Monocytes and Macrophages
Monocytes are a key target of the DENV and while being crucial for the host anti-viral response, they also contribute to ADE. [
43] Several monocyte subsets have been implicated in the pathogenesis of dengue,[
44] and the DENV appears to have a predilection to infect some subsets over others. The biological role of specific monocyte subsets in dengue remains an active field of research.[
45,
46] CD14+/CD16+ intermediate monocytes were found to be increased in paediatric patients with dengue up until day 3 of infection followed by a decline from day 4 onwards.[
47] Interestingly, intracellular staining demonstrated DENV infection in the classical (CD14bright/CD16-) and intermediate monocytes subsets but not the non-classical monocytes. The role of monocytes was reinforced in this study through the detection of raised IL-10 mRNA levels from CD14+ monocytes compared to other subsets of peripheral blood mononuclear cells. The increased IL-10 levels resulted in upregulation of pathways inhibiting the anti-viral response including the suppression of JAK-STAT signalling.[
48] Furthermore,
in -vitro infection of monocytes with the DENV resulted in suppression of Interferon-Beta and induced nitric oxide synthesis with sub-neutralising concentrations of DENV specific antibodies.[
49] Collectively, these data suggest that the DENV “hijacks” monocytes to downregulate innate anti-viral immune responses.
DENV infected macrophages have been detected in the tissues of patients with dengue, [
50]and macrophage recruitment to the endothelium has been implicated in the development of haemorrhage.[
51] It has been postulated that macrophages at different anatomic sites may have distinctive roles in the response to dengue infection, with some subsets being permissive to DENV.[
52] Indeed, it is possible that macrophage dysfunction underlies dengue related Hemophagocytic lymphohistiocytosis (HLH). HLH is characterized by inappropriate macrophage stimulation resulting in a cytokine storm and a severe systemic inflammatory response.[
53] Variable levels of haemo-phagocytosis were seen on post-mortem studies of patients with severe dengue, in contrast to those with mild disease.[
54] In a retrospective single centre study, mortality due to severe dengue was 22% and approximately 40% of these patients had dengue related HLH.[
55] The phenotype of macrophages in dengue associated HLH has not been adequately characterised and is an important area to be addressed for future research.
Further studies are required to further unravel the biological significance of the monocyte/macrophage lineage specially in the pathogenesis of severe dengue. Integration of single cell RNA sequencing with flow cytometry at serial time points of the disease would provide crucial information on the evolution of monocyte subsets and their function. Differential gene expression analysis between monocyte subsets infected by DENV and those which are not would also be of significant interest. Ideally, changes in monocytes subsets should be studied in the context of other immune cell populations including lymphocytes, as it is likely that a complex interplay exists between these populations.[
56]
Lymphocytes
Lymphocytes play a crucial role in the immune response against viral infections including dengue.[
57] Interestingly, a lower mean lymphocyte count was reported as a biomarker to distinguish dengue from non-dengue viral fevers[
58] , while a higher percentage of lymphocytes in the WBC differential was associated with a shorter hospital stay in a study from India.[
59]A lower absolute lymphocyte count was also reported as a predictor of severe dengue in a separate study.[
60]These conflicting data suggest that a specific subset of lymphocytes (rather than total lymphocytes) may be more relevant in the pathophysiology of dengue infection.
The finding of “atypical” or “reactive” lymphocytes in dengue infection has been well described for several years. [
61] Atypical lymphocytes (AL) are readily detected by evaluation of the peripheral blood film[
62] , and more recently by automated haematology analysers.[
63] Current analysers identify AL based on their light scatter properties which generates the AL count (a research parameter) as part of the full blood count read out. These data have been used by several groups to describe the kinetics of AL and their potential prognostic significance.[
64] AL can be detected at presentation, are known to peak during the defervescing phase of dengue, and are associated with more severe disease.
64 Specifically, the percentage of AL at presentation was associated with more severe thrombocytopaenia
64, as well as haemorrhage, shock, and respiratory compromise.[
65] It is reasonable to postulate that a higher AL count reflects greater immune dysregulation resulting in more severe thrombocytopaenia and other clinical manifestations.
