Effect of thymectomy on human peripheral blood T cell pools in myasthenia gravis

Comprehensive Flow Cytometric, Immunohistologic, and Molecular Assessment of Thymus Function in Rhesus Macaques

The critical importance of the thymus for generating new naive T cells that protect against novel infections and are tolerant to self-antigens has led to a recent revival of interest in monitoring thymic function in species other than humans and mice. Nonhuman primates such as rhesus macaques (Macaca mulatta) provide particularly useful animal models for translational research in immunology. In this study, we tested the performance of a 15-marker multicolor Ab panel for flow cytometric phenotyping of lymphocyte subsets directly from rhesus whole blood, with validation by thymectomy and T cell depletion. Immunohistochemical and multiplex RNA expression analysis of thymus tissue biopsies and molecular assays on PBMCs were used to further validate thymus function. Results identify Ab panels that can accurately classify rhesus naive T cells (CD3+CD45RA+CD197+ or CD3+CD28+CD95-) and recent thymic emigrants (CD8+CD28+CD95-CD103+CD197+) using just 100 µl of whole blood and commercially available fluorescent Abs. An immunohistochemical panel reactive with pan-cytokeratin (CK), CK14, CD3, Ki-67, CCL21, and TdT provides histologic evidence of thymopoiesis from formalin-fixed, paraffin-embedded thymus tissues. Identification of mRNAs characteristic of both functioning thymic epithelial cells and developing thymocytes and/or molecular detection of products of TCR gene rearrangement provide additional complementary methods to evaluate thymopoiesis, without requiring specific Abs. Combinations of multiparameter flow cytometry, immunohistochemistry, multiplex gene expression, and TCR excision circle assays can comprehensively evaluate thymus function in rhesus macaques while requiring only minimal amounts of peripheral blood or biopsied thymus tissue.

An angel or a devil? Current view on the role of CD8+ T cells in the pathogenesis of myasthenia gravis

BACKGROUND

Myasthenia gravis (MG) and the experimental autoimmune MG (EAMG) animal model are characterized by T-cell-induced and B-cell-dominated autoimmune diseases that affect the neuromuscular junction. Several subtypes of CD4+ T cells, including T helper (Th) 17 cells, follicular Th cells, and regulatory T cells (Tregs), contribute to the pathogenesis of MG. However, increasing evidence suggests that CD8+ T cells also play a critical role in the pathogenesis and treatment of MG.

MAIN BODY

Herein, we review the literature on CD8+ T cells in MG, focusing on their potential effector and regulatory roles, as well as on relevant evidence (peripheral, in situ, cerebrospinal fluid, and under different treatments), T-cell receptor usage, cytokine and chemokine expression, cell marker expression, and Treg, Tc17, CD3+CD8+CD20+ T, and CXCR5+ CD8+ T cells.

CONCLUSIONS

Further studies on CD8+ T cells in MG are necessary to determine, among others, the real pattern of the Vβ gene usage of autoantigen-specific CD8+ cells in patients with MG, real images of the physiology and function of autoantigen-specific CD8+ cells from MG/EAMG, and the subset of autoantigen-specific CD8+ cells (Tc1, Tc17, and IL-17+IFN-γ+CD8+ T cells). There are many reports of CD20-expressing T (or CD20 + T) and CXCR5+ CD8 T cells on autoimmune diseases, especially on multiple sclerosis and rheumatoid arthritis. Unfortunately, up to now, there has been no report on these T cells on MG, which might be a good direction for future studies.

Better safe than sorry: Naive T-cell dynamics in healthy ageing

It is well-known that the functioning of the immune system gradually deteriorates with age, and we are increasingly confronted with its consequences as the life expectancy of the human population increases. Changes in the T-cell pool are among the most prominent features of the changing immune system during healthy ageing, and changes in the naive T-cell pool in particular are generally held responsible for its gradual deterioration. These changes in the naive T-cell pool are thought to be due to involution of the thymus. It is commonly believed that the gradual loss of thymic output induces compensatory mechanisms to maintain the number of naive T cells at a relatively constant level, and induces a loss of diversity in the T-cell repertoire. Here we review the studies that support or challenge this widely-held view of immune ageing and discuss the implications for vaccination strategies.

Health Consequences of Thymus Removal in Adults

BACKGROUND

The function of the thymus in human adults is unclear, and routine removal of the thymus is performed in a variety of surgical procedures. We hypothesized that the adult thymus is needed to sustain immune competence and overall health.

METHODS

We evaluated the risk of death, cancer, and autoimmune disease among adult patients who had undergone thymectomy as compared with demographically matched controls who had undergone similar cardiothoracic surgery without thymectomy. T-cell production and plasma cytokine levels were also compared in a subgroup of patients.

RESULTS

After exclusions, 1420 patients who had undergone thymectomy and 6021 controls were included in the study; 1146 of the patients who had undergone thymectomy had a matched control and were included in the primary cohort. At 5 years after surgery, all-cause mortality was higher in the thymectomy group than in the control group (8.1% vs. 2.8%; relative risk, 2.9; 95% confidence interval [CI], 1.7 to 4.8), as was the risk of cancer (7.4% vs. 3.7%; relative risk, 2.0; 95% CI, 1.3 to 3.2). Although the risk of autoimmune disease did not differ substantially between the groups in the overall primary cohort (relative risk, 1.1; 95% CI, 0.8 to 1.4), a difference was found when patients with preoperative infection, cancer, or autoimmune disease were excluded from the analysis (12.3% vs. 7.9%; relative risk, 1.5; 95% CI, 1.02 to 2.2). In an analysis involving all patients with more than 5 years of follow-up (with or without a matched control), all-cause mortality was higher in the thymectomy group than in the general U.S. population (9.0% vs. 5.2%), as was mortality due to cancer (2.3% vs. 1.5%). In the subgroup of patients in whom T-cell production and plasma cytokine levels were measured (22 in the thymectomy group and 19 in the control group; mean follow-up, 14.2 postoperative years), those who had undergone thymectomy had less new production of CD4+ and CD8+ lymphocytes than controls (mean CD4+ signal joint T-cell receptor excision circle [sjTREC] count, 1451 vs. 526 per microgram of DNA [P = 0.009]; mean CD8+ sjTREC count, 1466 vs. 447 per microgram of DNA [P<0.001]) and higher levels of proinflammatory cytokines in the blood.

CONCLUSIONS

In this study, all-cause mortality and the risk of cancer were higher among patients who had undergone thymectomy than among controls. Thymectomy also appeared be associated with an increased risk of autoimmune disease when patients with preoperative infection, cancer, or autoimmune disease were excluded from the analysis. (Funded by the Tracey and Craig A. Huff Harvard Stem Cell Institute Research Support Fund and others.).

