T cell receptor gene deletion circles identify recent thymic emigrants in the peripheral T cell pool

Primary and secondary defects of the thymus

The thymus is the primary site of T-cell development, enabling generation, and selection of a diverse repertoire of T cells that recognize non-self, whilst remaining tolerant to self- antigens. Severe congenital disorders of thymic development (athymia) can be fatal if left untreated due to infections, and thymic tissue implantation is the only cure. While newborn screening for severe combined immune deficiency has allowed improved detection at birth of congenital athymia, thymic disorders acquired later in life are still underrecognized and assessing the quality of thymic function in such conditions remains a challenge. The thymus is sensitive to injury elicited from a variety of endogenous and exogenous factors, and its self-renewal capacity decreases with age. Secondary and age-related forms of thymic dysfunction may lead to an increased risk of infections, malignancy, and autoimmunity. Promising results have been obtained in preclinical models and clinical trials upon administration of soluble factors promoting thymic regeneration, but to date no therapy is approved for clinical use. In this review we provide a background on thymus development, function, and age-related involution. We discuss disease mechanisms, diagnostic, and therapeutic approaches for primary and secondary thymic defects.

Complete fatty degeneration of thymus associates with male sex, obesity and loss of circulating naïve CD8+ T cells in a Swedish middle-aged population

BACKGROUND

Fatty degeneration of thymus (or thymus involution) has long been considered a normal ageing process. However, there is emerging evidence that thymic involution is linked to T cell aging, chronic inflammation and increased morbidity. Other factors, aside from chronological age, have been proposed to affect the involution rate. In the present study, we investigated the imaging characteristics of thymus on computed tomography (CT) in a Swedish middle-aged population. The major aims were to establish the prevalence of fatty degeneration of thymus and to determine its associations with demographic, lifestyle and clinical factors, as well as inflammation, T cell differentiation and thymic output.

RESULTS

In total, 1 048 randomly invited individuals (aged 50-64 years, 49% females) were included and thoroughly characterized. CT evaluation of thymus included measurements of attenuation, size and a 4-point scoring system, with scale 0-3 based on the ratio of fat and soft tissue. A majority, 615 (59%) showed complete fatty degeneration, 259 (25%) predominantly fatty attenuation, 105 (10%) half fatty and half soft-tissue attenuation, while 69 (6.6%) presented with a solid thymic gland with predominantly soft-tissue attenuation. Age, male sex, high BMI, abdominal obesity and low dietary intake of fiber were independently associated with complete fatty degeneration of thymus. Also, fatty degeneration of thymus as well as low CT attenuation values were independently related to lower proportion of naïve CD8+ T cells, which in turn was related to lower thymic output, assessed by T-cell receptor excision circle (TREC) levels.

CONCLUSION

Among Swedish middle-aged subjects, nearly two-thirds showed complete fatty degeneration of thymus on CT. This was linked to depletion of naïve CD8+ T cells indicating that CT scans of thymus might be used to estimate immunological aging. Furthermore, our findings support the intriguing concept that obesity as well as low fiber intake contribute to immunological aging, thereby raising the possibility of preventive strategies.

Characteristic differences in the abundance of tumor-infiltrating lymphocytes and intratumoral developing T cells in thymoma, with special reference to PD-1 expression

PURPOSE

Though programmed cell death-1 (PD-1) inhibitors mainly target tumor-infiltrating lymphocytes (TILs) expressing PD-1, developing T cells in thymus also express PD-1 in their process of maturation. To predict the therapeutic effect of PD-1 inhibitors for thymoma, it is necessary to clarify the proportions of TILs and intratumoral developing T cells.

METHODS

The expressions of CD4, CD8, and PD-1 on T cells were analyzed by flow cytometry in 31 thymomas. The amount of T cell receptor excision circles (TRECs), which can be detected in newly formed naïve T cells in the thymus, was evaluated using sorted lymphocytes from thymomas by quantitative PCR. The expressions of granzyme B (GZMB) and lymphocyte activation gene-3 (LAG-3) in PD-1 + CD8 T cells were analyzed by image cytometry using multiplex immunohistochemistry.

RESULTS

The PD-1 + rate in both CD4 and CD8 T cells was significantly higher in type AB/B1/B2 than in type A/B3 thymomas. The amounts of TRECs in CD4 and CD8 T cells were significantly higher in type AB/B1/B2 than in type A/B3 thymomas and comparable to normal thymus. PD-1 expression at each stage of T cell development of type AB/B1/B2 thymomas was comparable to that of normal thymus. Both the percentages and cell densities of PD-1 + CD8 T cells expressing GZMB or LAG-3, which are known to contain tumor-reactive T cells, were significantly lower in type AB/B1/B2 thymomas.

CONCLUSION

Most PD-1 + T cells in type AB/B1/B2 thymomas are intratumoral developing T cells and are not TILs.

Clinical Practice Guidelines for the Immunological Management of Chromosome 22q11.2 Deletion Syndrome and Other Defects in Thymic Development

Current practices vary widely regarding the immunological work-up and management of patients affected with defects in thymic development (DTD), which include chromosome 22q11.2 microdeletion syndrome (22q11.2del) and other causes of DiGeorge syndrome (DGS) and coloboma, heart defect, atresia choanae, retardation of growth and development, genital hypoplasia, ear anomalies/deafness (CHARGE) syndrome. Practice variations affect the initial and subsequent assessment of immune function, the terminology used to describe the condition and immune status, the accepted criteria for recommending live vaccines, and how often follow-up is needed based on the degree of immune compromise. The lack of consensus and widely varying practices highlight the need to establish updated immunological clinical practice guidelines. These guideline recommendations provide a comprehensive review for immunologists and other clinicians who manage immune aspects of this group of disorders.

Blood levels of T-Cell Receptor Excision Circles (TRECs) provide an index of exposure to traumatic stress in mice and humans

Exposure to stress triggers biological changes throughout the body. Accumulating evidence indicates that alterations in immune system function are associated with the development of stress-associated illnesses such as major depressive disorder and post-traumatic stress disorder, increasing interest in identifying immune markers that provide insight into mental health. Recombination events during T-cell receptor rearrangement and T-cell maturation in the thymus produce circular DNA fragments called T-cell receptor excision circles (TRECs) that can be utilized as indicators of thymic function and numbers of newly emigrating T-cells. Given data suggesting that stress affects thymus function, we examined whether blood levels of TRECs might serve as a quantitative peripheral index of cumulative stress exposure and its physiological correlates. We hypothesized that chronic stress exposure would compromise thymus function and produce corresponding decreases in levels of TRECs. In male mice, exposure to chronic social defeat stress (CSDS) produced thymic involution, adrenal hypertrophy, and decreased levels of TRECs in blood. Extending these studies to humans revealed robust inverse correlations between levels of circulating TRECs and childhood emotional and physical abuse. Cell-type specific analyses also revealed associations between TREC levels and blood cell composition, as well as cell-type specific methylation changes in CD4T + and CD8T + cells. Additionally, TREC levels correlated with epigenetic age acceleration, a common biomarker of stress exposure. Our findings demonstrate alignment between findings in mice and humans and suggest that blood-borne TRECs are a translationally-relevant biomarker that correlates with, and provides insight into, the cumulative physiological and immune-related impacts of stress exposure in mammals.

