The effects of age, thymectomy, and HIV Infection on alpha and beta TCR excision circles in naive T cells

Exploiting Signal Joint T Cell Receptor Excision Circle to Investigate the Impact of COVID-19 and Autoimmune Diseases on Age Prediction and Immunosenescence

Signal joint T cell receptor excision circles (sjTRECs) are a promising marker for age estimation and immunosenescence in different ethnic groups. Several limitations are expected to overshadow their use as accurate markers for age prediction. The current study was conducted to determine the influence of immunologic disorders, such as autoimmune diseases and COVID-19, on the accuracy of sjTRECs as molecular markers for age estimation and immunosenescence among living Egyptians. Peripheral blood sjTRECs level was measured by qPCR in 90 autoimmune patients, 58 COVID-19 patients, and 85 healthy controls. The mean dCt values were significantly (p = 0.0002) different between the three groups, with the highest values in healthy subjects, followed by autoimmune and COVID-19 patients. A significant negative correlation was identified between the sjTRECs levels and ages in all studied cases. There were significant positive correlations between chronological age and predicted age for healthy individuals, autoimmune, and COVID-19 patients with mean absolute deviations (MAD) of 9.40, 11.04, and 9.71, respectively. The two patients' groups exhibited early immunosenescence, which was more noticeable among the young adults with COVID-19 and autoimmune patients of age range (18-49 years). Autoimmunity may represent a critical factor impacting the accuracy of sjTRECs quantitation for age prediction.

Adipose tissue, immune aging, and cellular senescence

Obesity represents a serious health problem as it is rapidly increasing worldwide. Obesity is associated with reduced healthspan and lifespan, decreased responses to infections and vaccination, and increased frequency of inflammatory conditions typical of old age. Obesity is characterized by increased fat mass and remodeling of the adipose tissue (AT). In this review, we summarize published data on the different types of AT present in mice and humans, and their roles as fat storage as well as endocrine and immune tissues. We review the age-induced changes, including those in the distribution of fat in the body, in abundance and function of adipocytes and their precursors, and in the infiltration of immune cells from the peripheral blood. We also show that cells with a senescent-associated secretory phenotype accumulate in the AT of mice and humans with age, where they secrete several factors involved in the establishment and maintenance of local inflammation, oxidative stress, cell death, tissue remodeling, and infiltration of pro-inflammatory immune cells. Not only adipocytes and pre-adipocytes but also immune cells show a senescent phenotype in the AT. With the increase in human lifespan, it is crucial to identify strategies of intervention and target senescent cells in the AT to reduce local and systemic inflammation and the development of age-associated diseases. Several studies have indeed shown that senescent cells can be effectively targeted in the AT by selectively removing them or by inhibiting the pathways that lead to the secretion of pro-inflammatory factors.

Thymic involution and rising disease incidence with age

For many cancer types, incidence rises rapidly with age as an apparent power law, supporting the idea that cancer is caused by a gradual accumulation of genetic mutations. Similarly, the incidence of many infectious diseases strongly increases with age. Here, combining data from immunology and epidemiology, we show that many of these dramatic age-related increases in incidence can be modeled based on immune system decline, rather than mutation accumulation. In humans, the thymus atrophies from infancy, resulting in an exponential decline in T cell production with a half-life of ∼16 years, which we use as the basis for a minimal mathematical model of disease incidence. Our model outperforms the power law model with the same number of fitting parameters in describing cancer incidence data across a wide spectrum of different cancers, and provides excellent fits to infectious disease data. This framework provides mechanistic insight into cancer emergence, suggesting that age-related decline in T cell output is a major risk factor.

