Neutrophil-derived apoptotic bodies function as decoy signals to preserve bone integrity during infectious osteomyelitis

Abstract

Bone infection represents a pathological intersection of immunity and skeletal remodeling, where inflammatory responses drive excessive bone loss. Neutrophils dominate the early immune infiltration during infection, yet how neutrophil fate within infected bone marrow influences disease progression remains poorly understood. Here, we identify neutrophil-derived apoptotic bodies (Neut-ABs) as key mediators of bone protection during infectious osteomyelitis. Bone infection triggers extensive neutrophil apoptosis, resulting in the accumulation of Neut-ABs. Integrated transcriptomic and proteomic profiling of these Neut-ABs revealed significant enrichment of TLR2 and the sialyltransferase ST3GAL1, and lectin blotting confirmed that TLR2 is modified with α2,3-linked sialic acids. Mechanistically, α2,3-sialylated TLR2 on Neut-ABs functions as a molecular decoy that competitively interferes with Siglec15–TLR2 interactions in osteoclast precursors, thereby preventing precursor fusion and suppressing bone-resorptive activity. This inhibitory effect requires both the formation and sialylation of Neut-ABs, as it was abolished in Fas-deficient (MRL/Lpr) mice, upon neuraminidase-mediated desialylation, or when using Neut-ABs derived from Tlr2^⁻/⁻ mice. Therapeutic delivery of Neut-ABs markedly attenuates osteolysis in infected mice without altering bacterial clearance. These findings uncover a neutrophil-to-bone signaling axis in which sialylated vesicular decoys selectively suppress osteoclast activation, illuminating a new paradigm linking innate immune turnover to skeletal homeostasis.

References

[1] Takayanagi H. Osteoimmunology: shared mechanisms and crosstalk between the immune and bone systems. Nat. Rev. Immunol., 2007, 7: 292-304

[2] Okamoto K, et al.. Osteoimmunology: the conceptual framework unifying the immune and skeletal systems. Physiol. Rev., 2017, 97: 1295-1349

[3] Walsh MC, et al.. Osteoimmunology: interplay between the immune system and bone metabolism. Annu Rev. Immunol., 2006, 24: 33-63

[4] Luthje FL, Jensen LK, Jensen HE, Skovgaard K. The inflammatory response to bone infection - a review based on animal models and human patients. APMIS, 2020, 128: 275-286

[5] Masters EA, et al.. Skeletal infections: microbial pathogenesis, immunity and clinical management. Nat. Rev. Microbiol, 2022, 20: 385-400

[6] Kumar G, Roger P-M. From crosstalk between immune and bone cells to bone erosion in infection. Int. J. Mol. Sci., 2019, 20: 5154

[7] Putnam NE, et al.. MyD88 and IL-1R signaling drive antibacterial immunity and osteoclast-driven bone loss during Staphylococcus aureus osteomyelitis. PLoS Pathog., 2019, 15: e1007744

[8] Campbell MJ, et al.. RANKL-mediated osteoclast formation is required for bone loss in a murine model of Staphylococcus aureus osteomyelitis. Bone, 2024, 187 117181

[9] Dong Q, et al.. A review of bacterial and osteoclast differentiation in bone infection. Micro. Pathog., 2024, 197 107102

[10] Rigby KM, DeLeo FR. Neutrophils in innate host defense against Staphylococcus aureus infections. Semin. Immunopathol., 2012, 34: 237-259

[11] Chen Y, et al.. The effect of Staphylococcus aureus on innate and adaptive immunity and potential immunotherapy for S. aureus-induced osteomyelitis. Front. Immunol., 2023, 14 1219895

[12] Quarato ER, et al.. Efferocytosis by bone marrow mesenchymal stromal cells disrupts osteoblastic differentiation via mitochondrial remodeling. Cell Death Dis., 2023, 14: 428

[13] Oliveira TC, Gomes MS, Gomes AC. The crossroads between infection and bone loss. Microorganisms, 2020, 8: 1765

[14] Tao S, et al.. Nanocapsuled neutrophil extracellular trap scavenger combating chronic infectious bone destruction diseases. Adv. Sci. (Weinh., Baden.-Wurtt. Ger.), 2025, 12e2411274

[15] Kolaczkowska E, Kubes P. Neutrophil recruitment and function in health and inflammation. Nat. Rev. Immunol., 2013, 13: 159-175

[16] Shi B, Phan TK, Poon IKH. Extracellular vesicles from the dead: the final message. Trends Cell Biol., 2025, 35: 439-452

