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

The contribution of the thymus to the recovery of peripheral naive T-cell numbers during antiretroviral treatment for HIV infection

The quantitative contribution of the thymus to the maintenance of peripheral populations of naive T cells is poorly understood. Several new lines of evidence indicate that thymic activity continues into adulthood, albeit at lower levels than in early life, and that this is important for a range of lymphopenic disorders. A measure of thymic activity that is often used is the quantification of T-cell receptor excision circles (TRECs). It has been shown that TREC levels decline after infection with HIV-1 and that they recover to above normal levels after antiretroviral treatment. The reasons for the latter observation are unknown. Here we quantitatively explore different possible causes for supranormal levels of TREC per cell and show that the small total number of cells involved in reconstituting the TREC+ T-cell pool of HIV-1-infected patients suffices to explain the observation. Even the expected small thymic outputs into a strongly depleted naive T-cell peripheral pool lead to a slow transient of elevated levels of TREC per cell. The main biological lesson from our quantitative modeling approach is that middle-aged human thymi continue to produce naive T cells and that this production can be demonstrated by tracking the increase of total TREC numbers (rather than the TREC content).

Premature aging of the immune system in children with juvenile idiopathic arthritis

OBJECTIVE

Juvenile idiopathic arthritis (JIA) is an autoimmune disease of the young. The pathogenesis is not completely understood. Premature aging, associated thymic involution, and compensatory autoproliferation could play important roles in the pathogenesis of autoimmunity. We undertook this study to determine whether patients with JIA demonstrate premature immunosenescence.

METHODS

To test this hypothesis, we measured 3 indicators of aging: the percentages and total counts of peripheral blood naive T cells, the frequency of T cell receptor excision circles (TRECs) in naive T cells, and telomeric erosion and Ki-67 expression as estimates of the replicative history of homeostatic proliferation.

RESULTS

JIA patients showed an accelerated loss of CD4+,CD45RA+,CD62L+ naive T cells with advancing age and a compensatory increase in the number of CD4+,CD45RO+ memory T cells. JIA patients demonstrated a significantly decreased frequency of TRECs in CD4+,CD45RA+ naive T cells compared with age-matched healthy donors (P = 0.002). TREC numbers correlated with age only in healthy donors (P = 0.0001). Telomeric erosion in CD4+,CD45RA+ naive T cells was increased in JIA patients (P = 0.01). The percentages of Ki-67-positive CD4+,CD45RA+ naive T cells were increased in JIA patients (P = 0.001) and correlated with disease duration (P = 0.003), which was also an independent factor contributing to telomeric erosion (P = 0.04).

CONCLUSION

Our findings suggest that age-inappropriate T cell senescence and disturbed T cell homeostasis may contribute to the development of JIA. In patients with JIA, dysfunction in the ability to reconstitute the T cell compartment should be considered when exploring new therapeutic strategies.

Sparse production but preferential incorporation of recently produced naive T cells in the human peripheral pool

In mice, recent thymic emigrants (RTEs) make up a large part of the naïve T cell pool and have been suggested to be a distinct short-lived pool. In humans, however, the life span and number of RTEs are unknown. Although (2)H(2)O labeling in young mice showed high thymic-dependent daily naïve T cell production, long term up- and down-labeling with (2)H(2)O in human adults revealed a low daily production of naïve T cells. Using mathematical modeling, we estimated human naïve CD4 and CD8 T cell half-lives of 4.2 and 6.5 years, respectively, whereas memory CD4 and CD8 T cells had half-lives of 0.4 and 0.7 year. The estimated half-life of recently produced naïve T cells was much longer than these average half-lives. Thus, our data are incompatible with a substantial short-lived RTE population in human adults and suggest that the few naïve T cells that are newly produced are preferentially incorporated in the peripheral pool.

Immunosenescence of ageing

Ageing is a complex process that negatively impacts the development of the immune system and its ability to function. The mechanisms that underlie these age-related defects are broad and range from defects in the haematopoietic bone marrow to defects in peripheral lymphocyte migration, maturation and function. The thymus is a central lymphoid organ responsible for production of naïve T cells, which play a vital role in mediating both cellular and humoral immunity. Chronic involution of the thymus gland is thought to be one of the major contributing factors to loss of immune function with increasing age. It has recently been demonstrated that thymic atrophy is mediated by a shift from a stimulatory to a suppressive cytokine microenvironment. In this review we present an overview of the morphological, cellular and biochemical changes that have been implicated in the decline of thymic and peripheral immune function with ageing. We conclude with the clinical implications of age-associated immunosenescence to vaccine development for tumours and infectious disease. A fundamental understanding of the complex mechanisms by which ageing attenuates immune function will enable translational research teams to develop new therapies and vaccines specifically aimed at overcoming these defects in immunological function in the aged.

Thymic output, ageing and zinc

The role of the thymus is vital for orchestration of T-cell development and maturation. With increasing age the thymus undergoes a process of involution which results in a reduction in thymic size, function and output. Until relatively recent it was not feasible to accurately measure the magnitude of age-related loss of thymic function. With the discovery of T-cell receptor excision circles (TRECs), which are the stable by-products of the newly generated T-cells, it is now possible to quantitatively measure the extent of thymic output. This review examines the available data on immune function and zinc deficiency and places them in the context of the aims of the ZINCAGE project which include the evaluation of the role played by zinc in maintaining thymic output in healthy elderly individuals.

Determining thymic output quantitatively: using models to interpret experimental T-cell receptor excision circle (TREC) data

T cells develop in the thymus and then are exported to the periphery. As one ages, the lymphoid mass of the thymus decreases, and a concomitant decrease in the ability to produce new T cells results. Human immunodeficiency virus (HIV) infects CD4(+) T cells and, hence, can also affect thymic function. Here we discuss experimental techniques and mathematical models that aim to quantify the rate of thymic export. We focus on a recent technique involving the quantification of T-cell receptor excision circles (TRECs). We discuss how proper interpretation of TREC data necessitates the critical development of appropriate mathematical models. We review the theory for interpretation of TREC data during aging, HIV infection, and anti-retroviral treatment. Also, we show how TRECs can be used to accurately quantify thymic output in the context of thymectomy experiments. We show that mathematical models are not only useful but absolutely necessary for these analyses. As such, they should be taken as just another tool in the immunologist's arsenal.

Dynamics of CD4+ T cells in HIV-1 infection

Mathematical modeling is becoming established in the immunologist's toolbox as a method to gain insight into the dynamics of the immune response and its components. No more so than in the case of the study of human immunodeficiency virus (HIV) infection, where earlier work on the viral dynamics brought significant advances in our understanding of HIV replication and evolution. Here, I review different areas of the study of the dynamics of CD4+ T cells in the setting of HIV, where modeling played important and diverse roles in helping us understand CD4+ T-cell homeostasis and the effect of HIV infection. As the experimental techniques become more accurate and quantitative, modeling should play a more important part in both experimental design and data analysis.

