# Targeting the colony-stimulating factor 1 axis for the treatment of Tenosynovial Giant Cell Tumors

Paul Humbert, Charlotte Vialle, Marie Chevalerias, Vincent Crenn, Denis

Waast, Marie-Astrid Boutet, Frederic Blanchard, Benoit Le Goff

► To cite this version:

Paul Humbert, Charlotte Vialle, Marie Chevalerias, Vincent Crenn, Denis Waast, et al. Targeting the colony-stimulating factor 1 axis for the treatment of Tenosynovial Giant Cell Tumors. Cytokine and Growth Factor Reviews, 2025, 86, pp.137-148. 10.1016/j.cytogfr.2025.10.007. hal-05390241

HAL Id: hal-05390241

https://hal.science/hal-05390241v1

Submitted on 1 Dec 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

# Targeting the colony-stimulating factor 1 axis for the treatment of Tenosynovial Giant Cell Tumors

Paul Humbert<sup>a, *, †</sup>, Charlotte Vialle<sup>a</sup>, Marie Chevalerias<sup>a</sup>, Vincent Crenn<sup>b, c</sup>, Denis Waast<sup>a</sup>, Marie-Astrid Boutet<sup>a, d</sup>, Frederic Blanchard<sup>a, *, 1</sup>, Benoit Le Goff<sup>a, 1</sup>

## ARTICLE INFO

Keywords:

Tenosynovial giant cell tumors
Colony stimulating factor 1
Macrophages
Tyrosine kinase inhibitors

## ABSTRACT

Diffuse and Localized Tenosynovial Giant Cell Tumors (D-/L-TGCT) are two closely related forms of benign tumors of the joint characterized by uncontrolled macrophage infiltration and proliferation. Both forms arise from similar translocation events in synovial fibroblasts involving the Colony Stimulating Factor 1 (CSF1) gene, the major regulator of myeloid cell survival and activity. This review first describes CSF1 gene structure and protein expression as well as signaling through its receptor (CSF1R). Then, genetic alterations observed in TGCT patients are described in depth. The main features are translocations that remove the 3' regulatory elements of CSF1 messenger RNA, driving its overexpression and the abnormal recruitment of non neoplastic macrophages leading to tumor growth. This review then focuses on the resulting histopathological features characterizing the disease, as well as the available treatments targeting the CSF1 axis. Systemic therapies targeting CSF1 or its receptor are valuable for relapsing or inoperable cases, especially D-TGCT, but their use is still limited by heavy side effects and unresponsive tumors. Overall, our understanding of the pathophysiology of these tumors, the efficacy and limitations of current therapeutic options, and the questions that remain unanswered open new avenues for research and provide opportunities to further improve patient care.

## 1. Introduction

Tenosynovial Giant Cell Tumor (TGCT) is a rare group of benign fibrohistiocytic tumors that arise from the synovium of joints, tendon sheaths or bursae and is characterized by synovial hyperplasia and hemosiderin deposition [1–3]. This disease mainly affects young people with a mean age at diagnosis between 35 and 50 years and is slightly more frequent in females (around 60 % of cases) [4]. The 2020 WHO Classification of Soft Tissue and Bone Tumours defines TGCT as a locally aggressive neoplasm, but sarcomatous transformation with metastatic spread is exceedingly rare with an incidence rate <1/1000,000 [5]. TGCT may cause chronic pain, and severely impact function and quality of life [4,6].

The TGCT family is divided in 2 subgroups represented by the localized and the diffuse forms. Localized TGCT (L-TGCT) (formerly

giant cell tumor of tendon sheath/nodular tenosynovitis) is characterized by a lobular or a nodular masse in one joint, with well-defined borders. L-TGCT commonly affects the digit (up to 85 % of cases) with an incidence rate of 30–34/1000,000. Diffuse TGCT (D-TGCT) (formerly pigmented villonodular synovitis/pigmented villonodular sclerosis) is characterized by more than one mass, can show extensive and infiltrative involvement of the synovium of the joint and/or tendon sheath, may extend into extra-articular structures or cartilage and can lead to bone erosion. D-TGCT has an incidence rate of 5–11/1000,000 and affects more often the knee (up to 64 % of cases) [3,4,6].

In both L- and D-TGCT, the histiocytic component is non neoplastic but is the main cellular contingent. It is composed of different types of myeloid cells such as foam cells, siderophages (hemosiderin-laden macrophages) and osteoclast-like giant cells [1,2]. Although the exact phenotype of neoplastic cells is still debated, it seems to be a small

<footnote><sup>a</sup> Nantes Université, ONIRIS, CHU Nantes, INSERM, Regenerative Medicine and Skeleton, RMeS, UMR, Nantes 1229, France</footnote>

<footnote><sup>b</sup> Nantes Université, CHU Nantes, Orthopedic and Traumatological Surgical Clinic, Nantes, France</footnote>

<footnote><sup>c</sup> Nantes Université, INSERM UMR 1307, CNRS UMR 6075, Université d'Angers, CRCI2NA, Nantes, France</footnote>

<footnote><sup>d</sup> Centre for Experimental Medicine and Rheumatology, William Harvey Research Institute, Barts and The London School of Medicine and Dentistry, Queen Mary University of London, London, United Kingdom</footnote>

<footnote>* Correspondence to: Regenerative Medicine and Skeleton, 1 place Alexis Ricordeau, Nantes 44042, France.
E-mail addresses: paul.humbert@univ-nantes.fr (P. Humbert), frederic.blanchard@univ-nantes.fr (F. Blanchard).</footnote>

<footnote n="1">¹ These authors contributed equally to this work</footnote>
contingent (2–30% of total cells) of synovial fibroblasts (fibroblast-like synoviocytes) presenting a translocation of their CSF1 gene [7–10]. This translocation would result in an over-expression of CSF1 (Colony Stimulating Factor 1 also known as M-CSF for Macrophage Colony Stimulating Factor). This cytokine is well known for its role as a positive regulator of recruitment, proliferation, differentiation and survival of macrophages expressing the CSF1 receptor (CSF1R) [11], and would therefore be responsible for the recruitment/accumulation of these macrophages within the synovium.

Most TGCT patients undergo surgery as first-line treatment, but recurrence is common. In D-TGCT, total resection is often incomplete and surgery can be morbid, leading to post-operative pain, stiffness, and loss of function [12]. In the TGCT Support Patient Registry, 79.1 % of all patients had surgical resection and 63 % of those patients reported tumor recurrence [6]. For those relapsing cases and patients who are not expected to clinically improve from surgical excision, inhibition of the CSF1/CSF1R axis often yields good radiological and clinical responses. However, adverse events may cause treatment discontinuation because of an unfavorable risk-benefit ratio in this benign disease [1,4,6,13].

The aim of this review is to highlight the current knowledge about the CSF1/CSF1R pathway and its role in the pathophysiology of TGCT, and emphasize the benefits and limitations of current therapies targeting CSF1 to suggest future treatment approaches.

## 2. Expression and function of CSF1/CSF1R

### 2.1. Structure and isoforms of CSF1

CSF1 (M-CSF) is a macrophage growth factor that is produced by several cell types such as fibroblasts, endothelial cells or osteoblasts. CSF1 stimulates the chemotaxis, survival, proliferation and differentiation of mononuclear phagocytes, in addition to the spreading and motility of macrophages, as previously reviewed by Pixley and Stanley [14]. It also synergizes with the receptor activator of NF-κB ligand (RANKL) to induce the differentiation of mononuclear phagocytes into multinucleated osteoclasts, the specialized myeloid cells responsible for bone resorption. The human CSF1 gene is located on chromosome 1 (1p13) and contains 10 exons. The five first exons code for the biologically active protein, the 6th to 8th exons for the cleavage, transmembrane and cytoplasmic domains, the 9th and 10th for two alternative 3' untranslated regions (UTR) of the CSF1 mRNA (Fig. 1A) [15,16]. The 3'-UTR encoded by the 10th exon is rich in negative regulatory element (AU-rich elements (ARE), miRNA target sites and a non-canonical G-quadruplex) responsible for a rapid turnover of CSF1 mRNA [17].

CSF1 is a disulfide linked homodimer and three different glycoprotein isoforms can be produced depending on the alternative splicing of mRNA precursors and different proteolytic cleavages: two soluble forms (a glycoprotein and a proteoglycan) and a cell-surface glycoprotein, as illustrated in Fig. 1B. Two different convertase sites dictate differential proteolysis in the secretory vesicle, i.e. either the short conventional CSF1 glycoprotein or the longer CSF1 proteoglycan isoform which carries chondroitin sulfate chains [18]. A specific alternative splicing (noted [1] in Fig. 1A) removes the two convertase sites and the chondroitin sulfate binding site, thereby forcing the transmembrane isoform. All three isoforms contain the amino-acid sequence required for receptor binding and biological activity but would be involved in different regulatory pathways as they are either found in the circulation (conventional glycoprotein isoform), or in tissues, associated to the extracellular matrix (proteoglycan isoform) or to the plasma membrane [16,18]. For example in Csf1-deficient osteopetrotic (op/op) mice, injection of soluble Csf1 completely rescues liver Kupffer cells, partially bone osteoclasts (and osteopetrosis) but has no effect on synovial macrophages. Inversely, forced expression of cell-associated Csf1 rescues synovial macrophages, partially osteoclasts but not Kupffer cells [19,20].

### 2.2. CSF1R and signal transduction

The CSF1 receptor (CSF1R/c-FMS/FMS), a transmembrane receptor with tyrosine kinase activity, is expressed by several cell types, especially monocytes, tissue macrophages and osteoclasts [11,21]. In mononuclear phagocytes, the CSF1R is involved in cell proliferation, survival, differentiation and chemotaxis/motility [11,21]. Fibroblast-macrophage interactions, notably via CSF1/CSF1R, are important to maintain homeostasis but also particulary involved in several pathological contexts, such as fibrosis and cancer [22,23].

The human CSF1R gene is located on chromosome 5 (5q32). The CSF1R possesses a glycosylated extracellular region comprised of five immunoglobulin domains (D1 to D5), a transmembrane domain, and an intracellular domain comprised of a juxtamembrane domain (JMD) and an intracellular tyrosine kinase domain that is interrupted by a kinase insert domain (Fig. 1C) [11]. CSF1R has two ligands, CSF1 and Interleukin-34 (IL-34), which induce the homodimerization of CSF1R and the activation of several downstream signaling pathways. Binding of CSF1 to the CSF1R is exclusively via the D2 and D3 domains, the D4 domains mediating CSF1R homotypic interactions [11]. The kinase domain of CSF1R is composed of the N and C lobes, providing a cleft for ATP binding. Upon activation, the Asp-Phe-Gly (DFG) motif moves into a "DFG-in" conformation and the activation loop (AL) is reorganized to allow substrate binding. The JMD normally prevents the AL from adopting this active conformation and thus confers a critical auto-inhibitory mechanism that is relieved upon CSF1R activation [11].

