{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,3]],"date-time":"2026-08-03T21:58:03Z","timestamp":1785794283240,"version":"3.56.0"},"reference-count":63,"publisher":"Frontiers Media SA","license":[{"start":{"date-parts":[[2021,8,4]],"date-time":"2021-08-04T00:00:00Z","timestamp":1628035200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":["frontiersin.org"],"crossmark-restriction":true},"short-container-title":["Front. Endocrinol."],"abstract":"<jats:p>Butyrophilins, which are members of the extended B7 family of immunoregulators structurally related to the B7 family, have diverse functions on immune cells as co-stimulatory and co-inhibitory molecules. Despite recent advances in the understanding on butyrophilins\u2019 role on adaptive immune cells during infectious or autoimmune diseases, nothing is known about their role in bone homeostasis. Here, we analyzed the role of one specific butyrophilin, namely Btn2a2, as we have recently shown that Btn2a2 is expressed on the monocyte\/macrophage lineage that also gives rise to bone degrading osteoclasts. We found that expression of Btn2a2 on monocytes and pre-osteoclasts is upregulated by the receptor activator of nuclear factor \u03ba-B ligand (RANKL), an essential protein required for osteoclast formation. Interestingly, in Btn2a2-deficient osteoclasts, typical osteoclast marker genes (Nfatc1, cathepsin K, TRAP, and RANK) were downregulated following RANKL stimulation. <jats:italic>In vitro<\/jats:italic> osteoclast assays resulted in decreased TRAP positive osteoclast numbers in Btn2a2-deficient cells. However, Btn2a2-deficient osteoclasts revealed abnormal fusion processes shown by their increased size. <jats:italic>In vivo<\/jats:italic> steady state \u00b5CT and histological analysis of bone architecture in complete Btn2a2-deficient mice showed differences in bone parameters further highlighting the fine-tuning effect of BTN2a2. Moreover, in rheumatoid arthritis patients and experimental arthritis, we detected significantly decreased serum levels of the secreted soluble Btn2a2 protein. Taken together, we identified the involvement of the immunomodulatory molecule Btn2a2 in osteoclast differentiation with potential future implications in basic and translational osteoimmunology.<\/jats:p>","DOI":"10.3389\/fendo.2021.685060","type":"journal-article","created":{"date-parts":[[2021,8,4]],"date-time":"2021-08-04T06:49:26Z","timestamp":1628059766000},"update-policy":"https:\/\/doi.org\/10.3389\/crossmark-policy","source":"Crossref","is-referenced-by-count":7,"title":["RANKL-Induced Btn2a2 \u2013 A T Cell Immunomodulatory Molecule \u2013 During Osteoclast Differentiation Fine-Tunes Bone Resorption"],"prefix":"10.3389","volume":"12","author":[{"given":"Michael","family":"Frech","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Gregor","family":"Schuster","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Fabian T.","family":"Andes","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Georg","family":"Schett","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mario M.","family":"Zaiss","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kerstin","family":"Sarter","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1965","published-online":{"date-parts":[[2021,8,4]]},"reference":[{"key":"B1","first-page":"i","article-title":"The Burden of Musculoskeletal Conditions at the Start of the New