Human thymus regeneration and T cell reconstitution

Analysis of early efficacy and immune reconstitution after autologous hematopoietic stem cell transplantation in multiple myeloma

This retrospective study aimed to stress the advantages of autologous hematopoietic stem cell transplantation (auto-HSCT) in treating primary MM. Ninety-four MM patients who underwent initial parallel sequential auto-HSCT were selected. Data on efficacy (efficacy evaluation, renal function and hemoglobin recovery), immune reconstitution (B-cell subsets, immunoglobulin levels, T-cell subsets, NK cells, neutrophil-to-lymphocyte ratio (NLR), lymphocyte-to-monocyte ratio (LMR)) and hematopoietic reconstitution times were collected and analyzed. Whether in all selected patients or in groups R-ISS II-III, there was a notable increase in the proportion of patients achieving in a very good partial response (VGPR) or better (P < 0.001, P = 0.02) and a complete response (CR) or better (P = 0.007, P = 0.014) after transplantation compared to the pre-transplant status. Post-Transplant Immune Reconstitution Analysis (Baseline vs. Pre-Transplant and Pre-Transplant vs. Post-Transplant): The level of CD19 + B cells, CD20 + B cells, CD22 + B cells, CD3 + T cells, IgG and LMR showed the same change trend, that is, it decreased before transplantation (P < 0.001, P < 0.001, P < 0.001, P < 0.001, P<0.007, P < 0.001) and then increased significantly after transplantation(P < 0.001, P < 0.001, P < 0.001, P < 0.001, P < 0.001, P < 0.001). CD3 + CD4 + T cells from 545.97 (342.11,708.60)/µL to 342.93 (168.38, 475.52)/µL (P < 0.001) and then to 251.48 (188.52, 406.98)/µL (P = 0.348); CD3 + CD8 + T cells from 391.36 (242.19, 563.37)/µL to 337.23 (192.54, 505.96)/µL (P = 0.065) and then to 797.96 (514.49, 1198.03)/µL (P < 0.001), so the CD3 + CD4+/CD3 + CD8 + T cell ratio still remained inverted post-transplant. NK cells changed from 309.86 (206.33, 460.96)/µL to 258.31 (160.75, 436.68)/ µL (P = 0.229) and then to 151.08 (108.17, 240.84)/µL (P = 0.007). Auto-HSCT can promote prolonged remission in patients with MM and also overcome some high-risk factors to achieve superior efficacy in group R-ISS II-III. Patients were immunodeficient before transplantation and auto-HSCT facilitated immune reconstitution.

The CXCR4-CXCL12 axis promotes T cell reconstitution via efficient hematopoietic immigration

T cells play a critical role in immunity to protect against pathogens and malignant cells. T cell immunodeficiency is detrimental, especially when T cell perturbation occurs during severe infection, irradiation, chemotherapy, and age-related thymic atrophy. Therefore, strategies that enhance T cell reconstitution provide considerable benefit and warrant intensive investigation. Here, we report the construction of a T cell ablation model in Tg(coro1a:DenNTR) zebrafish via metronidazole administration. The nascent T cells are mainly derived from the hematopoietic cells migrated from the kidney, the functional homolog of bone marrow and the complete recovery time is 6.5 days post-treatment. The cxcr4b gene is upregulated in the responsive hematopoietic cells. Functional interference of CXCR4 via both genetic and chemical manipulations does not greatly affect T lymphopoiesis, but delays T cell regeneration by disrupting hematopoietic migration. In contrast, cxcr4b accelerates the replenishment of hematopoietic cells in the thymus. Consistently, Cxcl12b, a ligand of Cxcr4, is increased in the thymic epithelial cells of the injured animals. Decreased or increased expression of Cxcl12b results in compromised or accelerated T cell recovery, respectively, similar to those observed with Cxcr4b. Taken together, our study reveals a role of CXCR4-CXCL12 signaling in promoting T cell recovery and provides a promising target for the treatment of immunodeficiency due to T cell injury.

