Peripheral T-cell Homeostasis Research Articles

SEL1L preserves CD8+ T-cell survival and homeostasis by fine-tuning PERK signaling and the IL-15 receptor-mediated mTORC1 axis.

SEL1L-mediated endoplasmic reticulum-associated degradation (ERAD) plays critical roles in controlling protein homeostasis by degrading misfolded or terminalunfolded proteins. However, it remains unclear how SEL1L regulates peripheral T-cell survival and homeostasis. Herein, we found that SEL1L deficiency led to a greatly reduced frequency and number of mature T cells, which was further validated by adoptive transfer experiments or bone marrow chimera experiments, accompanied by the induction of multiple forms of cell death. Furthermore, SEL1L deficiency selectively disrupted naïve CD8+ T-cell homeostasis, as indicated by the severe loss of the naïve T-cell subset but an increase in the memory T-cell subset. We also found that SEL1L deficiency fueled mTORC1/c-MYC activation and induced a metabolic shift, which was largely attributable to enhanced expression of the IL-15 receptor α and β chains. Mechanistically, single-cell transcriptomic profiling and biochemical analyses further revealed that Sel1l-/- CD8+ T cells harbored excessive ER stress, particularly aberrant activation of the PERK-ATF4-CHOP-Bim pathway, which was alleviated by supplementing IL-7 or IL-15. Importantly, PERK inhibition greatly resolved the survival defects of Sel1l-/- CD8+ T cells. In addition, IRE1α deficiency decreased mTORC1 signaling in Sel1l-/- naïve CD8+ T cells by downregulating the IL-15 receptor α chain. Altogether, these observations suggest that the ERAD adaptor molecule SEL1L acts as an important checkpoint for preserving the survival and homeostasis of peripheral T cells by regulating the PERK signaling cascade and IL-15 receptor-mediated mTORC1 axis.

Depletion of Ly6G-Expressing Neutrophilic Cells Leads to Altered Peripheral T-Cell Homeostasis and Thymic Development in Neonatal Mice.

Newborns and especially preterm infants are much more susceptible to infections than adults. Due to immature adaptive immunity, especially innate immune cells play an important role in a newborn's infection defense. Neonatal neutrophils exhibit profound differences in their functionality compared to neutrophils of adults. In particular, neonates possess a relevant population of suppressive neutrophils, which not only inhibit but also specifically modulate the function of T-cells. In this study, we investigated whether neonatal neutrophils are already involved in T-cell development in the thymus. For this purpose, we used a newly developed model of antibody-mediated immune cell depletion in which we administered a depleting antibody to pregnant and then lactating dams. Using this method, we were able to sufficiently deplete Ly6G-positive neutrophils in offspring. We demonstrated that the depletion of neutrophils in newborn mice resulted in altered peripheral T-cell homeostasis with a decreased CD4+/CD8+ T-cell ratio and decreased expression of CD62L. Neutrophil depletion even affected T-cell development in the thymus, with increased double positive thymocytes and a decreased CD4+/CD8+ single positive thymocyte ratio. Altogether, we demonstrated a previously unknown mechanism mediating neutrophils' immunomodulatory effects in newborns.

Cytokine levels of interleukin-2 and 7 amongst antiretroviral therapy success and failure HIV patients attending the University Teaching Hospital, Yaoundé, Cameroon.

