Protection from thymic epithelial cell injury by keratinocyte growth factor: a new approach to improve thymic and peripheral T-cell reconstitution after bone marrow transplantation

Primary and secondary defects of the thymus

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

Fish Collagen Peptides Enhance Thymopoietic Gene Expression, Cell Proliferation, Thymocyte Adherence, and Cytoprotection in Thymic Epithelial Cells via Activation of the Nuclear Factor-κB Pathway, Leading to Thymus Regeneration after Cyclophosphamide-Induced Injury

Prolonged thymic involution results in decreased thymopoiesis and thymic output, leading to peripheral T-cell deficiency. Since the thymic-dependent pathway is the only means of generating fully mature T cells, the identification of strategies to enhance thymic regeneration is crucial in developing therapeutic interventions to revert immune suppression in immunocompromised patients. The present study clearly shows that fish collagen peptides (FCPs) stimulate activities of thymic epithelial cells (TECs), including cell proliferation, thymocyte adhesion, and the gene expression of thymopoietic factors such as FGF-7, IGF-1, BMP-4, VEGF-A, IL-7, IL-21, RANKL, LTβ, IL-22R, RANK, LTβR, SDF-1, CCL21, CCL25, CXCL5, Dll1, Dll4, Wnt4, CD40, CD80, CD86, ICAM-1, VCAM-1, FoxN1, leptin, cathepsin L, CK5, and CK8 through the NF-κB signal transduction pathway. Furthermore, our study also revealed the cytoprotective effects of FCPs on TECs against cyclophosphamide-induced cellular injury through the NF-κB signaling pathway. Importantly, FCPs exhibited a significant capability to facilitate thymic regeneration in mice after cyclophosphamide-induced damage via the NF-κB pathway. Taken together, this study sheds light on the role of FCPs in TEC function, thymopoiesis, and thymic regeneration, providing greater insight into the development of novel therapeutic strategies for effective thymus repopulation for numerous clinical conditions in which immune reconstitution is required.

The Composite Immune Risk Score predicts overall survival after allogeneic hematopoietic stem cell transplantation: A retrospective analysis of 1838 cases

There has been little consensus on how to quantitatively assess immune reconstitution after hematopoietic stem cell transplantation (HSCT) as part of the standard of care. We retrospectively analyzed 11 150 post-transplant immune profiles of 1945 patients who underwent HSCT between 2012 and 2020. 1838 (94.5%) of the cases were allogeneic HSCT. Using the training set of patients (n = 729), we identified a composite immune signature (integrating neutrophil, total lymphocyte, natural killer, total T, CD4+ T, and B cell counts in the peripheral blood) during days 91-180 after allogeneic HSCT that was predictive of early mortality and moreover simplified it into a formula for a Composite Immune Risk Score. When we verified the Composite Immune Risk Score in the validation (n = 284) and test (n = 391) sets of patients, a high score value was found to be associated with hazard ratios (HR) of 3.64 (95% C.I. 1.55-8.51; p = .0014) and 2.44 (95% C.I., 1.22-4.87; p = .0087), respectively, for early mortality. In multivariate analysis, a high Composite Immune Risk Score during days 91-180 remained an independent risk factor for early mortality after allogeneic HSCT (HR, 1.80; 95% C.I., 1.28-2.55; p = .00085). In conclusion, the Composite Immune Risk Score is easy to compute and could identify the high-risk patients of allogeneic HSCT who require targeted effort for prevention and control of infection.

The potential role of the thymus in immunotherapies for acute myeloid leukemia

Understanding the factors which shape T-lymphocyte immunity is critical for the development and application of future immunotherapeutic strategies in treating hematological malignancies. The thymus, a specialized central lymphoid organ, plays important roles in generating a diverse T lymphocyte repertoire during the infantile and juvenile stages of humans. However, age-associated thymic involution and diseases or treatment associated injury result in a decline in its continuous role in the maintenance of T cell-mediated anti-tumor/virus immunity. Acute myeloid leukemia (AML) is an aggressive hematologic malignancy that mainly affects older adults, and the disease's progression is known to consist of an impaired immune surveillance including a reduction in naïve T cell output, a restriction in T cell receptor repertoire, and an increase in frequencies of regulatory T cells. As one of the most successful immunotherapies thus far developed for malignancy, T-cell-based adoptive cell therapies could be essential for the development of a durable effective treatment to eliminate residue leukemic cells (blasts) and prevent AML relapse. Thus, a detailed cellular and molecular landscape of how the adult thymus functions within the context of the AML microenvironment will provide new insights into both the immune-related pathogenesis and the regeneration of a functional immune system against leukemia in AML patients. Herein, we review the available evidence supporting the potential correlation between thymic dysfunction and T-lymphocyte impairment with the ontogeny of AML (II-VI). We then discuss how the thymus could impact current and future therapeutic approaches in AML (VII). Finally, we review various strategies to rejuvenate thymic function to improve the precision and efficacy of cancer immunotherapy (VIII).

Naive T Cells in Graft Versus Host Disease and Graft Versus Leukemia: Innocent or Guilty?

The outcome of allogeneic hematopoietic cell transplantation (allo-HCT) largely depends on the development and management of graft-versus-host disease (GvHD), infections, and the occurrence of relapse of malignancies. Recent studies showed a lower incidence of chronic GvHD and severe acute GvHD in patients receiving naive T cell depleted grafts compared to patients receiving complete T cell depleted grafts. On the other hand, the incidence of acute GvHD in patients receiving cord blood grafts containing only naive T cells is rather low, while potent graft-versus-leukemia (GvL) responses have been observed. These data suggest the significance of naive T cells as both drivers and regulators of allogeneic reactions. The naive T cell pool was previously thought to be a quiescent, homogenous pool of antigen-inexperienced cells. However, recent studies showed important differences in phenotype, differentiation status, location, and function within the naive T cell population. Therefore, the adequate recovery of these seemingly innocent T cells might be relevant in the imminent allogeneic reactions after allo-HCT. Here, an extensive review on naive T cells and their contribution to the development of GvHD and GvL responses after allo-HCT is provided. In addition, strategies specifically directed to stimulate adequate reconstitution of naive T cells while reducing the risk of GvHD are discussed. A better understanding of the relation between naive T cells and alloreactivity after allo-HCT could provide opportunities to improve GvHD prevention, while maintaining GvL effects to lower relapse risk.