A Proposed Link Between Acute Thymic Involution and Late Adverse Effects of Chemotherapy

Epidemiologic data suggest that cancer survivors tend to develop a protuberant number of adverse late effects, including second primary malignancies (SPM), as a result of cytotoxic chemotherapy. Besides the genotoxic potential of these drugs that directly inflict mutational burden on genomic DNA, the precise mechanisms contributing to SPM development are poorly understood. Cancer is nowadays perceived as a complex process that goes beyond the concept of genetic disease and includes tumor cell interactions with complex stromal and immune cell microenvironments. The cancer immunoediting theory offers an explanation for the development of nascent neoplastic cells. Briefly, the theory suggests that newly emerging tumor cells are mostly eliminated by an effective tissue immunosurveillance, but certain tumor variants may occasionally escape innate and adaptive mechanisms of immunological destruction, entering an equilibrium phase, where immunologic tumor cell death "equals" new tumor cell birth. Subsequent microenvironmental pressures and accumulation of helpful mutations in certain variants may lead to escape from the equilibrium phase, and eventually cause an overt neoplasm. Cancer immunoediting functions as a dedicated sentinel under the auspice of a highly competent immune system. This perspective offers the fresh insight that chemotherapy-induced thymic involution, which is characterized by the extensive obliteration of the sensitive thymic epithelial cell (TEC) compartment, can cause long-term defects in thymopoiesis and in establishment of diverse T cell receptor repertoires and peripheral T cell pools of cancer survivors. Such delayed recovery of T cell adaptive immunity may result in prolonged hijacking of the cancer immunoediting mechanisms, and lead to development of persistent and mortal infections, inflammatory disorders, organ-specific autoimmunity lesions, and SPMs. Acknowledging that chemotherapy-induced thymic involution is a potential risk factor for the emergence of SPM demarcates new avenues for the rationalized development of pharmacologic interventions to promote thymic regeneration in patients receiving cytoreductive chemotherapies.

Progressive multifocal leukoencephalopathy in a patient with mediastinal teratoma: a case report

Background

Progressive multifocal leukoencephalopathy (PML) is a rare demyelinating lytic brain infection caused by the John Cunningham virus (JCV). JCV manifests primarily in patients with innate immunodeficiency or taking immunomodulatory medications. In this case study, we report a PML patient with comorbid mediastinal teratoma and mild lymphopenia.

Case presentation

A 73-year-old female presented with a 3-month history of progressive hemiplegia, hemianopsia, and cognitive impairment. She was diagnosed as PML by cerebrospinal fluid metagenomics sequencing and brain biopsy. Extensive immunological tests did not reveal an apparent immunodeficiency, but further work-up revealed that the PML was most likely the first presentation of mediastinal teratoma and the mild lymphopenia. Mirtazapine and immunoglobulin were started, the patient’s condition was relatively stable and approved to be discharged from hospital. But unfortunately, she died of the lung infection 10 months after first presentation.

Conclusions

This case confirms that mediastinal teratoma may induce the lymphopenia and trigger PML, delayed or incorrect diagnosis may worsen the course of the disease and result in poor prognosis.

Attenuation of apoptotic cell detection triggers thymic regeneration after damage

The thymus, which is the primary site of T cell development, is particularly sensitive to insult but also has a remarkable capacity for repair. However, the mechanisms orchestrating regeneration are poorly understood, and delayed repair is common after cytoreductive therapies. Here, we demonstrate a trigger of thymic regeneration, centered on detecting the loss of dying thymocytes that are abundant during steady-state T cell development. Specifically, apoptotic thymocytes suppressed production of the regenerative factors IL-23 and BMP4 via TAM receptor signaling and activation of the Rho-GTPase Rac1, the intracellular pattern recognition receptor NOD2, and micro-RNA-29c. However, after damage, when profound thymocyte depletion occurs, this TAM-Rac1-NOD2-miR29c pathway is attenuated, increasing production of IL-23 and BMP4. Notably, pharmacological inhibition of Rac1-GTPase enhanced thymic function after acute damage. These findings identify a complex trigger of tissue regeneration and offer a regenerative strategy for restoring immune competence in patients whose thymic function has been compromised.