Replication history of B lymphocytes reveals homeostatic proliferation and extensive antigen-induced B cell expansion

The contribution of proliferation to B lymphocyte homeostasis and antigen responses is largely unknown. We quantified the replication history of mouse and human B lymphocyte subsets by calculating the ratio between genomic coding joints and signal joints on kappa-deleting recombination excision circles (KREC) of the IGK-deleting rearrangement. This approach was validated with in vitro proliferation studies. We demonstrate that naive mature B lymphocytes, but not transitional B lymphocytes, undergo in vivo homeostatic proliferation in the absence of somatic mutations in the periphery. T cell-dependent B cell proliferation was substantially higher and showed higher frequencies of somatic hypermutation than T cell-independent responses, fitting with the robustness and high affinity of T cell-dependent antibody responses. More extensive proliferation and somatic hypermutation in antigen-experienced B lymphocytes from human adults compared to children indicated consecutive responses upon additional antigen exposures. Our combined observations unravel the contribution of proliferation to both B lymphocyte homeostasis and antigen-induced B cell expansion. We propose an important role for both processes in humoral immunity. These new insights will support the understanding of peripheral B cell regeneration after hematopoietic stem cell transplantation or B cell-directed antibody therapy, and the identification of defects in homeostatic or antigen-induced B cell proliferation in patients with common variable immunodeficiency or another antibody deficiency.

Differential Biological Role of CD3 Chains Revealed by Human Immunodeficiencies

The biological role in vivo of the homologous CD3gamma and delta invariant chains within the human TCR/CD3 complex is a matter of debate, as murine models do not recapitulate human immunodeficiencies. We have characterized, in a Turkish family, two new patients with complete CD3gamma deficiency and SCID symptoms and compared them with three CD3gamma-deficient individuals belonging to two families from Turkey and Spain. All tested patients shared similar immunological features such as a partial TCR/CD3 expression defect, mild alphabeta and gammadelta T lymphocytopenia, poor in vitro proliferative responses to Ags and mitogens at diagnosis, and very low TCR rearrangement excision circles and CD45RA(+) alphabeta T cells. However, intrafamilial and interfamilial clinical variability was observed in patients carrying the same CD3G mutations. Two reached the second or third decade in healthy conditions, whereas the other three showed lethal SCID features with enteropathy early in life. In contrast, all reported human complete CD3delta (or CD3epsilon) deficiencies are in infants with life-threatening SCID and very severe alphabeta and gammadelta T lymphocytopenia. Thus, the peripheral T lymphocyte pool was comparatively well preserved in human CD3gamma deficiencies despite poor thymus output or clinical outcome. We propose a CD3delta >> CD3gamma hierarchy for the relative impact of their absence on the signaling for T cell production in humans.

Aging and T-cell diversity

Naïve and memory CD4 and CD8 T cells constitute a highly dynamic system with constant homeostatic and antigen-driven proliferation, influx, and loss of T cells. Thymic activity dwindles with age and essentially ceases in the later decades of life, severely constraining the generation of new T cells. Homeostatic control mechanisms are very effective at maintaining a large and diverse subset of naïve CD4 T cells through the 7th decade of life, but eventually and abruptly fail at about the age of 75 years. In contrast, the CD8 T cell compartment is more unstable, with progressive diminution of naïve T cells and increasing loss of diversity during mid adulthood. Vaccination strategies need to aim at developing a broad repertoire of memory T cells before the critical time period when the naïve CD4 T-cell repertoire collapses. Research efforts need to aim at understanding T-cell homeostatic control mechanisms to ultimately expand the time period of repertoire stability.

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

Due to homeostasis total naive T cell numbers remain fairly constant over life despite a gradual involution of the thymus. The contribution of the thymus to maintaining naive T cell pools is typically measured with TCR excision circles (TRECs) that are formed in thymocytes. The mechanisms underlying thymic involution are poorly understood. Some data suggest that thymocytes undergo fewer divisions in old (small) than young (large) thymi, and other data suggest that the number of TRECs per thymocyte is independent of age. If thymic involution were associated with a decreased number of divisions of the thymocytes, this would markedly complicate the interpretation of TREC data. To study this we develop a mathematical model in which the division rate of thymocytes decreases with increasing age. We describe the dilution of TRECs formed during the arrangement of both chains of the TCR by division of thymocytes, recent thymic emigrants, and mature naive T cells. The model behavior is complicated as TREC contents in naive T cells can increase with age due to decreased dilution in the thymus. Because our model is consistent with current data on the effects of age and thymectomy on TRECs in peripheral T cells, we conclude that aging may well affect thymocyte division, which markedly complicates the interpretation of TREC data. It is possible, but more difficult, to let the model be consistent with the rapid changes in alpha and beta TRECs observed shortly after HIV infection.

Association of CD4+ T-lymphocyte counts and new thymic emigrants in HIV-infected children during successful highly active antiretroviral therapy

BACKGROUND

In a cohort of children receiving highly active antiretroviral therapy (HAART) with sustained plasma HIV-1 RNA < 50 copies/mL, children who reached undetectable RNA after week 8 (slow responders, median: week 20) had higher HIV-1 intracellular DNA (HIV-1 DNA) and equal or greater CD4+ T-lymphocyte counts compared with children who reached undetectable plasma HIV-1 RNA by week 8 (rapid responders) throughout HAART.

OBJECTIVE

To determine whether levels of T-cell receptor excision circles (TRECs) could explain the apparent inconsistency between the quantity of HIV-1 DNA and CD4+ T-lymphocyte counts in HIV-1-infected children receiving HAART with sustained virologic suppression.

METHODS

T-cell receptor excision circles and HIV-1 DNA and plasma HIV-1 RNA were quantified longitudinally by PCR in 31 children (median age, 5.6 years) with sustained undetectable plasma HIV-1 RNA for >104 weeks of HAART.

RESULTS

There was a positive correlation between TREC and HIV-1 DNA during HAART, notably at weeks 48 and 80 (P < .004). During the early stage of HAART, TREC levels positively correlated with CD4+ T-lymphocyte percentages (P < .02) and naive CD4+ T-lymphocyte counts (P < .001) and percentages (P = .05). Median TREC levels were consistently equal or higher in slow responders compared with rapid responders (P < .001) despite slow responders having consistently greater quantities of HIV-1 DNA.

CONCLUSION

To maintain adequate levels of CD4+ T-lymphocytes, children with high HIV-1 DNA maintain high levels of TREC while receiving HAART. Thus, a thymic control mechanism is required to maintain new CD4+ T lymphocytes in the presence of persistent virus.

CLINICAL IMPLICATIONS

The TREC level is a useful marker of thymic function in HIV-infected children.