[17] Poon IKH, et al.. Moving beyond size and phosphatidylserine exposure: evidence for a diversity of apoptotic cell-derived extracellular vesicles in vitro. J. Extracell. Vesicles, 2019, 8 1608786

[18] Marki A, Ley K. The expanding family of neutrophil-derived extracellular vesicles. Immunol. Rev., 2022, 312: 52-60

[19] Shi J, Fujieda H, Kokubo Y, Wake K. Apoptosis of neutrophils and their elimination by Kupffer cells in rat liver. Hepatology, 1996, 24: 1256-1263

[20] Azcutia V, et al.. Sialylation regulates neutrophil transepithelial migration, CD11b/CD18 activation, and intestinal mucosal inflammatory function. JCI Insight, 2023, 8 e167151

[21] Nussbaumer K, Marth JD, Sperandio M. Posttranslational sialylation and its impact on neutrophil recruitment. J. Leukoc. Biol., 2025, 117 qiaf113

[22] Huang T, et al.. St3gal5-mediated sialylation of glyco-CD177 on neutrophils restricts neuroinflammation following CNS injury. Proc. Natl. Acad. Sci. USA, 2025, 122 e2426187122

[23] Kameda Y, et al.. Siglec-15 regulates osteoclast differentiation by modulating RANKL-induced phosphatidylinositol 3-kinase/Akt and Erk pathways in association with signaling Adaptor DAP12. J. Bone Miner. Res. Off. J. Am. Soc. Bone Miner. Res., 2013, 28: 2463-2475

[24] Lizcano A, et al.. Erythrocyte sialoglycoproteins engage Siglec-9 on neutrophils to suppress activation. Blood, 2017, 129: 3100-3110

[25] Lubbers J, Rodriguez E, van Kooyk Y. Modulation of immune tolerance via siglec-sialic acid interactions. Front. Immunol., 2018, 9: 2807

[26] Xu Y, Loison F, Luo HR. Neutrophil spontaneous death is mediated by down-regulation of autocrine signaling through GPCR, PI3Kgamma, ROS, and actin. Proc. Natl. Acad. Sci. USA, 2010, 107: 2950-2955

[27] Yu L, et al.. Apoptotic bodies: bioactive treasure left behind by the dying cells with robust diagnostic and therapeutic application potentials. J. Nanobiotechnol., 2023, 21 218

[28] Klopfenstein N, et al.. Murine models for staphylococcal infection. Curr. Protoc., 2021, 1 e52

[29] Villunger A, Scott C, Bouillet P, Strasser A. Essential role for the BH3-only protein Bim but redundant roles for Bax, Bcl-2, and Bcl-w in the control of granulocyte survival. Blood, 2003, 101: 2393-2400

[30] Chen T, Ren Q, Ma F. New insights into constitutive neutrophil death. Cell Death Discov., 2025, 11: 6

[31] O’Donnell JA, et al.. Fas regulates neutrophil lifespan during viral and bacterial infection. J. Leukoc. Biol., 2015, 97: 321-326

[32] Kennedy AD, DeLeo FR. Neutrophil apoptosis and the resolution of infection. Immunol. Res., 2009, 43: 25-61

[33] Fernandes P, et al.. Intestinal expression of Fas and Fas ligand is upregulated by bacterial signaling through TLR4 and TLR5, with activation of Fas modulating intestinal TLR-mediated inflammation. J. Immunol., 2014, 193: 6103-6113

[34] Nwakoby IE, et al.. Fas-mediated apoptosis of neutrophils in sera of patients with infection. Infect. Immun., 2001, 69: 3343-3349

[35] Dzhagalov I, St John A, He YW. The antiapoptotic protein Mcl-1 is essential for the survival of neutrophils but not macrophages. Blood, 2007, 109: 1620-1626

[36] Croker BA, et al.. Fas-mediated neutrophil apoptosis is accelerated by Bid, Bak, and Bax and inhibited by Bcl-2 and Mcl-1. Proc. Natl. Acad. Sci. USA, 2011, 108: 13135-13140

[37] Wang X, Yousefi S, Simon HU. Necroptosis and neutrophil-associated disorders. Cell Death Dis., 2018, 9: 111

[38] Szybinska A, Lesniak W. P53 dysfunction in neurodegenerative diseases—the cause or effect of pathological changes?. Aging Dis., 2017, 8: 506-518

[39] Aries ML, Hensley-McBain T. Neutrophils as a potential therapeutic target in Alzheimer’s disease. Front Immunol., 2023, 14 1123149

