Is thymocyte development functional in the aged?

Age-related thymic involution: Mechanisms and functional impact

The thymus is the primary immune organ responsible for generating self‐tolerant and immunocompetent T cells. However, the thymus gradually involutes during early life resulting in declined naïve T‐cell production, a process known as age‐related thymic involution. Thymic involution has many negative impacts on immune function including reduced pathogen resistance, high autoimmunity incidence, and attenuated tumor immunosurveillance. Age‐related thymic involution leads to a gradual reduction in thymic cellularity and thymic stromal microenvironment disruption, including loss of definite cortical‐medullary junctions, reduction of cortical thymic epithelial cells and medullary thymic epithelial cells, fibroblast expansion, and an increase in perivascular space. The compromised thymic microenvironment in aged individuals substantially disturbs thymocyte development and differentiation. Age‐related thymic involution is regulated by many transcription factors, micro RNAs, growth factors, cytokines, and other factors. In this review, we summarize the current understanding of age‐related thymic involution mechanisms and effects.

Thymic Engraftment by in vitro-Derived Progenitor T Cells in Young and Aged Mice

T cells play a critical role in mediating antigen-specific and long-term immunity against viral and bacterial pathogens, and their development relies on the highly specialized thymic microenvironment. T cell immunodeficiency can be acquired in the form of inborn errors, or can result from perturbations to the thymus due to aging or irradiation/chemotherapy required for cancer treatment. Hematopoietic stem cell transplant (HSCT) from compatible donors is a cornerstone for the treatment of hematological malignancies and immunodeficiency. Although it can restore a functional immune system, profound impairments exist in recovery of the T cell compartment. T cells remain absent or low in number for many months after HSCT, depending on a variety of factors including the age of the recipient. While younger patients have a shorter refractory period, the prolonged T cell recovery observed in older patients can lead to a higher risk of opportunistic infections and increased predisposition to relapse. Thus, strategies for enhancing T cell recovery in aged individuals are needed to counter thymic damage induced by radiation and chemotherapy toxicities, in addition to naturally occurring age-related thymic involution. Preclinical results have shown that robust and rapid long-term thymic reconstitution can be achieved when progenitor T cells, generated in vitro from HSCs, are co-administered during HSCT. Progenitor T cells appear to rely on lymphostromal crosstalk via receptor activator of NF-κB (RANK) and RANK-ligand (RANKL) interactions, creating chemokine-rich niches within the cortex and medulla that likely favor the recruitment of bone marrow-derived thymus seeding progenitors. Here, we employed preclinical mouse models to demonstrate that in vitro-generated progenitor T cells can effectively engraft involuted aged thymuses, which could potentially improve T cell recovery. The utility of progenitor T cells for aged recipients positions them as a promising cellular therapy for immune recovery and intrathymic repair following irradiation and chemotherapy, even in a post-involution thymus.

Anidulafungin Versus Micafungin in the Treatment of Candidemia in Adult Patients

Background

Echinocandins are recommended for the treatment of invasive candidiasis and candidemia. However, there are few studies comparing anidulafungin and micafungin in terms of efficacy and safety. The objective of this study was to evaluate the clinical efficacy and safety between anidulafungin and micafungin treatment for adult patients with candidemia.

Methods

This retrospective cohort study performed on adult candidemia patients diagnosed from January 2006 through December 2018 at a tertiary medical center. The study subjects included adult patients ≥ 19 years with candidemia who were only treated with anidulafungin or micafungin for ≥ 3 days. Clinical characteristics were collected and analyzed. Hepatotoxicity was assessed according to the Common Terminology Criteria for Adverse Events Version 5.0.

Results

A total of 98 patients with candidemia were treated with anidulafungin (n = 52, 53.1%) or micafungin (n = 46, 46.9%). There were no significant differences in age, sex, source of candidemia, and comorbidities between the anidulafungin and micafungin groups. Although there were more patients with abnormal baseline liver function test (LFT) in the anidulafungin group, the rate of clinical response (51.9% vs. 46.7%), mycological response (76.9% vs. 67.4%), and mortality (30-day mortality 26.9% vs. 21.7% and 90-day mortality 78.8% vs. 73.9%) was similar between the anidulafungin and micafungin groups. Also, there was no significant difference in terms of hepatotoxicity, even among the patients with abnormal baseline LFT between the two groups.