Immune reconstitution complications (IRC) are a major problem faced by HIV treated patients world wide. Interleukin (IL)-2 and IL-7 play vital roles in peripheral T-cell homeostasis. Our study objective was to measure and compare the blood plasma levels of IL-2 and IL-7 amongst antiretroviral therapy (ART) patients attending the Yaoundé University Teaching Hospital, Cameroon. We performed a cross-sectional study with 296 HIV positive patients enrolled between July 2017 and May 2018 at the Yaoundé University Teaching Hospital. IL-2, IL-7, T-cell profile counts and plasma viral load were measured on whole blood specimens. Data obtained were analyzed using Graph Pad Prism 5.0 and Epi info 7.0. Software. IL-2 and IL-7 plasma concentration levels were higher in patients with ART failure compared to ART success, with a mean SD of 19.4±8 and 17.1±6 pg /ml, 35.26±11 and 21.5±5 pg/ml, with p < 0.001 and < 0.001. There was a direct and significant correlation between viral load, IL-2 and IL-7 with p values = 0.028, and 0.020, respectively. There was an association between IL-2, IL-7 and viral load in relation to the duration on treatment (DT), with p values = 0.003 (R2=0.041, CI= 0.069 - 0.34) ,0.017 (R2=0.027, CI=-0.30 - 0.030), and 0.001 (R2=0.048, CI=-0.047-0.76). Considering that limited surrogate markers are availiable for monitoring immune reconstitution and high associated mortality rates, IL-2 and IL-7 could be a good immunological predictor for ART failure and success in HIV infected individuals.

Mucosal cell populations may contribute to peripheral immune abnormalities in HIV-infected subjects introducing cART with moderate immune-suppression.

HIV infection causes the progressive depletion of CD4+ T-lymphocytes and profound modifications of T-cell homeostasis, which persist despite virologically-suppressive treatment and have been linked to a worse clinical outcome. Enduring alterations of the gastrointestinal tract may represent the underlying pathogenic mechanisms of these phenomena. Twenty-six HIV-infected subjects were assessed over a 12-month period following the introduction of antiretroviral therapy. 18 uninfected individuals were enrolled as controls. Parameters of peripheral T-cell homeostasis (activation, maturation), gastrointestinal function (microbial translocation, gut inflammation, fecal microbiota composition) and mucosal immunity (CD4+CCR6+CD161+, CD4+CCR9+α4β7+, stem cell memory CD4+/CD8+ T-cells) were assessed. CD4+CCR6+CD161+ cells were depleted in HIV-infected untreated subjects and maintained significantly lower levels compared to controls, despite the introduction of effective antiviral treatment. The frequency of gut-homing CD4+CCR9+α4β7+ cells was also impaired in untreated infection and correlated with the HIV RNA load and CD4+HLADR+CD38+; during therapy, we observed a contraction of this pool in the peripheral blood and the loss of its correlation with antigenic exposure/immune activation. A partial correction of the balance between stem cell memory pools and T-cell homeostasis was registered following treatment. In HIV-infected subjects with moderate immune-suppression, antiretroviral therapy has a marginal impact on mucosal immune populations which feature distinctive kinetics in the periphery, possibly reflecting their diverse recruitment from the blood to the mucosa. The persistent defects in mucosal immunity may fuel peripheral T-cell abnormalities through diverse mechanisms, including the production of IL-17/IL-22, cellular permissiveness to infection and regulation of T-lymphocyte maturation.

Role of human forkhead box P3 in early thymic maturation and peripheral T-cell homeostasis

Forkhead box P3 (FOXP3) is a key transcription factor in regulatory T (Treg) cell function. FOXP3 gene mutations cause immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome, a fatal autoimmune syndrome. FOXP3 has also been proposed to act in effector T(Teff) cells, but to date, this role has not been confirmed. We sought to evaluate the effect of reduced FOXP3 expression on human Treg and Teff cell development and correlate it with IPEX syndrome immune pathology. We developed a model of humanized mice (huMice) in which the human hematopoietic system is stably knocked down or knocked out for the FOXP3 gene (knockdown [KD]/knockout [KO] huMice). Because FOXP3-KD/KO was not 100% effective, residual FOXP3 expression in hematopoietic stem progenitor cells was sufficient to give rise to Treg cells with normal expression of FOXP3. However, numerous defects appeared in the Teff cell compartment. Compared with control mice, FOXP3-KD/KO huMice showed altered thymocyte differentiation, with KD/KO thymocytes displaying significantly reduced T-cell receptor (TCR) signaling strength and increased TCR repertoire diversity. Peripheral KD/KO Teff cells were expanded and showed signs of homeostatic proliferation, such as a significantly contracted TCR repertoire, a severely reduced naive compartment, decreased telomeric repeat-binding factor 2 expression, and a skew toward a TH2 profile, resembling an aged immune system. Consistent with results in FOXP3-KD/KO huMice, analysis of patients with IPEX syndrome provided evidence of defects in the Teff cell compartment at both the thymic and peripheral levels. These findings support an intrinsic role for human FOXP3 in controlling thymocyte maturation and peripheral expansion of Teff cells and reveal a previously undescribed pathogenic mechanism through an altered Teff cell compartment in patients with IPEX syndrome.

