Insulin-like growth factor-I enhances lymphoid and myeloid reconstitution after allogeneic bone marrow transplantation

Disorganization of secondary lymphoid organs and dyscoordination of chemokine secretion as key contributors to immune aging

Aging is characterized by progressive loss of organ and tissue function, and the immune system is no exception to that inevitable principle. Of all the age-related changes in the body, reduction of the size of, and naïve T (Tn) cell output from, the thymus occurs earliest, being prominent already before or by the time of puberty. Therefore, to preserve immunity against new infections, over much of their lives, vertebrates dominantly rely on peripheral maintenance of the Tn cell pool in the secondary lymphoid organs (SLO). However, SLO structure and function subsequently also deteriorate with aging. Several recent studies have made a convincing case that this deterioration is of major importance to the erosion of protective immunity in the last third of life. Specifically, the SLO were found to accumulate multiple degenerative changes with aging. Importantly, the results from adoptive transfer and parabiosis studies teach us that the old microenvironment is the limiting factor for protective immunity in old mice. In this review, we discuss the extent, mechanisms, and potential role of stromal cell aging in the age-related alteration of T cell homeostatic maintenance and immune function decline. We use that discussion to frame the potential strategies to correct the SLO stromal aging defects - in the context of other immune rejuvenation approaches, - to improve functional immune responses and protective immunity in older adults.

Intrathymic somatotropic circuitry: consequences upon thymus involution

Growth hormone (GH) is a classic pituitary-derived hormone crucial to body growth and metabolism. In the pituitary gland, GH production is stimulated by GH-releasing hormone and inhibited by somatostatin. GH secretion can also be induced by other peptides, such as ghrelin, which interacts with receptors present in somatotropic cells. It is well established that GH acts directly on target cells or indirectly by stimulating the production of insulin-like growth factors (IGFs), particularly IGF-1. Notably, such somatotropic circuitry is also involved in the development and function of immune cells and organs, including the thymus. Interestingly, GH, IGF-1, ghrelin, and somatostatin are expressed in the thymus in the lymphoid and microenvironmental compartments, where they stimulate the secretion of soluble factors and extracellular matrix molecules involved in the general process of intrathymic T-cell development. Clinical trials in which GH was used to treat immunocompromised patients successfully recovered thymic function. Additionally, there is evidence that the reduction in the function of the somatotropic axis is associated with age-related thymus atrophy. Treatment with GH, IGF-1 or ghrelin can restore thymopoiesis of old animals, thus in keeping with a clinical study showing that treatment with GH, associated with metformin and dehydroepiandrosterone, could induce thymus regeneration in healthy aged individuals. In conclusion, the molecules of the somatotrophic axis can be envisioned as potential therapeutic targets for thymus regeneration in age-related or pathological thymus involution.

An ISCT Stem Cell Engineering Committee Position Statement on Immune Reconstitution: the importance of predictable and modifiable milestones of immune reconstitution to transplant outcomes

Allogeneic stem cell transplantation is a potentially curative therapy for some malignant and non-malignant disease. There have been substantial advances since the approaches first introduced in the 1970s, and the development of approaches to transplant with HLA incompatible or alternative donors has improved access to transplant for those without a fully matched donor. However, success is still limited by morbidity and mortality from toxicity and imperfect disease control. Here we review our emerging understanding of how reconstitution of effective immunity after allogeneic transplant can protect from these events and improve outcomes. We provide perspective on milestones of immune reconstitution that are easily measured and modifiable.

Insulin-like growth factor-1 and insulin-like growth factor binding protein-3: impact on early haematopoietic reconstitution following allogeneic haematopoietic stem cell transplantation

OBJECTIVES

The aim of the study was to investigate whether high endogenous levels of insulin-like growth factor-1 (IGF-1) and its binding protein-3 (IGFBP-3) were related to a faster reconstitution of different blood cell populations in the early phase after allogeneic myeloablative haematopoietic stem cell transplantation (HSCT).

METHODS

We measured IGF-1 and IGFBP-3 by chemiluminescence during the first three weeks after transplantation in 35 adult patients undergoing myeloablative HSCT and calculated area under the curve divided by time (AUC/t) for each patient.

RESULTS

Circulating levels of IGF-1 and IGFBP-3 correlated with counts of reticulocytes (r_s  = 0.44, p = .011 and r = 0.41, p = .017, respectively) and thrombocytes (r_s  = 0.38, p = .030 and r_s  = 0.56, p = .0008) three weeks post-transplant. Furthermore, high IGFBP-3 levels correlated with absolute lymphocyte counts 3 weeks post-HSCT (r_s  = 0.54, p = .012) and were associated with shorter time to neutrophil engraftment (r_s  = -0.35, p = .043). Both IGF-1 and IGFBP-3 levels were associated with the number of circulating natural killer cells one month after HSCT (r_s  = 0.42, p = .032 and r_s  = 0.57, p = .0026).

CONCLUSION

These data indicate that high levels of IGF-1 and IGFBP-3 relate to a faster haematopoietic reconstitution after HSCT and suggest a biological influence of these mediators in haematopoietic homeostasis in these patients.

Sirtuins and the prevention of immunosenescence

Aging of hematopoietic stem cells (HSCs) has been largely described as one underlying cause of senescence of the immune-hematopoietic system (immunosenescence). A set of well-defined hallmarks characterizes aged HSCs contributing to unbalanced hematopoiesis and aging-associated functional alterations of both branches of the immune system. In this chapter, the contribution of sirtuins, a family of conserved NAD⁺ dependent deacetylases with key roles in metabolism, genome integrity, aging and lifespan, to immunosenescence, will be addressed. In particular, the role of SIRT6 will be deeply analyzed highlighting a multifaceted part of this deacetylase in HSCs aging as well as in the immunosenescence of dendritic cells (DCs). These and other emerging data are currently paving the way for future design and development of rejuvenation means aiming at rescuing age-related changes in immune function in the elderly and combating age-associated hematopoietic diseases.