Millennium","volume":"919","year":"2003","journal-title":"World Health Organ Tech Rep Ser"},{"key":"B2","article-title":"Osteoporosis: Diagnosis and Treatment","volume":"46","author":"Lane","year":"1997","journal-title":"Instr Course Lect"},{"key":"B3","doi-asserted-by":"publisher","DOI":"10.1210\/er.2006-0029","article-title":"Insights Into Material and Structural Basis of Bone Fragility From Diseases Associated With Fractures: How Determinants of the Biomechanical Properties of Bone are Compromised by Disease","volume":"28","author":"Chavassieux","year":"2007","journal-title":"Endocr Rev"},{"key":"B4","doi-asserted-by":"publisher","DOI":"10.1126\/science.289.5484.1508","article-title":"Therapeutic Approaches to Bone Diseases","volume":"289","author":"Rodan","year":"2000","journal-title":"Science"},{"key":"B5","doi-asserted-by":"publisher","first-page":"16","DOI":"10.1038\/s41413-018-0019-6","article-title":"Paracrine and Endocrine Actions of Bone-the Functions of Secretory Proteins From Osteoblasts, Osteocytes, and Osteoclasts","volume":"6","author":"Han","year":"2018","journal-title":"Bone Res"},{"key":"B6","doi-asserted-by":"publisher","DOI":"10.1038\/nature01658","article-title":"Osteoclast Differentiation and Activation","volume":"423","author":"Boyle","year":"2003","journal-title":"Nature"},{"key":"B7","doi-asserted-by":"publisher","DOI":"10.1016\/S1097-2765(00)80232-4","article-title":"TRANCE, a TNF Family Member, Activates Akt\/PKB Through a Signaling Complex Involving TRAF6 and C-Src","volume":"4","author":"Wong","year":"1999","journal-title":"Mol Cell"},{"key":"B8","doi-asserted-by":"publisher","DOI":"10.1016\/j.biocel.2009.12.018","article-title":"Nfatc1: Functions in Osteoclasts","volume":"42","author":"Zhao","year":"2010","journal-title":"Int J Biochem Cell Biol"},{"key":"B9","doi-asserted-by":"publisher","DOI":"10.1038\/nature02444","article-title":"Costimulatory Signals Mediated by the ITAM Motif Cooperate With RANKL for Bone Homeostasis","volume":"428","author":"Koga","year":"2004","journal-title":"Nature"},{"key":"B10","doi-asserted-by":"publisher","DOI":"10.1002\/jcp.21511","article-title":"Epidermal Growth Factor Receptor Regulates Osteoclast Differentiation and Survival Through Cross-Talking With RANK Signaling","volume":"217","author":"Yi","year":"2008","journal-title":"J Cell Physiol"},{"key":"B11","doi-asserted-by":"publisher","DOI":"10.1007\/s00251-012-0619-z","article-title":"The Butyrophilin (BTN) Gene Family: From Milk Fat to the Regulation of the Immune Response","volume":"64","author":"Afrache","year":"2012","journal-title":"Immunogenetics"},{"key":"B12","doi-asserted-by":"publisher","DOI":"10.1016\/j.cyto.2009.03.009","article-title":"Regulation of Costimulation in the Era of Butyrophilins","volume":"46","author":"Arnett","year":"2009","journal-title":"Cytokine"},{"key":"B13","doi-asserted-by":"publisher","DOI":"10.4049\/jimmunol.0900416","article-title":"BTN1A1, the Mammary Gland Butyrophilin, and BTN2A2 are Both Inhibitors of T Cell Activation","volume":"184","author":"Smith","year":"2010","journal-title":"J Immunol"},{"key":"B14","doi-asserted-by":"publisher","DOI":"10.4049\/jimmunol.179.6.3804","article-title":"The B7 Homolog Butyrophilin BTN2A1 Is a Novel Ligand for DC-SIGN","volume":"179","author":"Malcherek","year":"2007","journal-title":"J Immunol"},{"key":"B15","doi-asserted-by":"publisher","DOI":"10.1074\/jbc.M109.020446","article-title":"The