Basic science and pathogenesis of ageing with HIV: potential mechanisms and biomarkers

: The increased prevalence of age-related comorbidities and mortality is worrisome in ageing HIV-infected patients. Here, we aim to analyse the different ageing mechanisms with regard to HIV infection. Ageing results from the time-dependent accumulation of random cellular damage. Epigenetic modifications and mitochondrial DNA haplogroups modulate ageing. In antiretroviral treatment-controlled patients, epigenetic clock appears to be advanced, and some haplogroups are associated with HIV infection severity. Telomere shortening is enhanced in HIV-infected patients because of HIV and some nucleoside analogue reverse transcriptase inhibitors. Mitochondria-related oxidative stress and mitochondrial DNA mutations are increased during ageing and also by some nucleoside analogue reverse transcriptase inhibitors. Overall, increased inflammation or 'inflammageing' is a major driver of ageing and could result from cell senescence with secreted proinflammatory mediators, altered gut microbiota, and coinfections. In HIV-infected patients, the level of inflammation and innate immunity activation is enhanced and related to most comorbidities and to mortality. This status could result, in addition to age, from the virus itself or viral protein released from reservoirs, from HIV-enhanced gut permeability and dysbiosis, from antiretroviral treatment, from frequent cytomegalovirus and hepatitis C virus coinfections, and also from personal and environmental factors, as central fat accumulation or smoking. Adaptive immune activation and immunosenescence are associated with comorbidities and mortality in the general population but are less predictive in HIV-infected patients. Biomarkers to evaluate ageing in HIV-infected patients are required. Numerous systemic or cellular inflammatory, immune activation, oxidative stress, or senescence markers can be tested in serum or peripheral blood mononuclear cells. The novel European Study to Establish Biomarkers of Human Ageing MARK-AGE algorithm, evaluating the biological age, is currently assessed in HIV-infected patients and reveals an advanced biological age. Some enhanced inflammatory or innate immune activation markers are interesting but still not validated for the patient's follow-up. To be able to assess patients' biological age is an important objective to improve their healthspan.

Mathematical Models for Immunology: Current State of the Art and Future Research Directions

The advances in genetics and biochemistry that have taken place over the last 10 years led to significant advances in experimental and clinical immunology. In turn, this has led to the development of new mathematical models to investigate qualitatively and quantitatively various open questions in immunology. In this study we present a review of some research areas in mathematical immunology that evolved over the last 10 years. To this end, we take a step-by-step approach in discussing a range of models derived to study the dynamics of both the innate and immune responses at the molecular, cellular and tissue scales. To emphasise the use of mathematics in modelling in this area, we also review some of the mathematical tools used to investigate these models. Finally, we discuss some future trends in both experimental immunology and mathematical immunology for the upcoming years.

Mathematics in modern immunology

Mathematical and statistical methods enable multidisciplinary approaches that catalyse discovery. Together with experimental methods, they identify key hypotheses, define measurable observables and reconcile disparate results. We collect a representative sample of studies in T-cell biology that illustrate the benefits of modelling–experimental collaborations and that have proven valuable or even groundbreaking. We conclude that it is possible to find excellent examples of synergy between mathematical modelling and experiment in immunology, which have brought significant insight that would not be available without these collaborations, but that much remains to be discovered.

Quantifying T lymphocyte turnover

Peripheral T cell populations are maintained by production of naive T cells in the thymus, clonal expansion of activated cells, cellular self-renewal (or homeostatic proliferation), and density dependent cell life spans. A variety of experimental techniques have been employed to quantify the relative contributions of these processes. In modern studies lymphocytes are typically labeled with 5-bromo-2'-deoxyuridine (BrdU), deuterium, or the fluorescent dye carboxy-fluorescein diacetate succinimidyl ester (CFSE), their division history has been studied by monitoring telomere shortening and the dilution of T cell receptor excision circles (TRECs) or the dye CFSE, and clonal expansion has been documented by recording changes in the population densities of antigen specific cells. Proper interpretation of such data in terms of the underlying rates of T cell production, division, and death has proven to be notoriously difficult and involves mathematical modeling. We review the various models that have been developed for each of these techniques, discuss which models seem most appropriate for what type of data, reveal open problems that require better models, and pinpoint how the assumptions underlying a mathematical model may influence the interpretation of data. Elaborating various successful cases where modeling has delivered new insights in T cell population dynamics, this review provides quantitative estimates of several processes involved in the maintenance of naive and memory, CD4(+) and CD8(+) T cell pools in mice and men.

