Prognostic role of PD-L1 and immune-related gene expression profiles in giant cell tumors of bone.

Giant cell tumor of bone (GCTB) is a locally aggressive and rarely metastatic tumor, with a relatively unpredictable clinical course. A retrospective series of 46 GCTB and a control group of 24 aneurysmal bone cysts (ABC) were selected with the aim of investigating the PD-L1 expression levels and immune-related gene expression profile, in correlation with clinicopathological features. PD-L1 and Ki67 were immunohistochemically tested in each case. Furthermore, comprehensive molecular analyses were carried out using NanoString technology and nCounter PanCancer Immune Profiling Panel, and the gene expression results were correlated with clinicopathological characteristics. PD-L1 expression was observed in 13/46 (28.3%) GCTB (and in 1/24, 4.2%, control ABC, only) and associated with a shorter disease free interval according to univariate analysis. Moreover, in PD-L1-positive lesions, three genes (CD27, CD6 and IL10) were significantly upregulated (p < 0.01), while two were downregulated (LCK and TLR8, showing borderline significance, p = 0.06). Interestingly, these genes can be related to maturation and immune tolerance of bone tissue microenvironment, suggesting a more immature/anergic phenotype of giant cell tumors. Our findings suggest that PD-L1 immunoreactivity may help to select GCTB patients with a higher risk of recurrence who could potentially benefit from immune checkpoint blockade.

Phenotypic and molecular differences between giant-cell tumour of soft tissue and its bone counterpart.

AIMS: Giant-cell tumour (GCT) of soft tissue (GCT-ST) is a primary soft tissue neoplasm that is histologically similar to GCT of bone (GCT-B). Recently, it has been reported that >90% of GCT-Bs have a driver mutation in the H3F3A gene. As the relationship between GCT-ST and GCT-B is unclear, the aim of this study was to compare a series of GCT-STs and GCT-Bs with regard to the presence of H3F3A mutations and several immunophenotypic markers. METHODS AND RESULTS: Eight GCT-STs were retrieved from our institutional archives. Fifteen GCT-Bs served as controls. Direct sequencing for H3F3A mutations in coding regions between codons 1 and 42, including the hotspot codons (28, 35, and 37), was performed on DNA extracted from formalin-fixed paraffin-embedded tissue. Tumours were studied immunohistochemically for the expression of CD14, CD33, RANKL, RANK, p63, and the osteoblastic markers SATB2 and RUNX2. None of the seven GCT-STs that could be analysed showed H3F3A mutations, whereas 14 GCT-Bs (93.3%) were mutated. All eight GCT-STs were positive for RANK and RUNX2, whereas RANKL and SATB2 were detected in only two cases (25%). CD14 was detected only in mononuclear elements, whereas multinucleated giant cells and a proportion of the mononuclear population expressed CD33. Few mononuclear cells of GCT-STs expressed p63. In comparison, GCT-Bs showed higher expression of p63 (14 of 15 cases with >50% of positive mononuclear cells), RANKL, and SATB2, whereas CD14, CD33, RANK and RUNX2 were similarly expressed. CONCLUSIONS: Although GCT-ST and GCT-B are similar in histological appearance, our results indicate that they are immunophenotypically and genetically distinct.

Expression of CD 68, CD 45 and human leukocyte antigen-DR in central and peripheral giant cell granuloma, giant cell tumor of long bones, and tuberculous granuloma: An immunohistochemical study.

BACKGROUND: Multinucleated giant cells (MNCs) form an integral part of numerous bone and soft tissue tumors, tumor-like lesions and are often associated with granulomas of immunological and nonimmunological origin. The presence of various types of giant cells depends on the lesions in which they are present which are difficult to be diagnosed under routine histological techniques. Immunohistochemistry can be used for a better diagnosis and understanding of the origin of various giant cells using various markers of immune response like human leukocyte antigen-DR (HLA-DR) and those expressed on monocytes and macrophages like CD 68 and leukocyte common antigen (LCA). MATERIALS AND METHODS: The study group consisted of 10 cases of giant cell tumor (GCT) of long bones, tuberculous granuloma, and giant cell granuloma to evaluate and analyze the expression pattern of LCA, CD 68, and HLA-DR in various giant cell lesions. RESULTS: Strong expression of CD 68 was observed in 80% of the lesions, strong and moderate expression of CD 45 observed in 70% of the lesions among and within the groups. In contrast, HLA-DR demonstrated negative expression in 80% of cases except for tuberculous granuloma where all the 10 cases showed moderate to strong immunoreactivity. CONCLUSION: CD 68 and CD 45 expression was found in central giant cell granuloma, peripheral giant cell granuloma and GCT, suggesting the origin from mononuclear phagocyte system and considering their clinical behavior of osteoclast type. High expressivity of HLA-DR in tuberculous granulomas which is an essential factor for presentation of the microbial antigen to CD 4 helper cells thus reassuring the fact that they are up-regulated in response to infection.