Treg and IL-1 receptor type 2-expressing CD4+ T cell-deleted CD4+ T cell fraction prevents the progression of age-related hearing loss in a mouse model

We investigated the association between cellular immunity and age-related hearing loss (ARHL) development using three CD4⁺ T cell fractions, namely, naturally occurring regulatory T cells (Treg), interleukin 1 receptor type 2-expressing T cells (I1R2), and non-Treg non-I1R2 (nTnI) cells, which comprised Treg and I1R2-deleted CD4⁺ T cells. Inoculation of the nTnI fraction into a ARHL murine model, not only prevented the development of ARHL and the degeneration of spiral ganglion neurons, but also suppressed serum nitric oxide, a source of oxidative stress. Further investigations on CD4⁺ T cell fractions could provide novel insights into the prevention of aging, including presbycusis.

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

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

Thymus involution sets the clock of the aging T-cell landscape: Implications for declined immunity and tissue repair

Aging is generally characterized as a gradual increase in tissue damage, which is associated with senescence and chronic systemic inflammation and is evident in a variety of age-related diseases. The extent to which such tissue damage is a result of a gradual decline in immune regulation, which consequently compromises the capacity of the body to repair damages, has not been fully explored. Whereas CD4 T lymphocytes play a critical role in the orchestration of immunity, thymus involution initiates gradual changes in the CD4 T-cell landscape, which may significantly compromise tissue repair. In this review, we describe the lifespan accumulation of specific dysregulated CD4 T-cell subsets and their coevolution with systemic inflammation in the process of declined immunity and tissue repair capacity with age. Then, we discuss the process of thymus involution-which appears to be most pronounced around puberty-as a possible driver of the aging T-cell landscape. Finally, we identify individualized T cell-based early diagnostic biomarkers and therapeutic strategies for age-related diseases.

When the Damage Is Done: Injury and Repair in Thymus Function

Even though the thymus is exquisitely sensitive to acute insults like infection, shock, or common cancer therapies such as cytoreductive chemo- or radiation-therapy, it also has a remarkable capacity for repair. This phenomenon of endogenous thymic regeneration has been known for longer even than its primary function to generate T cells, however, the underlying mechanisms controlling the process have been largely unstudied. Although there is likely continual thymic involution and regeneration in response to stress and infection in otherwise healthy people, acute and profound thymic damage such as that caused by common cancer cytoreductive therapies or the conditioning regimes as part of hematopoietic cell transplantation (HCT), leads to prolonged T cell deficiency; precipitating high morbidity and mortality from opportunistic infections and may even facilitate cancer relapse. Furthermore, this capacity for regeneration declines with age as a function of thymic involution; which even at steady state leads to reduced capacity to respond to new pathogens, vaccines, and immunotherapy. Consequently, there is a real clinical need for strategies that can boost thymic function and enhance T cell immunity. One approach to the development of such therapies is to exploit the processes of endogenous thymic regeneration into novel pharmacologic strategies to boost T cell reconstitution in clinical settings of immune depletion such as HCT. In this review, we will highlight recent work that has revealed the mechanisms by which the thymus is capable of repairing itself and how this knowledge is being used to develop novel therapies to boost immune function.