Homeostasis of lymphocytes and monocytes in frequent blood donors

Age-associated decline of immune function is believed to be mainly due to alterations of immune cells. However, longitudinal changes of human immune cells with age have not yet been adequately addressed. To test the hypothesis that regeneration of lymphocytes and monocytes is robust throughout most of adult life until advanced age, we examined six leukapheresis donors (3 young and 3 middle-aged/old) who donated approximately 10% of their peripheral blood mononuclear cells (PBMC) every other month over 3–5 years. We found the number of both lymphocytes and monocytes were quite stable in the blood of all six donors. As expected, young donors had more T cell receptor excision circles (TRECs), CD31{}⁺ cells (CD4 only) and longer telomeres in T cells than did the middle-aged donors. Interestingly, more variation in TREC number, Vβ usages, and telomere lengths were observed in young donors during the 3–5 years course of donation whereas the middle-aged/old donors showed a rather striking stability in these measurements. This may reflect a more prominent role of thymic output in T cell regeneration in young than in middle-aged/old donors. Together, these findings provide an in vivo glimpse into the homeostasis of lymphocytes and monocytes in the blood at different ages, and support the notion that regeneration of lymphocytes and monocytes is robust throughout adult life up to the early 70s.

Signal joint T-cell receptor excision circle assay in miniature swine

OBJECTIVE

Assays for T cell receptor excision circles (TREC) have been utilized in human, primate, and mouse models as a measure of thymic activity, but no comparable assay has been described in artiodactyls. We describe the development of the porcine signal joint (sj) TREC assay, and provide a likely reason for previous difficulties in its identification in artiodactyls.

DESIGN AND METHODS

Utilizing the homology between the known genomic sequences in sjTREC in human and mouse, polymerase chain reaction (PCR) primers were derived for the putative porcine sjTREC. Primers from the ψJα side of the sjTREC were derived from the known porcine sequence.

RESULTS

The sjTREC in two artiodactyls, swine and sheep, was identified using forward primers from the ψJα region, and reverse primers from the putative δ-rec region. Unlike in the detection of primate TRECs, initially the use of similar primers close to the δ-rec failed to yield the sjTREC product. Marching about 800 basepairs into δ-rec, primers derived from a homology region between human and mouse led to the detection of sjTREC. Comparing sjTREC amongst the species revealed highest homology between the two artiodactyls. A quantitative PCR (QPCR) assay of porcine sjTREC was also developed.

CONCLUSION

Identification and analysis of the sjTREC sequences in two artiodactyls suggested why previous attempts at cloning the pig TREC using known sjTREC sequences were unsuccessful. The development of the porcine signal joint TREC assay should enable a more direct quantification of thymic activity in porcine models of transplant biology.

Inhibition of transplantation tolerance by immune senescence is reversed by endocrine modulation

The senescent immune system responds poorly to new stimuli; thymic involution, accumulation of memory cells against other specificities, and general refractoriness to antigen signaling all may contribute to poor resistance to infection. These same changes may pose a significant clinical barrier to organ transplantation, as transplantation tolerance requires thymic participation and integrated, tolerance-promoting responses to novel antigens. We found that after the age of 12 months, mice became resistant to the tolerance-inducing capacity of the monoclonal antibody therapy anti-CD45RB. This resistance to tolerance to cardiac allografts could be overcome by surgical castration of male mice, a procedure that led to thymic regeneration and long-term graft acceptance. The potential for clinical translation of this endocrine-immune interplay was confirmed by the ability of Lupron Depot injections, which temporarily disrupt gonadal function, to restore tolerance in aged mice. Furthermore, we demonstrated that the restoration of tolerance after surgical or chemical castration depended on thymic production of regulatory T cells (T(regs)); thymectomy or T(reg) depletion abrogated tolerance restoration. The aging of the immune system ("immune senescence") is a significant barrier to immune tolerance, but this barrier can be overcome by targeting sex steroid production with commonly used clinical therapeutics.