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.

IL-22 Accelerates Thymus Regeneration via Stat3/Mcl-1 and Decreases Chronic Graft-versus-Host Disease in Mice after Allotransplants

High-dose chemotherapy and/or radiation given before an allogeneic hematopoietic cell transplantation severely damage thymic epithelial cells (TECs), resulting in poor post-transplant immune recovery. IL-22 mediates recovery of TECs via a proregenerative effect, but the precise mechanism by which this occurs is unknown. In this study, we found IL-22 improved thymus recovery after damage from irradiation in association with increased number of TECs. This effect was blocked by ruxolitinib, a JAK1/JAK2 inhibitor. IL-22 increased the number of TECs via a Stat3-dependent signaling in the mTEC1 murine thymic epithelial cell line. This, in turn, upregulated transcription of myeloid cell leukemia sequence 1 (Mcl1), resulting in increased number of TECs. Similar effects were seen in irradiated mice given IL-22. Defects in IL-22 resulted in delayed thymus recovery in irradiated mice and had an impact on levels of thymus function-related genes such as Foxn1, Aire, and Kgf. In mice, post-transplant use of IL-22 improved repair of TECs, increased the numbers of thymus T cells, increased the intrathymic levels of Aire, and increased the proportion of natural regulatory T cells, resulting in decreased severity of chronic graft-versus-host disease (GVHD). Our data highlight the critical role of the IL-22/Stat3/Mcl-1 pathway in the regeneration of TECs after damage from irradiation in mice and highlight circumstances where normalizing thymus T cell function with IL-22 decreases GVHD after allotransplants.

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.

Concise Review: Boosting T-Cell Reconstitution Following Allogeneic Transplantation-Current Concepts and Future Perspectives

Allogeneic hematopoietic stem cell transplantation (HSCT) is the treatment of choice for a large number of malignant and nonmalignant (inherited) diseases of the hematopoietic system. Nevertheless, non‐HLA identical transplantations are complicated by a severe T‐cell immunodeficiency associated with a high rate of infection, relapse and graft‐versus‐host disease. Initial recovery of T‐cell immunity following HSCT relies on peripheral expansion of memory T cells mostly driven by cytokines. The reconstitution of a diverse, self‐tolerant, and naive T‐cell repertoire, however, may take up to 2 years and crucially relies on the interaction of T‐cell progenitors with the host thymic epithelium, which may be altered by GvHD, age or transplant‐related toxicities. In this review, we summarize current concepts to stimulate reconstitution of a peripheral and polyclonal T‐cell compartment following allogeneic transplantation such as graft manipulation (i.e., T‐cell depletion), transfusion of ex vivo manipulated donor T cells or the exogenous administration of cytokines and growth factors to stimulate host‐thymopoiesis with emphasis on approaches which have led to clinical trials. Particular attention will be given to the development of cellular therapies such as the ex vivo generation of T‐cell precursors to fasten generation of a polyclonal and functional host‐derived T‐cell repertoire. Having been tested so far only in preclinical mouse models, clinical studies are now on the way to validate the efficacy of such T‐cell progenitors in enhancing immune reconstitution following HSCT in various clinical settings. stem cells translational medicine2019;00:1–8

New interleukin-15 superagonist (IL-15SA) significantly enhances graft-versus-tumor activity

Interleukin-15 (IL-15) is a potent cytokine that increases CD8⁺ T and NK cell numbers and function in experimental models. However, obstacles remain in using IL-15 therapeutically, specifically its low potency and short in vivo half-life. To help overcome this, a new IL-15 superagonist complex comprised of an IL-15N72D mutation and IL-15RαSu/Fc fusion (IL-15SA, also known as ALT-803) was developed. IL-15SA exhibits a significantly longer serum half-life and increased in vivo activity against various tumors. Herein, we evaluated the effects of IL-15SA in recipients of allogeneic hematopoietic stem cell transplantation. Weekly administration of IL-15SA to transplant recipients significantly increased the number of CD8⁺ T cells (specifically CD44⁺ memory/activated phenotype) and NK cells. Intracellular IFN-γ and TNF-α secretion by CD8⁺ T cells increased in the IL-15SA-treated group. IL-15SA also upregulated NKG2D expression on CD8⁺ T cells. Moreover, IL-15SA enhanced proliferation and cytokine secretion of adoptively transferred CFSE-labeled T cells in syngeneic and allogeneic models by specifically stimulating the slowly proliferative and nonproliferative cells into actively proliferating cells.

We then evaluated IL-15SA's effects on anti-tumor activity against murine mastocytoma (P815) and murine B cell lymphoma (A20). IL-15SA enhanced graft-versus-tumor (GVT) activity in these tumors following T cell infusion. Interestingly, IL-15 SA administration provided GVT activity against A20 lymphoma cells in the murine donor leukocyte infusion (DLI) model without increasing graft versus host disease. In conclusion, IL-15SA could be a highly potent T- cell lymphoid growth factor and novel immunotherapeutic agent to complement stem cell transplantation and adoptive immunotherapy.

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.

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.

