IL‐2 is positively involved in the development of colitogenic CD4+ IL‐7Rαhigh memory T cells in chronic colitis

IL‐2 and IL‐7 share a common γ‐chain receptor and are critical for T‐cell homeostasis. We aimed to clarify the reciprocal roles of IL‐2 and IL‐7 in the development and persistence of chronic colitis. We performed a series of adoptive transfers of IL‐2^?/? CD4⁺CD45RB^high T cells into RAG‐2^?/? mice and assessed the role of IL‐2 in the induction of IL‐7Rα on colitogenic CD4⁺ T cells and the development of chronic colitis. RAG‐2^?/? mice transferred with WT but not with IL‐2^?/? CD4⁺CD45RB^high T cells developed Th1/Th17‐mediated colitis. Consistently, re‐expression of IL‐7Rα was severely impaired on IL‐2^?/? but not on WT CD4⁺ T cells from the transferred mice. To exclude a contribution of the preclinical autoimmunity of IL‐2^?/?mice, WT Ly5.1⁺ or IL‐2^?/? Ly5.2⁺ CD4⁺CD45RB^high T cells from GFP mice previously transplanted with the same number of WT and IL‐2^?/? BM cells were transferred into RAG‐2^?/? mice. RAG‐2^?/? mice transferred with IL‐2^?/?‐derived CD4⁺CD45RB^high T cells did not develop colitis, but their splenic CD4⁺ T cells changed from effector‐memory to central‐memory type. These results show that IL‐2 is critically involved in the establishment and maintenance of IL‐7‐dependent colitogenic memory CD4⁺IL‐7Rα^high T cells.     

Competition between colitogenic Th1 and Th17 cells contributes to the amelioration of colitis

Th17 cells and Th1 cells coordinate to play a critical role in the formation of inflammatory bowel diseases. To examine how Th17 and Th1 cells are regulated at inflammatory sites, we used Th1‐dominant CD4⁺CD45RB^high T cell‐transferred RAG‐2^?/? and Th1/Th17‐mixed IL‐10^?/? mice. Interestingly, not only did colitic RAG‐2^?/? mice that were parabiosed with WT mice show significant amelioration of colitis, but amelioration of disease was also observed in those parabiosed with colitic IL‐10^?/? mice. To assess the interference between Th1 and Th17 colitogenic T cells, we co‐transferred colitogenic CD4⁺ T cells from the lamina propria (LP) of CD4⁺CD45RB^high T cell‐transferred RAG‐2^?/? mice and IL‐10^?/? mice into RAG‐2^?/? mice. Surprisingly, the co‐transferred RAG‐2^?/? mice showed a vast cellular infiltration of LP CD4⁺ T cells similar to that seen in RAG‐2^?/? mice re‐transferred with the cells from colitic RAG‐2^?/? mice alone, but the co‐transferred RAG‐2^?/? mice did not have the wasting symptoms, which are also absent in RAG‐2^?/? mice transferred with cells from colitic IL‐10^?/? mice alone. Furthermore, the percentages of Th1 and Th17 cells originating from IL‐10^?/? mice and those of Th1 cells originating from colitic RAG‐2^?/? mice were all significantly decreased in the co‐transferred mice as compared with the singly‐transferred paired RAG‐2^?/? mice, suggesting that Th1 and Th17 cells are in competition, and that their orchestration results in a merged clinical phenotype of the two types of murine colitis.     

IL‐7 is essential for lymphopenia‐driven turnover of colitogenic CD4+ memory T cells in chronic colitis

We previously demonstrated that IL‐7 is essential for the persistence of T‐cell‐mediated colitis, by showing that adoptive transfer of CD4⁺CD45RB^high T cells into IL‐7^?/?×RAG‐1^?/? mice did not induce colitis; and that intestinal IL‐7 is not essential for this colitis model, by showing that IL‐7^?/?×RAG‐1^?/? mice parabiosed with colitic CD4⁺CD45RB^high T‐cell‐transferred RAG‐1^?/? mice developed colitis. Here, we investigated the role of IL‐7 in the maintenance of colitogenic CD4⁺ T cells by surgically separating these parabionts. Surprisingly, the separated IL‐7^?/?×RAG‐1^?/? mice were consistently diseased after separation, although no IL‐7 mRNA was detected in the tissues of separated IL‐7^?/?×RAG‐1^?/? partners. CD4⁺ T cells isolated from the separated RAG‐1^?/? or IL‐7^?/?×RAG‐1^?/? mice were then transferred into new RAG‐1^?/? or IL‐7^?/?×RAG‐1^?/? mice. Regardless of the source of donor cells, RAG‐1^?/? recipients developed colitis, whereas IL‐7^?/?×RAG‐1^?/? recipients did not. Collectively, these results demonstrate that IL‐7 is essential for lymphopenia‐driven turnover of colitogenic CD4⁺ T cells rather than the maintenance of those cells in established colitic mice. They also provide a basis for the timing of IL‐7/IL‐7R blockade for the treatment of inflammatory bowel diseases.     

