Necroptotic-susceptible dendritic cells exhibit enhanced antitumor activities in mice.

INTRODUCTION: Priming of tumor-specific T cells is a key to antitumor immune response and inflammation, in turn, is crucial for proper T-cell activation. As antigen-presenting cells can activate T cells, dendritic cells (DCs) loaded with tumor antigens have been used as immunotherapeutics against certain cancer in humans but their efficacy is modest. Necroptosis is a form of programmed cell death that results in the release of inflammatory contents. We previously generated mice with DC deficiency in a negative regulator of necroptosis, Fas-associated death domain (FADD), and found that these mice suffer from systemic inflammation due to necroptotic DCs. We hypothesize that FADD-deficient DCs could serve as a better vaccine than wild-type (WT) DCs against tumors. MATERIALS AND METHODS: FADD-deficient and WT mouse DCs loaded with the relevant tumor peptide were injected onto mice before or after the syngeneic tumor challenge. DC vaccinations were repeated two more times and anti-PD-1 antibodies were coinjected in some experiments. Tumor sizes were measured by caliper, and the percentages of tumor-free mice or mice survived were examined over time. The cytometric analysis was carried out to analyze various immune populations. RESULTS: In two separate tumor models, we find that mice receiving FADD-deficient DCs as vaccine rejected tumors significantly better than those receiving a WT DC vaccine. Tumor growth was severely hampered, and survival extended in these mice. More activated CD8 T cells together with elevated cytokines were observed in mice receiving the FADD-deficient DC vaccine. Furthermore, we observed these effects were potent enough to protect against tumor challenge postinjection and can work in conjunction with anti-PD-1 antibodies to reduce the tumor growth. CONCLUSIONS: Necroptotic-susceptible DCs are better antitumor vaccines than WT DCs in mice. Our findings suggest that necroptosis-driven inflammation by DCs may be a novel avenue to generating a strong adaptive antitumor response in the clinical setting.

B cell lymphoma 2 (Bcl-2) residues essential for Bcl-2's apoptosis-inducing interaction with Nur77/Nor-1 orphan steroid receptors.

Apoptosis is mediated through the extrinsic or intrinsic pathway. Key regulators of the intrinsic apoptotic pathway are the family of B cell lymphoma 2 (Bcl-2) proteins. The activity of the prototypical Bcl-2 protein is usually considered antiapoptotic. However, under some conditions, Bcl-2 associates with the orphan nuclear hormone receptors Nur77 and Nor-1, converting Bcl-2 into a proapoptotic molecule. Expression of Nur77 and Nor-1 is induced by a variety of signals, including those leading to apoptosis. Translocation of Nur77/Nor-1 to mitochondria results in their association with Bcl-2, exposing the Bcl-2 homology (BH) 3 domain and causing apoptosis. However, the molecular details of this interaction are incompletely understood. Here, through extensive Bcl-2 mutagenesis and functional assays, we identified residues within Bcl-2 that are essential for its interaction with Nur77/Nor-1. Although an initial report has suggested that an unstructured loop region between the Bcl-2 BH4 and BH3 domains is required for Bcl-2's interaction with Nur77/Nor-1, we found that it is dispensable for this interaction. Instead, we found important interacting residues at the BH4 domain and crucial interacting residues between the BH1 and BH2 domains. Bcl-2 alanine mutants at this region could no longer interact with Nur77/Nor-1 and could not initiate Nur77/Bcl-2-mediated cell death. However, they still retained their anti-apoptotic capability in two different death assays. These results establish crucial residues in Bcl-2 required for Nur77/Nor-1-mediated apoptosis and point to potential new strategies for manipulating Bcl-2 function.

Induction of necroptotic cell death by viral activation of the RIG-I or STING pathway.