Following CSF1 binding and dimerization of the CSF1R, multiple intracellular tyrosine residues of CSF1R are phosphorylated, creating docking sites for a variety of downstream intracellular signaling molecules such as Src family kinases (SFK), phosphoinositol-3 kinase (PI3K) and Casitas B lineage (CBL) (Fig. 1C) [11,21]. Macrophage survival is notably increased by CSF1R activation of the PI3K/AKT pathway. Macrophage proliferation is stimulated by the activation of the SFK and ERK5 (extracellular-signal-related kinase 5) pathways. CSF1R activation in monocytes, in synergy with other cytokines, results in their differentiation into immunosuppressive M2 macrophages through activation of the PI3K/AKT and ERK1/2 pathways [11,24]. In arthritic joints, monocyte/macrophage stimulation by CSF1, in collaboration with RANKL and inflammatory cytokines, contributes to the differentiation into osteoclasts which are involved in bone destruction [25]. In macrophages and osteoclasts, CSF1 triggers a rapid membrane ruffling response followed by cell spreading and polarization, thus controlling cell adhesion, migration and phagocytosis. These processes involve a dynamic reorganization of the actin cytoskeleton and focal adhesions through PI3K, DNAX-associated protein 12 kD (DAP12), small GTPases (CDC42, Rac and Rho) and their downstream effectors [11,21].

Following CSF1 stimulation, CBL rapidly associates with CSF1R in macrophages and mediates CSF1R multiubiquitination. This CBL-dependant CSF1R ubiquitination promotes internalization of the CSF1/CSF1R complex via multivesicular bodies, targeting it into the lysosomal pathway where both the ligand and receptor are degraded [26].

## 3. CSF1 alterations in TGCT

The first evidences of chromosomal aberrations in TGCT dates back to the early 1990s with the detection of trisomy 5, trisomy 7 [27,28] and other patterns of aneuploidy [29]. Trisomy 7 turned out to be nonspecific as it could also be found in osteoarthritic and rheumatoid arthritic synovia [30]. However, recurrent genomic breakpoints were identified in the 1p11–13 region with various translocation partners such as 5q22–31, 8q21–22, and 11q11–12, the most frequent being 2q35–37 [31]. Then, an extremely high expression of CSF1R was observed by tissue microarray in TGCT in comparison to other soft-tissue tumors [7]. This initial finding led to the identification by in situ hybridization of a small subset of cells overexpressing the ligand CSF1. Based on the
previously described frequently translocated regions, the COL6A3 (2q37.3) locus was found to be rearranged with the region of CSF1 (1p13.3) in one third of the cases studied. The overexpression of CSF1 was further confirmed at the mRNA level by in-situ hybridization (ISH) and at the protein level by immunohistochemistry (IHC) in a larger cohort alongside rheumatoid arthritis and reactive synovitis samples [9] and, lately, in comparison with other giant cell-rich tumors of bone and soft tissues [32,33]. The expression of CSF1 has been shown to be high and spread throughout the tissue in TGCT samples while it is restricted to only synovial lining cells in reactive or chronic synovitis. This

expression is variable in other giant cell tumors [32,33].

Recently, the neoplastic cells overexpressing CSF1 were identified as lining fibroblast-like synovial cells coexpressing podoplanin and CD90 (PDPN<sup>high</sup>CD90<sup>+</sup>) using flow cytometry [34], and expressing the Platelet-Derived Growth Factor Receptor and the Glutamine-Fructose-6-Phosphate Transaminase 2 (PDGFR<sup>+</sup>GFPT2<sup>+</sup>) using single cell RNA sequencing (scRNAseq) [10]. The YAP/HIPPO and PDGFR signaling pathways were both identified as potent targets in neoplastic fibroblast-like cells [10]. In another study, tumor cells were annotated within other fibroblast subsets using large-scale chromosomal

<figure><img src="image_4.png" /></figure>

(caption on next page)
**Fig. 1.** CSF1 expression and signaling through CSF1R. A. The *CSF1* gene is composed of 10 exons, the translated sequence being restricted to the first eight. Four alternative splicing events can occur (dashed lines): the first [1] removes the two main convertase site of the protein sequence leading to a membrane-bound isoform (see 1B), the second [2] retains the first convertase site leading to the production of the shorter secreted glycoprotein, and the third [3] and fourth [4] give alternative 3'-UTR, the 10th exon being the most frequent. Negative regulatory elements (miRNA binding sites, G-quadruplex and AU-rich elements (ARE)) located in exon 10 confer instability to the *CSF1* transcript. The signal peptide (orange) is encoded by exons 1 and 2 and is followed by the active cytokine (red with a disulfide bond site, S-, and glycosylation sites, pink triangle) up to exon 5. Exon 6 contains most of the sequence for the spacer domain (green), containing a glycosylation site (pink triangle) and a site of glycosaminoglycan addition (blue diamond) between two convertase sites (double-ended arrows). The transmembrane domain (brown) and cytoplasmic domain (yellow) are encoded by exons 6–8. B. The CSF1 precursor is produced as a transmembrane homodimer linked by a disulfide bond (S-S). It is cleaved from the membrane at two convertase sites leading to either a short conventional glycoprotein (first convertase site) or a longer proteoglycan isoform (second convertase site). Both secreted forms can be found in the circulation and the proteoglycan can also be associated to the extracellular matrix. An important alternative splicing [1] removes both convertase sites leading to a membrane-bound isoform. C. CSF1R has an extracellular region comprised of five immunoglobulin (Ig)-like domains, with domains 2–3 responsible for CSF1 binding and domain 4 involved in stabilizing the receptor homodimer upon ligand fixation. The transmembrane domain is followed by an intracellular region divided in five regions: the juxtamembrane domain (JMD), the kinase N and C lobes separated by the kinase insert, and the carboxy-terminal tail. The kinase C lobe contains the Asp-Phe-Gly (DFG) motif (light blue) and the following activation loop (light purple). Upon activation, reorganization of this region opens the ATP-binding pocket between the N and C lobes. Several tyrosine (yellow dots with amino acid position indicated) and lysine (orange dots) residues that can be respectively phosphorylated (P, yellow) or ubiquitinated (Ub, orange) dictate signal transduction and receptor degradation. Notably, phosphorylation at tyrosines 561 and 969 are particularly important for CBL signaling, responsible for multiubiquitination of the receptor, its internalization and degradation. Other phosphorylation sites allow the activation of many signaling pathways simplified here in a few important ones participating in major functions, such as monocytes/macrophages survival and differentiation. For example, Src family kinases (SFK) and phosphoinositol-3 kinase (PI3K) through Cell Division Control protein 42 homolog (CDC42) participate in cell adhesion, migration and phagocytosis. Adapted from [11,21].

copy number variations (CNV) in scRNAseq expression data [35]. However, their CSF1 expression or *CSF1* gene translocation was not determined, questioning the true identity of those “tumor cells”. Nonetheless, because receptor tyrosine kinase-like orphan receptor 1 (ROR1) and serine/threonine-protein kinase D1 (PRKD1) were highly expressed in tumor cells from recurrent D-TGCT, they have been proposed as potential targets and biomarkers of tumor aggressiveness [35].

Finer localization of the breakpoints in *CSF1* and its putative partner gene *COL6A3* could first be obtained by RT-qPCR using several primer pairs at various location on both genes [36]. Using this approach, *COL6A3-CSF1* fusion transcripts were identified in 3 out of 6 cases. The breakpoints have been shown to be downstream of *CSF1* 5th exon, thus affecting the C-terminal sequence of the protein but not the active region binding to the receptor, as illustrated in Fig. 2A. However, the underlying mechanism leading to *CSF1* overexpression cannot be explained by these findings as the promoter was not affected by the translocation. The *S100A10* (1q21.3) gene and an intergenic region downstream of *CSF1* were later identified as other fusion partners by Panagopoulos and colleagues in 2014 [37]. These translocations occur after the 8th exon of *CSF1* which carries the stop codon, they therefore do not impact the protein sequence. Based on this observation, the authors speculated that the overexpression of *CSF1* could be solely due to the loss of the 3'-UTR of the wild-type *CSF1* as it carries microRNA target sequences and AU-rich elements, important for regulating the mRNA stability (Figs. 1A, 2A) [17,38,39]. The use of advanced RNA sequencing protocols have recently helped identifying many other fusion partner genes (or even intergenic non-protein-coding regions) and other translocation sites on *CSF1*. Overall, the removal of the 3'-UTR from the *CSF1* transcript is the only common feature, with events observed as far as the 10th exon [8, 10,40,41]. Different regions close to *CSF1* on chromosome 1 appear to be overrepresented fusion partners (Fig. 2B). In addition, genes known to be expressed in fibroblast-like synoviocytes such as *FN1*, *PDPN*, *PRG4* or collagens are frequent fusion partners, suggesting that spatial proximity and accessible chromatin favor the likeliness of a translocation event. The latest partner identified to date is an endogenous retrovirus gene (*ERV-LTR71B*), adding to the complexity of properly identifying fusion transcripts in TGCTs [42].

Apart from *CSF1* translocations, single nucleotide polymorphisms were detected on the *CBL* gene in 12 out of 34 samples (35 %) [41]. These mutations associate with a higher JAK2 expression and, most importantly, the recurrence rate of the disease. Results from an independent cohort reported 25 % of *CBL* mutations [8]. In both studies, some cases with *CBL* alteration also presented *CSF1* translocations, while others did not. *CBL* is a known proto-oncogene encoding a RING finger ubiquitin ligase involved in regulating signal transduction for several pathways, such as JAK2 [41,43,44]. In mice, Cbl has been shown

to participate in the ubiquitination of Csf1r and its subsequent internalization [26,45]. Consequently, Cbl⁻/⁻ macrophages are more proliferative while utilizing less Csf1 due to the overactivation of the receptor exposed extensively at the cell surface. In TGCT pathogenesis, this mechanism could also take place in subclones of macrophages. The emergence of CBL mutations could therefore be a secondary event after the initial recruitment and abnormal stimulation of macrophages by CSF1. However, one could also imagine a role for CBL in another pathway involved in the regulation of CSF1 expression in the mutated fibroblasts, potentiating the expression of a longer lasting CSF1-fusion mRNA.

The diversity of *CSF1* translocation sites and gene partners reported all lead to similar pathological features, suggesting that *CSF1* over-expression is the main, and perhaps only, driver of the disease. Reports of translocations not involving *CSF1* were often related to cases with atypical clinical presentations, suggesting that they should be considered as distinct tumor subtypes [46,47]. In addition, the sensitivity of the technical approaches used, combined with the scarcity of mutated cells could also prevent from identifying translocations in some studies. The increase in *CSF1* expression could also be due to other types of mutations yet to be identified.

Because all *CSF1* translocations are located after exon 5, they should not substantially impair CSF1 function, as the active portion recognized by the receptor remains intact. However, the different isoforms of the CSF1 protein may be impacted, leading for example to the absence of the membrane-bound isoform in the case of translocations before the transmembrane domain, as presented in Fig. 2A. Moreover, in-frame CSF1 fusion with another functional protein could also lead to different pathological features linked to the characteristics of the associated protein (interactions with other proteins, transmembrane domain etc.). For example, some overrepresented translocation partners are fused with the end of *CSF1* 5th exon, theoretically leading to uncleavable fusion proteins as the main convertase sites are removed. Notably, fusion with *FN1* (7 out of 77 cases) or *COL6A3* (8/77) could confer an affinity for the extracellular matrix, or fusion with *CD101* (8/77) could provide the sequences for a transmembrane domain (Fig. 2A, B). Such a link between the position of the translocation, a specific modification of the protein sequence, and the severity, the type of TGCT (localized or diffuse) or the response to treatment remains to be described [40,46].