Different Properties of Involuted Thymus upon Nutritional Deficiency in Young and Aged Mice

Immune suppression in elderly individuals is one of the most important hygienic problems in aged societies. The primary immune organ thymus is histologically and functionally reduced by aging, which is known as thymic involution. The thymus is also involuted by nutritional deficiency, which frequently occurs in elderly individuals. However, there is no information on the thymic changes caused by nutritional deficiency with aging. Therefore, this study was conducted to examine the histological and molecular responses of the thymus to nutritional deficiency in young and aged mice. The thymic size was significantly smaller in 16- or 18-week-old aged mice than in 7-week-old young mice. Dietary restriction for 48 h reduced the thymic size in young mice, but not in aged mice. Immunostaining with anti-keratin 5 antibody revealed that the integrity of the corticomedullary boundary was maintained in the aged thymus, whereas dietary restriction induced its disorganization in both young and aged thymus. The numbers of immunoglobulin G (IgG)-positive cells were increased upon dietary restriction in aged, but not in young, thymus. Dietary restriction, but not aging, upregulated the mRNA levels of T-helper 2 (Th2)-related Il5, Il6, and Il10, whereas aging increased that of Th1-related interferon-γ (Ifng). The dietary restriction-induced upregulation of prostanoid-synthesizing enzymes was clearly observed in the young thymus but attenuated in the aged thymus. Thus, nutritional deficiency and aging cause an involuted thymus with different properties. Moreover, the thymus in aged mice does not show further reduction in size by nutritional deficiency but still responds differently compared with that in young mice.

Poor responses and adverse outcomes of myasthenia gravis after thymectomy: Predicting factors and immunological implications

Myasthenia gravis (MG) has been recognized as a series of heterogeneous but treatable autoimmune conditions. As one of the indispensable therapies, thymectomy can achieve favorable prognosis especially in early-onset generalized MG patients with seropositive acetylcholine receptor antibody. However, poor outcomes, including worsening or relapse of MG, postoperative myasthenic crisis and even post-thymectomy MG, are also observed in certain scenarios. The responses to thymectomy may be associated with the general characteristics of patients, disease conditions of MG, autoantibody profiles, native or ectopic thymic pathologies, surgical-related factors, pharmacotherapy and other adjuvant modalities, and the presence of comorbidities and complications. However, in addition to these variations among individuals, pathological remnants and the abnormal immunological milieu and responses potentially represent major mechanisms that underlie the detrimental neurological outcomes after thymectomy. We underscore these plausible risk factors and discuss the immunological implications therein, which may be conducive to better managing the indications for thymectomy, to avoiding modifiable risk factors of poor responses and adverse outcomes, and to developing post-thymectomy preventive and therapeutic strategies for MG.

Thymoma-associated T-cell immunodeficiency after radiotherapy: A case report

Acquired immunodeficiency in thymoma (Good's syndrome) without hypogammaglobulinemia is a rare condition. Here we describe the case of a 29-year-old Japanese woman with thymoma-associated T cell immunodeficiency after radiation therapy. She was admitted to the hospital with refractory pneumonia, which resulted from as T cell immunodeficiency, as revealed through low peripheral lymphocytes and oral candidiasis triggered through radiotherapy and required long-term antimicrobial therapy. Although radiotherapy is commonly administered for thymoma, our findings suggest that physicians should consider carrying out lymphocyte counts during thymoma treatment.

Thymectomy in Juvenile Myasthenia Gravis Is Safe Regarding Long Term Immunological Effects

Thymectomy is an established treatment in adult MG and also recommended for the treatment of post-pubertal onset juvenile MG. Whether the youngest children should be thymectomized is still debated. Signs of premature aging of the immune system have been shown in studies on early perioperative thymectomy in children with congenital heart defect. In this retrospective cohort study the objective was to investigate the long-term effects of treatment related thymectomy on T cell subsets and T cell receptor rearrangement excision circles (TRECs) in peripheral blood of juvenile myasthenia gravis (MG) patients, as well as clinical occurrence of autoimmune disorders, malignancies and infectious diseases. Forty-seven patients with onset of myasthenia gravis before the age of 19 years were included; 32 (68.1%) had been thymectomized and 15 (31.8%) had not. They were studied at varying times after thymectomy (7–26 years). We found a significant lower number of naïve helper T cells (CD4+CD45RA+) with an increased proportion of memory helper T cells (CD4+CD45RO+), and a significant lower number of naïve cytotoxic T cells (CD8+CD27+CD28+) in the thymectomized patients. In addition they showed a significant reduction in the number of TRECs and proportion of recent thymic emigrants (RTE) compared to non-thymectomized patients. In none of them an increased frequency of malignancies or infections was found. Our findings indicate a premature aging of the immune system after thymectomy in juvenile MG, but associated clinical consequences could not be verified.

Assessment of Thymic Output Dynamics After in utero Infection of Mice With Coxsackievirus B4

The thymus is the main organ of the lymphatic system, in which T cells undergo a rigorous selection to ensure that their receptors (TCRs) will be functional and will not react against the self. Genes encoding for TCR chains are fragmented and must be rearranged by a process of somatic recombination generating TCR rearrangement excision circles (TRECs). We recently documented coxsackievirus B4 (CV-B4) infection of Swiss albino mouse thymus in the course of in utero transmission. In the current study, we intended to evaluate thymic output in this experimental model. For this purpose, pregnant Swiss albino mice were inoculated with CV-B4 at day 10 or 17 of gestation, and thymus and spleen were sampled from offspring at different time points and then subjected to quantification of TREC molecules and Ptk7 gene expression. Results showed a pronounced effect of in utero CV-B4 infection on the thymus with an increase in the cellularity and, consequently, the weight of the organ. sj and DβTREC analysis, by real-time PCR, revealed a significant decrease following CV-B4 infection compared to controls, a decrease which gets worse as time goes by, both in the thymus and in the periphery. Those observations reflect a disturbance in the export of T cells to the periphery and their accumulation within the thymus. The evaluation of Ptk7 transcripts in the thymus, for its part, showed a decrease in expression, especially following an infection at day 10 of gestation, which supports the hypothesis of T cell accumulation in a mature stage in the thymus. The various effects observed correlate either negatively or positively with the viral load in the thymus and spleen. Disruption in thymic export may indeed interfere with T cell maturation. We speculate that this may lead to a premature release of T cells and the possibility of circulating autoreactive or proliferation-impaired T cell clones.

Neonatal thymectomy in children-accelerating the immunologic clock?