FTY720-induced conversion of conventional Foxp3- CD4+ T cells to Foxp3+ regulatory T cells in NOD mice

Citation Sun Y, Wang W, Shan B, Di J, Chen L, Ren L, Li W, Li D‐J, Lin Y. FTY720‐induced conversion of conventional Foxp3^?CD4⁺ T cells to Foxp3⁺ regulatory T cells in NOD mice. Am J Reprod Immunol 2011; 66: 349–362 Problem FTY720 is known as an agonist of sphingosine‐1‐phosphate (S1P) receptor, but little is known about the possibility that FTY720 induces the conversion of conventional Foxp3^?CD4⁺ T cells to Foxp3⁺ regulatory T cells in non‐obese diabetic (NOD) mice. Method of study FTY720 treatment was performed using Foxp3^?CD4⁺ T cells purified from NOD mice. Results FTY720 caused an increase in Foxp3⁺ Treg cells in lymphoid organs in NOD mice. FTY720 effectively induced Foxp3 expression in Foxp3^?CD4⁺ T cells both in vitro and in vivo, an effect that was inhibited by a TGF‐β‐neutralizing antibody or the proinflammatory cytokine IL‐6. T‐cell‐mediated embryo rejection in NOD mice was prevented upon FTY720 treatment. Conclusions The use of FTY720 along with Ag administration may represent a useful therapeutic strategy to selectively expand Ag‐specific Foxp3⁺ Tregs to intervene autoimmune and infectious diseases.     

Homeostatic proliferation of naïve CD8+ T cells depends on CD62L/L‐selectin‐mediated homing to peripheral LN

Adoptive transfer of naïve CD8⁺ T cells into lymphopenic recipients results both in spontaneous proliferation and in partial activation of T cells, a phenomenon termed homeostatic proliferation (HP). HP of CD8⁺ T cells is dependent on host IL‐7, IL‐15, and MHC‐class I and has been shown to prevent T‐cell tolerance, reverse T‐cell anergy and support T‐cell‐mediated tumor control in vivo. However, the initial anatomic site of HP is still under debate. Since we observed that the earliest detectable HP occurs within LN and that T cells undergoing HP retain a CD62L^bright phenotype, we investigated the functional role of CD62L for this process. We found that CD62L‐expression on T cells is required for optimal HP and HP was impaired in lymphotoxin‐αβ^?/? mice, indicating the necessity for intact host secondary lymphoid organ structures. Use of the LN egression inhibitor FTY720 indicated that LN structures were pivotal to yield homeostatically proliferated T cells detected in other compartments. Consistent with these results, HP‐supported control of MC57‐SIY tumors depended on CD62L. Our data indicate a critical role for CD62L and LN homing for the process of HP, which has implications for adoptive immunotherapy approaches of cancer.     

Lack of TNFR2 expression by CD4+ T cells exacerbates experimental colitis

TNF plays fundamental roles in the induction and perpetuation of inflammation. The effects of TNF are mediated through TNF receptor (TNFR) 1 or 2. As these two receptors mediate different functions, selective targeting of one receptor may represent a more specific treatment for inflammatory disorders than the complete blocking of TNF. TNFR2 expression is up‐regulated in inflammatory bowel disease. Hence, we directly assessed the role of TNFR2 signaling in the CD4⁺ T‐cell transfer model of colitis using TNFR2^?/? or WT mice as donors of colitogenic CD4⁺CD45RB^hi T cells for transfer into syngeneic RAG2^?/? or RAG2^?/?TNFR2^?/? recipient mice. Although the absence of TNFR2 expression by non‐lymphoid cells of the recipient mice does not influence the course of colitis, transfer of TNFR2^?/? CD4⁺ T cells leads to an accelerated onset of disease and to more severe signs of inflammation. The enhanced colitogenic potential of TNFR2^?/? CD4⁺ T cells is associated with reduced activation‐induced cell death, resulting in an increased accumulation of TNFR2^?/? CD4⁺ T cells. Hence, TNFR2 signaling is crucial for the TNF‐dependent contraction of the disease‐inducing T cells. Therefore, a selective blocking of TNFR2 may lead to exacerbation rather than attenuation of T‐cell‐mediated inflammatory disorders.     

