Emerging patterns of regulatory T cell function in tuberculosis

People Living With HIV Have More Intact HIV DNA in Circulating CD4+ T Cells if They Have History of Pulmonary Tuberculosis

Background

A primary barrier to curing HIV is the HIV reservoir. The leading infectious cause of death worldwide for people living with HIV is tuberculosis (TB), but we do not know how TB impacts the HIV reservoir.

Methods

Participants in identification and validation cohorts were selected from previously enrolled studies at Groupe Haïtien d'Étude du Sarcome de Kaposi et des Infections Opportunistes (GHESKIO) in Port au Prince, Haiti. Intact and non-intact proviral DNA were quantified using droplet digital PCR of peripheral blood mononuclear cell (PBMC)-derived CD4+ T cells. Kruskal-Wallis tests were used to compare medians with tobit regression for censoring.

Results

In the identification cohort, we found that people living with HIV with a history of active pulmonary TB (n=19) had higher levels of intact provirus than people living with HIV without a history of active TB (n=47) (median 762; IQR, 183-1173 vs 117; IQR, 24-279 intact provirus per million CD4, respectively; P=0.0001). This difference also was seen in the validation cohort (n=31), (median 102; IQR, 0-737 vs 0; IQR, 0-24.5 intact provirus per million CD4, P=0.03) for TB vs no-TB history groups, respectively. The frequencies of CD4+ T cells with any detectable proviral fragment was directly proportional to the levels of interleukin-1 beta (r=0.524, P= 0.0025) and interleukin-2 (r=0.622, P=0.0002).

Conclusions

People living with HIV with a history of active pulmonary TB have more HIV pro-virus in their circulating CD4+ T cells, even years after TB cure. We need to characterize which CD4+ T cells are harboring intact provirus to consider the impact of T cell-targeting HIV cure interventions for people living in TB-endemic areas.

The clinical trajectory of peripheral blood immune cell subsets, T-cell activation, and cytokines in septic patients

OBJECTIVE AND DESIGN

Changes in the immune status of patients with sepsis may have a major impact on their prognosis. Our research focused on changes in various immune cell subsets and T-cell activation during the progression of sepsis.

METHODS AND SUBJECTS

We collected data from 188 sepsis patients at the First Affiliated Hospital of Zhejiang University School of Medicine. The main focus was on the patient's immunocyte subset typing, T-cell activation/Treg cell analysis, and cytokine assay, which can indicate the immune status of the patient.

RESULTS

The study found that the number of CD4+ T cells, CD8+ T cells, NK cells, and B cells decreased early in the disease, and the decrease in CD4+ and CD8+ T cells was more pronounced in the death group. T lymphocyte activation was inhibited, and the number of Treg cells increased as the disease progressed. T lymphocyte inhibition was more significant in the death group, and the increase in IL-10 was more significant in the death group. Finally, we used patients' baseline conditions and immunological detection indicators for modeling and found that IL-10, CD4+ Treg cells, CD3+HLA-DR+ T cells, and CD3+CD69+ T cells could predict patients' prognosis well.

CONCLUSION

Our study found that immunosuppression occurs in patients early in sepsis. Early monitoring of the patient's immune status may provide a timely warning of the disease.

Vaccine development against tuberculosis before and after Covid-19

Coronavirus disease (Covid-19) has not only shaped awareness of the impact of infectious diseases on global health. It has also provided instructive lessons for better prevention strategies against new and current infectious diseases of major importance. Tuberculosis (TB) is a major current health threat caused by Mycobacterium tuberculosis (Mtb) which has claimed more lives than any other pathogen over the last few centuries. Hence, better intervention measures, notably novel vaccines, are urgently needed to accomplish the goal of the World Health Organization to end TB by 2030. This article describes how the research and development of TB vaccines can benefit from recent developments in the Covid-19 vaccine pipeline from research to clinical development and outlines how the field of TB research can pursue its own approaches. It begins with a brief discussion of major vaccine platforms in general terms followed by a short description of the most widely applied Covid-19 vaccines. Next, different vaccination regimes and particular hurdles for TB vaccine research and development are described. This specifically considers the complex immune mechanisms underlying protection and pathology in TB which involve innate as well as acquired immune mechanisms and strongly depend on fine tuning the response. A brief description of the TB vaccine candidates that have entered clinical trials follows. Finally, it discusses how experiences from Covid-19 vaccine research, development, and rollout can and have been applied to the TB vaccine pipeline, emphasizing similarities and dissimilarities.