The thymus is critical for central tolerance and diverse T-lymphocyte repertoire development, to provide lifelong defense against pathogens while maintaining self-tolerance. Peak thymic output occurs in utero, during infancy, and in early childhood, diminishing throughout life. Infants with congenital heart disease requiring sternotomy often undergo thymectomy to clear the surgical field. The long-term effects of early thymectomy are just being appreciated. Many patients remain asymptomatic despite immunologic findings mirroring those of immunosenescence. Few develop increased infection or lymphoreticular malignancy risk. When considering the effects of infant thymectomy, patients with partial DiGeorge syndrome or hypomorphic recombination-activating gene (RAG) mutations may be instructive. These patients are lymphocytopenic, with increased early-onset infection and autoimmunity risk that is not seen in most patients who underwent thymectomy during infancy. The thymic structure of patients with partial DiGeorge syndrome or hypomorphic RAG is abnormal, with disrupted architecture inclining to perturbation of central tolerance. Similar findings may be seen in patients with myasthenia gravis, although disrupted peripheral tolerance may play a greater role in autoimmunity development. In conclusion, thymectomy during infancy may increase future risk of infection or autoimmunity, with premature immunosenescence mediated through disruption of central and peripheral tolerance mechanisms initiated by early cessation or diminution of thymic output. Ideally, some thymic tissue should be preserved at the time of surgery.

Mixed chimerism and acceptance of kidney transplants after immunosuppressive drug withdrawal

Preclinical studies have shown that persistent mixed chimerism is linked to acceptance of organ allografts without immunosuppressive (IS) drugs. Mixed chimerism refers to continued mixing of donor and recipient hematopoietic cells in recipient tissues after transplantation of donor cells. To determine whether persistent mixed chimerism and tolerance can be established in patients undergoing living donor kidney transplantation, we infused allograft recipients with donor T cells and hematopoietic progenitors after posttransplant lymphoid irradiation. In 24 of 29 fully human leukocyte antigen (HLA)-matched patients who had persistent mixed chimerism for at least 6 months, complete IS drug withdrawal was achieved without subsequent evidence of rejection for at least 2 years. In 10 of 22 HLA haplotype-matched patients with persistent mixed chimerism for at least 12 months, reduction of IS drugs to tacrolimus monotherapy was achieved. Withdrawal of tacrolimus during the second year resulted in loss of detectable chimerism and subsequent rejection episodes, unless tacrolimus therapy was reinstituted. Posttransplant immune reconstitution of naïve B cells and B cell precursors was more rapid than the reconstitution of naïve T cells and thymic T cell precursors. Robust chimerism was observed only among naïve T and B cells but not among memory T cells. No evidence of rejection was observed in all surveillance graft biopsies obtained from mixed chimeric patients withdrawn from IS drugs, and none developed graft-versus-host disease. In conclusion, persistent mixed chimerism established in fully HLA- or haplotype-matched patients allowed for complete or partial IS drug withdrawal without rejection.

Altered naive CD4+ T cell homeostasis in myasthenia gravis and thymoma patients

In Myasthenia Gravis (MG) thymic pathologies are often present and thymectomy is used as treatment. By flow cytometry we elucidated alterations of naïve CD4⁺ T cell homeostasis in MG patients and patients with thymoma. MG patients showed increased absolute numbers of CD31^{- central}naïve CD4⁺ T cells. Thymoma patients displayed a significantly higher fraction of peripheral blood CD31^{+ thymic}naive T cells. We show an altered naive CD4⁺ T cell homeostasis in MG patients that might predispose to autoimmunity. Aberrant generation of T cells in thymoma can be detected by an increased frequency of CD31^{+ thymic}naive CD4⁺ T cells in the periphery.

Longterm maintenance of human naive T cells through in situ homeostasis in lymphoid tissue sites

Naïve T cells develop in the thymus and coordinate immune responses to new antigens; however, mechanisms for their long-term persistence over the human lifespan remain undefined. Here, we investigated human naïve T cell development and maintenance in primary and secondary lymphoid tissues obtained from individual organ donors aged 3 months-73 years. In the thymus, the frequency of double-positive thymocytes declined sharply in donors over age 40 coincident with reduced recent thymic emigrants (RTE) in lymphoid tissues, while naïve T cells were functionally maintained predominantly in lymph nodes (LN). Analysis of TCR clonal distribution by CDR3 sequencing of naïve CD4⁺ and CD8⁺ T cells in spleen and LNs reveal site-specific clonal expansions of naïve T cells from individuals >40 years of age with minimal clonal overlap between lymphoid tissues. We also identified biased naïve T cell clonal distribution within specific lymph nodes based on VJ usage. Together these results suggest prolonged maintenance of naïve T cells through in situ homeostasis and retention in lymphoid tissue.

Decreased number of T cells bearing TCR rearrangement excision circles (TREC) in active recent onset systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is characterized by several T lymphocyte abnormalities. An indirect assessment of recent thymus emigrants (RTE) has been recently been made available by measuring the number of TCR recombination excision circles (TREC) in peripheral T cells. We studied TREC levels in peripheral blood mononuclear cells (PBMC) of 32 SLE patients with active disease and 32 normal age- and sex-matched controls. Signal-joint TREC concentration was determined by real-time quantitative-PCR as the number of TREC copies/μg PBMC DNA. SLE patients had lower TREC levels (4.1 ±3.9 ×104 TREC/μg DNA) than controls (8.9 ±7.9 ×104/μg DNA) ( P = 0.004). There was an inverse correlation between age and TREC levels in controls ( r = 20.41, P = 0.02) but not in SLE patients. No clinical association was observed between TREC levels and clinical and laboratory SLE manifestations. TREC levels tended to be lower in patients with SLEDAI above 20 than in the rest of the patients ( P = 0.08). The decreased PBMC TREC levels is indicative of a low proportion of RTE in SLE and could be caused by decreased RTE output and/or by increased peripheral T cell proliferation in this disease. The under-representation of RTE in the peripheral T cell pool may play a role in the immune tolerance abnormalities observed in SLE.

Neonatal thymectomy reveals differentiation and plasticity within human naive T cells

The generation of naive T cells is dependent on thymic output, but in adults, the naive T cell pool is primarily maintained by peripheral proliferation. Naive T cells have long been regarded as relatively quiescent cells; however, it was recently shown that IL-8 production is a signatory effector function of naive T cells, at least in newborns. How this functional signature relates to naive T cell dynamics and aging is unknown. Using a cohort of children and adolescents who underwent neonatal thymectomy, we demonstrate that the naive CD4+ T cell compartment in healthy humans is functionally heterogeneous and that this functional diversity is lost after neonatal thymectomy. Thymic tissue regeneration later in life resulted in functional restoration of the naive T cell compartment, implicating the thymus as having functional regenerative capacity. Together, these data shed further light on functional differentiation within the naive T cell compartment and the importance of the thymus in human naive T cell homeostasis and premature aging. In addition, these results affect and alter our current understanding on the identification of truly naive T cells and recent thymic emigrants.

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.

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.