Role of thymus in health and disease

The thymus is a primary lymphoid organ, essential for the development of T-cells that will protect from invading pathogens, immune disorders, and cancer. The thymus decreases in size and cellularity with age referred to as thymus involution or atrophy. This involution causes decreased T-cell development and decreased naive T-cell emigration to the periphery, increased proportion of memory T cells, and a restricted, altered T-cell receptor (TCR) repertoire. The changes in composition and function of the circulating T cell pool as a result of thymic involution led to increased susceptibility to infectious diseases including the recent COVID and a higher risk for autoimmune disorders and cancers. Thymic involution consisting of both structural and functional loss of the thymus has a deleterious effect on T cell development, T cell selection, and tolerance. The mechanisms which act on the structural (cortex and medulla) matrix of the thymus, the gradual accumulation of genetic mutations, and altered gene expressions may lead to immunosenescence as a result of thymus involution. Understanding the molecular mechanisms behind thymic involution is critical for identifying diagnostic biomarkers and targets for treatment help to develop strategies to mitigate thymic involution-associated complications. This review is focused on the consequences of thymic involution in infections, immune disorders, and diseases, identifying potential checkpoints and potential approaches to sustain or restore the function of the thymus particularly in elderly and immune-compromised individuals.

A Notch/IL-21 signaling axis primes bone marrow T cell progenitor expansion

Long-term impairment in T cell-mediated adaptive immunity is a major clinical obstacle following treatment of blood disorders with hematopoietic stem cell transplantation. Although T cell development in the thymus has been extensively characterized, there are significant gaps in our understanding of prethymic processes that influence early T cell potential. We have uncovered a Notch/IL-21 signaling axis in bone marrow common lymphoid progenitor (CLP) cells. IL-21 receptor expression was driven by Notch activation in CLPs, and in vivo treatment with IL-21 induced Notch-dependent CLP proliferation. Taking advantage of this potentially novel signaling axis, we generated T cell progenitors ex vivo, which improved repopulation of the thymus and peripheral lymphoid organs of mice in an allogeneic transplant model. Importantly, Notch and IL-21 activation were equally effective in the priming and expansion of human cord blood cells toward the T cell fate, confirming the translational potential of the combined treatment.

Impact of aging on gene expression response to x-ray irradiation using mouse blood

As a radiation biodosimetry tool, gene expression profiling is being developed using mouse and human peripheral blood models. The impact of dose, dose-rate, and radiation quality has been studied with the goal of predicting radiological tissue injury. In this study, we determined the impact of aging on the gene expression profile of blood from mice exposed to radiation. Young (2 mo) and old (21 mo) male mice were irradiated with 4 Gy x-rays, total RNA was isolated from whole blood 24 h later, and subjected to whole genome microarray analysis. Pathway analysis of differentially expressed genes revealed young mice responded to x-ray exposure by significantly upregulating pathways involved in apoptosis and phagocytosis, a process that eliminates apoptotic cells and preserves tissue homeostasis. In contrast, the functional annotation of senescence was overrepresented among differentially expressed genes from irradiated old mice without enrichment of phagocytosis pathways. Pathways associated with hematologic malignancies were enriched in irradiated old mice compared with irradiated young mice. The fibroblast growth factor signaling pathway was underrepresented in older mice under basal conditions. Similarly, brain-related functions were underrepresented in unirradiated old mice. Thus, age-dependent gene expression differences should be considered when developing gene signatures for use in radiation biodosimetry.

In Pursuit of Adult Progenitors of Thymic Epithelial Cells

Peripheral T cells capable of discriminating between self and non-self antigens are major components of a robust adaptive immune system. The development of self-tolerant T cells is orchestrated by thymic epithelial cells (TECs), which are localized in the thymic cortex (cortical TECs, cTECs) and medulla (medullary TECs, mTECs). cTECs and mTECs are essential for differentiation, proliferation, and positive and negative selection of thymocytes. Recent advances in single-cell RNA-sequencing technology have revealed a previously unknown degree of TEC heterogeneity, but we still lack a clear picture of the identity of TEC progenitors in the adult thymus. In this review, we describe both earlier and recent findings that shed light on features of these elusive adult progenitors in the context of tissue homeostasis, as well as recovery from stress-induced thymic atrophy.

The role of the thymus in allogeneic bone marrow transplantation and the recovery of the peripheral T-cell compartment

As the thymus represents the primary site of T-cell development, optimal thymic function is of paramount importance for the successful reconstitution of the adaptive immunity after allogeneic hematopoietic stem cell transplantation. Thymus involutes as part of the aging process and several factors, including previous chemotherapy treatments, conditioning regimen used in preparation to the allograft, occurrence of graft-versus-host disease, and steroid therapy that impair the integrity of the thymus, thus affecting its role in supporting T-cell neogenesis. Although the pathways governing its regeneration are still poorly understood, the thymus has a remarkable capacity to recover its function after damage. Measurement of both recent thymic emigrants and T-cell receptor excision circles is valuable tools to assess thymic output and gain insights on its function. In this review, we will extensively discuss available data on factors regulating thymic function after allogeneic hematopoietic stem cell transplantation, as well as the strategies and therapeutic approaches under investigation to promote thymic reconstitution and accelerate immune recovery in transplanted patients, including the use of cytokines, sex-steroid ablation, precursor T-cells, and thymus bioengineering. Although none of them is routinely used in the clinic, these approaches have the potential to enhance thymic function and immune recovery, not only in patients given an allograft but also in other conditions characterized by immune deficiencies related to a defective function of the thymus.

Immune Reconstitution After Allogeneic Haematopoietic Cell Transplantation: From Observational Studies to Targeted Interventions

Immune reconstitution (IR) after allogeneic haematopoietic cell transplantation (HCT) represents a central determinant of the clinical post-transplant course, since the majority of transplant-related outcome parameters such as graft-vs.-host disease (GvHD), infectious complications, and relapse are related to the velocity, quantity and quality of immune cell recovery. Younger age at transplant has been identified as the most important positive prognostic factor for favourable IR post-transplant and, indeed, accelerated immune cell recovery in children is most likely the pivotal contributing factor to lower incidences of GvHD and infectious complications in paediatric allogeneic HCT. Although our knowledge about the mechanisms of IR has significantly increased over the recent years, strategies to influence IR are just evolving. In this review, we will discuss different patterns of IR during various time points post-transplant and their impact on outcome. Besides IR patterns and cellular phenotypes, recovery of antigen-specific immune cells, for example virus-specific T cells, has recently gained increasing interest, as certain threshold levels of antigen-specific T cells seem to confer protection against severe viral disease courses. In contrast, the association between IR and a possible graft-vs. leukaemia effect is less well-understood. Finally, we will present current concepts of how to improve IR and how this could change transplant procedures in the near future.