Sex steroid ablation: an immunoregenerative strategy for immunocompromised patients

Age-related decline in thymic function is a well-described process that results in reduced T-cell development and thymic output of new naïve T cells. Thymic involution leads to reduced response to vaccines and new pathogens in otherwise healthy individuals; however, reduced thymic function is particularly detrimental in clinical scenarios where the immune system is profoundly depleted such as after chemotherapy, radiotherapy, infection and shock. Poor thymic function and restoration of immune competence has been correlated with an increased risk of opportunistic infections, tumor relapse and autoimmunity. Apart from their primary role in sex dimorphism, sex steroid levels profoundly affect the immune system in general and, in fact, age-related thymic involution has been at least partially attributed to the increase in sex steroids at puberty. Subsequently it has been demonstrated that the removal of sex steroids, or sex steroid ablation (SSA), triggers physiologic changes that ultimately lead to thymic re-growth and improved T-cell reconstitution in settings of hematopoietic stem cell transplant (HSCT). Although the cellular and molecular process underlying these regenerative effects are still poorly understood, SSA clearly represents an attractive therapeutic approach to enhance thymic function and restore immune competence in immunodeficient individuals.

Hormonal control of T-cell development in health and disease

The physiology of the thymus, the primary lymphoid organ in which T cells are generated, is controlled by hormones. Data from animal models indicate that several peptide and nonpeptide hormones act pleiotropically within the thymus to modulate the proliferation, differentiation, migration and death by apoptosis of developing thymocytes. For example, growth hormone and prolactin can enhance thymocyte proliferation and migration, whereas glucocorticoids lead to the apoptosis of these developing cells. The thymus undergoes progressive age-dependent atrophy with a loss of cells being generated and exported, therefore, hormone-based therapies are being developed as an alternative strategy to rejuvenate the organ, as well as to augment thymocyte proliferation and the export of mature T cells to peripheral lymphoid organs. Some hormones (such as growth hormone and progonadoliberin-1) are also being used as therapeutic agents to treat immunodeficiency disorders associated with thymic atrophy, such as HIV infection. In this Review, we discuss the accumulating data that shows the thymus gland is under complex and multifaceted hormonal control that affects the process of T-cell development in health and disease.

Effects of growth hormone therapeutic supplementation on hematopoietic stem/progenitor cells in children with growth hormone deficiency: focus on proliferation and differentiation capabilities

We investigated the direct effects of growth hormone (GH) replacement therapy (GH-RT) on hematopoiesis in children with GH deficiency (GHD) with the special emphasis on proliferation and cell cycle regulation. Peripheral blood (PB) was collected from sixty control individuals and forty GHD children before GH-RT and in 3rd and 6th month of GH-RT to measure hematological parameters and isolate CD34(+)-enriched hematopoietic progenitor cells (HPCs). Selected parameters of PB were analyzed by hematological analyzer. Moreover, collected HPCs were used to analyze GH receptor (GHR) and IGF1 expression, clonogenicity, and cell cycle activity. Finally, global gene expression profile of collected HPCs was analyzed using genome-wide RNA microarrays. GHD resulted in a decrease in several hematological parameters related to RBCs and significantly diminished clonogenicity of erythroid progenies. In contrast, GH-RT stimulated increases in clonogenic growth of erythroid lineage and RBC counts as well as significant up-regulation of cell cycle-propagating genes, including MAP2K1, cyclins D1/E1, PCNA, and IGF1. Likewise, GH-RT significantly modified GHR expression in isolated HPCs and augmented systemic IGF1 levels. Global gene expression analysis revealed significantly higher expression of genes associated with cell cycle, proliferation, and differentiation in HPCs from GH-treated subjects. (i) GH-RT significantly augments cell cycle progression in HPCs and increases clonogenicity of erythroid progenitors; (ii) GHR expression in HPCs is modulated by GH status; (iii) molecular mechanisms by which GH influences hematopoiesis might provide a basis for designing therapeutic interventions for hematological complications related to GHD.

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.

Sarcopenia, obesity, and natural killer cell immune senescence in aging: altered cytokine levels as a common mechanism

Human aging is characterized by both physical and physiological frailty. A key feature of frailty, sarcopenia is the age-associated decline in skeletal muscle mass, strength, and endurance that characterize even the healthy elderly. Increases in adiposity, particularly in visceral adipose tissue, are almost universal in aging individuals and can contribute to sarcopenia and insulin resistance by increasing levels of inflammatory cytokines known collectively as adipokines. Aging also is associated with declines in adaptive and innate immunity, known as immune senescence, which are risk factors for cancer and all-cause mortality. The cytokine interleukin-15 (IL-15) is highly expressed in skeletal muscle tissue and declines in aging rodent models. IL-15 inhibits fat deposition and insulin resistance, is anabolic for skeletal muscle in certain situations, and is required for the development and survival of natural killer (NK) lymphocytes. We review the effect that adipokines and myokines have on NK cells, with special emphasis on IL-15. We posit that increased adipokine and decreased IL-15 levels during aging constitute a common mechanism for sarcopenia, obesity, and immune senescence.

Improving immunity in the elderly: current and future lessons from nonhuman primate models

The immune system must overcome daily challenges from pathogens to protect the body from infection. The success of the immune response to infection relies on the ability to sense and evaluate microbial threats and organize their elimination, while limiting damage to host tissues. This delicate balance is achieved through coordinated action of the innate and adaptive arms of the immune system. Aging results in several structural and functional changes in the immune system, often described under the umbrella term "immune senescence". Age-related changes affect both the innate and adaptive arms of the immune system and are believed to result in increased susceptibility and severity of infectious diseases, which is further exacerbated by reduced vaccine efficacy in the elderly. Therefore, multiple strategies to improve immune function in the aged are being investigated. Traditionally, studies on immune senescence are conducted using inbred specific pathogen free (SPF) rodents. This animal model has provided invaluable insight into the mechanisms of aging. However, the limited genetic heterogeneity and the SPF status of this model restrict the successful transfer of immunological discoveries between murine models and the clinical setting. More recently, nonhuman primates (NHPs) have emerged as a leading translational model to investigate immune senescence and to test interventions aimed at delaying/reversing age-related changes in immune function. In this article, we review and summarize advances in immuno-restorative approaches investigated in the NHP model system and discuss where the NHP model can support the development of novel therapeutics.

