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Zhao J, Hu R, Lai KC, Zhang Z, Lai L. Recombinant FOXN1 fusion protein increases T cell generation in old mice. Front Immunol 2024; 15:1423488. [PMID: 39072332 PMCID: PMC11272594 DOI: 10.3389/fimmu.2024.1423488] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/25/2024] [Accepted: 07/02/2024] [Indexed: 07/30/2024] Open
Abstract
T cell development in the thymus is dependent on the thymic microenvironment, in which thymic epithelial cells (TECs) are the major component. However, TECs undergo both a qualitative and quantitative loss during aging, which is believed to be the major factor responsible for age-dependent thymic atrophy. FOXN1 plays a critical role in TEC development and adult TECs maintenance. We have previously reported that intrathymic injection of a recombinant (r) protein containing murine FOXN1 and a protein transduction domain increases the number of TECs in mice, leading to enhanced thymopoiesis. However, intrathymic injection may not be an ideal choice for clinical applications. In this study, we produced a rFOXN1 fusion protein containing the N-terminal of CCR9, human FOXN1 and a protein transduction domain. When injected intravenously into 14-month-old mice, the rFOXN1 fusion protein enters the thymus and TECs, and enhances thymopoiesis, resulting in increased T cell generation in the thymus and increased number of T cells in peripheral lymphoid organ. Our results suggest that the rFOXN1 fusion protein has the potential to be used in preventing and treating T cell immunodeficiency in older adults.
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Affiliation(s)
- Jin Zhao
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, United States
| | - Rong Hu
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, United States
| | - Kuan Chen Lai
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, United States
| | - Zhenzhen Zhang
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, United States
| | - Laijun Lai
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, United States
- Institute for Systems Genomics, University of Connecticut, Storrs, CT, United States
- University of Connecticut Stem Cell Institute, University of Connecticut, Storrs, CT, United States
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2
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Su M, Hu R, Song Y, Liu Y, Lai L. Targeted deletion of c-Met in thymic epithelial cells leads to an autoimmune phenotype. Immunol Cell Biol 2017; 96:229-235. [PMID: 29363160 PMCID: PMC5825253 DOI: 10.1111/imcb.1026] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2017] [Revised: 10/16/2017] [Accepted: 10/17/2017] [Indexed: 12/01/2022]
Abstract
Hepatocyte growth factor (HGF) and its receptor c-Met signaling have been implicated in regulating various types of cells including epithelial cells. We have previously reported that c-Met is expressed by thymic epithelial cells (TECs), and that in vivo administration of hybrid cytokines containing IL-7 and the beta- or alpha-chain of HGF significantly increase the number of TECs. In order to study the role of c-Met signaling in TECs, we generated conditional knockout (cKO) mice in which c-Met was specifically deleted in TECs using a Foxn1-Cre transgene. We show here that c-Met deficiency in TECs results in age-progressive reduction in TEC number and reduced number of regulatory T cells. Consequently, c-Met TEC cKO mice displayed an autoimmune phenotype. Thus, c-Met signaling in TECs is important for the maintenance of TECs and immune self-tolerance.
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Affiliation(s)
- Min Su
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA.,Guizhou Medical University, Guizhou, China
| | - Rong Hu
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA
| | - Yinhong Song
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA.,Medical college, Three Gorges University, Yichang, China
| | - Yalan Liu
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA
| | - Laijun Lai
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA.,University of Connecticut Stem Cell Institute, University of Connecticut, Storrs, CT, USA
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3
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Su M, Lin Y, Cui C, Tian X, Lu X, He Z, Lai L. ESC-derived thymic epithelial cells expressing MOG prevents EAE by central and peripheral tolerance mechanisms. Cell Immunol 2017; 322:84-91. [PMID: 29074250 DOI: 10.1016/j.cellimm.2017.10.007] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2017] [Revised: 09/28/2017] [Accepted: 10/17/2017] [Indexed: 12/20/2022]
Abstract
Experimental autoimmune encephalomyelitis (EAE) is an animal model for multiple sclerosis (MS), and is induced by immunization with disease-causative self-antigens such as myelin oligodendrocyte glycoprotein (MOG). We have previously reported that transplantation of MOG expressing thymic epithelial progenitors (TEPs) derived from 129S6SvEv Tac mouse embryonic stem cells (mESCs) prevented the development of EAE. In this study, we expand our previous studies to show that transplantation of MOG expressing mESC-TEPs derived from C57BL/6 mice also prevents EAE development. Furthermore, by using a MOG-specific T cell receptor (TCR) transgenic mouse model, we demonstrate that both central and peripheral tolerances are involved in the prevention of EAE induced by MOG expressing mESC-TEPs. Our results suggest that transplantation of human ESC-TEPs expressing MOG may provide an effective approach for the induction of MOG-specific immune tolerance, thereby the prevention and treatment of MS.
