T cells home to the thymus and control infection

Ruyong formula improves thymus function of CUMS-stimulated breast cancer mice

ETHNOPHARMACOLOGICAL RELEVANCE

Ruyong Formula (RYF) is a famous Chinese herbal formula composed of 10 traditional Chinese herbs. It has been used as a therapeutic agent for breast cancer patients with depressive symptoms in China. However, its underlying pharmacological mechanism remains unclear.

AIM OF THE STUDY

This study aimed to explore the mechanism of RYF on the changes of thymus immune function in breast cancer body under mood disorders such as depression/anxiety.

MATERIALS AND METHODS

The chronic unpredictable mild stress (CUMS) was used to stimulate 4T1 breast cancer mice. The behavioral changes, 5-hydroxytryptamine (5-HT) level in brain, cytokeratin 5 (CK5) and 8 (CK8) expression in thymus, the proportion of T cell subsets, the thymic output, phenotypic changes of thymus epithelial cells (TECs), the expression levels of immune-related factors and downstream proteins of TSLP were analyzed after RYF treatment.

RESULTS

In CUMS stimulated group, the level of 5-HT in brain was significantly increased after RYF treatment. The output function of the thymus was improved, and the number of TECs in the medulla (CK5+), the proportion of CD3+CD4-CD8- (Double negative) and CD3+CD4+CD8+ (Double positive) T cells were all increased. The mRNA level of TSLP in mouse thymus was significantly decreased, but increased for IL-7. The protein levels of TSLP and Vimentin were decreased, but increased for p-STAT3, p-JAK2, E-cadherin, and p-PI3K p55 in vivo. In vitro study was showed the levels of Snail 1, Zeb 1 and Smad increased significantly in TGF-β1 group, and RYF could reverse their expression.

CONCLUSIONS

RYF could restore the structure and function of the thymus in depressed breast cancer mice by reversing the phenotypic changes of TECs and activating the JAK2/STAT3/PI3K pathway.

Chronic Mycobacterium avium infection differentially affects the cytokine expression profile of three mouse strains, but has no effect on behavior

One of the most remarkable findings in the immunology and neuroscience fields was the discovery of the bidirectional interaction between the immune and the central nervous systems. This interplay is tightly regulated to maintain homeostasis in physiological conditions. Disruption in this interplay has been suggested to be associated with several neuropsychiatric disorders. Most studies addressing the impact of an immune system disruption on behavioral alterations focus on acute pro-inflammatory responses. However, chronic infections are highly prevalent and associated with an altered cytokine milieu that persists over time. Studies addressing the potential effect of mycobacterial infections on mood behavior originated discordant results and this relationship needs to be further addressed. To increase our understanding on the effect of chronic infections on the central nervous system, we evaluated the role of Mycobacterium avium infection. A model of peripheral chronic infection with M. avium in female from three mouse strains (Balb/c, C57BL/6, and CD-1) was used. The effect of the infection was evaluated in the cytokine expression profile (spleen and hippocampus), hippocampal cell proliferation, neuronal plasticity, serum corticosterone production and mood behavior. The results show that M. avium peripheral chronic infection induces alterations not just in the peripheral immune system but also in the central nervous system, namely in the hippocampus. Interestingly, the cytokine expression profile alterations vary between mouse strains, and are not accompanied by hippocampal cell proliferation or neuronal plasticity changes. Accordingly, no differences were observed in locomotor, anxious and depressive-like behaviors, in any of the mouse strains used. We conclude that the M. avium 2447 infection-induced alterations in the cytokine expression profile, both in the periphery and the hippocampus, are insufficient to alter hippocampal plasticity and behavior.

SARS-CoV-2 infection of thymus induces loss of function that correlates with disease severity

BACKGROUND

Lymphopenia, particularly when restricted to the T-cell compartment, has been described as one of the major clinical hallmarks in patients with coronavirus disease 2019 (COVID-19) and proposed as an indicator of disease severity. Although several mechanisms fostering COVID-19-related lymphopenia have been described, including cell apoptosis and tissue homing, the underlying causes of the decline in T-cell count and function are still not completely understood.

OBJECTIVE

Given that viral infections can directly target thymic microenvironment and impair the process of T-cell generation, we sought to investigate the impact of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on thymic function.

METHODS

We performed molecular quantification of T-cell receptor excision circles and κ-deleting recombination excision circles to assess, respectively, T- and B-cell neogenesis in SARS-CoV-2-infected patients. We developed a system for in vitro culture of primary human thymic epithelial cells (TECs) to mechanistically investigate the impact of SARS-CoV-2 on TEC function.