Necroptosis is a form of necrotic cell death that requires the activity of the death domain-containing kinase RIP1 and its family member RIP3. Necroptosis occurs when RIP1 is deubiquitinated to form a complex with RIP3 in cells deficient in the death receptor adapter molecule FADD or caspase-8. Necroptosis may play a role in host defense during viral infection as viruses like vaccinia can induce necroptosis while murine cytomegalovirus encodes a viral inhibitor of necroptosis. To see how general the interplay between viruses and necroptosis is, we surveyed seven different viruses. We found that two of the viruses tested, Sendai virus (SeV) and murine gammaherpesvirus-68 (MHV68), are capable of inducing dramatic necroptosis in the fibrosarcoma L929 cell line. We show that MHV68-induced cell death occurs through the cytosolic STING sensor pathway in a TNF-dependent manner. In contrast, SeV-induced death is mostly independent of TNF. Knockdown of the RNA sensing molecule RIG-I or the RIP1 deubiquitin protein, CYLD, but not STING, rescued cells from SeV-induced necroptosis. Accompanying necroptosis, we also find that wild type but not mutant SeV lacking the viral proteins Y1 and Y2 result in the non-ubiquitinated form of RIP1. Expression of Y1 or Y2 alone can suppress RIP1 ubiquitination but CYLD is dispensable for this process. Instead, we found that Y1 and Y2 can inhibit cIAP1-mediated RIP1 ubiquitination. Interestingly, we also found that SeV infection of B6 RIP3-/- mice results in increased inflammation in the lung and elevated SeV-specific T cells. Collectively, these data identify viruses and pathways that can trigger necroptosis and highlight the dynamic interplay between pathogen-recognition receptors and cell death induction.

MEF2C and EBF1 Co-regulate B Cell-Specific Transcription.

Hematopoietic stem cells are capable of self-renewal or differentiation along three main lineages: myeloid, erythroid, and lymphoid. One of the earliest lineage decisions for blood progenitor cells is whether to adopt the lymphoid or myeloid fate. Previous work had shown that myocyte enhancer factor 2C (MEF2C) is indispensable for the lymphoid fate decision, yet the specific mechanism of action remained unclear. Here, we have identified early B cell factor-1 (EBF1) as a co-regulator of gene expression with MEF2C. A genome-wide survey of MEF2C and EBF1 binding sites identified a subset of B cell-specific genes that they target. We also determined that the p38 MAPK pathway activates MEF2C to drive B cell differentiation. Mef2c knockout mice showed reduced B lymphoid-specific gene expression as well as increased myeloid gene expression, consistent with MEF2C's role as a lineage fate regulator. This is further supported by interaction between MEF2C and the histone deacetylase, HDAC7, revealing a likely mechanism to repress the myeloid transcription program. This study thus elucidates both activation and repression mechanisms, identifies regulatory partners, and downstream targets by which MEF2C regulates lymphoid-specific differentiation.

Deletion of FADD in macrophages and granulocytes results in RIP3- and MyD88-dependent systemic inflammation.

Myeloid cells, which include monocytes, macrophages, and granulocytes, are important innate immune cells, but the mechanism and downstream effect of their cell death on the immune system is not completely clear. Necroptosis is an alternate form of cell death that can be triggered when death receptor-mediated apoptosis is blocked, for example, in stimulated Fas-associated Death Domain (FADD) deficient cells. We report here that mice deficient for FADD in myeloid cells (mFADD-/-) exhibit systemic inflammation with elevated inflammatory cytokines and increased levels of myeloid and B cell populations while their dendritic and T cell numbers are normal. These phenotypes were abolished when RIP3 deficiency was introduced, suggesting that systemic inflammation is caused by RIP3-dependent necroptotic and/or inflammatory activity. We further found that loss of MyD88 can rescue the systemic inflammation observed in these mice. These phenotypes are surprisingly similar to that of dendritic cell (DC)-specific FADD deficient mice with the exception that DC numbers are normal in mFADD-/- mice. Together these data support the notion that innate immune cells are constantly being stimulated through the MyD88-dependent pathway and aberrations in their cell death machinery can result in systemic effects on the immune system.

Conserved and divergent aspects of human T-cell development and migration in humanized mice.

Humanized mice represent an important model to study the development and function of the human immune system. While it is known that mouse thymic stromal cells can support human T-cell development, the extent of interspecies cross-talk and the degree to which these systems recapitulate normal human T-cell development remain unclear. To address these questions, we compared conventional and non-conventional T-cell development in a neonatal chimera humanized mouse model with that seen in human fetal and neonatal thymus samples, and also examined the impact of a human HLA-A2 transgene expressed by the mouse stroma. Given that dynamic migration and cell-cell interactions are essential for T-cell differentiation, we also studied the intrathymic migration pattern of human thymocytes developing in a murine thymic environment. We found that both conventional T-cell development and intra-thymic migration patterns in humanized mice closely resemble human thymopoiesis. Additionally, we show that developing human thymocytes engage in short, serial interactions with other human hematopoietic-derived cells. However, non-conventional T-cell differentiation in humanized mice differed from both fetal and neonatal human thymopoiesis, including a marked deficiency of Foxp3(+) T-cell development. These data suggest that although the murine thymic microenvironment can support a number of aspects of human T-cell development, important differences remain, and additional human-specific factors may be required.