T-cells play a significant role in the host response to dengue infection. Patients with severe dengue were shown to have an increase in markers of T-cell activation, as well as cytolytic granule proteins.[
66] In contrast, other studies have shown that the inhibitory CTLA4 antigen was expressed at higher levels in T-cells from patients with dengue suggesting that T-cell responses against DENV maybe attenuated in some patients.[
67]It is noteworthy that T-regulatory (T-reg) cells are increased in dengue infection, however they are not associated with adverse clinical outcomes and are of the naïve T reg subtype with limited suppressive capacity.[
68] These data suggest a heterogeneity of T-cell subsets in dengue which maybe evolving as the disease progresses. Single-cell RNA sequencing (scRNAseq) at serial time points during dengue infection demonstrated unique effector T cell clusters expressing skin homing signature genes on day 1 of illness which was associated with defervescence.[
69] Further studies using high dimensional profiling of T-cells in dengue are required to elucidate the key T-cell subsets involved in the progression to severe disease. Prospective studies looking at the correlation between T-cell subsets and clinical outcomes DENV would be of significant interest and may generate new hypotheses to be addressed through translational research.
While B-cells are important in generating the humoral response against viral infections, they can also be a target for infection by the DENV. [
70] Indeed, virus-inclusive single-cell RNA-Seq (viscRNA-Seq) analysis demonstrated that naïve B-cells contained the highest amount of DENV RNA among the immune cells analysed in patients with severe dengue.[
71]. The DENV has been proposed to bind to B-cells via CD300a and is internalised via clathrin mediated endocytosis.[
72] Infection of B-cells by the DENV was shown to result in their proliferation as well as differentiation to a plasmablastic phenotype.[
73] Indeed, skin and gut homing genes were upregulated in plasma cells and plasmablasts of dengue patients during the febrile period [
74], which maybe a signature of entry into the critical phase. Furthermore, the Fcy receptors FcyRIIB and Fc like receptor LILRB1 expressed by B cells , are upregulated in dengue infection, and are implicated in ADE. [
75] It is possible that the AL detected by automated haematology analysers represent a subset of B-cells which have been infected by DENV. This may explain their association with more severe disease. Future studies should focus on delineating the phenotype of DENV infected B-cells in clinical samples at serial time points to confirm this hypothesis.
Natural killer (NK) cells are a key part of the innate immune response against viral infections and are charactered by their cytotoxic function.[
76] NK cells secrete cytolytic granules to eliminate dengue infected cells.[
77] Costa et al demonstrated that in a murine model, IFN-y production by human NK cells was important in controlling DENV replication.[
78] The significance of NK cells in the immune response against dengue has since been corroborated in clinical studies. The CD56 bright immunoregulatory subset of NK cells was found to be increased in patients with mild disease but notably not in those with severe dengue.[
79] Similarly, the cytotoxic CD56 dim NK cell subset showed increased cytolytic capability ( based on the expression of CD69, NKP30,Granzyme B and IFNγ) in those with mild compared to severe dengue. [
79] Importantly, the impaired cytotoxic capacity of NK cells in those with severe disease persisted even up to a week after infection.[
79] Mass cytometry by time of flight (CyTOF) analysis was able to delineate unique NK cell signatures associated with dengue infection in paediatric and adult patients, respectively.[
80] Similar studies evaluating NK cell subsets via high dimensional immune profiling at presentation and at serial time points during the disease would be of significant value, especially if NK cell phenotypes at presentation can predict severity of disease. The impact of dengue vaccines on T/NK cell subsets and their function is another important area to be addressed by future research.
Neutrophils
Neutropenia is a well-recognised feature of many viral infections including dengue.[
81] The neutrophil nadir in dengue infection typically occurs on day 4-6 and may coincide with defervescence.[
82] It is noteworthy that severe neutropenia is not associated with disease severity or mortality.[
82] Although neutropenia is common in dengue, there is clear evidence of neutrophil activation during dengue infection as evidenced by elevated levels of IL-8 and TNF alpha.[
83,
84]
Neutrophils in dengue also over-express the activation marker CD66B, which potentiates endothelial adhesion and the generation of reactive oxygen species. [
85] Patients with severe dengue also have increased neutrophil elastase levels, which implies that enhanced neutrophil activation can also be associated with severe disease. The generation of neutrophil extracellular traps (NETs) (web like chromatin structures which trap and destroy pathogens) has recently been demonstrated in dengue. [
86] In addition to their antimicrobial functions, NETs are also proposed to have pro-inflammatory properties, and may exacerbate vascular permeability. Indeed, increased levels of NETs components were found in the serum of patients with severe dengue.
The occurrence of NETs in dengue is thought to be mediated by platelet extracellular vesicles which in turn promote signalling via C-type lectin domain containing 5 A (CLEC5A) and Toll Like Receptor 2 (TLR2)[
87]. Dual blockade of CLEC5A and TLR2 reduced virus induced inflammation in a murine model.
87 Collectively, these data support a prominent role for neutrophils in generating the pro inflammatory milieu associated with severe dengue. Future studies should address the neutrophil subsets involved in this process and how they interact with other immune cells as well as virus infected cells in driving the pathogenesis of severe dengue.