Bystander T cells in human immune responses to dengue antigens

Proteomic analysis of circulating immune cells identifies cellular phenotypes associated with COVID-19 severity

Certain serum proteins, including C-reactive protein (CRP) and D-dimer, have prognostic value in patients with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Nonetheless, these factors are non-specific, providing limited mechanistic insight into the peripheral blood mononuclear cell (PBMC) populations that drive the pathogenesis of severe COVID-19. To identify cellular phenotypes associated with disease, we performed a comprehensive, unbiased analysis of total and plasma-membrane PBMC proteomes from 40 unvaccinated individuals with SARS-CoV-2, spanning the whole disease spectrum. Combined with RNA sequencing (RNA-seq) and flow cytometry from the same donors, we define a comprehensive multi-omic profile for each severity level, revealing that immune-cell dysregulation progresses with increasing disease. The cell-surface proteins CEACAMs1, 6, and 8, CD177, CD63, and CD89 are strongly associated with severe COVID-19, corresponding to the emergence of atypical CD3+CD4+CEACAM1/6/8+CD177+CD63+CD89+ and CD16+CEACAM1/6/8+ mononuclear cells. Utilization of these markers may facilitate real-time patient assessment by flow cytometry and identify immune populations that could be targeted to ameliorate immunopathology.

Cytokine storm and COVID-19: a chronicle of pro-inflammatory cytokines

Coronavirus disease 2019 (COVID-19) has swept the world, unlike any other pandemic in the last 50 years. Our understanding of the disease has evolved rapidly since the outbreak; disease prognosis is influenced mainly by multi-organ involvement. Acute respiratory distress syndrome, heart failure, renal failure, liver damage, shock and multi-organ failure are strongly associated with morbidity and mortality. The COVID-19 disease pathology is plausibly linked to the hyperinflammatory response of the body characterized by pathological cytokine levels. The term 'cytokine storm syndrome' is perhaps one of the critical hallmarks of COVID-19 disease severity. In this review, we highlight prominent cytokine families and their potential role in COVID-19, the type I and II interferons, tumour necrosis factor and members of the Interleukin family. We address various changes in cellular components of the immune response corroborating with changes in cytokine levels while discussing cytokine sources and biological functions. Finally, we discuss in brief potential therapies attempting to modulate the cytokine storm.

Bystander CD4+ T cells: crossroads between innate and adaptive immunity

T cells are the central mediators of both humoral and cellular adaptive immune responses. Highly specific receptor-mediated clonal selection and expansion of T cells assure antigen-specific immunity. In addition, encounters with cognate antigens generate immunological memory, the capacity for long-term, antigen-specific immunity against previously encountered pathogens. However, T-cell receptor (TCR)-independent activation, termed “bystander activation”, has also been found. Bystander-activated T cells can respond rapidly and secrete effector cytokines even in the absence of antigen stimulation. Recent studies have rehighlighted the importance of antigen-independent bystander activation of CD4⁺ T cells in infection clearance and autoimmune pathogenesis, suggesting the existence of a distinct innate-like immunological function performed by conventional T cells. In this review, we discuss the inflammatory mediators that activate bystander CD4⁺ T cells and the potential physiological roles of these cells during infection, autoimmunity, and cancer.

Immune cells that become activated in the absence of antigen stimulation could be harnessed in the fight against infection, autoimmunity, and cancer. Je-Min Choi and colleagues from Hanyang University in Seoul, South Korea, review how the immune system can deploy helper T cells through an unusual process called bystander activation. Most T cells become activated only after receptors on their surface bind to specific cognate antigen. In contrast, bystander T cells are activated non-specifically in response to cytokines and other pro-inflammatory mediators. Studies have shown that this cell population has a variety of protective and pathogenic functions, for example, guarding against multiple sclerosis, aggravating the symptoms of parasitic infections and promoting antitumor immunity. A better understanding of these immune cells could lead to new therapeutic options for these diseases.