Conclusions

Our results suggest that clinical efficacy and safety may be similar between anidulafungin and micafungin treatment for adult patients with candidemia.

Electronic supplementary material

The online version of this article (10.1007/s11046-020-00471-8) contains supplementary material, which is available to authorized users.

Phosphate Transporter Profiles in Murine and Human Thymi Identify Thymocytes at Distinct Stages of Differentiation

Thymocyte differentiation is dependent on the availability and transport of metabolites in the thymus niche. As expression of metabolite transporters is a rate-limiting step in nutrient utilization, cell surface transporter levels generally reflect the cell's metabolic state. The GLUT1 glucose transporter is upregulated on actively dividing thymocytes, identifying thymocytes with an increased metabolism. However, it is not clear whether transporters of essential elements such as phosphate are modulated during thymocyte differentiation. While PiT1 and PiT2 are both phosphate transporters in the SLC20 family, we show here that they exhibit distinct expression profiles on both murine and human thymocytes. PiT2 expression distinguishes thymocytes with high metabolic activity, identifying immature murine double negative (CD4⁻CD8⁻) DN3b and DN4 thymocyte blasts as well as immature single positive (ISP) CD8 thymocytes. Notably, the absence of PiT2 expression on RAG2-deficient thymocytes, blocked at the DN3a stage, strongly suggests that high PiT2 expression is restricted to thymocytes having undergone a productive TCRβ rearrangement at the DN3a/DN3b transition. Similarly, in the human thymus, PiT2 was upregulated on early post-β selection CD4⁺ISP and TCRαβ⁻CD4^hiDP thymocytes co-expressing the CD71 transferrin receptor, a marker of metabolic activity. In marked contrast, expression of the PiT1 phosphate importer was detected on mature CD3⁺ murine and human thymocytes. Notably, PiT1 expression on CD3⁺DN thymocytes was identified as a biomarker of an aging thymus, increasing from 8.4 ± 1.5% to 42.4 ± 9.4% by 1 year of age (p < 0.0001). We identified these cells as TCRγδ and, most significantly, NKT, representing 77 ± 9% of PiT1⁺DN thymocytes by 1 year of age (p < 0.001). Thus, metabolic activity and thymic aging are associated with distinct expression profiles of the PiT1 and PiT2 phosphate transporters.

Estrogen, estrogen-like molecules and autoimmune diseases

In western countries, the slope of autoimmune disease (AD) incidence is increasing and affects 5-8% of the population. Mainly prevalent in women, these pathologies are due to thymic tolerance processes breakdown. The female sex hormone, estrogen, is involved in this AD female susceptibility. However, predisposition factors have to act in concert with unknown triggering environmental factors (virus, microbiota, pollution) to initiate AD. Individuals are exposed to various environmental compounds that display endocrine disruption abilities. The cellular effects of some of these molecules may be mediated through the aryl hydrocarbon receptor (AhR). Here, we review the effects of these molecules on the homeostasis of the thymic cells, the immune tolerance intrinsic factors (transcription factors, epigenetic marks) and on the immune tolerance extrinsic factors (microbiota, virus sensibility). This review highlights the contribution of estrogen and endocrine disruptors on the dysregulation of mechanisms sustaining AD development.