Prolonged severe immunodeficiency following thymectomy and radiation: a case report

Introduction

Immunodeficiency can occur both in patients undergoing radiation therapy, as well as in patients who have had thymectomies. However, few studies have examined the immune recovery of a patient following both procedures. We aim to emphasize the need for assessment and consistent monitoring of patients with thymoma prior to and after combined treatment of thymectomy and radiation, both of which are likely to result in an increased risk for immunodeficiency.

Case presentation

We describe the longitudinal progress of a 59-year-old Asian male who underwent thymectomy followed by radiation therapy and subsequently presented with generalized urticaria. Revelation of a low absolute lymphocyte count (615 cells/mcL) on initial evaluation prompted further analysis of his immunoglobulin levels and antigen response to a polysaccharide pneumococcal vaccine (PneumoVax-23). Although his immunoglobulin levels were unremarkable, he failed to respond to 11 of 12 serotypes of the pneumococcal vaccine. As a result, he was placed on Bactrim^® (trimethoprim-sulfamethoxazole) prophylaxis to prevent opportunistic infections, and his CD4+ and CD8+ counts were monitored over the course of 8 years. His lymphocyte counts 87 months after thymectomy and 85 months after radiation therapy were as follows: absolute lymphocyte count 956 cells/mcL, absolute CD3+/CD4+ 164/mm³ (16%) and absolute CD3+/CD8+ 257/mm³ (25%). The patient was able to discontinue Bactrim^® (trimethoprim-sulfamethoxazole) prophylaxis after 9 years of treatment.

Conclusions

The lymphocytopenia, low CD4+ count, and failed response to pneumococcal vaccination that presented in our patient are consistent with immunodeficiency. After radiation alone, a recovery of T-lymphocytes is usually observed after approximately 3 weeks. Over the course of 8 years, he has still not made a full recovery according to laboratory markers, which seem to have stabilized at chronically low levels. To prevent serious complications, we suggest that patients who have undergone both thymectomy and radiation therapy be monitored for immunodeficiency. This case report informs the practices of allergists, oncologists, and neurologists in the continuing care of patients with thymoma.

FK506 attenuates thymic output in patients with myasthenia gravis

INTRODUCTION

Myasthenia gravis (MG) is an antibody-mediated, T-cell-dependent autoimmune disease. The symptoms are caused by high-affinity IgG against the muscle acetylcholine receptor (AChR) at the neuromuscular junction. The production of these antibodies in B-cells depends on AChR-specific CD4(+) T-cells and the thymus gland seems to play a significant role in the pathogenesis of MG. Altered thymic T-cell export seems to be associated with a pathological mechanism in myasthenia gravis. Tacrolimus (FK506) has recently been used to treat MG.

MATERIAL AND METHODS

We examined the effects of tacrolimus on thymic T-cell export in patients with MG. Sixteen patients with nonthymomatous and/or thymectomized MG were treated with oral administrations of tacrolimus. To assess the effect of tacrolimus on the thymic output, we assayed the levels of T-cell receptor excision circle (TREC), a molecular marker of thymus emigrants.

RESULTS

T-cell receptor excision circle was not significantly different from those in age-matched controls before tacrolimus therapy, but they were partially decreased 4 months after tacrolimus therapy. T-cell receptor excision circle levels were significantly decreased in the thymomatous group (p < 0.05), but not in the nonthymomatous group. Tacrolimus treatment significantly attenuated TREC levels in cultured CD4(-)CD8(+) cells (p < 0.05), but total cell counts were not significantly changed.

CONCLUSIONS

These results indicate that TREC levels may become a marker of the curative effect of tacrolimus therapy for thymomatous MG, and that tacrolimus suppresses not only activating T-lymphocytes, but also naïve T-cells.

Late-onset myasthenia gravis - CTLA4(low) genotype association and low-for-age thymic output of naïve T cells

Late-onset myasthenia gravis (LOMG) has become the largest MG subgroup, but the underlying pathogenetic mechanisms remain mysterious. Among the few etiological clues are the almost unique serologic parallels between LOMG and thymoma-associated MG (TAMG), notably autoantibodies against acetylcholine receptors, titin, ryanodine receptor, type I interferons or IL-12. This is why we checked LOMG patients for two further peculiar features of TAMG - its associations with the CTLA4(high/gain-of-function) +49A/A genotype and with increased thymic export of naïve T cells into the blood, possibly after defective negative selection in AIRE-deficient thymomas. We analyzed genomic DNA from 116 Caucasian LOMG patients for CTLA4 alleles by PCR/restriction fragment length polymorphism, and blood mononuclear cells for recent thymic emigrants by quantitative PCR for T cell receptor excision circles. In sharp contrast with TAMG, we now find that: i) CTLA4(low) +49G(+) genotypes were more frequent (p = 0.0029) among the 69 LOMG patients with age at onset ≥60 years compared with 172 healthy controls; ii) thymic export of naïve T cells from the non-neoplastic thymuses of 36 LOMG patients was lower (p = 0.0058) at diagnosis than in 77 age-matched controls. These new findings are important because they suggest distinct initiating mechanisms in TAMG and LOMG and hint at aberrant immuno-regulation in the periphery in LOMG. We therefore propose alternate defects in central thymic or peripheral tolerance induction in TAMG and LOMG converging on similar final outcomes. In addition, our data support a 60-year-threshold for onset of 'true LOMG' and an LOMG/early-onset MG overlapping group of patients between 40 and 60.

Association of Thymectomy With Infection Following Congenital Heart Surgery

Background: Congenital absence of the thymus can lead to profound immunodeficiency, suggesting that thymic function during fetal development is essential to normal lymphocyte development. How vital the thymus after birth is to human immune competence and regulation is not known. Routine thymectomy, especially at an early age, may influence immunity, and therefore the risk of infection, autoimmunity, or malignancy. Methods: A retrospective review of cardiac surgery patients followed at Seattle Children’s Hospital was performed. The primary outcome was rate of serious infections requiring hospitalization. Secondary analyses included age, type of infection, cardiac diagnosis, surgical procedure, and comorbidities. Results: Patients fell into 2 groups: 60 with complete thymectomy and 35 with partial or no thymectomy. There was no statistical difference between groups in the overall prevalence of serious infections (16.7% vs 17.2%, P = 1.0). There was a nonsignificant trend toward reduced time between surgery and onset of first infection in patients in the total thymectomy group versus those without thymectomy (1.7 years vs 4.6 years, P = .07). Total thymectomy before 6 months of age also tended to increase infection rate, but the effect was not significant (0.09/year vs 0.02, P = .14). Gastroesophageal reflux in patients with total thymectomy increased the risk of infection ( P = .013), suggesting a cumulative effect. Conclusions: Though infections occurred frequently in the childhood cardiac surgery population, total thymectomy was not associated with increased risk of serious infection. Comorbid conditions may be more important contributing factors increasing the risk of infection in this complex and vulnerable population.