PRY\/SPRY\/B30.2 Domain of Butyrophilin 1A1 (BTN1A1) Binds to Xanthine Oxidoreductase: Implications for the Function of BTN1A1 in the Mammary Gland and Other Tissues","volume":"284","author":"Jeong","year":"2009","journal-title":"J Biol Chem"},{"key":"B16","doi-asserted-by":"publisher","DOI":"10.4049\/jimmunol.1300658","article-title":"Butyrophilin 3A1 Plays an Essential Role in Prenyl Pyrophosphate Stimulation of Human Vgamma2Vdelta2 T Cells","volume":"191","author":"Wang","year":"2013","journal-title":"J Immunol"},{"key":"B17","doi-asserted-by":"publisher","first-page":"490","DOI":"10.1016\/j.immuni.2014.03.003","article-title":"The Intracellular B30.2 Domain of Butyrophilin 3A1 Binds Phosphoantigens to Mediate Activation of Human Vgamma9Vdelta2 T Cells","volume":"40","author":"Sandstrom","year":"2014","journal-title":"Immunity"},{"key":"B18","doi-asserted-by":"publisher","DOI":"10.1038\/nri3715","article-title":"Immune Modulation by Butyrophilins","volume":"14","author":"Arnett","year":"2014","journal-title":"Nat Rev Immunol"},{"key":"B19","doi-asserted-by":"publisher","DOI":"10.1038\/gene.2009.13","article-title":"Reproducible Association With Type 1 Diabetes in the Extended Class I Region of the Major Histocompatibility Complex","volume":"10","author":"Viken","year":"2009","journal-title":"Genes Immun"},{"key":"B20","doi-asserted-by":"publisher","DOI":"10.18632\/oncotarget.165","article-title":"CD277 Is a Negative Co-Stimulatory Molecule Universally Expressed by Ovarian Cancer Microenvironmental Cells","volume":"1","author":"Cubillos-Ruiz","year":"2010","journal-title":"Oncotarget"},{"key":"B21","doi-asserted-by":"publisher","DOI":"10.4049\/jimmunol.1203325","article-title":"Butyrophilin Btn2a2 Inhibits TCR Activation and Phosphatidylinositol 3-Kinase\/Akt Pathway Signaling and Induces Foxp3 Expression in T Lymphocytes","volume":"190","author":"Ammann","year":"2013","journal-title":"J Immunol"},{"key":"B22","doi-asserted-by":"publisher","DOI":"10.1084\/jem.20150435","article-title":"Btn2a2, a T Cell Immunomodulatory Molecule Coregulated With MHC Class II Genes","volume":"213","author":"Sarter","year":"2016","journal-title":"J Exp Med"},{"key":"B23","doi-asserted-by":"publisher","DOI":"10.1038\/35046102","article-title":"T-Cell-Mediated Regulation of Osteoclastogenesis by Signalling Cross-Talk Between RANKL and IFN-Gamma","volume":"408","author":"Takayanagi","year":"2000","journal-title":"Nature"},{"key":"B24","doi-asserted-by":"publisher","DOI":"10.1084\/jem.20061775","article-title":"Th17 Functions as an Osteoclastogenic Helper T Cell Subset That Links T Cell Activation and Bone Destruction","volume":"203","author":"Sato","year":"2006","journal-title":"J Exp Med"},{"key":"B25","doi-asserted-by":"publisher","DOI":"10.1136\/gutjnl-2014-306947","article-title":"Bone Marrow Th17 Tnfalpha Cells Induce Osteoclast Differentiation, and Link Bone Destruction to IBD","volume":"64","author":"Ciucci","year":"2015","journal-title":"Gut"},{"key":"B26","doi-asserted-by":"publisher","DOI":"10.1007\/s11914-017-0356-1","article-title":"T Regulatory Cells in Bone Remodelling","volume":"15","author":"Bozec","year":"2017","journal-title":"Curr Osteoporos Rep"},{"key":"B27","doi-asserted-by":"publisher","DOI":"10.4049\/jimmunol.0903841","article-title":"Regulatory T Cells Protect From Local and Systemic Bone Destruction in