Histoarchitectural Deterioration of Lymphoid Tissues in HIV-1 Infection and in Aging

Impaired immunity is a common symptom of aging and advanced Human Immunodeficiency Virus type 1 (HIV-1) disease. In both diseases, a decline in lymphocytic function and cellularity leads to ineffective adaptive immune responses to opportunistic infections and vaccinations. Furthermore, despite sustained myeloid cellularity there is a background of chronic immune activation and a decrease in innate immune function in aging. In HIV-1 disease, myeloid cellularity is often more skewed than in normal aging, but similar chronic activation and innate immune dysfunction typically arise. Similarities between aging and HIV-1 infection have led to several investigations into HIV-1-mediated aging of the immune system. In this article, we review various studies that report alterations of leukocyte number and function during aging, and compare those alterations with those observed during progressive HIV-1 disease. We pay particular attention to changes within lymphoid tissue microenvironments and how histoarchitectural changes seen in these two diseases affect immunity. As we review various immune compartments including peripheral blood as well as primary and secondary lymphoid organs, common themes arise that help explain the decline of immunity in the elderly and in HIV-1-infected individuals with advanced disease. In both conditions, lymphoid tissues often show signs of histoarchitectural deterioration through fat accumulation and/or fibrosis. These structural changes can be attributed to a loss of communication between leukocytes and the surrounding stromal cells that produce the extracellular matrix components and growth factors necessary for cell migration, cell proliferation, and lymphoid tissue function. Despite the common general impairment of immunity in aging and HIV-1 progression, deterioration of immunity is caused by distinct mechanisms at the cellular and tissue levels in these two diseases.

Investigating Factors Associated with Thymic Regeneration after Chemotherapy in Patients with Lymphoma

The factors involved in thymus regeneration after chemotherapy has not been sufficiently explored. This study was aimed to identify the clinical characteristics and single-nucleotide polymorphisms in the gene (IL7R) encoding IL-7Rα associated with thymus renewal after chemotherapy in Chinese Han individuals with lymphoma. The dynamics of thymic activity in 134 adults with Hodgkin lymphoma (HL) and B cell lymphoma from baseline to 12 months post-chemotherapy were analyzed by assessing thymic structural changes using serial computed tomography scans and correlating these with measurements of thymic output by concurrent analysis of single-joint T-cell receptor excision circles (sjTREC) and CD31⁺ recent thymic emigrants (RTE) in peripheral blood. The association of clinical variables and IL7R polymorphisms with the occurrence of rebound thymic hyperplasia (TH) and the recovery of thymic output following chemotherapy were evaluated. Thymic regeneration was observed, with the evidence that TH occurred in 38/134 (28.4%) cases, and thymic output, assessed by CD31⁺ RTE numbers and sjTREC content, recovered to baseline levels within 1 year after the end of therapy. The frequencies of the T allele and TT + GT genotype of rs7718919 located in the promoter of IL7R were significantly higher in patients with TH compared with those without TH (P = 0.031 and 0.027, respectively). In contrast, no significant difference was found between two groups with respect to the distribution of allele and genotype frequencies of rs6897932. By general linear models repeated-measure analysis, rs7718919 and rs6897932 were determined to exert no significant effects on the recovery of thymic output after therapy. Univariate analysis revealed host age under 30, the diagnosis of HL, baseline thymic index and CD31⁺ RTE counts, and rs7718919 genotype as potential predictors for TH after chemotherapy (P < 0.05); after multivariate adjustment, only host age was independently associated with the occurrence of TH (odds ratios = 4.710, 95% confidence intervals: 1.727–12.845, P = 0.002). These findings indicate that patient age is an independent predictor for thymic regrowth after chemotherapy, which should promote awareness among physicians to make a timely diagnosis of TH in young adults and help physicians to prioritize intervention strategies for thymus rejuvenation in this population.