Analysis of the TREC and KREC Levels in the Dried Blood Spots of Healthy Newborns with Different Gestational Ages and Weights

Inborn errors of immunity can be detected by evaluating circular DNA (cDNA) fragments of T- and B-cell receptors (TREC and KREC) resulting from the receptor gene rearrangement in T and B cells. Maturation and activation of the fetal immune system is known to proceed gradually according to the gestational age, which highlights the importance of the immune status in premature infants at different gestational ages. In this article, we evaluated TREC and KREC levels in infants of various gestational ages by real-time PCR with taking into account the newborn’s weight and sex. The 95% confidence intervals for TREC and KREC levels (expressed in the number of cDNA copies per 105 cells) were established for different gestational groups. The importance of studying immune system development in newborns is informed by the discovered dependence of the level of naive markers on the gestational stage in the early neonatal period.

Early Thymectomy Is Associated With Long-Term Impairment of the Immune System: A Systematic Review

Background and Aims

Congenital heart diseases (CHDs) are diagnosed in approximately 9 in 1,000 newborns, and early cardiac corrective surgery often requires partial or complete thymectomy. As the long-term effect of early thymectomy on the subsequent development of the immune system in humans has not been completely elucidated, the present study aimed to evaluate the effects of thymus removal on the functional capacity of the immune system after different periods.

Methods

A systematic review of the literature was performed using MEDLINE, EMBASE, LILACS and Scopus. The inclusion criteria were original studies that analyzed any component of the immune system in patients with CHD who had undergone thymectomy during cardiac surgery in the first years of life. The results were evaluated for the quality of evidence.

Results

Twenty-three studies were selected and showed that patients who underwent a thymectomy in the first years of life tended to exhibit important alterations in the T cell compartment, such as fewer total T cells, CD4+, CD8+, naïve and CD31+ T cells, lower TRECs, decreased diversity of the TCR repertoire and higher peripheral proliferation (increased Ki-67 expression) than controls. However, the numbers of memory T cells and Treg cells differed across the selected studies.

Conclusions

Early thymectomy, either partial or complete, may be associated with a reduction in many T cell subpopulations and TCR diversity, and these alterations may persist during long-term follow-up. Alternative solutions should be studied, either in the operative technique with partial preservation of the thymus or through the autograft of fragments of the gland.

Systematic Review Registration

Prospero [157188].

Congenital Athymia: Genetic Etiologies, Clinical Manifestations, Diagnosis, and Treatment

Congenital athymia is an ultra-rare disease characterized by the absence of a functioning thymus. It is associated with several genetic and syndromic disorders including FOXN1 deficiency, 22q11.2 deletion, CHARGE Syndrome (Coloboma, Heart defects, Atresia of the nasal choanae, Retardation of growth and development, Genitourinary anomalies, and Ear anomalies), and Complete DiGeorge Syndrome. Congenital athymia can result from defects in genes that impact thymic organ development such as FOXN1 and PAX1 or from genes that are involved in development of the entire midline region, such as TBX1 within the 22q11.2 region, CHD7, and FOXI3. Patients with congenital athymia have profound immunodeficiency, increased susceptibility to infections, and frequently, autologous graft-versus-host disease (GVHD). Athymic patients often present with absent T cells but normal numbers of B cells and Natural Killer cells (T-B+NK+), similar to a phenotype of severe combined immunodeficiency (SCID); these patients may require additional steps to confirm the diagnosis if no known genetic cause of athymia is identified. However, distinguishing athymia from SCID is crucial, as treatments differ for these conditions. Cultured thymus tissue is being investigated as a treatment for congenital athymia. Here, we review what is known about the epidemiology, underlying etiologies, clinical manifestations, and treatments for congenital athymia.

Dysregulation of Systemic Immunity in Aging and Dementia

Neuroinflammation and the tissue-resident innate immune cells, the microglia, respond and contribute to neurodegenerative pathology. Although microglia have been the focus of work linking neuroinflammation and associated dementias like Alzheimer's Disease, the inflammatory milieu of brain is a conglomerate of cross-talk amongst microglia, systemic immune cells and soluble mediators like cytokines. Age-related changes in the inflammatory profile at the levels of both the brain and periphery are largely orchestrated by immune system cells. Strong evidence indicates that both innate and adaptive immune cells, the latter including T cells and B cells, contribute to chronic neuroinflammation and thus dementia. Neurodegenerative hallmarks coupled with more traditional immune system stimuli like infection or injury likely combine to trigger and maintain persistent microglial and thus brain inflammation. This review summarizes age-related changes in immune cell function, with special emphasis on lymphocytes as a source of inflammation, and discusses how such changes may potentiate both systemic and central nervous system inflammation to culminate in dementia. We recap the understudied area of AD-associated changes in systemic lymphocytes in greater detail to provide a unifying perspective of inflammation-fueled dementia, with an eye toward evidence of two-way communication between the brain parenchyma and blood immune cells. We focused our review on human subjects studies, adding key data from animal models as relevant.

Dynamics of thymus function and T cell receptor repertoire breadth in health and disease

T cell recognition of unknown antigens relies on the tremendous diversity of the T cell receptor (TCR) repertoire; generation of which can only occur in the thymus. TCR repertoire breadth is thus critical for not only coordinating the adaptive response against pathogens but also for mounting a response against malignancies. However, thymic function is exquisitely sensitive to negative stimuli, which can come in the form of acute insult, such as that caused by stress, infection, or common cancer therapies; or chronic damage such as the progressive decline in thymic function with age. Whether it be prolonged T cell deficiency after hematopoietic cell transplantation (HCT) or constriction in the breadth of the peripheral TCR repertoire with age; these insults result in poor adaptive immune responses. In this review, we will discuss the importance of thymic function for generation of the TCR repertoire and how acute and chronic thymic damage influences immune health. We will also discuss methods that are used to measure thymic function in patients and strategies that have been developed to boost thymic function.

Telomere length and signal joint T-cell receptor rearrangement excision circles as biomarkers for chronological age estimation

BACKGROUND

Chronological age estimation is a challenging marker in the field of forensic medicine. The current study aimed to investigate the accuracy of signal joint T-cell receptor rearrangement excision circles (sjTRECs) quantification and telomere length measurement as methods for estimating chronological age.