RESULTS

We showed that patients with COVID-19 had reduced thymic function that was inversely associated with the severity of the disease. We found that angiotensin-converting enzyme 2, through which SARS-CoV-2 enters the host cells, was expressed by thymic epithelium, and in particular by medullary TECs. We also demonstrated that SARS-CoV-2 can target TECs and downregulate critical genes and pathways associated with epithelial cell adhesion and survival.

CONCLUSIONS

Our data demonstrate that the human thymus is a target of SARS-CoV-2 and thymic function is altered following infection. These findings expand our current knowledge of the effects of SARS-CoV-2 infection on T-cell homeostasis and suggest that monitoring thymic activity may be a useful marker to predict disease severity and progression.

Thymus, undernutrition, and infection: Approaching cellular and molecular interactions

Undernutrition remains a major issue in global health. Low protein-energy consumption, results in stunting, wasting and/or underweight, three deleterious forms of malnutrition that affect roughly 200 million children under the age of five years. Undernutrition compromises the immune system with the generation of various degrees of immunodeficiency, which in turn, renders undernourished individuals more sensitive to acute infections. The severity of various infectious diseases including visceral leishmaniasis (VL), influenza, and tuberculosis is associated with undernutrition. Immunosuppression resulting from protein-energy undernutrition severely impacts primary and secondary lymphoid organs involved in the response to related pathogens. The thymus-a primary lymphoid organ responsible for the generation of T lymphocytes-is particularly compromised by both undernutrition and infectious diseases. In this respect, we will discuss herein various intrathymic cellular and molecular interactions seen in undernutrition alone or in combination with acute infections. Many examples illustrated in studies on humans and experimental animals clearly revealed that protein-related undernutrition causes thymic atrophy, with cortical thymocyte depletion. Moreover, the non-lymphoid microenvironmental compartment of the organ undergoes important changes in thymic epithelial cells, including their secretory products such as hormones and extracellular matrix proteins. Of note, deficiencies in vitamins and trace elements also induce thymic atrophy. Interestingly, among the molecular interactions involved in the control of undernutrition-induced thymic atrophy is a hormonal imbalance with a rise in glucocorticoids and a decrease in leptin serum levels. Undernutrition also yields a negative impact of acute infections upon the thymus, frequently with the intrathymic detection of pathogens or their antigens. For instance, undernourished mice infected with Leishmania infantum (that causes VL) undergo drastic thymic atrophy, with significant reduction in thymocyte numbers, and decreased levels of intrathymic chemokines and cytokines, indicating that both lymphoid and microenvironmental compartments of the organ are affected. Lastly, recent data revealed that some probiotic bacteria or probiotic fermented milks improve the thymus status in a model of malnutrition, thus raising a new field for investigation, namely the thymus-gut connection, indicating that probiotics can be envisioned as a further adjuvant therapy in the control of thymic changes in undernutrition accompanied or not by infection.

Alterations in thymocyte populations under conditions of endotoxin tolerance

Background:

Endotoxin tolerance (ET) is a protective phenomenon in which pre-treatment with a tolerance dose of lipopolysaccharide (LPS) leads to dramatically elevated survival. Accumulating evidence has shown that peripheral T cells contribute to the induction of ET. However, what happens to T cell development in the thymus under ET conditions remains unclear. The purpose of this study was to analyze the alterations in thymocyte populations (double-positive [DP] and single-positive [SP] cells) under ET conditions.

Methods:

Mice were intraperitoneally injected with LPS at a concentration of 5 mg/kg to establish an LPS tolerance model and were divided into two groups: a group examined 72 h after LPS injection (72-h group) and a group examined 8 days after LPS injection (8-day group). Injection of phosphate-buffered saline was used as a control (control group). Changes in thymus weight, cell counts, and morphology were detected in the three groups. Moreover, surface molecules such as CD4, CD8, CD44, CD69, and CD62L were analyzed using flow cytometry. Furthermore, proliferation, apoptosis, cytokine production, and extracellular signal-regulated kinase (ERK) pathway signaling were analyzed in thymocyte populations. The polymorphism and length of the T-cell receptor (TCR) β chain complementarity-determining region 3 (CDR3) were analyzed using capillary electrophoresis DNA laser scanning analysis (ABI 3730).