Role of insulin-like growth factor-1 (IGF-1) pathway in the pathogenesis of Graves' orbitopathy

The etiology of Graves' orbitopathy (GO) remains enigmatic and thus controversy surrounds its pathogenesis. The role of the thyroid stimulating hormone receptor (TSHR) and activating antibodies directed against it in the hyperthyroidism of Graves' disease (GD) is firmly established. Less well elucidated is what part the TSHR pathway might play in the development of GO. Also uncertain is the participation of other cell surface receptors in the disease. Elevated levels of insulin-like growth factor-1 receptor (IGF-1R) have been found in orbital fibroblasts as well as B and T cells from patients with GD. These abnormal patterns of IGF-1R display are also found in rheumatoid arthritis and carry functional consequences. In addition, activating IgGs capable of displacing IGF-1 from IGF-1R have also been detected in patients with these diseases. IGF-1R forms a complex with TSHR which is necessary for at least some of the non-canonical signaling observed following TSHR activation. Functional TSHR and IGF-1R have also been found on fibrocytes, CD34⁺ bone marrow-derived cells from the monocyte lineage. Levels of TSHR on fibrocytes greatly exceed those found on orbital fibroblasts. When ligated by TSH or M22, a TSHR-activating monoclonal antibody, fibrocytes produce extremely high levels of several cytokines and chemokines. Moreover, fibrocytes infiltrate both the orbit and thyroid in GD. In sum, based on current evidence, IGF-1R and TSHR can be thought of as "partners in crime". Involvement of the former probably transcends disease boundaries, while TSHR may not.

Greater than the sum of their parts: combination strategies for immune regeneration following allogeneic hematopoietic stem cell transplantation

Cytoreductive conditioning regimes designed to allow for successful allogeneic hematopoietic stem cell transplantation (allo-HSCT) paradoxically are also detrimental to recovery of the immune system in general but lymphopoiesis in particular. Post-transplant immune depletion is particularly striking within the T cell compartment which is exquisitely sensitive to negative regulation, evidenced by the profound decline in thymic function with age. As a consequence, regeneration of the immune system remains a significant unmet clinical need. Over the past decade studies have revealed several promising therapeutic strategies to address ineffective lymphopoiesis and post-transplant immune deficiency. These include the use of cytokines such as IL-7, IL-12 and IL-15; growth factors and hormones like keratinocyte growth factor (KGF), insulin-like growth factor (IGF)-1 and growth hormone (GH); adoptive transfer of ex vivo-generated precursor T cells (pre-T) and sex steroid ablation (SSA). Moreover, recently several novel approaches have been proposed to generate whole thymii ex vivo using stem cell technologies and bioscaffolds. Increasingly, however, when transferred to the clinic, these strategies alone are not sufficient to restore thymopoiesis in all patients leading to the potential of combination strategies as a way to reign in non-responders. Synergistic enhancement in combination may be due to differential targets may therefore be effective in improving clinical outcomes in the transplant settings as well as in other lymphopenic states induced by high dose chemotherapy/radiation therapy or HIV, and may also be useful in improving responses to vaccination and augmenting anti-tumor immunotherapy.

Hematopoietic progenitor migration to the adult thymus

Although most hematopoietic lineages develop in the bone marrow (BM), T cells uniquely complete their development in the specialized environment of the thymus. Hematopoietic stem cells with long-term self-renewal capacity are not present in the thymus. As a result, continuous T cell development requires that BM-derived progenitors be imported into the thymus throughout adult life. The process of thymic homing begins with the mobilization of progenitors out of the BM, continues with their circulation in the bloodstream, and concludes with their settling in the thymus. This review will discuss each of these steps as they occur in the unirradiated and postirradiation scenarios, focusing on the molecular mechanisms of regulation. Improved knowledge about these early steps in T cell generation may accelerate the development of new therapeutic options in patients with impaired T cell number or function.

Advances in adoptive immunotherapy to accelerate T-cellular immune reconstitution after HLA-incompatible hematopoietic stem cell transplantation

Although partially HLA-mismatched hematopoietic stem cell transplantation (HSCT) has become an important therapeutic option for children with primary immunodeficiencies, delayed reconstitution of the T-cell compartment remains a major clinical concern. Adoptive immunotherapies to provide recipients with a protective and diverse T-cell repertoire in the months following HSCT are warranted. In order to improve T-cell reconstitution after T-cell-depleted HSCT, different strategies are currently being studied. Some are based on administration of modified mature T cells (e.g., allodepleted T cells or pathogen-specific T cells). Others aim at accelerating de novo thymopoiesis from donor-derived hematopoietic stem cells in vivo via the administration of thymopoietic agents or the transfer of large numbers of T-cell precursors generated ex vivo. The present article will provide a brief summary of recent advances in the field of allodepletion and adoptive transfer of pathogen-specific T cells and a detailed discussion of strategies for enhancing thymopoiesis in vivo.

Rejuvenation of the aging thymus: growth hormone-mediated and ghrelin-mediated signaling pathways

One of the major fundamental causes for the aging of the immune system is the structural and functional involution of the thymus, and the associated decline in de novo naïve T-lymphocyte output. This loss of naïve T-cell production weakens the ability of the adaptive immune system to respond to new antigenic stimuli and eventually leads to a peripheral T-cell bias to the memory phenotype. While the precise mechanisms responsible for age-associated thymic involution remain unknown, a variety of theories have been forwarded including the loss of expression of various growth factors and hormones that influence the lymphoid compartment and promote thymic function. Extensive studies examining two hormones, namely growth hormone (GH) and ghrelin (GRL), have demonstrated their contributions to thymus biology. In the current review, we discuss the literature supporting a role for these hormones in thymic physiology and age-associated thymic involution and their potential use in the restoration of thymic function in aged and immunocompromised individuals.

