No one knows what makes a mild dengue viral infection morph into a severe and sometimes deadly dengue hemorrhagic fever/dengue shock syndrome.
Experts previously believed the likely cause was ramped up activity of T cells, which can massively boost an immune response to a virus.
Now, however, researchers at the La Jolla Institute for Immunology (LJI), have found definitive evidence that CD4 T cells, one of two main subtypes of T cells, are not to blame.
The finding, reported in the December 24, 2019, issue of Cell Reports, is important to both the basic understanding of this disease – the world’s most common mosquito-borne illness – and to the hunt for an effective vaccine for dengue.
“We found no evidence to support the common dogma that these T cells are responsible for turning a mild infection to a severe one.
This will help us narrow the search for the true culprit,” says the study’s lead investigator Yuan Tian, Ph.D., an AAI Intersect Fellow and a Bioinformatics Student at LJI. He is also a postdoctoral fellow in the lab of Alessandro Sette, Dr. Sci. Biol, a co-author of the study.
These issues are serious. Dengue fever is spreading. Infected mosquitos have expanded beyond their established tropical and subtropical territories in South East Asia and Latin America to new continents, including Europe and the United States. More than half of the world’s population is now at risk; already, 390 million infections occur annually, according to public health experts.
The goal of the LJI study was to define the molecular pattern of dengue-specific CD4 T cells and to investigate whether there is a difference in the T cell response between patients with mild dengue fever or with severe dengue hemorrhagic fever.
When analyzing dengue-specific CD4 T cells, the researchers realized that the responding CD4 T cells, have both a pro-inflammatory function (regulated by the cytokine interferon gamma, or IFN-γ) and an anti-inflammatory function (regulated by the cytokine interleukin 10, or IL-10) which is typically not seen in acute viral infections.
To comprehensively define these dengue-virus specific T cells in hospitalized patients, researchers used whole transcriptome analysis to determine if there was a difference in the quality of the increased response.
This approach allows to identify all RNA transcripts – produced when a gene’s DNA sequence is copied, or transcribed – within the transcriptome of dengue-specific CD4 T cells in hospitalized patients being treated for either mild or for severe dengue infection. These patients were being treated in Sri Lanka, where dengue fever is endemic.
“This is a very powerful approach to detect gene expression activity because all genes upregulated in response to the virus can be identified.
It is completely unbiased and does not rely on pre-selected genes,” says the study’s senior investigator, Daniela Weiskopf, Ph.D., an instructor at LJI.
The research team, to their surprise, detected no difference in the genomic profile of dengue-virus specific CD4 T cells regardless if they isolated them form patients with mild or severe dengue infection.
“The CD4 T cell response in the severe disease does not look different so that cannot be the switch we are all looking for,” Tian says.
“In fact, based on some intriguing preliminary findings, we speculate that to counteract the severe immune response occurring in acute cases, these dengue-specific CD4 cells may have gradually acquired the ability to produce more IL-10 by converting IFN-γ.
It is as if they are trying to calm themselves, calm the inflammation. The double positive CD4 T cells could actually be helping, rather than hurting.”
Tian adds that he hopes these findings will serve to “help guide efforts to develop effective dengue vaccines by improving our understanding of this novel T cell response.”
DENV Infection and the Complex Roles of T Cells
Dengue virus (DENV) belongs to the genus Flavivirus and is closely related to several other flaviviruses including Zika virus (ZIKV), yellow fever virus (YFV), Japanese encephalitis virus (JEV), and West Nile virus (WNV) (1).
DENV is a serious public health issue especially in tropical and subtropical areas, and it is estimated that ~390 million people are infected yearly with DENV (2). DENV infection is associated with a range of clinical manifestations, from asymptomatic to more severe presentations including dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). There is currently no specific therapy available for the treatment of dengue diseases other than supportive care.
Furthermore, Dengvaxia® (Sanofi Pasteur), the first licensed DENV vaccine, is associated with efficacy and safety concerns (3–7). Sridhar et al. integratively analyzed data from three clinical trials and reported that Dengvaxia® increases the risk of severe dengue and hospitalization among vaccinees who have not been exposed to DENV before the vaccination (8). In order to develop effective DENV therapeutics and vaccines, it is important to define immunological correlates of protection against DENV infection as well as biomarkers that can be used to access their safety and efficacy.