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.     

B and CD4+ T‐cell expression of TLR2 is critical for optimal induction of a T‐cell‐dependent humoral immune response to intact Streptococcus pneumoniae

TLR2^?/? mice immunized with Streptococcus pneumoniae (Pn) elicit normal IgM, but defective CD4⁺ T‐cell‐dependent type 1 IgG isotype production, associated with a largely intact innate immune response. We studied the T‐cell‐dependent phosphorylcholine (PC)‐specific IgG3 versus the T‐cell‐independent IgM response to Pn to determine whether TLR2 signals directly via the adaptive immune system. Pn‐activated TLR2^?/? BMDC have only a modest defect in cytokine secretion, undergo normal maturation, and when transferred into naïve WT mice elicit a normal IgM and IgG3 anti‐PC response, relative to WT BMDC. Pn synergizes with BCR and TCR signaling for DNA synthesis in purified WT B and CD4⁺T cells, respectively, but is defective in cells lacking TLR2. Pn primes TLR2^?/? mice for a normal CD4⁺ T‐cell IFN‐γ recall response. Notably, TLR2^?/? B cells transferred into RAG‐2^?/? mice with WT CD4⁺T cells, or TLR2^?/? CD4⁺T cells transferred into athymic nude mice, each elicit a defective IgG3, in contrast to normal IgM, anti‐PC response relative to WT cells. These data are the first to demonstrate a major role for B‐cell and CD4⁺ T‐cell expression of TLR2 for eliciting an anti‐bacterial humoral immune response.     

IL-10 contributes to the suppressive function of tumour-associated myeloid cells and enhances myeloid cell accumulation in tumours

Studies have revealed that tumour‐associated myeloid cells (TAMC) are one of the major sources of IL‐10 in tumour‐bearing mice. However, the significance of TAMC‐derived IL‐10 in tumour immunity is poorly understood. Here, we show that IL‐10 blockade or IL‐10 deficiency reduces the capacity of TAMC in suppressing the proliferation of P1A‐specific CD8 T cells. In the spleen, IL‐10‐deficient and wild‐type (WT) mice bearing large tumour burdens have similar TAMC populations. The tumours from IL‐10‐deficient mice, however, have reduced numbers of TAMC compared with tumours from their WT counterparts. IL‐10^?/?RAG‐2^?/? mice also had reduced numbers of TAMC compared with tumours from IL‐10^+/+RAG‐2^?/? mice; therefore, the reduction in TAMC in IL‐10‐deficient tumours was not because of adaptive immune response in tumours. Adoptively transferred tumour antigen–specific CD8 T cells expanded more efficiently within tumours in IL‐10^?/?RAG‐2^?/? mice than in tumours from IL‐10^+/+RAG‐2^?/? mice. Cytotoxic T lymphocyte adoptive transfer therapy prevented tumour evasion in IL‐10^?/?RAG‐2^?/? mice more efficiently than in IL‐10^+/+RAG‐2^?/? mice. Thus, IL‐10 enhances the accumulation of myeloid cells in tumours, and TAMC‐derived IL‐10 suppresses the activation and expansion of tumour antigen–specific T cells.     