Insulin-like growth factor-I regulation of immune function: a potential therapeutic target in autoimmune diseases?

This topically limited review explores the relationship between the immune system and insulin-like growth factors (IGF-I and IGF-II) and the proteins through which they act, including IGF-I receptor (IGF-IR) and the IGF-I binding proteins. The IGF/IGF-IR pathway plays important and diverse roles in tissue development and function. It regulates cell cycle progression, apoptosis, and the translation of proteins. Many of the consequences ascribed to IGF-IR activation result from its association with several accessory proteins that are either identical or closely related to those involved in insulin receptor signaling. Relatively recent awareness that IGF-I and IGF-IR regulate immune function has cast this pathway in an unexpected light; it may represent an important switch governing the quality and amplitude of immune responses. IGF-I/IGF-IR signaling may also participate in the pathogenesis of autoimmune diseases, although its relationship with these processes seems complex and relatively unexplored. On the one hand, IGF-I seems to protect experimental animals from developing insulin-deficient diabetes mellitus. In contrast, activating antibodies directed at IGF-IR have been detected in patients with Graves' disease, where the receptor is overexpressed by multiple cell types. The frequency of IGF-IR+ B and T cells is substantially increased in patients with that disease. Potential involvement of IGF-I and IGF-IR in the pathogenesis of autoimmune diseases suggests that this pathway might constitute an attractive therapeutic target. IGF-IR has been targeted in efforts directed toward drug development for cancer, employing both small-molecule and monoclonal antibody approaches. These have been generally well-tolerated. Recognizing the broader role of IGF-IR in regulating both normal and pathological immune responses may offer important opportunities for therapeutic intervention in several allied diseases that have proven particularly difficult to treat.

Rejuvenation of the aging T cell compartment

The elderly face significant risk for susceptibility to infection and cancer because of declining immune function. Various agents used in the setting of bone marrow transplantation and aging studies represent promising approaches to combating T cell defects in the aging population. Preclinical and clinical studies on the T cell reconstitution effects of sex steroid ablation, keratinocyte growth factor, the growth hormone pathway, and the cytokines interleukin-7, interleukin-12, and interleukin-15 indicate that these strategies may be used to alleviate the effects of T cell deficiencies in the elderly.

Role of interleukin-15 in umbilical cord blood transplantation

Owing to its easier accessibility and less severe graft-versus-host disease, umbilical cord blood (UCB) has been increasingly used as an alternative to bone marrow for hematopoietic stem-cell transplantation. Naiveté of UCB lymphocytes, however, results in delayed immune reconstitution and infection-related mortality in transplant recipients. This article reviews UCB immunology and addresses the potential therapeutic role of interleukin (IL)-15, a pleiotropic gamma chain signaling cytokine, in modulating immune reconstitution, graft-versus-host disease (GVHD), graft-versus-leukemia effect, and infection susceptibility during the post-UCB transplant period. Cytokine immunotherapy using IL-15 simultaneously modulates several immune compartments, thus holds promise for facilitating post-transplant recovery and augmenting antitumor effect without aggravating GVHD in the setting of UCB transplantation.

Variation in the IGF-1 gene is associated with lymphocyte subset counts in neonates: the Generation R Study

OBJECTIVE

IGF-1 stimulates growth, development and function of lymphocytes. The aim of this study was to examine whether functional variants of the IGF-1 gene are associated with absolute lymphocyte subset counts in neonates.

STUDY DESIGN AND MEASUREMENTS

This study was embedded in the Generation R Study, a prospective cohort study from foetal life onwards. A polymorphism in the IGF-1 promoter region was genotyped in cord blood DNA. Lymphocytes (T, B and NK) and T lymphocyte subsets (helper, cytotoxic, naive and memory) in cord blood were immunophenotyped in 380 neonates by six-colour flow cytometry.

RESULTS

In total, 39% of the neonates were homozygous for the 192-bp allele (wild-type), 48% were heterozygous and 13% were noncarrier. No differences in absolute lymphocyte and T lymphocyte subset counts were observed between the 192-bp allele heterozygous and homozygous groups. In noncarriers, we found 15% lower T lymphocyte (P = 0.03), 22% lower B lymphocyte (P = 0.04) and 10% lower NK lymphocyte counts (P = 0.36) than in the 192-bp allele homozygous group. Analyses of T lymphocyte subsets showed 16% lower helper T lymphocyte counts (P = 0.01) in noncarriers. No significant differences were found for cytotoxic, naive and memory T lymphocyte counts. All associations were adjusted for gravidity, mode of delivery, gestational age, birth weight, gender and 1- and 5- min Apgar scores.

CONCLUSIONS

Our study showed associations between this IGF-1 promoter region polymorphism and absolute lymphocyte subset counts in neonates. These results should be regarded as hypothesis generating until they have been replicated in other studies.

B cells from patients with Graves' disease aberrantly express the IGF-1 receptor: implications for disease pathogenesis

Graves' disease (GD) is an autoimmune process involving the thyroid and connective tissues in the orbit and pretibial skin. Activating anti-thyrotropin receptor Abs are responsible for hyperthyroidism in GD. However, neither these autoAbs nor the receptor they are directed against have been convincingly implicated in the connective tissue manifestations. Insulin-like growth factor-1 receptor (IGF-1R)-bearing fibroblasts overpopulate connective tissues in GD and when ligated with IgGs from these patients, express the T cell chemoattractants, IL-16, and RANTES. Disproportionately large fractions of peripheral blood T cells also express IGF-1R in patients with GD and may account, at least in part, for expansion of IGF-1R(+) memory T cells. We now report a similarly skewed B cell population exhibiting the IGF-1R(+) phenotype from the blood, orbit, and bone marrow of patients with GD. This expression profile exhibits durability in culture and is maintained or increased with CpG activation. Moreover, IGF-1R(+) B cells produce pathogenic Abs against the thyrotropin receptor. In lymphocytes from patients with GD, IGF-1 enhanced IgG production (p < 0.05) and increased B cell expansion (p < 0.02) in vitro while those from control donors failed to respond. These findings suggest a potentially important role for IGF-1R display by B lymphocytes in patients with GD in supporting their expansion and abnormal Ig production.