Results:

Thymus weight and cell counts were decreased in the early stage but recovered by the late stage in a murine model of LPS-induced ET. Moreover, the proportions of DP cells (control: 72.130 ± 4.074, 72-h: 10.600 ± 3.517, 8-day: 84.770 ± 2.228), CD4⁺ SP cells (control: 15.770 ± 4.419, 72-h: 44.670 ± 3.089, 8-day: 6.367 ± 0.513), and CD8⁺ SP cells (control: 7.000 ± 1.916, 72-h: 34.030 ± 3.850, 8-day: 5.133 ± 0.647) were obviously different at different stages of ET. The polymorphism and length of TCR β chain CDR3 also changed obviously, indicating the occurrence of TCR rearrangement and thymocyte diversification. Further analysis showed that the expression of surface molecules, including CD44, CD69, and CD62L, on thymocyte populations (DP and SP cells) were changed to different degrees. Finally, the proliferation, apoptosis, cytokine production, and ERK pathway signaling of thymocyte populations were changed significantly.

Conclusion:

These data reveal that alterations in thymocyte populations might contribute to the establishment of ET.

Chronic virus infection drives CD8 T cell-mediated thymic destruction and impaired negative selection

Chronic infection provokes alterations in inflammatory and suppressive pathways that potentially affect the function and integrity of multiple tissues, impacting both ongoing immune control and restorative immune therapies. Here we demonstrate that chronic lymphocytic choriomeningitis virus infection rapidly triggers severe thymic depletion, mediated by CD8 T cell-intrinsic type I interferon (IFN) and signal transducer and activator of transcription 2 (Stat2) signaling. Occurring temporal to T cell exhaustion, thymic cellularity reconstituted despite ongoing viral replication, with a rapid secondary thymic depletion following immune restoration by anti-programmed death-ligand 1 (PDL1) blockade. Therapeutic hematopoietic stem cell transplant (HSCT) during chronic infection generated new antiviral CD8 T cells, despite sustained virus replication in the thymus, indicating an impairment in negative selection. Consequently, low amounts of high-affinity self-reactive T cells also escaped the thymus following HSCT during chronic infection. Thus, by altering the stringency and partially impairing negative selection, the host generates new virus-specific T cells to replenish the fight against the chronic infection, but also has the potentially dangerous effect of enabling the escape of self-reactive T cells.

Thymic Microenvironment Is Modified by Malnutrition and Leishmania infantum Infection

Detrimental effects of malnutrition on immune responses to pathogens have long been recognized and it is considered a main risk factor for various infectious diseases, including visceral leishmaniasis (VL). Thymus is a target of both malnutrition and infection, but its role in the immune response to Leishmania infantum in malnourished individuals is barely studied. Because we previously observed thymic atrophy and significant reduction in cellularity and chemokine levels in malnourished mice infected with L. infantum, we postulated that the thymic microenvironment is severely compromised in those animals. To test this, we analyzed the microarchitecture of the organ and measured the protein abundance in its interstitial space in malnourished BALB/c mice infected or not with L. infantum. Malnourished-infected animals exhibited a significant reduction of the thymic cortex:medulla ratio and altered abundance of proteins secreted in the thymic interstitial fluid. Eighty-one percent of identified proteins are secreted by exosomes and malnourished-infected mice showed significant decrease in exosomal proteins, suggesting that exosomal carrier system, and therefore intrathymic communication, is dysregulated in those animals. Malnourished-infected mice also exhibited a significant increase in the abundance of proteins involved in lipid metabolism and tricarboxylic acid cycle, suggestive of a non-proliferative microenvironment. Accordingly, flow cytometry analysis revealed decreased proliferation of single positive and double positive T cells in those animals. Together, the reduced cortical area, decreased proliferation, and altered protein abundance suggest a dysfunctional thymic microenvironment where T cell migration, proliferation, and maturation are compromised, contributing for the thymic atrophy observed in malnourished animals. All these alterations could affect the control of the local and systemic infection, resulting in an impaired response to L. infantum infection.

Role of arginine deiminase in thymic atrophy during experimental Streptococcus pyogenes infection

Expression of gene of arginine deiminase (AD) allows adaptation of Streptococcus pyogenes to adverse environmental conditions. AD activity can lead to L-arginine deficiency in the host cells' microenvironment. Bioavailability of L-arginine is an important factor regulating the functions of the immune cells in mammals. By introducing a mutation into S pyogenes M46-16, we obtained a strain with inactivated arcA/sagp gene (M49-16 delArcA), deficient in AD. This allowed elucidating the function of AD in pathogenesis of streptococcal infection. The virulence of the parental and mutant strains was examined in a murine model of subcutaneous streptococcal infection. L-arginine concentration in the plasma of mice infected with S pyogenes M49-16 delArcA remained unchanged in course of the entire experiment. At the same time mice infected with S pyogenes M49-16 demonstrated gradual diminution of L-arginine concentration in the blood plasma, which might be due to the activity of streptococcal AD. Mice infected with S pyogenes M49-16 delArcA demonstrated less intensive bacterial growth in the primary foci and less pronounced bacterial dissemination as compared with animals infected with the parental strain S pyogenes M46-16. Similarly, thymus involution, alterations in apoptosis, thymocyte subsets and Treg cells differentiation were less pronounced in mice infected with S pyogenes M49-16 delArcA than in those infected with the parental strain. The results obtained showed that S pyogenes M49-16 delArcA, unable to produce AD, had reduced virulence in comparison with the parental S pyogenes M49-16 strain. AD is an important factor for the realization of the pathogenic potential of streptococci.