Reconstitution of Th17, Tc17 and Treg cells after paediatric haematopoietic stem cell transplantation: Impact of interleukin-7

Successful reconstitution of T lymphocytes after allogeneic haematopoietic stem cell transplantation (HSCT) is needed to establish the graft-versus-leukaemia effect and an effective anti-microbial defense, but the ratio between functionally different T-cell subsets needs to be balanced to avoid graft-versus-host disease (GVHD).IL-7 is essential for T-cell generation in the thymus and peripheral T-cell homeostasis. High IL-7 levels have been associated with impaired T-cell reconstitution, increased risk of acute GVHD and treatment-related mortality, but the underlying cellular mechanisms behind these associations have not been investigated previously. We hypothesized that increased levels of IL-7 post-transplant alters the balance between immune-regulatory T cell subsets during the post-transplant lymphocyte recovery towards a more pro-inflammatory profile.We quantified Th17 cells, Tc17 cells and Tregs in 29 children following HSCT. Th17 cell and Treg counts rose significantly from day +90 to +180 post-HSCT, and prior acute GVHD was associated with significant changes in the concentration of Tregs (9.4×106/L vs. 1.3×106/L, P=0.0052) and the Th17/Treg ratio (1.5 vs. 4.2, P=0.025). The plasma level of IL-7 at day +90 correlated inversely with Th17 cell counts (rs=−0.65, P=0.0002) and the proportion of Tc17 cells (rs=0.64, P=0.0005) at day +90, but not with Tregs. Furthermore, high IL-7 levels at day +7 were predictive of a less naïve T-cell phenotype at day +90.These findings add further evidence that IL-7 is a key regulatory factor that may tune the balance between functionally different T-cell subsets following HSCT.

Thymic epithelial cells require p53 to support their long-term function in thymopoiesis in mice

AbstractThymic epithelial cells (TECs) provide crucial microenvironments for T-cell development and tolerance induction. As the regular function of the thymus declines with age, it is of fundamental and clinical relevance to decipher new determinants that control TEC homeostasis in vivo. Beyond its recognized tumor suppressive function, p53 controls several immunoregulatory pathways. To study the cell-autonomous role of p53 in thymic epithelium functioning, we developed and analyzed mice with conditional inactivation of Trp53 in TECs (p53cKO). We report that loss of p53 primarily disrupts the integrity of medullary TEC (mTEC) niche, a defect that spreads to the adult cortical TEC compartment. Mechanistically, we found that p53 controls specific and broad programs of mTEC differentiation. Apart from restraining the expression and responsiveness of the receptor activator of NF-κB (RANK), which is central for mTEC differentiation, deficiency of p53 in TECs altered multiple functional modules of the mTEC transcriptome, including tissue-restricted antigen expression. As a result, p53cKO mice presented premature defects in mTEC-dependent regulatory T-cell differentiation and thymocyte maturation, which progressed to a failure in regular and regenerative thymopoiesis and peripheral T-cell homeostasis in the adulthood. Lastly, peripheral signs of altered immunological tolerance unfold in mutant mice and in immunodeficient mice that received p53cKO-derived thymocytes. Our findings position p53 as a novel molecular determinant of thymic epithelium function throughout life.