Exogenous insulin-like growth factor 1 enhances thymopoiesis predominantly through thymic epithelial cell expansion

Insulin-like growth factor 1 (IGF-1) enhances thymopoiesis but given the broad distribution of IGF-1 receptors (IGF-1Rs), its mechanism of action has remained unclear. To identify points of thymic regulation by IGF-1, we examined its effects on T-cell precursors, thymocytes, and thymic epithelial cells (TECs) in normal and genetically altered mice. In thymus-intact but not thymectomized mice, IGF-1 administration increased peripheral naive and recent thymic emigrant (RTE) populations, demonstrating its effect on T-cell production, not peripheral expansion. IGF-1 administration increased bone marrow LSK (lineage(-), Sca-1(+), c-kit(+)) precursor proliferation and peripheral LSK populations, increased thymocyte populations in a sequential wave of expansion, and proportionately expanded TEC subpopulations and enhanced their chemokine expression. To separate IGF-1's effects on thymocytes and TECs, we generated mice lacking IGF-1R on thymocytes and T cells. Thymocyte and RTE numbers were decreased in these mice, but IGF-1 treatment produced comparable thymocyte numbers to similarly treated wild-type mice. We additionally separated thymic- from LSK-specific effects by demonstrating that IGF-1 increased thymocyte numbers despite impaired early thymic progenitor (ETP) importation in PSGL-1KO mice. These results indicate the critical point thymic function regulation by IGF-1 involves TEC expansion regulating thymocyte precursor entry and facilitating thymocyte development.

Survival and cellular immune functions in septic mice treated with growth hormone (GH) and insulin-like growth factor-I (IGF-I)

OBJECTIVE

GH was used to counteract the catabolic metabolism in critically ill patients until it was demonstrated that administration of GH was associated with an increased morbidity due to uncontrolled infections and sepsis. The immunomodulatory effect of GH and its main mediator IGF-I during systemic inflammation remain to be established. We therefore investigated the effect of GH and IGF-I on cellular immune functions in a murine model of sepsis.

DESIGN

Randomized animal study. Septic mice were treated with either saline, GH (1mg/kg/24h s.c.), IGF-I (4mg/kg/24h), or GH in combination with IGF-I over a 48h period.

SETTING

Level 1 trauma center, university research laboratory.

OBJECTS

Male NMRI mice.

MEASUREMENTS

clinical parameters (survival rate, body weight, body temperature, fluid intake, food intake, blood glucose levels) and cellular immune functions (splenocyte proliferation by using a (3)H-thymidine incorporation assay, splenocyte apoptosis by determination of Annexin V binding capacity, splenocyte IL-2, IL-6, and TNF-alpha release by using ELISA, and distribution of circulating immune cell subsets by FACScan).

RESULTS

Administration of GH did not affect clinical parameters or cellular immune functions in septic mice. In contrast, treatment with IGF-I alone or in combination with GH improved splenocyte proliferation and increased the ability of splenocytes to release IL-2 and IL-6 without affecting the survival rate or any other clinical parameter determined.

CONCLUSION

GH did not affect cellular immune functions or the survival rate in our murine sepsis model. In contrast, IGF-I improved splenocyte proliferation and cytokine release independently of GH but did not affect the determined clinical parameters of septic mice.

Neuroendocrine hormones such as growth hormone and prolactin are integral members of the immunological cytokine network

Neuroendocrine hormones such as growth hormone (GH) and prolactin (PRL) have been demonstrated to accelerate the recovery of the immune response after chemotherapy and bone marrow transplantation and to enhance the restoration of immunity in individuals infected with HIV and in normal individuals with compromised immune systems associated with aging. As the mechanism of action of these hormones has been elucidated, it has become clear that they are integral members of the immunological cytokine/chemokine network and share regulatory mechanisms with a wide variety of cytokines and chemokines. The members of this cytokine network induce and can be regulated by members of the suppressor of cytokine signaling (SOCS) family of intracellular proteins. In order to take advantage of the potential beneficial effects of hormones such as GH or PRL, it is essential to take into consideration the overall cytokine network and the regulatory effects of SOCS proteins.

Clinical strategies to enhance T cell reconstitution

Strategies to enhance T cell recovery are of increasing clinical importance to overcome long lasting T cell deficiencies, which occur in association with infections, autoimmunity and chemo/radiotherapy as well as aging of the immune system. In this review we discuss those strategies that are close to or in the clinic. Interleukin-7, sex steroid modulation, keratinocyte growth factor, growth hormone and cellular therapies using ex vivo generated T-cell precursors are currently being tested in recipients of a hematopoietic stem cell transplantation and patients with malignancies or HIV/AIDS.

Enhancing T cell reconstitution after hematopoietic stem cell transplantation: a brief update of the latest trends

Hematopoietic stem cell transplantation (HSCT) is associated with a period of immune incompetence that particularly affects the T cell lineage. Strategies to enhance T cell reconstitution could significantly improve the survival of HSCT recipients by decreasing the incidence of fatal infectious complications and by enhancing graft-versus-tumor activity. In recent years, a variety of promising strategies have been established in preclinical models to improve T cell recovery in particular after allogeneic T cell-depleted HSCT, without aggravating graft-versus-host disease while preserving or even improving graft-versus-tumor activity. These therapies include treatment with keratinocyte growth factor (KGF), growth hormone (GH), LHRH agonists, interleukin 7 (IL-7) and interleukin 15 (IL-15). Thanks to the establishment of Notch-based culture systems, adoptive cellular therapies with T lineage-committed precursor cells have become feasible, since early T cell progenitors can now easily be generated in vitro in large quantities and have been proven to be very effective in enhancing T cell reconstitution and anti-tumor activity after allogeneic T cell-depleted HSCT. The translation of most of these strategies into clinical trials is likely and in some cases Phase I/II studies are already underway.