A novel experimental model of Cryptococcus neoformans‐related immune reconstitution inflammatory syndrome (IRIS) provides insights into pathogenesis

Antiretroviral therapy (ART) has yielded major advances in fighting the HIV pandemic by restoring protective immunity. However, a significant proportion of HIV patients co‐infected with the opportunistic fungal pathogen Cryptococcus neoformans paradoxically develops a life‐threatening immune reconstitution inflammatory syndrome (IRIS) during antiretroviral therapy. Despite several clinical studies, the underlying pathomecha‐nisms are poorly understood. Here, we present the first mouse model of cryptococcal IRIS that allows for a detailed analysis of disease development. Lymphocyte‐deficient RAG‐1^?/? mice are infected with C. neoformans and 4 weeks later adoptively transferred with purified CD4⁺ T cells. Reconstitution of CD4⁺ T cells is sufficient to induce a severe inflammatory disease similar to clinical IRIS in C. neoformans‐infected RAG‐1^?/? mice of different genetic backgrounds and immunological phenotypes (i.e. C57BL/6 and BALB/c). Multiorgan inflammation is accompanied by a systemic release of distinct proinflammatory cytokines, i.e. IFN‐γ, IL‐6, and TNF‐α. IRIS development is characterized by infection‐dependent activation of donor CD4⁺ T cells, which are the source of IFN‐γ. Interestingly, IFN‐γ‐mediated effects are not required for disease induction. Taken together, this novel mouse model of cryptococcal IRIS provides a useful tool to verify potential mechanisms of pathogenesis, revealing targets for diagnosis and therapeutic interventions.     

A stromal cell free culture system generates mouse pro‐T cells that can reconstitute T‐cell compartments in vivo

T‐cell lymphopenia following BM transplantation or diseases such as AIDS result in immunodeficiency. Novel approaches to ameliorate this situation are urgently required. Herein, we describe a novel stromal cell free culture system in which Lineage^?Sca1⁺c‐kit⁺ BM hematopoietic progenitors very efficiently differentiate into pro‐T cells. This culture system consists of plate‐bound Delta‐like 4 Notch ligand and the cytokines SCF and IL‐7. The pro‐T cells developing in these cultures express CD25, CD117, and partially CD44; express cytoplasmic CD3ε; and have their TCRβ locus partially D–J rearranged. They could be expanded for over 3 months and used to reconstitute the T‐cell compartments of sublethally irradiated T‐cell‐deficient CD3ε^?/? mice or lethally irradiated WT mice. Pro‐T cells generated in this system could partially correct the T‐cell lymphopenia of pre‐Tα^?/? mice. However, reconstituted CD3ε^?/? mice suffered from a wasting disease that was prevented by co‐injection of purified CD4⁺ CD25^high WT Treg cells. In a T‐cell‐sufficient or T‐lymphopenic setting, the development of disease was not observed. Thus, this in vitro culture system represents a powerful tool to generate large numbers of pro‐T cells for transplantation and possibly with clinical applications.     

Effects of the novel immunosuppressant FTY720 in a murine rheumatoid arthritis model

We investigated the effects and mechanisms by which FTY720 (FTY) inhibits arthritis development in the SKG mouse rheumatoid arthritis (RA) model. FTY (1 mg/kg/day) administration suppressed the progression of laminarin-induced arthritis in SKG mice. FTY treatment decreased IL-6 and TNF-α expression in synovial fibroblast cells and the number of inflammatory cells overall. Bone destruction was also suppressed by treatment with FTY. The numbers of CD4⁺ and CD8⁺ T cells were significantly increased in the thymus and decreased in the spleen in FTY-treated SKG mice. FTY enhanced IL-4 production by CD4⁺ T cells stimulated by allogeneic spleen cells and inhibited prostaglandin E₂ (PGE₂) production by a TNF-α-stimulated synovial fibroblast cell line. These results indicate that FTY can inhibit arthritis in SKG mice via sequestration of autoimmune CD4⁺ T cells in the thymus, enhancement of Th2 immune responses, and inhibition of PGE₂ production by synovial cells.     

Lung-resident CD4+ T cells are sufficient for IL-4Rα-dependent recall immunity to Nippostrongylus brasiliensis infection

Immunity to Nippostrongylus brasiliensis reinfection requires pulmonary CD4⁺ T-cell responses. We examined whether secondary lymphoid recruited or pre-existing lung CD4⁺ T-cell populations coordinated this immunity. To do this, we blocked T-cell egress from lymph nodes using Fingolimod (FTY720). This impaired host ability to resolve a primary infection but did not change effectiveness of recall immunity. Associated with this effective recall immunity was the expansion and T helper type 2 polarization of a pre-existing pulmonary CD4⁺ T-cell population. LTβR-Ig (lymphotoxin beta-receptor fusion protein)-mediated disruption of stromal cell organization of immune cells did not disrupt this recall immunity, suggesting that protection was mediated by a pulmonary interstitial residing CD4⁺ T-cell population. Adoptive transfer of N. brasiliensis-experienced pulmonary CD4⁺ T cells from FTY720-treated wild-type or T-cell interleukin (IL)-4Rα-deficient mice demonstrated protection to be IL-4Rα dependent. These results show that pre-existing CD4⁺ T cells can drive effective recall immunity to N. brasiliensis infection independently of T-cell recruitment from secondary lymphoid organs.     

