to disrupt the toll-like receptor (TLR) signalling cascade whereas Atg5-Atg12 and DAK function to inhibit the RIG-I/MDA-5 signalling cascade. This disabled the production of type I IFN, resulting in suppression of antiviral responses. Second, ADE reduced the production of inflammatory mediators such as interleukin-12 (IL-12) and IFN-Ɣ, but the production of immunosuppressive mediators such as interleukin-10 (IL-10) which induced the expression of the suppressor of cytokine signalling 3 (SOCS-3) gene, resulting in inhibition of the JAK-STAT pathway (Ye et al., 2013). This resulted in an increase in DENV viral loads, leading to severe disease manifestations. Apart from that, IL-10 could also suppress T cell activation, degranulation and cytokine production. The production of cytokines such as B-cell activating factor (BAFF) and proliferation-inducing ligand (APRIL) which played a role in stimulating the transformation of resting B cells into plasma cells might also be responsible for further production of DENV-specific antibodies, leading to ADE (Malavige et al., 2020).

It is a requirement to cleave the prM protein for the activation of DENV infectivity. However, studies have shown that the process of maturation and cleavage of prM for DENV was inefficient (van der Schaar et al., 2007; Zybert et al., 2008). Incomplete prM cleavage was associated with the presence of an acidic residue at position P3 within the 13 amino acid sequence proximal to the prM protein (Keelapang et al., 2004; Junjhon et al., 2008). Studies have found that the presence of prM antibodies caused immature DENV particles to become highly infectious. The prM antibodies facilitated viral entry into cells followed by virus maturation catalysed by endosomal furin, and activation of its membrane-fusion machinery (Rodenhuis-Zybert et al., 2010; Rodenhuis-Zybert et al., 2011). The prM antibodies could enhance the infectivity of prM-containing DENV particles due to interactions with Fc receptors expressed on immune cells and heat shock protein 60 which was expressed on certain Fc receptor-deficient cells (Huang et al., 2006; Rodenhuis-Zybert et al., 2010; Rodenhuis-Zybert et al., 2011). Immature DENV particles were not able to bind efficiently to cells in the absence of antibodies and thus they did not affect disease pathogenesis during primary DENV infections. However, during secondary infections, the prM-specific antibody responses would induce the infectivity of immature DENV particles, thereby increasing the number of infectious DENV virions in the blood circulation (Rodenhuis-Zybert et al., 2011). This indicated that prM-containing viruses were more prone to cause ADE in contrast to fully mature DENV particles. The findings of the infective potential of immature DENV virions in the presence of antibodies not only suggested that incomplete cleavage of prM occurred during natural DENV infections, but it is also possible that it could be a mechanism to evade humoral immune responses (Ye et al., 2013).

Apart from humoral immunity, activation of cell-mediated immune responses are also vital for clearance of established DENV infections. The recognition of viral peptides presented by major histocompatibility complex (MHC) class I molecules on cytotoxic T lymphocytes (CTLs) is a significant event in eliminating cells which produce viral proteins during DENV infections. CTLs play a vital role in controlling viral infections, especially as a long-term immune surveillance effector that can react rapidly against the same virus during a secondary infection (Ye et al., 2013).

For example, to establish an infection, DENV requires T helper 1 (T_H1) responses which stimulate the production of interleukin-12 (IL-12), interleukin-18 (IL-18), tumor necrosis factor (TNF), and IFN-γ. Both TNF and IFN-γ are known to be associated with DENV disease severity. However, it has also been reported that T_H1 cells may play a role in limiting DENV pathogenesis, suggesting that the balance in induction of T helper cells is of vital importance (Rivino et al., 2013; Mathew et al., 2014; Rivino, 2016; Rivino and Lim, 2017). Studies have shown that co-culturing DENV-infected dendritic cells (DCs) and naive T helper cells resulted in the formation of T_H1 cells which secreted IFN-γ. This process is dependent on the activation of RIG-I-like receptors (RLRs) during DENV replication. The differentiation of T_H1 cells is induced by IL-12, a cytokine consisting of IL-12p35 and IL-12p40 subunits. However, triggering of RLRs during DENV infection did not induce the production of IL-12 as RLR-activated IRF3 inhibited IL-12p40. Type 1 IFN and interleukin-27 (IL-27) are also known to induce the differentiation of T_H1 cells. RLR activation during DENV infection induced the secretion of both type 1 IFN and IL-27 and they acted as mediators for T_H1 polarization, resulting in production of DHF- and DSS-associated inflammatory mediators, TNF and IFN-γ (Sprokholt et al., 2017a; Sprokholt et al., 2017b; Sprokholt et al., 2017).