Improvement of glucocorticoid-impaired thymus function by dihydromyricetin via up-regulation of PPARγ-associated fatty acid metabolism

T lymphocytes produced by the thymus are essential mediators of immunity. Accelerated thymic atrophy appears in the patients with administration of glucocorticoids (GCs) which are commonly-used drugs to treat autoimmune and infectious diseases, leading to dysregulation of immunity with manifestation of progressive diminution of new T cell production. However, there is no ideal method to overcome such side effects of GCs. In the current study, we proposed a composition of dexamethasone (DEX) and dihydromyricetin (DMY) derived from a medicinal plant, which could protect from DEX-induced thymus damage and simultaneously enhance the anti-inflammatory effect of DEX. In the current study, we found that DEX-damaged thymic cellularity and architecture, reduced thymocyte numbers, induced thymocyte apoptosis and dropped CD4+ and CD8+ double positive T cell numbers in thymus which was effectively improved by co-treatment with DMY. Quantification of signal joint TCR delta excision circles (TRECs) and Vβ TCR spectratyping analysis were employed to determine the thymus function with indicated treatments. The results showed that DEX-impaired thymus output and decreased TCR cell diversity which was ameliorated by co-treatment with DMY. iTRAQ 2D LC-MS/MS was applied to analyze the proteomic profiling of thymus of mice treated with or without indicated agents, followed by informatics analysis to identify the correlated signaling pathway. After validated by Western blotting and Real-time PCR, we found that PPARγ-associated fatty acid metabolism was increased in the thymic tissues of the animals treated with DMY plus DEX than the animals treated with DEX alone. The agonist and antagonist of PPARγ were further employed to verify the role of PPARγ in the present study. Furthermore, DMY demonstrated a synergistic effect with co-administration of DEX on suppressing inflammation in vivo. Collectively, DMY relieved thymus function damaged by DEX via regulation of PPARγ-associated fatty acid metabolism. Our findings may provide a new strategy on protection of thymus from damage caused by GCs by using appropriate adjuvant natural agents through up-regulation of PPARγ-associated fatty acid metabolism.

Ag85-focused T-cell immune response controls Mycobacterium avium chronic infection

CD4⁺ T cells are essential players for the control of mycobacterial infections. Several mycobacterial antigens have been identified for eliciting a relevant CD4⁺ T cell mediated-immune response, and numerous studies explored this issue in the context of Mycobacterium tuberculosis infection. Antigen 85 (Ag85), a highly conserved protein across Mycobacterium species, is secreted at the early phase of M. tuberculosis infection leading to the proliferation of Ag85-specific CD4⁺ T cells. However, in the context of Mycobacterium avium infection, little is known about the expression of this antigen and the elicited immune response. In the current work, we investigated if a T cell receptor (TCR) repertoire mostly, but not exclusively, directed at Ag85 is sufficient to mount a protective immune response against M. avium. We show that P25 mice, whose majority of T cells express a transgenic TCR specific for Ag85, control M. avium infection at the same level as wild type (WT) mice up to 20 weeks post-infection (wpi). During M. avium infection, Ag85 antigen is easily detected in the liver of 20 wpi mice by immunohistochemistry. In spite of the propensity of P25 CD4⁺ T cells to produce higher amounts of interferon-gamma (IFNγ) upon ex vivo stimulation, no differences in serum IFNγ levels are detected in P25 compared to WT mice, nor enhanced immunopathology is detected in P25 mice. These results indicate that a T cell response dominated by Ag85-specific T cells is appropriate to control M. avium infection with no signs of immunopathology.

Tuberculosis, the Disrupted Immune-Endocrine Response and the Potential Thymic Repercussion As a Contributing Factor to Disease Physiopathology

Upon the pathogen encounter, the host seeks to ensure an adequate inflammatory reaction to combat infection but at the same time tries to prevent collateral damage, through several regulatory mechanisms, like an endocrine response involving the production of adrenal steroid hormones. Our studies show that active tuberculosis (TB) patients present an immune-endocrine imbalance characterized by an impaired cellular immunity together with increased plasma levels of cortisol, pro-inflammatory cytokines, and decreased amounts of dehydroepiandrosterone. Studies in patients undergoing specific treatment revealed that cortisol levels remained increased even after several months of initiating therapy. In addition to the well-known metabolic and immunological effects, glucocorticoids are involved in thymic cortical depletion with immature thymocytes being quite sensitive to such an effect. The thymus is a central lymphoid organ supporting thymocyte T-cell development, i.e., lineage commitment, selection events and thymic emigration. While thymic TB is an infrequent manifestation of the disease, several pieces of experimental and clinical evidence point out that the thymus can be infected by mycobacteria. Beyond this, the thymic microenvironment during TB may be also altered because of the immune-hormonal alterations. The thymus may be then an additional target of organ involvement further contributing to a deficient control of infection and disease immunopathology.