The functional relationship between hematopoietic stem cells and developing T lymphocytes

In contrast to all other blood and immune cells, T lymphocytes do not develop in the bone marrow (BM), but in the specialized microenvironment provided by the thymus. Similar to the other lineages, however, all T cells arise from multipotent hematopoietic stem cells (HSCs) that reside in the BM. Not all HSCs give rise to T cells; but how many and what kind of developmental checkpoints are located along this intricate differentiation path is the subject of intense research. Traditionally, this process has been studied almost exclusively using mouse cells, but recent advances in immunodeficient mouse models, high-speed cell sorting, lentiviral transduction protocols, and deep sequencing techniques have allowed these questions to be addressed using human cells. Here we review the process of thymic seeding by BM-derived cells and T cell commitment in humans, discussing recent insights into the clonal composition of the thymus and the definition of developmental checkpoints, on the basis of insights from human severe combined immunodeficiency patients.

Fetal thymus graft enables recovery from age-related hearing loss and expansion of CD4-Positive T cells expressing IL-1 receptor type 2 and regulatory T Cells

Background

Accumulating evidence has indicated the relationship between the systemic immune system and the central nervous system including the inner ear.

Results

We have shown that age-related developments of T-cell dysfunction, hearing loss, and degeneration of cochlear spiral ganglion (SG) neurons observed in 6-month-old mice were recovered in 12 months old mice which previously given fetal thymus transplants twice. We have also demonstrated that CD4⁺ T cells expressing interleukin 1 receptor type 2 (IL-1R2) and naturally occurring regulatory T cells (nTregs), which expanded in aged 12-month-old mice, were reduced in the thymus-grafted mice of the same age.

Conclusion

It is conceivable that the rejuvenation of systemic immune function by fetal thymus grafts contributes not only to the activation of cellular immunity but also to the decrease of IL-1R2⁺ CD4⁺ T cells or nTregs, which cells accelerate both age-related hearing loss (AHL) and neurodegeneration of the cochlear neurons. Further studies on the interactions among IL-1R2 expression on CD4⁺ T cells, Tregs, and neuronal cells and also on the relationships between fetal thymus grafting and the rejuvenation of systemic immunity should be designed in order to advance towards therapeutic effects on neurosenescence, including AHL.

Interleukin-22: immunobiology and pathology

Interleukin-22 (IL-22) is a recently described IL-10 family cytokine that is produced by T helper (Th) 17 cells, γδ T cells, NKT cells, and newly described innate lymphoid cells (ILCs). Knowledge of IL-22 biology has evolved rapidly since its discovery in 2000, and a role for IL-22 has been identified in numerous tissues, including the intestines, lung, liver, kidney, thymus, pancreas, and skin. IL-22 primarily targets nonhematopoietic epithelial and stromal cells, where it can promote proliferation and play a role in tissue regeneration. In addition, IL-22 regulates host defense at barrier surfaces. However, IL-22 has also been linked to several conditions involving inflammatory tissue pathology. In this review, we assess the current understanding of this cytokine, including its physiologic and pathologic effects on epithelial cell function.

Concise review: Immune recognition of induced pluripotent stem cells

Autologous-induced pluripotent stem cells (iPSCs) may eventually be used in cell replacement therapies to treat a wide range of diseases and have been touted as a solution to the vexing problem of immune rejection in this context. Emerging evidence suggests, however, that ostensibly histocompatible iPSCs may be rejected following transplantation. Here, we review the mechanisms that contribute to immunogenicity in iPSCs and forward approaches to permit their acceptance in potential cell replacement therapies.

Mechanisms of thymus organogenesis and morphogenesis

The thymus is the primary organ responsible for generating functional T cells in vertebrates. Although T cell differentiation within the thymus has been an area of intense investigation, the study of thymus organogenesis has made slower progress. The past decade, however, has seen a renewed interest in thymus organogenesis, with the aim of understanding how the thymus develops to form a microenvironment that supports T cell maturation and regeneration. This has prompted modern revisits to classical experiments and has driven additional genetic approaches in mice. These studies are making significant progress in identifying the molecular and cellular mechanisms that control specification, early organogenesis and morphogenesis of the thymus.