Arthritis","volume":"184","author":"Zaiss","year":"2010","journal-title":"J Immunol"},{"key":"B28","doi-asserted-by":"publisher","DOI":"10.1016\/j.immuni.2008.06.018","article-title":"The Cytokine RANKL Produced by Positively Selected Thymocytes Fosters Medullary Thymic Epithelial Cells That Express Autoimmune Regulator","volume":"29","author":"Hikosaka","year":"2008","journal-title":"Immunity"},{"key":"B29","doi-asserted-by":"publisher","DOI":"10.3389\/fimmu.2012.00261","article-title":"Emerging Functions of RANKL in Lymphoid Tissues","volume":"3","author":"Mueller","year":"2012","journal-title":"Front Immunol"},{"key":"B30","doi-asserted-by":"publisher","DOI":"10.1038\/ni.3732","article-title":"Identification of Subepithelial Mesenchymal Cells That Induce Iga and Diversify Gut Microbiota","volume":"18","author":"Nagashima","year":"2017","journal-title":"Nat Immunol"},{"key":"B31","doi-asserted-by":"publisher","first-page":"e74","DOI":"10.1002\/cpim.74","article-title":"Generation and Analysis of Human and Murine Osteoclasts","volume":"125","author":"Steffen","year":"2019","journal-title":"Curr Protoc Immunol"},{"key":"B32","doi-asserted-by":"publisher","first-page":"108108","DOI":"10.1016\/j.celrep.2020.108108","article-title":"Malic Enzyme Couples Mitochondria With Aerobic Glycolysis in Osteoblasts","volume":"32","author":"Lee","year":"2020","journal-title":"Cell Rep"},{"key":"B33","doi-asserted-by":"publisher","DOI":"10.1038\/nmeth.4197","article-title":"Salmon Provides Fast and Bias-Aware Quantification of Transcript Expression","volume":"14","author":"Patro","year":"2017","journal-title":"Nat Methods"},{"key":"B34","doi-asserted-by":"publisher","first-page":"550","DOI":"10.1186\/s13059-014-0550-8","article-title":"Moderated Estimation of Fold Change and Dispersion for RNA-Seq Data With Deseq2","volume":"15","author":"Love","year":"2014","journal-title":"Genome Biol"},{"key":"B35","doi-asserted-by":"publisher","first-page":"S1","DOI":"10.1186\/ar2165","article-title":"Biology of RANK, RANKL, and Osteoprotegerin","author":"Boyce","year":"2007","journal-title":"Arthritis Res Ther"},{"key":"B36","doi-asserted-by":"publisher","DOI":"10.1073\/pnas.95.7.3597","article-title":"Osteoclast Differentiation Factor Is a Ligand for Osteoprotegerin\/Osteoclastogenesis-Inhibitory Factor and is Identical to TRANCE\/RANKL","volume":"95","author":"Yasuda","year":"1998","journal-title":"Proc Natl Acad Sci USA"},{"key":"B37","doi-asserted-by":"publisher","DOI":"10.1101\/gad.13.18.2412","article-title":"RANK is Essential for Osteoclast and Lymph Node Development","volume":"13","author":"Dougall","year":"1999","journal-title":"Genes Dev"},{"key":"B38","doi-asserted-by":"publisher","DOI":"10.1002\/jbmr.5650050716","article-title":"Impairment of Macrophage Colony-Stimulating Factor Production and Lack of Resident Bone Marrow Macrophages in the Osteopetrotic Op\/Op Mouse","volume":"5","author":"Felix","year":"1990","journal-title":"J Bone Miner Res"},{"key":"B39","doi-asserted-by":"publisher","DOI":"10.1126\/science.7939685","article-title":"C-Fos: A Key Regulator of Osteoclast-Macrophage Lineage Determination and Bone Remodeling","volume":"266","author":"Grigoriadis","year":"1994","journal-title":"Science"},{"key":"B40","doi-asserted-by":"publisher","DOI":"10.1038\/nm1197-1285","article-title":"Osteopetrosis in Mice Lacking NF-KappaB1 and NF-KappaB2","volume":"3","author":"Iotsova","year":"1997","journal-title":"Nat