T cell regeneration after immunological injury

Following periods of haematopoietic cell stress, such as after chemotherapy, radiotherapy, infection and transplantation, patient outcomes are linked to the degree of immune reconstitution, specifically of T cells. Delayed or defective recovery of the T cell pool has significant clinical consequences, including prolonged immunosuppression, poor vaccine responses and increased risks of infections and malignancies. Thus, strategies that restore thymic function and enhance T cell reconstitution can provide considerable benefit to individuals whose immune system has been decimated in various settings. In this Review, we focus on the causes and consequences of impaired adaptive immunity and discuss therapeutic strategies that can recover immune function, with a particular emphasis on approaches that can promote a diverse repertoire of T cells through de novo T cell formation.

Generation and Regeneration of Thymic Epithelial Cells

The thymus is unique in its ability to support the maturation of phenotypically and functionally distinct T cell sub-lineages. Through its combined production of MHC-restricted conventional CD4⁺ and CD8⁺, and Foxp3⁺ regulatory T cells, as well as non-conventional CD1d-restricted iNKT cells and invariant γδT cells, the thymus represents an important orchestrator of immune system development and control. It is now clear that thymus function is largely determined by the availability of stromal microenvironments. These specialized areas emerge during thymus organogenesis and are maintained throughout life. They are formed from both epithelial and mesenchymal components, and collectively they support a stepwise program of thymocyte development. Of these stromal cells, cortical, and medullary thymic epithelial cells represent functional components of thymic microenvironments in both the cortex and medulla. Importantly, a key feature of thymus function is that levels of T cell production are not constant throughout life. Here, multiple physiological factors including aging, stress and pregnancy can have either short- or long-term detrimental impact on rates of thymus function. Here, we summarize our current understanding of the development and function of thymic epithelial cells, and relate this to strategies to protect and/or restore thymic epithelial cell function for therapeutic benefit.

In silico mutagenesis in recombinant human keratinocyte growth factor: Improvement of stability and activity in addition to decrement immunogenicity

The recombinant human keratinocyte growth factor (rhKGF) is clinically applied to decrease the incidence and duration of cancer therapeutic agents. Particularly, it is extensively used for oral mucositis after chemotherapy-induced damage of different human cancers. However, the usage of rhKGF in treatment is limited owing to its short half-life, poor stability, immunogenicity, tendency to aggregate, and side effects. Therefore, there is a need to enhance the stability and to reduce immunogenicity of rhKGF for therapeutic applications. In this study, the stability, activity, and immunogenicity of rhKGF were improved using computational methods. The several mutations were generated based on sequence alignment, amino acids physic-chemical properties, and the structure simulation. The 3D structure of rhKGF and proposed mutants were predicted by Modeller v9.15 program, and then were evaluated using PROSESS, PROCHECK, and ProSA web tools. Afterwards, the effect of these mutants on rhKGF structure, stability, activity, and its interaction with fibroblast growth factor receptor2-IIb (FGFR2-IIb) was analyzed through utilizing GROMACS molecular dynamics simulations and docking tools, respectively. Also, binding free energies were calculated by the Molecular Mechanics/Poisson-Boltzmann Surface Area (MM/PBSA) method. We found that F63Y, R121K, and combine1 (K38R, F63Y, K72E, N105S) mutants lead to reduction of the number of T-cell epitopes. However, all of the selected mutants, except for R121K, could considerably increase stability and affinity of the rhKGF to FGFR2-IIb, in silico. In conclusion, this study, for the first time, offered that the combine1 and F63Y mutants could highly improve the stability and activity of rhKGF and even reduce immunogenicity without having any significant effect on the biological functions of rhKGF.

Restoring thymic function: Then and now

Thymic vulnerability, a leading cause of defective immunity, was discovered decades ago. To date, several strategies have been investigated to unveil any immunorestorative capacities they might confer. Studies exploiting castration, transplantation, adoptive cell therapies, hormones/growth factors, and cytokines have demonstrated enhanced in vitro and in vivo thymopoiesis, albeit with clinical restrictions. In this review, we will dissect the thymus on a physiological and pathological level and discuss the pros and cons of several strategies esteemed thymotrophic from a pre-clinical perspective. Finally, we will shed light on interleukin (IL)-21, a pharmacologically-promising cytokine with a significant thymotrophic nature, and elaborate on its potential clinical efficacy and safety in immune-deficient subjects.

Keratinocyte growth factor impairs human thymic recovery from lymphopenia

BACKGROUND

The lymphocyte-depleting antibody alemtuzumab is a highly effective treatment of relapsing-remitting multiple sclerosis (RRMS); however 50% of patients develop novel autoimmunity post-treatment. Most at risk are individuals who reconstitute their T-cell pool by proliferating residual cells, rather than producing new T-cells in the thymus; raising the possibility that autoimmunity might be prevented by increasing thymopoiesis. Keratinocyte growth factor (palifermin) promotes thymopoiesis in non-human primates.

METHODS

Following a dose-tolerability sub-study, individuals with RRMS (duration ≤10 years; expanded disability status scale ≤5·0; with ≥2 relapses in the previous 2 years) were randomised to placebo or 180mcg/kg/day palifermin, given for 3 days immediately prior to and after each cycle of alemtuzumab, with repeat doses at M1 and M3. The interim primary endpoint was naïve CD4+ T-cell count at M6. Exploratory endpoints included: number of recent thymic-emigrants (RTEs) and signal-joint T-cell receptor excision circles (sjTRECs)/mL of blood. The trial primary endpoint was incidence of autoimmunity at M30.

FINDINGS

At M6, individuals receiving palifermin had fewer naïve CD4+T-cells (2.229x107/L vs. 7.733x107/L; p=0.007), RTEs (16% vs. 34%) and sjTRECs/mL (1100 vs. 3396), leading to protocol-defined termination of recruitment. No difference was observed in the rate of autoimmunity between the two groupsConclusion: In contrast to animal studies, palifermin reduced thymopoiesis in our patients. These results offer a note of caution to those using palifermin to promote thymopoiesis in other settings, particularly in the oncology/haematology setting where alemtuzumab is often used as part of the conditioning regime.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01712945Funding: MRC and Moulton Charitable Foundation.