Enhanced renewal of regulatory T cells in relation to CD4+ conventional T lymphocytes in the peripheral compartment

CD4(+) Foxp3(+) regulatory T (Treg) cells are necessary for the maintenance of self-tolerance and T-cell homeostasis. This population is kept at stable frequencies in secondary lymphoid organs for the majority of the lifetime, despite permanent thymic emigration or in the face of thymic involution. Continuous competition is expected to occur between recently thymus-emigrated and resident Treg cells (either natural or post-thymically induced). In the present work, we analysed the renewal dynamics of Treg cells compared with CD4(+) Foxp3- conventional T cells (Tconv), using protocols of single or successive T-cell transfers into syngeneic euthymic or lymphopenic (nu/nu or RAG2(-/-)) mice, respectively. Our results show a higher turnover for Treg cells in the peripheral compartment, compared with Tconv cells, when B cell-sufficient euthymic or nude hosts are studied. This increased renewal within the Treg pool, shown by the greater replacement of resident Treg cells by donor counterparts, correlates with augmented rates of proliferation and is not modified following temporary environmental perturbations induced by inflammatory state or microbiota alterations. Notably, the preferential substitution of Treg lymphocytes was not observed in RAG2(-/-) hosts. We showed that limited B-cell replenishment in the RAG2(-/-) hosts decisively contributed to the altered peripheral T-cell homeostasis. Accordingly, weekly transfers of B cells to RAG2(-/-) hosts rescued the preferential substitution of Treg lymphocytes. Our study discloses a new aspect of T-cell homeostasis that depends on the presence of B lymphocytes to regulate the relative incorporation of recently arrived Treg and Tconv cells in the peripheral compartment.

IL‐7 induces clathrin‐mediated endocytosis of CD127 and subsequent degradation by the proteasome in primary human CD8 T cells

Interleukin-7 (IL-7), a key immunoregulatory cytokine, plays an essential role in peripheral T-cell homeostasis and function. Signaling via the IL-7 receptor is tightly regulated and we and others have shown IL-7 provides negative feedback on its own signaling by downregulating expression of the IL-7 receptor alpha-chain (CD127) through both suppression of CD127 gene transcription and by internalization of existing CD127 proteins from the cell membrane. We show here for the first time in primary human CD8 T cells that upon stimulation with IL-7, CD127 is internalized through clathrin-coated pits, a process dependent on both lipid-raft formation and the activity of dynamin. As visualized by confocal microscopy, CD127 shows increased co-localization with clathrin within 5 min of IL-7 stimulation and within 15-30 min is seen in multiple intracellular punctae co-localizing with the early endosomal marker EEA1. By 2 h after addition of IL-7, CD127 staining associates with the late endosomal marker RAB7 and with the proteasomal 20S subunit. By inducing receptor internalization and translocation from early endosomes to the proteasome, IL-7 directly influences its receptor density on the cell surface and thus regulates the intensity of its own signaling cascades. Given the important role IL-7 plays in T-cell development, homeostasis and function, deciphering how expression of its receptor is controlled on the cell surface is essential in understanding how T-cell activity can be regulated in different microenvironments and in response to different pathogens.

Autophagy-independent functions of UVRAG are essential for peripheral naive T-cell homeostasis

UV radiation resistance-associated gene (UVRAG) encodes a tumor suppressor with putative roles in autophagy, endocytic trafficking, and DNA damage repair but its in vivo role in T cells is unknown. Because conditional homozygous deletion of Uvrag in mice results in early embryonic lethality, we generated T-cell-specific UVRAG-deficient mice that lacked UVRAG expression specifically in T cells. This loss of UVRAG led to defects in peripheral homeostasis that could not be explained by the increased sensitivity to cell death and impaired proliferation observed for other autophagy-related gene knockout mice. Instead, UVRAG-deficient T-cells exhibited normal mitochondrial clearance and activation-induced autophagy, suggesting that UVRAG has an autophagy-independent role that is critical for peripheral naive T-cell homeostatic proliferation. In vivo, T-cell-specific loss of UVRAG dampened CD8(+) T-cell responses to LCMV infection in mice, delayed viral clearance, and impaired memory T-cell generation. Our data provide novel insights into the control of autophagy in T cells and identify UVRAG as a new regulator of naïve peripheral T-cell homeostasis.