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Affiliation(s)
- Min Su
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA; Tissue Engineering and Stem Cell Research Center, School of Basic Medical Sciences, Guizhou Medical University, Guizhou, China
| | - Yujun Lin
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA
| | - Cheng Cui
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA
| | - Xiaohong Tian
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA
| | - Xiuling Lu
- Department of Pharmaceutical Sciences, University of Connecticut, Storrs, CT, USA
| | - Zhixu He
- Tissue Engineering and Stem Cell Research Center, School of Basic Medical Sciences, Guizhou Medical University, Guizhou, China; Pediatric Department of the Affiliated Hospital, Guizhou Medical University, Guizhou, China.
| | - Laijun Lai
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA; University of Connecticut Stem Cell Institute, University of Connecticut, Storrs, CT, USA.
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4
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Hu R, Liu Y, Su M, Song Y, Rood D, Lai L. Transplantation of Donor-Origin Mouse Embryonic Stem Cell-Derived Thymic Epithelial Progenitors Prevents the Development of Chronic Graft-versus-Host Disease in Mice. Stem Cells Transl Med 2016; 6:121-130. [PMID: 28170174 PMCID: PMC5442732 DOI: 10.5966/sctm.2016-0012] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2016] [Accepted: 06/16/2016] [Indexed: 01/03/2023] Open
Abstract
Allogeneic hematopoietic stem cell transplantation (HSCT) is a potentially curative therapy for many malignant and nonmalignant diseases. However, chronic graft-versus-host disease (cGVHD) remains a significant cause of late morbidity and mortality after allogeneic HSCT. cGVHD often manifests as autoimmune syndrome. Thymic epithelial cells (TECs) play a critical role in supporting negative selection and regulatory T-cell (Treg) generation. Studies have shown that damage in TECs is sufficient to induce cGVHD. We have previously reported that mouse embryonic stem cells (mESCs) can be selectively induced to generate thymic epithelial progenitors (TEPs) in vitro. When transplanted in vivo, mESC-TEPs further develop into TECs that support T-cell development. We show here that transplantation of donor-origin mESC-TEPs into cGVHD recipients induces immune tolerance to both donor and host antigens and prevents the development of cGVHD. This is associated with more TECs and Tregs. Our results suggest that embryonic stem cell-derived TEPs may offer a new tool to control cGVHD. Stem Cells Translational Medicine 2017;6:121-130.
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Affiliation(s)
- Rong Hu
- Department of Allied Health Sciences, University of Connecticut, Storrs, Connecticut, USA
- Guizhou Medical University, Guizhou, People's Republic of China
| | - Yalan Liu
- Department of Allied Health Sciences, University of Connecticut, Storrs, Connecticut, USA
| | - Min Su
- Department of Allied Health Sciences, University of Connecticut, Storrs, Connecticut, USA
- Guizhou Medical University, Guizhou, People's Republic of China
| | - Yinhong Song
- Department of Allied Health Sciences, University of Connecticut, Storrs, Connecticut, USA
| | - Debra Rood
- Department of Allied Health Sciences, University of Connecticut, Storrs, Connecticut, USA
| | - Laijun Lai
- Department of Allied Health Sciences, University of Connecticut, Storrs, Connecticut, USA
- University of Connecticut Stem Cell Institute, University of Connecticut, Storrs, Connecticut, USA
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5
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Song Y, Su M, Zhu J, Di W, Liu Y, Hu R, Rood D, Lai L. FOXN1 recombinant protein enhances T-cell regeneration after hematopoietic stem cell transplantation in mice. Eur J Immunol 2016; 46:1518-28. [PMID: 27125859 DOI: 10.1002/eji.201546196] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/16/2015] [Revised: 02/22/2016] [Accepted: 04/22/2016] [Indexed: 12/26/2022]
Abstract
A prolonged period of T-cell recovery is the major challenge in hematopoietic stem cell transplantation (HSCT). Thymic epithelial cells (TECs) are the major component of the thymic microenvironment for T-cell generation. However, TECs undergo degeneration over time. FOXN1 plays a critical role in TEC development and is required to maintain adult TECs for thymopoiesis. To investigate the potential application of FOXN1, we have cloned and expressed recombinant FOXN1 protein (rFOXN1) that was fused with cell-penetrating peptides. We show here that the rFOXN1 protein can translocate from the cell surface into the cytoplasm and nucleus. Administration of rFOXN1 into both congenic and allogeneic HSCT recipient mice increased the number of TECs, resulting in enhanced thymopoiesis that led to an increased number of functional T cells in the periphery. The increased number of TECs is due to the enhanced survival and proliferation of TECs. Our results suggest that rFOXN1 has the potential to be used in enhancing T-cell regeneration in patients following HSCT.