Impaired NK cell activation during acute dengue virus infection: A contributing factor to disease severity

Dengue viral (DENV) infection has a broad clinical spectrum ranging from classical febrile illness to life-threatening disease. Literature suggests that spectrum of illness could be due to differences in innate immune-responses; however, the knowledge is still at infancy. Amongst the various cells involved in innate immune responses, NK cells play a central role, particularly in anti-viral immunity. Thus in this study we have evaluated the role of NK-cells during acute-DENV infection and its influence on severity of disease, by analyzing activation, cytotoxic receptors, cytolytic granule contents and degranulation markers on NK-cells during different stages of infection. Based on the clinical manifestations and severity of the disease, DENV patients were classified into patients with dengue without warning signs (DF), dengue with warning signs (DFWS) and severe dengue (SD) patients. During acute-DENV infection, though there was no alteration in frequency of NK-cells, significant increase in frequency of CD56^bright subset in DF patients (p < 0.05) was observed, while it remained unaltered in SD patients. We also found that, CD56^dim NK-cell subset of DF patients had elevated CD69 expression, granzyme B and intracellular IFN-γ levels compared to SD patients (p < 0.05). Amongst the NK-cell cytotoxicity receptor (NCR), NKp30 receptor was significantly elevated in DF patients (p < 0.05), however in SD patients it was comparable to healthy controls. This receptor is essential for dendritic cells-NK-cells crosstalk for initiating adaptive immune response. IL-15 is known to induce NKp30 expression, which was also seen to be elevated in DF patients (p < 0.05) but unaltered in SD patients. In SD patients, even post-6 days of infection i.e. during recovery phase, CD69 and NKp30 expression did not raise, suggesting impaired NK-cell response in these patients. To summarize, our study reports, that efficient NK cell response during acute phase of DENV infection is crucial for preventing severity of the disease. This study helps in understanding the dynamics of NK cell response in immunopathogenesis of DENV infection; which is crucial for development of efficacious therapeutics as well as vaccine.

Vaccines Against Dengue and West Nile Viruses in India: The Need of the Hour

The circulation of flaviviruses, dengue (DEN), Japanese encephalitis (JE) and West Nile (WN) viruses, and others, is generating a major concern in many countries. Both JE along with DEN have been endemic in large regions of India. WN virus infection, although circulating in southern regions for many years, in recent years, WN encephalitis patients have been demonstrated. While vaccines against JE have been developed and decrease outbreaks, in case of DEN and WN, vaccines are still in developing level, especially, it has been difficult to achieve the long-term protective immune response. The first licensed DEN vaccine, which is a live attenuated vaccine, was administered in countries where the virus is endemic, and has a potential to cause serious side effects, especially when administered to younger population as observed in the Philippines vaccination drive. In the case of WN, although the purified inactivated virion-based vaccine worked effectively as a veterinary vaccine for horses, no effective vaccine has yet been licensed for humans. The induction of CD4⁺ and CD8⁺ T cell responses is essential to complete protection by these viruses, as evidenced by responses to asymptomatic infections. Many studies have shown that neutralizing antibody (NAb) response is against surface structural proteins; CD4⁺ and CD8⁺ responses are mainly directed against nonstructural proteins rather than NAb response. New data suggest that encapsulating virus vaccines in nanoparticles (NPs) will direct antigen in cytoplasmic compartment by antigen-presenting cells, which will improve presentation to CD4⁺ and CD8⁺ T cells. Since tissue culture-derived, purified inactivated viruses are easier to manufacture and safer than developing live virus vaccines, inclusion of NP provides an attractive alternative for generating robust flaviviral vaccines that are affordable with long-lived protection.

Hantavirus-Driven PD-L1/PD-L2 Upregulation: An Imperfect Viral Immune Evasion Mechanism

Viruses often subvert antiviral immune responses by taking advantage of inhibitory immune signaling. We investigated if hantaviruses use this strategy. Hantaviruses cause viral hemorrhagic fever (VHF) which is associated with strong immune activation resulting in vigorous CD8+ T cell responses. Surprisingly, we observed that hantaviruses strongly upregulate PD-L1 and PD-L2, the ligands of checkpoint inhibitor programmed death-1 (PD-1). We detected high amounts of soluble PD-L1 (sPD-L1) and soluble PD-L2 (sPD-L2) in sera from hantavirus-infected patients. In addition, we observed hantavirus-induced PD-L1 upregulation in mice with a humanized immune system. The two major target cells of hantaviruses, endothelial cells and monocyte-derived dendritic cells, strongly increased PD-L1 and PD-L2 surface expression upon hantavirus infection in vitro. As an underlying mechanism, we found increased transcript levels whereas membrane trafficking of PD-L1 was not affected. Further analysis revealed that hantavirus-associated inflammatory signals and hantaviral nucleocapsid (N) protein enhance PD-L1 and PD-L2 expression. Cell numbers were strongly reduced when hantavirus-infected endothelial cells were mixed with T cells in the presence of an exogenous proliferation signal compared to uninfected cells. This is compatible with the concept that virus-induced PD-L1 and PD-L2 upregulation contributes to viral immune escape. Intriguingly, however, we observed hantavirus-induced CD8+ T cell bystander activation despite strongly upregulated PD-L1 and PD-L2. This result indicates that hantavirus-induced CD8+ T cell bystander activation bypasses checkpoint inhibition allowing an early antiviral immune response upon virus infection.