METHODS

Telomere length was estimated in the DNA derived from the buccal cells through estimating the telomeric restriction fragment (TRF) length using TeloTTAGGG Telomere Length Assay while the sjTRECs quantification was carried out on DNA isolated from the blood samples using qPCR.

RESULTS

The TRF length was shortened with increased age (r = -0.722, p < 0.001). The sjTRECs were also decreased with increased age (r = -0.831, p < 0.001). Stronger coefficient and lower standard error of the estimate was obtained when multiple regression analysis for age prediction based on the values of both methods was applied (r = -0.876, p < 0.001).

Long-term robustness of a T-cell system emerging from somatic rescue of a genetic block in T-cell development

BACKGOUND

The potential of a single progenitor cell to establish and maintain long-term protective T-cell immunity in humans is unknown. For genetic disorders disabling T-cell immunity, somatic reversion was shown to support limited T-cell development attenuating the clinical phenotype. However, the cases reported so far deteriorated over time leaving unanswered the important question of long-term activity of revertant precursors and the robustness of the resulting T-cell system.

METHODS

We applied TCRβ-CDR3 sequencing and mass cytometry on serial samples of a now 18 year-old SCIDX1 patient with somatic reversion to analyse the longitudinal diversification and stability of a T-cell system emerging from somatic gene rescue.

FINDINGS

We detected close to 105 individual CDR3β sequences in the patient. Blood samples of equal size contained about 10-fold fewer unique CDR3β sequences compared to healthy donors, indicating a surprisingly broad repertoire. Despite dramatic expansions and contractions of individual clonotypes representing up to 30% of the repertoire, stable diversity indices revealed that these transient clonal distortions did not cause long-term repertoire imbalance. Phenotypically, the T-cell system did not show evidence for progressive exhaustion. Combined with immunoglobulin substitution, the limited T-cell system in this patient supported an unremarkable clinical course over 18 years.

INTERPRETATION

Genetic correction in the appropriate cell type, in our patient most likely in a T-cell biased self-renewing hematopoietic progenitor, can yield a diverse T-cell system that provides long-term repertoire stability, does not show evidence for progressive exhaustion and is capable of providing protective and regulated T-cell immunity for at least two decades.

FUNDING

DFG EH 145/9-1, DFG SCHW 432/4-1 and the German Research Foundation under Germany's Excellence Strategy-EXC-2189-Project ID: 390939984.

Establishing Simultaneous T Cell Receptor Excision Circles (TREC) and K-Deleting Recombination Excision Circles (KREC) Quantification Assays and Laboratory Reference Intervals in Healthy Individuals of Different Age Groups in Hong Kong

The clinical experience gathered throughout the years has raised awareness of primary immunodeficiency diseases (PIDD). T cell receptor excision circles (TREC) and kappa-deleting recombination excision circles (KREC) assays for thymic and bone marrow outputs measurement have been widely implemented in newborn screening (NBS) programs for Severe Combined Immunodeficiency. The potential applications of combined TREC and KREC assay in PIDD diagnosis and immune reconstitution monitoring in non-neonatal patients have been suggested. Given that ethnicity, gender, and age can contribute to variations in immunity, defining the reference intervals of TREC and KREC levels in the local population is crucial for setting up cut-offs for PIDD diagnosis. In this retrospective study, 479 healthy Chinese sibling donors (240 males and 239 females; age range: 1 month-74 years) from Hong Kong were tested for TREC and KREC levels using a simultaneous quantitative real-time PCR assay. Age-specific 5th-95th percentile reference intervals of TREC and KREC levels (expressed in copies per μL blood and copies per 106 cells) were established in both pediatric and adult age groups. Significant inverse correlations between age and both TREC and KREC levels were observed in the pediatric age group. A significant higher KREC level was observed in females than males after 9-12 years of age but not for TREC. Low TREC or KREC levels were detected in patients diagnosed with mild or severe PIDD. This assay with the established local reference intervals would allow accurate diagnosis of PIDD, and potentially monitoring immune reconstitution following haematopoietic stem cell transplantation or highly active anti-retroviral therapy in the future.

The ESC: The Dangerous By-Product of V(D)J Recombination

V(D)J recombination generates antigen receptor diversity by mixing and matching individual variable (V), diversity (D), and joining (J) gene segments. An obligate by-product of many of these reactions is the excised signal circle (ESC), generated by excision of the DNA from between the gene segments. Initially, the ESC was believed to be inert and formed to protect the genome from reactive broken DNA ends but more recent work suggests that the ESC poses a substantial threat to genome stability. Crucially, the recombinase re-binds to the ESC, which can result in it being re-integrated back into the genome, to cause potentially oncogenic insertion events. In addition, very recently, the ESC/recombinase complex was found to catalyze breaks at recombination signal sequences (RSSs) throughout the genome, via a “cut-and-run” mechanism. Remarkably, the ESC/recombinase complex triggers these breaks at key leukemia driver genes, implying that this reaction could be a significant cause of lymphocyte genome instability. Here, we explore these alternate pathways and discuss their relative dangers to lymphocyte genome stability.

Polymorphisms in the canine IL7R 3'UTR are associated with thymic output in Labrador retriever dogs and influence post-transcriptional regulation by microRNA 185

Interleukin-7 (IL-7) and its receptor (IL-7R) are essential for T cell development in the thymus, and changes in the IL-7/IL-7R pathway have been implicated in age-associated thymic involution which results in a reduction of naïve T cell output. The aim of this study was to investigate the relationship between IL7 and IL7R genetic variation and thymic output in dogs. No single nucleotide polymorphisms (SNPs) were identified in the canine IL7 gene, but a number were present in the canine IL7R gene. Polymorphisms in the IL7R exon 8 and 3'UTR were found to be associated with signal joint T cell receptor excision circle (sj-TREC) values (a biomarker of thymic output) in young and geriatric Labrador retrievers. Additionally, one of the SNPs in the IL7R 3'UTR (SNP 14 c.1371 + 446 A > C) was found to cause a change in the seed-binding site for microRNA 185 which, a luciferase reporter assay demonstrated, caused changes in post-transcriptional regulation, and therefore might be capable of influencing IL-7R expression. The research findings suggest a genetic link between IL7R genotype and thymic output in dogs, which might impact on immune function as these animals age and provide further evidence of the involvement of IL-7/IL-7R pathway in age-associated thymic involution.

A comparison of TRECs and flow cytometry for naive T cell quantification

Assessment of thymic output by measurement of naive T cells is carried out routinely in clinical diagnostic laboratories, predominantly using flow cytometry with a suitable panel of antibodies. Naive T cell measurements can also be made using molecular analyses to quantify T cell receptor excision circle (TRECs) levels in sorted cells from peripheral blood. In this study we have compared TRECs levels retrospectively with CD45RA⁺ CD27⁺ T cells and also with CD45RA⁺ CD31⁺ T cells in 134 patient samples at diagnosis or during follow-up. Both panels provide naive T cell measurements that have a strongly positive correlation with TRECs numbers and are suitable for use with enumerating naive T cell levels in a clinical laboratory.