Immune aging and rheumatoid arthritis

Immunologic models of rheumatoid arthritis (RA) have to take into account that the disease occurs at an age when immunocompetence is declining and in a host whose immune system shows evidence of accelerated immune aging. By several immune aging biomarkers, the immune system in patients with RA is prematurely aged by more than 20 years. One major pathogenetic mechanism is a defect in telomere maintenance and DNA repair that causes accelerated cell death. These findings in RA are reminiscent of murine autoimmunity models, in which lymphopenia was identified as a major risk factor for autoimmunity. Progress in the understanding of how accelerated immune aging is pathogenetically involved in RA may allow development of new therapeutic approaches that go beyond the use of anti-inflammatory agents and eventually could open new avenues for preventive intervention.

Thymic involution and proliferative T-cell responses in multiple sclerosis

We investigated naïve CD4 T-cell homeostasis in relapsing-remitting multiple sclerosis (RRMS). Quantification of signal joint T-cell receptor excision circles in FACS-isolated CD31hi cells, which correspond closely to CD4 recent thymic emigrants (RTEs), indicates that young patients have reduced generation of CD4 RTEs compared to age-matched controls. In RRMS, compared to controls, CXCR4 analyses indicate age-associated thymic output of progressively immature CD4 RTEs, and Ki-67 data demonstrate altered T-cell proliferative responses that fail to maintain naïve CD4 T-cell numbers with age. Thus, RRMS patients have early thymic involution with compensatory homeostatic peripheral T-cell proliferative responses that may predispose patients to autoreactivity.

Thymectomy in early childhood: a model for premature T cell immunosenescence?

The thymus is the main source of recent thymic emigrants (RTE) and naïve T cells. The aging of the immune system (immunosenescence) is characterized by loss of thymic function, decreased numbers of RTE, peripheral proliferation of mature T cells, and oligoclonal expansions of specific T cell subpopulations. As shown in several studies, thymectomized patients demonstrate signs of premature immunosenescence reminiscent of aged people, such as decreased proportions of naïve T cells and RTE, a compensatory increase of mature T cell subpopulations with increased proliferation rates, restriction of the T cell receptor repertoire, and a delayed response to new antigens and vaccinations. This review demonstrates that, despite some limitations, childhood thymectomy may serve as an useful model for premature immunosenescence, mimicking changes expected after physiological thymus involution in the elderly. Thus, it may prove an insightful tool for obtaining better understanding of human naïve T cell development, thymic function, and maintenance of the naïve T cell pool.

Developments in the scientific understanding of rheumatoid arthritis

Rheumatoid arthritis (RA) is recognized to be an autoimmune disease that causes preclinical systemic abnormalities and eventually leads to synovial inflammation and destruction of the joint architecture. Recently identified genetic risk factors and novel insights from animal models of spontaneous arthritis have lent support to the concept that thymic selection of an autoreactive T-cell repertoire is an important risk factor for this disease. With advancing age, defects in the homeostatic control of the T-cell pool and in the setting of signaling thresholds lead to the accumulation of pro-inflammatory T-effector cell populations and loss of tolerance to neo-antigens, such as citrullinated peptides. As the breakdown of tolerance to modified self-antigens can precede synovitis by decades, repair of homeostatic defects may open a unique window of opportunity for preventive interventions in RA. The end result of RA, destruction of cartilage and bone, appears to be driven by cytokine- and cell contact-induced activation of synoviocytes and monocytic cells, some of which differentiate into tissue-destructive osteoclasts. Targeting mediators involved in this process has greatly improved the management of this chronic inflammatory syndrome.