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.

BACKGROUND: Diffuse-type tenosynovial giant cell tumour (dt-GCT) of the soft tissue (alternatively known as pigmented villonodular synovitis), an orphan disease with unmet medical need, is characterised by an overexpression of colony-stimulating factor 1 (CSF1), and is usually caused by a chromosomal translocation involving CSF1. CSF1 receptor (CSF1R) activation leads to the recruitment of CSF1R-expressing cells of the mononuclear phagocyte lineage that constitute the tumor mass in dt-GCT. Emactuzumab (RG7155) is a novel monoclonal antibody that inhibits CSF1R activation. We have assessed the safety, tolerability and activity of emactuzumab in patients with Dt-GCT of the soft tissue. METHODS: In this phase 1, first-in-human dose-escalation and dose-expansion study, eligible patients were aged 18 years or older with dt-GCT of the soft tissue with locally advanced disease or resectable tumours requiring extensive surgery, an Eastern Cooperative Oncology Group performance status of 1 or less, measurable disease according to Response Evaluation Criteria In Solid Tumors version 1.1, and adequate end-organ function. Patients with GCT of the bone were not eligible. Patients received intravenous emactuzumab at 900 mg, 1350 mg, or 2000 mg every 2 weeks in the dose-escalation phase and at the optimal biological dose in a dose-expansion phase. The primary objective was to evaluate the safety and tolerability of emactuzumab, and to determine the maximum tolerated dose or optimal biological dose. All treated patients were included in the analyses. Expansion cohorts are currently ongoing. This study is registered with ClinicalTrials.gov, number NCT01494688. FINDINGS: Between July 26, 2012, and Oct 21, 2013, 12 patients were enrolled in the dose-escalation phase. No dose-limiting toxicities were noted in the dose-escalation cohort; on the basis of pharmacokinetic, pharmacodynamic, and safety information, we chose a dose of 1000 mg every 2 week for the dose-expansion cohort, into which 17 patients were enrolled. Owing to different cutoff dates for safety and efficacy readouts, the safety population comprised 25 patients. Common adverse events after emactuzumab treatment were facial oedema (16 [64%] of 25 patients), asthenia (14 [56%]), and pruritus (14 [56%]). Five serious adverse events (periorbital oedema, lupus erythematosus [occurring twice], erythema, and dermohypodermitis all experienced by one [4%] patient each) were reported in five patients. Three of the five serious adverse events-periorbital oedema (one [4%]), lupus erythematosus (one [4%]), and dermohypodermitis (one [4%])-were assessed as grade 3. Two other grade 3 events were reported: mucositis (one [4%]) and fatigue (one [4%]). 24 (86%) of 28 patients achieved an objective response; two (7%) patients achieved a complete response. INTERPRETATION: Further study of dt-GCT is warranted and different possibilities, such as an international collaboration with cooperative groups to assure appropriate recruitment in this rare disease, are currently being assessed. FUNDING: F Hoffmann-La Roche.

Benign giant-cell tumor of the common bile duct: a case report.

Primary giant-cell tumors rarely arise in the common bile duct. We herein report a case of primary giant-cell tumor of the common bile duct. The patient was an 81-year-old male who was diagnosed with a well-defined 1.2-cm mass projecting into the lumen of the middle common bile duct. Excision of the gallbladder and extrahepatic bile duct and a Roux-en-Y cholangiojejunostomy were performed. Histologically, the tumor had no association with carcinomas of epithelial origin and was similar to giant-cell tumors of the bone. The tumor consisted of a mixture of mononuclear and multinucleated osteoclast-like giant cells. The mononuclear cells showed no atypical features, and their nuclei were similar to those of the multinucleated giant cells. CD68 was expressed on the mononuclear and multinucleated osteoclast-like giant cells, whereas CD163 immunoreactivity was restricted to the mononuclear cells. Six months after the operation, the patient was still alive and had no recurrence. The interest of this case lies in the rarity of this entity, the difficulty of preoperative diagnosis, and this tumor's possible confusion with other malignant tumors.