Overcoming tumor immune evasion with an unique arbovirus

Combining dendritic cell vaccination with the adjuvant effect of a strain of dengue virus may be a way to overcome known tumor immune evasion mechanisms. Dengue is unique among viruses as primary infections carry lower mortality than the common cold, but secondary infections carry significant risk of hypovolemic shock. While current immuno-therapies rely on a single axis of attack, this approach combines physiological (hyperthermic reduction of tumor perfusion), immunological (activation of effector cells of the adaptive and innate immune system), and apoptosis-inducing pathways (sTRAIL) to destroy tumor cells. The premise of using multiple mechanisms of action in synergy with a decline in the ability of the tumor cells to employ resistance methods suggests the potential of this combination approach in cancer immunotherapy.

NK Cells during Dengue Disease and Their Recognition of Dengue Virus-Infected cells

The innate immune response, in addition to the B- and T-cell response, plays a role in protection against dengue virus (DENV) infection and the degree of disease severity. Early activation of natural killer (NK) cells and type-I interferon-dependent immunity may be important in limiting viral replication during the early stages of DENV infection and thus reducing subsequent pathogenesis. NK cells may also produce cytokines that reduce inflammation and tissue injury. On the other hand, NK cells are also capable of inducing liver injury at early-time points of DENV infection. In vitro, NK cells can kill antibody-coated DENV-infected cells through antibody-dependent cell-mediated cytotoxicity. In addition, NK cells may directly recognize DENV-infected cells through their activating receptors, although the increase in HLA class I expression may allow infected cells to escape the NK response. Recently, genome-wide association studies have shown an association between MICB and MICA, which encode ligands of the activating NK receptor NKG2D, and dengue disease outcome. This review focuses on recognition of DENV-infected cells by NK cells and on the regulation of expression of NK cell ligands by DENV.

Rapid and massive virus-specific plasmablast responses during acute dengue virus infection in humans

Humoral immune responses are thought to play a major role in dengue virus-induced immunopathology; however, little is known about the plasmablasts producing these antibodies during an ongoing infection. Herein we present an analysis of plasmablast responses in patients with acute dengue virus infection. We found very potent plasmablast responses that often increased more than 1,000-fold over the baseline levels in healthy volunteers. In many patients, these responses made up as much 30% of the peripheral lymphocyte population. These responses were largely dengue virus specific and almost entirely made up of IgG-secreting cells, and plasmablasts reached very high numbers at a time after fever onset that generally coincided with the window where the most serious dengue virus-induced pathology is observed. The presence of these large, rapid, and virus-specific plasmablast responses raises the question as to whether these cells might have a role in dengue immunopathology during the ongoing infection. These findings clearly illustrate the need for a detailed understanding of the repertoire and specificity of the antibodies that these plasmablasts produce.

CD16(+) natural killer cells play a limited role against primary dengue virus infection in tamarins

CD16 is a major molecule expressed on NK cells. To directly assess the role of natural killer (NK) cells in dengue virus (DENV) infection in vivo, CD16 antibody-treated tamarins were inoculated with a DENV-2 strain. This resulted in the transient depletion of CD16(+) NK cells, whereas no significant effects on the overall levels or kinetics of plasma viral loads and antiviral antibodies were observed in the treated monkeys when compared to control monkeys. It remains elusive whether the CD16(-) NK subpopulation could play an important role in the control of primary DENV infection.

Human CD8+ T cells display a differential ability to undergo cytokine-driven bystander activation

A subset of CD44(hi)CD8+ T cells in some, but not all mice, can be induced to rapidly secrete IFNγ during infection with Listeria monocytogenes. This response is dependent on the presence of both IL-12 and IL-18 and does not require engagement of the T cell receptor. In this study, we demonstrate that human CD8+ T cells also vary widely in their ability to secrete IFNγ within 15h of either Listeria infection or cytokine stimulation. The magnitude of the rapid IFNγ response correlated more closely with the intrinsic responsiveness of the T cells to cytokine stimulation rather than the amount of IL-12 produced. CD8+ T cells from 2 out of 16 blood donors (12.5%) failed to generate a significant IFNγ response. These results demonstrate that bystander activation of CD8+ T cells varies among individuals and validate further study of the differential responses observed using BALB/c vs. C57BL/6 mice.