Thymic output in aged mice

Using GFP to mark recent thymic emigrants (RTEs) in mice carrying a GFP transgene driven by the recombination-activating gene 2 promoter, we demonstrate that RTEs are readily detectable even in 2-year-old mice, despite the fact that the proportion of the peripheral T cell pool comprised of RTEs declines with age. Although the number of RTEs decreases after reaching a peak at 6 weeks of age, thymic output as a function of thymic size is surprisingly age-independent. The CD4:CD8 ratio of RTEs declines with age, partly because of a striking decrease in steady-state proliferation of CD4+ RTEs in older mice. RTEs in aged mice undergo phenotypic maturation in the lymphoid periphery with delayed kinetics compared with young mice. RTEs from aged mice secrete less IL-2, proliferate less well, and achieve only weak expression of early-activation markers compared with more mature naïve peripheral T cells from the same mice. The proportion of GFP- cells in the CD4+ and CD8+ thymic compartments increases with age, partly as a result of leakiness in the aged thymus, allowing reentry of naïve peripheral T cells.

Imbalance of regulatory T cells in human autoimmune diseases

The breakdown of mechanisms assuring the recognition of self and non-self is a hallmark feature of autoimmune diseases. In the past 10 years, there has been a steadily increasing interest in a subpopulation of regulatory T cells, which exert their suppressive function in vitro in a contact-dependent manner and preferentially express high levels of CD25 and forkhead and winged-helix family transcription factor forkhead box P3 (FOXP3) (TREGs). Recent findings of changed prevalences and functional efficiencies indicate that these TREGs play a unique role in autoimmune diseases. Clinical findings in patients with mutated FOXP3 genes and a specific polymorphism in the promotor region of FOXP3 also support the role of FOXP3 as a 'master control gene' in the development and functioning of TREGs. Both altered generation of TREGs and insufficient suppression of inflammation in autoimmune diseases are considered to be crucial for the initiation and perpetuation of disease. TREG-related somatic cell therapy is considered as an intriguing new intervention to approach autoimmune diseases.

Levels of recent thymic emigrant cells decrease in children undergoing partial thymectomy during cardiac surgery

The human thymus is required for establishment of a T-cell pool in fetal life, but postnatal thymectomy is not known to cause immunodeficiency. T-cell emigration from thymus (thymic recent emigrants [TRECs]) is a continuous thymic-dependent process. We studied TREC levels pre- and post-partial thymectomy in children undergoing cardiac surgery. TRECs were quantitated by real-time PCR in peripheral blood lymphocytes of 24 children (0 to 12 years). TREC values were 47916 +/- 9271 pre-partial thymectomy and 33157 +/- 8479 post-partial thymectomy in 11 paired patients (P = 0.014). Interval between pre- and post-partial thymectomy was 8.8 days +/- 5.8 days. Another group of 8 children had 30384 +/- 9748 TRECs 16 days to 6 years post-partial thymectomy. There was a significant drop in TREC values post-partial thymectomy in the immediate postoperative period compared to prethymectomy TREC levels. While decreased thymic output may persist, the long-term implications were not evaluated in this patient population.

T cell development and receptor diversity during aging

The T cell system is a complex and highly dynamic system that is amazingly robust over many decades of human life. Its functional competence is determined not only by its size but also by its diversity. Homeostatic control mechanisms have to secure sufficient T cell replenishment while preventing loss of clonal diversity. Major homeostatic challenges include profound expansion and shrinkage of T cell clonotypes upon antigenic triggering and, more importantly, age-related changes in T cell regeneration. The ability of the thymus to rebuild a diverse repertoire ceases in the fifth decade of life. Emerging data suggest that the end of the 7th decade of life defines a critical time period when T cell homeostasis is no longer guaranteed and diversity of the naïve T-cell repertoire collapses. Thus, failure of T cell homeostasis appears to result from cumulative defects of T cell generation. Elucidation of the underlying mechanisms will allow for extending this turning point to later in life; ultimately, interventions have to aim at restoring thymic function and complementary modes of T cell reconstitution.

Decreased T-Cell Receptor Excision Circles in Cutaneous T-Cell Lymphoma

PURPOSE

The T cell repertoire in patients with advanced cutaneous T cell lymphoma (CTCL) is significantly contracted despite the presence of relatively normal absolute numbers of T cells. We propose that many normal T cells were being lost in patients with CTCL, with the remaining normal T cells expanding clonally to fill the T cell compartment. T-cell receptor excision circles (TREC) form as a result of the initial gene rearrangement in naïve T cells. Although they are stable, they do not replicate and are subsequently diluted with the expansion of a population of T cells. Their concentration is therefore a measure of unexpanded naïve T cells relative to T cells that have undergone expansion.

EXPERIMENTAL DESIGN

We analyzed TRECs from unfractionated peripheral blood T cells from 108 CTCL patients by quantitative PCR. In patients with obvious peripheral blood involvement, we also analyzed TRECs from clonal and nonclonal T cells.

RESULTS

We found a decrease in the number of TRECs in peripheral blood of patients with CTCL at all stages of disease, and this decrease was proportional to the loss of complexity of the T cell repertoire as measured by complementarity-determining region 3 spectratyping. In patients with leukemic CTCL and a numerically expanded clone, we also found a significantly lower-than-expected number of TRECs in the nonclonal normal T cells.

CONCLUSIONS

We hypothesize that the nonmalignant T cells have proliferated to fill the empty T cell repertoire space left by the loss of other T cells, leading to diminished TRECs and loss of T-cell receptor diversity.

Peripheral T-cell expansion and low infection rate after reduced-intensity conditioning and allogeneic blood stem cell transplantation

Peripheral blood stem cell transplantation after reduced-intensity conditioning (RIC-PBSCT) regimen is an alternative to conventional regimens with less immediate toxicity. Since immune recovery is of crucial importance for the control of infections, we retrospectively studied the recovery of T-, B- and NK cell subsets in 20 consecutive patients undergoing RIC-PBSCT. We also studied the thymic output using T-cell receptor excision circle assay. Engraftment was rapid and few infectious complications were seen: three early (before 2.5 months) cases of asymptomatic cytomegalovirus reactivation, two late Gram-negative bacterial infections and no fungal infection. While CD4+ T-cell reconstitution was slow, CD8+ T-cell counts were close to normal values at 4 months. Median CD19+ B-cell counts reached normal values at 11 months. Rapid CD56+ NK cell reconstitution was noticed as early as 1.5 months. Low T-cell receptor excision circle numbers and preponderance of memory-type subsets among T cells further suggested that CD8+ T-cell reconstitution resulted predominantly from peripheral expansion and that thymic-dependent reconstitution was severely impaired. In conclusion, large peripheral T-cell expansion may compensate for late thymic-dependent lymphopoiesis, and may, with other factors such as NK and B-cell reconstitution and careful antiinfectious prophylaxis, help limit the incidence of severe infections after RIC-PBSCT.