Androgens Contribute to Age-Associated Changes in Peripheral T-Cell Homeostasis Acting in a Thymus-Independent Way

Objective: Considering a causal role of androgens in thymic involution, age-related remodeling of peripheral T-cell compartments in the absence of testicular hormones was evaluated. Methods: Rats were orchidectomized (ORX) at the age of 1 month, and T-peripheral blood lymphocytes (PBLs) and splenocytes from young (75-day-old) and aged (24-month-old) rats were examined for differentiation/activation and immunoregulatory marker expression. Results: In ORX rats, following the initial rise, the counts of CD4+ and CD8+ PBLs diminished with aging. This reflected the decline in thymic export as shown by recent thymic emigrant (RTE) enumeration. Orchidectomy increased the count of both of the major T-splenocyte subsets in young rats, and they (differently from controls) remained stable with aging. The CD4+:CD8+ T-splenocyte ratio in ORX rats shifted towards CD4+ cells compared to age-matched controls. Although in the major T-cell subsets in the blood and spleen from aged ORX rats the numbers of RTEs were comparable to the corresponding values in age-matched controls, the numbers of mature naïve and memory/activated cells substantially differed. Compared with age-matched controls, in aged ORX rats the numbers of CD4+ mature naïve PBLs and splenocytes were reduced, whereas those of CD4+ memory/activated cells (predictive of early mortality) were increased. Additionally, in spleens from aged ORX rats, despite unaltered thymic export, CD4+CD25+FoxP3+ and natural killer T cell counts were greater than in age-matched controls. Conclusion: (i) Age-related decline in thymopoietic efficacy is not dependent on androgen presence, and (ii) androgens are involved in the maintenance of peripheral T-cell (particularly CD4+ cell) homeostasis during aging.

Premature Thymic Involution and Multiple Sclerosis

It is uncertain as to whether similar or different mechanisms contribute to different subtypes of multiple sclerosis (MS). A unifying concept proposes that patients with both relapsing-remitting MS (RRMS) and primary progressive MS (PPMS) have premature thymic involution, which alters peripheral T-cell homeostasis and leads to T-cell alterations that contribute to the pathogenesis of both MS subtypes.

LKB1 and AMPK: central regulators of lymphocyte metabolism and function

When T cells encounter foreign antigen and appropriate costimulatory signals from professional antigen-presenting cells (APCs), they initiate a coordinated program of rapid proliferation and differentiation, leading to the development of activated T cells with specific effector functions tailored toward pathogen clearance or control. One of the fundamental programs that underpin T-cell proliferation and function is the regulation of cellular metabolism. Recent efforts to identify the signal transduction pathways that regulate T-cell metabolism have led to the identification of liver kinase B1 (LKB1) and AMP-activated protein kinase (AMPK) as key regulators of T-cell metabolism. LKB1 and AMPK are part of an evolutionarily conserved signal transduction pathway that monitors cellular energy status. AMPK senses bioenergetic fluctuations in cells and works in concert with LKB1 to maintain cellular energy homeostasis by promoting catabolic pathways of ATP production and limiting processes that consume ATP. Recent data indicate that LKB1 and AMPK can influence diverse aspects of T-cell biology beyond metabolism, including T-cell development, peripheral T-cell homeostasis, and T-cell effector function. In this review, we focus on the regulation of lymphocyte metabolism by this energy-sensing pathway and discuss its influence on T-cell function.

