The systemic administration of keratinocyte growth factor (KGF) enhances T-cell lymphopoiesis in normal mice and mice that received a bone marrow transplant. KGF exerts protection to thymic stromal cells from cytoablative conditioning and graft-versus-host disease-induced injury. However, little is known regarding KGF's molecular and cellular mechanisms of action on thymic stromal cells. IntroductionDecreased T-cell cellularity and a skewed TCR repertoire are hallmarks of an immune deficiency commonly observed in old age, as a consequence of general infectious diseases and aggressive lymphocyte-depleting therapies for diverse malignancies. [1][2][3][4] The regeneration of a phenotypically and functionally normal T-cell compartment is curtailed for an extended period of time in patients receiving a hematopoietic stem cell transplant (HSCT). [5][6][7] This lack in T-cell reconstitution is associated with opportunistic infections, the reactivation of latent viral and parasitic infections, chronic inflammation, and autoimmunity. 3,4 Following cytoablative therapy, the recovery of the T-cell compartment relies on 2 independent pathways, that is, the expansion of peripheral T cells and, alternatively, the de novo production of T cells in the thymus. 1,2,7-10 The latter assures the generation of a population of naive T cells expressing a diverse repertoire of TCR specificities. 5,7,8,10,11 The extent of thymusdependent T-cell reconstitution correlates directly with thymic size following immune ablation and hematopoietic stem cell (HSC)-derived reconstitution 7,12 but is inversely related to age and transplant-related toxicities such as graft-versus-host disease (GVHD). 10,[13][14][15][16][17] The generation of new T cells of donor origin depends on the migration of hematopoietic precursors to the thymus. Normal thymic T-cell development is in turn contingent on the regular maintenance of the stromal microenvironment. However, age-related thymic involution 18 and injury from radiation, 19 GVHD, 20 chemotherapy, 12,21 or infection 3,4,12,[18][19][20][21][22][23] preclude normal thymopoiesis to occur as they directly affect thymic epithelial cells (TECs). There has been considerable interest in identifying strategies to prevent TEC injury. Recently, robust T-cell lymphopoiesis has been maintained in myeloablated HSCT recipients by pretransplantation administration of different factors such as 24,25 androgen antagonists, 26 and fibroblast growth factor 7 (Fgf7; aka, keratinocyte growth factor [KGF]). 20,27-29 KGF belongs to the family of the structurally related Fgfs and is a potent epithelial cell mitogen. 27,30 KGF is expressed under physiological conditions within the thymus both by mesenchymal cells and by T cells at specific developmental stages. To exert its biologic activity, KGF activates the IIIb variant of the FgfR2 receptor (FgfR2IIIb), which is expressed within the thymus exclusively on TECs. 31 Experiments using mice deficient for FgfR2IIIb or the removal of mesenchyme from normal embryos revealed the importa...

Cytoreductive conditioning regimens used in the context of allogeneic hematopoietic cell transplantation (HCT) elicit deficits in innate and adaptive immunity, which predispose patients to infections. As such, transplantation outcomes depend vitally on the successful reconstruction of immune competence. Restoration of a normal peripheral T-cell pool after HCT is a slow process that requires the de novo production of naive T cells in a functionally competent thymus. However, there are several challenges to this regenerative process. Most notably, advanced age, the cytotoxic pretransplantation conditioning, and posttransplantation alloreactivity are risk factors for T-cell immune deficiency as they independently interfere with normal thymus function. Here, we discuss preclinical allogeneic HCT models and clinical observations that have contributed to a better understanding of the transplant-related thymic dysfunction. The identification of the cellular and molecular mechanisms that control regular thymopoiesis but are altered in HCT patients is expected to provide the basis for new therapies that improve the regeneration of the adaptive immune system, especially with functionally competent, naive T cells. (Blood. 2011;117(25): 6768-6776) IntroductionAllogeneic hematopoietic cell transplantation (HCT) offers an effective treatment for a broad spectrum of malignant and nonmalignant disorders. 1,2 The rates for 1-year and disease-free survival have significantly improved over the last few years. [3][4][5][6] However, transplant-related complications, such as opportunistic infections and GVHD, continue to seriously affect the patients' quality of life. 7,8 Why are transplant recipients prone to develop infections? The answer to this question lies, at least in large part, in how allogeneic HCT is performed. Eligible patients are first treated with chemotherapy and/or radiotherapy before they receive the hematopoietic cell graft. This preconditioning can be done at different intensities and results in cytoreduction or ablation. 9 The objectives are to reduce the bulk of malignant cells in instances where allogeneic HCT is used for cancer treatment, to decrease the risk of graft rejection via general host immune suppression, and to improve the engraftment of donor hematopoietic stem cells (HSCs) by evacuating the host marrow. As a side effect, the preconditioning also invariably compromises natural and adaptive immune responses of HCT recipients, predisposing them to infectious complications that may contribute to poor clinical outcome.The universal observation is made that innate immunity is rapidly restored after allogeneic HCT. Although studies are heterogeneous given the divergent clinical situations, the blood counts of myeloid lineage cells of the natural immune system (ie, neutrophils, monocytes, natural killer cells) often normalize in humans within 2 to 4 weeks after myeloablative allogeneic HCT. [10][11][12] Bacterial infections are frequent during this aplastic phase and may be observed at later time points, e...