The influence of age on T cell generation and TCR diversity

The ability to mount protective immune responses depends on the diversity of T cells. T cell diversity may be compromised by the declining thymic output of new T cells. The aging process imposes a threat to diversity, because thymic function deteriorates. In this study we have examined the relationship between thymic production, homeostatic T cell proliferation and TCR beta-chain diversity in young (approximately 25 years), middle-aged ( approximately 60 years), and elderly adults (approximately 75 years). TCR excision circles (TREC) as a marker of thymic output exponentially decreased by >95% between 25 and 60 years of age. The frequency of Ki67(+) cycling CD4 T cells remained steady, and surprisingly, the diversity of the naive CD4 T cell repertoire was maintained at approximately 2 x 10(7) different TCR beta-chains. After the age of 70 years, TRECs only slightly declined, but homeostatic proliferation doubled. The diversity of the T cell pool drastically contracted to 200,000 TCR beta-chains. Also, the phenotypic distinction between naive and memory CD4 T cells became fuzzy. The collapse in CD4 T cell diversity during the seventh and eighth decades indicates substantial T cell loss and implies that therapeutic measures to improve vaccine responses will have to include strategies for T cell replenishment.

Rheumatoid arthritis

Therapeutic efficacy of depleting B cells or blocking T-cell costimulation in rheumatoid arthritis (RA) has confirmed the critical pathogenic role of adaptive immune responses. Yet, RA preferentially affects elderly individuals, in whom adaptive immunity to exogenous antigens begins to fail. Here, we propose that senescence of the immune system is a risk factor for RA, with chronic inflammation resulting from the accumulation of degenerate T cells that have a low threshold for activation and utilize a spectrum of novel receptors to respond to microenvironmental cues. The process of immunosenescence is accelerated in RA and precedes the onset of disease, the acceleration, in part, being conferred by the HLA-DR4 haplotype. Naive CD4(+) T cells in RA are contracted in diversity and restricted in clonal burst. Senescence of effector CD4(+) T cells is associated with the loss of CD28 and the de novo expression of KIR2DS2, NKG2D, and CX(3)CR1, all of which function as costimulatory molecules and reduce the threshold for T-cell activation. The synovial microenvironment promotes chronic persistent immune responses by facilitating ectopic lymphoid neogenesis, such as the formation of aberrant germinal centers. With the propensity to develop complex lymphoid architectures and to provide optimal activation conditions for senescent CD4(+) T cells, the synovium becomes a natural target for pathogenic immune responses in prematurely aged individuals.

Is tolerance induction the answer to adolescent non-adherence?

By definition, tolerance will eliminate the problem of adolescent medication non-adherence. Although adolescents' propensity toward non-adherence makes them at first glance to be particularly attractive candidates for tolerance trials, there are also immunologic, psychosocial and ethical barriers that temper enthusiasm for their inclusion at present. Limits in emotional and cognitive maturity are combined during the teenage years with adult-like immunologic maturity to lessen the potential for successful implementation of tolerance and near tolerance strategies. Alternatively, an interval step to tolerance in adolescents is to eliminate the medications most likely contributing to non-adherence through harsh side effects such as steroids and calcineurin inhibitors. This manuscript will review the general topic of transplantation tolerance with specific attention given to the application of pro-tolerant therapies in adolescent recipients.

Insights into thymic aging and regeneration

The deterioration of the immune system with progressive aging is believed to contribute to morbidity and mortality in elderly humans due to the increased incidence of infection, autoimmunity, and cancer. Dysregulation of T-cell function is thought to play a critical part in these processes. One of the consequences of an aging immune system is the process termed thymic involution, where the thymus undergoes a progressive reduction in size due to profound changes in its anatomy associated with loss of thymic epithelial cells and a decrease in thymopoiesis. This decline in the output of newly developed T cells results in diminished numbers of circulating naive T cells and impaired cell-mediated immunity. A number of theories have been forwarded to explain this 'thymic menopause' including the possible loss of thymic progenitors or epithelial cells, a diminished capacity to rearrange T-cell receptor genes and alterations in the production of growth factors and hormones. Although to date no interventions fully restore thymic function in the aging host, systemic administration of various cytokines and hormones or bone marrow transplantation have resulted in increased thymic activity and T-cell output with age. In this review, we shall examine the current literature on thymic involution and discuss several interventional strategies currently being explored to restore thymic function in elderly subjects.

Limited role for the thymus in SIV pathogenesis

The role of the thymus in the pathogenesis of AIDS is a frequently discussed and controversial topic. Tuttleton Arron et al. studied the role of thymic output in SIV infection directly, by comparing the dynamics of TCR excision circles and CD4(+ )and CD8(+) T cell numbers in healthy and in SIV-infected euthymic and thymectomized rhesus macaques. In this issue of the European Journal of Immunology, they report that complete abrogation of thymic output in juvenile rhesus macaques has very little impact on the peripheral T cell compartment, both in healthy and in SIV-infected macaques. Their data therefore suggest that the main cause of CD4(+) T cell loss during SIV infection is the peripheral effect of SIV, and not its effect on thymic output.

Chromosomal excision of TCRδ chain genes is dispensable for αβ T cell lineage commitment

TCRbeta, delta and gamma chain genes are assembled and expressed in double-negative thymocytes prior to alphabeta or gammadelta T cell lineage commitment. Thus, cells committed to the alphabeta T cell lineage can possess completely assembled TCRdelta and/or TCRgamma chain genes. However, these genes are not expressed. TCRgamma chain gene expression may be silenced through the activity of a cis-acting silencer element. In the TCRalpha/delta locus, the TCRdelta genes lie between the Valpha and Jalpha gene segments, which rearrange by deletion. Moreover, Valpha to Jalpha rearrangements occur on both alleles in essentially all developing alphabeta T cells. Consequently, both TCRdelta chain genes are excised from the chromosome and placed on extrachromosomal circles in mature alphabeta T cells. It has been proposed that this excision process is important for silencing TCRdelta gene expression and permitting alphabeta T cell lineage commitment. A gene-targeting Cre-loxP strategy was used to invert a 75-kb region of the TCRalpha/delta locus encompassing all the Jalpha gene segments, generating the TCRalpha/delta(I) allele. Initial Valpha to Jalpha rearrangements on the TCRalpha/delta(I) allele occur by inversion, resulting in chromosomal retention of TCRdelta chain genes. These TCRdelta chain genes can be productively rearranged and are expressed at levels similar to TCRdelta chain genes in gammadelta T cells. However, alphabeta T cell development appears unperturbed in TCRalpha/delta(I/I) mice. Thus, excision of TCRdelta genes from the chromosome per se is not required for commitment of developing lymphocytes to the alphabeta T cell lineage.