Molecular pathways involved in synovial cell inflammation and tumoral proliferation in diffuse pigmented villonodular synovitis.

Diffuse-type tenosynovial giant cell tumors, also known as pigmented villonodular synovitis, are unique mesenchymal lesions that arise from the synovial tissue of the joints. They are predominantly intraarticular, aggressive, infiltrative processes, characterized by both inflammatory or neoplastic properties and local destructive progression. The pattern of synovial gene and protein expressions in pigmented villonodular synovitis, similar to those in activated macrophages in rheumatoid arthritis, and the phenotype of multinucleated giant cells, characteristic of osteoclasts, suggest that there is a common autocrine mechanism in osteoclast differentiation in both diseases and indicate the potential utility of tumor necrosis factor (TNF)-alpha blockade. High synovial colony stimulating factor 1 (CSF1) messenger RNA (m RNA) expression in pigmented villonodular synovitis, unrelated to a chromosomal translocation involving CSF1 locus, may indicate that there is a synergic paracrine loop mediated by TNF-alpha and CSF1, as shown in both inflammatory and neoplastic conditions. The effects of a new therapeutic approach consisting in intraarticular TNF-alpha blockade were studied in four pigmented villonodular synovitis knees. Knee injections produced a rapid reduction in clinical and sonographic indexes and immunohistological alterations, confirmed by arthroscopic synovectomy. A delayed relapse in one of the four knees and unaltered synovial CSF1 expression were other important findings. In the light of these observations, CSF1/CSF1R interaction probably represents a more sensible therapeutic target than TNF-alpha blockade in the diffuse form of pigmented villonodular synovitis.

Malignant diffuse-type tenosynovial giant cell tumors: a series of 7 cases comparing with 24 benign lesions with review of the literature.

BACKGROUND: Malignant diffuse-type tenosynovial giant cell tumor (D-TSGCT), an unusual sarcoma with concurrent or previous benign D-TSGCTs, poses challenges to diagnosis and prognostication. METHODS: We described the radiologic, clinicopathologic, and immunophenotypical findings of 5 primary and 2 metachronous malignant D-TSGCTs and reviewed published cases to better delineate their morphologic spectrum and behavior. Twenty-four benign D-TSGCTs were also statically compared to analyze the diagnostic values of various variables. RESULTS: The 7 malignant cases affected 4 females and 3 males aged 45 to 78 (mean, 60.9) years, which included 1 intraarticular and 6 extra-articular lesions. These tumors were 5 to 17 cm (mean, 9.4) and located within or near the large joints of extremities. Magnetic resonance imaging revealed expansile or infiltrative masses with frequent lobulation and heterogeneous signals. Histologically, areas of benign D-TSGCTs blended abruptly or gradually with frank sarcomas composed of pleomorphic, spindle, or enlarged oval cells, forming malignant fibrous histiocytomalike (n = 4), fibrosarcomatous (n = 1), myxosarcomatous (n = 1), or giant cell tumorlike (n = 1) patterns. One patient experienced recurrences twice, and another 3 developed metastases to the lymph nodes (n = 2), lung (n = 1), or vertebrae (n = 1), with 1 dying from disseminated diseases. An older age (P = 0.003), a larger size (P = 0.036), tumor necrosis (P < 0.001), atypical mitoses (P < 0.001), and Ki-67 overexpression (P < 0.001) appeared preferentially in malignant lesions, but these parameters had overlap between few benign and malignant tumors. CONCLUSIONS: Malignant D-TSGCTs are a distinct sarcoma with considerable morphologic variability, metastatic propensity, and lethality. Altered architecture with anaplastic cells represents an important distinguishing feature, while abnormalities of other parameters should not be directly equated with malignancy.

Dermatofibrosarcoma protuberans and giant cell fibroblastoma exhibit CD99 positivity.