Med"},{"key":"B41","doi-asserted-by":"publisher","DOI":"10.1038\/386081a0","article-title":"Osteopetrosis in Mice Lacking Haematopoietic Transcription Factor PU.1","volume":"386","author":"Tondravi","year":"1997","journal-title":"Nature"},{"key":"B42","doi-asserted-by":"publisher","DOI":"10.1002\/jcb.22640","article-title":"Recent Advances in Understanding the Mechanisms of Osteoclast Precursor Fusion","volume":"110","author":"Oursler","year":"2010","journal-title":"J Cell Biochem"},{"key":"B43","doi-asserted-by":"publisher","first-page":"791","DOI":"10.1038\/nm1593","article-title":"Skeletal Remodeling in Health and Disease","volume":"13","author":"Zaidi","year":"2007","journal-title":"Nat Med"},{"key":"B44","doi-asserted-by":"publisher","DOI":"10.1074\/jbc.273.43.28355","article-title":"The TRAF Family of Signal Transducers Mediates NF-Kappab Activation by the TRANCE Receptor","volume":"273","author":"Wong","year":"1998","journal-title":"J Biol Chem"},{"key":"B45","doi-asserted-by":"publisher","DOI":"10.1016\/S0945-053X(00)00108-6","article-title":"Osteopontin: A Versatile Regulator of Inflammation and Biomineralization","volume":"19","author":"Giachelli","year":"2000","journal-title":"Matrix Biol"},{"key":"B46","doi-asserted-by":"publisher","DOI":"10.1359\/jbmr.1998.13.7.1101","article-title":"Mice Lacking Osteopontin Show Normal Development and Bone Structure But Display Altered Osteoclast Formation in Vitro","volume":"13","author":"Rittling","year":"1998","journal-title":"J Bone Miner Res"},{"key":"B47","doi-asserted-by":"publisher","DOI":"10.1080\/08916930701694667","article-title":"Tartrate-Resistant Acid Phosphatase (TRAP) and the Osteoclast\/Immune Cell Dichotomy","volume":"41","author":"Hayman","year":"2008","journal-title":"Autoimmunity"},{"key":"B48","doi-asserted-by":"publisher","DOI":"10.1002\/jcb.20835","article-title":"Secretion of Tartrate-Resistant Acid Phosphatase by Osteoclasts Correlates With Resorptive Behavior","volume":"98","author":"Kirstein","year":"2006","journal-title":"J Cell Biochem"},{"key":"B49","doi-asserted-by":"publisher","DOI":"10.1359\/jbmr.2001.16.10.1747","article-title":"Osteoclastic Resorption, and Osteoporosis Therapy","volume":"16","author":"Zaidi","year":"2001","journal-title":"J Bone Miner Res"},{"key":"B50","doi-asserted-by":"publisher","DOI":"10.1006\/bbrc.2001.5127","article-title":"New Insights Into the Regulation of Cathepsin K Gene Expression by Osteoprotegerin Ligand","volume":"285","author":"Corisdeo","year":"2001","journal-title":"Biochem Biophys Res Commun"},{"key":"B51","doi-asserted-by":"publisher","DOI":"10.1084\/jem.20050645","article-title":"DC-STAMP is Essential for Cell-Cell Fusion in Osteoclasts and Foreign Body Giant Cells","volume":"202","author":"Yagi","year":"2005","journal-title":"J Exp Med"},{"key":"B52","doi-asserted-by":"publisher","DOI":"10.1359\/jbmr.060308","article-title":"RANKL-Induced Expression of Tetraspanin CD9 in Lipid Raft Membrane Microdomain is Essential for Cell Fusion During Osteoclastogenesis","volume":"21","author":"Ishii","year":"2006","journal-title":"J Bone Miner Res"},{"key":"B53","doi-asserted-by":"publisher","DOI":"10.1038\/nm1514","article-title":"V-Atpase V0 Subunit D2-Deficient Mice Exhibit Impaired Osteoclast Fusion and Increased Bone Formation","volume":"12","author":"Lee","year":"2006","journal-title":"Nat