Mast Cells Differentiated in Synovial Fluid and Resident in Osteophytes Exalt the Inflammatory Pathology of Osteoarthritis

Abstract

Introduction: Osteophytes are a prominent feature of osteoarthritis (OA) joints and one of the clinical hallmarks of the disease progression. Research on osteophytes is fragmentary and modes of its contribution to OA pathology are obscure. Aim: To elucidate the role of osteophytes in OA pathology from a perspective of molecular and cellular events. Methods: RNA-seq of fully grown osteophytes, collected from tibial plateau of six OA patients revealed patterns corresponding to active extracellular matrix re-modulation and prominent participation of mast cells. Presence of mast cells was further confirmed by immunohistochemistry, performed on the sections of the osteophytes using anti-tryptase alpha/beta-1 and anti-FC epsilon RI antibodies and the related key up-regulated genes were validated by qRT-PCR. To test the role of OA synovial fluid (SF) in mast cell maturation as proposed by the authors, hematopoietic stem cells (HSCs) and ThP1 cells were cultured in a media supplemented with 10% SF samples, obtained from various grades of OA patients and were monitored using specific cell surface markers by flow cytometry. Proteomics analysis of SF samples was performed to detect additional markers specific to mast cells and inflammation that drive the cell differentiation and maturation. Results: Transcriptomics of osteophytes revealed a significant upregulation of mast cells specific genes such as chymase 1 (CMA1; 5-fold) carboxypeptidase A3 (CPA3; 4-fold), MS4A2/FCERI (FCERI; 4.2-fold) and interleukin 1 receptor-like 1 (IL1RL1; 2.5-fold) indicating their prominent involvement. (In IHC, anti-tryptase alpha/beta-1 and anti- FC epsilon RI-stained active mast cells were seen populated in cartilage, subchondral bone, and trabecular bone.) Based on these outcomes and previous learnings, the authors claim a possibility of mast cells invasion into osteophytes is mediated by SF and present in vitro cell differentiation assay results, wherein ThP1 and HSCs showed differentiation into HLA-DR+/CD206+ and FCERI+ phenotype, respectively, after exposing them to medium containing 10% SF for 9 days. Proteomics analysis of these SF samples showed an accumulation of mast cell-specific inflammatory proteins. Conclusions: RNA-seq analysis followed by IHC study on osteophyte samples showed a population of mast cells resident in them and may further accentuate inflammatory pathology of OA. Besides subchondral bone, the authors propose an alternative passage of mast cells invasion in osteophytes, wherein OA SF was found to be necessary and sufficient for maturation of mast cell precursor into effector cells.

Osteoarthritis: New Insight on Its Pathophysiology

“…Furthermore, it has been shown that modulating these cells or intervening with factors that modify their phenotypic state may be a promising approach to slowing down OA development [ 52 , 53 , 54 , 55 , 56 ]. Lastly, different recent studies suggested the involvement of mast cells in OA pathology due to the reported presence of these cells and their degranulation products in OA synovium and synovial fluid [ 57 , 58 , 59 , 60 ]. In particular, a central role for IgE-dependent mast cell activation has been proposed in the pathogenesis of osteoarthritis [ 58 ].…”

“…It has also been proposed that osteophyte formation may play a compensatory role in redistributing forces to protect articular cartilage [ 73 ]. For instance, a link between the immunological mechanisms related to OA and osteophyte pathology has been proposed by a recent work, which provided evidence for mast cell presence in osteophytes of OA joints and demonstrated their maturation mediated by the synovial fluid synchronously with disease progression [ 57 ].…”

Increased NMUR1 Expression in Mast Cells in the Synovial Membrane of Obese Osteoarthritis Patients

“…According to a prior study in mice, activation of the NMU/NMUR1 pathway in MCs results in degranulation and neutrophil infiltration [ 40 ]. Additionally, MCs and their degranulation products were observed in the SM and synovial fluid of KOA patients [ 26 ]. Moreover, the number of synovial MCs correlates with the KOA patients’ synovitis score [ 25 ].…”

“…In addition to being found in the synovial membrane (SM), MCs are elevated in patients with rheumatoid arthritis (RA) [ 19 , 20 , 21 ] and OA [ 22 , 23 , 24 ], suggesting that MCs could constitute a crucial component of the mechanism underlying both acute and chronic inflammation. More recent studies have demonstrated a potential association between MCs and knee OA (KOA) severity [ 25 , 26 ]. Given that we also previously reported increased MC marker expression in obese KOA patients [ 6 , 27 , 28 ], we hypothesized that MCs may contribute to the NMU/NMURs system in the osteoarthritic synovium of obese individuals.…”

Targeting Neuroinflammation in Osteoarthritis with Intra-Articular Adelmidrol

“…Additionally, MCs may participate in joint destruction by the induction of MMPs from fibroblast-like synoviocytes, chondrocytes, and other cell types in the arthritic cartilage [ 42 , 43 ]. MCs have been detected even in osteophytes, where they play a role in accentuating the inflammatory pathology of OA [ 44 ]. Similarly, in the rheumatoid-arthritis (RA) joint, activated MCs release pro-inflammatory and immunomodulatory mediators (TNF-A, IL-6, IL-8, VEGF, histamine, heparin, tryptase) that induce angiogenesis, promote cartilage erosions, synoviocyte proliferation, and pain sensitization.…”