Long-term restoration of the human T-cell compartment after thymectomy during infancy: a role for thymic regeneration?

Thymectomy during early childhood is generally thought to have serious consequences for the establishment of the T-cell compartment. In the present study, we investigated the composition of the T-cell pool in the first 3 decades after thymectomy during infancy due to cardiac surgery. In the first 5 years after thymectomy, naive and total CD4(+) and CD8(+) T-cell numbers in the blood and T-cell receptor excision circle (TREC) levels in CD4(+) T cells were significantly lower than in healthy age-matched controls. In the first years after thymectomy, plasma IL-7 levels were significantly elevated and peripheral T-cell proliferation levels were increased by ∼ 2-fold. From 5 years after thymectomy onward, naive CD4(+) and CD8(+) T-cell counts and TRECs were within the normal range. Because TREC levels are expected to decline continuously in the absence of thymic output, we investigated whether normalization of the naive T-cell pool could be due to regeneration of thymic tissue. In the majority of individuals who had been thymectomized during infancy, thymic tissue could indeed be identified on magnetic resonance imaging scans. Whereas thymectomy has severe effects on the establishment of the naive T-cell compartment during early childhood, our data suggest that functional regrowth of thymic tissue can limit its effects in subsequent years.

Protein tyrosine kinase 7: a novel surface marker for human recent thymic emigrants with potential clinical utility

Recent thymic emigrants (RTEs) are antigenically naive T cells that have recently completed intrathymic maturation and have emigrated from the thymus to the periphery. RTEs are clinically and immunologically important as they are essential for maintaining peripheral T cells in sufficient numbers in order to recognize, by their αβT-cell receptors (TCRs), a diverse array of foreign peptide antigens. However, RTE frequency and function has been poorly understood because of a lack of surface markers to distinguish them from older non-RTE naive T cells. This review summarizes the biology of the intrathymic generation and function of RTEs, including the recent identification of protein tyrosine kinase 7 (PTK7) as a novel marker for human RTEs of the CD4 (helper) T-cell lineage. PTK7+ RTEs in adults have a reduced capacity for activation-induced proliferation and cytokine production (interleukin-2 and interferon-γ) than older PTK7- naive CD4 T cells. Importantly, this immaturity in CD4 RTE effector function may contribute to the reduced adaptive immune responses observed in situations in which CD4 RTEs predominate, including the fetus, neonate and young infant, and following immune reconstitution, such as post-hematopoietic stem cell transplant. The ability to identify viable CD4+ RTEs based on PTK7 surface staining may be particularly useful in the infant for better defining the impact of nutritional and environmental factors on thymic output, peripheral T-cell function and adaptive immune responses to vaccination and infection.

Low thymic output in the 22q11.2 deletion syndrome measured by CCR9+CD45RA+ T cell counts and T cell receptor rearrangement excision circles

Thymic hypoplasia is a frequent feature of the 22q11.2 deletion syndrome, but we know little about patients' age-related thymic output and long-term consequences for their immune system. We measured the expression of T cell receptor rearrangement excision circles (TREC) and used flow cytometry for direct subtyping of recent thymic emigrant (RTE)-related T cells in 43 patients (aged 1-54 years; median 9 years) from all over Norway and in age-matched healthy controls. Thymic volumes were estimated by ultrasound in patients. TREC levels correlated well with RTE-related T cells defined by co-expression of CD3, CD45RA and CCR9 (r=0.84) as well as with the CD4+ and CD8+ T cell subtypes. RTE-related T cell counts also paralleled age-related TREC reductions. CD45RA+ T cells correlated well with absolute counts of CD4+ (r=0.87) and CD8+ (r=0.75) RTE-related T cells. Apart from CD45RA- T cells, all T cell subsets were lower in patients than in controls. Thymic volumes correlated better with RTE-related cells (r=0.46) than with TREC levels (r=0.38). RTE-related T cells and TREC levels also correlated well (r=0.88) in patients without an identifiable thymus. Production of RTEs is impaired in patients with a 22q11.2 deletion, and CCR9 appears to be a good marker for RTE-related T cells.

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.

Quantifying thymic export: combining models of naive T cell proliferation and TCR excision circle dynamics gives an explicit measure of thymic output

Understanding T cell homeostasis requires knowledge of the export rate of new T cells from the thymus, a rate that has been surprisingly difficult to estimate. TCR excision circle (TREC) content has been used as a proxy for thymic export, but this quantity is influenced by cell division and loss of naive T cells and is not a direct measure of thymic export. We present in this study a method for quantifying thymic export in humans by combining two simple mathematical models. One uses Ki67 data to calculate the rate of peripheral naive T cell production, whereas the other tracks the dynamics of TRECs. Combining these models allows the contributions of the thymus and cell division to the daily production rate of T cells to be disentangled. The method is illustrated with published data on Ki67 expression and TRECs within naive CD4+ T cells in healthy individuals. We obtain a quantitative estimate for thymic export as a function of age from birth to 20 years. The export rate of T cells from the thymus follows three distinct phases, as follows: an increase from birth to a peak at 1 year, followed by rapid involution until approximately 8 years, and then a more gradual decline until 20 years. The rate of involution shown by our model is compatible with independent estimates of thymic function predicted by thymic epithelial space. Our method allows nonintrusive estimation of thymic output on an individual basis and may provide a means of assessing the role of the thymus in diseases such as HIV.

Rituximab for the treatment of thymoma-associated and de novo myasthenia gravis: 3 cases and review

OBJECTIVES

Myasthenia gravis (MG) is an immune-mediated disorder associated with autoantibodies against postsynaptic nicotinic acetylcholine receptors at neuromuscular junctions. Rituximab, a monoclonal antibody specific for CD20, is used primarily to treat B-cell non-Hodgkin lymphoma. Although it has been used for treatment of a number of autoimmune diseases, there is limited experience in MG.

METHODS

Three patients with refractory MG (2 with concurrent thymoma) were given rituximab.

RESULTS

Symptoms stabilized and reductions in immunosuppressive medications were tolerated for extended periods, without adverse effects or infectious complications.

CONCLUSIONS

These observations support the concept that rituximab may be helpful for the treatment of MG. Remissions in patients with or without thymoma are achievable with rituximab given in combination with commonly used modalities. Furthermore, rituximab is not necessarily contraindicated for the treatment of MG in patients being treated for thymoma. Controlled studies are called for to define its role in the treatment of refractory MG.

Thymic function in juvenile idiopathic arthritis

OBJECTIVE

Thymic function declines exponentially with age. Impaired thymic function has been associated with autoimmune disease in adults but has never been formally assessed in childhood autoimmunity. Therefore, thymic function in children with the autoimmune disease juvenile idiopathic arthritis (JIA) was determined.