Lymph nodes as barriers to T-cell rejuvenation in aging mice and nonhuman primates

In youth, thymic involution curtails production of new naïve T cells, placing the onus of T-cell maintenance upon secondary lymphoid organs (SLO). This peripheral maintenance preserves the size of the T-cell pool for much of the lifespan, but wanes in the last third of life, leading to a dearth of naïve T cells in blood and SLO, and contributing to suboptimal immune defense. Both keratinocyte growth factor (KGF) and sex steroid ablation (SSA) have been shown to transiently increase the size and cellularity of the old thymus. It is less clear whether this increase can improve protection of old animals from infectious challenge. Here, we directly measured the extent to which thymic rejuvenation benefits the peripheral T-cell compartment of old mice and nonhuman primates. Following treatment of old animals with either KGF or SSA, we observed robust rejuvenation of thymic size and cellularity, and, in a reporter mouse model, an increase in recent thymic emigrants (RTE) in the blood. However, few RTE were found in the spleen and even fewer in the lymph nodes, and SSA-treated mice showed no improvement in immune defense against West Nile virus. In parallel, we found increased disorganization and fibrosis in old LN of both mice and nonhuman primates. These results suggest that SLO defects with aging can negate the effects of successful thymic rejuvenation in immune defense.

Nonhuman Primate Models of Immunosenescence

Due to a dramatic increase in life expectancy, the number of individuals aged 65 and older is rapidly rising. This presents considerable challenges to our health care system since advanced age is associated with a higher susceptibility to infectious diseases due to immune senescence. However, the mechanisms underlying age-associated dysregulated immunity are still incompletely understood. Advancement in our comprehension of mechanisms of immune senescence and development of interventions to improve health span requires animal models that closely recapitulate the physiological changes that occur with aging in humans. Nonhuman primates (NHPs) are invaluable preclinical models to study the underlying causal mechanism of pathogenesis due to their outbred nature, high degree of genetic and physiological similarity to humans, and their susceptibility to human pathogens. In this chapter, we review NHP models available for biogerontology research, advantages and challenges they present, and advances they facilitated. Furthermore, we emphasize the utility of NHPs in characterizing immune senescence, evaluating interventions to reverse aging of the immune system, and development of vaccine strategies that are better suited for this vulnerable population.

Sublethal Total Body Irradiation Causes Long-Term Deficits in Thymus Function by Reducing Lymphoid Progenitors

Total body irradiation (TBI) damages hematopoietic cells in the bone marrow and thymus; however, the long-term effects of irradiation with aging remain unclear. In this study, we found that the impact of radiation on thymopoiesis in mice varied by sex and dose but, overall, thymopoiesis remained suppressed for ≥12 mo after a single exposure. Male and female mice showed a long-term dose-dependent reduction in thymic cKit⁺ lymphoid progenitors that was maintained throughout life. Damage to hematopoietic stem cells (HSCs) in the bone marrow was dose dependent, with as little as 0.5 Gy causing a significant long-term reduction. In addition, the potential for T lineage commitment was radiation sensitive with aging. Overall, the impact of irradiation on the hematopoietic lineage was more severe in females. In contrast, the rate of decline in thymic epithelial cell numbers with age was radiation-sensitive only in males, and other characteristics including Ccl25 transcription were unaffected. Taken together, these data suggest that long-term suppression of thymopoiesis after sublethal irradiation was primarily due to fewer progenitors in the BM combined with reduced potential for T lineage commitment. A single irradiation dose also caused synchronization of thymopoiesis, with a periodic thymocyte differentiation profile persisting for at least 12 mo postirradiation. This study suggests that the number and capability of HSCs for T cell production can be dramatically and permanently damaged after a single relatively low TBI dose, accelerating aging-associated thymic involution. Our findings may impact evaluation and therapeutic intervention of human TBI events.

Loss of thymic innate lymphoid cells leads to impaired thymopoiesis in experimental graft-versus-host disease

Graft-versus-host disease (GVHD) and posttransplant immunodeficiency are frequently related complications of allogeneic hematopoietic transplantation. Alloreactive donor T cells can damage thymic epithelium, thus limiting new T-cell development. Although the thymus has a remarkable capacity to regenerate after injury, endogenous thymic regeneration is impaired in GVHD. The mechanisms leading to this regenerative failure are largely unknown. Here we demonstrate in experimental mouse models that GVHD results in depletion of intrathymic group 3 innate lymphoid cells (ILC3s) necessary for thymic regeneration. Loss of thymic ILC3s resulted in deficiency of intrathymic interleukin-22 (IL-22) compared with transplant recipients without GVHD, thereby inhibiting IL-22-mediated protection of thymic epithelial cells (TECs) and impairing recovery of thymopoiesis. Conversely, abrogating IL-21 receptor signaling in donor T cells and inhibiting the elimination of thymic ILCs improved thymopoiesis in an IL-22-dependent fashion. We found that the thymopoietic impairment in GVHD associated with loss of ILCs could be improved by restoration of IL-22 signaling. Despite uninhibited alloreactivity, exogenous IL-22 administration posttransplant resulted in increased recovery of thymopoiesis and development of new thymus-derived peripheral T cells. Our study highlights the role of innate immune function in thymic regeneration and restoration of adaptive immunity posttransplant. Manipulation of the ILC-IL-22-TEC axis may be useful for augmenting immune reconstitution after clinical hematopoietic transplantation and other settings of T-cell deficiency.

Immune Reconstitution after Allogeneic Hematopoietic Stem Cell Transplantation: Time To T Up the Thymus

The success of allogeneic hematopoietic stem cell transplantation, a key treatment for many disorders, is intertwined with T cell immune reconstitution. The thymus plays a key role post allogeneic hematopoietic stem cell transplantation in the generation of a broad but self-tolerant T cell repertoire, but it is exquisitely sensitive to a range of insults during the transplant period, including conditioning regimens, corticosteroids, infections, and graft-versus-host disease. Although endogenous thymic repair is possible it is often suboptimal, and there is a need to develop exogenous strategies to help regenerate the thymus. Therapies currently in clinical trials in the transplant setting include keratinocyte growth factor, cytokines (IL-7 and IL-22), and hormonal modulation including sex steroid inhibition and growth hormone administration. Such regenerative strategies may ultimately enable the thymus to play as prominent a role after transplant as it once did in early childhood, allowing a more complete restoration of the T cell compartment.

T cell progenitor therapy-facilitated thymopoiesis depends upon thymic input and continued thymic microenvironment interaction

Infusion of in vitro-derived T cell progenitor (proT) therapy with hematopoietic stem cell transplant aids the recovery of the thymus damaged by total body irradiation. To understand the interaction between proTs and the thymic microenvironment, WT mice were lethally irradiated and given T cell-deficient (Rag1-/-) marrow with WT in vitro-generated proTs, limiting mature T cell development to infused proTs. ProTs within the host thymus led to a significant increase in thymic epithelial cells (TECs) by day 21 after transplant, increasing actively cycling TECs. Upon thymus egress (day 28), proT TEC effects were lost, suggesting that continued signaling from proTs is required to sustain TEC cycling and cellularity. Thymocytes increased significantly by day 21, followed by a significant improvement in mature T cell numbers in the periphery by day 35. This protective surge was temporary, receding by day 60. Double-negative 2 (DN2) proTs selectively increased thymocyte number, while DN3 proTs preferentially increased TECs and T cells in the spleen that persisted at day 60. These findings highlight the importance of the interaction between proTs and TECs in the proliferation and survival of TECs and that the maturation stage of proTs has unique effects on thymopoiesis and peripheral T cell recovery.