Although the paracrine dogma is more and more accepted in TGCT pathophysiology, i.e. fibroblast overexpression of CSF1 induces CSF1R⁺ macrophagic infiltration, a few studies described contrasting results and suggest that an autocrine loop involving the CSF1/CSF1R axis in neoplastic cells is also possible. For example, a translocation of the *CSF1* gene in macrophagic cells expressing the CSF1R [7] or co-expression of
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Translocations of the *CSF1* gene in TGCT. A. Depending on its localization (dashed lines), the translocation of the *CSF1* gene will dictate the production of distinct CSF1 protein isoforms. The prevalence of the translocation in TGCT patients (%) according to its localization is indicated (n = 77 patients from references [8, 10,37,40–42,46,47,51]). A CSF1 fusion protein with another sequence (known protein fragment if in-frame translocation or new protein sequence, noted as “?”) can occur when the translocation happens before the end of the 8th exon. It can be exclusively secreted (translocation occurring before the transmembrane domain) or with an altered cytoplasmic domain. In the case of secreted fusion proteins, the new sequence can be cleaved or not depending on the removal of the convertase sites. Often, the translocation happens after the 8th exon and does not impact the protein sequence at all. In all cases, the loss of negative regulatory elements in exon 10 will lead to CSF1 overexpression. B. Human karyotype showing the localization of *CSF1* (orange) on chromosome 1 and its reported translocation partners in TGCT, both known genes (green with *gene name*) and intergenic regions (pink). Chromosomes 1 and 2 are heavily overrepresented with around 50 % reported translocation partners on chromosome 1, with a variety of partners altought the region between *CSF1* and the centromere is particularly affected, and 25 % on chromosome 2, but with 3 recurent genes *COL3A1*, *FN1* and *COL6A3*.</figcaption></figure>
mesenchymal (ACTA2, FAP) and macrophage (CD68, CSF1R) markers in neoplastic cells in culture [48] were previously observed.

## 4. Histopathological features associated with CSF1 overexpression

The knowledge on the CSF1/CSF1R axis and its dysregulation in TGCT can partially explain histopathological features of the disease, notably the origin and development of the vast majority of cells constituting the neoplastic tissue. Apart from the fibroblastic population and its CSF1-translocated subpopulation, most of the other cells are of myeloid origin, although they exhibit a wide range of phenotypes (foam cells, siderophages and osteoclast-like giant cells). Recent studies used scRNAseq to improve the knowledge on TGCT tissue composition mostly based on histological observations so far [10,35,49–51]. As expected, two major populations of CSF1-expressing fibroblasts and CSF1R-expressing macrophages were described. Only small numbers of the CSF1-translocated fibroblasts have been detected using standard scRNAseq [10] and single-cell long-read RNA sequencing would be required to confirm these observations. Different studies identified clusters of endothelial cells, M1 pro-inflammatory and M2 alternative macrophages, giant cells/osteoclast-like cells, pericytes, smooth muscle cells, dendritic cells, mast cells and T cells [10,35,49–51]. A proliferating cluster of macrophages also is interestingly always observed in TGCT tissues. Whether these highly diverse macrophage subsets arise from infiltrating blood monocytes and/or from proliferation and local expansion of tissue resident macrophages is currently unknown, and their exact ontogeny in TGCT will require additional investigations.

### 4.1. Macrophages

Early reports suggested two main cellular components in TGCT, a fibroblastic (clusterin⁺ PDPN⁺) and a macrophagic (CD68⁺ CD163⁺) population, in line with the healthy composition of the synovial tissue [52–54]. In TGCT, proliferative cells were identified as part of the monocyte/macrophage lineage, as proliferation markers PCNA and Ki67 were co-expressed preferentially with HAM56 (macrophages) rather than vimentin (mesenchymal cells) [55]. While several subtypes of macrophages can now be distinguished, the majority expresses CSF1R and concurrently have a rather anti-inflammatory profile characterized by the expression of CD163 [7,50]. The detection by IHC of programmed cell death ligand 1 (PD-L1)-expressing macrophages, foam cells and giant cells in some TGCT samples further reinforce this immunosuppressive phenotype [56]. A parallel could also easily be drawn with tumor-associated macrophages (TAMs) from other types of tumors, bringing novel therapeutic opportunities for TGCT [57] but with specific challenges that will be developed in the next section. Therefore, the majority of myeloid cells in TGCT could derive from monocytes/macrophages “overstimulated” with CSF1, leading to pathogenic phenotypes [10,50]. Other ligands produced by neoplastic fibroblasts, such as CXCL12, could also participate in macrophage infiltration and activation [35,50].

### 4.2. Giant cells

Physiological osteoclast differentiation relies on both CSF1 and RANKL [25]. Giant cells in TGCT are believed to originate from the same lineage as mononuclear macrophages (CD14 and CD68 positive) and share features with osteoclasts such as the expression of tartrate-resistant acid phosphatase, matrix metalloproteinase 9 and calcitonin receptor [10,58–60]. In addition, they have the ability to create resorption pits on bone slices in vitro following RANKL treatment or co-culture with CD14 negative stromal cells from TGCT expressing CSF1 and RANKL [60,61]. While their presence is a histological hallmark of TGCTs, in scRNAseq studies these multinucleated giant cells can be lost within the microfluidic systems due to their size or considered as

multiplets and filtered out during the bioinformatic analysis. Whether they are mononuclear pre-osteoclasts or true multinucleated osteoclast-like giant cells, they express canonical osteoclast markers such as CSF1R, SPI1, TNFRSF11A (RANK), SPP1 and CTSK [10,49,62]. In the other closely related Giant Cell Tumor of Bone (GCTB), multinucleated giant cells are sensitive to RANKL inhibition (Denosumab treatment) [63] but their dependency to RANKL in TGCT is a matter of debate, although the use of Denosumab for treating osteolytic bone lesions has been reported in TGCT [64]. Indeed, multinucleated giant cells capable of bone resorption can form independently of RANKL in the presence of TNFα, IL-1β, IL-4 or IL-13 in inflammatory diseases or as a foreign body response for example [25,65,66].

### 4.3. Siderophages

Siderophages are macrophages that accumulate hemosiderin, an insoluble iron-containing protein. They are thus mostly observed in pathologies related to iron metabolism or hemorrhage. For examples, they can be observed in the lungs of patients with idiopathic pulmonary hemosiderosis [67], in the cerebrospinal fluid following subarachnoid hemorrhage [68] or in the liver of patients with hemochromatosis, although hepatocytes can also present iron deposition [69,70]. While they are useful for the histopathological diagnosis of some diseases, their phenotype and role beyond erythrocyte clearance are poorly studied. A few in vitro studies suggested that M1 macrophages present lower iron intake and release capacities compared to M2 macrophages [71,72]. Reciprocally, iron loading favors an M2 phenotype and reduces LPS-induced inflammation, both in vitro and in vivo [73]. Hemorrhage constitutes a classic TGCT imaging hallmark, especially in diffuse forms, related to hemosiderin deposits [74]. The identification of a causal relationship between endothelial cell dysfunction, tumor bleeding and the presence of siderophages could foster the development of anti-angiogenic therapies, such as anti-VEGF treatments, for TGCT [1].

### 4.4. Foam cells

Foam cells are macrophages with dysregulated lipid metabolism leading to the accumulation of cytoplasmic lipid droplets. They are observed in several pathological context such as atherosclerosis, various cancers as well as tuberculosis and other infections. The review by Guerrini and Gennaro [75] emphasizes that foam cells are not associated to a single macrophage polarization profile and can exhibit different phenotypes, partially depending on the type of lipid stored as exemplified by the storage of cholesterol in atherosclerosis and triglyceride in tuberculosis. The presence of lipid droplets is not just a reflection of a metabolic switch, but can be involved in several other functions such as antigen presentation or the production of inflammatory mediators [76]. The key role of the nuclear receptor PPARγ (peroxisome proliferator-activated receptor gamma) in the regulation of lipid metabolism in foam cells [77] supports the use of PPARγ activators for the treatment of TGCT [1]. In TGCT, foam cells also express PD-L1, suggesting that anti-PD-L1 treatment may have a therapeutic potential [56].

## 5. Treatments targeting the CSF1 pathway: efficacy and mechanisms of action

For many years, surgical excision was the primary and only treatment for TGCT, though no consensus still exists on the optimal surgical approach. The high recurrence rates (10 % in L-TGCT, 33–55 % in D-TGCT) and the need for multiple surgeries have highlighted the necessity to develop new systemic or intra-articular therapies [78,79]. Therapies targeting either CSF1 or its receptor with distinct mechanisms of action are currently used: small molecule tyrosine kinase inhibitors (TKI) and monoclonal antibodies, as illustrated in Fig. 3 and summarized in Table 1.
## 5.1. Kinase inhibitors

Imatinib was the first targeted therapy used in TGCT. Its effect on the CSF1 pathway was discovered when it was shown to inhibit the growth of non-malignant hematopoietic cells, including monocytes/macrophages, via tyrosine phosphorylation inhibition [80]. This TKI also targets the Abelson proto-oncogene (ABL), c-Kit proto-oncogene (KIT),

platelet-derived growth factor receptor (PDGFR), and Discoidin domain receptor (DDR) (Fig. 3A) [81,82]. The largest retrospective analyses reported an overall response rate (ORR) of 31 % and a disease control rate (DCR) over 90 % (Table 1) [83,84]. Nilotinib, another multi-TKI including CSF1R among its targets (Fig. 3A), showed efficacy in a phase 2 trial (NCT01261429). Although the ORR at 12 weeks was low (0–6.3 %), volumetric reductions and high DCRs (up to 96 %) were

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Specificity and mode of action of CSF1/CSF1R inhibitors. A. Small molecule tyrosine kinase inhibitors (TKI) used to treat TGCT have different selectivity for protein kinases. On the right, the whole human kinome is represented as a phylogenetic tree with TKI bioactivity shown as a colored disc (Red for imatinib, Green for nilotinib, Blue for pexidartinib and Grey for vimseltinib) with a size proportional to the corresponding inhibition constant [81,87,93]. On the left, close-up views on the tyrosine kinase group (TK) for each TKI, from nilotinib, the least specific to FMS (CSF1R), to vimseltinib, the most specific. Whereas vimseltinib preferentially inhibits the CSF1R, pexidartinib also inhibits other related kinases such as KIT, FLT3 or CDK11 (CDK19), imatinib also inhibits PDGFRa,b, DDR1,2, ABL or LCK, and nilotinib also inhibits several ephrins, LYN or p38b (MAPK11). The activity of pimicotinib or HMPL-653 on other kinases is poorly described. Tyrosine kinase (TK), Tyrosine kinase-like (TKL), Homologs of yeast Sterile 7, Sterile 11, Sterile 20 kinases (STE), Group containing CDK, MAPK, GSK3 and CLK families (CMGC), Casein kinase 1 (CK1), Group containing PKA, PKG and PKC families (AGC), Calcium/calmodulin-dependent protein kinase (CAMK). Generated with KinMap, excluding atypical protein kinases unaffected by TKIs (http://www.kinhub.org/kinmap/). B. Monoclonal antibodies (lacnotuzumab, cabiralizumab, AMB-05X and emactuzumab) and TKIs (imatinib, nilotinib, pexidartinib, vimseltinib) of CSF1/CSF1R have different mechanisms of action, from inhibition of ligand binding, inhibition of CSF1R dimerization, JMD autoinhibition to AL inactivation. The exact mechanism of action of the TKIs pimicotinib and HMPL-653 are currently not known (indicated as “?”). Juxtamembrane domain (JMD), DFG motif (light blue), activation loop (AL, light purple).</figcaption></figure>
<caption>**Table 1**</caption>

Clinical trials targeting the CSF1 pathway in TGCT. ND, not determined; TKI, tyrosine kinase inhibitor; mAb, monoclonal antibody; IV, intra-venous; IA, intra-articular; PO, per os; ORR, overall response rate; DCR, disease control rate; *, congress abstract, not peer-reviewed.