METHODS

Thymic function was measured in 70 children and young adults with JIA (age range 2.1-30.8 (median 10.4)) and 110 healthy age-matched controls using four independent assays. T cell receptor excision circles (WBLogTREC/ml) and the proportion of CD4(+) CD45RA(+)CD31(+) T cells (representing recent thymic emigrants; %RTEs) were quantified and intrathymic proliferation measured by calculating the alphaTREC/SigmabetaTREC ratio. Lastly, regulatory T cells (T(Reg)) of thymic origin (CD4(+)FOXP3(+)) were quantified in peripheral blood to assess the ability of the thymus in JIA to generate this T cell subset.

RESULTS

Thymic function was equivalent by all four parameters in JIA when compared with the control population. Furthermore, there was no consistent effect of JIA subtype on thymic function, although intrathymic proliferation was higher in the small rheumatoid factor (RF)(+) polyarticular group. There were no significant effects of disease-modifying antirheumatic drugs (DMARDs) or oral corticosteroids on thymic function, although those with the worst prognostic ILAR (International League of Associations for Rheumatology) subtypes were also those most likely to be on a DMARD.

CONCLUSIONS

It is demonstrated that children and young adults with JIA, unlike adults with autoimmune diseases, have thymic function that is comparable with that of healthy controls. The varied pathologies represented by the term "JIA" suggest this observation may not be disease specific and raises interesting questions about the aetiology of thymic impairment in adult autoimmunity.

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.

Longitudinal analysis of immune function in the first 3 years of life in thymectomized neonates during cardiac surgery

The purpose of this study is to evaluate the effects of neonatal thymectomy in the functional capacity of the immune system. We selected a group of 23 subjects, who had undergone thymectomy in their first 30 days of life, during an intervention for congenital heart disease. Several parameters of the immune system were evaluated during their first 3 years of life. Lymphocyte populations and subpopulations (including naive, memory and effector subpopulations), T cell receptor (TCR) Vbeta repertoire, response of T cells following in vitro stimulation by mitogen, quantification of immunoglobulins, TCR excision circles (TRECS) and interleukin (IL)-7 were measured. We found that neonatal thymectomy produces long-term diminution in total lymphocyte counts, especially in naive CD4+ and CD8+ T cells. Additionally, TRECS were decreased, and plasma IL-7 levels increased. A statistically significant negative correlation was found between absolute CD4+ T cells and IL-7 (r = -0.470, P = 0.02). The patients did not suffer more infectious events than healthy control children, but thymectomy in neonates resulted in a significant decrease in T lymphocyte levels and TRECS, consistent with cessation of thymopoiesis. This could produce a compromise in immune function later in life, especially if the patients suffer T cell depletion and need a reconstitution of immune function.

Rebooting the immune system with high-dose cyclophosphamide for treatment of refractory myasthenia gravis

A small but important proportion of patients with myasthenia gravis (MG) are refractory to conventional immunotherapy. We have treated 12 such patients by "rebooting" the immune system with high-dose cyclophosphamide (Hi Cy, 200 mg/kg), which largely eliminates the mature immune system, while leaving hematopoietic precursors intact. The objective of this report is to describe the clinical and immunologic results of Hi Cy treatment of refractory MG. We have followed 12 patients clinically for 1-9 years, and have analyzed their humoral and cellular immunologic parameters. Hi Cy is safe and effective. All but one of the patients experienced dramatic clinical improvement for variable periods from 5 months to 7.5 years, lasting for more than 1 year in seven of the patients. Two patients are still in treatment-free remission at 5.5 and 7.5 years, and five have achieved responsiveness to immunosuppressive agents that were previously ineffective. Hi Cy typically reduced, but did not completely eliminate, antibodies to the autoantigen AChR or to tetanus or diphtheria toxin; re-immunization with tetanus or diphtheria toxoid increased the antibody levels. Despite prior thymectomy, T cell receptor excision circles, generally considered to reflect thymic emigrant T cells, were produced by all patients. Hi Cy treatment results in effective, but often not permanent, remission in most refractory myasthenic patients, suggesting that the immune system is in fact "rebooted," but not "reformatted." We therefore recommend that treatment of refractory MG with Hi Cy be followed with maintenance immunotherapy.

Life after the thymus: CD31+ and CD31- human naive CD4+ T-cell subsets

Early in life, thymic export establishes the size and the diversity of the human naive T-cell pool. Yet, on puberty thymic activity drastically decreases. Because the overall size of the naive T-cell pool decreases only marginally during ageing, peripheral postthymic expansion of naive T cells has been postulated to account partly for the maintenance of T-cell immunity in adults. So far, the analysis of these processes had been hampered by the inability to distinguish recent thymic emigrants from proliferated, peripheral, naive T cells. However, recently, CD31 has been introduced as a marker to distinguish 2 subsets of naive CD4(+) T cells with distinct T-cell receptor excision circle (TREC) content in the peripheral blood of healthy humans. Here, we review studies that have characterized TREC(hi) CD31(+ thymic)naive CD4(+) T cells and have accordingly used the assessment of this distinct subset of naive CD4(+) T cells as a correlate of thymic activity. We will discuss further potential clinical applications and how more research on CD31(+ thymic)naive and CD31(- central)naive CD4(+) T cells may foster our knowledge of the impact of thymic involution on immune competence.

Tolerance and chimerism after renal and hematopoietic-cell transplantation

We describe a recipient of combined kidney and hematopoietic-cell transplants from an HLA-matched donor. A post-transplantation conditioning regimen of total lymphoid irradiation and antithymocyte globulin allowed engraftment of the donor's hematopoietic cells. The patient had persistent mixed chimerism, and the function of the kidney allograft has been normal for more than 28 months since discontinuation of all immunosuppressive drugs. Adverse events requiring hospitalization were limited to a 2-day episode of fever with neutropenia. The patient has had neither rejection episodes nor clinical manifestations of graft-versus-host disease.