Although T cells have important functions in combating viral pathogens, both pathological and protective effects of T cells have been reported in the context of DENV infection (9–14). According to T cell original antigenic sin, cross-reactive T cells that are specific for a primary DENV serotype become predominant during a secondary heterologous infection (9–16). Consequently, the expansion of preexisting cross-reactive and low-affinity memory T cells results in ineffective viral control and contributes to immunopathology and severe dengue disease through excessive production of inflammatory cytokines (9–16).
In contrast to the implications of original antigenic sin, several lines of evidence indicate that T cells contribute to the control of DENV infection. Murine studies demonstrate that CD4 T cells and especially CD8 T cells can play a protective role against DENV challenge (17–24). Furthermore, HLA alleles associated with protection from severe dengue disease are also associated with strong and multifunctional T cell responses, supporting the notion that T cells have protective functions during DENV infection (25–28).
The main characteristic of an efficient vaccine is the prophylactic effect provided by protective neutralizing antibodies. Therefore, it is possible that in Dengvaxia® vaccines, native conserved masked conformational DENV (1–4) epitopes are not unmasked and therefore not accessible for highly neutralizing and broadly protective antibodies. Nevertheless, Dengvaxia® is a yellow fever dengue chimeric vaccine and lacks DENV non-structural (NS) proteins that contain a large proportion of T cell epitopes (25, 28, 29).
Therefore, the suboptimal efficacy of Dengvaxia® may partially due to its defective ability to induce T cell responses (30). Indeed, a single dose of the live attenuated tetravalent DENV vaccine TV003 provides complete protection against infection with a DENV-2 challenge virus (31), potentially highlighting the importance of harnessing the protective functions of both humoral and cellular antiviral immunity.
Characterization of Human DENV-Specific T cell Responses
Megapool Approach to Detect DENV-Specific T Cell Responses
Epitope identification studies have provided the basis for phenotyping DENV-specific T cell responses directly ex vivo without the need for in vitro stimulation that could potentially alter T cell phenotypes. Utilizing the knowledge of the epitopes recognized we developed the megapool approach, which allows for combining a large number of peptides into one peptide pool based on sequential lyophilization. This enables detection of DENV-specific T cell responses irrespective of HLA types and DENV serotypes in various immunological contexts where only small amounts of blood are available (78).
DENV megapools have been generated for both CD4 and CD8 T cells, which consist of 180 and 268 peptides, respectively (25, 27, 28, 73, 77). These peptides are pooled, lyophilized, and resuspended to form a master mix, which is then used to stimulate T cell ex vivo (86). DENV CD4 and CD8 megapools account for 62 and 90% of the IFN-γ response in Sri Lankan and Nicaraguan cohorts, respectively, and have been validated in different geographical locations supporting their global applicability (25, 27, 28, 73, 77).
DENV-Specific CD8 T Cells: Activated, Skin-Homing, and Functional
Using tetramers incorporating three variants of the HLA-A*1101-restricted DENV NS3133−142 epitope, Friberg et al. reported that cross-reactive CD8 T cells develop following both primary and secondary DENV infections and that the magnitude of tetramer+ CD8 T cell response does not correlated with disease severity (35). Although tetramer+ CD8 T cells upregulate the activation marker CD38 during the acute phase of infection (35), the phenotype of these DENV-specific CD8 T cells was not further assessed in this study.
More recent studies using DENV peptide pools shows that higher magnitude and more polyfunctional CD8 T cell responses correlate with HLA alleles that are associated with reduced risk of severe dengue disease (27, 28), which is consistent with the report that the frequency of DENV-specific cytokine-producing CD8 T cells is higher among children who subsequently developed subclinical secondary infection than those who developed symptomatic secondary infection (87).
Using a pool containing 268 CD8 T cell epitopes derived from DENV (termed megapool), de Alwis et al. demonstrated that the majority of DENV-specific IFN-γ+ CD8 T cells have a CD45RA−CCR7− effector memory (Tem) or CD45RA+CCR7− effector memory re-expressing CD45RA (Temra) phenotype (27).
Notably, DENV-specific CD8 T cells are also associated with increased PD-1 expression in donors expressing the immunodominant allele HLA-B*35:01. In contrast to classical exhausted CD8 T cells, these DENV-specific PD-1+ CD8 T cells do not co-express other exhaustion makers and are apparently proliferative and functional (27), suggesting that PD-1 may serve as a marker of activated and highly functional antigen-specific CD8 T cells in the context of DENV infection.