Premalignant PTEN-deficient thymocytes activate microRNAs miR-146a and miR-146b as a cellular defense against malignant transformation.

Cancer develops by a multistep process during which cells acquire characteristics that allow them to evade apoptosis and proliferate unchecked. Sequential acquisition of genetic alterations drives this process but also causes cellular stress, frequently prompting cells to enter a premalignant period during which they mount a defense against transformation. T cell-specific deletion of the tumor suppressor PTEN in mice induces premalignancy in the thymus and development of CD4(+) T-cell lymphomas in the periphery. Here we sought to identify factors mediating the cellular defense against transformation during the premalignant period. We identified several microRNAs upregulated specifically in premalignant thymocytes, including miR-146a, miR-146b, and the miR-183/96/182 cluster. CD4-driven T cell-specific transgenic overexpression of mir-146a and mir-146b significantly delayed PTEN-deficient lymphomagenesis and delayed c-myc oncogene induction, a key driver of transformation in PTEN-deficient T-cell malignancies. We found that miR-146a and miR-146b targeting of Traf6 attenuates TCR signaling in the thymus and inhibits downstream NF-κB-dependent induction of c-myc. Additionally, c-myc repression in mature CD4 T cells by miR-146b impaired TCR-mediated proliferation. Hence, we have identified 2 miRNAs that are upregulated as part of the cellular response against transformation that, when overrepresented, can effectively inhibit progression to malignancy in the context of PTEN deficiency.

Comparative study of hematopoietic differentiation between human embryonic stem cell lines.

Directed differentiation of human embryonic stem cells (hESCs) into any desired cell type has been hailed as a therapeutic promise to cure many human diseases. However, substantial roadblocks still exist for in vitro differentiation of hESCs into distinct cell types, including T lymphocytes. Here we examined the hematopoietic differentiation potential of six different hESC lines. We compare their ability to develop into CD34(+) or CD34(+)CD45(+) hematopoietic precursor populations under several differentiation conditions. Comparison of lymphoid potential of hESC derived- and fetal tissue derived-hematopoietic precursors was also made. We found diverse hematopoietic potential between hESC lines depending on the culture or passage conditions. In contrast to fetal-derived hematopoietic precursors, none of the CD34(+) precursors differentiated from hESCs were able to develop further into T cells. These data underscore the difficulties in the current strategy of hESC forward differentiation and highlight distinct differences between CD34(+) hematopoietic precursors generated in vitro versus in vivo.

CD11c+ dendritic cells and B cells contribute to the tumoricidal activity of anti-DR5 antibody therapy in established tumors.

The selective targeting of the tumor-associated death-inducing receptors DR4 and DR5 with agonistic mAbs has demonstrated preclinical and clinical antitumor activity. However, the cellular and molecular mechanisms contributing to this efficacy remain poorly understood. In this study, using the first described C57BL/6 (B6) TRAIL-sensitive experimental tumor models, we have characterized the innate and adaptive immune components involved in the primary rejection phase of an anti-mouse DR5 (mDR5) mAb, MD5-1 in established MC38 colon adenocarcinomas. FcR mediated cross-linking of MD5-1 significantly inhibited the growth of MC38 colon adenocarcinomas through the induction of TRAIL-R-dependent tumor cell apoptosis. The loss of host DR5, TRAIL, perforin, FasL, or TNF did not compromise anti-DR5 therapy in vivo. By contrast, anti-DR5 therapy was completely abrogated in mice deficient of B cells or CD11c(+) dendritic cells (DCs), providing the first direct evidence that these cells play a critical role. Importantly, the requirement for an intact B cell compartment for optimal anti-DR5 antitumor efficacy was also observed in established AT-3 mammary tumors. Interestingly, MD5-1-mediated apoptosis as measured by early TUNEL activity was completely lost in B cell-deficient microMT mice, but intact in mice deficient in CD11c(+) DCs. Overall, these data show that Ab-mediated targeting of DR5 triggers tumor cell apoptosis in established tumors in a B cell-dependent manner and that CD11c(+) DCs make a critical downstream contribution to anti-DR5 antitumor activity.