<table><thead><tr><th>Drug</th><th>Class</th><th>Route</th><th>Study</th><th>Ref</th><th>Type of study</th><th>Phase</th><th>Year</th><th>N</th><th>Main outcome</th><th>ORR</th><th>DCR</th><th>Toxicity</th></tr></thead><tbody><tr><td rowspan="3">IMATINIB</td><td rowspan="3">TKI</td><td rowspan="3">PO</td><td>-</td><td>[123]</td><td>Retrospective</td><td>-</td><td>2011</td><td>27</td><td>Response</td><td>19 %</td><td>93 %</td><td>Grade 3 ≥ 3 %</td></tr><tr><td>-</td><td>[83]</td><td>Retrospective</td><td>-</td><td>2019</td><td>58</td><td>Response</td><td>31 %</td><td>96 %</td><td>Grade 3 ≥ 11 %</td></tr><tr><td>-</td><td>[84]</td><td>Retrospective</td><td>-</td><td>2020</td><td>25</td><td>Response</td><td>ND</td><td>99 %</td><td>Grade 3 ≥ 12 %</td></tr><tr><td rowspan="11">NILOTINIB</td><td rowspan="11">TKI</td><td rowspan="11">PO</td><td>NCT01261429</td><td>[85]</td><td>Interventional</td><td>2</td><td>2018</td><td>56</td><td>Response</td><td>0 % at 12 weeks</td><td>96 % at 12 weeks</td><td>Grade 3 ≥ 11 %</td></tr><tr><td>NCT01261429</td><td>[86]</td><td>Interventional</td><td>2</td><td>2022</td><td>48</td><td>Response</td><td>6.3 %</td><td>93.8 %</td><td>ND</td></tr><tr><td>NCT01004861</td><td>[87]</td><td>Interventional</td><td>1</td><td>2015</td><td>41 / 23</td><td>Safety / Response</td><td>52 %</td><td>83 %</td><td>Grade 3 ≥ 46 %</td></tr><tr><td>NCT01004861</td><td>[40]</td><td>Interventional</td><td>1</td><td>2022</td><td>39</td><td>Response</td><td>62 %</td><td>82 %</td><td>ND</td></tr><tr><td>NCT02371369</td><td>[89, 90]</td><td>Interventional</td><td>3</td><td>2019</td><td>120</td><td>Response</td><td>39 / 60 %</td><td>79 / 85 %</td><td>Grade 3 ≥ 44 %</td></tr><tr><td>NCT02371369</td><td>[124]</td><td>Interventional</td><td>3</td><td>2023</td><td>120</td><td>Pain</td><td>ND</td><td>ND</td><td>ND</td></tr><tr><td>NCT04526704</td><td>[91]</td><td>Interventional</td><td>4</td><td>2020</td><td>32</td><td>Response</td><td>39 %</td><td>45.5 %</td><td>Grade 3 ≥ 25 %</td></tr><tr><td>Pooled studies</td><td>[92]</td><td>Pooled analysis</td><td>-</td><td>2017</td><td>130</td><td>Response</td><td>60 %</td><td>ND</td><td>ND</td></tr><tr><td>NCT04488822</td><td></td><td>Interventional</td><td>3</td><td>2020</td><td>40</td><td>Response</td><td>Ongoing</td><td>Ongoing</td><td>Ongoing</td></tr><tr><td>NCT04703322</td><td></td><td>Interventional</td><td>2</td><td>2021</td><td>21</td><td>Response</td><td>Ongoing</td><td>Ongoing</td><td>Ongoing</td></tr><tr><td>NCT04635111</td><td></td><td>Interventional</td><td>4</td><td>2021</td><td>30</td><td>Safety</td><td>Ongoing</td><td>Ongoing</td><td>Ongoing</td></tr><tr><td rowspan="4">VIMSEL TinIB</td><td rowspan="4">TKI</td><td rowspan="4">PO</td><td>NCT03069469</td><td>[94]</td><td>Interventional</td><td>1/2</td><td>2021</td><td>32 / 45</td><td>Safety / Response</td><td>72 %</td><td>97 %</td><td>Grade 3 ≥ 25 %</td></tr><tr><td>NCT05059262</td><td>[95]</td><td>Interventional</td><td>3</td><td>2022</td><td>123</td><td>Response</td><td>40 %</td><td>ND</td><td>Grade 3 ≥ 37 %</td></tr><tr><td>NCT04192344</td><td>[96]*</td><td>Interventional</td><td>1</td><td>2023</td><td>49</td><td>Response</td><td>77 %</td><td>ND</td><td>Grade 3 ≥ 8.2 %</td></tr><tr><td>NCT05804045</td><td>[97]</td><td>Interventional</td><td>3</td><td>2024</td><td>90</td><td>Response</td><td>Ongoing</td><td>Ongoing</td><td>Ongoing</td></tr><tr><td rowspan="2">HMPL-653</td><td rowspan="2">TKI</td><td rowspan="2">PO</td><td>NCT05277454</td><td>[98]*</td><td>Interventional</td><td>1</td><td>2022</td><td>113</td><td>Safety / Response</td><td>Ongoing</td><td>Ongoing</td><td>Ongoing</td></tr><tr><td>NCT01494688</td><td>[101]</td><td>Interventional</td><td>1</td><td>2015</td><td>28</td><td>Response</td><td>86 %</td><td>ND</td><td>Grade 3 ≥ 12 %</td></tr><tr><td rowspan="2">EMACTUZUMAB</td><td rowspan="2">mAb</td><td rowspan="2">IV</td><td>NCT01494688</td><td>[101]</td><td>Interventional</td><td>1</td><td>2015</td><td>63</td><td>Response</td><td>71 %</td><td>98 %</td><td>Grade 3 ≥ 8 %</td></tr><tr><td>NCT05417789</td><td>[102]</td><td>Interventional</td><td>3</td><td>2024</td><td>128</td><td>Safety / Response</td><td>Ongoing</td><td>Ongoing</td><td>Ongoing</td></tr><tr><td rowspan="2">CABIRALIZUMAB</td><td rowspan="2">mAb</td><td rowspan="2">IV</td><td>NCT02471716</td><td>[103]</td><td>Interventional</td><td>1/2</td><td>2017</td><td>22</td><td>Safety / Response</td><td>ND</td><td>ND</td><td>ND</td></tr><tr><td>NCT04938180</td><td>*</td><td>Interventional</td><td>2</td><td>2022</td><td>4</td><td>Safety</td><td>Ongoing</td><td>Ongoing</td><td>Ongoing</td></tr><tr><td rowspan="2">AMB-05X</td><td rowspan="2">mAb</td><td>IV</td><td>NCT04731675</td><td>[104]</td><td>Interventional</td><td>2</td><td>2022</td><td>11</td><td>Safety / Response</td><td>36–45 % at 12 weeks</td><td>91–100 % at 12 weeks</td><td>Grade 3 ≥ 18 %</td></tr><tr><td>IA</td><td>NCT05349643</td><td>[105]</td><td>Interventional</td><td>2</td><td>2023</td><td>20</td><td>Response</td><td>Ongoing</td><td>Ongoing</td><td>Ongoing</td></tr><tr><td rowspan="2">LACNOTUZUMAB</td><td rowspan="2">mAb</td><td rowspan="2">IV</td><td>NCT01643850</td><td>[107]</td><td>Interventional</td><td>2</td><td>2012</td><td>36</td><td>Response</td><td>ND</td><td>ND</td><td>ND</td></tr><tr><td>NCT01643850</td><td>*</td><td>Interventional</td><td>2</td><td>2012</td><td>36</td><td>Response</td><td>ND</td><td>ND</td><td>ND</td></tr></tbody></table>

noted [85,86]. Imatinib and nilotinib are so-called type II inhibitors that bind to the ATP-binding site and trap targeted tyrosine kinases in an inactive state (DFG-out) (Fig. 3B).

Pexidartinib, an oral selective CSF1R inhibitor, was the first FDA-approved drug for TGCT. It was derived from other TKIs to provide stronger specificity for CSF1R through binding to the juxtamembrane domain and freezing the CSF1R in its autoinhibition state (Fig. 3B) [87]. However, pexidartinib also inhibits other kinases such as KIT, Fms-like tyrosine kinase 3 (FLT3), Cyclin-dependent kinase 19 (CDK19, alias CDK11) and Kinase insert domain receptor (KDR, alias VEGFR2) (Fig. 3A) [87,88]. The phase 3 ENLIVEN trial (NCT02371369) established its clinical value, demonstrating a 39 % ORR versus placebo [89] and a 60 % ORR in a long-term study [90]. Similar response rates were obtained in other cohorts [91,92]. More recently, vimseltinib, an even more specific inhibitor (Fig. 3A), was identified as a "switch-control" kinase inhibitor that forces the activation loop of CSF1R (or activation "switch") into an inactive conformation (Fig. 3B) [93,94]. In the phase 3 MOTION trial, vimseltinib showed an ORR of 40 % and a clinical benefit rate of 67 % [95]. Pimicotinib, currently in phase 3 clinical trial, was also proposed as a selective CSF1R kinase inhibitor with minimum inhibition of KIT and PDGFR, but its specificity and mode of action remain poorly described to date [96,97]. Similarly, only a few data are available concerning HMPL-653, the latest reported TKI under early clinical evaluation (Phase 1, NCT05277454) [98]. Overall, the efficacy of CSF1R-targeting small molecules for TGCT treatment represents a major advance, especially for diffuse, non-operable, or relapsed cases. However, response rates can vary depending on the drug and remain around

40 % even when more specific molecules are used.

## 5.2. Monoclonal antibodies

Monoclonal antibodies against CSF1 or its receptor have also been developed. As IL-34 also signals through CSF1R, the functional consequences of targeting the ligand versus the receptor may differ. IL-34 has been shown to rescue bone, osteoclast, tissue macrophage, and fertility defects in Csf1 op/op mice [99], suggesting that receptor blockade might have stronger biological effects. Several CSF1R-targeting monoclonal antibodies are currently under development. Emactuzumab, the most advanced candidate, is a recombinant humanized IgG1 monoclonal antibody specifically targeting CSF1R, inhibiting its dimerization which is necessary for intracellular signaling (Fig. 3B) [1,23,100]. Early data from a phase 1 trial (NCT01494688) showed notable antitumor activity, with an ORR of 86 % [101]. Extended follow-up reported an ORR of 71 % and a DCR of 98 % [102]. Emactuzumab is now being evaluated in a phase 3 randomized trial (NCT05417789, TANGENT) (Table 1).

Other monoclonal antibodies in early-phase trials include cabiralizumab and AMB-05X, both anti-CSF1R antibodies that inhibit ligand binding (Fig. 3B) [103–105].