Acute Thymic Involution and Mechanisms for Recovery

Acute thymic involution (ATI) is usually regarded as a virulence trait. It is caused by several infectious agents (bacteria, viruses, parasites, fungi) and other factors, including stress, pregnancy, malnutrition and chemotherapy. However, the complex mechanisms that operate during ATI differ substantially from each other depending on the causative agent. For instance, a transient reduction in the size and weight of the thymus and depletion of populations of T cell subsets are hallmarks of ATI in many cases, whereas severe disruption of the anatomical structure of the organ is also associated with some factors, including fungal, parasitic and viral infections. However, growing evidence shows that ATI may be therapeutically halted or reversed. In this review, we highlight the current progress in this field with respect to numerous pathological factors and discuss the possible mechanisms. Moreover, these new observations also show that ATI can be mechanistically reversed.

Protein malnutrition promotes dysregulation of molecules involved in T cell migration in the thymus of mice infected with Leishmania infantum

Protein malnutrition, the most deleterious cause of malnutrition in developing countries, has been considered a primary risk factor for the development of clinical visceral leishmaniasis (VL). Protein malnutrition and infection with Leishmania infantum leads to lymphoid tissue disorganization, including changes in cellularity and lymphocyte subpopulations in the thymus and spleen. Here we report that protein malnutrition modifies thymic chemotactic factors by diminishing the CCL5, CXCL12, IGF1, CXCL9 and CXCL10 protein levels in infected animals. Nevertheless, T cells preserve their migratory capability, as they were able to migrate ex vivo in response to chemotactic stimuli, indicating that malnutrition may compromise the thymic microenvironment and alter in vivo thymocyte migration. Decrease in chemotactic factors protein levels was accompanied by an early increase in the parasite load of the spleen. These results suggest that the precondition of malnutrition is affecting the cell-mediated immune response to L. infantum by altering T cell migration and interfering with the capacity of protein-deprived animals to control parasite spreading and proliferation. Our data provide evidence for a disturbance of T lymphocyte migration involving both central and peripheral T-cells, which likely contribute to the pathophysiology of VL that occurs in malnourished individuals.

The human thymus perivascular space is a functional niche for viral-specific plasma cells

The human thymus is susceptible to viral infections that can severely alter thymopoiesis and compromise the mechanisms of acquired tolerance to self-antigens. In humans, plasma cells residing primarily in the bone marrow confer long-lasting protection to common viruses by secreting antigen-specific antibodies. Since the thymus also houses B cells, we examined the phenotypic complexity of these thymic resident cells and their possible protective role against viral infections. Using tissue specimens collected from subjects ranging in age from 5 days to 71 years, we found that starting during the first year of life, CD138⁺ plasma cells (PC) begin accumulating in the thymic perivascular space (PVS) where they constitutively produce IgG without the need for additional stimulation. These, thymic PC secrete almost exclusively IgG1 and IgG3, the two main complement-fixing effector IgG subclasses. Moreover, using antigen-specific ELISpot assays, we demonstrated that thymic PC include a high frequency of cells reactive to common viral proteins. Our study reveals an unrecognized role of the PVS as a functional niche for viral-specific PCs. The PVS is located between the thymic epithelial areas and the circulation. PCs located in this compartment may therefore provide internal protection against pathogen infections and preserve the integrity and function of the organ.

Systemic toxoplasma infection triggers a long-term defect in the generation and function of naive T lymphocytes

Kugler et al. show that systemic infection with Toxoplasma gondii triggers a long-term impairment in thymic function, which leads to an immunodeficient state reflected in decreased antimicrobial resistance.