Cytokine modulation, oxidative stress and thymic dysfunctions: Role of age-related changes in the experimental Trypanosoma cruzi infection: Age-related thymic dysfunctions and Trypanosoma cruzi infection

Aging is linked with a thymic oxidative damage and some infectious diseases such as Chagas' disease may aggravate this process. The aim of this study was to evaluate the production of distinct cytokines as well as the antioxidant/oxidant status of the thymus and thymocytes populations during Trypanosoma cruzi (T. cruzi) infection. Young (5 weeks old) and aged (18 weeks old) male Wistar rats were inoculated with blood trypomastigotes forms of the Y strain of T. cruzi. On the 16th day after T. cruzi infection, increased concentrations of transforming growth factor β (TGF-β), interleukin (IL)-12, IL-17 were detected in aged infected subjects as compared to young infected ones. Interestingly, a reduction in the production of tumor necrose factor (TNF)-α was observed in aged infected rats when compared to young infected subjects. Aged-infected rats presented increased O₂^- levels, compared to young counterparts. Significant raise in the generation of O₂^- in aged infected animals, as compared to uninfected counterparts was observed. Up-regulated expression of Nox2 in the thymus of young and aged infected animals was observed. An increased SOD2 expression was detected in the thymus of young animals infected with T. cruzi, when compared to uninfected young rats. Aged animals showed reduced thymus weight and the number of thymocytes. Decreased percentages of SPCD4⁺ and SPCD8⁺T cells were detected in aged and control groups when compared to young counterparts. In summary, this is the first data to directly examine the influence of aging on age-related dysfunctions during the acute phase of experimental Chagas disease. Concerning to oxidative stress, it is clear from our analysis that aged infected rats suffer a more intense oxidative damage when compared to young and infected ones. Age and infection triggered a dynamic interplay of cytokines, oxidative stress and thymic dysfunctions which led to impaired response from aged and infected rats. Such findings may have significant functional relevance in therapeutic strategies in order to reestablish the thymic immunological function which occurs in aged and T. cruzi infected subjects.

Thymus: the next (re)generation

As the primary site of T-cell development, the thymus plays a key role in the generation of a strong yet self-tolerant adaptive immune response, essential in the face of the potential threat from pathogens or neoplasia. As the importance of the role of the thymus has grown, so too has the understanding that it is extremely sensitive to both acute and chronic injury. The thymus undergoes rapid degeneration following a range of toxic insults, and also involutes as part of the aging process, albeit at a faster rate than many other tissues. The thymus is, however, capable of regenerating, restoring its function to a degree. Potential mechanisms for this endogenous thymic regeneration include keratinocyte growth factor (KGF) signaling, and a more recently described pathway in which innate lymphoid cells produce interleukin-22 (IL-22) in response to loss of double positive thymocytes and upregulation of IL-23 by dendritic cells. Endogenous repair is unable to fully restore the thymus, particularly in the aged population, and this paves the way toward the need for exogenous strategies to help regenerate or even replace thymic function. Therapies currently in clinical trials include KGF, use of the cytokines IL-7 and IL-22, and hormonal modulation including growth hormone administration and sex steroid inhibition. Further novel strategies are emerging in the preclinical setting, including the use of precursor T cells and thymus bioengineering. The use of such strategies offers hope that for many patients, the next regeneration of their thymus is a step closer.

Viral Profile of COPD Exacerbations According to Patients

Background :

To compare the differences between elderly and non-elderly patients with acute exacerbations of chronic obstructive pulmonary disease (AECOPD) due to viral infections.

Methods :

Patients with chronic obstructive pulmonary disease (COPD) exacerbation were recruited and classified as elderly (>65 years) and non-elderly (≤ 65 years). Sputum and oropharyngeal samples were assessed, PCR for respiratory viruses and cultures for common pathogens were performed.

Results :

247 patients (median age: 69.3±9.5 years) were recruited and categorized into group A: non-elderly patients [n=81 (32.8%), median age 58±5.99] and group B: elderly patients [n=166 (67.2%), median age 74.8±4.8] years. In 133 (53.8%) patients a viral infection was identified and in 34 (13.8%) a bacterial pathogen was isolated from cultures. In 18 (7.3%) patients a double infection (bacterial+viral) was identified. In group B, the presence of cardiac failure (46.6% vs 28.3%, p<0.001), renal failure (10.5% vs 4%, p=0.03), bacterial co-infection (13.8% vs 7.4%, p=0.04), influenza vaccination rates (45.5% vs 215, p<0.001), and longer hospital stay (8.4±4.4 vs 7.5±3.2 days, p=0.02) were higher than group A. The overall rate of viral infections did not differ according to age. A trend to higher rates of infection with parainfluenza 3 [19 (20%) patients in group B vs3 (7.5%) patients in group A, p=0.04] was observed in older patients.