Axin expression in thymic stromal cells contributes to an age-related increase in thymic adiposity and is associated with reduced thymopoiesis independently of ghrelin signaling

The adipocytes are the predominant cell types that constitute the bulk of the thymic microenvironment by the fifth decade of life in healthy humans. An age-related increase in thymic adiposity is associated with reduced thymopoiesis and compromised immune surveillance in the elderly. However, the mechanisms regulating the generation of intrathymic adipocytes during aging remain to be elucidated. Here, we report that the CD45- thymic stromal cells (TSCs) are amenable to adipogenesis. We identified that the Wnt inhibitor axin is expressed in the lymphoid as well as stromal cells of the thymus with increased expression in CD45- TSCs of older mice. Knockdown of axin by RNA interference in CD45- primary TSCs led to a marked reduction in adipogenesis with significantly lower expression of adipogenic transcripts peroxisome proliferator-activated receptor 2 (PPAR), adipocyte fatty acid-binding protein (aP2), and perilipin. Age-related elevated axin expression was increased specifically in thymic fibroblasts and medullary thymic epithelial cells (TECs) but not in the cortical TEC or CD45+ cells. Consistent with a role of axin in promoting thymic adipogenesis, axin expression was also colocalized with lipid-expressing adipogenic cells in aging thymus. The prolongevity intervention, caloric restriction (CR), prevented the age-related increase in axin and the adipogenic cell in the thymus together with increase in thymic output. We have recently demonstrated that CR induces ghrelin, which can partially reverse thymic involution. Here, we show that axin expression is not affected by ablation of ghrelin receptors in aging mice, suggesting a ghrelin-independent mechanism for regulation of axin. Our data are consistent with the hypothesis that blocking the specific proadipogenic signals in the thymus may complement the present approaches to rejuvenate thymic function during aging.

Impact of growth hormone (GH) deficiency and GH replacement upon thymus function in adult patients

BACKGROUND

Despite age-related adipose involution, T cell generation in the thymus (thymopoiesis) is maintained beyond puberty in adults. In rodents, growth hormone (GH), insulin-like growth factor-1 (IGF-1), and GH secretagogues reverse age-related changes in thymus cytoarchitecture and increase thymopoiesis. GH administration also enhances thymic mass and function in HIV-infected patients. Until now, thymic function has not been investigated in adult GH deficiency (AGHD). The objective of this clinical study was to evaluate thymic function in AGHD, as well as the repercussion upon thymopoiesis of GH treatment for restoration of GH/IGF-1 physiological levels.

METHODOLOGY/PRINCIPAL FINDINGS

Twenty-two patients with documented AGHD were enrolled in this study. The following parameters were measured: plasma IGF-1 concentrations, signal-joint T-cell receptor excision circle (sjTREC) frequency, and sj/beta TREC ratio. Analyses were performed at three time points: firstly on GH treatment at maintenance dose, secondly one month after GH withdrawal, and thirdly one month after GH resumption. After 1-month interruption of GH treatment, both plasma IGF-1 concentrations and sjTREC frequency were decreased (p<0.001). Decreases in IGF-1 and sjTREC levels were correlated (r = 0.61, p<0.01). There was also a decrease in intrathymic T cell proliferation as indicated by the reduced sj/beta TREC ratio (p<0.01). One month after reintroduction of GH treatment, IGF-1 concentration and sjTREC frequency regained a level equivalent to the one before GH withdrawal. The sj/beta TREC ratio also increased with GH resumption, but did not return to the level measured before GH withdrawal.

CONCLUSIONS

In patients with AGHD under GH treatment, GH withdrawal decreases thymic T cell output, as well as intrathymic T cell proliferation. These parameters of thymus function are completely or partially restored one month after GH resumption. These data indicate that the functional integrity of the somatotrope GH/IGF-1 axis is important for the maintenance of a normal thymus function in human adults.

TRIAL REGISTRATION

ClinicalTrials.gov NTC00601419.