Protection of immunocompromised mice from fungal infection with a thymus growth-stimulatory component from Selaginella involvens, a fern

CONTEXT

Recent studies have shown that the water extract of Selaginella involvens (Sw.) Spring, a wild fern, exhibits thymus growth-stimulatory activity in adult mice (reversal of involution of thymus) and remarkable anti-lipid peroxidation activity. Follow-up studies were carried out in the present study.

MATERIALS AND METHODS

Activity-guided isolation of the active component (AC) was carried out. The effect of AC on immune function was studied using fungal (Aspergillus fumigatus) challenge in cortisone-treated mice. The in vitro antifungal activity of AC was assayed using disc diffusion assay. In vitro and in vivo effect of AC on DNA synthesis in thymus was studied using (3)H-thymidine incorporation. In in vitro anti-lipid peroxidation, hydroxyl radical scavenging and inhibition of superoxide production were assayed.

RESULTS

The active principle/component (AC) was isolated in a chromatographically pure form from the water extract of S. involvens. AC showed positive reaction to glycosides. AC possessed both thymus growth-stimulatory and antioxidant properties. It protected cortisone-treated mice from A. fumigatus challenge. It did not exhibit in vitro antifungal activity. Increased (3)H-thymidine incorporation was observed in the reticuloepithelium of thymus obtained from AC-treated mice. However, in vitro AC treatment to thymus for 5 h did not result in an increase in (3)H-thymidine incorporation.

DISCUSSION AND CONCLUSION

AC (named as Selagin), from S. involvens, could reverse involution of thymus to a large extent, exhibit remarkable antioxidant activity, and protect immunocompromised mice from fungal infection. Therefore, it is very promising for the development of a drug to ameliorate old age-related health problems and prolong lifespan.

Thymopoietic and bone marrow response to murine Pneumocystis pneumonia

CD4(+) T cells play a key role in host defense against Pneumocystis infection. To define the role of naïve CD4(+) T cell production through the thymopoietic response in host defense against Pneumocystis infection, Pneumocystis murina infection in the lung was induced in adult male C57BL/6 mice with and without prior thymectomy. Pneumocystis infection caused a significant increase in the number of CCR9(+) multipotent progenitor (MPP) cells in the bone marrow and peripheral circulation, an increase in populations of earliest thymic progenitors (ETPs) and double negative (DN) thymocytes in the thymus, and recruitment of naïve and total CD4(+) T cells into the alveolar space. The level of murine signal joint T cell receptor excision circles (msjTRECs) in spleen CD4(+) cells was increased at 5 weeks post-Pneumocystis infection. In thymectomized mice, the numbers of naïve, central memory, and total CD4(+) T cells in all tissues examined were markedly reduced following Pneumocystis infection. This deficiency of naïve and central memory CD4(+) T cells was associated with delayed pulmonary clearance of Pneumocystis. Extracts of Pneumocystis resulted in an increase in the number of CCR9(+) MPPs in the cultured bone marrow cells. Stimulation of cultured bone marrow cells with ligands to Toll-like receptor 2 ([TLR-2] zymosan) and TLR-9 (ODN M362) each caused a similar increase in CCR9(+) MPP cells via activation of the Jun N-terminal protein kinase (JNK) pathway. These results demonstrate that enhanced production of naïve CD4(+) T lymphocytes through the thymopoietic response and enhanced delivery of lymphopoietic precursors from the bone marrow play an important role in host defense against Pneumocystis infection.

Expression profiling of immune-associated genes in peripheral blood mononuclear cells reveals baseline differences in co-stimulatory signalling between nonagenarians and younger controls: the vitality 90+ study