Conclusion :

No differences on the rate and type of viral infections were noted for elderly vs non elderly patients. However, they tended to have more bacterial co-infections that led to AECOPD and longer hospitalization stays compared to non-elderly patients.

Kinetin inhibits apoptosis of aging spleen cells induced by D-galactose in rats

Kinetin (Kn) is a cytokinin growth factor that exerts several anti-aging and antioxidant effects on cells and organs. To investigate the mechanism underlying apoptotic events in aging cells induced by D-galactose (D-gal), we examined the effect of Kn delivered via nuchal subcutaneous injection on D-gal-induced aging and apoptosis in rats. Our results showed that interleukin (IL)-2 levels and mitochondrial membrane potential (ΔΨm) were decreased by Kn in aging rats while IL-6 production and apoptosis increased. In addition, the expression of anti-apoptotic Bcl-2 was low while that of Bax was high in the aging group. After treated with Kn, compared with aging group, there showed obvious difference in Kn group with elevated IL-2, proliferation index, Bcl-2, ΔΨm and decreased IL-6 and Bax in splenic lymphocyte. Based on these results, we concluded that Kn can effectively protect the rat spleen from aging, apoptosis, and atrophy.

Senescence-accelerated OXYS rats: a model of age-related cognitive decline with relevance to abnormalities in Alzheimer disease

Senescence-accelerated OXYS rats are an experimental model of accelerated aging that was established from Wistar stock via selection for susceptibility to cataractogenic effects of a galactose-rich diet and via subsequent inbreeding of highly susceptible rats. Currently, we have the 102nd generation of OXYS rats with spontaneously developing cataract and accelerated senescence syndrome, which means early development of a phenotype similar to human geriatric disorders, including accelerated brain aging. In recent years, our group found strong evidence that OXYS rats are a promising model for studies of the mechanisms of brain aging and neurodegenerative processes similar to those seen in Alzheimer disease (AD). The manifestation of behavioral alterations and learning and memory deficits develop since the fourth week of age, i.e., simultaneously with first signs of neurodegeneration detectable on magnetic resonance imaging and under a light microscope. In addition, impaired long-term potentiation has been demonstrated in OXYS rats by the age of 3 months. With age, neurodegenerative changes in the brain of OXYS rats become amplified. We have shown that this deterioration happens against the background of overproduction of amyloid precursor protein (AβPP), accumulation of β-amyloid (Aβ), and hyperphosphorylation of the tau protein in the hippocampus and cortex. The development of AMD-like retinopathy in OXYS rats is also accompanied by increased accumulation of Aβ in the retina. These published data suggest that the OXYS strain may serve as a spontaneous rat model of AD-like pathology and could help to decipher the pathogenesis of AD.

Changes in the frequencies of human hematopoietic stem and progenitor cells with age and site

This study enumerated CD45(hi)/CD34(+) and CD45(hi)/CD133(+) human hematopoietic stem cells (HSCs) and progenitor granulocyte-macrophage colony forming cells (GM-CFCs) in blood and trochanteric and femoral bone marrow in 233 individuals. Stem cell frequencies were determined with multiparameter flow cytometry and using an internal control to determine the intrinsic variance of the assays. Progenitor cell frequency was determined using a standard colony assay technique. The frequency of outliers from undetermined methodological causes was highest for blood, but less than 5% for all values. The frequency of CD45(hi)/CD133(+) cells correlated highly with the frequency of CD45(hi)/CD34(+) cells in trochanteric and femoral bone marrow. The frequency of these HSC populations in trochanteric and femoral bone marrow rose significantly with age. In contrast, there was no significant trend of either of these cell populations with age in the blood. Trochanteric marrow progenitor GM-CFCs showed no significant trends with age, but femoral marrow GM-CFCs trended downward with age, potentially because of the reported conversion of red marrow at this site to fat with age. Hematopoietic stem and progenitor cells exhibited changes in frequencies with age that differed between blood and bone marrow. We previously reported that side population (SP) multipotential HSC, which includes the precursors of CD45(hi)/CD133(+) and CD45(hi)/CD34(+), decline with age. Potentially the increases in stem cell frequencies in the intermediate compartment between SP and GM progenitor cells observed in this study represent a compensatory increase for the loss of more potent members of the HSC hierarchy.