An Age-Associated Decline in Thymic Output Differs in Dog Breeds According to Their Longevity

The age associated decline in immune function is preceded in mammals by a reduction in thymic output. Furthermore, there is increasing evidence of a link between immune competence and lifespan. One approach to determining thymic output is to quantify signal joint T cell receptor excision circles (sj-TRECs), a method which has been developed and used in several mammalian species. Life expectancy and the rate of aging vary in dogs depending upon their breed. In this study, we quantified sj-TRECs in blood samples from dogs of selected breeds to determine whether there was a relationship between longevity and thymic output. In Labrador retrievers, a breed with a median expected lifespan of 11 years, there was an age-associated decline in sj-TREC values, with the greatest decline occurring before 5 years of age, but with sj-TREC still detectable in some geriatric animals, over 13 years of age. In large short-lived breeds (Burnese mountain dogs, Great Danes and Dogue de Bordeaux), the decline in sj-TREC values began earlier in life, compared with small long-lived breeds (Jack Russell terriers and Yorkshire terriers), and the presence of animals with undetectable sj-TRECs occurred at a younger age in the short-lived breeds. The study findings suggest that age-associated changes in canine sj-TRECs are related to breed differences in longevity, and this research highlights the use of dogs as a potential model of immunosenescence.

Transcriptome analysis indicated that Salmonella lipopolysaccharide-induced thymocyte death and thymic atrophy were related to TLR4-FOS/JUN pathway in chicks

Background

Thymus is the crucial site for T cell development and once believed to be immune privileged. Recently, thymus has gained special attention as it is commonly targeted by infectious agents which may cause pathogenic tolerance and subsequent immunosuppression.

Results

We analyzed thymic responses to the challenge with Salmonella typhimurium (STm) or lipopolysaccharide (LPS) derived from STm in chicks. Newly hatched chicks were injected intraperitoneally with 5 × 10⁴ CFU/mL STm or 50 mg/kg LPS. After LPS treatment, maximum thymocyte death (3 ~ 5-fold change) compared to controls was found at 12 h, and maximum loss of thymic weight (35 %) and reduced thymic index (20 %) were found at 36 h. After STm infection, maximum thymocyte death and thymic atrophy occurred at 36 and 72 h, respectively. No significant changes of thymic structure, chT1+ and CD4+/CD8+ T cell ratio were observed in thymus or spleen tissues after LPS treatment. Furthermore, transcriptome analysis revealed important roles for the TLR4-FOS/JUN signaling pathway in thymic injury. Thus, the major process of thymic atrophy in this study first involved activation of transcriptional factors FOS/JUN upon LPS binding to TLR4 that caused release of inflammatory factors, thereby inducing inflammatory responses and DNA damage and ultimately cell cycle arrest and thymic injury.

Conclusions

STm and Salmonella LPS could induce acute chick thymic injury. LPS treatment acted faster than STm. TLR4-FOS/JUN pathway may play an important role in LPS induced chick thymic injury.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2674-6) contains supplementary material, which is available to authorized users.

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.

Evaluation of bovine thymic function by measurement of signal joint T-cell receptor excision circles

A signal joint T-cell receptor excision circle (sjTREC) is a circular DNA produced by T-cell receptor α gene rearrangement in the thymus. Measurements of sjTREC values have been used to evaluate thymic function. We recently established a quantitative PCR (QPCR) assay of bovine sjTREC. In the present study, we used this QPCR assay to measure the sjTREC value in bovine peripheral blood mononuclear cells and we then evaluated the relationships between sjTREC values and peripheral blood T-cell number, growth stage, gender, and meteorological season. The sjTREC value was highest at the neonatal stage, and its value subsequently decreased with age. On the other hand, the peripheral T-cell number increased with age. The sjTREC value in calves up to 50-days old was significantly higher for males than for females, suggesting that thymic function might differ by gender. In addition, the sjTREC value and the peripheral T-cell number were significantly higher in calves in the summer season than in calves in the winter season. These data suggest that bovine thymic function is highly variable and varies according to the growth stage, gender, and environmental factors such as air temperature or the UV index.

Thymic output: Assessment of CD4+ recent thymic emigrants and T-Cell receptor excision circles in infants

BACKGROUND

CD4⁺ recent thymic emigrants (CD4⁺ RTEs) constitute a subset of T cells recently generated in the thymus and exported into peripheral blood. CD4⁺ RTEs have increased copy numbers of T-cell receptor excision circles (TREC). They are characterized by the expression of CD31 on naïve CD4 T-cells. We aimed to validate a flow-cytometry assay to enumerate CD4⁺ RTEs and assess its performance in relation to TREC measurement.

METHODS

CD4⁺ RTEs cell count in peripheral blood was measured to determine sample stability, precision, linearity, and to establish reference ranges. TRECs were measured using qPCR assay performed with DNA isolated from peripheral blood. CD4⁺ RTEs, TRECs, and flow cytometry results for major T-cell markers were assessed in 50 infants less than 2 years of age.

RESULTS

Inter-and intra-assay precisions (% CV) were 1.5-12.2 and 1.5-7.0, respectively. Linearity studies showed that the results are linear over a range of 0.7 to 403.0 CD4⁺ RTEs/μL of blood. There was 84% agreement (42 of 50) between CD4⁺ RTEs and TRECs qualitative results for the infant samples. CD4⁺ RTEs reference ranges in 17 healthy children was in agreement with published data, while that of the healthy adults were 51-609 cells/μL of blood.

CONCLUSION

The validation results provide acceptable measures of the CD4⁺ RTEs test performance within CAP/CLIA frameworks. CD4⁺ RTEs and TRECs assays show high agreement in the infant population. The CD4⁺ RTEs test can be used as a confirmation for the TREC results along with or as an alternative to T-cell phenotyping in infants with repeatedly low TRECs concentrations. © 2015 International Clinical Cytometry Society.