Since DENV infection initiates at the site of the mosquito bite in the host skin, it is possible that CD8 T cells may migrate to the site of infection and mediate localized responses. Indeed, DENV NS3 27-specific CD8 T cells in the periphery blood upregulate the expression of several chemokine receptors including CCR5, CXCR3, and CXCR6 as well as the skin-homing molecule cutaneous lymphocyte-associated antigen (CLA) during acute DENV infection (67).
Moreover, DENV-specific CD8 T cells are readily detectable in the skin of DENV-infected individuals at the acute stage (67), suggesting that these cells may exert effector functions at the site of infection. Tissue-resident memory T (Trm) cells reside in non-lymphoid tissues including the skin and can serve as a front line of defense against invading pathogens such as vaccinia virus (88). It would be interesting to investigate whether DENV-specific CD8 T cells could differentiate into Trm cells in the skin that may mount rapid and localized protective immunity upon reinfection.
Comprehensive transcriptomic profiling of DENV-specific CD8 T cells has also been carried out. Chandele et al. performed microarray analysis on HLA-DR+CD38+ activated CD8 T cells isolated from the PBMCs of DENV-infected patients and found that these cells upregulate genes involved in T cell proliferation, activation, migration and cytotoxicity (89). Interestingly, HLA-DR+CD38+ CD8 T cells also display increased expression of multiple inhibitory receptors and downregulate several genes that are involved in TCR signaling (89). A more recent study from our group characterized the transcriptomic profiles of DENV-specific CD8 Tem and Temra subsets identified by their production of IFN-γ following simulation with the megapool of DENV-derived epitopes (90).
DENV-specific Tem and especially Temra cells display specialized gene expression profiles and upregulated genes that are associated with activation, co-stimulation, and effector functions (90), which is consistent with the previous study from Chandele et al. Since these DENV-specific Tem and Temra cells were isolated from healthy donors with secondary DENV infection, these studies suggest that DENV-specific CD8 T cell populations may maintain an activated phenotype in donors that have been infected multiple times with DENV. Interestingly, DENV-specific Temra cells may have higher expression of a few killer cell immunoglobulin-like receptor (KIR) genes including KIR2DL3 by comparison with DENV-specific Tem cells (90). In addition, DENV-specific CD8 T cells may show preferential usage of TCR beta-chain variable (TRBV) genes (90), which is in line with the report that DENV NS3133-specific CD8 T cells targeting HLA-A*11:01-restricted epitope variants derived from DENV1, DENV3, and DENV4 but not DENV2 preferentially use a few TRBV segments including TRBV9, TRBV12-3/4, and especially TRBV11-2 (33).
DENV-Specific CD4 T Cells: A Tale of Cytotoxicity
The majority of antigen-specific CD4 T cells differentiate into T helper type 1 (Th1) and follicular helper T (Tfh) cells following viral infections and provide help to CD8 T cells and B cells (91–93). Indeed, DENV-specific CD4 T cells produce Th1 cell-associated cytokines including IFN-γ, TNF-α, and IL-2 following both infection and vaccination (87, 94, 95). In addition, DENV-specific CD4 T cells with cytotoxic activity have been reported by numerous studies (12) and their frequency may be lower in patients with more severe dengue disease (96).
Interestingly, a subset human CD4 T cells, which is CD45RA+CCR7− and termed effector memory re-expressing CD45RA T (Temra) cells, expands in individuals that have been infected with DENV multiple times, and the frequency of DENV-specific CD4 Temra cells is higher in donors expressing an HLA allele associated with protection from severe dengue disease (26). Despite the production of IFN-γ, these cells may not represent classical Th1 cells as they lack the expression of CXCR3 (26). CD4 Temra cells have increased expression of several cytotoxic molecules including CD107a, perforin, granzyme B as well as the CX3CL1 (fractalkine) receptor, CX3CR1 (26). Notably, CX3CR1 has recently been reported to be a member of a 20-gene set that can predict severe dengue disease (97).