According to most authors, dermatofibrosarcoma protuberans (DFSP) and giant cell fibroblastoma (GCF) represent the adult and juvenile forms, respectively, of the same disease entity, as evidenced by similar morphology, an identical chromosomal translocation, and CD34 positivity. It has been shown that DFSP and nuchal-type fibroma (NTF) (which is also CD34-positive) are related lesions, and that there might possibly be a continuum between the two. In addition, NTF exhibits CD99 positivity. It was therefore, hypothesized that both DFSP and GCF would show similar immunopositivity for CD99. Archives of pathology at several institutions were searched for DFSP and GCF tissue blocks. A total of 29 DFSP and 5 GCF were analyzed by immunohistochemistry for expression of CD99. Twenty-three of 29 DFSP (79%) and 2 of 5 GCP (40%) expressed CD99. Comparison of CD99 and CD34 showed that the non-tumoral periphery of DFSP was less probable to be CD99 positive, but this finding was not statistically significant.

Apolipoprotein D in CD34-positive and CD34-negative cutaneous neoplasms: a useful marker in differentiating superficial acral fibromyxoma from dermatofibrosarcoma protuberans.

More recent techniques to characterize the genetic profile of soft-tissue tumors include the use of gene arrays. Using this technique, Apolipoprotein D (Apo D), a 33-kDa glycoprotein component of high-density lipoprotein, has been found to be highly expressed in dermatofibrosarcoma protuberans. To corroborate these results, we sought to ascertain the utility of Apo D by investigating its sensitivity and specificity in a variety of CD34-positive and CD34-negative cutaneous neoplasms including superficial acral fibromyxoma, sclerotic fibromas, and cellular dermatofibromas. Of interest, we found absence of Apo D expression in all four cases of superficial acral fibromyxoma. Of the remaining CD34-positive lesions, Apo D expression was noted in 35/36 (97%) cases of dermatofibrosarcoma protuberans, 3/5 (60%) giant-cell fibroblastomas, 4/4 (100%) sclerotic fibromas, 8/8 (100%) neurofibromas, and 1/1 (100%) solitary fibrous tumor. Of the CD34-negative lesions, Apo D expression was noted in 2/22 (9%) regular dermatofibroma, 23/45 (51%) cellular dermatofibroma, 10/10 (100%) malignant fibrous histiocytoma, 9/10 (90%) atypical fibroxanthoma, 7/8 (86%) cellular neurothekeoma, 9/9 (100%) malignant melanoma, 8/8 (100%) melanocytic nevi (100%), 0/2 superficial angiomyxoma, 0/15 fibromatosis, 0/1 nodular fasciitis, and 1/2 (50%) desmoplastic fibroblastomas. In summary, our findings indicate that Apo D expression is not specific to dermatofibrosarcoma protuberans. Its principal use as an immunohistochemical adjunct lies in its utility in differentiating superficial acral fibromyxoma from dermatofibrosarcoma protuberans. Although strong positive staining of Apo D in a markedly atypical fibrohistiocytic lesion is suggestive of atypical fibroxanthoma and/or malignant fibrous histiocytoma, further studies with the inclusion of other atypical spindled cell neoplasms are required to conclusively prove the same.

Osteoclast-like giant-cell tumor of the parotid with salivary duct carcinoma: case report and cytologic, histologic, and immunohistochemical findings.

Primary giant-cell tumor of the salivary gland is a rare lesion with an incompletely characterized histogenesis. To the best of our knowledge, only 16 cases have been previously documented in the English-language literature. We report a new case, which occurred in a 75-year-old man who presented with a parotid mass and cervical lymphadenopathy. The patient underwent a left total parotidectomy and cervical lymph node dissection. As far as we know, ours is the only reported case of a primary giant-cell tumor of the salivary gland in which the patient presented with lymph node metastasis. Because so little is known about giant-cell tumor of the salivary gland, we use the occasion of this case report to describe the cytologic, histologic, and immunohistochemical characteristics that we observed.

Unusual aggressive course of a giant cell tumor of soft tissue during immunosuppressive therapy.

Giant cell tumor of soft tissue with low malignant potential (GCT-ST) is a low-grade, primary soft tissue sarcoma with histological and clinical features similar to giant cell tumor of the bone. The main tumor localizations are the extremities, but it may also occur in the head and neck region. GCT-ST shows a recurrence rate of approximately 15%, but it very rarely metastasizes. The risk of cancer development, especially of the skin, is up to fivefold increased in immunosuppressed patients after organ transplantation. The association of sarcomas and immunosuppressive therapy is best known for Kaposi sarcomas, whereas other types of sarcomas are rarely found. We report of a GCT-ST of low malignant potential, which developed under long-term immunosuppression in a patient 12 years after heart transplantation. The tumor presented with an unusual aggressive course and metastatic site: the parotid gland. Therefore, we suggest that in patients with immunosuppression, even low malignant cancerous lesions should be carefully observed, as their local behavior may be aggressive with development of metastasis.