Medullary thymic epithelial stem cells: role in thymic epithelial cell maintenance and thymic involution

The thymus consists of two distinct anatomical regions, the cortex and the medulla; medullary thymic epithelial cells (mTECs) play a crucial role in establishing central T-cell tolerance for self-antigens. Although the understanding of mTEC development in thymic organogenesis as well as the regulation of their differentiation and maturation has improved, the mechanisms of postnatal maintenance remain poorly understood. This issue has a central importance in immune homeostasis and physiological thymic involution as well as autoimmune disorders in various clinicopathological settings. Recently, several reports have demonstrated the existence of TEC stem or progenitor cells in the postnatal thymus, which are either bipotent or unipotent. We identified stem cells specified for mTEC-lineage that are generated in the thymic ontogeny and may sustain mTEC regeneration and lifelong central T-cell self-tolerance. This finding suggested that the thymic medulla is maintained autonomously by its own stem cells. Although several issues, including the relationship with other putative TEC stem/progenitors, remain unclear, further examination of mTEC stem cells (mTECSCs) and their regulatory mechanisms may contribute to the understanding of postnatal immune homeostasis. Possible relationships between decline of mTECSC activity and early thymic involution as well as various autoimmune disorders are discussed.

Early memory phenotypes drive T cell proliferation in patients with pediatric malignancies

Engineered T cell therapies have begun to demonstrate impressive clinical responses in patients with B cell malignancies. Despite this efficacy, many patients are unable to receive T cell therapy because of failure of in vitro expansion, a necessary component of cell manufacture and a predictor of in vivo activity. To evaluate the biology underlying these functional differences, we investigated T cell expansion potential and memory phenotype during chemotherapy in pediatric patients with acute lymphoblastic leukemia (ALL) and non-Hodgkin lymphoma (NHL). We found that patients with T cell populations enriched for early lineage cells expanded better in vitro and that patients with ALL had higher numbers of these cells with a corresponding enhancement in expansion as compared to cells from patients with NHL. We further demonstrated that early lineage cells were selectively depleted by cyclophosphamide and cytarabine chemotherapy and that culture with interleukin-7 (IL-7) and IL-15 enriched select early lineage cells and rescued T cell expansion capability. Thus, early lineage cells are essential to T cell fitness for expansion, and enrichment of this population either by timing of T cell collection or culture method can increase the number of patients eligible to receive highly active engineered cellular therapies.

Strategies before, during, and after hematopoietic cell transplantation to improve T-cell immune reconstitution

T-cell immune reconstitution (IR) after allogeneic hematopoietic cell transplantation (allo-HCT) is highly variable between patients and may take several months to even years. Patients with delayed or unbalanced T-cell IR have a higher probability of developing transplantation-related morbidity, mortality, and relapse of disease. Hence, there is a need for strategies to better predict and improve IR to reduce these limitations of allo-HCT. In this review, we provide an update of current and in-near-future clinically relevant strategies before, during, and after transplantation to achieve successful T-cell IR. Potent strategies are choosing the right HCT source (eg, donor-recipient matching, cell dose, graft manipulation), individualized conditioning and serotherapy (eg, antithymocyte globulin), nutritional status, exercise, home care, modulation of microbiota, enhancing homeostatic peripheral expansion, promoting thymopoiesis, and the use of adjuvant-targeted cellular immunotherapies. Strategies to prevent graft-versus-host disease are important as well because this complication and the subsequent need for immunosuppression affects T-cell IR and function. These options aim for personalized precision transplantation, where allo-HCT therapy is designed to boost a well-balanced T-cell IR and limit complications in individual patients, resulting in overall lower morbidity and higher survival chances.

Transplantation of Donor-Origin Mouse Embryonic Stem Cell-Derived Thymic Epithelial Progenitors Prevents the Development of Chronic Graft-versus-Host Disease in Mice

Allogeneic hematopoietic stem cell transplantation (HSCT) is a potentially curative therapy for many malignant and nonmalignant diseases. However, chronic graft-versus-host disease (cGVHD) remains a significant cause of late morbidity and mortality after allogeneic HSCT. cGVHD often manifests as autoimmune syndrome. Thymic epithelial cells (TECs) play a critical role in supporting negative selection and regulatory T-cell (Treg) generation. Studies have shown that damage in TECs is sufficient to induce cGVHD. We have previously reported that mouse embryonic stem cells (mESCs) can be selectively induced to generate thymic epithelial progenitors (TEPs) in vitro. When transplanted in vivo, mESC-TEPs further develop into TECs that support T-cell development. We show here that transplantation of donor-origin mESC-TEPs into cGVHD recipients induces immune tolerance to both donor and host antigens and prevents the development of cGVHD. This is associated with more TECs and Tregs. Our results suggest that embryonic stem cell-derived TEPs may offer a new tool to control cGVHD. Stem Cells Translational Medicine 2017;6:121-130.

Homology modeling and virtual screening studies of FGF-7 protein-a structure-based approach to design new molecules against tumor angiogenesis

Keratinocyte growth factor (KGF) protein is a member of the fibroblast growth factor (FGF) family, which is also known as FGF-7. The FGF-7 plays an important role in tumor angiogenesis. In the present work, FGF-7 is treated as a potential therapeutic target to prevent angiogenesis in cancerous tissue. Computational techniques are applied to evaluate and validate the 3D structure of FGF-7 protein. The active site region of the FGF-7 protein is identified based on hydrophobicity calculations using CASTp and Q-site Finder active site prediction tools. The protein-protein docking study of FGF-7 with its natural receptor FGFR2b is carried out to confirm the active site region in FGF-7. The amino acid residues Asp34, Arg67, Glu116, and Thr194 in FGF-7 interact with the receptor protein (FGFR2b). A grid is generated at the active site region of FGF-7 using Glide module of Schrödinger suite. Subsequently, a virtual screening study is carried out at the active site using small molecular structural databases to identify the ligand molecules. The binding interactions of the ligand molecules, with piperazine moiety as a pharmacophore, are observed at Arg67 and Glu149 residues of the FGF-7 protein. The identified ligand molecules against the FGF-7 protein show permissible pharmacokinetic properties (ADME). The ligand molecules with good docking scores and satisfactory pharmacokinetic properties are prioritized and identified as novel ligands for the FGF-7 protein in cancer therapy.