Lacnotuzumab, which targets CSF1 directly, has demonstrated dose-dependent effects on downstream biomarkers, with mostly mild and reversible adverse events [106]. A phase 2 clinical trial (NCT01643850) evaluating lacnotuzumab in inoperable D-TGCT reported an ORR of 100 % in four patients. Updated results are anticipated shortly and will provide further insight into the efficacy and safety of this approach [1,
107]. Overall, this therapeutic strategy has shown promise, with encouraging response rates observed in early-phase clinical trials.

### 5.3. Prevalence and underlying mechanisms of adverse events

A major challenge with systemic therapies targeting the CSF1 pathway is their poor tolerability, a summary of the toxicity of all cited agents is presented in Table 1. Imatinib commonly causes side effects, though rarely grade 3–4, which often lead to treatment discontinuation. Adverse events were reported in 78 % of patients, with edema (48 %) and fatigue (50 %) being the most frequent, mostly grade 1–2 (89 %) [83]. Pexidartinib is associated with fatigue, hair color changes, cytopenias, and liver toxicity [89]. Vimseltinib appears to have a more favorable profile, although 45 % of patients reported periorbital edema and 30 % reported fatigue, pruritus, or headache [95]. Emactuzumab also causes frequent side effects, including pruritus (70 %) and various forms of oedema: facial (49 %), peripheral (44 %), periorbital (43 %), and eyelid (37 %) [102].

Oedema is likely a class effect related to direct CSF1 inhibition rather than a drug-specific toxicity. One study showed that, in mice, CSF1R antibody treatment led to fluid retention, increased MMP expression, and hyaluronan accumulation due to reduced LYVE1⁺ (Lymphatic vessel endothelial hyaluronan receptor 1) macrophages, which degrade hyaluronan [108]. Hepatotoxicity is also linked to CSF1R inhibition in liver-resident macrophages (Kupffer cells), which are involved in enzyme clearance [109]. Some adverse events, such as hair color changes and dysgeusia, appear specific to pexidartinib, likely resulting from off-target effects, such as KIT inhibition [110].

The report of such side effects has motivated the development of intra-articular therapies. A phase 2 open-label study assessed intra-articular injections of the anti-CSF1R antibody (AMB-05X) injections in patients with TGCT of the knee (NCT04731675) [104]. Higher synovial versus systemic levels of the antibody were observed, leading to accumulation of CSF1 locally in the synovial fluid. Despite a favorable ORR of 36–45 %, adverse events such as oedema and liver enzyme elevations were still observed.

### 5.4. Prediction of treatment response

The advent of systemic therapies and the better understanding of the pathophysiology of the disease raises important questions about predictive biomarkers of response and the potential for personalized treatment in TGCT. Unlike gastrointestinal stromal tumours, where specific mutations guide therapy, no genetic markers have yet been identified to predict response to CSF1R-targeted treatments in TGCT [111]. Notably, some patient who were resistant to imatinib or nilotinib responded to emactuzumab, suggesting that the latter may exert stronger CSF1R inhibition [102]. Despite these observations, macrophage depletion in synovial tissue does not appear to correlate with clinical response. In a cohort of 36 patients with paired biopsies, a significant reduction in CSF1R⁺ and CD68/CD163⁺ macrophages was observed in 61 % of cases, however, this was not related to a reduction in tumor size or clinical benefit [102]. In the MOTION trial, clinically meaningful improvement in functional or symptom-based measures was reported in 43 % of patients with stable disease, indicating that mechanisms beyond tumor volume reduction may contribute to symptom relief in TGCT [95].

Overall, the development of CSF1 pathway-targeting therapies represents a major advancement in the management of TGCT. However, integration in multimodal treatment strategies (e.g., neo/adjuvant to surgery) remain to be fully defined. In addition, managing adverse events remains a major challenge for broader implementation.

## 6. Open questions and future directions

The CSF1/CSF1R axis is central to TGCT pathogenesis, and an

abnormal overexpression of CSF1 due to the translocation of its 3'-UTR has been revealed as the main driver of both the localized and diffuse forms. Therapies targeting this pathway are important assets to treat diffuse, inoperable or relapsing cases despite varying efficacy and substantial side effects. Moreover, several questions remain to be addressed, specifically related to the potential involvement of other genes such as CBL or the rationale for defining the right therapy for the right patient.

Initially considered as distinct entities, D- and L-TGCT actually present very similar histological and genetic features [112]. However, given that CBL mutations were found associated with a higher relapse rate [41], it is possible that secondary genetic alterations that remain to be fully identified are responsible for the development of more diffuse, painful, destructive or difficult to treat forms of TGCT. TGCT could therefore be initially a single disease that may secondarily develop into a localized form or spread to a diffuse form that is more difficult to treat. It therefore appears crucial to improve the early detection of this pathology. In addition, deciphering the mechanisms underlying the development of both forms could help expand the therapeutic options beyond the inhibition of CSF1R signaling. Osteoclast-related markers, such as SPP1, MMP9 or CTSK, are known to be upregulated in TGCT compared to rheumatoid arthritis [62], osteoarthritis [49] or synovial chondromatosis [113], but the number of studies comparing L- and D-TGCT is more limited. Recently, the description of fibroblast subsets that are specifically found in D-TGCT tissues, and could participate in tumor aggressiveness, and the identification of ROR1 and PRKD1 as other kinases associated with disease recurrence [35] broadened our understanding of the disease. In vitro, synovial fibroblasts from D-TGCT produce more IL-6, as compared to those isolated from L-TGCT, and could therefore exhibit a more inflammatory and deleterious phenotype [114].

With the identification of new pathways and considering that therapies targeting the CSF1/CSF1R axis primarily prevent macrophage infiltration without eradicating the neoplastic fibroblast-like cells, there is therapeutic interest in exploring additional drugs, either alone or in combination. Multi-TKI may offer greater value as they are not specific for CSF1R⁺ macrophages but can also target fibroblasts by inhibiting PDFGRα/β or DDR1/2 for example (Fig. 3A). Similar to rheumatoid arthritis, TGCT is characterized by inflammation, elevated production of cytokines, including TNF, IL6 and CCL20, and activation of T lymphocytes, particularly Th17 cells [62,114,115]. Although anti-TNF treatments reduce synovial fibrosis and vasculogenesis in TGCT, suggesting a synergistic paracrine loop mediated by TNF and CSF1, they do not decrease CSF1 production or immune cell infiltration, and are unlikely to induce stable remission [116]. Nonetheless, anti-TNF agents may represent an alternative approach for some TGCT patients [117]. Other potential therapeutic approaches include anti-angiogenic treatments (anti-VEGF, bevacizumab) [117,118], nonsteroidal anti-inflammatory drugs (Zaltoprofen) [119], immune checkpoint inhibitor therapy targeting for example PD-1/PD-L1 [56], or JAK inhibitors targeting JAK2 [41] or TYK2 [120]. These treatments could also be combined with CSF1R inhibitors to help tackle the disease through multiple complementary mechanisms.

In summary, the highest specificity is achieved with monoclonal antibodies targeting CSF1 or its receptor. These agents not only mitigate off-target effects commonly seen with TKI, but are also under evaluation for intra-articular delivery to limit systemic exposure and minimize their class effect on other tissue resident macrophages such as Kupffer cells or perivascular macrophages [105,108,109]. The intra-articular injection route could similarly also be applied to TKI and other molecules of interest, especially when combined with hydrogel-based strategies for local and sustained drug delivery. However, these approaches carry risks and limitations that must be carefully considered given the rare and benign nature of TGCT [121,122]. Advances in understanding the molecular and cellular mechanisms of TGCT are thus guiding the development of therapies targeting the CSF1 axis but also other complementary pathways, paving the way for more effective and
personalized treatment strategies for this challenging disease.

### CRediT authorship contribution statement

**Paul Humbert:** Investigation, Visualization, Writing – original draft.
**Charlotte Vialle:** Investigation, Writing – review and editing.
**Marie Chevalerias:** Investigation, Writing – review and editing.
**Vincent Crenn:** Writing – review and editing.
**Denis Waast:** Writing – review and editing.
**Marie-Astrid Boutet:** Writing – review and editing.
**Frederic Blanchard:** Funding acquisition, Conceptualization, Investigation, Visualization, Writing – original draft.
**Benoit Le Goff:** Funding acquisition, Conceptualization, Investigation, Writing – original draft.

### Funding

The funding for this work was provided by French « Agence Nationale de la Recherche » (ANR, grant ANR-23-CE17-0037), Société Française de Rhumatologie (SFR), Fondation de l'Avenir (grant AP-RM-23-008) and Région Pays de la Loire (Trajectoires project).

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### References

[1] G. Spierenburg, L. van der Heijden, K. van Langevelde, K. Szuhai, J.V.G.M. Bovée, M.A.J. van de Sande, et al., Tenosynovial giant cell tumors (TGCT): molecular biology, drug targets and non-surgical pharmacological approaches, Expert Opin. Ther. Targets 26 (4) (2022) 333–345.

[2] W.D. Tap, J.H. Healey, Role of colony-stimulating factor 1 in the neoplastic process of tenosynovial giant cell tumor, Tumor Biol. 44 (1) (2022) 239–248.

[3] M. Kager, R. Kager, P. Falek, A. Falek, G. Szczypiór, J. Niemunis-Sawicka, et al., Tenosynovial giant cell tumor, Folia Med. Cracov. 62 (2) (2022) 93–107.

[4] S. Stacchiotti, H.R. Dür, I.M. Schaefer, K. Woertler, R. Haas, A. Trama, et al., Best clinical management of tenosynovial giant cell tumour (TGCT): a consensus paper from the community of experts, Cancer Treat. Rev. 112 (2023) 102491.

[5] M. Sbaraglia, E. Bellan, A.P.D. Tos, The 2020 WHO classification of soft tissue tumours: news and perspectives, Pathologica 113 (2) (2020) 70.

[6] S. Stern, P.M. McKenzie, N. Bernthal, S. O'Neill, E. Palmerini, R.L. Randall, et al., Localized and diffuse tenosynovial giant cell tumor: real-world results from a patient observational registry, Future Oncol. Lond. Engl. (2025) 1–10.

[7] R.B. West, B.P. Rubin, M.A. Miller, S. Subramanian, G. Kaygusuz, K. Montgomery, et al., A landscape effect in tenosynovial giant-cell tumor from activation of CSF1 expression by a translocation in a minority of tumor cells, Proc. Natl. Acad. Sci. 103 (3) (2006) 690–695.

[8] J. Ho, T. Peters, B.C. Dickson, D. Swanson, A. Fernandez, A. Frova-Seguin, et al., Detection of CSF1 rearrangements deleting the 3' utr in tenosynovial giant cell tumors, Genes Chromosomes Cancer 59 (2) (2020) 96–105.

[9] J.S. Cupp, M.A. Miller, K.D. Montgomery, T.O. Nielsen, J.X. O'Connell, D. Huntsman, et al., Translocation and expression of CSF1 in pigmented villonodular synovitis, tenosynovial giant cell tumor, rheumatoid arthritis and other reactive synovitides, Am. J. Surg. Pathol. 31 (6) (2007) 970–976.

[10] D.G.P. van IJzendoorn, M. Matusiak, G.W. Charville, G. Spierenburg, S. Varma, D. R.C. Colburg, et al., Interactions in CSF1-Driven tenosynovial giant cell tumors, Clin. Cancer Res. 28 (22) (2022) 4934–4946.

[11] E.R. Stanley, V. Chitu, CSF-1 receptor signaling in myeloid cells, Cold Spring Harb. Perspect. Biol. 6 (6) (2014) a021857.