Single-cell RNA-sequencing reveals heterogeneity and intercellular crosstalk in human tuberculosis lung

Lung inflammation indicated by 18F-labeled fluorodeoxyglucose (FDG) in patients with tuberculosis is associated with disease severity and relapse risk upon treatment completion. We revealed the heterogeneity and intercellular crosstalk in lung tissues with 18F-FDG avidity and adjacent uninvolved tissues from 6 tuberculosis patients by single-cell RNA-sequencing. Tuberculous lungs had an influx of regulatory T cells (Treg), exhausted CD8 T cells, immunosuppressive myeloid cells, conventional DC, plasmacytoid DC, and neutrophils. Immune cells in inflamed lungs showed general up-regulation of ATP synthesis and interferon-mediated signaling. Immunosuppressive myeloid and Treg cells strongly displayed transcriptions of genes related to tuberculosis disease progression. Intensive crosstalk between IL4I1-expressing myeloid cells and Treg cells involving chemokines, costimulatory molecules, and immune checkpoints, some of which are specific in 18F-FDG-avid lungs, were found. Our analysis provides insights into the transcriptomic heterogeneity and cellular crosstalk in pulmonary tuberculosis and guides unveiling cellular and molecular targets for tuberculosis therapy.

Mycobacterium tuberculosis-specific memory T cells in bronchoalveolar lavage of patients with pulmonary tuberculosis

BACKGROUND

Mycobacterium tuberculosis(MTB) most often infects the lungs and results in pulmonary tuberculosis(TB). MTB-specific memory T cells are able to respond quickly against antigens and help reduce the burden of pulmonary bacteria. The characteristics, function and chemotaxis axis of memory T cells in the lung remain unclear. The current study aimed to clarify the classification, function and recruitment of local antigen-specific memory T cells in the lung and the periphery blood of patients with pulmonary TB.

METHODS

A total of 85 patients with active pulmonary TB were included in the study. Bronchoalveolar lavage fluid (BALF) and Peripheral blood were collected for further detection. The cell-surface markers and intracellular staining of memory T cell subtypes were measured by flow cytometry. The level of CXCL9, CXCL10 and CXCL11 in Bronchoalveolar lavage fluid cells and peripheral blood mononuclear cells (PBMC) were measured by Real-time PCR.

RESULTS

The ratio of effective Memory T cells (TEM) were the highest in BALF of patients with pulmonary TB. In patients, CXCR3 and its ligands was increased in memory T cells of BALF compared with PBMC. IFN-γ+TNF-α+ effective Memory T cells and central memory T cells from BALF were increased after antigen stimulation. CXCR3 was higher in IFN-γ+ compared with IFN-γ- in CD4+ TCM and TEM from BALF of patients. Compared with PBMC, the PD-1 levels of terminal effector memory RA+(TEMRA) and TEM cells in CD4+ memory T cells of BALF were significantly increased. In addition, PD-1 was increased in IFN-γ+ compared with IFN-γ- in CD4+TEM from BALF of patients. There was no difference in Treg ratio between PBMC and BALF of TB patients.

CONCLUSIONS

The CXCL9/CXCL11-CXCR3 axis may participate in the chemotaxis of memory T cells from the peripheral to lung. CD4+TEM and TEMRA in BALF may have exhausted, especially the cytokine producing TEM. Our study clarified the characteristics of antigen-specific memory T cells in local lung and may have impact on strategies of therapy and vaccine.

Maintenance and recall of memory T cell populations against tuberculosis: Implications for vaccine design

Despite the widespread use of standardised drug regimens, advanced diagnostics, and Mycobacterium bovis Bacille-Calmette-Guérin (BCG) vaccines, the global tuberculosis (TB) epidemic remains uncontrollable. To address this challenge, improved vaccines are urgently required that can elicit persistent immunologic memory, the hallmark of successful vaccines. Nonetheless, the processes underlying the induction and maintenance of immunologic memory are not entirely understood. Clarifying how memory T cells (Tm cells) are created and survive long term may be a crucial step towards the development of effective T cell–targeted vaccines. Here, we review research findings on the memory T cell response, which involves mobilization of several distinct Tm cell subsets that are required for efficient host suppression of M. tuberculosis (Mtb) activity. We also summaries current knowledge related to the T cell response-based host barrier against Mtb infection and discuss advantages and disadvantages of novel TB vaccine candidates.

