IL-7 Is the Limiting Homeostatic Factor that Constrains Homeostatic Proliferation of CD8+ T Cells after Allogeneic Stem Cell Transplantation and Graft-versus-Host Disease.

Immune reconstitution after allogeneic hematopoietic stem cell transplantation relies primarily on homeostatic proliferation (HP) of mature T lymphocytes, but this process is typically impaired during graft-versus-host disease (GVHD). We previously showed that low IL-7 levels combined with lack of dendritic cell (DC) regeneration constrain CD4+ T cell HP during GVHD. However, it is not clear whether these alterations to the peripheral CD4+ T cell niche also contribute to impair CD8+ T cell regeneration during GVHD. We found that IL-7 therapy was sufficient for restoring CD8+ T cell HP in GVHD hosts while forcing DC regeneration with Flt3-L had only a modest effect on CD8+ T cell HP in IL-7 treated mice. Using bone marrow chimeras, we showed that HP of naïve CD8+ T cells is primarily regulated by MHC class I on radio-resistant stromal cells, yet optimal recovery of CD8+ T cell counts still requires expression of MHC class I on both radio-resistant and radio-sensitive hematopoietic cells. Thus, IL-7 level is the primary limiting factor that constrains naïve CD8+ T cell HP during GVHD, and accessibility of MHC class I on stromal cells explains how IL-7 therapy, as a single agent, can induce robust CD8 + T cell HP in the absence of DCs.

Stromal-Derived Factor-1α and Interleukin-7 Treatment Improves Homeostatic Proliferation of Naïve CD4(+) T Cells after Allogeneic Stem Cell Transplantation.

Graft-versus-host disease (GVHD) impairs immune reconstitution after allogeneic stem cell transplantation (allo-SCT) and effective therapies aimed at restoring T cell counts in GVHD patients have yet to be developed. During GVHD, CD4(+) T cell reconstitution is particularly affected and current models hold that GVHD insult to the peripheral lymphoid niche is responsible for this effect. Here, we show that naïve CD4(+) T cell homeostatic proliferation (HP) is lost during GVHD because of low systemic IL-7 and impaired dendritic cell (DC) regeneration. We assessed factors involved in DC differentiation and found that although fms-like tyrosine kinase 3 ligand (Flt3-L) levels were normal, stromal-derived factor-1α (SDF-1α) was diminished in the blood of GVHD mice. Unlike Flt3-L treatment, the administration of SDF-1α specifically increased CD8α(+) DC numbers and did not worsen GVHD. Importantly, CD4(+) T cell HP was enhanced only when IL-7 and SDF-1α or Flt3L were coadministered, confirming the crucial role of DCs and IL-7 in restoring CD4(+) T cell regeneration during GVHD. Altogether, our results indicate that CD8α(+) DCs are part of the peripheral niche that controls CD4(+) T cell HP and that their depletion, combined with low systemic IL-7, explains how GVHD constrains naïve CD4(+) T cell reconstitution after allo-SCT.

TCR triggering modulates the responsiveness and homeostatic proliferation of CD4+ thymic emigrants to IL-7 therapy.

Interleukin-7 (IL-7) is currently used in clinical trials to augment T-cell counts. Paradoxically, elevated systemic IL-7 found in lymphopenic humans is typically insufficient for CD4(+) T-cell regeneration, and thymopoiesis becomes critical in this process. Here we show that the proliferative effect of IL-7 is more pronounced on CD4(+)CD8(-) thymocytes compared with peripheral CD4(+) T cells. These cells express miR181a at higher levels and respond to lower concentrations of IL-7. As single-positive CD4(+) thymocytes (CD4(+)(SPT)) exit the thymus, they rapidly diminish their proliferation to IL-7 therapy, and this is mediated, at least in part, by major histocompatibility complex class II distribution outside the thymus. Interestingly, increasing T-cell receptor (TCR) stimulation augments IL-7 responsiveness and proliferation of peripheral CD4(+) T cells, whereas failure to stimulate TCR abrogates proliferation induced by IL-7. Finally, we demonstrated that IL-7 enhances the proliferation of CD4(+) T cells that undergo "slow proliferation" in lymphopenic hosts. To date, our results indicate that TCR signaling is a major controlling factor for CD4 responsiveness and proliferation to IL-7 therapy.

SMAD3 prevents graft-versus-host disease by restraining Th1 differentiation and granulocyte-mediated tissue damage.

Gene expression profiling of human donor T cells before allogeneic hematopoietic cell transplantation revealed that expression of selected genes correlated with the occurrence of graft-versus-host disease (GVHD) in recipients. The gene with the best GVHD predictive accuracy was SMAD3, a core component of the transforming growth factor-ß signaling pathway, whose expression levels vary more than a 6-fold range in humans. The putative role of SMAD3 in the establishment of graft-host tolerance remained elusive. We report that SMAD3-KO mice present ostensibly normal lymphoid and myeloid cell subsets. However, the lack of SMAD3 dramatically increased the frequency and severity of GVHD after allogeneic hematopoietic cell transplantation into major histocompatibility complex-identical recipients. Lethal GVHD induced by SMAD3-KO donors affected mainly the intestine and resulted from massive tissue infiltration by T-bet(+) CD4 T cells and granulocytes that caused tissue damage by in situ release of Th1 cytokines and oxidative-nitrosative mediators, respectively. Our report reveals the nonredundant roles of SMAD3 in the development of tolerance to the host. Furthermore, our data support the concept that SMAD3 levels in donor cells dictate the risk of GVHD and that SMAD3 agonists would be attractive for prevention of GVHD.

Studying Peripheral T Cell Homeostasis in Mice: A Concise Technical Review.

For several years, it was believed that the thymus was entirely responsible for maintaining T cell homeostasis. Today, it is well-known that homeostatic peripheral mechanisms are essential in order to maintain T cell numbers and diversity constant in the periphery. Naïve and memory T cells require continual access to self-peptide MHC class I and II molecules and/or cytokines to survive in the periphery. Under normal conditions, homeostatic resources are low, and lymphocytes undergo very slow proliferation and survive. Following T cell depletion, the bioavailability of homeostatic resources is significantly increased, and T cell proliferation is dramatically augmented. The development of lymphopenic mouse models has helped our current understanding of factors involved in the regulation of peripheral T cell homeostasis. In this minireview, we will give a brief overview about basic techniques used to study peripheral T cell homeostasis in mice.