AM-3K, an anti-macrophage antibody, recognizes CD163, a molecule associated with an anti-inflammatory macrophage phenotype.

CD163 is a member of the scavenger receptor cysteine-rich superfamily restricted to the monocyte/macrophage lineage and is thought to be a useful marker for anti-inflammatory or alternatively activated macrophages. In this study we used mass spectrometric analysis to determine that the antigen recognized by the antibody AM-3K, which we previously generated as a tissue macrophage-specific monoclonal antibody, was CD163. An anti-inflammatory subtype of macrophages stimulated by dexamethasone or interleukin-10 showed strong reactivity for AM-3K and increased expression of CD163 mRNA. Immunohistochemical staining of routinely processed pathological specimens revealed that AM-3K recognized a specialized subpopulation of macrophages. In granulomatous diseases such as tuberculosis, sarcoidosis, or foreign body reactions, tissue macrophages around granulomas, but not component cells of the granulomas such as epithelioid cells and multinucleated giant cells, showed positive staining for AM-3K. In atherosclerotic lesions, scattered macrophages in diffuse intimal lesions were strongly positive for AM-3K, whereas foamy macrophages in atheromatous plaques demonstrated only weak staining. We therefore suggest that, in routine pathological specimens, AM-3K is a useful marker for anti-inflammatory macrophages because these cells can be distinguished from inflammatory or classically activated macrophages. Because AM-3K cross-reacts with macrophage subpopulations in different animal species including rats, guinea pigs, rabbits, cats, dogs, goats, pigs, bovine species, horses, monkeys, and cetaceans, it will have wide application for detection of CD163 in various animals.

Osteoclast-like giant cell tumor in mucinous cystadenocarcinoma of the pancreas: an immunohistochemical and molecular analysis.

Osteoclast-like giant cell tumors (OLGT) are rare neoplasms of the pancreas and mostly associated with ductal adenocarcinomas. In this report, we present the rare case of OLGT associated with mucinous cystadenocarcinoma (MCC). We investigated the expression profile of both tumors by methods of molecular biology and immunohistochemistry. The panel of markers included osteopontin, her2/neu, mismatch repair genes, K-ras, p53, E-cadherin, VEGF-C, and podoplanin. Osteopontin was expressed by the osteoclast-like giant cells but not by the mononuclear tumor cells of the OLGT. We detected an amplification and overexpression of her2/neu in the MCC but not in the OLGT. Although we observed an immunohistochemical expression of hMSH2 and hMLH1 in the OLGT, we were not able to confirm this result by western blot analysis. We also did not find any microsatellite instability (D2S123, BAT26). While mutation of K-ras codon 12 was found in both tumor components, there was wild-type DNA of p53. E-cadherin was expressed in MCC but not in OLGT. VEGF-C was only positive in osteoclast-like giant cells and some of the mononuclear cells of OLGT. The vessel-rich stroma of OLGT did not present any podoplanin-positive lymphatic vessel. The observation of our case and others in the published literature may indicate separating OLGT with undifferentiated carcinoma from OLGT with MCC for the better clinical outcome of the latter.

Giant-cell fibroblastoma in a patient with a bone-marrow transplant.

A case of an epigastric giant-cell fibroblastoma is reported in a 6-year-old girl who had undergone a bone-marrow transplant for severe combined immunodeficiency secondary to adenosine deaminase deficiency. A small subcutaneous nodule had been excised from the epigastrium at age 12 months.

Apoptosis in giant cell tumors of bone.