Thymic Activity and T Cell Repertoire Recovery after Autologous Hematopoietic Stem Cell Transplantation Preceded by Myeloablative Radiotherapy or Chemotherapy

It was previously postulated that pretransplant myeloablative treatment may impair thymopoiesis, contributing in this way to delayed reconstitution of T cells after hematopoietic stem cell transplantation (HSCT). On the other hand, de novo generation of T cells after HSCT requires a competent thymus. Various myeloablative conditioning regimens (total body irradiation [TBI] or high-dose chemotherapy) routinely used in clinical practice may have potentially different impacts on the thymus. However, no comparative study on thymic output and T cell repertoire in autologous (auto)HSCT model has been presented so far. Here we evaluated thymic output and TCR diversity in 45 lymphoma patients submitted to autoHSCT differing in respect to conditioning regimen: high-dose chemotherapy as monotherapy (BEAM, n = 22) or combination of total body irradiation with cyclophosphamide chemotherapy: Cy/TBI (n = 23). Thymic output was assessed before and on days +100, +180, and +365 after autoHSCT by flow cytometric counts of recent thymic emigrant (RTE) cells (CD31(+) CD62L(+) CD45RA(+) CD4(+)) and quantification of signal joint TCR receptor excision circles (sjTRECs) by quantitative PCR. T cell repertoire diversity was analyzed on day +365 after autoHSCT by spectra-typing of the CDR3 region in the TCRVβ chain. The BEAM group, in contrast to the Cy/TBI group, manifested significantly higher proportions of RTE cells and sjTREC copy numbers on days +100 and +180. Analysis of TCRVβ spectra-types on day +365 revealed more restricted (monoclonal or oligoclonal) T cell repertoires in the Cy/TBI versus BEAM group (48.8% versus 18.2%, P = .0002). In conclusion, the conditioning scheme based on BEAM chemotherapy may be performed with lower risk of thymic destruction and T cell repertoire distortion than Cy/TBI scheme. This finding may help to potentially improve conditioning schemes to efficiently perform myeloablation and maintain active thymopoiesis.

Reduced T-Cell Thymic Export Reflected by sj-TREC in Patients with Coronary Artery Disease

AIM

Immunologic dysfunction was recently found to be one of the most important mechanisms underlying the initiation and development of atherosclerosis. Thymus involution can contribute to immune disturbance and disequilibrium of T-cell subsets. This study aimed to explore whether recent thymic emigration (RTE) is impaired in patients with coronary artery disease (CAD).

METHODS

Content of signal-joint T cell receptor excision circles (sj-TREC) in T lymphocytes, a molecular marker of RTE, was assessed among CAD patients and age-matched controls. Monochrome multiplex quantitative PCR method was used to assess the samples' telomere length in order to exclude the potential influence of T cell proliferation on the dilution of sj-TREC. Patients were grouped according to Gensini score (GS) (low, GS ＜18; intermediate, GS 18-41; high, GS ＞41). Ordinary logistic regression models were used to determine potential risk factors for CAD and GS tertiles.

RESULTS

Average copy numbers of sj-TREC per 10(6) T lymphocytes among patients with unstable angina, stable angina, and controls were 726±429, 1213±465, and 1795±838, respectively (P＜0.001). However, there was no significant difference in telomere length among groups. Moreover, the content of sj-TREC in the high GS group was most significantly reduced than the low GS group (P＜0.001). Multivariate logistic regression analysis revealed that lower sj-TREC was independently associated with the progression of CAD (OR=0.44, P＜0.001) and higher GS (OR=0.4, P＜0.001).

CONCLUSION

Impaired RTE could be partly responsible for CAD development. Mechanisms may be involved in the disturbance of T lymphocyte compartment and interruption of maintained immune tolerance resulting from thymus involution.

Molecular biomarkers for chronological age in animal ecology

The chronological age of an individual animal predicts many of its biological characteristics, and these in turn influence population-level ecological processes. Animal age information can therefore be valuable in ecological research, but many species have no external features that allow age to be reliably determined. Molecular age biomarkers provide a potential solution to this problem. Research in this area of molecular ecology has so far focused on a limited range of age biomarkers. The most commonly tested molecular age biomarker is change in average telomere length, which predicts age well in a small number of species and tissues, but performs poorly in many other situations. Epigenetic regulation of gene expression has recently been shown to cause age-related modifications to DNA and to cause changes in abundance of several RNA types throughout animal lifespans. Age biomarkers based on these epigenetic changes, and other new DNA-based assays, have already been applied to model organisms, humans and a limited number of wild animals. There is clear potential to apply these marker types more widely in ecological studies. For many species, these new approaches will produce age estimates where this was previously impractical. They will also enable age information to be gathered in cross-sectional studies and expand the range of demographic characteristics that can be quantified with molecular methods. We describe the range of molecular age biomarkers that have been investigated to date and suggest approaches for developing the newer marker types as age assays in nonmodel animal species.

Establishment and Maintenance of the Human Naïve CD4+ T-Cell Compartment

The naïve CD4⁺ T-cell compartment is considered essential to guarantee immune competence throughout life. Its replenishment with naïve cells with broad diverse receptor repertoire, albeit with reduced self-reactivity, is ensured by the thymus. Nevertheless, cumulative data support a major requirement of post-thymic proliferation both for the establishment of the human peripheral naïve compartment during the accelerated somatic growth of childhood, as well as for its lifelong maintenance. Additionally, a dynamic equilibrium is operating at the cell level to fine-tune the T-cell receptor threshold to activation and survival cues, in order to counteract the continuous naïve cell loss by death or conversion into memory/effector cells. The main players in these processes are low-affinity self-peptide/MHC and cytokines, particularly IL-7. Moreover, although naïve CD4⁺ T-cells are usually seen as a homogeneous population regarding stage of maturation and cell differentiation, increasing evidence points to a variety of phenotypic and functional subsets with distinct homeostatic requirements. The paradigm of cells committed to a distinct lineage in the thymus are the naïve regulatory T-cells, but other functional subpopulations have been identified based on their time span after thymic egress, phenotypic markers, such as CD31, or cytokine production, namely IL-8. Understanding the regulation of these processes is of utmost importance to promote immune reconstitution in several clinical settings, namely transplantation, persistent infections, and aging. In this mini review, we provide an overview of the mechanisms underlying human naïve CD4⁺ T-cell homeostasis, combining clinical data, experimental studies, and modeling approaches.

Detection and quantification of bovine signal joint T-cell receptor excision circles

A signal joint (sj) T-cell receptor excision circle (TREC) is produced by T-cell receptor (TCR) gene rearrangements during αβ T-cell maturation in the thymus. sjTREC have been studied as a marker of thymic function in several spices. We designed specific primers for δrec-ψJα sj region to identify the location of the bovine sjTREC region and determined the nucleotide sequence of the PCR product. The obtained sequences were subjected to a BLAST search, which identified a matching region. This matching region contained TCR δ genes and was identified on bovine chromosome 10. We also confirmed the polymorphism of the sj region by sequencing of 10 PCR products, and observed irregular insertion of bases in the δrec-ψJα recombination signal sequence. We then developed a quantitative PCR (QPCR) assay for evaluation of sjTRECs level in order to evaluate bovine thymic function for application in the veterinary clinic. This QPCR assay specifically amplified the sj region of bovine sjTREC and could detected 10(1)-10(7) copy numbers of sjTRECs. Using this assay we found that the number of sjTRECs in peripheral blood mononuclear cells was less than 10% that of the thymus.