Insulin-like growth factor-1 receptor RNA expression in hematopoietic stem cell transplanted patients does not correlate with graft-versus-host disease

In vivo activated T lymphocytes exhibit altered expression of insulin-like growth factor-1 receptor (IGF-1R) compared to the naïve T lymphocyte pool. The objective of this study was to investigate the expression of IGF-1R RNA in CD4 and CD8 positive cells after allogeneic hematopoietic stem cell transplantation (HSCT) in patients with and without GVHD. For this purpose we isolated RNA from CD4 and CD8 positive cells, sorted with immunomagnetic beads. We used real-time PCR for RNA quantification. We demonstrate a significantly decreased expression of IGF-1R RNA in both CD4 and CD8 positive cells up to 12 months after HSCT. We could not demonstrate a correlation between the IGF-1R RNA expression and T cell activating processes like GVHD, expansion of CD4 or CD8 populations or virus infections.

Aberrant expression of the insulin-like growth factor-1 receptor by T cells from patients with Graves' disease may carry functional consequences for disease pathogenesis

Graves' disease (GD), an autoimmune process involving thyroid and orbital tissue, is associated with lymphocyte abnormalities including expansion of memory T cells. Insulin-like growth factor receptor-1 (IGF-1R)-bearing fibroblasts overpopulate connective tissues in GD. IGF-1R on fibroblasts, when ligated with IgGs from these patients, results in the expression of the T cell chemoattractants, IL-16 and RANTES. We now report that a disproportionately large fraction of peripheral blood T cells express IGF-1R (CD3+IGF-R+). CD3+IGF-1R+ T cells comprise 48 +/- 4% (mean +/- SE; n = 33) in patients with GD compared with 15 +/- 3% (n = 21; p < 10(-8)) in controls. This increased population of IGF-1R+ T cells results, at least in part, from an expansion of CD45RO+ T cells expressing the receptor. In contrast, the fraction of CD45RA+IGF-1R+ T cells is similar in GD and controls. T cells harvested from affected orbital tissues in GD reflect similar differences in the proportion of IGF-1R+CD3+ and IGF-1R+CD4+CD3+ cells as those found in the peripheral circulation. GD-derived peripheral T cells express durable, constitutive IGF-1R expression in culture and receptor levels are further up-regulated following CD3 complex activation. IGF-1 enhanced GD-derived T cell incorporation of BrdU (p < 0.02) and inhibited Fas-mediated apoptosis (p < 0.02). These findings suggest a potential role for IGF-1R displayed by lymphocytes in supporting the expansion of memory T cells in GD.

Adoptive precursor cell therapy to enhance immune reconstitution after hematopoietic stem cell transplantation

Strategies to enhance post-transplant immune reconstitution without aggravating graft-vs-host disease (GVHD) can improve the outcome of allogeneic hematopoietic stem cell transplantation. Recent preclinical studies demonstrated that the use of T cell depleted allografts supplemented with committed progenitor cells (vs stem cells only) allows enhanced immune reconstitution of specific hematopoietic lineages including myeloid, B, T, and natural killer lineages in the absence of GVHD. This novel adoptive therapy resulted in significantly improved resistance to microbial pathogens and could, in some cases, even mediate tumor immunity. Clinical protocols using adoptive transfer of committed hematopoietic progenitor cells are currently being evaluated.

Immunosenescence: emerging challenges for an ageing population

It is now becoming apparent that the immune system undergoes age-associated alterations, which accumulate to produce a progressive deterioration in the ability to respond to infections and to develop immunity after vaccination, both of which are associated with a higher mortality rate in the elderly. Immunosenescence, defined as the changes in the immune system associated with age, has been gathering interest in the scientific and health-care sectors alike. The rise in its recognition is both pertinent and timely given the increasing average age and the corresponding failure to increase healthy life expectancy. This review attempts to highlight the age-dependent defects in the innate and adaptive immune systems. While discussing the mechanisms that contribute to immunosenescence, with emphasis on the extrinsic factors, particular attention will be focused on thymic involution. Finally, we illuminate potential therapies that could be employed to help us live a longer, fuller and healthier life.

Immune restoration following hematopoietic stem cell transplantation: an evolving target

Hematopoietic stem cell transplantation (HSCT) is the definitive cure for many malignant and nonmalignant diseases. However, delays in immune reconstitution (IR) following HSCT significantly limit the success of transplantation and increase the risk for infection and disease relapse in the transplant recipient. Therefore, ways to measure and to manipulate immune recovery following HSCT are emerging and their success depends directly upon an enhanced understanding for the underlying mechanisms responsible for reconstituted immunity and hematopoiesis. Recent discoveries in the activation, function, and regulation of dendritic cell (DC), natural killer (NK) cell, and T-lymphocyte subtypes have been critical in developing immunotherapies used to prevent graft-versus-host disease and to enhance graft-versus-leukemia. For example, regulatory T cells that induce tolerance and NK receptor-tumor ligand disparities that result in tumor lysis are being used to minimize GVHD and tumor burden, respectively. Furthermore, expansion and modulation of immune effector cells are being used to augment hematopoietic and immune recovery and to decrease transplant-related toxicity in the transplant recipient. Specifically, DC expansion and incorporation into antitumor and anti-microbial vaccines is fast approaching application into clinical trials. This paper will review our current understanding for IR following HSCT and the novel ways in which to restore immune function and decrease transplant-related toxicity in the transplant recipient.