Thymopoiesis, regulatory T cells, and TCRVbeta expression in thymoma with and without myasthenia gravis, and modulatory effects of steroid therapy

We analyzed thymocyte and thymic regulatory T cell (CD4SPCD25+Foxp3+cells, Treg) development in thymoma with and without myasthenia gravis (MG, MG-thymoma, non-MG-thymoma) and in MG-associated non-neoplastic thymus (MG-NNT). An increased number of immature CD4+CD8(-)CD3(-) thymocytes through the CD4+CD8+ to CD4+CD8(-) transition and an abnormal T cell receptor Vbeta (TCRVbeta) development through the CD4+CD8+ to CD4(-)CD8+ transition were seen both in MG-and non-MG-thymomas. Terminal thymopoiesis, i.e., CD45RA+ cells within the CD4+CD8(-)CD3+ and CD8+CD4(-)CD3+ subsets, was skewed towards the CD4+ compartment in MG-thymoma and CD8+ compartment in non-MG-thymoma, but thymic export was increased only in the latter in keeping with the hypothesis that CD8+ lymphocytes may play a role in the initial stages of autosensitization and in disagreement with the relevance of an increased output of CD4+ T lymphocytes in paraneoplastic MG. Treg level in normal thymus and MG-NNT and both MG- and non-MG-thymoma was similar, and TCRVbeta development in Treg cells was slightly altered in thymoma but irrespective of MG presence. Thus, the relevance of a defective Treg development in MG context remains to be established. Most alterations in thymopoiesis were corrected by therapeutic corticosteroid administration, and the effects of steroid administration may be mediated by thymic microenvironment.

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.

Recent thymic emigrants in lymphoma patients with and without human immunodeficiency virus infection candidates for autologous peripheral stem cell transplantation

Signal joint T cell receptor excision circles (sjTRECs) have been reported as a clinical marker to measure the potential for recovery of the immune system after immunosuppressive treatments. The aim of this study was to investigate the thymic regenerative potential in 55 human immunodeficiency virus (HIV)-1 infected (HIV(+)) and non-infected (HIV(-)) lymphoma patients, candidates for autologous stem cell transplantation (ASCT). Moreover, the possible associations between sjTRECs and other immunological and clinical parameters were examined. SjTRECs levels in peripheral blood mononuclear cells (PBMCs) were quantified by real-time polymerase chain reaction and T lymphocyte subsets were analysed by flow cytometry. Our data showed that sjTRECs were reduced in lymphoma patients compared to healthy controls, although a weak significant association between low sjTRECs levels and increasing age was maintained [odds ratio (OR) = 4.00; 95% confidence interval (CI) 1.09-17.17]. We found that different chemotherapeutic treatments seem to induce similar effects on the thymic reservoir, independently from their intensity (type and number of cycles of previous chemotherapy). Results from multivariate models including adjustment for patients' sex, type of lymphoma and type of chemotherapy showed that thymic output was independent from HIV infection (OR, 0.95; 95% CI 0.20-4.48). SjTRECs levels correlated with naive T cell subsets in overall lymphoma patients and after stratification by HIV infection (r > 0.37). HIV replication should be maximally suppressed to properly evaluate thymic output by sjTREC markers. Our results suggested that de novo T cell generation is maintained partially in pretreated recurrent lymphoma patients, candidates for ASCT, and could contribute to restore the immune function after transplantation.

Immunological system status and the appearance of respiratory system disturbances in thymectomized patients

Introduction

Adult-onset thymoma may be responsible for several diseases, such as pure red cell aplasia, myasthenia gravis, and immunodeficiency (Good’s syndrome). Thymectomy does not always improve the patient’s condition, and may even produce additional symptoms. Its pathogenesis is still not entirely understood, but autoimmunological processes and bone marrow defect are the most frequently suggested.

Materials and Methods

Eleven patients (mean age: 56.2 ± 15.5 years) were analyzed 6 months to 10 years after thymectomy due to thymoma as were 25 healthy persons serving as controls. Enzyme-linked immunosorbent assay (ELISA) and flow cytometry techniques were used to evaluate the immunological status of the subjects.

Results

Good’s syndrome was diagnosed in one patient, 4 subjects suffered from myasthenia gravis, and recurrent infections of upper and lower respiratory tract appeared in 9 patients. The immunological analyses (ELISA and flow cytometry) revealed a significantly lower IgG level (p < 0.05), percentage of peripheral blood B lymphocytes (p < 0.0005), and CD4:CD8 ratio (p < 0.05) in thymectomized patients compared with the healthy controls. The percentages of CD4⁺ and CD8⁺ T lymphocytes expressing CD28 antigen were significantly lower in thymectomized patients than in healthy subjects (p < 0.005 and p < 0.01, respectively). The percentage of naïve T helper lymphocytes was significantly lower in the patients than in the control group (p < 0.05).

Conclusions

Immunodeficiency and recurrent infections may be the first symptoms of immunological disturbances after thymectomy in adults. It is suggested that regular medical monitoring of these patients is important in preventing further complications, which may result in irreversible lung tissue destruction.

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.

Interleukin-7 induced immunopathology in arthritis

Interleukin (IL)-7 is a potent immunoregulatory cytokine that is detected in joints of patients with rheumatoid and juvenile idiopathic arthritis and which correlates with parameters of disease. Several synovial cell types that play an important role in inflammation and immunopathology, such as macrophages, dendritic cells, and fibroblasts, produce IL-7. IL-7 induces cytokines produced by arthritogenic T cells (for example, interferon gamma (IFNgamma), IL-17), T cell differentiating factors (for example, IL-12), chemokines capable of attracting inflammatory cells (for example, macrophage induced gene (MIG), macrophage inflammatory protein (MIP)-1alpha) as well as molecules involved in cell adhesion, migration, and costimulation (for example, lymphocyte function associated antigen (LFA)-1, CD40, CD80). In addition, IL-7 can induce bone loss by stimulating osteoclastogenesis that is dependent on receptor activator of nuclear factor kappaB ligand (RANKL). IL-7 induces tumour necrosis factor alpha (TNFalpha) secretion by T cells and by monocytes after T cell dependent monocyte/macrophage activation. Importantly, induction of both IL-7 and IL-7 induced effects seems to be able to operate independent of TNFalpha. Together this suggests that IL-7 is an important cytokine in several rheumatic conditions, capable of inducing inflammation and immunopathology. Thus it may be an important target for immunotherapy.

Lymphocyte numbers and subsets in the human blood. Do they mirror the situation in all organs?

Lymphocyte numbers in the blood are used to evaluate the immune status on a daily basis in medicine. Several studies have documented the normal ranges of lymphocytes and lymphocyte subsets in the peripheral blood. A variety of techniques and criteria have revealed clear differences between the lymphocyte subsets in childhood and adolescence. Race and gender are also variables for blood lymphocytes, and even environmental factors seem to influence the numbers of some lymphocyte populations. However, do all these variations in lymphocyte subsets in the peripheral blood mirror changes in the lymphocyte populations of the whole body, or is it just a result of different migratory habits of cells? The factors influencing the distribution of lymphocytes in the peripheral blood with regard to the different abilities of T and B cells to migrate to distinct lymphoid or non-lymphoid tissue are summarized. In addition it will be described how the removal of organs (e.g. thymus, spleen, liver) influences the distribution of lymphocytes in the blood. All these parameters should be considered not only in the clinical situation when the immune status of a patient is extrapolated from the lymphocyte numbers in the blood, but also when interpreting treatment effects in patients.

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.