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Affiliation(s)
- Yinhong Song
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA
| | - Min Su
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA
| | - Jing Zhu
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA.,Department of Obstetrics and Gynecology, Shanghai JiaoTong University School of Medicine, Shanghai, China
| | - Wen Di
- Department of Obstetrics and Gynecology, Shanghai JiaoTong University School of Medicine, Shanghai, China
| | - Yalan Liu
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA
| | - Rong Hu
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA
| | - Debra Rood
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA
| | - Laijun Lai
- Department of Allied Health Sciences, University of Connecticut, Storrs, CT, USA.,University of Connecticut Stem Cell Institute, University of Connecticut, Storrs, CT, USA
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6
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Administration of embryonic stem cell-derived thymic epithelial progenitors expressing MOG induces antigen-specific tolerance and ameliorates experimental autoimmune encephalomyelitis. J Autoimmun 2015; 58:36-47. [DOI: 10.1016/j.jaut.2015.01.002] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2014] [Revised: 12/24/2014] [Accepted: 01/05/2015] [Indexed: 12/29/2022]
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7
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Good Syndrome, a rare cause of refractory chronic diarrhea and recurrent pneumonia in a Chinese patient after thymectomy. CLINICAL AND VACCINE IMMUNOLOGY : CVI 2013; 20:1097-8. [PMID: 23697574 DOI: 10.1128/cvi.00141-13] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
The diagnosis of Good syndrome is very difficult. It has various symptoms, and these symptoms can be present at different periods. In this report we present a patient with refractory chronic diarrhea, recurrent pneumonia, and dysgammaglobulinemia after thymectomy, who was finally then diagnosed with Good syndrome.
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8
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Le Douarin NM, Dieterlen-Lièvre F. How studies on the avian embryo have opened new avenues in the understanding of development: a view about the neural and hematopoietic systems. Dev Growth Differ 2012; 55:1-14. [PMID: 23278669 DOI: 10.1111/dgd.12015] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2012] [Revised: 10/16/2012] [Accepted: 10/17/2012] [Indexed: 01/26/2023]
Abstract
The chick embryo is as ancient a source of knowledge on animal development as the very beginning of embryology. Already, at the time of Caspar Friedrich Wolff, contemplating the strikingly beautiful scenario of the germ deploying on the yellow background of the yolk inspired and supported the tenants of epigenesis at the expense of the preformation theory. In this article, we shall mention some of the many problems of developmental biology that were successfully clarified by research on chick embryos. Two topics, the development of the neural system and that of blood and blood vessels, familiar to the authors, will be discussed in more detail.