The magnitude of thymic output is genetically determined through controlled intrathymic precursor T cell proliferation

The thymus plays a crucial role in providing the immune system with naive T cells showing a diverse TCR repertoire. Whereas the diversity of thymic production is mainly ensured by TCR rearrangement at both the TRA and TRB loci, the number of cells reaching the double-positive differentiation stage defines the extent of thymic output. A quantitative analysis of TCR excision circles (TREC; signal-joint TRECs and DJbetaTRECs) produced at different stages of thymopoiesis was performed in nine laboratory mouse strains. The results clearly demonstrate that the magnitude of thymic output is directly proportional to the extent of proliferation in the double-negative 4 thymocyte subset. Strikingly, intrathymic precursor T cell proliferation was found to be strain dependent, thus suggesting a genetic regulation of thymic output. The inherited character of thymic output was further confirmed by the transmission of the phenotype in a recessive fashion in F(1) progeny of the different parental strains. Our results provide the first demonstration of the genetic regulation of thymic output.

Thymectomy in early childhood: significant alterations of the CD4(+)CD45RA(+)CD62L(+) T cell compartment in later life

The study was aimed to assess indicators of immunosenescence, such as the total counts of peripheral blood CD4(+)CD45RA(+)CD62L(+) (naive) T cells, the numbers of T cell receptor excision circles (TRECs), and Ki67-expression as marker of peripheral replication in thymectomized patients (TP) (n=101) compared to age-matched healthy donors (HD) (n=81). In TP, there was an inverse correlation between naive T cells and chronological age (p<0.001) or time post thymectomy (p<0.001). TP demonstrated lower TREC numbers in naive T cells compared to HD (p<0.001). TREC numbers negatively correlated with time post thymectomy (p<0.001). Percentages of Ki67-expresssing naive T cells were higher in TP compared to HD (p<0.05). The findings of the presented long-term follow up cohort of thymectomized patients indicate that changes of the peripheral naive T cell subset in TP may resemble the findings of an aging immune system in elderly persons after thymic involution. Our data provide evidence that peripheral T cell homeostasis in TP is maintained at minimal levels mainly by extrathymic expansion of existing naive T cells in the periphery to compensate the diminished thymic output.

The role of sex steroids and gonadectomy in the control of thymic involution

A major underlying cause for aging of the immune system is the structural and functional atrophy of the thymus, and associated decline in T cell genesis. This loss of naïve T cells reduces adaptive immunity to new stimuli and precipitates a peripheral bias to memory cells against prior antigens. Whilst multiple mechanisms may contribute to this process, the temporal alliance of thymic decline with puberty has implicated a causative role for sex steroids. Accordingly ablation of sex steroids induces profound thymic rejuvenation. Although the thymus retains some, albeit highly limited, function in healthy adults, this is insufficient for resurrecting the T cell pool following cytoablative treatments such as chemo- and radiation-therapy and AIDS. Increased risk of opportunistic infections and cancer relapse or appearance, are a direct consequence. Temporary sex steroid ablation may thus provide a clinically effective means to regenerate the thymus and immune system in immunodeficiency states.

Efficient thymopoiesis contributes to the maintenance of peripheral CD4 T cells during chronic human immunodeficiency virus type 2 infection

Human immunodeficiency virus type 2 (HIV-2) infection leads to a lifelong asymptomatic period in the majority of patients. Even in patients with progressive disease, a slow CD4 count decline characterizes the chronic phase of HIV-2 infection, suggesting that peripheral T-cell homeostasis is controlled better following HIV-2 infection than following HIV-1 infection. Herein we showed that, in contrast to HIV-1-infected patients, HIV-2-infected patients demonstrate enhanced thymic function compared to age-matched healthy individuals. The correlation between higher thymic production and lower CD4 T-cell loss in these patients suggests that efficient thymopoiesis is implicated in the long-lasting maintenance of CD4 T-cell counts in HIV-2 disease.