Protein kinase C regulates mitochondrial targeting of Nur77 and its family member Nor-1 in thymocytes undergoing apoptosis.

Nur77 orphan steroid receptor and its family member Nor-1 are required for apoptosis of developing T cells. In thymocytes, signals from the TCR complex induce Nur77 and Nor-1 expression followed by translocation from the nucleus to mitochondria. Nur77 and Nor-1 associate with Bcl-2 in the mitochondria, resulting in a conformation change that exposes the Bcl-2 BH3 domain, a presumed pro-apoptotic molecule of Bcl-2. As Nur77 and Nor-1 are heavily phosphorylated, we examined the requirement of Nur77 and Nor-1 phosphorylation in mitochondria translocation and Bcl-2 BH3 exposure. We found that HK434, a PKC agonist, in combination with calcium ionophore, can induce Nur77 and Nor-1 phosphorylation, translocation, Bcl-2 BH3 exposure and thymocyte apoptosis. Inhibitors of both classical and novel forms of PKC were able to block this process. In contrast, only the general but not classical PKC-specific inhibitors were able to block the same process initiated by PMA, a commonly used PKC agonist. These data demonstrate a differential activation of PKC isoforms by PMA and HK434 in thymocytes, and show the importance of PKC in mitochondria translocation of Nur77/Nor-1 and Bcl-2 conformation change during TCR-induced thymocyte apoptosis.

FoxM1, a forkhead transcription factor is a master cell cycle regulator for mouse mature T cells but not double positive thymocytes.

FoxM1 is a forkhead box transcription factor and a known master regulator required for different phases of the cell cycle. In cell lines, FoxM1 deficient cells exhibit delayed S phase entry, aneuploidy, polyploidy and can't complete mitosis. In vivo, FoxM1 is expressed mostly in proliferating cells but is surprisingly also found in non-proliferating CD4(+)CD8(+) double positive thymocytes. Here, we addressed the role of FoxM1 in T cell development by generating and analyzing two different lines of T-cell specific FoxM1 deficient mice. As expected, FoxM1 is required for proliferation of early thymocytes and activated mature T cells. Defective expression of many cell cycle proteins was detected, including cyclin A, cyclin B1, cdc2, cdk2, p27 and the Rb family members p107 and p130 but surprisingly not survivin. Unexpectedly, loss of FoxM1 only affects a few cell cycle proteins in CD4(+)CD8(+) thymocytes and has little effect on their sensitivity to apoptosis and the subsequent steps of T cell differentiation. Thus, regulation of cell cycle genes by FoxM1 is stage- and context-dependent.

The role of the PI3K-AKT kinase pathway in T-cell development beyond the beta checkpoint.

The PI3K-AKT pathway can mediate diverse biological responses and is crucial for optimal immune responses and lymphocyte development. Deletion of PI3K subunits or AKT leads to blockage of T-cell development at the TCR-beta checkpoint. Studies with over-expression of constitutively activated AKT have implicated this pathway in anti-apoptosis of developing thymocytes and in development of regulatory T cells. However, the role of endogenous PI3K-AKT in T-cell development beyond the TCR-beta checkpoint remains unclear. Here, we inhibited the endogenous PI3K-AKT pathway in thymocytes after double negative stages by expressing the negative regulator, PTEN. These mice exhibit normal early T-cell development, but the transition from intermediate single positive to double positive (DP) thymocytes is inhibited, leading to a significantly decreased number of DP, single positive thymocytes and peripheral T cells. Proliferation of peripheral T cells is reduced but apoptosis of DP cells and subsequent T-cell maturation, including regulatory T cells, are normal. AKT phosphorylation can be readily observed in most WT T-cell compartments but not DP thymocytes in response to TCR activation. Thus, the PI3K-AKT pathway is crucial for the transition of intermediate single positive to DP thymocytes but is dispensable for apoptosis and maturation of developing thymocytes.

During negative selection, Nur77 family proteins translocate to mitochondria where they associate with Bcl-2 and expose its proapoptotic BH3 domain.