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Sairafi D, Mattsson J, Uhlin M, Uzunel M. Thymic function after allogeneic stem cell transplantation is dependent on graft source and predictive of long term survival. Clin Immunol 2011; 142:343-50. [PMID: 22227522 DOI: 10.1016/j.clim.2011.12.001] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2011] [Revised: 11/28/2011] [Accepted: 12/03/2011] [Indexed: 01/23/2023]
Abstract
T-cell deficiency after allogeneic stem cell transplantation (ASCT) is common and has major impact on clinical outcome. In this retrospective study 210 patients were analyzed with regards to levels of T-cell receptor excision circles (TRECs) during the first 24 months after transplantation. We could for the first time show a significant correlation between the use of bone marrow grafts and higher TREC levels >6 months post-ASCT (p<0.001). Treatment with anti-thymocyte globulin was correlated with lower TREC levels ≤6 months post-ASCT (p<0.001). Patients with TREC levels above median at 3 months had a superior overall survival, 80% vs. 56% (p=0.002), and lower transplantation-related mortality, 7% vs. 21% (p=0.01). We conclude that graft source and conditioning regimen may have a significant effect on T-cell reconstitution after ASCT and can thus affect outcome. These results strongly support the use of TREC measurement as part of the standard repertoire of immunological monitoring after ASCT.
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Affiliation(s)
- Darius Sairafi
- Center for Allogeneic Stem Cell Transplantation, B87, Karolinska University Hospital, SE-141 86 Stockholm, Sweden.
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10
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Yuan FL, Li X, Lu WG, Xu RS, Zhao YQ, Li CW, Li JP, Chen FH. Regulatory T cells as a potent target for controlling bone loss. Biochem Biophys Res Commun 2010; 402:173-6. [PMID: 20920469 DOI: 10.1016/j.bbrc.2010.09.120] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2010] [Accepted: 09/28/2010] [Indexed: 01/02/2023]
Abstract
Metabolic bone diseases, such as rheumatoid arthritis (RA) and osteoporosis, affect hundreds and millions of people worldwide leading causes of long-term pain and disability. Effective clinical treatment for bone destruction in bone diseases is lacking because the knowledge about molecular mechanisms leading to bone destruction are incompletely understood. Recently, it has been confirmed that regulatory T cells (Tregs) play a crucial role in suppressing the immune response in the pathogenesis of various autoimmune diseases. In vitro, Tregs directly inhibit osteoclasts and differentiation and function. In mice, the injection of Tregs into the TNF transgenic results in enhanced systemic bone density. In addition, it has been shown that increase of Tregs numbers by overexpressing the FoxP3 is effective in the prevention of local and systemic bone destruction. In vivo treatment with anti-CD28 superagonist antibody leading to a stronger increase in Tregs numbers protect against TNF-a-induced bone loss in TNF-transgenic mice. In agreement, Tregs can control ovariectomy-induced bone loss in FoxP3-transgenic mice. In this paper, we will briefly discuss the biological features of Tregs and summarize recent advances on the role of Tregs in the pathogenesis and treatment of bone loss in metabolic bone diseases.
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Affiliation(s)
- Feng-Lai Yuan
- The Third Hospital Affiliated to Nantong University, Wuxi, Jiangsu 214041, China.
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11
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Giraud S, Barrou B, Sebillaud S, Debré P, Klatzmann D, Thomas-Vaslin V. Transient depletion of dividing T lymphocytes in mice induces the emergence of regulatory T cells and dominant tolerance to islet allografts. Am J Transplant 2008; 8:942-53. [PMID: 18341686 DOI: 10.1111/j.1600-6143.2008.02195.x] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
We previously showed that transient depletion of dividing T cells at the time of an allogeneic transplantation induces long-term tolerance to the allograft. Here we investigated the role of homeostatic perturbation and regulatory T cells (Treg) in such tolerance. Transient depletion of dividing T cells was induced at the time of an allogeneic pancreatic islets graft, by administration of ganciclovir for 14 days, into diabetic transgenic mice expressing a thymidine kinase (TK) conditional suicide gene in T cells. Allograft tolerance was obtained in 63% of treated mice. It was not due to global immunosuppression, permanent deletion or anergy of donor-alloantigens specific T cells but to a dominant tolerance process since lymphocytes from tolerant mice could transfer tolerance to naïve allografted recipients. The transient depletion of dividing T cells induces a 2- to 3-fold increase in the proportion of CD4(+)CD25(+)Foxp3(+) Treg, within 3 weeks that persisted only in allograft-bearing mice but not in nongrafted mice. Tolerance with similar increased proportion of Treg cells was also obtained after a cytostatic hydroxyurea treatment in normal mice. Thus, the transient depletion of dividing T cells represents a novel means of immuno-intervention based on disturbance of T-cell homeostasis and subsequent increase in Treg proportion.