Immune system and bone metabolism: Does thymectomy influence postmenopausal bone loss in humans?

Recent studies of animal models have suggested that an increase in the number of T cells due to both peripheral expansion and increased thymic T cell output plays a key role in the regulation of bone loss after ovariectomy. Osteoclastogenic cytokines which are either produced by T cells or activate T cells have also been implicated in ovx induced bone loss. Among them are TNF alpha and IL-7. The present study investigates the role of thymectomy (THX) and IL-7 in bone metabolism in humans. We studied T cells subsets, cytokine production and bone metabolism in 13 women thymectomized for Myasthenia gravis as compared to healthy controls. Our data demonstrate that the number of CD4+ and TNF-producing T cells is lower in THX women as compared to euthymic controls. However in THX women the residual T cells produce higher levels of IL-7 and RANKL. Furthermore, flow cytometry shows that IL-7 is produced by T and B cells. Serum levels of TNF alpha were unaffected by THX and both serum TNF alpha and the RANKL/OPG correlated inversely with BMD. There were no differences in bone turnover and bone mineral density between THX women and the controls. These data suggest that THX decreases the number of TNF-producing CD4+ T cells but does not alters serum TNF levels. The RANKL/OPG ratio and indices of bone metabolisms are also not affected by THX, although THX increases the levels of IL-7 and RANKL. Further studies are needed to clarify the role of thymus in bone metabolism and osteoclastogenesis in postmenopausal women.

CDR3 Spectratyping Analysis of the TCR Repertoire in Myasthenia Gravis

Because myasthenia gravis (MG) is an autoimmune disease mediated by Abs specific for the acetylcholine receptor, helper T cells play a role in Ab production. In this study, we have performed large-scale cross-sectional and longitudinal TCR studies by CDR3 spectratyping using PBL and thymus tissues from MG patients. We found that there was no preferential usage of any particular TCR beta-chains that was identical among MG patients. However, the longitudinal study clearly demonstrated that one or more TCR Vbeta expansions persisted frequently in MG patients. Importantly, persistent TCR expansions correlated with clinical severity and high anti-acetylcholine receptor Ab titer. Finally, examinations of T cells expressing CXCR5, i.e., follicular B-helper T cells, revealed that spectratype expansions in MG patients were detected mainly in the CD4+ CXCR5+ T cell populations, whereas CD8+ T cells were the major source of clonal expansion in healthy subjects. These findings suggest that persistent clonal expansions of T cells in MG patients are associated with the development and maintenance of MG. Close examination of pathogenic T cells in MG provides useful information to elucidate the pathogenesis and to estimate the disease status.

Thymic volume, T-cell populations, and parameters of thymopoiesis in adolescent and adult survivors of HIV infection acquired in infancy

OBJECTIVES

Antiretroviral therapy has significantly prolonged the lifespan of children who acquire HIV infection in infancy, but the impact of HIV on thymus-mediated maintenance of T lymphocytes has not been studied. To examine the long-term effects of HIV infection in childhood on thymopoiesis, thymic volume and parameters of thymic function from clinically stable adolescents and young adults with HIV infection acquired in infancy were compared with those from uninfected controls.

METHODS

Thymic volume was determined using three-dimensional reconstruction and volumetric analysis of non-contrast enhanced computed tomography images of the upper chest. The degree of fat involution was assessed using a semiquantitive scoring system. CD4 and CD8 T cell populations and T cell receptor recombination excision circles (TREC) concentrations in peripheral blood lymphocytes were measured in all subjects.

RESULTS

Twenty youths (aged 17.6 +/- 2.5 years) with HIV infection acquired perinatally (n = 18) or by neonatal transfusion (n = 2) were enrolled whose HIV plasma viral load had been undetectable for a median of 3.1 years, along with 18 seronegative healthy young adults (aged 20.6 +/- 1.3 years). HIV infected subjects and controls had indistinguishable CD4 T cell counts, thymus volumes (20.5 versus 15.8 cm), thymic index scores, and TREC values. Thymic volume correlated with the number and percentage of CD4 T lymphocytes in the control group and with the number of TREC in CD4 lymphocytes in the HIV infected group.

CONCLUSIONS

Long term survivors of pediatric HIV infection appear to have retained or recovered thymic volume and thymic activity approximating uninfected youths.

Rat thymectomy effects on leptin receptor and T-bet: erythroid hyperplasia with maturation arrest and suppressed T-cell-mediated hepatotoxicity

Thymectomy is an inevitable therapy for thymoma. Therefore, determining hemato-immune changes post-thymectomy is important. Twenty-six normal LEW/Sea rats thymectomized (Tx) at the ages of 38 +/- 5 days were followed without any treatment for 4 months (experiments [Exp] A and B). In addition, 16 LEW/Sea rats Tx at the age of 42 days (Exp C and D) and 10 non-Tx control LEW/Sea rats (Exp E) were immunized with syngeneic male liver cells 2 to 3 times and followed 2.7 months after the first immunization. Flow cytometric (FCM) analysis of mesenteric lymph nodes (MLN) and peripheral blood (PB) showed as follows: among the 26 Tx rats (Exp A and B), MLN lymphocyte population at 4 months postthymectomy was characterized by decreased numbers of CD4+ cells (22%-36%) or alphabeta T-cell receptor (TCR)-positive cells (16%-54%) and increased numbers of interleukin 2 receptors (IL-2R) (>90%). In the 16 Tx-immunized rats (Exp C and D), both alphabeta TCR (MLN) and CD45R (PB) expression on lymphocytes was suppressed with rather high numbers of CD4. Bone marrow (BM) and PB hematological studies of the partially and totally Tx rats indicated the following: 8 BM of the 10 totally Tx males (Exp A) and the 7/10 male spleen showed erythroid hyperplasia with maturation arrest at the stage of basophilic erythroblasts and reticulocytopenia in the PB. One of the 10 males, which had both BM myeloid to erythroid (M/E) ratio of 4.3 and spleen myeopoiesis, was in a more advanced stage, a prestage of pure red cell aplasia (PRCA) with 22% of CD4+ MLN cells. Syngeneic liver cell immunization resulted in the Tx rats as follows: hepatotoxicity based on the immunization was weaker in the 16 Tx rats (Exp C and D) than in the 10 non-Tx rats (Exp E). Polymerase chain reaction (PCR) of PB and MLN showed compensatory activated leptin receptor (LR) and T-bet DNA in 19 Tx rats (Exp A and B) with abnormal FCM findings. The ineffective erythropoiesis at 4 months after thymectomy was explained by the erythroblast LR/erythropoietin receptor (EPOR) dysfunction. The low grade hepatotoxicity in the Tx rats (Exp C and D) was explained by the disturbed Th1 reactions (or the disturbed T-bet gene transcription) at postthymectomy.