Assessing thymopoiesis in patients with common variable immunodeficiency as measured by T-cell receptor excision circles

BACKGROUND

Common variable immunodeficiency (CVID) is one of the most common primary immune deficiencies. The immunologic hallmark of CVID is failure of B-cell differentiation and impaired secretion of immunoglobulins. However, there is mounting evidence of accompanying T-cell dysregulation, which could be due to abnormal thymic function because the thymus plays a crucial role in T-cell development. Recently, it was shown that the human thymus remains functional well into adulthood. Current data show that the level of T-cell receptor excision circles (TRECs) correlates well with active thymopoiesis.

OBJECTIVE

To determine whether thymic dysfunction contributes to the pathogenesis of CVID.

METHODS

We evaluated 15 patients, aged 19 to 65 years, previously diagnosed as having CVID. Genomic DNA was isolated from peripheral blood mononuclear cells of each patient. Thymic output was evaluated by measuring coding joint TRECs in the total T-cell population using real-time polymerase chain reaction.

RESULTS

Results were compared with known age-matched reference values. The median TREC level in patients with CVID (82,034 copies/microg of DNA) was significantly higher than that in the healthy cohort (43,000 copies/microg of DNA) (P < .001). In examining the relationship between TREC levels and age, we noted that TREC levels significantly declined faster with age in patients with CVID vs the healthy cohort.

CONCLUSIONS

In these patients, thymic dysregulation may be a factor in CVID, with an accelerated rate of TREC loss with age compared with healthy adults.

HIV Infection Rapidly Induces and Maintains a Substantial Suppression of Thymocyte Proliferation

The supply of naive T cells by the thymus normally requires precursor T cell proliferation within the thymus and would be particularly important in the setting of HIV infection when both naive and memory T cells are progressively depleted. As a robust, quantitative index of intrathymic proliferation, the ratio of different T cell receptor excision circles (TRECs), molecular markers of distinct T cell receptor rearrangements occurring at different stages of thymocyte development, was measured in peripheral blood-mononuclear cells (PBMCs). This ratio has the virtue that it is a "signature" of thymic emigrants throughout their entire life and, thus, can be measured in peripheral cell populations that are easy to obtain. Using the new assay, we evaluated the effect of HIV infection on intrathymic precursor T cell proliferation by longitudinal analysis of PBMCs from recently infected individuals. Our findings reveal a substantial reduction in intrathymic proliferation. The analysis also indicates the existence of a compensatory mechanism acting to sustain the numbers of recent thymic emigrants (RTEs) in the periphery.

T-cell regulation in rheumatoid arthritis

PURPOSE OF REVIEW

Rheumatoid arthritis (RA) is characterized by a chronic T-cell response that has escaped normal control mechanisms. This review summarizes recent insights in pathways that are functional in RA and that favor continuous and pathogenic T-cell activation.

RECENT FINDINGS

T-cell activation is ultimately determined by positive signals from costimulatory molecules and negative signals from regulatory T cells. Blockade of the classic costimulatory pathway, CD28-CD80 or CD86, is beneficial in RA. Additional pathways that predominantly control the activation of memory and effector T cells are functionally important in synovial inflammation. Some of these costimulatory molecules (such as stimulatory killer cell immunoglobulin-like receptors and NKG2D) appear to be relatively specific for RA and not to play a role in normal immune responses. In addition to this predominance of positive signals, age-disproportionate decline in thymic activity in RA may lead to a diminution of regulatory T cells and loss of their negative signals.

SUMMARY

The successful treatment trial of RA with CTLA-4Ig clearly documents the importance of T-cell costimulation in RA disease activity. Novel costimulatory pathways may be of even greater significance than CD28 in RA and may represent promising new therapeutic targets. The finding of reduced thymic activity in RA is exciting and will stimulate further studies of T-cell homeostasis and the function of regulatory cells.

Continued maturation of thymic emigrants in the periphery

Developing thymocytes are selected for recognition of molecules encoded by the major histocompatibility complex, purged of self-reactive cells and committed to either the CD4 or CD8 lineage. The 1% of thymocytes that complete these tasks emigrate and join the population of peripheral lymphocytes. Whether T cell maturation is complete at the time of thymic exit has been a subject of debate. Using mice transgenic for green fluorescent protein driven by the recombination activating gene 2 promoter to identify recent thymic emigrants, we now show that T cell differentiation continues post-thymically, with progressive maturation of both surface phenotype and immune function. In addition, the relative contribution of CD4 and CD8 recent thymic emigrants was modulated as they entered the peripheral T cell pool. Thus, T cell maturation and subset contribution are both finalized in the lymphoid periphery.

Thymic function and immunoglobulin mutation genotype in B-cell chronic lymphocytic leukemia patients

B-cell chronic lymphocytic leukemia (B-CLL) is characterized by a profound dysregulation of the host's immune system at both the humoral and cellular level. We investigated to see if this dysregulation could be due partly to thymus dysfunction by quantifying the number of signal joint T-cell receptor excision circles (sjTRECs) in peripheral blood mononuclear cells of 30 untreated B-CLL patients at diagnosis and in age-matched healthy controls. sjTRECs were found decreased, normal and elevated in 19, 9 and 2 patients, respectively, in comparison to age-matched controls. We next speculated that sjTREC levels might be related to an accredited B-CLL prognostic marker represented by the somatic hypermutation (SHM) status of the variable heavy chain (V(H)) genes. Eight of 17 patients with SHMs had sjTREC levels in the range of or higher than normal donors, whereas only 3 of the 13 patients lacking SHMs had normal sjTREC levels. After a 5-year observation period in 16 patients for whom a clinical follow-up was available, only 2 of 10 patients with SHMs progressed vs. 5 of 6 patients without SHMs and 3 of 7 patients with normal or higher sjTREC levels progressed vs. 4 of 9 with low sjTREC levels. Our study demonstrates that sjTREC levels are decreased in >60% of B-CLL patients and suggests a potential role of thymus in the immune dysfunction of these patients.

Premature telomeric loss in rheumatoid arthritis is genetically determined and involves both myeloid and lymphoid cell lineages

In rheumatoid arthritis, peripheral blood T cells have age-inappropriate telomeric erosion. We examined whether HLA-DRB1*04 alleles, the major susceptibility genes for this disease, confer risk for T cell senescence. In healthy individuals, HLA-DRB1*04 alleles were associated with excessive loss of telomeres in CD4+ T cells. Accelerated telomeric erosion occurred during the first two decades of life and was followed by reduced homeostatic T cell proliferation during adulthood. Premature telomeric loss also affected granulocytes, suggesting that the hematopoietic stem cell is the primary target. Telomeric repair mechanisms were intact in HLA-DRB1*04+ donors. We propose that HLA-DRB1*04 alleles or genes in linkage disequilibrium regulate stem cell replication and contribute to the accumulation of senescent and autoreactive T cells in rheumatoid arthritis.