Altered Thymic Function during Interferon Therapy in HCV-Infected Patients

Interferon alpha (IFNα) therapy, despite good efficacy in curing HCV infection, leads to major side effects, in particular inducement of a strong peripheral T-cell lymphocytopenia. We here analyze the early consequences of IFNα therapy on both thymic function and peripheral T-cell homeostasis in patients in the acute or chronic phase of HCV-infection as well as in HIV/HCV co-infected patients. The evolution of T-cell subsets and T-cell homeostasis were estimated by flow cytometry while thymic function was measured through quantification of T-cell receptor excision circles (TREC) and estimation of intrathymic precursor T-cell proliferation during the first four months following the initiation of IFNα therapy. Beginning with the first month of therapy, a profound lymphocytopenia was observed for all T-cell subsets, including naïve T-cells and recent thymic emigrants (RTE), associated with inhibition of intrathymic precursor T-cell proliferation. Interleukin (IL)-7 plasma concentration rapidly dropped while lymphocytopenia progressed. This was neither a consequence of higher consumption of the cytokine nor due to its neutralization by soluble CD127. Decrease in IL-7 plasma concentration under IFNα therapy correlated with the decline in HCV viral load, thymic activity and RTE concentration in blood. These data demonstrate that IFNα-based therapy rapidly impacts on thymopoiesis and, consequently, perturbs T-cell homeostasis. Such a side effect might be detrimental for the continuation of IFNα therapy and may lead to an increased level of infectious risk, in particular in HIV/HCV co-infected patients. Altogether, this study suggests the therapeutic potential of IL-7 in the maintenance of peripheral T-cell homeostasis in IFNα-treated patients.

IL-15 transpresentation promotes both human T-cell reconstitution and T-cell–dependent antibody responses in vivo

Cytokine immunotherapies targeting T lymphocytes are attractive clinical interventions against viruses and tumors. In the mouse, the homeostasis of memory α/β CD8(+) T cells and natural killer (NK) cells is significantly improved with increased IL-15 bioavailability. In contrast, the role of "transpresented" IL-15 on human T-cell development and homeostasis in vivo is unknown. We found that both CD8 and CD4 T cells in human immune system (HIS) mice are highly sensitive to transpresented IL-15 in vivo, with both naïve (CD62L(+)CD45RA(+)) and memory phenotype (CD62L(-)CD45RO(+)) subsets being significantly increased following IL-15 "boosting." The unexpected global improvement in human T-cell homeostasis involved enhanced proliferation and survival of both naïve and memory phenotype peripheral T cells, which potentiated B-cell responses by increasing the frequency of antigen-specific responses following immunization. Transpresented IL-15 did not modify T-cell activation patterns or alter the global T-cell receptor (TCR) repertoire diversity. Our results indicate an unexpected effect of IL-15 on human T cells in vivo, in particular on CD4(+) T cells. As IL-15 promotes human peripheral T-cell homeostasis and increases the frequency of neutralizing antibody responses in HIS mice, IL-15 immunotherapy could be envisaged as a unique approach to improve vaccine responses in the clinical setting.

Interleukin-7 Regulates Bim Proapoptotic Activity in Peripheral T-Cell Survival

Interleukin-7 (IL-7) is critical for T-cell development and peripheral T-cell homeostasis. The survival of pro-T cells and mature T cells requires IL-7. The survival function of IL-7 is accomplished partly through induction of the antiapoptotic protein Bcl-2 and inhibition of proapoptotic proteins Bax and Bad. We show here that the proapoptotic protein Bim, a BH3-only protein belonging to the Bcl-2 family, also plays a role in peripheral T-cell survival. Deletion of Bim partially protected an IL-7-dependent T-cell line and peripheral T cells, especially cells with an effector memory phenotype, from IL-7 deprivation. However, T-cell development in the thymus was not restored in IL-7(-/-) Rag2(-/-) mice reconstituted with Bim(-/-) bone marrow. IL-7 withdrawal altered neither the intracellular location of Bim, which was constitutively mitochondrial, nor its association with Bcl-2; however, a reduction in its association with the prosurvival protein Mcl-1 was observed. IL-7 withdrawal did not increase Bim mRNA or protein expression but did induce changes in the isoelectric point of Bim(EL) and its reactivity with an antiphosphoserine antibody. Our findings suggest that the maintenance of peripheral T cells by IL-7 occurs partly through inhibition of Bim activity at the posttranslational level.