The human immune system, especially the adaptive branch, substantially declines with ageing. Several distinct immunosenescent events have already been described, yet data regarding to age-associated baseline alterations in immune cell function is limited. Therefore, by using the TaqMan Human Immune Arrays we conducted a preliminary gene expression profiling of immune-related genes in the peripheral blood mononuclear cells of young individuals (aged 22–37 years, n = 13) and nonagenarians (n = 12), the latter being part of the Vitality 90+ Study. We also analysed the correlations between significantly regulated genes. The results revealed a significantly decreased expression of CCR7, CD19, CD28, CD40LG, ICOS, IL4, IL6 and LTA as well as significantly increased expression of FN1 in the nonagenarians as compared to the controls. Significant direct correlations were observed between the expression of CCR7 and CD19, CCR7 and ICOS, ICOS and CD19, ICOS and CD40LG, as well as CD40LG and CD28 in the nonagenarians but not in the controls. These results suggest that the key players of adaptive immunity i.e. the factors required for full lymphocyte activation are markedly and coordinately down-modulated in the very old individuals. Further research is, however, required to establish the relationship between these changes and the mechanisms of immunosenescence.

Generation and identification of thymic epithelial progenitor cells pTEC by in-vitro processing of human thymic fragments for allotransplantation

The procedure of generation and identification of stromal progenitor cells derived from human thymic fragments (PL patent 378431) has been described in this article. Our aim was to prepare material for transplantation in elderly people. The method is based on in-vitro processing of thymic fragments to get rid of all immunogenic elements of lymphocytes, endothelial cells, macrophages, and fibroblasts. In the thymic culture process, this organ dies out in the incubation medium and epithelial cells emerge out of the organ. After about 4 weeks from the start of the culture, the population of various developmental forms of epithelial cells was generated, namely CK AE1/AE3+, SDF-1 alpha+ and a weak expression of FGF+ S-100+. Finally, we obtained approximately 3 million cells as a monolayer. The progenitor cells were experimentally transplanted into a 72-year-old volunteer in order to prove that they do not induce neither a local nor a systemic rejection response.

The role of the thymus in tolerance

The thymus serves as the central organ of immunologic self-nonself discrimination. Thymocytes undergo both positive and negative selection, resulting in T cells with a broad range of reactivity to foreign antigens but with a lack of reactivity to self-antigens. The thymus is also the source of a subset of regulatory T cells that inhibit autoreactivity of T-cell clones that may escape negative selection. As a result of these functions, the thymus has been shown to be essential for the induction of tolerance in many rodent and large animal models. Proper donor antigen presentation in the thymus after bone marrow, dendritic cell, or solid organ transplantation has been shown to induce tolerance to allografts. The molecular mechanisms of positive and negative selection and regulatory T-cell development must be understood if a tolerance-inducing therapeutic intervention is to be designed effectively. In this brief and selective review, we present some of the known information on T-cell development and on the role of the thymus in experimental models of transplant tolerance. We also cite some clinical attempts to induce tolerance to allografts using pharmacologic or biologic interventions.

IL-7 enhances thymic human T cell development in "human immune system" Rag2-/-IL-2Rgammac-/- mice without affecting peripheral T cell homeostasis

IL-7 is a central cytokine in the development of hematopoietic cells, although interspecies discrepancies have been reported. By coculturing human postnatal thymus hematopoietic progenitors and OP9-huDL1 stromal cells, we found that murine IL-7 is approximately 100-fold less potent than human IL-7 for supporting human T cell development in vitro. We investigated the role of human IL-7 in newborn BALB/c Rag2(-/-)gamma(c)(-/-) mice transplanted with human hematopoietic stem cells (HSC) as an in vivo model of human hematopoiesis using three approaches to improve IL-7 signaling: administration of human IL-7, ectopic expression of human IL-7 by the transplanted human HSC, or enforced expression of a murine/human chimeric IL-7 receptor binding murine IL-7. We show that premature IL-7 signaling at the HSC stage, before entrance in the thymus, impeded T cell development, whereas increased intrathymic IL-7 signaling significantly enhanced the maintenance of immature thymocytes. Increased thymopoiesis was also observed when we transplanted BCL-2- or BCL-x(L)-transduced human HSC. Homeostasis of peripheral mature T cells in this humanized mouse model was not improved by any of these strategies. Overall, our results provide evidence for an important role of IL-7 in human T cell development in vivo and highlight the notion that IL-7 availability is but one of many signals that condition peripheral T cell homeostasis.