Antibiotic Resistance to Mycobacterium tuberculosis and Potential Use of Natural and Biological Products as Alternative Anti-Mycobacterial Agents

Background: Tuberculosis (TB) is an infectious disease caused by the bacillus Mycobacterium tuberculosis (Mtb). TB treatment is based on the administration of three major antibiotics: isoniazid, rifampicin, and pyrazinamide. However, multi-drug resistant (MDR) Mtb strains are increasing around the world, thus, allowing TB to spread around the world. The stringent response is demonstrated by Mtb strains in order to survive under hostile circumstances, even including exposure to antibiotics. The stringent response is mediated by alarmones, which regulate bacterial replication, transcription and translation. Moreover, the Mtb cell wall contributes to the mechanism of antibiotic resistance along with efflux pump activation and biofilm formation. Immunity over the course of TB is managed by M1-macrophages and M2-macrophages, which regulate the immune response against Mtb infection, with the former exerting inflammatory reactions and the latter promoting an anti-inflammatory profile. T helper 1 cells via secretion of interferon (IFN)-gamma, play a protective role in the course of TB, while T regulatory cells secreting interleukin 10, are anti-inflammatory. Alternative therapeutic options against TB require further discussion. In view of the increasing number of MDR Mtb strains, attempts to replace antibiotics with natural and biological products have been object of intensive investigation. Therefore, in this review the anti-Mtb effects exerted by probiotics, polyphenols, antimicrobial peptides and IFN-gamma will be discussed. All the above cited compounds are endowed either with direct antibacterial activity or with anti-inflammatory and immunomodulating characteristics.

Ozone Pollution, Oxidative Stress, Regulatory T Cells and Antioxidants

Ozone pollution, is a serious health problem worldwide. Repeated exposure to low ozone doses causes a loss of regulation of the oxidation–reduction systems, and also induces a chronic state of oxidative stress. This fact is of special importance for the regulation of different systems including the immune system and the inflammatory response. In addition, the oxidation–reduction balance modulates the homeostasis of these and other complex systems such as metabolism, survival capacity, cell renewal, and brain repair, etc. Likewise, it has been widely demonstrated that in chronic degenerative diseases, an alteration in the oxide-reduction balance is present, and this alteration causes a chronic loss in the regulation of the immune response and the inflammatory process. This is because reactive oxygen species disrupt different signaling pathways. Such pathways are related to the role of regulatory T cells (Treg) in inflammation. This causes an increase in chronic deterioration in the degenerative disease over time. The objective of this review was to study the relationship between environmental ozone pollution, the chronic state of oxidative stress and its effect on Treg cells, which causes the loss of regulation in the inflammatory response as well as the role played by antioxidant systems in various pathologies.

Clinical Significance of Negative Costimulatory Molecule PD-1/PD-L1 on Peripheral Blood Regulatory T Cell Levels among Patients with Pulmonary Tuberculosis

Objective. This study aimed to investigate the expression and clinical significance of negative costimulatory molecules programmed death-1 (PD-1) and programmed death ligand 1 (PD-L1) on CD4+CD25+CD127low regulatory T cells (Tregs) in peripheral blood of patients with active pulmonary tuberculosis (TB). Methods. A total of 30 patients with active pulmonary TB and 20 healthy controls were enrolled. The proportions of peripheral blood CD4+CD25+CD127low Tregs and the expression of PD-1 and PD-L1 on CD4+CD25+CD127low Tregs were detected among active pulmonary TB patients using flow cytometry. The associations of proportions of CD4+CD25+CD127low Tregs with the demographic and clinical characteristics of active pulmonary TB patients were evaluated, and the correlation between PD-1/PD-L1 expression and proportions of peripheral blood CD4+CD25+CD127low Tregs was examined among patients with active pulmonary TB using Pearson correlation analysis. Results. Flow cytometry detected a significantly higher proportion of peripheral blood CD4+CD25+CD127low Tregs in the TB group than in the control group (9.14% ± 2.66% vs. 6.39% ± 1.73%; t = 4.067, $P<0.001$ ), and a higher proportion of peripheral blood CD4+CD25+CD127low Tregs among active pulmonary TB patients with a positive anti-M. tuberculosis antibody than in those with a negative antibody (Figure 2(a)); however, there were no gender, M. tuberculosis culture, tuberculin test, CT examination, or sputum smear test-specific proportions of CD4+CD25+CD127low Tregs among patients with active pulmonary TB. The PD-1 (6.13% ± 3.53% vs. 24.78% ± 7.73%, $P<0.05$ ) and PD-L1 levels (2.97% ± 2.00% vs. 9.23% ± 5.76%, $P<0.05$ ) were lower on peripheral blood CD4+CD25+CD127low Tregs among the TB group than in the control group. In addition, Pearson correlation analysis revealed a positive correlation between PD-1 and PD-L1 expression on peripheral blood CD4+CD25+CD127low Tregs among patients with active pulmonary TB (r = 0.435, $P=0.016$ ) and a negative correlation between the proportion of peripheral blood CD4+CD25+CD127low Tregs and PD-1 (r = ‒0.344, $P=0.024$ ) and PD-L1 expression among patients with active pulmonary TB (r = ‒0.310, $P=0.043$ ). Conclusion. The proportion of CD4+CD25+CD127low Tregs is higher in patients with active pulmonary TB than in healthy controls, and the negative costimulatory signal PD-1/PD-L1 expression is downregulated among active pulmonary TB patients. Our findings provide insights into the illustration of pathogenic mechanisms and immunotherapy of active pulmonary TB.