Neuregulin-1 suppresses muscarinic receptor expression and acetylcholine-activated muscarinic K+ channels in cardiac myocytes

The neuregulin-1 family of growth factors regulates nicotinic acetylcholine receptor synthesis in skeletal muscle, but its role in cardiac myogenesis remains unclear. Here, we investigate the involvement of neuregulins in the development of cardiac cholinergic responsiveness. Treatment of chick cardiac myocytes with neuregulin-1 inhibited mRNA expression of the M4 muscarinic receptor, but not the M2 receptor. In addition, mRNA levels of GIRK1 were reduced in myocytes by treatment with neuregulin-1. Activation of cholinergic receptors in cultured chick atrial myocytes by carbachol produced an outward potassium current (I(K(ACh))), which was attenuated by 24-48-h pre-treatment with neuregulin-1. These data suggest that neuregulins can regulate cardiac parasympathetic tone and may be involved in the pathogenesis of cardiac arrhythmias and heart failure.

A trial of IRX-2 in patients with squamous cell carcinomas of the head and neck

A Phase II trial in 42 patients with squamous cell cancer of the head and neck (H and NSCC) was performed using a combination immunotherapy with 10-20 days of perilymphatic injections of a natural cytokine mixture (NCM: IRX-2; 200 units IL-2 equivalence) preceded by low dose cyclophosphamide (CY; 300 mg/m(2)) and followed by daily oral indomethacin (25 mg t.i.d.) and zinc (65 mg in a multivitamin preparation). Thirty-nine patients underwent subsequent surgical resection and 22 stage IV patients received additional radiotherapy. Forty-two percent were adjudged to have complete and partial clinical responses (>50% tumor reduction); an additional five patients had minor responses for a total of 58%. Comparison of post-treatment biopsies or surgical specimens showed 90% of patients had reduction in tumor area from 79% to 48% (over half of which was fragmented) and increased area of leukocyte infiltration from 9% to 32% (79% of which was lymphoid). The treatment with IRX-2 was not associated with significant side effects and 24 of patients showed improvement in eating, breathing or phonation or reduced pain and bleeding. Fifteen patients with lymphocytopenia (lymphocyte counts [LC] less than or equal to 1500 mm(3)) showed significant increases in LC, CD3+, CD4+ and CD8+ T lymphocytes of 401, 147, 95 and 100/mm(3), respectively. Analysis of outcome of 32 on protocol patients after 36 months versus 32 concurrent institutional H and NSCC controls showed delayed recurrences and significant increases in mean survival time (MST) and survival (p's<0.02). The data document immunotherapy induced regression of H and NSCC with delayed recurrence and improved mean survival time.

Thymic export function and T cell homeostasis in patients with relapsing remitting multiple sclerosis

Multiple sclerosis (MS) is an inflammatory and possibly autoimmune mediated demyelinating disease of the CNS. Autoimmunity within the CNS may be triggered by dysfunction of peripheral immune tolerance mechanisms via changes in the homeostatic composition of peripheral T cells. We have assessed the release of naive T lymphocytes from the thymus in patients with relapsing remitting MS (RRMS) to identify alterations in the equilibrium of the peripheral T cell compartment. Thymic T cell production was estimated by measuring TCR excision circles (TRECs) as a traceable molecular marker in recent thymic emigrants. A total of 46 treatment-naive patients with active RRMS and 49 gender- and age-matched healthy persons were included in the study. The levels of TREC-expressing CD4(+) and CD8(+) T lymphocytes were significantly decreased in MS patients, and TREC quantities overall matched those of 30 years older healthy individuals. The average concentrations of TRECs/10(6) CD4(+) and CD8(+) T lymphocytes derived from MS patients and healthy donors were 26 x 10(3)/10(6) and 28 x 10(3)/10(6) vs 217 x 10(3)/10(6) and 169 x 10(3)/10(6), respectively. To account for any influence of T cell proliferation on TREC levels, we assayed T lymphocytes from additional patients with MS and normal individuals for telomere length (n = 20) and telomerase activity (8 MS patients, 16 controls), respectively. There were no significant differences between CD4(+) and CD8(+) T cells from MS patients and controls. Altogether, our findings suggest that an impaired thymic export function and, as a consequence, altered ability to maintain T cell homeostasis and immune tolerance may play an important pathogenic role in RRMS.

T-cell receptor excisional circles, telomere length, proliferation and apoptosis in peripheral blood mononuclear cells of human immunodeficiency virus-infected individuals after 18 months of treatment induced viral suppression

This study evaluated the effect of highly active antiretroviral therapy (HAART)-induced viral suppression on T-cell receptor excisional circles (TRECs), telomere length, proliferative responses and spontaneous as well as phytohaemagglutinin (PHA)-stimulated lymphocyte apoptosis in 27 human immunodeficiency virus (HIV)-infected individuals followed for 18 months during HAART. Our results show that HAART significantly increased the level of TRECs in CD4+ cells (P = 0.003) after 18 months of almost continuously suppressed HIV-RNA levels. Lymphocyte proliferative responses and apoptosis levels in patients were significantly lower and significantly higher, respectively, compared with healthy controls. The proliferative response and apoptosis levels did not change during follow up. Changes in telomere length were observed in CD4+ and in CD8+ T cells. The study demonstrated that HAART induces normal TREC levels in the CD4+ T-cell pool. However, the other perturbed functions in T cells indicate that immune reconstitution is incomplete and may need longer viral suppression.

Quantification of T-cell receptor excision circle DNA using fluorescence resonance energy transfer and the LightCycler system

T-cell receptor excision circles (TRECs) are circular, stable extrachromosomal DNA fragments and are generated during V(D)J gene recombination, a process responsible for diversity of the T-cell receptor repertoire. Here we describe a sensitive, rapid and easy to perform real-time PCR assay based on the LightCycler technique for the quantification of TRECs among peripheral blood cells and a comparison of this assay with conventional PCR-ELISA. By LightCycler, a sensitivity of 20 copies of plasmid deltaRec-psiJalpha Signal Joint TREC DNA was achieved whereas by PCR-ELISA, a detection limit of 2 copies was demonstrated. In blood samples from healthy individuals (n=52) a median TREC count of 1.6x10(4) copies [range 2x10(1) to 2x10(5)]/2x10(5) peripheral blood mononuclear cells (PBMNCs) and in cord blood, a median TREC count of 1.45x10(5) copies [range 1.2x10(5) to 1.6x10(5)]/2x10(5) PBMNCs) could be detected. No significant difference was found in 15 individuals when unfractionated PBMNCs (median count of 1.1x10(5) copies/2x10(5) PBMNCs) or magnetic associated cell sorter (MACS)-sorted CD45RA+ T-cells (median count of 2.5x10(5) copies/2x10(5) cells) were analyzed for TREC counts. In addition, we examined the number of deltaRec-psiJalpha Signal Joint TREC in paediatric (n=6) and adult patients (n=7) after allogeneic stem cell transplantation. In children, we observed a median TREC count of 5.7x10(4) copies/2x10(5) PBMNCs after 1x10(4) and 5.6x10(4) copies/2x10(5) PBMNCs after 2 years, and in adults, a median count of 3.6x10(4) copies/2x10(5) PBMNCs after 1x10(4) and 1.1x10(4) copies/2x10(5) PBMNCs after 2 years. In conclusion, the LightCycler-based real-time PCR assay described offers a very sensitive and rapid tool for the quantification of TREC DNA.