Because antigen-stimulated naive T cells either die as effectors or enter the activated/memory pool, continuous egress of new T lymphocytes from thymus is essential for maintenance of peripheral immune homeostasis. Unexpectedly, we found that systemic infection with the protozoan Toxoplasma gondii triggers not only a transient increase in activated CD4⁺ Th1 cells but also a persistent decrease in the size of the naive CD4⁺ T lymphocyte pool. This immune defect is associated with decreased thymic output and parasite-induced destruction of the thymic epithelium, as well as disruption of the overall architecture of that primary lymphoid organ. Importantly, the resulting quantitative and qualitative deficiency in naive CD4⁺ T cells leads to an immunocompromised state that both promotes chronic toxoplasma infection and leads to decreased resistance to challenge with an unrelated pathogen. These findings reveal that systemic infectious agents, such as T. gondii, can induce long-term immune alterations associated with impaired thymic function. When accumulated during the lifetime of the host, such events, even when occurring at low magnitude, could be a contributing factor in immunological senescence.

Severe Changes in Thymic Microenvironment in a Chronic Experimental Model of Paracoccidioidomycosis

T cell maturation takes place within the thymus, a primary lymphoid organ that is commonly targeted during infections. Previous studies showed that acute infection with Paracoccidioides brasiliensis (Pb), the causative agent of paracoccidioidomycosis (PCM), promotes thymic atrophy that is associated with the presence of yeast cells in the organ. However, as human PCM is a chronic infection, it is imperative to investigate the consequences of Pb infection over the thymic structure and function in chronic infection. In this sense, we developed a new experimental model where Pb yeast cells are injected through the intraperitoneal route and mice are evaluated over 120 days of infection. Thymuses were analyzed in chronically infected mice and we found that the thymus underwent extensive morphological alterations and severe infiltration of P. brasiliensis yeast cells. Further analyses showed an altered phenotype and function of thymocytes that are commonly found in peripheral mature T lymphocytes. We also observed activation of the NLRP3 inflammasome in the thymus. Our data provide new information on the severe changes observed in the thymic microenvironment in a model of PCM that more closely mimics the human infection.

TNF superfamily members play distinct roles in shaping the thymic stromal microenvironment

The differentiation and proper function of thymic epithelial cells (TECs) depend on various tumor necrosis factor superfamily (TNFSF) signals that are needed to maintain the thymic stromal microenvironment. Nevertheless, the direct transcriptional effects of these signals on TECs remain unclear. To address this issue, we stimulated murine embryonic thymus tissue with selected TNFSF ligands and performed a gene expression profiling study. We show that Aire expression is a direct and specific effect of RANKL stimulation, whereas LTβ and TNFα are major inducers of chemokines in the thymic stroma and we propose differential NF-κB binding as one possible cause of these gene expression patterns. Our work provides further insight into the complex molecular pathways that shape the thymic microenvironment and maintain central tolerance.

The Attenuated Live Yellow Fever Virus 17D Infects the Thymus and Induces Thymic Transcriptional Modifications of Immunomodulatory Genes in C57BL/6 and BALB/C Mice

Thymus is involved in induction of self-tolerance in T lymphocytes, particularly due to Aire activity. In peripheral tissues, Treg cells and immunomodulatory molecules, like the major histocompatibility complex (MHC) class Ib molecules, are essential for maintenance of autotolerance during immune responses. Viral infections can trigger autoimmunity and modify thymic function, and YFV17D immunization has been associated with the onset of autoimmunity, being contraindicated in patients with thymic disorders. Aiming to study the influence of YFV17D immunization on the transcriptional profiles of immunomodulatory genes in thymus, we evaluated the gene expression of AIRE, FOXP3, H2-Q7 (Qa-2/HLA-G), H2-T23 (Qa-1/HLA-E), H2-Q10, and H2-K1 following immunization with 10,000 LD50 of YFV17D in C57BL/6 and BALB/c mice. The YFV17D virus replicated in thymus and induced the expression of H2-Q7 (Qa-2/HLA-G) and H2-T23 (Qa-1/HLA-E) transcripts and repressed the expression of AIRE and FOXP3. Transcriptional expression varied according to tissue and mouse strain analyzed. Expression of H2-T23 (Qa-1/HLA-E) and FOXP3 was induced in thymus and liver of C57BL/6 mice, which exhibited defective control of viral load, suggesting a higher susceptibility to YFV17D infection. Since the immunization with YFV17D modulated thymus gene expression in genetically predisposed individuals, the vaccine may be related to the onset of autoimmunity disorders.