The effect of age on thymic function

Age-related regression of the thymus is associated with a decline in naïve T cell output. This is thought to contribute to the reduction in T cell diversity seen in older individuals and linked with increased susceptibility to infection, autoimmune disease, and cancer. Thymic involution is one of the most dramatic and ubiquitous changes seen in the aging immune system, but the mechanisms which underlying this process are poorly understood. However, a picture is emerging, implicating the involvement of both extrinsic and intrinsic factors. In this review we assess the role of the thymic microenvironment as a potential target that regulates thymic involution, question whether thymocyte development in the aged thymus is functionally impaired, and explore the kinetics of thymic involution.

Treatment of invasive candidiasis in the elderly: a review

Fungi are major causes of infections among immunocompromised or hospitalized patients with serious underlying diseases and comorbidities. Candida species remain the most important cause of opportunistic infections worldwide, affecting predominantly patients over 65 years old, while they are considered to be the fourth most common cause of nosocomial bloodstream infections. The rapidly growing elderly population has specific physiological characteristics, which makes it susceptible to colonization and subsequent infection due to Candida species. Comorbidities and multidrug use should be taken into account any time the therapeutic regimen is under consideration. Different classes of antifungal drugs are available for the treatment of invasive fungal infections but echinocandins, apart from their activity against resistant strains (Candida glabrata and Candida krusei), seem to be safe, with limited adverse events and minimal drug-drug interactions in comparison to the other regimens. Therefore, these agents are strongly recommended when dealing with elderly patients suffering from an invasive form of Candida infection.

[Candida infections in the elderly]

Candida infections of the skin, mucous membranes as well as of internal organs may be more frequent and more serious in the elderly. There are several biological reasons for this, for example, alterations in the immune system. Whereas Candida albicans remains the major pathogen, there has been a relative increase of Candida glabrata infections. This species is associated with higher mortality. Furthermore, C. glabrata is in general less susceptible to fluconazole, so that this drug does not represent the agent of first choice for the treatment of yeast infections. The choice of the antimycotic agent must take into consideration their inherent side effects and interaction profiles; echinocandins play a particular role in the treatment of yeast infections in the elderly. These drugs have low toxicities, low potential for interactions with co-medications, as well as broad and good activities against yeasts.

Age-Related Disruption of Steady-State Thymic Medulla Provokes Autoimmune Phenotype via Perturbing Negative Selection

The hymic medulla plays an essential role in the generation of central tolerance by eliminating self-reactive T-cell clones through thymic negative selection and developing natural regulatory T cells. Age-related FoxN1 decline induces disruption of medullary thymic epithelial cells (mTECs). However, it is unknown whether this perturbs central tolerance to increase autoimmune predisposition in the elderly. Using a loxP-floxed-FoxN1 (FoxN1(flox)) mouse model, which exhibits a spontaneous ubiquitous deletion of FoxN1 with age to accelerate thymic aging, we investigated whether disruption of steady-state thymic medulla results in an increase of autoimmune-prone associated with age. We demonstrated age-associated ubiquitous loss of FoxN1(flox)-formed two-dimensional thymic epithelial cysts were primarily located in the medulla. This resulted in disruption of thymic medullary steady state, with evidence of perturbed negative selection, including reduced expression of the autoimmune regulator (Aire) gene and disrupted accumulation of thymic dendritic cells in the medulla, which are required for negative selection. These provoke autoimmune phenotypes, including increased inflammatory cell infiltration in multiple organs in these mice. This finding in an animal model provides a mechanistic explanation of increased susceptibility to autoimmunity in aged humans, although they may not show clinic manifestations without induction.