Dynamics of T cells and TCR excision circles differ after treatment of acute and chronic HIV infection

We quantified T cell proliferation and thymic function in primary HIV infection (PHI; n = 19) and chronic HIV infection (CHI; n = 14) by measuring Ki67 staining and TCR excision circle (TREC) number. After antiretroviral therapy of PHI there is a profound decrease in the number and percentage of Ki67(+) T cells (<6% Ki67(+)) with no significant increase in TREC per million cells and a transient increase in TREC per milliliter. In contrast, after antiretroviral therapy of CHI there is a reduction in the percentage but little change in the total number of Ki67(+)CD4(+) T cells associated with increases in both TREC per million cells and TREC per milliliter. Using a mathematical model that accounts for proliferation, death, and redistribution of T cells, we find that redistribution is consistent with the TREC changes observed during treatment of PHI and that an increase in thymic output is needed to explain the increase in TREC during treatment of CHI. Consideration of TREC per milliliter shows that changes in proliferation alone cannot explain the changes in TREC. In addition, although increased proliferation of memory cells in HIV infection has been established, we find no difference in TREC per million CD45RA(-) "memory" T cells between healthy and infected individuals (p = 0.154 for CD4(+); p = 0.383 for CD8(+)). Finally, although the number of TREC per million cells is always much lower in memory T cells than in naive T cells, in the setting of HIV infection, given that memory cells make up a larger proportion of total T cells, we find that 50% of TREC per milliliter in CD4(+) T cells is harbored in the CD45RA(-) "memory" subset of our infected subjects.

Development of alphabeta T cells in the human thymus

The thymus is the main producer of alphabeta T cells and is, therefore, crucial for a normal immune system. The intrathymic developmental pathway of human alphabeta T cells has now been delineated. The production of new T cells by the thymus decreases with age, and the thymus was thought to be redundant in adults once the peripheral T-cell pool has been formed early in life. However, recent work has shown that the thymus can function even at an advanced age. Research into the production of T cells in clinical settings that are associated with loss of T cells in the periphery has sparked renewed interest in the function of the human thymus.

Recent thymic emigrants and prognosis in T- and B-cell childhood hematopoietic malignancies

The concentration of T-cell receptor rearrangement excision DNA circles (TRECs) in peripheral blood mononuclear cells (PBMCs) is currently known to be a marker of recent thymic emigrants. We evaluated the hypothesis that TREC values would be lower in childhood T-cell hematopoietic malignancies than in childhood B-cell acute lymphoblastic leukemia (ALL) or healthy controls because the former category may reflect compromised thymic function. From the Greek national childhood leukemia/lymphoma database we obtained all 30 available T-cell leukemia/non-Hodgkin's lymphoma cases, 30 age- and sex-matched childhood B-cell origin cases of ALL and 60 healthy hospital controls. We compared TREC levels in PBMCs using a real-time PCR assay. There was highly significant reduction of TREC values in children with T-cell malignancies (median 3,100 TRECs/10(6) PBMCs), whereas children with B-cell origin ALL had slightly but nonsignificantly lower TREC values compared to healthy children (medians 19,300 and 22,500 TRECs/10(6) PBMCs, respectively). During a median follow-up period of about 19 months, only 4 children died. All of them had a T-cell hematopoietic malignancy and relatively low TREC values. The number of TRECs was higher among healthy girls than among healthy boys, and a similar pattern was evident in T-cell malignancies. It appears that there is a pattern of concordance of high TREC values with better disease prognosis in hematologic childhood malignancies. This applies to specific disease entities with better prognosis (B-cell origin ALL having higher TREC values than T-cell leukemia/lymphoma) and to gender, another important predictor of prognosis conditional on disease entity (girls having higher TREC values than boys); however, it may also be true for the survival of individual patients. These preliminary findings can be used as hypothesis-generating indications that should be confirmed in larger data sets.

Assessment of thymic output in common variable immunodeficiency patients by evaluation of T cell receptor excision circles

Common variable immunodeficiency (CVID) is a heterogeneous syndrome characterized by repeated infections and hypogammaglobulinaemia. Additionally, T-cell abnormalities including lymphopenia, decreased proliferation to mitogens and antigens, and the reduced production and expression of cytokines, have also been observed. In this study we have investigated the expression of naive, memory and activation markers in T-cell subpopulations in 17 CVID patients in comparison to age-matched normal controls. The numbers of CD4+ T cells, including CD45RA+CD62L+ and, to a lesser extent, CD45RA-CD62L+/RA+CD62L- were significantly reduced in patients, whereas CD8+ T cells were within normal range. In contrast, HLA-DR+ cells were increased both in CD4+ and CD8+ T cells. To assess the thymic output, we analysed the presence of T-cell receptor excision circles (TRECs) in CD4+ and CD8+ T cells by quantitative PCR. TRECs were decreased significantly in patients and the rate of TREC loss was higher with increasing age. TRECs correlated with naive CD4+ T cells, whereas there was an inverse relationship between TRECs and CD8+HLA-DR+ and CD8+CD45RA-CD62L+/RA+CD62L- T cells. Our results suggest the presence of a defect in the naive T cell compartment with origin at the thymic level in CVID, and indicate that TREC may be a useful marker to monitor thymic function in this primary immunodeficiency.

Quantification of newly developed T cells in mice by real-time quantitative PCR of T-cell receptor rearrangement excision circles

OBJECTIVE

Thymic output of newly developed ab T cells in humans can be measured via signal joint T-cell receptor rearrangement excision circles (sjTRECs). Deletion of the TCRD locus via dRec to psiJa recombination during TCRA rearrangement results in the production of such sjTRECs. The deleting elements dRec and psiJa are highly conserved between humans and mice and used in a comparable manner. We developed and evaluated a real-time quantitative PCR (RQ-PCR) to detect and quantify dRec-psiJa sjTRECs in murine peripheral blood leukocytes for estimation of thymic output of newly developed ab T cells in mice.