[40] Mittelbrunn M, Sanchez-Madrid F. Intercellular communication: diverse structures for exchange of genetic information. Nat. Rev. Mol. Cell Biol., 2012, 13: 328-335

[41] Liu YJ, Wang C. A review of the regulatory mechanisms of extracellular vesicles-mediated intercellular communication. Cell Commun. Signal, 2023, 21: 77

[42] Gatenby RA. The role of cell membrane information reception, processing, and communication in the structure and function of multicellular tissue. Int. J. Mol. Sci., 2019, 20: 3609

[43] Oikawa T, Kuroda Y, Matsuo K. Regulation of osteoclasts by membrane-derived lipid mediators. Cell. Mol. Life Sci., 2013, 70: 3341-3353

[44] Tian M, et al.. Glycosylation as an intricate post-translational modification process takes part in glycoproteins related immunity. Cell Commun. Signal., 2025, 23: 214

[45] Hiruma Y, Hirai T, Tsuda E. Siglec-15, a member of the sialic acid-binding lectin, is a novel regulator for osteoclast differentiation. Biochem Biophys. Res. Commun., 2011, 409: 424-429

[46] Dou C, et al.. Sialylation of TLR2 initiates osteoclast fusion. Bone Res., 2022, 10: 24

[47] Angata T. Siglec-15: a potential regulator of osteoporosis, cancer, and infectious diseases. J. Biomed. Sci., 2020, 27 10

[48] Kameda Y, et al.. Siglec-15 regulates osteoclast differentiation by modulating RANKL-induced phosphatidylinositol 3-Kinase/Akt and Erk pathways in association with signaling adaptor DAP12. J. Bone Miner. Res., 2013, 28: 2463-2475

[49] Chang L, et al.. Identification of siglec ligands using a proximity labeling method. J. Proteome Res., 2017, 16: 3929-3941

[50] Hiruma Y, et al.. Impaired osteoclast differentiation and function and mild osteopetrosis development in Siglec-15-deficient mice. Bone, 2013, 53: 87-93

[51] Muneyama T, et al.. Histochemical assessment on osteoclasts in long bones of toll-like receptor 2 (TLR2) deficient mice. J. Oral. Biosci., 2023, 65: 163-174

[52] Venkatakrishnan V, et al.. Glycan analysis of human neutrophil granules implicates a maturation-dependent glycosylation machinery. J. Biol. Chem., 2020, 295: 12648-12660

[53] Takegahara N, Kim H, Choi Y. Unraveling the intricacies of osteoclast differentiation and maturation: insight into novel therapeutic strategies for bone-destructive diseases. Exp. Mol. Med., 2024, 56: 264-272

[54] Carmona-Rivera C, Kaplan MJ, O’Neil LJ. Neutrophils in inflammatory bone diseases. Curr. Osteoporos. Rep., 2024, 22: 280-289

[55] Chapple ILC, Hirschfeld J, Kantarci A, Wilensky A, Shapira L. The role of the host-Neutrophil biology. Periodontology 2000, 2023, 00: 1-47

[56] Kalluri R. The biology and function of extracellular vesicles in immune response and immunity. Immunity, 2024, 57: 1752-1768

[57] Crespo HJ, Lau JT, Videira PA. Dendritic cells: a spot on sialic Acid. Front Immunol., 2013, 4: 491

[58] Laubli H, Varki A. Sialic acid-binding immunoglobulin-like lectins (Siglecs) detect self-associated molecular patterns to regulate immune responses. Cell. Mol. Life Sci., 2020, 77: 593-605

[59] Jiang HS, Liang SY, Chou YC, Angata T. Comprehensive identification of the genes involved in the expression of Siglec-15 ligands on osteoclast precursors. Cell Rep., 2026, 45 116880

[60] Kakarla R, Hur J, Kim YJ, Kim J, Chwae YJ. Apoptotic cell-derived exosomes: messages from dying cells. Exp. Mol. Med., 2020, 52: 1-6

[61] Gregory CD, Rimmer MP. Extracellular vesicles arising from apoptosis: forms, functions, and applications. J. Pathol., 2023, 260: 592-608

[62] Miao X, et al.. Tailoring of apoptotic bodies for diagnostic and therapeutic applications: advances, challenges, and prospects. J. Transl. Med., 2024, 22 810

Funding

National Natural Science Foundation of China (National Science Foundation of China)(82572785)

Chongqing Health Sector National Talent Reserve Program (524Z28D)

Southwest Hospital Boqing Innovation Fund (2024BQCXJJ-9)

NSFC Key projects of the Regional Innovation and Development Joint Fund (U23A20413)

Rights & permissions

The Author(s)