Alpha-Particle-Induced Complex Chromosome Exchanges Transmitted through Extra-Thymic Lymphopoiesis In Vitro Show Evidence of Emerging Genomic Instability

Human exposure to high-linear energy transfer α-particles includes environmental (e.g. radon gas and its decay progeny), medical (e.g. radiopharmaceuticals) and occupational (nuclear industry) sources. The associated health risks of α-particle exposure for lung cancer are well documented however the risk estimates for leukaemia remain uncertain. To further our understanding of α-particle effects in target cells for leukaemogenesis and also to seek general markers of individual exposure to α-particles, this study assessed the transmission of chromosomal damage initially-induced in human haemopoietic stem and progenitor cells after exposure to high-LET α-particles. Cells surviving exposure were differentiated into mature T-cells by extra-thymic T-cell differentiation in vitro. Multiplex fluorescence in situ hybridisation (M-FISH) analysis of naïve T-cell populations showed the occurrence of stable (clonal) complex chromosome aberrations consistent with those that are characteristically induced in spherical cells by the traversal of a single α-particle track. Additionally, complex chromosome exchanges were observed in the progeny of irradiated mature T-cell populations. In addition to this, newly arising de novo chromosome aberrations were detected in cells which possessed clonal markers of α-particle exposure and also in cells which did not show any evidence of previous exposure, suggesting ongoing genomic instability in these populations. Our findings support the usefulness and reliability of employing complex chromosome exchanges as indicators of past or ongoing exposure to high-LET radiation and demonstrate the potential applicability to evaluate health risks associated with α-particle exposure.

Gene analysis of signal-joint T cell receptor excision circles and their relationship to age in dogs

The quantification of DNA excision circles produced during T cell receptor (TCR) rearrangement, termed signal joint TCR rearrangement excision circles (sjTRECs), has been employed as a measure of age and thymic function in humans and animals. δRec-ψJα sjTRECs are ring-shaped DNAs that are generated during TCRδ locus deletion that occurs at a late stage of T cell development. In this study, the nucleotide sequences of δRec-ψJα signal joints of canine δRec-ψJα sjTRECs were analyzed. The gene structure of canine δRec-ψJα signal joints was found to be similar to that of humans and mice. However, diversity of signal joints was detected and found to derive from N nucleotide insertions, recombination signal sequence combinational diversity and single-base substitutions at the recombination signal sequence. In addition, an adenine insertion or deletion was found approximately 280 bases from the ψJα signal end. Blood samples were collected from 46 dogs, ranging in age from 3 to 192 months, with a mean age of 96.4 and a SD of 51.5 months. Although δRec-ψJα sjTRECs were detectable in most of the dogs evaluated, the level did not significantly correlate with age. These results indicated that δRec-ψJα sjTREC levels were ineffective as a measure of age in dogs.

Premature senescence of T-cell subsets in axial spondyloarthritis

OBJECTIVE

To investigate the possible occurrence of early thymic failure and premature senescence of naïve and memory T-cells in patients with axial spondyloarthritis (aSpA).

METHODS

Prospective, cross-sectional study of consecutive patients with aSpA (n=51), rheumatoid arthritis (RA, n=51) and healthy controls (HCs, n=50). Demographic, clinical and laboratory parameters were collected in all patients and we isolated naïve (CD45RA(+)) and memory (CD45RO(+)) CD4(+) and CD8(+) T-cell subsets by MACS technology. T-cell receptor rearrangement excision circle (TREC) and telomere length were measured by real-time PCR. We used TRECs as a surrogate for thymus function and telomere length as an indicator of cellular senescence. Telomerase activity was analysed with the Telomeric Repeat Amplification Protocols.

RESULTS

We observed a premature decline of thymic output in patients with aSpA and patients with RA compared with HCs as indicated by a reduction of TREC levels in naive T-cells (aSpA: age adjusted regression coefficient (regcoeff) for CD4(+)CD45RA(+) T-cells -2.566, p=0.023; RA regcoeff=-2.844, p=0.008). Telomere length of all CD4(+) and CD8(+) T-cell subsets was reduced in young patients with aSpA compared with HCs, whereas data for patients with RA were comparable with HCs. Telomerase activity was inversely correlated with telomere length in HCs (correlation coefficient (corcoeff)=-0.532, p<0.001) but not in patients with aSpA (corcoeff=-0.056, p=0.697) and RA (corcoeff=-0.003, p=0.982).

CONCLUSIONS

Our data indicate an age-inappropriate shrinkage of thymic output, an inappropriate shortening of telomeres in young patients with aSpA and an impaired telomerase enzyme in patients with aSpA and RA.

Disruption of the TSLP-TSLPR-LAP signaling between epithelial and dendritic cells through hyperlipidemia contributes to regulatory T-Cell defects in atherosclerotic mice

Regulatory T-Cells (Tregs) play a protective role against the development of atherosclerosis. Moreover, thymic stromal lymphopoietin (TSLP)/thymic stromal lymphopoietin receptor (TSLPR) signaling in myeloid dendritic cells (DCs) promote Treg differentiation. Here, we examined the potential role of TSLP/TSLPR on Treg homeostasis in atherosclerosis. The frequencies of both latency-associated peptide (LAP)(+) and Foxp3(+) Tregs were reduced in the thymus and spleen of ApoE(-/-) mice compared with C57BL/6 mice, and this effect was associated with decreased thymic output. The tolerogenic function of DCs obtained from ApoE(-/-) mice was compromised compared with those from C57BL/6 mice. The expression of TSLP and TSLPR was also inhibited in ApoE(-/-) mice. In addition, we found that ox-LDL attenuated TSLP expression in cultured thymic epithelial cells (TECs) through the activation of retinoid X receptor alpha (RXRA) and IL-1β and decreased LAP and PD-L1 expression in oxLDL-activated DCs while both were up-regulated in TSLP-activated DCs. We also observed that the TSLP-DCs mediated differentiation of Tregs was abrogated through LAP neutralization. Furthermore, TSLP injection rescued Treg defects in ApoE(-/-) mice. These findings suggest that Treg defects in ApoE(-/-) mice might partially be attributed to the disruption of TSLP-TSLPR-LAP signaling in epithelial cells (ECs) and DCs.

Routine thymectomy in congenital cardiac surgery changes adaptive immunity without clinical relevance

The actual importance of the thymus in both children and adults is largely unclear. In congenital cardiac surgery, a partial or total thymectomy is frequently performed to improve access to the heart and great vessels. We performed a literature search to evaluate the effect on the adaptive immune system of the removal of thymus tissue in patients with congenital heart disease. A PubMed search according to Dunning's standard provided 149 articles, of which 13 addressed our search question. Each study has been tabulated with author, cases, controls, follow-up, methods, results and limitations. A first group of articles repeatedly showed the effect on the T-cell compartment, including the impact on subgroups of this compartment. More recent studies, usually with a longer follow-up, confirm that the earlier changes in T-cell population appear to be permanent. Only one author found a normalization of T-cell population five years after thymectomy. In contrast to these clear changes in T-cell population, there is currently no clear clinical relevance. A literature search on thymectomy in congenital cardiac surgery revealed clear changes in T-cell-related immunity; however, there is a lack of clinical relevance. Further investigation of the adaptive immune system is required to explain this discrepancy.