Fingolimod targeting protein phosphatase 2A differently affects IL‐33 induced IL‐2 and IFN‐γ production in CD8+ lymphocytes

Multiple sclerosis patients are treated with fingolimod (FTY720), a prodrug that acts as an immune modulator. FTY720 is first phosphorylated to FTY720‐P and then internalizes sphingosine‐1‐phosphate receptors, preventing lymphocyte sequestration. IL‐33 is released from necrotic endothelial cells and contributes to MS severity by coactivating T cells. Herein we analyzed the influence of FTY720, FTY720‐P, and S1P on IL‐33 induced formation of IL‐2 and IFN‐ γ , by using IL‐33 receptor overexpressing EL4 cells, primary CD8 ⁺ T cells, and splenocytes. EL4‐ST2 cells released IL‐2 after IL‐33 stimulation that was inhibited dose‐dependently by FTY720‐P but not FTY720. In this system, S1P increased IL‐2, and accordingly, inhibition of S1P producing sphingosine kinases diminished IL‐2 release. In primary CD8 ⁺ T cells and splenocytes IL‐33/IL‐12 stimulation induced IFN‐ γ , which was prevented by FTY720 but not FTY720‐P, independently from intracellular phosphorylation. The inhibition of IFN‐ γ by nonphosphorylated FTY720 was mediated via the SET/protein phosphatase 2A (PP2A) pathway, since a SET peptide antagonist also prevented IFN‐ γ formation and the inhibition of IFN‐ γ by FTY720 was reversible by a PP2A inhibitor. While our findings directly improve the understanding of FTY720 therapy in MS, they could also contribute to side effects of FTY720 treatment, like progressive multifocal leukoencephalopathy, caused by an insufficient immune response to a viral infection.     

NR4A1‐dependent Ly6Clow monocytes contribute to reducing joint inflammation in arthritic mice through Treg cells

Monocytes are central to the physiopathology of arthritis, but their roles in progression and resolution of the disease remain to be clarified. Using NR4A1^?/? mice, which lack patrolling lymphocyte antigen 6C (Ly6C^low) monocytes, we found that inflammatory Ly6C^high monocytes contribute to rapid development of arthritis in a serum transfer‐induced arthritis (STIA) model. Our experiments suggest that patrolling monocytes do not promote the initiation and progression of arthritis in mice, as severity of symptoms was amplified in NR4A1^?/? mice. Moreover, we show that treatment of arthritic wild type (WT) mice with cytosporone B (Csn‐B), a NR4A1‐specific agonist, significantly reduces severity of disease. Effects of Csn‐B were absent in monocyte‐depleted mice treated with clodronate until Ly6C^low monocytes were restored. Adoptive transfer of Ly6C^low monocytes in arthritic NR4A1^?/? mice treated with Csn‐B reduces joint inflammation, supporting the regulatory role of Ly6C^low subset on disease development. Our results also reveal that administration of Csn‐B to arthritic mice enhances levels of circulating CD4⁺CD25⁺FoxP3⁺ Treg cells, a process requiring the presence of Ly6C^low monocytes. Together, these data indicate that Ly6C^high monocytes are involved in the initiation and progression of arthritis and Ly6C^low monocytes contribute to reduce joint inflammation through the mobilization of Treg cells.     