Thymus-dependent reconstitution of the peripheral T-cell compartment is critical for the successful outcome of bone marrow transplantation. However, graft-versus-host disease (GVHD) affects thymic stromal function and thus prevents normal T-cell maturation and selection. To determine whether cytoprotection of thymic epithelial cells (TECs) by keratinocyte growth factor (KGF) averts GVHD-related injury to the thymus, a nonirradiated murine parent-->F(1) transplantation model was investigated. Administration of KGF between days -3 and +3 of GVHD induction preserved normal thymic size, cellularity, and thymocyte phenotype when measured 2 weeks after transplantation and compared with saline-treated parent-->F(1) mice that received allogeneic transplants. Moreover, the characteristic GVHD-induced impairment in cell cycle progression of pro- and pre-T cells was prevented by KGF. However, the normal phenotypic and functional status of the thymus did not correlate with the higher number of GVHD-inducing mature donor T cells in thymi of KGF-treated mice. Importantly, extensive analysis of the different TEC populations within the thymic cortex and medulla revealed an almost normal stromal architecture and composition in GVHD mice treated with KGF. These observations are likely to reflect an indirect effect of KGF on thymopoiesis as KGF-receptor expression was demonstrated to be restricted to TECs. Thus, pharmacologic doses of KGF appear to exert a potent effect on TEC function, which in turn allows for normal T lymphopoiesis to occur during acute GVHD.

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Graft-versus-host disease (GVHD) is the major complication after allogeneic bone marrow transplantation (BMT) and is initiated by alloreactive donor T cells recognizing foreign histocompatibility antigens of the host. There is now substantial experimental and clinical evidence to implicate a dysregulation of cytokine networks as a primary cause for the induction and maintenance of GVHD. In this article, current knowledge of the involvement of cytokines in GVHD is reviewed. The balance between type 1 cytokines (interleukin-2, interferon-gamma) and type 2 cytokines (interleukin-4, interleukin-10) is hypothesized to govern the extent to which a cell-mediated immune response and a systemic inflammatory response develop after allogeneic BMT. Because type 2 cytokines can inhibit the production of the proinflammatory cytokines interleukin-1 and tumor necrosis factor-alpha, a type 1 to type 2 shift in the initial response of donor T cells to host alloantigens may interrupt the cytokine cascade after allogeneic BMT and may offer a new approach to the prevention and treatment of acute GVHD. Interventions to specifically eliminate or modify the response of donor T cells to alloantigens in order to reduce GVHD may obviate the need for T cell depletion in clinical BMT and thus avoid the increased risk of relapse of malignancy and impairment of donor cell engraftment.

Thymic T cell development is dependent on a specialized epithelial microenvironment mainly composed of cortical and medullary thymic epithelial cells (TECs). The molecular programs governing the differentiation and maintenance of TECs remain largely unknown. Wnt signaling is central to the development and maintenance of several organ systems but a specific role of this pathway for thymus organogenesis has not yet been ascertained. In this report, we demonstrate that activation of the canonical Wnt signaling pathway by a stabilizing mutation of β-catenin targeted exclusively to TECs changes the initial commitment of endodermal epithelia to a thymic cell fate. Consequently, the formation of a correctly composed and organized thymic microenvironment is prevented, thymic immigration of hematopoietic precursors is restricted, and intrathymic T cell differentiation is arrested at a very early developmental stage causing severe immunodeficiency. These results suggest that a precise regulation of canonical Wnt signaling in thymic epithelia is essential for normal thymus development and function.

The major complication after allogeneic bone marrow transplantation (BMT) is the development of graft‐versus‐host‐disease (GVHD). This disease is initiated during the conditioning of the recipient, when host tissues are damaged. During the afferent phase of the disease, alloreactive donor T cells recognize foreign major and minor histocompatibility antigens of host tissues. The efferent phase includes activation of inflammatory effector cells as well as the secretion of cytopathic molecules which induce pathology in skin, gastrointestinal tract, liver, lung, and the immune system. Substantial experimental and clinical evidence now indicates a central role of cytokines in the immunopathophysiology of acute GVHD, which forms the basis of this review. The balance between cytokines released by T helper 1 (Th1) cells (interleukin 2, interferon‐γ) or by T helper 2 (Th2) cells (interleukin 4, interleukin 10) after allogeneic BMT is hypothesized to govern the extent of the systemic inflammatory response. Because Th2 cytokines can inhibit the production of proinflammatory cytokines such as interleukin 1 and tumor necrosis factor‐α, a Th1→Th2 shift in the initial response of donor T cells may interrupt the cytokine cascade and thus offer a new approach to the prevention and treatment of acute GVHD. Successful interventions to modify the response of donor T cells may obviate the need for T cell depletion and thereby avoid the increased risk of relapse of malignancy and impairment of donor cell engraftment.