Thymus: the next (re)generation

As the primary site of T-cell development, the thymus plays a key role in the generation of a strong yet self-tolerant adaptive immune response, essential in the face of the potential threat from pathogens or neoplasia. As the importance of the role of the thymus has grown, so too has the understanding that it is extremely sensitive to both acute and chronic injury. The thymus undergoes rapid degeneration following a range of toxic insults, and also involutes as part of the aging process, albeit at a faster rate than many other tissues. The thymus is, however, capable of regenerating, restoring its function to a degree. Potential mechanisms for this endogenous thymic regeneration include keratinocyte growth factor (KGF) signaling, and a more recently described pathway in which innate lymphoid cells produce interleukin-22 (IL-22) in response to loss of double positive thymocytes and upregulation of IL-23 by dendritic cells. Endogenous repair is unable to fully restore the thymus, particularly in the aged population, and this paves the way toward the need for exogenous strategies to help regenerate or even replace thymic function. Therapies currently in clinical trials include KGF, use of the cytokines IL-7 and IL-22, and hormonal modulation including growth hormone administration and sex steroid inhibition. Further novel strategies are emerging in the preclinical setting, including the use of precursor T cells and thymus bioengineering. The use of such strategies offers hope that for many patients, the next regeneration of their thymus is a step closer.

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.

Prolongevity hormone FGF21 protects against immune senescence by delaying age-related thymic involution

Age-related thymic degeneration is associated with loss of naïve T cells, restriction of peripheral T-cell diversity, and reduced healthspan due to lower immune competence. The mechanistic basis of age-related thymic demise is unclear, but prior evidence suggests that caloric restriction (CR) can slow thymic aging by maintaining thymic epithelial cell integrity and reducing the generation of intrathymic lipid. Here we show that the prolongevity ketogenic hormone fibroblast growth factor 21 (FGF21), a member of the endocrine FGF subfamily, is expressed in thymic stromal cells along with FGF receptors and its obligate coreceptor, βKlotho. We found that FGF21 expression in thymus declines with age and is induced by CR. Genetic gain of FGF21 function in mice protects against age-related thymic involution with an increase in earliest thymocyte progenitors and cortical thymic epithelial cells. Importantly, FGF21 overexpression reduced intrathymic lipid, increased perithymic brown adipose tissue, and elevated thymic T-cell export and naïve T-cell frequencies in old mice. Conversely, loss of FGF21 function in middle-aged mice accelerated thymic aging, increased lethality, and delayed T-cell reconstitution postirradiation and hematopoietic stem cell transplantation (HSCT). Collectively, FGF21 integrates metabolic and immune systems to prevent thymic injury and may aid in the reestablishment of a diverse T-cell repertoire in cancer patients following HSCT.

FSP1(+) fibroblast subpopulation is essential for the maintenance and regeneration of medullary thymic epithelial cells

Thymic epithelial cells (TECs) form a 3-dimentional network supporting thymocyte development and maturation. Besides epithelium and thymocytes, heterogeneous fibroblasts are essential components in maintaining thymic microenvironments. However, thymic fibroblast characteristics, development and function remain to be determined. We herein found that thymic non-hematopoietic CD45(-)FSP1(+) cells represent a unique Fibroblast specific protein 1 (FSP1)(-)fibroblast-derived cell subset. Deletion of these cells in FSP1-TK transgenic mice caused thymus atrophy due to the loss of TECs, especially mature medullary TECs (MHCII(high), CD80(+) and Aire(+)). In a cyclophosphamide-induced thymus injury and regeneration model, lack of non-hematopoietic CD45(-)FSP1(+) fibroblast subpopulation significantly delayed thymus regeneration. In fact, thymic FSP1(+) fibroblasts released more IL-6, FGF7 and FSP1 in the culture medium than their FSP1(-) counterparts. Further experiments showed that the FSP1 protein could directly enhance the proliferation and maturation of TECs in the in vitro culture systems. FSP1 knockout mice had significantly smaller thymus size and less TECs than their control. Collectively, our studies reveal that thymic CD45(-)FSP1(+) cells are a subpopulation of fibroblasts, which is crucial for the maintenance and regeneration of TECs especially medullary TECs through providing IL-6, FGF7 and FSP1.

Identification of MiR-205 As a MicroRNA That Is Highly Expressed in Medullary Thymic Epithelial Cells

Thymic epithelial cells (TECs) support T cell development in the thymus. Cortical thymic epithelial cells (cTECs) facilitate positive selection of developing thymocytes whereas medullary thymic epithelial cells (mTECs) facilitate the deletion of self-reactive thymocytes in order to prevent autoimmunity. The mTEC compartment is highly dynamic with continuous maturation and turnover, but the genetic regulation of these processes remains poorly understood. MicroRNAs (miRNAs) are important regulators of TEC genetic programs since miRNA-deficient TECs are severely defective. However, the individual miRNAs important for TEC maintenance and function and their mechanisms of action remain unknown. Here, we demonstrate that miR-205 is highly and preferentially expressed in mTECs during both thymic ontogeny and in the postnatal thymus. This distinct expression is suggestive of functional importance for TEC biology. Genetic ablation of miR-205 in TECs, however, neither revealed a role for miR-205 in TEC function during homeostatic conditions nor during recovery from thymic stress conditions. Thus, despite its distinct expression, miR-205 on its own is largely dispensable for mTEC biology.

Sex steroid blockade enhances thymopoiesis by modulating Notch signaling

Velardi et al. show that sex steroids regulate thymopoiesis by directly modulating Notch signaling, and provide a novel clinical strategy to boost immune regeneration.

Paradoxical to its importance for generating a diverse T cell repertoire, thymic function progressively declines throughout life. This process has been at least partially attributed to the effects of sex steroids, and their removal promotes enhanced thymopoiesis and recovery from immune injury. We show that one mechanism by which sex steroids influence thymopoiesis is through direct inhibition in cortical thymic epithelial cells (cTECs) of Delta-like 4 (Dll4), a Notch ligand crucial for the commitment and differentiation of T cell progenitors in a dose-dependent manner. Consistent with this, sex steroid ablation (SSA) led to increased expression of Dll4 and its downstream targets. Importantly, SSA induced by luteinizing hormone-releasing hormone (LHRH) receptor antagonism bypassed the surge in sex steroids caused by LHRH agonists, the gold standard for clinical ablation of sex steroids, thereby facilitating increased Dll4 expression and more rapid promotion of thymopoiesis. Collectively, these findings not only reveal a novel mechanism underlying improved thymic regeneration upon SSA but also offer an improved clinical strategy for successfully boosting immune function.