[12] M.J.L. Mastboom, E. Palmerini, F.G.M. Verspoor, A.J. Rueten-Budde, S. Stacchiotti, E.L. Staals, et al., Surgical outcomes of patients with diffuse-type tenosynovial giant-cell tumours: an international, retrospective, cohort study, Lancet Oncol. 20 (6) (2019) 877–886.

[13] T. Assi, T. Moussa, C. Ngo, M. Faron, B. Verret, A. Lévy, et al., Therapeutic advances in tenosynovial giant cell tumor: targeting the CSF1/CSF1R axis, Cancer Treat. Rev. 134 (2025) 102904.

[14] F.J. Pixely, E.R. Stanley, CSF-1 regulation of the wandering macrophage: complexity in action, Trends Cell Biol. 14 (11) (2004) 628–638.

[15] M.B. Ladner, G.A. Martin, J.A. Noble, D.M. Nikoloff, R. Tal, E.S. Kawasaki, et al., Human CSF-1: gene structure and alternative splicing of mRNA precursors, EMBO J. 6 (9) (1987) 2693–2698.

[16] T.G. Douglass, L. Driggers, J.G. Zhang, N. Hoa, C. Delgado, C.C. Williams, et al., Macrophage colony stimulating factor: not just for macrophages anymore! a gateway into complex biologies, Int. Immunopharmacol. 8 (10) (2008) 1354–1376.

[17] H.H. Woo, T. Baker, C. Laszlo, S.K. Chambers, Nucleolin mediates MicroRNA-directed CSF-1 mRNA deadenylation but increases translation of CSF-1 mRNA*, Mol. Cell Proteom. 12 (6) (2013) 1661–1677.

[18] F. Kimura, S. Suzu, Y. Nakamura, N. Wakimoto, Y. Kanatani, N. Yanai, et al., Structural analysis of proteoglycan macrophage colony-stimulating factor, J. Biol. Chem. 269 (31) (1994) 19751–19756.

[19] X.M. Dai, X.H. Zong, V. Sylvestre, E.R. Stanley, Incomplete restoration of colony-stimulating factor 1 (CSF-1) function in CSF-1-deficient Csf1op/Csf1op mice by transgenic expression of cell surface CSF-1, Blood 103 (3) (2004) 1114–1123.

[20] S. Nandi, M.P. Akhter, M.F. Seifert, X.M. Dai, E.R. Stanley, Developmental and functional significance of the CSF-1 proteoglycan chondroitin sulfate chain, Blood 107 (2) (2006) 786–795.

[21] S.H. Mun, P.S.U. Park, K.H. Park-Min, The M-CSF receptor in osteoclasts and beyond, Exp. Mol. Med. 52 (8) (2020) 1239–1254.

[22] M.B. Buechler, W. Fu, S.J. Turley, Fibroblast-macrophage reciprocal interactions in health, fibrosis, and cancer, Immunity 54 (5) (2020) 903–915.

[23] P. Ordentlich, Clinical evaluation of colony-stimulating factor 1 receptor inhibitors, Semin Immunol. 54 (2021) 101514.

[24] I. Ushach, A. Zlotnik, Biological role of granulocyte macrophage colony-stimulating factor (GM-CSF) and macrophage colony-stimulating factor (M-CSF) on cells of the myeloid lineage, J. Leukoc. Biol. 100 (3) (2016) 481–489.

[25] I.E. Adamopoulos, E.D. Mellins, Alternative pathways of osteoclastogenesis in inflammatory arthritis, Nat. Rev. Rheumatol. 11 (3) (2015) 189–194.

[26] P.S.W. Lee, Y. Wang, M.G. Dominguez, Y. Yeung, M.A. Murphy, D.D.L. Bowtell, et al., The cbl protooncoprotein stimulates CSF-1 receptor multiubiquitination and endocytosis, and attenuates macrophage proliferation, EMBO J. 18 (13) (1999) 3616–3628.

[27] J.A. Fletcher, C. Henkle, L. Atkins, A.E. Rosenberg, C.C. Morton, Trisomy 5 and trisomy 7 are nonrandom aberrations in pigmented villonodular synovitis: confirmation of trisomy 7 in uncultured cells, Genes Chromosomes Cancer 4 (3) (1992) 264–266.

[28] R.A. Ray, C.C. Morton, K.K. Lipinski, J.M. Corson, J.A. Fletcher, Cytogenetic evidence of clonality in a case of pigmented villonodular synovitis, Cancer 67 (1) (1992) 121–125.

[29] F.W. Abdul-Karim, A.K. El-Naggar, M.J. Joyce, J.T. Makley, J.R. Carter, Diffuse and localized tenosynovial giant cell tumor and pigmented villonodular synovitis: a clinicopathologic and flow cytometric DNA analysis, Hum. Pathol. 23 (7) (1992) 729–735.

[30] K. Broberg, S. Toksvig-Larsen, A. Lindstrand, F. Mertens, Trisomy 7 accumulates with age in solid tumors and non-neoplastic synovia, Genes Chromosomes Cancer 30 (3) (2001) 310–315.

[31] M. Nilsson, M. Höglund, I. Panagopoulos, R. Sciot, P. Dal Cin, M. Debiec-Rychter, et al., Molecular cytogenetic mapping of recurrent chromosomal breakpoints in tenosynovial giant cell tumors, Virchows Arch. 441 (5) (2002) 475–480.

[32] S. Sugita, T. Takenami, T. Kido, T. Aoyama, M. Hosaka, K. Segawa, et al., Diagnostic utility of CSF1 immunohistochemistry in tenosynovial giant cell tumor for differentiating from giant cell-rich tumors and tumor-like lesions of bone and soft tissue, Diagn. Pathol. 17 (1) (2022) 1–10.

[33] J.J. Thangaiah, J.W. Koeplin, A.L. Folpe, RNAscope CSF1 chromogenic in situ hybridization: a potentially useful tool in the differential diagnosis of tenosynovial giant cell tumor, Hum. Pathol. 115 (2021) 1–9.

[34] A.C. Chandler, M. Yakoub, T. Fujiwara, L.T. Donlin, P.E. Purdue, J.H. Healey, Neoplastic synovial lining cells that coexpress podoplanin and CD90 overproduce CSF-1, driving tenosynovial giant cell tumor, J. Orthop. Res Publ. Orthop. Res Soc. 40 (8) (2022) 1918–1925.

[35] Y. Xie, C. Chen, F. Wu, Y. Peng, J. Su, Y. Zhao, et al., Single-Cell analysis clarifies pathological heterogeneity in tenosynovial giant cell tumor and identifies biomarkers for predicting disease recurrence, Adv. Sci. Weinh. Baden. Wurtt. Ger. (2025) e2415835.

[36] E. Möller, N. Mandahl, F. Mertens, I. Panagopoulos, Molecular identification of COL6A3-CSF1 fusion transcripts in tenosynovial giant cell tumors, Genes Chromosomes Cancer 47 (1) (2008) 21–25.

[37] I. Panagopoulos, P. Brandal, L. Gorunova, B. Bjerkehagen, S. Heim, Novel CSF1-S100A10 fusion gene and CSF1 transcript identified by RNA sequencing in tenosynovial giant cell tumors, Int. J. Oncol. 44 (5) (2014) 1425–1432.

[38] N. Bonafé, M. Gilmore-Hebert, N.L. Folk, M. Azodi, Y. Zhou, S.K. Chambers, Glyceraldehyde-3-Phosphate dehydrogenase binds to the AU-Rich 3' untranslated region of Colony-Stimulating Factor-1 (CSF-1) messenger RNA in human ovarian cancer cells: possible role in CSF-1 posttranscriptional regulation and tumor phenotype, Cancer Res. 65 (9) (2005) 3762–3771.

[39] H.H. Woo, C.F. László, S. Greco, S.K. Chambers, Regulation of colony stimulating factor-1 expression and ovarian cancer cell behavior in vitro by miR-128 and miR-152, Mol. Cancer 11 (1) (2012) 1–11.

[40] W.D. Tap, A.S. Singh, S.P. Anthony, M. Sterba, C. Zhang, J.H. Healey, et al., Results from phase I extension study assessing pexidartinib treatment in six cohorts with solid tumors including TGCT, and abnormal CSF1 transcripts in TGCT, Clin. Cancer Res. J. Am. Assoc. Cancer Res. 28 (2) (2022) 298–307.

[41] Y. Tsuda, M. Hirata, K. Katayama, T. Motoi, D. Matsubara, Y. Oda, et al., Massively parallel sequencing of tenosynovial giant cell tumors reveals novel CSF1 fusion transcripts and novel somatic CBL mutations, Int. J. Cancer 145 (12) (1991) 3276–3284.

[42] A. Lipplaa, D. Meijer, M.A.J. van de Sande, H. Gelderblom, J.V.M.G. Bovée, H. Mei, et al., A novel colony-stimulating factor 1 (CSF1) translocation involving human endogenous retroviral element in a tenosynovial giant cell tumor, Genes Chromosomes Cancer 62 (4) (2023) 223–230.
[43] D. Leardini, D. Messelodi, E. Muratore, F. Baccelli, S.N. Bertuccio, L. Anselmi, et al., Role of CBL mutations in cancer and Non-Malignant phenotype, Cancers 14 (3) (2022) 839.

[44] M.H.H. Schmidt, I. Dikic, The cbl interactome and its functions, Nat. Rev. Mol. Cell Biol. 6 (12) (2005 Dec) 907-919.

[45] L. Huang, N.W. Thielx, J. Lou, G. Ahmad, W. An, S.T. Low-Nam, et al., The ubiquitin ligases cbl and Cbl-b regulate macrophage growth by controlling CSF1R import into macropinosomes, Mol. Biol. Cell 35 (3) (2024) ar38.

[46] T. Gauduchon, H. Vanacker, D. Pissaloux, P. Cassier, A. Dufresne, M. Karanian, et al., Expanding the molecular spectrum of tenosynovial giant cell tumors, Front Oncol. 12 (2022) 1012527.

[47] T. Vougiouklakis, G. Shen, X. Feng, T. S. Hoda, G. Jour, Molecular profiling of atypical tenosynovial giant cell tumors reveals novel Non-CSF1 fusions, Cancers 12 (1) (2020) 100.

[48] S. Thongchot, S. Duangkaew, W. Yotchai, S. Maungsomboon, R. Phimolsarnti, A. Asavamongkolkul, et al., Novel CSF1R-positive tenosynovial giant cell tumor cell lines and their pexidartinib (PLX3397) and sotuletinib (BLZ945)-induced apoptosis, Hum. Cell (2022).

[49] C. Chen, L. Zheng, G. Zeng, Y. Chen, W. Liu, W. Song, Identification of potential diagnostic biomarkers for tenosynovial giant cell tumour by integrating microarray and single-cell RNA sequencing data, J. Orthop. Surg. 18 (1) (2023) 905.

[50] Y. Tang, M. Ma, R. Mi, W. Liu, J. Hou, Y. Feng, et al., Single-cell RNA-seq analysis reveals aberrant CSF1 expression in disease-causing synovial fibroblasts of pigmented villonodular synovitis [Internet], Mol. Biol. (2021). (http://biorxiv.org/lookup/doi/10.1101/2021.09.29.462128).

[51] A. Fähnrich, Z. Gasimova, Y. Maluje, F. Ott, H. Sievert, S. Fliedner, et al., Case report: tenosynovial giant cell tumor, Front Oncol. 14 (2024) 1445427.