A discrete population of IFN λ-expressing BDCA3hi dendritic cells is present in human thymus

Human thymus contains two major subpopulations of dendritic cells (DCs), conventional DCs (cDCs) and plasmacytoid DCs (pDCs), which are mainly involved in central tolerance and also in protecting the thymus against infections. In blood and peripheral organs cDCs include the subpopulation of BDCA3(hi) DCs, considered as equivalents to mouse CD8α(+) DCs. In this study we describe in human thymus the presence of a discrete population of BDCA3(hi) DCs that, like their peripheral counterparts, express CD13, low-intermediate levels of CD11c, CLEC9A, high levels of XCR1, IRF8 and TLR3, and mostly lack the expression of CD11b, CD14 and TLR7. Thymic BDCA3(hi) DCs display immature features with a low expression of costimulatory molecules and HLA-DR, and a low allostimulatory capacity. Also, BDCA3(hi) DCs exhibit a strong response to TLR3 stimulation, producing high levels of interferon (IFN)-λ1 and CXCL10, which indicates that, similarly to thymic pDCs, BDCA3(hi) DCs can have an important role in thymus protection against viral infections.

Human and Murine Clonal CD8+ T Cell Expansions Arise during Tuberculosis Because of TCR Selection

The immune system can recognize virtually any antigen, yet T cell responses against several pathogens, including Mycobacterium tuberculosis, are restricted to a limited number of immunodominant epitopes. The host factors that affect immunodominance are incompletely understood. Whether immunodominant epitopes elicit protective CD8+ T cell responses or instead act as decoys to subvert immunity and allow pathogens to establish chronic infection is unknown. Here we show that anatomically distinct human granulomas contain clonally expanded CD8+ T cells with overlapping T cell receptor (TCR) repertoires. Similarly, the murine CD8+ T cell response against M. tuberculosis is dominated by TB10.4_4-11-specific T cells with extreme TCRβ bias. Using a retrogenic model of TB10.4_4-11-specific CD8+ T cells, we show that TCR dominance can arise because of competition between clonotypes driven by differences in affinity. Finally, we demonstrate that TB10.4-specific CD8+ T cells mediate protection against tuberculosis, which requires interferon-γ production and TAP1-dependent antigen presentation in vivo. Our study of how immunodominance, biased TCR repertoires, and protection are inter-related, provides a new way to measure the quality of T cell immunity, which if applied to vaccine evaluation, could enhance our understanding of how to elicit protective T cell immunity.

While T cells are required for protection against Mycobacterium tuberculosis infection, attempts to prevent tuberculosis by vaccines designed to elicit memory T cells have only been partially successful. Several vaccine candidates are in clinical trials, but progress has been slow because their ability to prevent disease must be empirically tested. There is little understanding of why certain antigens are targets of protective immunity. We have characterized an immunodominant CD8⁺ T cell response to the M. tuberculosis antigen TB10.4 (EsxH). CD8⁺ T cells specific for the TB10.4_4–11 epitope are primed early during infection and account for 30–50% of lung CD8⁺ T cells during chronic infection. Now we have used deep sequencing to characterize the TCR repertoire of TB10.4_4-11-specific CD8⁺ T cells in the lungs of infected mice. Interestingly, TB10.4_4-11-specific CD8⁺ T cells exhibit extreme clonal expansion of certain TCRβ with common structural features, most likely because of affinity selection. Affinity selection of T cells is more important when antigen presentation is limiting. Although the lung contains numerous bacteria during infection, antigen-presentation by infected APC may be limiting, mimicking a “low antigen” state. Thus, even T cells that have the potential to mediate protection may function inefficiently because of suboptimal T cell activation.

Anxiety- and depression-like behavior in mice lacking the CD157/BST1 gene, a risk factor for Parkinson's disease

CD157, known as bone marrow stromal cell antigen-1, is a glycosylphosphatidylinositol-anchored ADP-ribosyl cyclase that supports the survival and function of B-lymphocytes and hematopoietic or intestinal stem cells. Although CD157/Bst1 is a risk locus in Parkinson's disease (PD), little is known about the function of CD157 in the nervous system and contribution to PD progression. Here, we show that no apparent motor dysfunction was observed in young knockout (CD157 (-/-)) male mice under less aging-related effects on behaviors. CD157 (-/-) mice exhibited anxiety-related and depression-like behaviors compared with wild-type mice. These behaviors were rescued through treatment with anti-psychiatric drugs and oxytocin. CD157 was weakly expressed in the amygdala and c-Fos immunoreactivity in the amygdala was less evident in CD157 (-/-) mice than in wild-type mice. These results demonstrate for the first time that CD157 plays a role as a neuro-regulator and suggest a potential role in pre-motor symptoms in PD.

Immunotherapy of tuberculosis with Mycobacterium leprae Hsp65 as a DNA vaccine triggers cross-reactive antibodies against mammalian Hsp60 but not pathological autoimmunity

Despite substantial efforts in recent years toward the development of new vaccines and drugs against tuberculosis (TB), success has remained elusive. Immunotherapy of TB with mycobacterial Hsp65 as a DNA vaccine (DNA-hsp65) results in a reduction of systemic bacterial loads and lung tissue damage, but the high homology of Hsp65 with the mammalian protein raises concern that pathological autoimmune responses may also be triggered. We searched for autoimmune responses elicited by DNA-hsp65 immunotherapy in mice chronically infected with TB by evaluating the humoral immune response and comprehensive histopathology using stereology. Cross-reactive antibodies between mycobacterial and mammalian Hsp60/65 were detected; however, no signs of pathological autoimmunity were found up to 60 days after the end of the therapy.