Maturation and migration of murine CD4 single positive thymocytes and thymic emigrants

T lymphopoiesis in the thymus was thought to be completed once they reach the single positive (SP) stage, when they are “fully mature” and wait to be exported at random or follow a “first in-first out” manner. Recently, accumulating evidence has revealed that newly generated SP thymocytes undergo further maturation in the thymic medulla before they follow a tightly regulated emigrating process to become recent thymic emigrants (RTEs). RTEs in the periphery then experience a post-thymic maturation and peripheral tolerance and eventually become licensed as mature naïve T cells. This review summarizes the recent progress in the late stage T cell development in and outside of the thymus. The regulation of this developmental process is also discussed.

Visualization and quantification of monoallelic TCRα gene rearrangement in αβ T cells

T-cell receptor α (TCRα) chain rearrangement is not constrained by allelic exclusion and thus αβ T cells frequently have rearranged both alleles of this locus. Thereby, stepwise secondary rearrangements of both TCRα loci further increase the odds for generation of an α-chain that can be positively selected in combination with a pre-existing TCRβ chain. Previous studies estimated that approximately 2-12% of murine and human αβ T cells still carry one TCRα locus in germline configuration, which must comprise a partially or even fully rearranged TCRδ locus. However, these estimates are based on a relatively small amount of individual αβ T-cell clones and αβ T-cell hybridomas analyzed to date. To address this issue more accurately, we made use of a mouse model, in which a fluorescent reporter protein is introduced into the constant region of the TCRδ locus. In this TcrdH2BeGFP system, fluorescence emanating from retained TCRδ loci enabled us to quantify monoallelically rearranged αβ T cells on a single-cell basis. Via fluorescence-activated cell sorting analysis, we determined the frequency of monoallelic TCRα rearrangements to be 1.7% in both peripheral CD4(+) and CD8(+) αβ T cells. Furthermore, we found a skewed 5' Jα gene utilization of the rearranged TCRα allele in T cells with monoallelic TCRα rearrangements. This is in line with previous descriptions of a tight interallelic positional coincidence of Jα gene segments used on both TCRα alleles. Finally, analysis of T cells from transgenic mice harboring only one functional TCRα locus implied the existence of very rare unusual translocation or episomal reintegration events of formerly excised TCRδ loci.

Clinical Implications of T Cell Receptor Repertoire Analysis after Allogeneic Stem Cell Transplantation

Stem cell transplantation (SCT) constitutes a major challenge to the immune system. Long-term impairment of immunity against various common infectious stimuli leads to increased susceptibility to infectious diseases; in contrast, an immune response against the recipient may cause the devastating graft-versus-host disease (GvHD). Recovery of the immune system (both qualitative and quantitative) after SCT is perhaps the most important factor in determining the clinical outcome. Consequently, immune reconstitution has been extensively studied using different approaches, including quantitative analysis of immune cells as well as their phenotypic characterization. Analysis of diversity and clonality is an important tool in determining competence of the immune system, assuming that a broad diversity assures efficient response to different stimuli and clonal dominance reflects ongoing, potentially relevant immune responses. Detailed analysis of the immune repertoire through the flow cytometric and molecular study of the T cell receptor repertoire has been applied to gain quantitative and qualitative insights about the T cell immune competence and responsiveness. After SCT, a contraction of the T cell pool and a reduction in T cell receptor diversity is clearly associated with clinical immunodeficiency. Reconstitution of the immune system is often characterized by dominance of oligoclonal T cell populations, reflecting specific antigen-driven immune responses. Detailed characterization of T lymphocytes by T cell receptor analysis is possible, and may lead to the identification of individual clones involved in specific immune reactions, such as alloresponses in GvHD, the closely related graft-versus-leukemia effect and opportunistic viral agents such as CMV or EBV.

Impaired thymic export and increased apoptosis account for regulatory T cell defects in patients with non-ST segment elevation acute coronary syndrome

Regulatory T (Treg) cells play a protective role against the development of atherosclerosis. Previous studies have revealed Treg cell defects in patients with non-ST elevation acute coronary syndrome (NSTACS), but the mechanisms underlying these defects remain unclear. In this study, we found that the numbers of peripheral blood CD4(+)CD25(+)CD127(low) Treg cells and CD4(+)CD25(+)CD127(low)CD45RA(+)CD45RO(-) naive Treg cells were lower in the NSTACS patients than in the chronic stable angina (CSA) and the chest pain syndrome (CPS) patients. However, the number of CD4(+)CD25(+)CD127(low)CD45RA(-)CD45RO(+) memory Treg cells was comparable in all of the groups. The frequency of CD4(+)CD25(+)CD127(low)CD45RO(-)CD45RA(+)CD31(+) recent thymic emigrant Treg cells and the T cell receptor excision circle content of purified Treg cells were lower in the NSTACS patients than in the CSA patients and the CPS controls. The spontaneous apoptosis of Treg cells (defined as CD4(+)CD25(+)CD127(low)annexin V(+)7-AAD(-)) was increased in the NSTACS patients compared with the CSA and CPS groups. Furthermore, oxidized LDL could induce Treg cell apoptosis, and the oxidized LDL levels were significantly higher in the NSTACS patients than in the CSA and CPS groups. In accordance with the altered Treg cell levels, the concentration of TNF-α was increased in the NSTACS patients, resulting in a decreased IL-10/TNF-α ratio. These findings indicate that the impaired thymic output of Treg cells and their enhanced susceptibility to apoptosis in the periphery were responsible for Treg cell defects observed in the NSTACS patients.