Effect of recombinant human keratinocyte growth factor in inducing Ras-Raf-Erk pathway-mediated cell proliferation in emphysematous mice lung

CONTEXT

Pulmonary emphysema is resulted due to destruction of the structure of the alveoli. Recently, exogenous recombinant human Keratinocyte growth factor (rHuKGF) has been reported to induce the regeneration of gas exchange structures. However, the molecular mechanisms governing this process are so far unknown.

OBJECTIVE

The objective of this study was to investigate the effect of rHuKGF in the lungs of emphysema-challenged mice on Ras-Raf-Erk (Erk, extracellular signal-regulated kinase) mediated signaling pathway that regulates alveolar epithelial cell proliferation.

METHODS

Three experimental groups (i.e. emphysema, therapy and control group) were prepared. Lungs of mice were therapeutically treated at three occasions by oropharyngeal instillation of 10 mg rHuKGF per kg body weight after induction of emphysema by porcine pancreatic elastase (PPE). Subsequently, lung tissues from each mouse were collected for histopathology and molecular biology studies.

RESULTS AND DISCUSSION

Histopathology photomicrographs and Destructive Index analysis have shown that elastase induced airspace enlargement and loss of alveoli were recovered in therapy group. Moreover, proliferating cell nuclear antigen (PCNA) at mRNA and protein expression level was markedly increased in therapy group than emphysema group. Upon validation at mRNA level, expressions of FGF-7, FGF-R, Ras, c-Raf, Erk-1, Erk-2, c-Myc and were significantly increased, whereas Elk-1 was notably decreased in therapy group when compared with emphysema group and were well comparable with the control group.

CONCLUSION

Therapeutic supplementation of rHuKGF rectifies the deregulated Ras-Raf-Erk pathway in emphysema condition, resulting in alveolar epithelium regeneration. Hence, rHuKGF may prove to be a potential drug in the treatment of emphysema.

The role of the thymus in T-cell immune reconstitution after umbilical cord blood transplantation

Umbilical cord blood (UCB) is an alternative source of hematopoietic stem cells for patients without HLA-matched adult donors. UCB contains a low number of nucleated cells and mostly naive T cells, resulting in prolonged time to engraftment and lack of transferred T-cell memory. Although the first phase of T-cell reconstitution after UCB transplantation (UCBT) depends on peripheral expansion of transferred T cells, permanent T-cell reconstitution is mediated via a central mechanism, which depends on de novo production of naive T lymphocytes by the recipient's thymus from donor-derived lymphoid-myeloid progenitors (LMPs). Thymopoiesis can be assessed by quantification of recent thymic emigrants, T-cell receptor excision circle levels, and T-cell receptor repertoire diversity. These assays are valuable tools for monitoring posttransplantation thymic recovery, but more importantly they have shown the significant prognostic value of thymic reconstitution for clinical outcomes after UCBT, including opportunistic infections, disease relapse, and overall survival. Strategies to improve thymic entry and differentiation of LMPs and to accelerate recovery of the thymic stromal microenvironment may improve thymic lymphopoiesis. Here, we discuss the mechanisms and clinical implications of thymic recovery and new approaches to improve reconstitution of the T-cell repertoire after UCBT.

A randomized controlled trial of palifermin (recombinant human keratinocyte growth factor) for the treatment of inadequate CD4+ T-lymphocyte recovery in patients with HIV-1 infection on antiretroviral therapy

BACKGROUND

Poor CD4 lymphocyte recovery on antiretroviral therapy (ART) is associated with reduced function of the thymus. Palifermin (keratinocyte growth factor), by providing support to the thymic epithelium, promotes lymphopoiesis in animal models of bone marrow transplantation and graft-versus-host disease.

METHODS

In AIDS Clinical Trials Group A5212, a randomized, double-blind, placebo-controlled study, 99 HIV-infected patients on ART with plasma HIV-1 RNA levels ≤200 copies per milliliter for ≥6 months and CD4 lymphocyte counts <200 cells per cubic milliliter were randomized 1:1:1:1 to receive once daily intravenous administration of placebo or 20, 40, or 60 μg/kg of palifermin on 3 consecutive days.

RESULTS

The median change in the CD4 T-cell count from baseline to week 12 was not significantly different between the placebo arm [15 (-16, 23) cells/mm] and the 20-μg/kg dose [11 (2, 32) cells/mm], the 40-μg/kg dose [12 (-2, 25) cells/mm], or the 60-μg/kg dose arm [8 (-13, 35) cells/mm] of palifermin. No significant changes were observed in thymus size or in the number of naive T cells or recent thymic emigrants.

CONCLUSIONS

Palifermin in the doses studied was not effective in improving thymic function and did not raise CD4 lymphocyte counts in HIV-infected patients with low CD4 cell counts despite virologically effective ART.

Chemokine treatment rescues profound T-lineage progenitor homing defect after bone marrow transplant conditioning in mice

Development of T cells in the thymus requires continuous importation of T-lineage progenitors from the bone marrow via the circulation. Following bone marrow transplant, recovery of a normal peripheral T-cell pool depends on production of naïve T cells in the thymus; however, delivery of progenitors to the thymus limits T-lineage reconstitution. Here, we examine homing of intravenously delivered progenitors to the thymus following irradiation and bone marrow reconstitution. Surprisingly, following host conditioning by irradiation, we find that homing of lymphoid-primed multipotent progenitors and common lymphoid progenitors to the thymus decreases more than 10-fold relative to unirradiated mice. The reduction in thymic homing in irradiated mice is accompanied by a significant reduction in CCL25, an important chemokine ligand for thymic homing. We show that pretreatment of bone marrow progenitors with CCL25 and CCL21 corrects the defect in thymic homing after irradiation and promotes thymic reconstitution. These data suggest new therapeutic approaches to promote T-cell regeneration.