Comparative Profiling of Circulating Exosomal Small RNAs Derived From Peruvian Patients With Tuberculosis and Pulmonary Adenocarcinoma

Tuberculosis (TB) is one of the most fatal infectious diseases, caused by the aerobic bacteria Mycobacterium tuberculosis. It is estimated that one-third of the world’s population is infected with the latent (LTB) version of this disease, with only 5-10% of infected individuals developing its active (ATB) form. Pulmonary adenocarcinoma (PA) is the most common and diverse form of primary lung carcinoma. The simultaneous or sequential occurrence of TB and lung cancer in patients has been widely reported and is known to be an issue for diagnosis and surgical treatment. Raising evidence shows that patients cured of TB represent a group at risk for developing PA. In this work, using sRNA-sequencing, we evaluated the expression patterns of circulating small RNAs available in exosomes extracted from blood samples of Peruvian patients affected by latent tuberculosis, active tuberculosis, or pulmonary adenocarcinoma. Differential expression analysis revealed a set of 24 microRNAs perturbed in these diseases, revealing potential biomarker candidates for the Peruvian population. Most of these miRNAs are normally expressed in healthy lung tissue and are potential regulators of different shared and unique KEGG pathways related to cancers, infectious diseases, and immunology.

Precision Vaccine Development: Cues From Natural Immunity

Traditional vaccine development against infectious diseases has been guided by the overarching aim to generate efficacious vaccines normally indicated by an antibody and/or cellular response that correlates with protection. However, this approach has been shown to be only a partially effective measure, since vaccine- and pathogen-specific immunity may not perfectly overlap. Thus, some vaccine development strategies, normally focused on targeted generation of both antigen specific antibody and T cell responses, resulting in a long-lived heterogenous and stable pool of memory lymphocytes, may benefit from better mimicking the immune response of a natural infection. However, challenges to achieving this goal remain unattended, due to gaps in our understanding of human immunity and full elucidation of infectious pathogenesis. In this review, we describe recent advances in the development of effective vaccines, focusing on how understanding the differences in the immunizing and non-immunizing immune responses to natural infections and corresponding shifts in immune ontogeny are crucial to inform the next generation of infectious disease vaccines.

TB and COVID-19: An Exploration of the Characteristics and Resulting Complications of Co-infection

Tuberculosis (TB) and Coronavirus Disease-19 (COVID-19) infection are two respiratory diseases that are of particular concern epidemiologically. Tuberculosis is one of the oldest diseases recorded in the history of mankind dating back thousands of years. It is estimated that approximately one quarter of the world’s population is infected with latent Mycobacterium tuberculosis (LTBI). This contrasts with COVID-19, which emerged in late 2019. Data continues to accumulate and become available on this pathogen, but the long-term side effect of fibrotic damage in COVID-19 patients evokes parallels between this novel coronavirus and its ancient bacterial affiliate. This similarity as well as several others may incite inquiries on whether coinfection of individuals with latent TB and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) lead to excessive fibrosis in the lungs and thus the emergence of an active TB infection. While it is well understood how TB leads to structural and immunological lung complications including granuloma formation, fibrosis, and T cell exhaustion, less is known about the disease course when coinfection with SARS-CoV-2 is present. Past and present research demonstrate that IL-10, TNF-α, IFN class I-III, TGF-β, IL-35, and Regulatory T cells (T-regs) are all important contributors of the characteristics of host response to mycobacterium tuberculosis. It has also been noted with current research that IL-10, TNF-α, IFN class I, II, and III, TGF-β, ACE-2, and T-regs are also important contributors to the host response to the SARS-CoV-2 virus in different ways than they are to the TB pathogen. Both pathogens may lead to an unbalanced inflammatory immune response, and together a shared dysregulation of immune response suggests an increased risk of severity and progression of both diseases. We have reviewed 72 different manuscripts between the years 1992 and 2021. The manuscripts pertaining to the SARS-COV-2 virus specifically are from the years 2020 and 2021. Our literature review aims to explore the biomolecular effects of these contributors to pathogenicity of both diseases along with current publications on TB/COVID-19 coinfection, focusing on the pathogenicity of SARS-CoV-2 infection with both latent and active TB, as well as the challenges in treating TB during the COVID-19 pandemic. The compiled material will then aid the latticework foundation of knowledge for future research leading to a hopeful improved system of therapeutic strategies for coinfection.