[52] A. Alguacil-Garcia, K.K. Unni, J.R. Goellner, Giant cell tumor of tendon sheath and pigmented villonodular synovitis: an ultrastructural study, Am. J. Clin. Pathol. 69 (1) (1978) 6-17.

[53] J.X. O'connell, J.C. Fanburg, A.E. Rosenberg, Giant cell tumor of tendon sheath and pigmented villonodular synovitis: immunophenotype suggests a synovial cell origin, Hum. Pathol. 26 (7) (1995) 771-775.

[54] J.M. Boland, A.L. Folpe, J.L. Hornick, K.L. Grogg, Clusterin is expressed in normal synoviocytes and in tenosynovial giant cell tumors of localized and diffuse types: diagnostic and histogenic genetic implications, Am. J. Surg. Pathol. 33 (8) (2009) 1225-1229.

[55] H. Tashiro, H. Iwasaki, M. Kikuchi, K. Ogata, M. Okazaki, Giant cell tumors of tendon sheath: a single and multiple immunostaining analysis, Pathol. Int. 45 (2) (1995) 147-155.

[56] B. Zheng, L. Yu, J. Hu, H. Xu, J. Wang, Y. Shi, et al., Expression of PD-L1 in mononuclear cells, multinucleated cells, and foam cells in tenosynovial giant cell tumors, Int. J. Clin. Exp. Pathol. 12 (3) (2009) 876.

[57] A. Mantovani, P. Allavena, F. Marchesi, C. Garlanda, Macrophages as tools and targets in cancer therapy, Nat. Rev. Drug Discov. 21 (11) (2022) 799-820.

[58] J.M. Darling, S.R. Goldring, Y. Harada, M.L. Handel, J. Glowacki, E. M. Gravallese, Multinucleated cells in pigmented villonodular synovitis and giant cell tumor of tendon sheath express features of osteoclasts, Am. J. Pathol. 150 (4) (1997) 1383.

[59] W. Yoshida, M. Uzuki, A. Kurose, M. Yoshida, J. Nishida, T. Shimamura, et al., Cell characterization of mononuclear and giant cells constituting pigmented villonodular synovitis, Hum. Pathol. 34 (1) (2003) 65-73.

[60] R. Taylor, T.G. Kashima, H. Knowles, C.L.M.H. Gibbons, D. Whitwell, N. A. Athanasou, Osteoclast formation and function in pigmented villonodular synovitis, J. Pathol. 225 (1) (2011) 151-156.

[61] N.A. Athanasou, J. Quinn, D.J.P. Ferguson, J. McGee, Bone resorption by macrophage polykaryons of giant cell tumour of tendon sheath, Br. J. Cancer 63 (4) (1991) 527-533.

[62] Y. Zhao, J. Lv, H. Zhang, J. Xie, H. Dai, X. Zhang, Gene expression profiles analyzed using integrating RNA sequencing, and microarray reveals increased inflammatory response, proliferation, and osteoclastogenesis in pigmented villonodular synovitis, Front Immunol. 12 (2021) 665442.

[63] L. van der Heijden, P.D.S. Dijkstra, J.Y. Blay, H. Gelderblom, Giant cell tumour of bone in the denosumab era, Eur. J. Cancer 77 (2017) 75-83.

[64] Y. Hirata, T. Nagase, S. Sasaki, Y. Ayada, H. Miyake, C. Sugahara, et al., Treatment of tenosynovial giant cell tumor of the cervical spine with postoperative Anti-RANKL antibody (Denosumab) administration, Acta Med. Okayama 78 (6) (2024) 469-474.

[65] I.E. Adamopoulos, A. Sabokbar, B.P. Wordsworth, A. Carr, D.J. Ferguson, N. A. Athanasou, Synovial fluid macrophages are capable of osteoclast formation and resorption, J. Pathol. 208 (1) (2006) 35-43.

[66] F. Cai, B. Jiang, F. He, Formation and biological activities of foreign body giant cells in response to biomaterials, Acta Biomater. 188 (2024) 1-26.

[67] B.K. Saha, Idiopathic pulmonary mesosclerosis: a state of the art review, Respir. Med. 176 (2021) 106234.

[68] J. Rahimi, A. Woehrer, Chapter 35 - overview of cerebrospinal fluid cytology, in: G.G. Kovacs, I. Alafuzoff (Eds.), Handbook of Clinical Neurology [Internet], 145, Elsevier, 2018, pp. 563-571. (https://www.sciencedirect.com/science/article/pii/B9780128023952000353).

[69] K.P. Batts, Iron overload syndromes and the liver, Mod. Pathol. 20 (2007) S31-S39.

[70] I. De Domenico, D.M. Ward, G. Musci, J. Kaplan, Iron overload due to mutations in ferroportin, Haematologica 91 (1) (2006) 92-95.

[71] G. Corna, L. Campana, E. Pignatti, A. Castiglioni, E. Tagliafico, L. Bosurgi, et al., Polarization dictates iron handling by inflammatory and alternatively activated macrophages, Haematologica 95 (11) (2010) 1814-1822.

[72] S. Recalcati, M. Locati, A. Marini, P. Santambrogio, F. Zaninotto, M. De Pizzol, et al., Differential regulation of iron homeostasis during human macrophage polarized activation, Eur. J. Immunol. 40 (3) (2010) 824-835.

[73] R. Agoro, M. Taleb, V.F.J. Quesniaux, C. Mura, Cell iron status influences macrophage polarization, PLOS One 13 (5) (2018) e0196921.

[74] G. Spierenburg, C. Suevos Ballesteros, B.C. Stoel, A. Navas Cañete, H. Gelderblom, M.A.J. van de Sande, et al., MRI of diffuse-type tenosynovial giant cell tumour in the knee: a guide for diagnosis and treatment response assessment, Insights Imaging 14 (1) (2023) 22.

[75] V. Guerrini, M.L. Gennaro, Foam cells: one size doesn't fit all, Trends Immunol. 40 (12) (2019) 1163-1179.

[76] M.H. den Brok, T.K. Raaijmakers, E. Collado-Camps, G.J. Adema, Lipid droplets as immune modulators in myeloid cells, Trends Immunol. 39 (5) (2018) 380-392.

[77] G. Chinetti, S. Lestavel, V. Bocher, A.T. Remaley, B. Neve, I.P. Torra, et al., PPAR-α and PPAR-γ activators induce cholesterol removal from human macrophage foam cells through stimulation of the ABCA1 pathway, Nat. Med. 7 (1) (2001) 53-58.

[78] E. Palmerini, E.L. Staals, R.G. Maki, S. Pengo, A. Cioffi, M. Gambarotti, et al., Tenosynovial giant cell tumour/pigmented villonodular synovitis: outcome of 294 patients before the era of kinase inhibitors, Eur. J. Cancer 51 (2) (2015) 210-217.

[79] N.M. Bernthal, G. Spierenburg, J.H. Healey, E. Palmerini, S. Bauer, TOPP study group, et al. The diffuse-type tenosynovial giant cell tumor (dt-TGCT) patient journey: a prospective multicenter study, Orphanet J. Rare Dis. 16 (1) (2021) 191.

[80] A.L. Dewar, A.C. Cambareri, A.C.W. Zannettino, B.L. Miller, K.V. Doherty, T. P. Hughes, et al., Macrophage colony-stimulating factor receptor c-fms is a novel target of imatinib, Blood 105 (8) (2005) 3127-3132.

[81] M.I. Davis, J.P. Hunt, S. Herrgard, P. Ciceri, L.M. Wodicka, G. Pallares, et al., Comprehensive analysis of kinase inhibitor selectivity, Nat. Biotechnol. 29 (11) (2011) 1046-1051.

[82] E. Buchdunger, T. O'Reilly, J. Wood, Pharmacology of imatinib (STI571), Eur. J. Cancer Oxf. Engl. 1990 38 (5) (2002) S28-S36.

[83] F.G.M. Verspoor, M.J.L. Mastboom, G. Hannink, R.G. Maki, A. Wagner, E. Bompas, et al., Long-term efficacy of imatinib mesylate in patients with advanced tenosynovial giant cell tumor, Sci. Rep. 9 (1) (2019) 14551.

[84] M.J.L. Mastboom, W. Lips, K. van Langevelde, M. Mifsud, C. Ng, C.L. McCarthy, et al., The effect of imatinib mesylate in diffuse-type tenosynovial giant cell tumours on MR imaging and PET-CT, Surg. Oncol. 35 (2020) 261-267.

[85] H. Gelderblom, C. Cropet, C. Chevreau, R. Boyle, M. Tattersall, S. Stacchiotti, et al., Nilotinib in locally advanced pigmented villonodular synovitis: a multicenter, open-label, single-arm, phase 2 trial, Lancet Oncol. 19 (5) (2018) 639-648.

[86] G. Spierenburg, P. Grimison, C. Chevreau, S. Stacchiotti, S. Piperno-Neumann, A. Le Cesne, et al., Long-term follow-up of nilotinib in patients with advanced tenosynovial giant cell tumours: Long-term follow-up of nilotinib in TGCT, Eur. J. Cancer 173 (2022) 219-228.

[87] W.D. Tap, Z.A. Wainberg, S.P. Anthony, P.N. Ibrahim, C. Zhang, J.H. Healey, et al., Structure-Guided blockade of CSF1R kinase in tenosynovial Giant-Cell tumor, New Engl. J. Med. 373 (5) (2015) 428-437.

[88] E. Hacıosmanoğlu-Aldoğan, D. Lama, H.İ. Yetke, H. Şenol, F.D. Yöntem, Necroptotic suppression of lung cancer cell proliferation and migration: a comprehensive in vitro and in silico study to determine new molecular targets for pexidartinib, Cell Biochem. Funct. 43 (3) (2025) e70068.

[89] W.D. Tap, H. Gelderblom, E. Palmerini, J. Desai, S. Bauer, J.Y. Blay, et al., Pexidartinib versus placebo for advanced tenosynovial giant cell tumour (ENLIVEN): a randomised phase 3 trial, Lancet Lond. Engl. 394 (10197) (2019) 478-487.

[90] A.J. Wagner, W.D. Tap, S. Bauer, J.Y. Blay, J. Desai, H. Gelderblom, et al., Long-term efficacy and safety of pexidartinib in patients with tenosynovial giant cell tumor: final results of the ENLIVEN study, Oncologist 30 (7) (2025) oyae345.

[91] J. Desai, A.J. Wagner, I. Carrasco Garcia, M. Czelari, M. Gordon, C.C. Lin, et al., A phase 4, multicenter, global clinical study to evaluate discontinuation and rechallenge of pexidartinib in patients with tenosynovial giant cell tumor previously treated with pexidartinib, Cancer (2024).

[92] H. Gelderblom, A.J. Wagner, W.D. Tap, E. Palmerini, Z.A. Wainberg, J. Desai, et al., Long-term outcomes of pexidartinib in tenosynovial giant cell tumors, Cancer 127 (6) (2021) 884-893.

[93] T.M. Caldwell, Y.M. Ahn, S.L. Bulfer, C.B. Leary, M.M. Hood, W.P. Lu, et al., Discovery of vimseltinib (DCC-3014), a highly selective CSF1R switch-control kinase inhibitor, in clinical development for the treatment of tenosynovial giant cell tumor (TGCT), Bioorg. Med. Chem. Lett. 74 (2022) 128928.