Apart from T_H1 cells, the expansion of cytotoxic CD4⁺ and CD8⁺ T cells from naive T cells was also detected during DENV infection. These cells acquire a high affinity for the primary infecting DENV serotype. The triggering of T-cell receptors (TCRs) of DENV-specific cytotoxic T cells resulted in the production of IFN-γ and an increase in the expression of CD107a on the cell surface (Gagnon et al., 1999; Tian et al., 2016). The production of IFN-γ and upregulation of CD107a expression led to lysis of DENV-infected cells and the amount of cytotoxic CD4⁺ and CD8⁺ T cells which expressed CD107a were associated with protection against DENV (Duangchinda et al., 2010; Sprokholt et al., 2017). Hence, due to the various roles played by CTLs during DENV pathogenesis, evasion of CTLs is a requirement for productive DENV viral replications (Ye et al., 2013).

Dendritic cells (DCs) play an important role in bridging both innate and adaptive immune responses by integrating signals from pathogen-associated microbial patterns (PAMPs) with pathogen-derived antigens to stimulate antigen-specific B cell and T cell responses. DCs carry out functions such as (a) uptake and processing of antigens, followed by presentation of the antigen-derived peptides on MHC class I molecules to CD8⁺ T cells or on MHC class II molecules to CD4⁺ T cells; (b) expressing co-stimulatory molecules for activation of T cells; and (c) secreting cytokines and chemokines to lure T cells and modulate priming of T cells (Schmid et al., 2014).

Since the survival of DCs is important for optimal T cell activation, DENV-induced apoptosis of DENV-infected DCs could inhibit the priming of adaptive immune responses. Studies have reported that non-infected bystander monocyte-derived DCs (moDCs) upregulated MHC class I and II molecules and co-stimulatory molecules such as CD80 (B7-1), CD83 and CD86 (B7-2) after exposure to DENV. However, within the same culture, DENV blocked the activation and maturation of infected moDCs (Libraty et al., 2001; Palmer et al., 2005). In another study, it was also observed that non-infected bystander monocytes, moDCs and classical DCs (cDCs) expressed higher levels of CD80 and CD86 as compared to DENV-infected cells in infected mice deficient in IFN-α/β receptor (Schmid and Harris, 2014). These observations suggested that DENV induced apoptosis in DENV-infected cells, but increased its survival in non-infected bystander cells. Additionally, these results also indicated that the blockage of activation in DENV-infected DCs by DENV might reduce the priming of CD4⁺ or CD8⁺ T cells, whereas non-infected bystander cells could still be activated (Schmid et al., 2014).

Studies have also shown that DENV infection reduced the capacity of moDCs and DCs isolated from human skin explants to prime DENV-naïve CD4⁺ T cells, suggesting that DENV-infected DCs had an impaired to activate CD4⁺ T cells (Palmer et al., 2005; Nightingale et al., 2008; Cerny et al., 2014). Others have also reported similar results that DENV-infected moDCs could stimulate CD4⁺ T cells, but with reduced T cell effector functions, such as reduced secretions of IFN-γ and TNFα (Chase et al., 2011). The impaired ability of DENV-infected moDCs to produce IFN-α and IFN-β might also explain their reduced ability to prime T cell responses (Rodriguez-Madoz et al., 2010). Therefore, inhibition of antigen presentation and functionality of DENV-infected DCs might be a viral evasion strategy to reduce T cell responses.

Apart from enabling viral escape from neutralizing antibodies, viral quasispecies formed during DENV infections might also facilitate evasion of the recognition by MHC molecules or T cell receptors. Viral quasispecies formed during infection might also reduce the amount of T cell epitopes that could promote effector functions during the infection (Diamond, 2003). A study by Erickson et al. (2001) revealed that the hepatitis C virus (HCV) quasispecies acquired mutations in multiple epitopes which resulted in impairment of MHC class I binding and CTL recognition (Erickson et al., 2001). For the case of DENV infections, studies have shown that complexity in the pattern of functional immune responses to heterologous DENV serotypes were highly dependent on the amino acid sequences and epitopes of the variant peptides (Bashyam et al., 2006). This might also justify the strong correlation of DHF with high cytokine levels and heterologous secondary DENV infections. It has been demonstrated that dengue-specific memory T cells have high binding affinities to dengue antigens from primary infections and poor binding affinities to dengue antigens from secondary infections caused by heterologous serotypes, resulting in inadequate immune responses. This phenomenon is known as the “original antigenic sin” (Mongkolsapaya et al., 2003).