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Affiliation(s)
- S Giraud
- UPMC Univ Paris 06, U543, Laboratoire d'Immunologie Cellulaire et Tissulaire, Paris F-75013 France
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Yildiz O, Ozguroglu M, Turna H, Yanmaz T, Kaynak K, Akman C, Cetin SE, Oz B, Celik A. Thymoma with chronic diarrhea: report of two cases and review of the literature. Med Oncol 2007; 24:119-23. [PMID: 17673822 DOI: 10.1007/bf02685913] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2006] [Revised: 11/30/1999] [Accepted: 08/30/2006] [Indexed: 11/27/2022]
Abstract
Thymoma associated with hypogammaglobulinemia and profound susceptibility to recurrent and serious infections was first reported by Good in 1954, after whom it was named as Good's syndrome. Chronic diarrhea associated with thymoma is almost always seen in patients with hypogammaglobulinemia. However, chronic diarrhea in a setting of normal gammaglobulins have been rarely reported. We hereby report two cases of thymoma with normal immune functions, presenting with chronic diarrhea as the only symptom of thymic malignancy. We discussed the etiopathogenic relation between thymic pathology and diarrhea and review the cases of thymoma associated with chronic diarrhea in the English literature.
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Affiliation(s)
- Ozcan Yildiz
- Cerrahpasa Medical Faculty, Department of Internal Medicine, Division of Medical Oncology, Istanbul University, Istanbul, Turkey
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13
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Corbel C, Lemarchandel V, Thomas-Vaslin V, Pelus AS, Agboton C, Roméo PH. Neuropilin 1 and CD25 co-regulation during early murine thymic differentiation. DEVELOPMENTAL AND COMPARATIVE IMMUNOLOGY 2007; 31:1082-94. [PMID: 17374393 DOI: 10.1016/j.dci.2007.01.009] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/28/2006] [Revised: 01/17/2007] [Accepted: 01/26/2007] [Indexed: 05/14/2023]
Abstract
Neuropilin 1 (NP1) is a receptor for both semaphorin and vascular endothelial growth factor expressed by subpopulations of neuronal and endothelial cells. In the immune system, NP1 is present on dendritic and regulatory T cells. Here, we show that NP1 is expressed in the murine thymus, starting on day 12.5 of gestation. In the adult, NP1 is mainly expressed by CD4(-)CD8(-) double negative cells, CD4+CD8+ double positive cells, and CD4+CD25+ regulatory T cells but barely detected in single CD4+ and CD8+ positive thymocytes. Within the CD4(-)CD8(-)CD3(-) (triple-negative, TN) immature cells, NP1 expression starts in TN3 (CD44(-)CD25+) and increases in TN4 (CD44(-)CD25(-)) cells. In order to study the role of NP1 in thymocyte differentiation, we generated mice in which the np1 gene is selectively disrupted in the T-cell lineage. The mutant mice display normal thymocyte, peripheral, conventional and CD4+CD25+Foxp3+ regulatory T-cell populations. However, we observe a down-regulation of the CD25 expression between the TN3 and TN4 stages that is (i) correlated to increased expression of NP1 in control mice and (ii) altered in mutant mice, suggesting that NP1 is co-regulated with CD25 during early immature thymocyte differentiation.
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Affiliation(s)
- Catherine Corbel
- Institut Cochin, Université Paris Descartes, CNRS (UMR 8104), Paris, France.
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14
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Abstract
Recent elucidation of the role of central tolerance in preventing organ-specific autoimmunity has changed our concepts of self/nonself discrimination. This paradigmatic shift is largely attributable to the discovery of promiscuous expression of tissue-restricted self-antigens (TRAs) by medullary thymic epithelial cells (mTECs). TRA expression in mTECs mirrors virtually all tissues of the body, irrespective of developmental or spatio-temporal expression patterns. This review summarizes current knowledge on the cellular and molecular regulation of TRA expression in mTECs, outlines relevant mechanisms of antigen presentation and modes of tolerance induction, and discusses implications for the pathogenesis of autoimmune diseases and other biological processes such as fertility, pregnancy, puberty, and tumor defense.
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Affiliation(s)
- Bruno Kyewski
- Division of Developmental Immunology, Tumor Immunology Program, German Cancer Research Center, 69120 Heidelberg, Germany.
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