De novo generation of cells within human nurse macrophages and consequences following HIV-1 infection

Nurse cells are defined as those that provide for the development of other cells. We report here, that in vitro, human monocyte-derived macrophages can behave as nurse cells with functional capabilities that include de novo generation of CD4+ T-lymphocytes and a previously unknown small cell with monocytoid characteristics. We named these novel cells “self-renewing monocytoid cells” (SRMC), because they could develop into nurse macrophages that produced another generation of SRMC. SRMC were not detectable in blood. Their transition to nurse behavior was characterized by expression of CD10, a marker of thymic epithelium and bone marrow stroma, typically absent on macrophages. Bromodeoxyuridine labeling and immunostaining for cdc6 expression confirmed DNA synthesis within nurse macrophages. T-cell excision circles were detected in macrophages, along with expression of pre-T-cell receptor alpha and recombination activating gene 1, suggesting that genetic recombination events associated with generation of the T-cell receptor were occurring in these cells. SRMC expressed CCR5, the coreceptor for R5 HIV-1 isolates, and were highly susceptible to HIV-1 entry leading to productive infection. While expressing HIV-1, SRMC could differentiate into nurse macrophages that produced another generation of HIV-1-expressing SRMC. The infected nurse macrophage/SRMC cycle could continue in vitro for multiple generations, suggesting it might represent a mechanism whereby HIV-1 can maintain persistence in vivo. HIV-1 infection of nurse macrophages led to a decline in CD4+ T-cell production. There was severe, preferential loss of the CCR5+ CD4+ T-cell subpopulation. Confocal microscopy revealed individual HIV-1-expressing nurse macrophages simultaneously producing both HIV-1-expressing SRMC and non-expressing CD3+ cells, suggesting that nurse macrophages might be a source of latently infected CD4+ T-cells. Real-time PCR experiments confirmed this by demonstrating 10-fold more HIV-1-genome-harboring T-cells, than virus-expressing ones. These phenomena have far-reaching implications, and elicit new perspectives regarding HIV pathogenesis and T-cell and hematopoietic cell development.

Immune aging and autoimmunity

Age is an important risk for autoimmunity, and many autoimmune diseases preferentially occur in the second half of adulthood when immune competence has declined and thymic T cell generation has ceased. Many tolerance checkpoints have to fail for an autoimmune disease to develop, and several of those are susceptible to the immune aging process. Homeostatic T cell proliferation which is mainly responsible for T cell replenishment during adulthood can lead to the selection of T cells with increased affinity to self- or neoantigens and enhanced growth and survival properties. These cells can acquire a memory-like phenotype, in particular under lymphopenic conditions. Accumulation of end-differentiated effector T cells, either specific for self-antigen or for latent viruses, have a low activation threshold due to the expression of signaling and regulatory molecules and generate an inflammatory environment with their ability to be cytotoxic and to produce excessive amounts of cytokines and thereby inducing or amplifying autoimmune responses.

Post-thymic maturation: young T cells assert their individuality

T cell maturation was once thought to occur entirely within the thymus. Now, evidence is mounting that the youngest peripheral T cells in both mice and humans comprise a distinct population from their more mature, yet still naive, counterparts. These cells, termed recent thymic emigrants (RTEs), undergo a process of post-thymic maturation that can be monitored at the levels of cell phenotype and immune function. Understanding this final maturation step in the process of generating useful and safe T cells is of clinical relevance, given that RTEs are over-represented in neonates and in adults recovering from lymphopenia. Post-thymic maturation may function to ensure T cell fitness and self tolerance.

Rapid migration of thymic emigrants to the colonic mucosa in ulcerative colitis patients

Inflammatory bowel disease (IBD) is associated with imbalances of the local intestinal immune responses, with dysregulated CD4(+) T cells contributing to the chronic inflammation. Having demonstrated altered T cell maturation in the thymus in two different mouse models of colitis, we set out to investigate whether abnormalities in T cell maturation is present in patients with ulcerative colitis (UC) or Crohn's disease (CD). Specimens were obtained from peripheral blood (CD; n = 14, UC; n = 22), colon and small intestinal specimens (CD; n = 6, UC; n = 13). As controls, peripheral blood specimens were obtained from healthy volunteers, patients with adenocarcinomas (n = 18) and colonic specimens from patients with adenocarcinomas (n = 14). Recent thymic emigrants were estimated by analysis of the normalized ratio of T cell receptor excision circles (TRECs) by real-time polymerase chain reaction (PCR). The frequency of naive- and proliferating T lymphocytes and markers of extrathymic T cell maturation in the mucosa was analyzed by flow cytometry and real time-PCR. TREC levels in peripheral blood T lymphocytes were similar between IBD patients and controls. In contrast, UC patients demonstrated significantly increased levels of TRECs both in intraepithelial and lamina propria lymphocytes from the colonic mucosa compared to patients with adenocarcinomas and CD. However, markers for extrathymic T cell maturation in the mucosa were not different between controls and IBD patients. The increased TREC levels in mucosal but not peripheral blood lymphocytes in UC patients in the absence of increased extrathymic maturation in situ in the mucosa together demonstrate that recent thymic emigrants are recruited rapidly to the inflamed mucosa of these patients.

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.

A life of adventure in immunobiology

This article outlines my early start in medicine, a late start in immunology research, and my efforts to integrate the two activities. I first describe some of the background information, excitement, and implications of the recognition of T and B cells as separate but functionally intertwined arms of the adaptive immune system. The article continues with a brief account of my colleagues' and my efforts to use the model of hematopoietic stem cell differentiation along T and B cell lines to gain a better understanding of immunodeficiency diseases and lymphoid malignancies. It concludes with the discovery of a more ancient adaptive immune system in which T-like and B-like cells in jawless vertebrates use variable lymphocyte receptors constructed with leucine-rich-repeat sequences to recognize antigens.

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.

Multiple relapses of visceral leishmaniasis in an adolescent with idiopathic CD4+ lymphocytopenia associated with novel immunophenotypic and molecular features

An adolescent with idiopathic CD4 lymphocytopenia suffered from 4 visceral leishmaniasis relapses despite appropriate treatment. CD8 lymphocytopenia and abnormal expansion of TCRalphabeta, CD4, CD8 cells were consistently detected together with reduced export of mature T cells from thymus. This novel form of idiopathic CD4 lymphocytopenia may predispose to multiple visceral leishmaniasis relapses.

IL-7 sustains CD31 expression in human naive CD4+ T cells and preferentially expands the CD31+ subset in a PI3K-dependent manner

The CD31(+) subset of human naive CD4(+) T cells is thought to contain the population of cells that have recently emigrated from the thymus, while their CD31(-) counterparts have been proposed to originate from CD31(+) cells after homeostatic cell division. Naive T-cell maintenance is known to involve homeostatic cytokines such as interleukin-7 (IL-7). It remains to be investigated what role this cytokine has in the homeostasis of naive CD4(+) T-cell subsets defined by CD31 expression. We provide evidence that IL-7 exerts a preferential proliferative effect on CD31(+) naive CD4(+) T cells from adult peripheral blood compared with the CD31(-) subset. IL-7-driven proliferation did not result in loss of CD31 expression, suggesting that CD31(+) naive CD4(+) T cells can undergo cytokine-driven homeostatic proliferation while preserving CD31. Furthermore, IL-7 sustained or increased CD31 expression even in nonproliferating cells. Both proliferation and CD31 maintenance were dependent on the activation of phosphoinositide 3-kinase (PI3K) signaling. Taken together, our data suggest that during adulthood CD31(+) naive CD4(+) T cells are maintained by IL-7 and that IL-7-based therapies may exert a preferential effect on this population.

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.