[94] H. Gelderblom, A.A. Razak, M.H. Taylor, T.M. Bauer, B. Wilky, J. Martín-Broto, et al., CSF1R inhibition in patients with advanced solid tumors or tenosynovial giant cell tumor: a phase 1 study of vimseltinib, Clin. Cancer Res. J. Am. Assoc. Cancer Res. (2024).

[95] H. Gelderblom, V. Bhadri, S. Stacchiotti, S. Bauer, A.J. Wagner, M. van de Sande, et al., Vimseltinib versus placebo for tenosynovial giant cell tumour (MOTION): a multicenter, randomised, double-blind, placebo-controlled, phase 3 trial, Lancet Lond. Engl. 403 (10445) (2024) 2709-2719.

[96] L. Yao, Y. Zhou, B. Li, X. Xie, X. Du, T. Yuan, et al., Efficacy and safety profile of pimotinib (ABSK021) in tenosynovial giant cell tumor (TGCT): phase 1b update, J. Clin. Oncol. 41 (16) (2023) 11559.
[97] X. Niu, V. Ravi, B. Shan, Q. Guo, H. Shi, Q. Zou, et al., MANEUVER: a phase III study of pimicotinib to assess efficacy and safety in tenosynovial giant cell tumor patients, Future Oncol. Lond. Engl. (2024) 1–8.

[98] J. Hu, L. Ge, H. Zhang, A. Jiang, J. Yu, W. Xue, et al., Abstract C160: HMPL-653, a highly potent and selective CSF-1R inhibitor, targeting both tumor cells and tumor microenvironment, Mol. Cancer Ther. 22 (12) (2023) C160.

[99] S. Wei, S. Nandi, V. Chitu, Y.G. Yeung, W. Yu, M. Huang, et al., Functional overlap but differential expression of CSF-1 and IL-34 in their CSF-1 receptor-mediated regulation of myeloid cells, J. Leukoc. Biol. 88 (3) (2010) 495–505.

[100] A.S. Chan, V. Katiyar, P. Dy, V. Singh, Updates on the treatment of tenosynovial giant cell tumor, Hematol. Oncol. Stem Cell Ther. 16 (4) (2023) 307–315.

[101] P.A. Cassier, A. Italiano, C.A. Gomez-Roca, C. Le Tourneau, M. Toulmonde, M. A. Cannarile, et al., CSF1R inhibition with emactuzumab in locally advanced diffuse-type tenosynovial giant cell tumours of the soft tissue: a dose-escalation and dose-expansion phase 1 study, Lancet Oncol. 16 (8) (2015) 949–956.

[102] P.A. Cassier, A. Italiano, C. Gomez-Roca, C. Le Tourneau, M. Toulmonde, S. P. D'Angelo, et al., Long-term clinical activity, safety and patient-reported quality of life for emactuzumab-treated patients with diffuse-type tenosynovial giant-cell tumour, Eur. J. Cancer 141 (2020) 162–170.

[103] K.K. Sankhala, J.Y. Blay, K.N. Ganjoo, A. Italiano, A.B. Hassan, T.M. Kim, et al., A phase I/II dose escalation and expansion study of cabiralizumab (cabira; FPA-008), an anti-CSF1R antibody, in tenosynovial giant cell tumor (TGCT, diffuse pigmented villonodular synovitis D-PVNS), J. Clin. Oncol. 35 (15) (2017) 11078.

[104] M. van de Sande, K. Johnson, K. van Langevelde, T. Scharschmidt, L. Alani, D. Nguyen, et al., Pharmacokinetics and pharmacodynamics of an anti-CSF1 receptor antibody in the intra-articular treatment of tenosynovial giant cell tumor, Eur. J. Cancer Oxf. Engl. 1990 229 (2025) 115756.

[105] H. Gelderblom, V. Bhadri, R.L. Randall, T. Scharschmidt, W.D. Tap, L. Alani, et al., A phase 2, open-label, adaptive, dose-ranging study with long-term extension to evaluate the safety, tolerability, efficacy, and pharmacokinetics of intra-articular AMB-05X injections in patients with tenosynovial giant cell tumor, J. Clin. Oncol. 41 (16) (2023) TPS11585.

[106] F. Pognan, P. Couttet, I. Demin, B. Jaitner, Y. Pang, R. Roubenoff, et al., Colony-Stimulating Factor-1 antibody lacnotuzumab in a phase 1 healthy volunteer study and mechanistic investigation of safety outcomes, J. Pharm. Exp. Ther. 369 (3) (2019) 428–442.

[107] E. Cheng, A. Kudlidjian, J. Block, S. Gitelis, R. Henshaw, B. Wilky, et al., MCS110, an anti-CSF-1 antibody, for the treatment of pigmented villonodular synovitis (PVNS), ISOLS/MSTS 2015 Annu. Meet. (2015).

[108] S. Bissinger, C. Hage, V. Wagner, I.P. Maser, V. Brand, M. Schmittmaegel, et al., Macrophage depletion induces edema through release of matrix-degrading proteases and proteoglycan deposition, Sci. Transl. Med. 13 (598) (2021) eabd4550.

[109] F. Pognan, C. Buono, P. Couttet, J.R. Galarneau, Y. Timsit, A. Wolf, Liver enzyme delayed clearance in rat treated by CSF1 receptor specific antagonist sotuletinib, Curr. Res. Toxicol. 3 (2022) 100091.

[110] M. Picardo, G. Cardinali, The genetic determination of skin pigmentation: *kidg* and the *kidg*-c-Kit pathway as key players in the onset of human familial pigmentary diseases, J. Invest Dermatol. 131 (6) (2011) 1182–1185.

[111] G. Calderillo-Ruíz, E.A. Pérez-Yepez, M.A. García-Gámez, O. Millan-Catalan, C. Díaz-Romero, P. Ugalde-Silva, et al., Genomic profiling in GIST: implications in clinical outcome and future challenges, Neoplasia New Y N. 48 (2024) 100959.

[112] M.J.L. Mastboom, D.M. Hoek, J.V.M.G. Bovée, M.A.J. van de Sande, K. Szuhai, Does CSF1 overexpression or rearrangement influence biological behaviour in tenosynovial giant cell tumours of the knee? Histopathology 74 (2) (2019) 332–340.

[113] W. Han, T. Xu, Z. He, H. Luo, C. Guo, J. Meng, Single-Cell RNA sequencing study on two synovial derived tumors in the temporomandibular joint, Oral. Dis. (2025).

[114] M. Noack, P. Miossec, IFNγ production during cell interactions distinguishes localized from diffuse pigmented villonodular synovitis and rheumatoid arthritis, Arthritis Res. Ther. 27 (1) (2025) 134.

[115] M. Robert, H. Farese, P. Miossec, Update on tenosynovial giant cell tumor, an inflammatory arthritis with neoplastic features, Front Immunol. 13 (2022) 820046.

[116] U. Fiocco, P. Sfriso, F. Lunardi, E. Pagnin, F. Oliviero, E. Scaglioni, et al., Molecular pathways involved in synovial cell inflammation and tumoral proliferation in diffuse pigmented villonodular synovitis, Autoimmun. Rev. 9 (11) (2010) 780–784.

[117] V. Dania, N.A. Stavropoulos, P. Gavriil, I. Trikoupis, P. Koulouvaris, O. D. Savvidou, et al., Treatment modalities for Refractory-Recurrent tenosynovial giant cell tumor (TGCT): an update, Med Kaunas. Lith. 60 (10) (2024) 1675.

[118] M.J. Nissen, A. Boucher, L. Brulhart, J. Menetrey, C. Gabay, Efficacy of intra-articular bevacizumab for relapsing diffuse-type giant cell tumour, Ann. Rheum. Dis. 73 (5) (2014) 947–948.

[119] A. Takeuchi, M. Endo, A. Kawai, Y. Nishida, R. Terauchi, A. Matsumine, et al., Randomized placebo-controlled double-blind phase II study of zaloprofen for patients with diffuse-type and unresectable localized tenosynovial giant cell tumors: the REALIZE study, Front Oncol. 12 (2022) 900010.

[120] S. Lu, C. Cao, W. Zhang, J. Li, J. Yang, Z. Huang, et al., Peficitinib suppresses diffuse-type tenosynovial giant cell tumor by targeting TYK2 and JAK/STAT signaling, Sci. China Life Sci. (2025).

[121] I.A. Jones, R. Togashi, M.L. Wilson, N. Heckmann, C.T. Vangsness, Intra-articular treatment options for knee osteoarthritis, Nat. Rev. Rheuma 15 (2) (2019) 77–90.

[122] W. Back, J.H. Park, Hydrogel-Based strategies for managing rheumatoid arthritis: from sustained drug release to Cell-Based therapies, ACS Biomater. Sci. Eng. 11 (6) (2025) 3262–3275.

[123] P.A. Cassier, H. Gelderblom, S. Stacchiotti, D. Thomas, R.G. Maki, J.R. Kroep, et al., Efficacy of imatinib mesylate for the treatment of locally advanced and/or metastatic tenosynovial giant cell tumor/pigmented villonodular synovitis, Cancer 118 (6) (2012) 1649–1655.

[124] J.H. Healey, W.D. Tap, H.L. Gelhorn, X. Ye, R.M. Speck, E. Palmerini, et al., Pexidartinib provides modest pain relief in patients with tenosynovial giant cell tumor: results from ENLIVEN, Clin. Orthop. 481 (1) (2023) 107–116.

<figure><img src="image_7.png" /></figure>

Paul Humbert, PhD, is a research engineer at the Regenerative Medicine and Skeleton lab (RMeS) in Nantes, France. He obtained his PhD in biology from Nantes University, studying the role of osteoclasts and other multinucleated giant cells in the response to implanted biomaterials for bone repair. At RMeS, he started working on intervertebral disc regeneration strategies before joining the project on tenosynovial giant cell tumors.

<figure><img src="image_8.png" /></figure>

Frederic Blanchard, PhD, is the deputy director of the Regenerative Medicine and Skeleton Lab at the Nantes University and Senior Research Director at the National Institute of Health and Medical Research (INSERM, France). He earned his PhD in Medical Sciences from the Nantes University, followed by postdoctoral research at the Roswell Park Cancer Institute in Buffalo (New-York). His research focuses on cytokine signaling, cellular communication, joint histopathology and spatial biology, with a particular emphasis on inflammatory rheumatic diseases and tumors of soft tissue and bone. His work has highlighted the key role of myeloid-stromal cell crosstalk in chronic inflammation, translating this knowledge into refined and personalized therapeutic approaches.

<figure><img src="image_9.png" /></figure>

Benoit Le Goff, MD, PhD, is Professor of Medicine at the Rheumatology Department at Nantes University Hospital, France. He completed his medical studies at René Descartes University (Paris V) and Nantes University, followed by a rheumatology fellowship. He obtained a PhD in biology from Nantes University, and completed a postdoctoral fellowship at Melbourne University, Australia. He is also Vice Dean for Medical Education, Pedagogy and Innovation at Nantes Medical School and a member of the European education committee. His research, conducted within the Regenerative Medicine and Skeleton Lab (INSERM UMRS 1229), focuses on the synovial membrane and the mechanisms driving joint inflammation, with a particular emphasis on synovial tissue

characterization in rheumatoid arthritis. More recently, his work has expanded to include the study of tenosynovial giant cell tumor, combining clinical, histopathological and translational approaches.