Secondary immunodeficiencies: An overview

OBJECTIVE

To review the different causes of secondary immunodeficiencies and provide clinicians with an updated overview of potential factors that contribute to immunodeficiency.

DATA SOURCES

Recent published literature obtained through PubMed database searches, including research articles, review articles, and case reports.

STUDY SELECTIONS

PubMed database searches were conducted using the following keywords: immunodeficiency, antibody deficiency, immunosuppressive drugs, genetic syndrome, malignancy, HIV infection, viral infection, secondary immunodeficiency, nutrition, prematurity, aging, protein-losing enteropathy, nephropathy, trauma, space travel, high altitude, and ultraviolet light. Studies published in the last decade and relevant to the pathogenesis, epidemiology, and clinical characteristics of secondary immunodeficiencies were selected and reviewed.

RESULTS

Researchers continue to investigate and report abnormal immune parameters in the different entities collectively known as secondary immunodeficiencies. Immunodeficiency might occur as a consequence of malnutrition, metabolic disorders, use of immunosuppressive medications, chronic infections, malignancies, severe injuries, and exposure to adverse environmental conditions. The neonate and the elderly may have decreased immune responses relative to healthy adults. Each of these conditions may present with different immune defects of variable severity. The acquired immunodeficiency syndrome results from infections by the human immunodeficiency virus, which targets CD4 T cells leading to defective immune responses. Rituximab is a monoclonal antibody that targets CD20 B cells, and its use might result in persistent hypogammaglobulinemia.

CONCLUSION

Clinicians should consider secondary immunodeficiencies in the differential diagnosis of a patient with recurrent infections and abnormal immunologic evaluation. The use of biological agents for the treatment of inflammatory conditions and malignancies is an increasingly important cause of secondary immunodeficiency.

High occurrence of simultaneous multiple-drug hypersensitivity syndrome induced by first-line anti-tuberculosis drugs

Background

Multiple drug hypersensitivity syndrome (MDHS) results in treatment delay or failure and often results in severe drug hypersensitivity reactions. There are few reports of MDHS in response to anti-tuberculosis drugs; however, clinical information is scarce. Understanding the frequency and clinical characteristics of simultaneous MDHS against first-line anti-tuberculosis drugs in patients with non-severe drug hypersensitivity reactions is necessary.

Methods

We reviewed 27 patients with drug fever or maculopapular exanthem in response to first-line anti-tuberculosis drugs between January 2010 and June 2019. Drug fever or maculopapular exanthem occurred when isoniazid, rifampin, ethambutol, and pyrazinamide were administered simultaneously. Drug provocation tests for the 4 drugs were performed to identify the culprit drugs.

Results

All patients showed positive reactions to 1 or more drugs. MDHS was diagnosed in 13 (48%) patients, of whom 11 and 2 patients reacted to 2 and 3 drugs, respectively. In comparison to the patients with single-drug hypersensitivity, the patients with MDHS did not exhibit any differences in characteristics. Ethambutol and rifampin were the common drugs that induced a reaction, and the combination of these 2 drugs induced MDHS most frequently. Among the patients with MDHS, there were no differences between the drugs that caused drug fever and maculopapular exanthem. All patients with MDHS were successfully treated with alternative drugs.

Conclusions

Simultaneous MDHS may occur frequently in patients with drug fever or maculopapular exanthem caused by first-line anti-tuberculosis drugs, indicating the need to evaluate the allergy responses for all 4 drugs, even in patients without severe drug hypersensitivity. The combination of ethambutol and rifampin was the most common trigger that induced MDHS.