Apoptosis accompanying negative selection is a central but poorly understood event in T cell development. The Nur77 nuclear steroid receptor and Bim, a proapoptotic BH3-only member of the Bcl-2 family, are two molecules implicated in this process. However, how they relate to each other and how Nur77 induces apoptosis remain unclear. In thymocytes, Nur77 has been shown to induce cell death through a transcriptional-dependent pathway, but in cancer cell lines, Nur77 was reported to induce apoptosis through conversion of Bcl-2 into a killer protein at the mitochondria. Whether this Nur77 transcriptional-independent pathway actually occurs in vivo remains controversial. Using an optimized fractionation protocol for thymocytes, here we report that stimulation of CD4(+)CD8(+) thymocytes results in translocation of Nur77 and its family member Nor-1 to the mitochondria, leading to their association with Bcl-2 and exposure of the Bcl-2 proapoptotic BH3 domain. In two T cell receptor transgenic models of negative selection, F5 and HY, a conformational change of the Bcl-2 molecule in the negatively selected T cell population was similarly observed. Thus, the Nur77 family and Bim pathways converge at mitochondria to mediate negative selection.

Normal development is an integral part of tumorigenesis in T cell-specific PTEN-deficient mice.

PTEN is a tumor suppressor gene but whether cancer can develop in all PTEN-deficient cells is not known. In T cell-specific PTEN-deficient (tPTEN-/-) mice, which suffer from mature T cell lymphomas, we found that premalignancy, as defined by elevated AKT and senescence pathways, starts in immature T cell precursors and surprisingly not in mature T cells. Premalignancy only starts in 6-week-old mice and becomes much stronger in 9-week-old mice although PTEN is lost since birth. tPTEN-/- immature T cells do not become tumors, and senescence has no role in this model because these cells exist in a novel cell cycle state, expressing proliferating proteins but not proliferating to any significant degree. Instead, the levels of p27(kip1), which is lower in tPTEN-/- immature T cells and almost nonexistent in tPTEN-/- mature T cells, correlate with the proliferation capability of these cells. Interestingly, transient reduction of these cancer precursor cells in adult tPTEN-/- mice within a crucial time window significantly delayed lymphomas and mouse lethality. Thus, loss of PTEN alone is not sufficient for cells to become cancerous, therefore other developmental events are necessary for tumor formation.

Apoptosis during negative selection of autoreactive thymocytes.

Recent investigations have solidified the importance of negative selection in controlling autoimmunity. Loss of autoimmune regulator (AIRE), required for thymic stromal-cell differentiation and thymic expression of peripheral antigens, results in multi-organ autoimmunity. Mice with AIRE/Foxp3 double mutations suffer from exacerbated autoimmunity when compared with mice with only one mutation, supporting the important contributions of both central and peripheral tolerance. In thymocytes, Cbl is a negative regulator of thymocyte apoptosis while MINK, a MEKK kinase, is required for negative selection. This is consistent with the requirement of JNK, p38 and possibly ERK5 MAP kinases in thymocyte apoptosis. ERK5 induces the Nur77 orphan steroid receptor family members. In cell lines, Nur77 interaction with Bcl-2 turns Bcl-2 into a pro-apoptotic molecule. This and other possibilities will be discussed to explain the unresolved finding that negative selection is defective in Bim(-/-) but is not efficiently blocked in Bcl-2 transgenic mice.

Constitutive phosphorylation mutation in Fas-associated death domain (FADD) results in early cell cycle defects.

Fas-associated death domain (FADD) is an adaptor molecule for the death receptor subfamily of the tumor necrosis factor receptor superfamily, but it is also required for cell proliferation. Cell cycle-specific regulation of FADD phosphorylation plays an important role in FADD proliferative function since mice with a mutant form of FADD mimicking constitutive phosphorylation at serine 191 (FADD-D) exhibit defective T cell proliferation. Here we characterized these mice in detail and found that T cell development in 2-4-week-old mice is relatively normal, although mature FADD-D T cells manifest defective G(0) and G(1) to S transition with abnormalities in regulation of p130, p27 degradation, retinoblastoma protein phosphorylation, and CDK2 kinase activity. These downstream defects are further associated with the failure to up-regulate the forkhead box M1 cell cycle transcription factor, FoxM1. FADD-D protein is also mislocalized during cell cycle progression. Thus, regulation of FADD phosphorylation is crucial for proper cell cycle entry.

Transcriptional regulation of tissue-specific genes by the ERK5 mitogen-activated protein kinase.

