SPP1 Macrophage Research: Immune and Tissue Repair Roles

SPP1, also known as osteopontin, is a multifunctional glycoprotein involved in immune modulation and tissue repair. Highly expressed by macrophages in various physiological and pathological conditions, it influences inflammation, wound healing, and fibrosis. Understanding SPP1-expressing macrophages provides insights into their roles in both homeostasis and disease progression.

Research on SPP1 in macrophages has expanded due to its involvement in immune regulation and extracellular matrix remodeling. Identifying macrophage subsets expressing SPP1, their spatial distribution, and interactions with other cells clarifies their impact on health and disease.

Origins And Molecular Profile

SPP1, encoded by the SPP1 gene on chromosome 4 in humans, is a secreted phosphoprotein undergoing extensive post-translational modifications, including phosphorylation, glycosylation, and proteolytic cleavage. These modifications influence its interactions with cellular receptors such as integrins and CD44, mediating cell adhesion, migration, and signaling. Extracellular SPP1 is a key component of the extracellular matrix, binding calcium and hydroxyapatite to support tissue remodeling and mineralization.

Its molecular structure features an intrinsically disordered region, allowing flexibility in ligand interactions across different microenvironments. The presence of arginine-glycine-aspartate (RGD) motifs enables engagement with integrins like αvβ3, αvβ5, and α4β1, supporting cell adhesion and survival. Thrombin cleavage sites generate functional fragments with varying receptor affinities, expanding SPP1’s regulatory potential.

SPP1 expression is tightly controlled by cytokines, growth factors, and mechanical stress. Promoter regions contain binding sites for transcription factors such as SP1, AP-1, and NF-κB, which modulate its expression in response to environmental cues. Epigenetic modifications, including DNA methylation and histone acetylation, further regulate transcription, particularly during inflammation or tissue injury. Alternative splicing produces multiple isoforms with distinct functions, allowing SPP1 to participate in diverse physiological processes.

Macrophage Subsets Expressing SPP1

Distinct macrophage subsets express SPP1 based on their tissue environment and activation state. These cells emerge in response to cytokine gradients, mechanical stress, and metabolic conditions, contributing to tissue remodeling, fibrosis, and inflammatory resolution.

SPP1 expression is elevated in macrophages within fibrotic tissues, tumor-associated macrophages (TAMs), and osteoarthritic joints. Single-cell RNA sequencing has identified SPP1 as a marker for macrophage subsets in fibrotic lung diseases and liver cirrhosis, where they contribute to extracellular matrix deposition. In tumors, SPP1-expressing macrophages often adopt an immunosuppressive phenotype, promoting stromal remodeling and angiogenesis, correlating with poor prognosis and therapy resistance.

Their differentiation and maintenance are driven by signaling pathways such as TGF-β, IL-4, and CSF-1, which shape their transcriptional profile. These macrophages upregulate tissue repair and fibrosis-associated genes, including COL1A1 and FN1, while shifting their metabolism toward oxidative phosphorylation and lipid metabolism. This metabolic adaptation supports their survival in fibrotic and hypoxic environments, reinforcing their role in tissue remodeling.

Spatial Localization And Cell-Cell Interactions

SPP1-expressing macrophages accumulate in regions undergoing active remodeling, such as fibrotic lesions, tumor stroma, and sites of chronic inflammation. Their localization is influenced by chemotactic factors like CCL2 and CSF-1, which recruit monocytes to specific niches where they differentiate into functionally distinct macrophages. In fibrotic organs, these cells are often found in perivascular regions and interstitial spaces, interacting with fibroblasts and endothelial cells to modulate extracellular matrix dynamics.

Direct interactions with stromal cells shape their behavior and influence tissue architecture. Through integrin-mediated binding, they adhere to fibronectin-rich matrices, reinforcing matrix deposition and remodeling. In fibrotic tissues, macrophage-fibroblast crosstalk via paracrine signaling promotes fibroblast activation and collagen synthesis, perpetuating structural changes. Similarly, in tumors, SPP1-expressing macrophages interact with cancer-associated fibroblasts, enhancing tumor stiffness and supporting malignant progression.