Med"},{"key":"B54","doi-asserted-by":"publisher","DOI":"10.3389\/fcell.2017.00054","article-title":"Regulation of Osteoclast Growth and Fusion by mTOR\/Raptor and mTOR\/Rictor\/Akt","volume":"5","author":"Tiedemann","year":"2017","journal-title":"Front Cell Dev Biol"},{"key":"B55","doi-asserted-by":"publisher","DOI":"10.1038\/nature07019","article-title":"Osteoclast Size Is Controlled by Fra-2 Through LIF\/LIF-Receptor Signalling and Hypoxia","volume":"454","author":"Bozec","year":"2008","journal-title":"Nature"},{"key":"B56","doi-asserted-by":"publisher","first-page":"1330","DOI":"10.1016\/j.cell.2021.02.002","article-title":"Osteoclasts Recycle via Osteomorphs During RANKL-Stimulated Bone Resorption","volume":"184","author":"McDonald","year":"2021","journal-title":"Cell"},{"key":"B57","doi-asserted-by":"publisher","DOI":"10.1038\/s41586-019-1105-7","article-title":"Developmental Origin, Functional Maintenance and Genetic Rescue of Osteoclasts","volume":"568","author":"Jacome-Galarza","year":"2019","journal-title":"Nature"},{"key":"B58","doi-asserted-by":"publisher","DOI":"10.1006\/bbrc.1999.1623","article-title":"Activated T Lymphocytes Support Osteoclast Formation In Vitro","volume":"265","author":"Horwood","year":"1999","journal-title":"Biochem Biophys Res Commun"},{"key":"B59","doi-asserted-by":"publisher","DOI":"10.1038\/46303","article-title":"Activated T Cells Regulate Bone Loss and Joint Destruction in Adjuvant Arthritis Through Osteoprotegerin Ligand","volume":"402","author":"Kong","year":"1999","journal-title":"Nature"},{"key":"B60","doi-asserted-by":"publisher","DOI":"10.1002\/art.23138","article-title":"Treg Cells Suppress Osteoclast Formation: A New Link Between the Immune System and Bone","volume":"56","author":"Zaiss","year":"2007","journal-title":"Arthritis Rheum"},{"key":"B61","doi-asserted-by":"publisher","DOI":"10.1002\/art.27535","article-title":"Increased Bone Density and Resistance to Ovariectomy-Induced Bone Loss in Foxp3-Transgenic Mice Based on Impaired Osteoclast Differentiation","volume":"62","author":"Zaiss","year":"2010","journal-title":"Arthritis Rheum"},{"key":"B62","doi-asserted-by":"publisher","first-page":"115335","DOI":"10.1016\/j.bone.2020.115335","article-title":"Type 2 Innate Lymphoid Cells Inhibit the Differentiation of Osteoclasts and Protect From Ovariectomy-Induced Bone Loss","volume":"136","author":"Omata","year":"2020","journal-title":"Bone"},{"key":"B63","doi-asserted-by":"publisher","DOI":"10.1016\/j.celrep.2018.06.005","article-title":"Group 2 Innate Lymphoid Cells Attenuate Inflammatory Arthritis and Protect From Bone Destruction in Mice","volume":"24","author":"Omata","year":"2018","journal-title":"Cell Rep"}],"container-title":["Frontiers in Endocrinology"],"original-title":[],"link":[{"URL":"https:\/\/www.frontiersin.org\/articles\/10.3389\/fendo.2021.685060\/full","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2021,8,4]],"date-time":"2021-08-04T06:49:31Z","timestamp":1628059771000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.frontiersin.org\/articles\/10.3389\/fendo.2021.685060\/full"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2021,8,4]]},"references-count":63,"alternative-id":["10.3389\/fendo.2021.685060"],"URL":"https:\/\/doi.org\/10.3389\/fendo.2021.685060","relation":{},"ISSN":["1664-2392"],"issn-type":[{"value":"1664-2392","type":"electronic"}],"subject":[],"published":{"date-parts":[[2021,8,4]]},"article-number":"685060"}}