Injection of glycosylated recombinant simian IL-7 provokes rapid and massive T-cell homing in rhesus macaques

AbstractInterleukin-7 (IL-7), the principal cytokine implicated in thymopoiesis and peripheral T-cell homeostasis, is presently under evaluation in human diseases characterized by persistent lymphopenia. Unexpectedly, before the eventual IL-7–driven T-cell expansion, all treated patients showed a profound T-cell depletion 24 hours after injection. The current study uses the rhesus macaque model to investigate the mechanisms involved in this IL-7–induced T-cell depletion. We identify a new critical function of IL-7 that induces massive and rapid T-cell migration from the blood into various organs, including lymph nodes, parts of the intestine, and the skin. This homing process was initiated after the induction of chemokine receptor expression by circulating T cells and the production of corresponding chemokines in target organs. Finally, we demonstrate that the IL-7–induced cell cycling is initiated within these organs before T cells migrate back into the bloodstream, indicating that T-cell homing is required for in vivo IL-7 function.

Essential role for Bclaf1 in lung development and immune system function

Bcl-2 associated factor 1 (Bclaf1) is a nuclear protein that was originally identified in a screen of proteins that interact with the adenoviral bcl-2 homolog E1B19K. Overexpression of Bclaf1 was shown to result in apoptosis and transcriptional repression that was reversible in the presence of Bcl-2 or Bcl-x(L). Furthermore, antiapoptotic members, but not proapoptotic members of the Bcl-2 protein family, were shown to interact with Bclaf1 and prevent its localization to the nucleus. Bclaf1 has also recently been identified as a binding partner for Emerin, a nuclear membrane protein that is mutated in X-linked recessive Emery-Dreifuss muscular dystrophy. To ascertain the in vivo function of Bclaf1, we have generated mice that carry a targeted mutation of the bclaf1 locus. In this study, we show that Bclaf1 is required for proper spatial and temporal organization of smooth muscle lineage during the saccular stage of lung development. We also show that Bclaf1 is dispensable for thymocyte development but is essential for peripheral T-cell homeostasis. Despite its postulated role as a proapoptotic protein, Bclaf1-deficient cells did not show any defect in cell death linked to development or after exposure to various apoptotic stimuli. Our findings show a critical role for Bclaf1 in developmental processes independent of apoptosis.

43: Administration of rhIL-7 Increases TCR Repertoire Diversity Through Preferential Expansion of Naive T Cells

Interleukin-7 (IL-7) is a multifunctional cytokine with critical and non-redundant roles in thymopoiesis and peripheral T-cell homeostasis. We previously reported preliminary results of the first Phase I study of recombinant human IL-7 (rhIL-7), demonstrating that two weeks of alternate day treatment with rhIL-7 produced a marked increase in the number of CD4+ and CD8+ T cells. This increase was maintained in follow up assays at 6 to 12 weeks post treatment. Furthermore, rhIL-7 therapy disproportionately increased CD27+CD45RA+ naive cells, which represent the most diverse elements of the mature T cell receptor (TCR) repertoire, at the expense of CD27-CD45RA+/- effector populations, which are often oligoclonal.

41: Pre-Transplant Administration of Keratinocyte Growth Factor Affects Peripheral T-cell Homeostasis Through Increased Recent Thymic Emigrant Export and Affects the Course of Murine Chronic Graft-Vs.-Host Disease

Accelerated recovery of thymic function following allogeneic hematopoietic stem cell transplantation (allo-HSCT) not only provides a sufficiently broad repertoire of T-cell responses to pathogens, but also is thought to play a role in affecting the outcome of graft-vs.-host disease (GVHD) through the restoration of central tolerance and/or the production of regulatory cell populations that may blunt the effect of donor-derived alloreactive T-cell populations. Keratinocyte growth factor (KGF) has been shown in murine models to accelerate thymic function and ameliorate acute GVHD, but it is unclear whether the latter involves a thymic-dependent mechanism of increased T-cell production and/or cytoprotection of epithelial cells in target organs of GVHD.