The ERK5 mitogen-activated protein kinase (MAPK) differs from other MAPKs in possessing a potent transcriptional activation domain. ERK5-/- embryos die from angiogenic defects, but the precise physiological role of ERK5 remains poorly understood. To elucidate molecular functions of ERK5 in the development of vasculature and other tissues, we performed gene profile analyses of erk5-/- mouse embryos and erk5-/- fibroblast cells reconstituted with ERK5 or ERK5(1-740), which lacks the transactivation domain. These experiments revealed several potential ERK5 target genes, including a proapoptotic gene bnip3, known angiogenic genes flt1 and lklf (lung Krüppel-like factor), and genes that regulate cardiovascular development. Among these, LKLF, known for its roles in angiogenesis, T-cell quiescence, and survival, was found to be absolutely dependent on ERK5 for expression in endothelial and T cells. We show that ERK5 drives lklf transcription by activating MEF2 transcription factors. Expression of erk5 short hairpin or a dominant-negative form of the ERK5 upstream activator, MEK5, in T cells led to downregulation of LKLF, increased cell size and upregulation of activation markers. Thus, through its kinase and transcriptional activation domains, ERK5 regulates transcriptional responses of cell survival and quiescence critical for angiogenesis and T-cell function.

Phosphorylation of FADD at serine 194 by CKIalpha regulates its nonapoptotic activities.

FADD is essential for death receptor (DR)-induced apoptosis. However, it is also critical for cell cycle progression and proliferation, activities that are regulated by phosphorylation of its C-terminal Ser194, which has also been implicated in sensitizing cancer cells to chemotherapeutic drugs and in regulating FADD's intracellular localization. We now demonstrate that casein kinase Ialpha (CKIalpha) phosphorylates FADD at Ser194 both in vitro and in vivo. FADD-CKIalpha association regulates the subcellular localization of FADD, and phosphorylated FADD was found to colocalize with CKIalpha on the spindle poles in metaphase. Inhibition of CKIalpha diminished FADD phosphorylation, prevented the ability of Taxol to arrest cells in mitosis, and blocked mitogen-induced proliferation of mouse splenocytes. In contrast, a low level of cycling splenocytes from mice expressing FADD with a mutated phosphorylation site was insensitive to CKI inhibition. These data suggest that phosphorylation of FADD by CKI is a crucial event during mitosis.

TRAIL-R as a negative regulator of innate immune cell responses.

TRAIL receptor (TRAIL-R) signaling has been implicated in inducing apoptosis in tumor cells, but little is understood about its physiological function. Here, we report the generation and characterization of TRAIL-R(-/-) mice, which develop normal lymphocyte populations but possess enhanced innate immune responses. TRAIL-R(-/-) mice exhibited increased clearance of murine cytomegalovirus that correlated with increased levels of IL-12, IFN-alpha, and IFN-gamma. Stimulation of macrophages with Mycobacterium and Toll-like receptor (TLR)-2, -3, and -4, but not TLR9, ligands resulted in high levels of TRAIL upregulation and enhanced cytokine production in TRAIL-R(-/-) cells. The immediate-early TLR signaling events in TRAIL-R(-/-) macrophages and dendritic cells are normal, but I kappa B-alpha homeostatic regulation and NF-kappa B activity at later time points is perturbed. These data suggest that TRAIL-R negatively regulates innate immune responses.

Essential role of survivin, an inhibitor of apoptosis protein, in T cell development, maturation, and homeostasis.

Survivin is an inhibitor of apoptosis protein that also functions during mitosis. It is expressed in all common tumors and tissues with proliferating cells, including thymus. To examine its role in apoptosis and proliferation, we generated two T cell-specific survivin-deficient mouse lines with deletion occurring at different developmental stages. Analysis of early deleting survivin mice showed arrest at the pre-T cell receptor proliferating checkpoint. Loss of survivin at a later stage resulted in normal thymic development, but peripheral T cells were immature and significantly reduced in number. In contrast to in vitro studies, loss of survivin does not lead to increased apoptosis. However, newborn thymocyte homeostatic and mitogen-induced proliferation of survivin-deficient T cells were greatly impaired. These data suggest that survivin is not essential for T cell apoptosis but is crucial for T cell maturation and proliferation, and survivin-mediated homeostatic expansion is an important physiological process of T cell development.