Apart from regulating innate immunity, miRNAs have been widely associated with regulating adaptive immunity by modulating the survival, activation, development, and proliferation of B- and T-cells (Raisch et al., 2013).

T-cell development involves the roles of various signaling cascades which are mediated by miRNAs. It has been discovered that disruption in the biogenesis of miRNAs might cause a conditional removal of dicer in the early stage of T-cell development, resulting in a reduction of T-cell counts. Additionally, it has also been observed that this disruption might also lead to deviant cytokine production and differentiation of T-helper cells as well as reduced survival of αβ-expressing thymocytes. Poor proliferations, reductions in number, and increased apoptosis were also observed in peripheral T-cells (Koenecke and Krueger, 2018; Rodríguez-Galán et al., 2018). miRNAs such as the miRNA-181 family, miRNA-17-92 clusters, miRNA-214, miRNA-146a, miRNA-29, let-7, miRNA-125, miRNA-155, and miRNA-216 were observed to play a role in the signalling cascade downstream of T-cell activation (Luan et al., 2018). For example, miRNA-155 has been shown to play a critical role in the differentiation of CD4+ T-cells. Overexpression of miRNA-155 is associated with the differentiation of CD4+ T cells into Th1 cells and reduced expression of miRNA-155 demonstrated a bias toward cell differentiation (Baumjohann and Ansel, 2013). miRNA-155 is expressed abundantly in dendritic cells, macrophages, as well as active B and T cells (Pareek et al., 2014; Xia et al., 2018; Ekiz et al., 2019; Wang et al., 2019). A study by Su et al. (2020) reported that the expression of miRNA-155 was downregulated during DENV infection, thereby suggesting its possible role against oxidative stress in DENV-induced pathogenesis. Their results demonstrated that miRNA-155 inhibited DENV replication via induction of the heme oxygenase-1 (HO-1) signalling pathway and enhanced antiviral IFN responses. They also observed that the exogenous expression of miRNA-155 in the brains of ICR suckling mice which provided a protective effect against life-threatening DENV infection (Su et al., 2020). Another miRNA, miRNA-146, modulated immune responses by targeting tumor necrosis factor receptor-associated factor 6 (TRAF6) and interleukin-1 receptor-associated kinase 1 (IRAK1) of the NF-κβ signalling in activated T-cells (Saki et al., 2015). (Pu et al., 2017) demonstrated that inhibition of autophagic activity in DENV-2-infected A549 and THP-1 cells by miR-146a was due to targeting TRAF6 which resulted in a reduction of IFN-β (Pu et al., 2017). In addition, miRNAs also play an important role in regulating the function of Treg cells. Treg cells have important functions in maintaining immune cell homeostasis by preventing autoimmunity and limiting immune responses. Various miRNAs such as miRNA-10a, miRNA-146a, and miRNA-155 have been found to contribute to Treg homeostasis and functions (Hippen et al., 2018; Chandan et al., 2019).

The expression of miRNAs in B cells have been observed which implies its role in the development and maturation of B cells (Chen et al., 2018). For example, it was found that miRNA-155 suppressed the expression of c-MAF (transcription factor) and IFNγ receptor 1 in naive CD4+ T cells whereas it blocked the expression of SH-2 domain containing inositol 5’ polyphosphatase 1 (SHIP1) and PU.1 (transcription factor) in B cells. However, in the absence of miRNA-155, defective antibodies were produced and thus, impairing immune responses to antigenic stimulation. Additionally, murine models deficient in miRNA-155 were observed to show an increased number of germinal central B cells as well as reduced IgG production and maturation. The regulation of germinal central B cells was mediated by regulating PU.1 during the post-transcriptional stage (Danger et al., 2014). Apart from that, it was discovered that the expression pattern of C-Myb which plays a critical role in B cell development was inversely correlated to the expression of miRNA-150 in B cells. miRNA-150 was shown to be highly expressed in progenitor B cells whereas it was downregulated in mature B cells. High levels of miRNA-150 were necessary for the conversion of pre-B to mature B cells, so as to downregulate the expression of C-Myb for normal B cell development (Tan et al., 2009; Chandan et al., 2019). However, not much is known regarding the role of miRNAs in regulating B cell immune responses during DENV infections.