Subsequent transcriptomic profiling studies further revealed the gene expression patterns and heterogeneity of CD4 Temra cells and identified additional phenotypic markers such as GPR56 and CD244 that are specifically expressed by cytotoxic CD4 Temra cells and confirmed their expression on DENV-specific Temra cells (98, 99). Additionally, cytotoxic CD4 Temra cells may have undergone extensive clonal expansions based upon TCR analysis (98, 99), supporting the notion that these cells are induced by repeated DENV infections.
Both Foxp3+ regulatory T (Treg) cells and Foxp3− type 1 regulatory T (Tr1) cells can suppress inflammation and exert immunoregulatory effects (100, 101). However, their functional significance in the context of DENV infection is less well-defined (14). It has been reported that the frequency of Treg cells and the ratio of Treg cells to effector T cells are significantly higher during acute DENV infection than after recovery in patients with mild disease but not in those with severe disease (102).
However, subsequent studies indicate that the frequency of Treg cells is not associated with viral load or disease severity (103). Therefore, whether and how Treg and Tr1 cells influence antiviral immune response and disease progression during DENV infection warrants further investigation.
T cells help to B cells is provided by a CD4 T cells subset termed follicular T helper cells (Tfh) (104). Tfh cells have been associated with protective roles in human infectious disease (105–107) and vaccinees (108–110). They provide several forms of T cell help to B cells such as signals that promote survival, proliferation, plasma cell differentiation, hypermutation, class-switch recombination, adhesion and chemoattraction (cell migration) (111).
Tfh cells are essential for the generation of most isotype switched and affinity matured antibodies, and therefore they have an obvious role in protective immunity against pathogens. A recent breakthrough has been the ability to detect Tfh cells in peripheral blood (105, 112) thus allowing their assessment in PBMC samples based on surface markers. A central marker of Tfh cells is the CXC-chemokine receptor 5 (CXCR5) which is required for T and B cells to enter into follicles. OX40, and PD-L1 have further been identified as TCR activation-dependent markers of human Tfh cells (113, 114). Recent studies have reported an expansion of peripheral Tfh cells in DENV-infected children during the acute phase (115).
Furthermore, Tfh cells are more abundant in patients with secondary DENV infection and in those who developed a more severe dengue disease (115). Although Tfh cells has been shown to promote DENV-specific antibody responses in mice (116), the differentiation and functional significance of DENV-specific Tfh cells in humans warrants further investigation.
Conclusion and Perspective
All DENV-specific T cell phenotypes discussed in this review as well as the markers they express are summarized in Figure 2 and Table 2, respectively. Comprehensive epitope identification over the last few years has provided the tools that allow one to probe for DENV specific T cell responses in donors exposed to natural infection and vaccination. Global assessment of dengue virus-specific CD4 and CD8 T cell responses in dengue-endemic areas led to the development of new tools (megapools) that allow analysis of small samples typically available from pediatric and hospital cohorts.
It has also been demonstrated that DENV-specific CD4 and CD8 responses are more complex than previously thought, with different subsets revealed by in depth phenotypic and transcriptomic analyses. The hypothesis that these different subsets have unique roles and dictate and shape clinical outcomes and vaccine efficacy will have to be explored in the near future.
Markers expressed by DENV-specific T cell populations.
|T cell population||Makers|
|Skin-homing CD8 T cells||CCR5, CXCR3, CXCR6, CLA|
|Cytotoxic CD8 T cells||Perforin, Granzyme B, PD-1|
|Cytotoxic CD4 T cells||Perforin, Granzyme B, CX3CR1, GPR56, CD244|
|Th1 cells||IFN-γ, TNF-α, and IL-2|
|Tfh cells||CXCR5, PD-1, OX40, PD-L1|
More information: Yuan Tian, Grégory Seumois, Luzia M. De-Oliveira-Pinto, Jose Mateus, Sara Herrera-de la Mata, Cheryl Kim, Denise Hinz, N.D. Suraj Goonawardhana, Aruna D. de Silva, Sunil Premawansa, Gayani Premawansa, Ananda Wijewickrama, Angel Balmaseda, Alba Grifoni, Pandurangan Vijayanand, Eva Harris, Bjoern Peters, Alessandro Sette, and Daniela Weiskopf. “Molecular Signatures of Dengue Virus-Specific IL-10/IFN-γ Co-producing CD4 T Cells and their Association with Dengue Disease.” Cell Reports, 2019. DOI: 10.1016/j.celrep.2019.11.098