Effect of recombinant human GH on circulating granulocyte colony-stimulating factor and neutrophils in patients with adult GH deficiency

BACKGROUND

We previously reported that short-term continuous subcutaneous infusion (CSI) of recombinant human growth hormone (rhGH) increased plasma erythropoietin levels and hemoglobin concentrations in patients with adult GH deficiency. In the present study, we investigated the effect of rhGH on plasma granulocyte colony-stimulating factor (G-CSF) levels and neutrophil counts in patients with adult GH deficiency.

METHODS

rhGH was administrated for 1 year in six patients with adult GH deficiency (age range, 24-69 years; mean +/- S.E.M., 51.7 +/- 5.8 years; two males and four females) by means of CSI at a rate of 0.25 U/kg per week. Blood samples were obtained in the morning after overnight fasting every month before and after the start of rhGH administration. Plasma GH, insulin-like growth factor I (IGF-I) and G-CSF levels, and neutrophil counts, were measured.

RESULTS

Mean ( +/- S.E.M.) plasma GH levels increased from 0.26 +/- 0.14 to 2.28 +/- 0.20 microg/l 1 month after the start of rhGH administration. An increase of the plasma GH levels was accompanied by an increase in the plasma IGF-I levels from 64.7 +/- 8.5 to 293.3 +/- 80.6 microg/l. Plasma G-CSF levels increased at 2, 3, 8, 9 and 10 months after the start of rhGH administration compared with 28.6 +/- 11.0 ng/l at time 0. The neutrophil counts increased at 2, 3, 7, 8, 9, 11 and 12 months after the start of rhGH administration compared with 2822 +/- 377 neutrophils/microl at time 0.

CONCLUSION

rhGH administration increased plasma G-CSF levels and neutrophil counts. GH and/or IGF-I might stimulate neutrophil production and/or release via G-CSF.

Strategies to enhance T-cell reconstitution in immunocompromised patients

Immune deficiency, together with its associated risks such as infections, is becoming an increasingly important clinical problem owing to the ageing of the general population and the increasing number of patients with HIV/AIDS, malignancies (especially those treated with intensive chemotherapy or radiotherapy) or transplants (of either solid organs or haematopoietic stem cells). Of all immune cells, T cells are the most often affected, leading to a prolonged deficiency of T cells, which has important clinical consequences. Accordingly, strategies to improve the recovery and function of T cells, as we discuss here, should have a direct impact on reducing the morbidity and mortality of many patients and should increase the efficacy of therapeutic and prophylactic vaccinations against microbial pathogens or tumours.

A single-platform approach using flow cytometry and microbeads to evaluate immune reconstitution in mice after bone marrow transplantation

The monitoring of immune reconstitution in murine models of HC transplantation, using accurate and automated methods, is necessary in view of the recent developments of hematopoietic cell (HC) transplantation (including reduced intensity conditioning regimens) as well as emerging immunological concepts (such as the involvement of dendritic cells or regulatory T cells). Here, we describe the use of a single-platform approach based on flow cytometry and tubes that contain a defined number of microbeads to evaluate absolute blood cell counts in mice. This method, previously used in humans to quantify CD34+ stem cells or CD4+ T cells in HIV infected patients, was adapted for mouse blood samples. A CD45 gating strategy in this "lyse no wash" protocol makes it possible to discriminate erythroblasts or red blood cell debris from CD45+ leukocytes, thus avoiding cell loss. Tubes contain a lyophilized brightly fluorescent microbead pellet permitting the acquisition of absolute counts of leukocytes after flow cytometric analysis. We compared this method to determine absolute counts of circulating cells with another method combining Unopette reservoir diluted blood samples, hemocytometer, microscopic examination and flow cytometry. The sensitivity of this single-platform approach was evaluated in different situations encountered in allogeneic HC transplantation, including immune cell depletion after different conditioning regimens, activation status of circulating cells after transplantation, evaluation of in vivo cell depletion and hematopoietic progenitor mobilization in the periphery. This single-platform flow cytometric assay can also be proposed to standardize murine (or other mammalian species) leukocyte count determination for physiological, pharmacological/toxicological and diagnostic applications in veterinary practice.

GITR activation induces an opposite effect on alloreactive CD4(+) and CD8(+) T cells in graft-versus-host disease

Glucocorticoid-induced tumor necrosis factor receptor family-related gene (GITR) is a member of the tumor necrosis factor receptor (TNFR) family that is expressed at low levels on unstimulated T cells, B cells, and macrophages. Upon activation, CD4⁺ and CD8⁺ T cells up-regulate GITR expression, whereas immunoregulatory T cells constitutively express high levels of GITR. Here, we show that GITR may regulate alloreactive responses during graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation (BMT). Using a BMT model with major histocompatibility complex class I and class II disparity, we demonstrate that GITR stimulation in vitro and in vivo enhances alloreactive CD8⁺CD25⁻ T cell proliferation, whereas it decreases alloreactive CD4⁺CD25⁻ proliferation. Allo-stimulated CD4⁺CD25⁻ cells show increased apoptosis upon GITR stimulation that is dependent on the Fas–FasL pathway. Recipients of an allograft containing CD8⁺CD25⁻ donor T cells had increased GVHD morbidity and mortality in the presence of GITR-activating antibody (Ab). Conversely, recipients of an allograft with CD4⁺CD25⁻ T cells showed a significant decrease in GVHD when treated with a GITR-activating Ab. Our findings indicate that GITR has opposite effects on the regulation of alloreactive CD4⁺ and CD8⁺ T cells.