The in vivo induction of lymphocyte apoptosis in MRL-lpr/lpr mice treated with FTY720

The in vitro treatment of lpr thymocytes with FTY720 resulted in a dose-dependent reduction in cell viability due to apoptosis. In order to study the effect of FTY720 in vivo, lpr mice received an oral daily dose of 1 mg/kg FTY720 for 14 days, beginning at 16 weeks of age which was when the animals were developing massive lymphoadenopathy. Compared with untreated lpr mice, FTY720-treated lpr mice had significantly prolonged lives. At 24 weeks of age, treated mice demonstrated markedly reduced weights of the spleen and lymph nodes, and the proportion of CD3⁺B220⁺ and CD4⁻CD8⁻ cells in the thymus, spleen and lymph nodes decreased markedly. In addition, in these mice the percentage of CD4⁺CD8⁺ and CD3⁻B220⁻ cells in the thymus and the percentage of CD4⁺CD8⁻, CD4⁻CD8⁺, CD3⁺B220⁻ and CD3⁻B220⁺ cells in the spleen returned to almost the normal values observed in wild-type mice. Histological observation 1 day after the final administration of FTY720 revealed a remarkable infiltration of neutrophils in the lymphoid organs. Apoptotic cells were detected in all the lymphoid organs using in situDNA nick-end labelling. Electron microscopy showed that the apoptotic cells were ingested by phagocytes. FTY720 therapy is thus highly effective in Fas-mutant animals with abnormally expanding lymphocytes.     

人源化NOD/SCID小鼠免疫细胞的动态变化与鉴定

目的： 比较脐血干细胞与单个核细胞移植NOD/SCID鼠所建立的人源化SCID模型，分析人源化淋巴细胞重建。方法： 磁珠分选法分离脐血中CD34⁺细胞，淋巴细胞分层液分离脐血单个核细胞，分别经尾静脉输入NOD/SCID小鼠。每隔2周采血至10周，流式细胞术动态检测人源淋巴细胞CD45、CD19、CD3抗原。第10周处死小鼠收集外周血、骨髓、胸腺组织，RT-PCR检测模型鼠组织中人β₂M基因及RAG2基因。结果： 两种类型细胞移植均可重建人源免疫细胞，人源淋巴细胞表达水平均在第8周达高峰。骨髓中人源淋巴细胞表达水平明显高于外周血。RT-PCR在外周血与骨髓检测到人β₂M基因及RAG2基因标志。结论： CD34⁺细胞移植重建人源化NOD/SCID免疫系统模型效果要好于脐血单个核细胞。人源T淋巴细胞在模型鼠骨髓中分化成熟。     

CD137 ligand signaling enhances myelopoiesis during infections

CD137 and its ligand are expressed in the BM, and conflicting data exist on the regulation of myelopoiesis by the CD137 receptor–ligand system. CD137^?/? mice have increased numbers of myeloid cells in the BM, indicating an inhibitory influence of CD137 on myelopoiesis. However, CD137 also induces proliferation of hematopoietic progenitor cells and their myeloid differentiation, arguing for an enhancing effect. Here we hypothesized that this latter case represents the situation during infections since expression of CD137 is activation dependent and strongly enhanced during inflammation. Indeed, infections with Influenza, Bordetella pertussis, Mycobacterium bovis, Bacille Calmette‐Guérin or Escherichia coli or i.p. injection of LPS led to increased numbers of CD137‐expressing cells, especially of CD4⁺ T cells in the BM of mice. Coculture experiments confirmed that CD137 expression enables CD4⁺ T cells to induce proliferation and myeloid differentiation of BM and hematopoietic progenitor cells. CD137 also enhances myelopoiesis in vivo since the infection‐induced increase in myeloid cell proliferation and total myeloid cell numbers in the BM were significantly lower in CD137^?/? mice. This study reconciles earlier conflicting data by demonstrating that while CD137–CD137L interactions inhibit myelopoiesis during steady‐state conditions they increase myelopoiesis during infection.     

CD4+ T‐cell development in a mouse expressing a transgenic TCR derived from a Treg

CD4⁺Foxp3⁺ Treg maintain peripheral tolerance and influence immune responses to foreign antigens. The thymus is an important source of Treg, but controversy exists as to whether T cells are selected into the Treg lineage based on signals received through TCR specific for self‐peptides. To examine the specificity of TCR expressed by Treg and its effect on CD4⁺ T‐cell development, we generated Treg‐TCR transgenic mice. Deletion of >90% of CD4⁺ T cells in RAG‐sufficient mice, and nearly 100% deletion in RAG^?/? mice expressing this TCR indicate that the TCR is specific for an unknown, naturally expressed peptide in the thymus. Deletion occurs late in development, suggesting this peptide is presented by APC in the thymic medulla. These studies are the first to describe the effects of expressing a Treg‐TCR on CD4⁺ T‐cell development. The implications of our data for models of Treg selection are discussed.