In bone, these macrophages localize near osteoclasts and osteoblasts, participating in bone resorption and formation. Their proximity to mineralized surfaces allows them to regulate osteoclast differentiation through direct contact and secreted factors, including SPP1 itself. This positioning is particularly evident in osteoporosis and osteoarthritis, where macrophage-osteoclast interactions contribute to pathological bone loss. Their ability to sense mechanical signals from the extracellular matrix further refines their function, enabling dynamic responses to tissue stiffness.

Roles In Immune Regulation And Tissue Repair

SPP1-expressing macrophages coordinate tissue repair by clearing necrotic debris and apoptotic cells through efferocytosis, limiting secondary damage and priming the microenvironment for regeneration. As they transition from an inflammatory to a reparative phenotype, SPP1 secretion increases, promoting fibroblast recruitment and extracellular matrix remodeling. The glycoprotein binds integrins and CD44 on fibroblasts and epithelial cells, enhancing adhesion and migration necessary for wound closure.

Beyond its structural role, SPP1 modulates signaling pathways that regulate fibroblast activation and myofibroblast differentiation. Its interaction with αvβ3 and αvβ5 integrins stimulates fibronectin and type I collagen production, reinforcing tissue architecture. In fibrotic conditions, excessive matrix deposition leads to organ dysfunction, whereas in controlled settings like wound healing, SPP1 supports inflammation resolution and epithelial restoration. Murine models have shown that SPP1 deficiency delays re-epithelialization and granulation tissue formation, underscoring its importance in normal repair processes.

Relevance In Pathological Conditions

SPP1-expressing macrophages are key players in diseases driven by chronic inflammation and fibrosis. In idiopathic pulmonary fibrosis (IPF), liver cirrhosis, and systemic sclerosis, they contribute to excessive extracellular matrix deposition. In IPF, these macrophages localize within fibrotic foci, interacting with fibroblasts to sustain a pro-fibrotic microenvironment through TGF-β and SPP1 secretion. Elevated SPP1 levels in bronchoalveolar lavage fluid and lung tissue correlate with disease severity, reinforcing its role in fibrotic remodeling. Similar trends are observed in liver cirrhosis, where SPP1-expressing macrophages activate hepatic stellate cells, driving collagen accumulation and liver dysfunction.

In cancer, these macrophages play a significant role in solid tumors with desmoplastic responses, such as pancreatic ductal adenocarcinoma and triple-negative breast cancer. They enhance stromal stiffness, induce angiogenesis, and suppress cytotoxic T-cell activity, creating a tumor-supportive niche. Their presence often predicts poor prognosis by facilitating immune evasion and metastasis. Targeting SPP1-related pathways has emerged as a therapeutic strategy, with studies exploring monoclonal antibodies and small-molecule inhibitors to block its interactions with integrins and CD44. Beyond fibrosis and cancer, these macrophages also contribute to neuroinflammatory disorders like multiple sclerosis, where SPP1 influences microglial activation and blood-brain barrier integrity.

Methods Of Laboratory Identification

Studying SPP1-expressing macrophages requires precise laboratory techniques to characterize their expression patterns, functions, and spatial distribution. Immunohistochemistry (IHC) and immunofluorescence (IF) are commonly used to visualize SPP1 in tissue sections, often co-staining with macrophage markers such as CD68, CD163, and CD206. Advanced imaging techniques, including confocal and multiplexed fluorescent microscopy, enhance spatial resolution, revealing macrophage interactions with surrounding cells.

At the transcriptional level, quantitative PCR (qPCR) and RNA sequencing (RNA-seq) assess SPP1 mRNA expression, particularly in single-cell analyses that define macrophage heterogeneity. Single-cell RNA-seq has identified SPP1-expressing macrophage clusters across different disease states, revealing distinct transcriptional signatures linked to fibrosis, cancer, and chronic inflammation. Flow cytometry and mass cytometry (CyTOF) quantify protein expression in individual cells, using fluorophore- or metal-conjugated antibodies to detect SPP1 alongside macrophage surface markers. Functional assays, such as in vitro macrophage differentiation models, help researchers manipulate signaling pathways like TGF-β and IL-4 to study SPP1 regulation.