METHODS

The threshold cycle (Ct) of the sjTREC RQ-PCR was related to the Ct value of an endogenous reference gene. The difference in Ct value (DCt) was correlated to the absolute numbers of CD45+ and CD3+ cells per mL of blood, as obtained by a single platform flow cytometric assay, resulting in the frequency of sjTRECs in CD45+ and CD3+ cells.

RESULTS

The RQ-PCR proved to be sensitive with a detection level of approximately one sjTREC copy in 100 ng of DNA. SjTRECs could not be detected in peripheral blood leukocytes of RAG-1(-/-) mice, demonstrating the specificity of the assay. As in humans and primates, sjTREC levels declined in aging and thymectomized mice. Remarkably, significant mouse strain-dependent differences in sjTREC levels were observed. 129Sv and C57BL/6 mice had significantly lower sjTREC levels in blood than Balb/c and DBA2 mice.

CONCLUSION

Quantification of murine sjTRECs by RQ-PCR may allow for accurate assessment of thymic output in mice.

Cellular immunology in a historical perspective

Bruton's XLA and DiGeorge syndrome patients show that two basic immune systems are distinct from each other in humans - thymus-dependent cell-mediated immunodeficiencies vs. antibody-based immunodeficiencies. The appendix-sacculus lymphoid organ of rabbits, like the bursa of Fabricius, represents a central lymphoid organ. Chronic granulomatous disease of childhood (CGD) revealed that phagocytosis killing of catalase-positive microorganisms employ oxidative burst. Bone marrow transplantation (BMT) proved life saving in severe combined immunodeficiency (SCID). The first BMT cured XSCID and the second BMT cured a complicating aplastic anemia launching BMT as a treatment of many diseases. Now 75 fatal diseases have been cured by myeloablative BMT. BMT also cured experimental autoimmune diseases. BMT alone did not cure lupus with polyarthritis in MRL/lpr mice or polyarthritis in NZB/KN mice, but BMT plus bone (stromal cell) transplants cured these diseases. Autoimmune diseases and lethal glomerulonephritis were prevented or cured in BXSB mice by mixed allogeneic plus syngeneic BMT. X-linked Hyper IgM syndrome (XHIM) was also cured by BMT from a 2-year-old MHC-matched sibling donor. Nonmyeloablative BMT plus mesenchymal stem cells (stromal cells) was effective treatment for a form of collagen-vascular disease and also a lethal form of hypophosphatasia. Mannan-binding lectin, an opsonin that activates the complement system when mutated and at low levels in blood, opens a door to frequent infections throughout childhood and adult life. This new immunodeficiency is based on genetic mutations that involve a native defense system.

Exploring lymphocyte differentiation pathways

Highlights in a 4-decade exploration of lymphocyte differentiation begin with comparative studies in birds and mammals leading to recognition of the separate T- and B-cell differentiation pathways and their cooperative interaction. The global effects of aborting IgM B-cell development with anti-mu antibodies indicated that B cells can undergo immunoglobulin isotype switching. A search for the mammalian bursa equivalent that began with an extended excursion through the gut-associated lymphoepithelial tissues ultimately led to the hematopoietic tissue origin of mammalian B cells. The identification of the precursors of B cells in hematopoietic tissues provided an expanded view of the life history of B cells. A recurring theme in this essay is the interplay between understanding normal lymphocyte differentiation and the defects that underlie immunodeficiency diseases and lymphoid malignancies.

Reversible disruption of thymic function by steroid treatment

The effect of steroid treatment on the thymic output of T cells was examined in an avian model. Recent thymic emigrants in chickens transiently express the chicken T cell Ag 1 thymocyte marker, and thymic function can be monitored indirectly by measuring the levels of TCR gene rearrangement excision circles in peripheral T cells. Both parameters were used to show that intensive steroid treatment induces thymic involution and a profound reduction in the supply of naive T cells to the periphery. Conversely, resident T cells in the peripheral lymphocyte pool were relatively spared. Thymopoiesis immediately recovered following cessation of steroid treatment, concurrent with restoration of the thymic output of newly formed T cells. Repopulation of the peripheral T cell pool recapitulated the ontogenetic pattern of gamma delta T cell replenishment before alpha beta T cell reseeding, thereby indicating the complete recovery of thymic function after a course of steroid treatment.

Reevaluation of T cell receptor excision circles as a measure of human recent thymic emigrants

The human thymus exports newly generated T cells to the periphery. As no markers have been identified for these recent thymic emigrants (RTE), it is presently impossible to measure human thymic output. T cell receptor excision circles (TREC) have been recently used to assess thymic output during both health and disease. Using a mathematical model, we quantify age-dependent changes both in the number of RTE generated per day and in TREC concentration during an 80-year lifespan. Through analyses, we demonstrate that RTE and peripheral T cell division have the same potential to affect TREC concentration at any age in healthy people. T cell death also influences TREC concentration, but to a lesser extent. During aging, our results indicate that thymic involution primarily induces an age-dependent decline in TREC concentrations within both CD4(+) and CD8(+) T cell populations. We further apply this model for studying TREC concentration during HIV-1 infection. Our analyses reveal that a decrease in thymic output is the major contributor to the decline in TREC concentration within CD4(+) T cells, whereas both increased peripheral T cell division and decreased thymic output induce the decline in TREC concentration within CD8(+) T cells. Therefore, we suggest that T cell turnover should be examined together with TREC concentration as a measure of RTE. If peripheral T cell division remains relatively unchanged, then TREC concentration indeed reflects thymic output.

T-cell receptor excision circle and T-cell dynamics after allogeneic stem cell transplantation are related to clinical events

It is generally believed that homeostatic responses regulate T-cell recovery after peripheral stem cell transplantation (PSCT). We studied in detail immune recovery in relation to T-cell depletion and clinical events in a group of adult patients who underwent PSCT because of hematologic malignancies. Initially, significantly increased proportions of dividing naive, memory, and effector CD4(+) and CD8(+) T cells were found that readily declined, despite still very low numbers of CD4(+) and CD8(+) T cells. After PSCT, increased T-cell division rates reflected immune activation because they were associated with episodes of infectious disease and graft-versus-host disease (GVHD). T-cell receptor excision circles (TRECs) were measured to monitor thymic output of naive T cells. Mean TREC content normalized rapidly after PSCT, long before naive T-cell numbers had significantly recovered. This is compatible with the continuous thymic production of TREC(+) naive T cells and does not reflect homeostatic increases of thymic output. TREC content was decreased in patients with GVHD and infectious complications, which may be explained by the dilution of TRECs resulting from increased proliferation. Combining TREC and Ki67 analysis with repopulation kinetics led to the novel insight that recovery of TREC content and increased T-cell division during immune reconstitution after transplantation are related to clinical events rather than to homeostatic adaptation to T-cell depletion. (Blood. 2002;99:3449-3453)