Anti-tuberculosis chemotherapy alters TNFR2 expression on CD4+ lymphocytes in both drug-sensitive and -resistant tuberculosis: however, only drug-resistant tuberculosis maintains a pro-inflammatory profile after a long time

Background

Tuberculosis (TB) is an infectious disease. During TB, regulatory T cells (Treg) are related to poor prognosis. However, information about conventional and unconventional Treg (cTreg and uTreg, respectively) is limited. The tumour necrosis factor (TNF) and its receptors (TNFR1 and TNFR2) are necessary for mycobacterial infection, and TNFR2 signalling is required to maintain Treg.

Methods

A blood sample of drug-susceptible (DS-TB) and drug-resistant tuberculosis (DR-TB) patients was obtained before (basal) and after 2 and 6 months of anti-TB therapy. Expression of TNF, TNFR1, and TNFR2 (transmembrane form, tm) on cTreg, uTreg, activated CD4+ (actCD4+), and CD4+ CD25− (CD4+) T cell subpopulations were evaluated. The main objective was to identify immunological changes associated with sensitive/resistant Mtb strains and with the use of anti-TB therapy.

Results

We found that after 6 months of anti-TB therapy, both DS- and DR-TB patients have decreased the frequency of cTreg tmTNF+, CD4+ tmTNFR1+ and CD4+ tmTNFR2+. Nevertheless, after 6 months of therapy, only DR-TB patients decreased the frequency of actCD4+ tmTNF+ and actCD4+ tmTNFR2+, exhibited a systemic inflammatory status (high levels of TNF, IFN-γ and IL-12), and their purified CD4+ T cells showed that TNF and TNFR2 are up-regulated at the transcriptional level. Moreover, DS- and DR-TB down-regulated TNFR1 and other proteins associated with Treg (FOXP3 and TGFβ1) in response to the anti-TB therapy.

Conclusion

These results partially explain the differences in the immune response of DS-TB vs DR-TB. The frequency of actCD4+ tmTNFR2+ cells and inflammatory status should be considered in the follow-up of therapy in DR-TB patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10020-021-00320-4.

A century of BCG: Impact on tuberculosis control and beyond

BCG turns 100 this year and while it might not be the perfect vaccine, it has certainly contributed significantly towards eradication and prevention of spread of tuberculosis (TB). The search for newer and better vaccines for TB is an ongoing endeavor and latest results from trials of candidate TB vaccines such as M72AS01 look promising. However, recent encouraging data from BCG revaccination trials in adults combined with studies on mucosal and intravenous routes of BCG vaccination in non-human primate models have renewed interest in BCG for TB prevention. In addition, several well-demonstrated non-specific effects of BCG, for example, prevention of viral and respiratory infections, give BCG an added advantage. Also, BCG vaccination is currently being widely tested in human clinical trials to determine whether it protects against SARS-CoV-2 infection and/or death with detailed analyses and outcomes from several ongoing trials across the world awaited. Through this review, we attempt to bring together information on various aspects of the BCG-induced immune response, its efficacy in TB control, comparison with other candidate TB vaccines and strategies to improve its efficiency including revaccination and alternate routes of administration. Finally, we discuss the future relevance of BCG use especially in light of its several heterologous benefits.

The Complex Role of Regulatory T Cells in Immunity and Aging

The immune system is a tightly regulated network which allows the development of defense mechanisms against foreign antigens and tolerance toward self-antigens. Regulatory T cells (Treg) contribute to immune homeostasis by maintaining unresponsiveness to self-antigens and suppressing exaggerated immune responses. Dysregulation of any of these processes can lead to serious consequences. Classically, Treg cell functions have been described in CD4⁺ T cells, but other immune cells also harbour the capacity to modulate immune responses. Regulatory functions have been described for different CD8⁺ T cell subsets, as well as other T cells such as γδT cells or NKT cells. In this review we describe the diverse populations of Treg cells and their role in different scenarios. Special attention is paid to the aging process, which is characterized by an altered composition of immune cells. Treg cells can contribute to the development of various age-related diseases but they are poorly characterized in aged individuals. The huge diversity of cells that display immune modulatory functions and the lack of universal markers to identify Treg make the expanding field of Treg research complex and challenging. There are still many open questions that need to be answered to solve the enigma of regulatory T cells.