Although giant cell tumor of bone (GCT) is characterized by the extensive multinucleated giant cells among mononuclear stromal cells, proliferation of these cells and multinucleation are not without limit in certain cases. Few studies on oncogenesis of GCT have focused on the negative growth control, including growth arrest and apoptosis. The purpose of this study was to investigate the mechanism of cell death in multinucleated giant cells and stromal cells of GCT. In this study, we have demonstrated that GCT cells can undergo apoptosis. The cells in surgical specimen were positively stained in situ nick end labeling methods, and electron micrographs showed the morphological changes associated with apoptosis in some of stromal cells and multinucleated giant cells. A candidate responsible for this apoptosis was then examined using cultured GCT cells. We focused on Fas that is a major trigger of apoptosis. Cultured GCT cells expressed detectable amount of Fas on their surface. Although GCT cells did a little undergo apoptosis following treatment with anti-Fas alone, combination treatment with cyclohexamide led to an increase in apoptosis of the GCT cells. These data suggested that the sensitizing activity of cyclohexamide on anti-Fas mediated cytotoxicity could happen in vitro.

Extraorbital giant cell angiofibroma.

A number of intriguing fibrovascular mesenchymal proliferations with benign or low grade malignant potential have recently been described. Giant cell angiofibroma was introduced as an entity by Dei Tos et al. in 1995 and initially considered to be a lesion of the orbit. We describe an extraorbital example, indicating that giant cell angiofibroma is not confined to the orbit. Immunologically, giant cell angiofibroma is positive for CD 34, bcl-2 and vimentin, and negative for epithelial and muscle markers, and S-100. The tumor shares several morphological and immunological properties with giant cell fibroblastoma and solitary fibrous tumor, yet it features a histology sufficiently characteristic to allow its categorization as a separate entity. The recommended treatment is complete but conservative excision. Metastases have not been reported.

Induction of human osteoclast-like cells by treatment of blood monocytes with anti-fusion regulatory protein-1/CD98 monoclonal antibodies.

We have developed a new and simple system of human osteoclast formation by fusing peripheral blood monocytes with anti-Fusion Regulatory Protein-1 (anti-FRP-1) monoclonal antibody (mAb). When human blood monocytes were cultured in the presence of anti-FRP-1/CD98 mAbs, polykaryocytes began to appear at approximately 15 h and increased in size with time until 3-4 days of incubation with anti-FRP-1 mAb. These fused cells showed positive staining in tartrate-resistant acid phosphatase, possessed numerous calcitonin receptors, and were capable of bone resorption. These results strongly suggest that anti-FRP-1 antibody-induced multinucleated cells are osteoclasts. Furthermore, FRP-1 antigens were detected in osteoclasts isolated from human bone and in the osteoclast-like cells obtained from human giant cell tumors of bone.

Giant cell tumor of the ovary. Immunohistochemical evidence of origin from stromal ovarian cells.

Giant cell tumor (GCT) of the ovary is a rare condition, found almost invariably in the context of a mucinous tumor and presenting a microscopic picture indistinguishable from GCT of bone. We describe a case of GCT in the wall of a serous cyst of the ovary. An immunohistochemical study was performed using a panel of antibodies to epithelial, mesenchymal and leukocyte antigens. Mononuclear and giant tumor cells were positive for vimentin; CD 68 and LCA were found only in giant cells whereas actin was only found in mononuclear tumor cells. The immunophenotypic profile of the stromal cells of the residual ovary was identical to that of mononuclear tumor cells. The presented data suggest that GCT of the ovary is probably a non-neoplastic lesion of the mesenchymal stromal cells that react against substances of the associated tumor or cyst.

Transforming growth factor alpha and CD68 immunoreactivity in giant cell tumours of bone: a study on the nature of stromal and giant cells, and their interrelations.

To clarify the nature of neoplastic cells, 17 giant cell tumours of bone were studied histologically and immunohistochemically. L1 antigen and S-100 protein were not detected in the tumour giant cells and stromal cells, although present in non-neoplastic macrophages. The giant cells in all the lesions, some stromal cells, and osteoclasts in the normal bone showed CD68 and transforming growth factor alpha (TGF alpha) immunoreactivity. Fibrohistiocytic antigen, factor XIIIa, was expressed in large numbers of stromal cells in all lesions. Some stromal cells expressed alpha-smooth muscle actin and osteocalcin. These immunohistochemical results suggested that the stromal cells of giant cell tumours of bone showed histiocytic and occasional myofibroblastic and osteoblastic differentiation. Proliferating cell nuclear antigen was demonstrated in the nuclei of the stromal cells only, indicating that these were the sole proliferating elements. TGF alpha produced by the giant cells and some stromal cells may play a role as a mediator for the attraction and/or proliferation of the precursor cells, and may suppress the activity of osteoblastic stromal cells, resulting in restricted bone formation in giant cell tumours.