Mature peripheral T cells are important to preserve thymus function and selection of thymocytes during Mycobacterium tuberculosis infection

Evaluation of: Nobrega C, Nunes-Alves C, Cerqueira-Rodrigues B et al. T cells home to the thymus and control infection. J. Immunol. 190, 1646-1658 (2013). It is well documented that the thymus is a target organ for a large variety of pathogens (virus, bacteria, fungi and protozoa). Moreover, the presence of pathogen-derived antigens in the thymus of infected mice seems to interfere with the capacity of mature T cells to respond to the invading organism. In this way, Nobrega and colleagues demonstrated in 2010 that Mycobacterium avium infection in the thymus leads to the appearance of differentiated T cells tolerogenic for bacterial antigens. In the present and elegant study, the same group demonstrates that T-cell recirculation from the periphery to the thymus is a mechanism that allows the immune system to respond to thymic infection. A Mycobacterium-infected thymus increases the production of Th1-effector chemokines, such as CXCL9 and CXCL10, which in turn recruit CXCR3(+) peripheral T cells involved in intrathymic bacterial control. Taken together, these findings may represent an important issue of the host response, in terms of different pathogens able to infect the thymus.

Tolerance has its limits: how the thymus copes with infection

The thymus is required for T cell differentiation; a process that depends on which antigens are encountered by thymocytes, the environment surrounding the differentiating cells, and the thymic architecture. These features are altered by local infection of the thymus and by the inflammatory mediators that accompany systemic infection. Although once believed to be an immune privileged site, it is now known that antimicrobial responses are recruited to the thymus. Resolving infection in the thymus is important because chronic persistence of microbes impairs the differentiation of pathogen-specific T cells and diminishes resistance to infection. Understanding how these mechanisms contribute to disease susceptibility, particularly in infants with developing T cell repertoires, requires further investigation.

Repairing thymic function

PURPOSE OF REVIEW

Maintenance of T-cell function and modulation of tolerance are critical issues in organ transplantation. The thymus is the primary organ for T-cell generation, and a preserved thymic function is essential for a self-tolerant diverse T-cell repertoire. Transplant procedures and related immunosuppressive drugs may hinder thymic integrity and function. We review here the recent advances in understanding the regulation of the unique thymic microenvironment with relevance for the field of transplantation.

RECENT FINDINGS

Recent studies have assigned a role for IL-22 in the regeneration of thymic epithelium, and for microRNAs in the modulation of its survival and function. The interplay of key molecules in the cross-talk between thymic epithelial cells and thymocytes was depicted, opening new perspectives for the in-vitro recapitulation of T-cell development and for thymic transplantation. Additionally, the thymus was shown to be able to sustain thymocyte progenitor renewal.

SUMMARY

These findings open new venues of research toward therapeutic interventions in the endogenous thymus to modulate or reconstitute the immune system; thymic transplantation; and the future development of artificial thymus, which would represent an important tool to achieve tolerance across the histocompatibility barriers.

Thymus-resident memory CD8+ T cells mediate local immunity

The thymus is a primary lymphoid organ responsible for production and selection of T cells. Nonetheless, mature T cells and in particular activated T cells can reenter the thymus. Here, we identified memory CD8(+) T cells specific for lymphocytic choriomeningitis virus or vaccinia virus in the thymus of mice long-time after the infection. CD8(+) T cells were mainly located in the thymic medulla, but also in the cortical areas. Interestingly, virus-specific memory CD8(+) T cells in the thymus expressed the cell surface markers CD69 and CD103 that are characteristic of tissue-resident memory T cells in a time-dependent manner. Kinetic analyses and selective depletion of peripheral CD8(+) T cells by antibodies further revealed that thymic virus-specific memory CD8(+) T cells did not belong to the circulating pool of lymphocytes. Finally, we demonstrate that these thymus-resident virus-specific memory CD8(+) T cells efficiently mounted a secondary proliferative response, exhibited immediate effector functions and were able to protect the thymus from lymphocytic choriomeningitis virus reinfection. In conclusion, the present study not only describes for the first time virus-specific memory CD8(+) T cells with characteristics of tissue-resident memory T (T(RM)) cells in a primary lymphoid organ but also extends our knowledge about local T-cell immunity in the thymus.