Magnetic resonance imaging of radiation-induced thymic atrophy as a model for pathologic changes in acute feline immunodeficiency virus infection

The development of a protocol to reproducibly induce thymic atrophy, as occurs in feline immunodeficiency virus (FIV) infection and other immunosuppressive diseases, and to consistently estimate thymic volume, provides a valuable tool in the search of innovative and novel therapeutic strategies. Magnetic resonance imaging (MRI) using the short tau inversion recovery (STIR) technique, with fat suppression properties, was determined to provide an optimized means of locating, defining, and quantitatively estimating thymus volume in young cats. Thymic atrophy was induced in four, 8-10-week-old kittens with a single, directed 500 cGy dose of 6 MV X-rays from a clinical linear accelerator, and sequential MR images of the cranial mediastinum were collected at 2, 7, 14, and 21 days post irradiation (PI). Irradiation induced a severe reduction in thymic volume, which was decreased, on average, to 47% that of normal, by 7 days PI. Histopathology confirmed marked, diffuse thymic atrophy, characterized by reduced thymic volume, decreased overall cellularity, increased apoptosis, histiocytosis, and reduced distinction of the corticomedullary junction, comparable to that seen in acute FIV infection. Beginning on day 7 PI, thymic volumes rebounded slightly and continued to increase over the following 14 days, regaining 3-35% of original volume. These findings demonstrate the feasibility and advantages of using this non-invasive, in vivo imaging technique to measure and evaluate changes in thymic volume in physiologic and experimental situations. All experimental protocols in this study were approved by the Institutional Animal Care and Use Committee (IACUC) at Auburn University.

Age-dependent spatial memory loss can be partially restored by immune activation

Aging is often associated with a decline in hippocampus-dependent spatial memory. Here, we show that functional cell-mediated immunity is required for the maintenance of hippocampus-dependent spatial memory. Sudden imposition of immune compromise in young mice caused spatial memory impairment, whereas immune reconstitution reversed memory deficit in immune-deficient mice. Analysis of hippocampal gene expression suggested that immune-dependent spatial memory performance was associated with the expression of insulin-like growth factor (Igf1) and of genes encoding proteins related to presynaptic activity (Syt10, Cplx2). We further showed that memory loss in aged mice could be attributed to age-related attenuation of the immune response and could be reversed by immune system activation. Homeostatic-driven proliferation of lymphocytes, which expands the existing T cell repertoire, restored spatial memory deficits in aged mice. Thus, our results identify a novel function of the immune system in the maintenance of spatial memory and suggest an original approach for arresting or reversing age-associated memory loss.

Determination of thymic function directly from peripheral blood: a validated modification to an established method

The thymus contributes naïve, self MHC reactive, self tolerant T cells to the peripheral immune system throughout life, albeit with a log-linear decline with age. Quantification of thymic function is clinically relevant in the setting of lymphoablation, but a phenotypic marker distinguishing recent thymic emigrants from long lived naïve T cells remains elusive. T cell receptor excision circles (TREC) are present in thymocytes exiting the thymus and quantification of the most frequent of these, the deltarec-psiJalpha rearrangement has been widely used as a measure of recent thymic function. However, interpretation of results presented as TREC per cell has been criticised on the basis that extra-thymic cellular proliferation impacts on peripherally determined TREC numbers. TREC/ml is now considered to be more representative of thymic function than TREC/cell, especially where significant cellular proliferation occurs (e.g. during reconstitution following stem cell transplantation). Here we describe the validation of a novel variation to the established assay, directly quantifying TREC/ml from 300 microl whole blood. We show the assay to be reproducible, robust and stable longitudinally and we show equivalence of performance when compared with more standard assays. This assay particularly lends itself to the measurement of thymic function in children and where monitoring clinical variables is limited by tissue availability.