Palifermin for the protection and regeneration of epithelial tissues following injury: new findings in basic research and pre-clinical models

Keratinocyte growth factor (KGF) is a paracrine-acting epithelial mitogen produced by cells of mesenchymal origin, that plays an important role in protecting and repairing epithelial tissues. Pre-clinical data initially demonstrated that a recombinant truncated KGF (palifermin) could reduce gastrointestinal injury and mortality resulting from a variety of toxic exposures. Furthermore, the use of palifermin in patients with hematological malignancies reduced the incidence and duration of severe oral mucositis experienced after intensive chemoradiotherapy. Based upon these findings, as well as the observation that KGF receptors are expressed in many, if not all, epithelial tissues, pre-clinical studies have been conducted to determine the efficacy of palifermin in protecting different epithelial tissues from toxic injury in an attempt to model various clinical situations in which it might prove to be of benefit in limiting tissue damage. In this article, we review these studies to provide the pre-clinical background for clinical trials that are described in the accompanying article and the rationale for additional clinical applications of palifermin.

Improving engraftment and immune reconstitution in umbilical cord blood transplantation

Umbilical cord blood (UCB) is an important source of hematopoietic stem cells (HSC) for allogeneic transplantation when HLA-matched sibling and unrelated donors (MUD) are unavailable. Although the overall survival results for UCB transplantation are comparable to the results with MUD, UCB transplants are associated with slow engraftment, delayed immune reconstitution, and increased opportunistic infections. While this may be a consequence of the lower cell dose in UCB grafts, it also reflects the relative immaturity of cord blood. Furthermore, limited cell numbers and the non-availability of donor lymphocyte infusions currently prevent the use of post-transplant cellular immunotherapy to boost donor-derived immunity to treat infections, mixed chimerism, and disease relapse. To further develop UCB transplantation, many strategies to enhance engraftment and immune reconstitution are currently under investigation. This review summarizes our current understanding of engraftment and immune recovery following UCB transplantation and why this differs from allogeneic transplants using other sources of HSC. It also provides a comprehensive overview of promising techniques being used to improve myeloid and lymphoid recovery, including expansion, homing, and delivery of UCB HSC; combined use of UCB with third-party donors; isolation and expansion of natural killer cells, pathogen-specific T cells, and regulatory T cells; methods to protect and/or improve thymopoiesis. As many of these strategies are now in clinical trials, it is anticipated that UCB transplantation will continue to advance, further expanding our understanding of UCB biology and HSC transplantation.

Thymic epithelial cell development and its dysfunction in human diseases

Thymic epithelial cells (TECs) are the key components in thymic microenvironment for T cells development. TECs, composed of cortical and medullary TECs, are derived from a common bipotent progenitor and undergo a stepwise development controlled by multiple levels of signals to be functionally mature for supporting thymocyte development. Tumor necrosis factor receptor (TNFR) family members including the receptor activator for NFκB (RANK), CD40, and lymphotoxin β receptor (LTβR) cooperatively control the thymic medullary microenvironment and self-tolerance establishment. In addition, fibroblast growth factors (FGFs), Wnt, and Notch signals are essential for establishment of functional thymic microenvironment. Transcription factors Foxn1 and autoimmune regulator (Aire) are powerful modulators of TEC development, differentiation, and self-tolerance. Dysfunction in thymic microenvironment including defects of TEC and thymocyte development would cause physiological disorders such as tumor, infectious diseases, and autoimmune diseases. In the present review, we will summarize our current understanding on TEC development and the underlying molecular signals pathways and the involvement of thymus dysfunction in human diseases.

GSK-3β inhibition preserves naive T cell phenotype in bone marrow reconstituted mice

Hematopoietic stem cell transplantation (HSCT) is used in the treatment of hematologic and nonhematologic disorders. PostHSCT immunologic reconstitution is a critical component for successful outcome. Pretransplant conditioning impairs thymic function, leading to delayed T cell regeneration. Thymus-independent T cell expansion is associated with defective generation of naive T cells and memory T cell skewing, resulting in decreased diversity in the T cell repertoire, thus attenuating the immune responses and increasing the risk of opportunistic infections and leukemia relapse. Wingless (Wnt) signaling has been identified as an important regulator of T cell development and function. Activated Wnt signaling inhibited differentiation of mature T cells in transgenic mouse models. The effect of Wnt activation on T cell regeneration following HSCT was not investigated. In this study, we demonstrate that the GSK-3β inhibitor 6-bromoindirubin 3'-oxime (BIO) activates Wnt/β-catenin signaling, elevates the proportion of naive T cells, and delays T cell differentiation during homeostatic T cell expansion in lymphodepleted mice transplanted with human hematopoietic stem cells. In vitro BIO-treatment promoted naive T cell expansion following mitogenic stimulation and improved proliferative responses of T cells to allogeneic stimuli. Treatment with BIO acts to expand the IL7Rα(+) subset of naive T cells, suggesting the potential mechanism driving T cell expansion during IL-7-dependent T cell proliferation. BIO downregulated expression of genes activated during effector cell differentiation and preserved naive T cell gene expression. We propose that administration of GSK-3β inhibitor increases the potency of T cells in recipients of HSCT by expansion of naive T cell subsets with a diverse T cell receptor repertoire.

Clinical strategies to enhance thymic recovery after allogeneic hematopoietic stem cell transplantation

The thymus is particularly sensitive to injury caused by cytoreductive chemo- or radiation therapy, shock, infection and graft versus host disease. Insufficient thymic recovery has been directly correlated with increased risk of opportunistic infections and poor clinical outcomes in recipients of allogeneic hematopoietic stem cell transplantation (allo-HSCT). Prolonged immune deficiency is particularly pronounced in older patients whose thymi are already significantly impaired due to age-related thymic involution. Preclinical and clinical studies have revealed several strategies that can enhance thymic function and immune reconstitution after transplant, including sex steroid ablation, growth factors (growth hormone, keratinocyte growth factor, insulin-like growth factor 1, interleukin-7) and ex vivo generated precursor T cells. In addition, recent studies have shown that other approaches, such as interleukein-22 and nutritional changes, may represent additional candidates to enhance thymic regeneration. In this review we provide updates on these strategies and comment on their potential to be translated into clinical therapies.

Thymic epithelial cell development and differentiation: cellular and molecular regulation

Thymic epithelial cells (TECs) are one of the most important components in thymic microenvironment supporting thymocyte development and maturation. TECs, composed of cortical and medullary TECs, are derived from a common bipotent progenitor, mediating thymocyte positive and negative selections. Multiple levels of signals including intracellular signaling networks and cell-cell interaction are required for TEC development and differentiation. Transcription factors Foxn1 and autoimmune regulator (Aire) are powerful regulators promoting TEC development and differentiation. Crosstalks with thymocytes and other stromal cells for extrinsic signals like RANKL, CD40L, lymphotoxin, fibroblast growth factor (FGF) and Wnt are also definitely required to establish a functional thymic microenvironment. In this review, we will summarize our current understanding about TEC development and differentiation, and its underlying multiple signal pathways.