The origin and implication of thymic involution

Age-related regression of the thymus is associated with a decline in naïve T cell output which is thought to contribute to the reduction in T cell diversity in older individuals that is partially responsible for an increase in susceptibility and severity of infections, cancers and autoimmune diseases. Thymic involution is one of the most dramatic and ubiquitous changes in the ageing immune system, but the precise regulators remain anonymous. However, a picture is emerging, implicating extrinsic and intrinsic factors that may contribute towards age-associated thymic involution. In this review we assess the role of the thymic microenvironment as a possible target of thymic involution, question whether thymocyte development in the aged thymus is functional and explore why the thymus involutes.

It's not all equal: a multiphasic theory of thymic involution

Regression of the thymus is a key feature of immunosenescence, which coincides with a decrease in T cell output and contributes to the restriction of the T cell repertoire in the elderly, leading to increased susceptibility to illness and disease. However, the mechanisms involved in thymic involution are still not fully known. Although, it is often believed that thymic involution occurs during the onset of puberty, increasing data suggests alterations to the thymus happen much earlier in life. Therefore, the changes in the thymus and subsequent thymic function may not just be an ageing phenomenon. In this article, we propose that there are several, non-linear, phases to thymic atrophy, which are regulated by different mechanisms, including the familiar age-dependent thymic involution and a much earlier growth-dependent thymic involution.

The effect of age on the phenotype and function of developing thymocytes

The immune system declines with age leading to a progressive deterioration in the ability to respond to infection and vaccination. Age-associated thymic involution is one of the most recognized changes in the ageing immune system and is believed to be a major contributor towards immunosenescence; however, the precise mechanisms involved in age-associated thymic involution remain unclear. In order to gain further insight into the effect of ageing on T-cell development, steady-state thymopoiesis was studied in mice ranging from 1 to 18 months of age. There was a decrease in thymic cellularity with age, but the most dramatic loss occurred early in life. Although there were no alterations in the proportion of the major thymocyte subsets, there was a significant decline in the expression of other key molecules including CD3 and CD24. There was a decline in the ability of thymocytes from older mice to respond to mitogens, which was demonstrated by a failure to up-regulate expression of the activation marker CD69 and to enter the G(2)--M phase of the cell cycle. This was concurrent with an increased resistance to apoptosis in thymocytes from aged animals. Together, these results suggest that T cells may be flawed even before exiting to the periphery and that this could contribute to the age-associated decline in immune function.

An evolutionary perspective on the mechanisms of immunosenescence

There is an accumulating body of evidence that a decline in immune function with age is common to most if not all vertebrates. For instance, age-associated thymic involution seems to occur in all species that possess a thymus, indicating that this process is evolutionary ancient and conserved. The precise mechanisms regulating immunosenescence remain to be resolved, but much of what we do know is consistent with modern evolutionary theory. In this review, we assess our current knowledge from an evolutionary perspective on the occurrence of immunosenescence, we show that life history trade-offs play a key role and we highlight the possible advantages of the age-related decline in thymic function.

Mitochondria-targeted antioxidant SkQ1 inhibits age-dependent involution of the thymus in normal and senescence-prone rats

One of the most striking changes during mammal aging is a progressive involution of the thymus, associated with an increase in susceptibility to infections, autoimmune diseases and cancer. In order to delay age-related processes, we have developed mitochondria-targeted antioxidant plastoquinonyl decyltriphenyl phosphonium (SkQ1). Here we report that, at low doses, SkQ1 (250 nmol/kg per day) inhibited age-dependent involution of the thymus in normal (Wistar) and senescence-prone (OXYS) rats. SkQ1 preserved total weight and volume of the organ, the volume of thymic cortex and medulla, the thymic cellularity, and the number of CD3+, CD4+, and CD8+ cells in the thymus. Moreover, SkQ1 was especially effective in senescence-prone rats.   Thus SkQ1 slows down age-linked decline of the immune system, explaining prevention by this compound of infection-caused death in rodents, previously described in our group.