Morphological Changes in the Infrapatellar Fat Pad During Walking Detected by Dynamic Ultrasound in Healthy Volunteers.

Aim This study aimed to verify specific morphological changes in the infrapatellar fat pad (IFP) during walking in healthy young participants. Methods A total of 17 healthy young participants (mean age, 22.8 ± 0.9 years) were recruited in this cross-sectional study. The IFP was evaluated using ultrasonography in three conditions: supine, standing, and walking. The IFP value was described as the thickness of the distal section of the IFP. Additionally, in the walking condition, the IFP was captured in video mode on ultrasonography, and its dynamics were recorded. The waveform of the IFP was produced using the sequence of the IFP thickness on each image. The morphological change of IFP (ΔIFP) was calculated in the IFP waveform and was shown as the difference in IFP thickness between the maximum and minimum at the beginning of the early stance phase. Moreover, kinematics and kinetic data were evaluated using a three-dimensional motion system, and the knee flexion angle (KFA) and knee flexion moment (KFM) were obtained. Results The thickness of the IFP during walking was significantly greater than that during the supine and standing conditions (p < 0.001 for both). The IFP waveform during walking showed a gradual increase during the stance phase and a decrease during the swing phase of the gait cycle. ΔIFP was 1.35 ± 0.42 mm and significantly correlated with the KFM (r = 0.59, p = 0.007). Conclusions Dynamic ultrasonography revealed a specific morphological change in the IFP during walking, which correlated with the KFM.

The biphasic role of the infrapatellar fat pad in osteoarthritis.

Osteoarthritis (OA) is a progressive degenerative disease resulting in joint deterioration. It is a whole organ disease characterized by cartilage degeneration and varying degrees of synovitis, involving pathological changes in all joint tissues, such as cartilage, subchondral bone, ligaments, meniscus, synovium, and infrapatellar fat pad (IPFP). IPFP is the largest adipose tissue structure in the knee joint and is composed of fat cells, immune cells and blood vessels. Moreover, IPFP is located close to the cartilage and bone surface so that it may reduce the impact of loading and absorb forces generated through the knee joint, and may have a protective role in joint health. IPFP has been shown to release various cytokines and adipokines that play pro-inflammatory and pro-catabolic roles in cartilage, promoting OA progression. Intra-articular injections of IPFP-derived mesenchymal stem cells and exosomes have been shown to reduce pain and prevent OA progression in patients with knee OA. Previous studies have shown that IPFP has a biphasic effect on OA progression. This article reviews the latest research progress of IPFP, discusses the role and mechanism of IPFP in OA, provide new intervention strategies for the treatment of OA. This article will also discuss the handling of IPFP during the procedure of total knee arthroplasty.

Evaluation of the therapeutic potential of infrapatellar fat pad adipose-derived stem cells and their secretome for regenerating knee articular cartilage in a rat model of osteoarthritis.

Background: Mesenchymal stem cell (MSC) therapy has ameliorative effects for treating knee osteoarthritis (KOA) disease. Moreover, there is a growing interest in using MSCs-derived secretome (Sec) containing trophic factors secreted by MSCs for KOA treatment. Recently, some studies have suggested that the combination of MSCs and Sec has the potential to treat the diseases. Aims: This study aimed to evaluate the ameliorative effects of combined administration of infrapatellar fat pad (IPFP)-derived MSCs, a type of adipose-derived stem cells (ASCs), for treating degenerated cartilage in a rat model of KOA. Methods: IPFP-ASCs were isolated from the IPFP of male rats. Sec was obtained from IPFP-ASCs in the fourth passage. Eight weeks after the induction of KOA by collagenase II, the rats were divided into 5 groups (n=5), including a control group with no treatment, and four experimental groups that received sodium hyaluronate (Hyalgan®, Hya), ASCs, Sec, and IPFP-ASCs+Sec, respectively by an infrapatellar injection. To perform the pathological and radiological evaluations, the animals were sacrificed 8 weeks later. Results: Our findings indicated that combined administration of the IPFP-ASCs and Sec statistically (P<0.05) improved scores of medial tibial and femoral condyles and medial fabella osteophytes. Also, it statistically (P<0.05) enhances the cartilage surface, matrix, cell distribution and population viability, and subchondral bone indices. No statistical difference was observed between IPFP-ASCs+Sec and IPFP-ASCs. Conclusion: Administration of IPFP-ASCs+Sec has a therapeutic potential to treat KOA in rats. However, there is no difference in the combined administration of IPFP-ASCs and Sec with IPFP-ASCs alone.

A CCL2/MCP-1 antagonist attenuates fibrosis of the infrapatellar fat pad in a rat model of arthritis.

BACKGROUND: Fibrosis of the infrapatellar fat pad (IFP) is a feature of osteoarthritis and contributes substantially to the pain and dysfunction in patients' joints. However, the underlying mechanisms remain unclear. C-C motif chemokine ligand-2 (CCL2) plays a central role in tissue fibrosis. Thus, we aimed to investigate the role of CCL2 in the development of IFP fibrosis in a rat model of arthritis, hypothesizing that a CCL2 antagonist could mitigate fibrotic progression. METHODS: We induced arthritis in male Wistar rats using intra-articular injections of carrageenan. Furthermore, to evaluate the effects of a CCL2 antagonist on protein expression and collagen deposition in the IFP of the rats, we transferred an N-terminal-truncated CCL2 gene into a rat model via electroporation-mediated intramuscular injection. Macrophage infiltration and collagen deposition in the IFP were analyzed in vivo. Groups were compared using the Mann-Whitney U test and Student's t-test. RESULTS: We identified infiltrating macrophages as well as increases in CCL2 and TGF-ß levels as collagen deposition progressed. Gene transfer of the CCL2-antagonist before arthritis induction attenuated collagen deposition remarkably. CONCLUSIONS: We provide initial evidence that anti-CCL2 gene therapy can effectively suppress the development of IFP fibrosis in a rat model. Thus, targeting CCL2 holds promise as a therapeutic strategy for managing tissue fibrosis in osteoarthritis patients.

Disentangling the detrimental effects of local from systemic adipose tissue dysfunction on articular cartilage in the knee.

OBJECTIVE: Obesity increases osteoarthritis (OA) risk due to adipose tissue dysfunction with associated metabolic syndrome and excess weight. Lipodystrophy syndromes exhibit systemic metabolic and inflammatory abnormalities similar to obesity without biomechanical overloading. Here, we used lipodystrophy mouse models to investigate the effects of systemic versus intra-articular adipose tissue dysfunction on the knee. METHODS: Intra-articular adipose tissue development was studied using reporter mice. Mice with selective lipodystrophy of intra-articular adipose tissue were generated by conditional knockout (cKO) of Bscl2 in Gdf5-lineage cells, and compared with whole-body Bscl2 knockout (KO) mice with generalised lipodystrophy and associated systemic metabolic dysfunction. OA was induced by surgically destabilising the medial meniscus (DMM) and obesity by high-fat diet (HFD). Gene expression was analysed by quantitative RT-PCR and tissues were analysed histologically. RESULTS: The infrapatellar fat pad (IFP), in contrast to overlying subcutaneous adipose tissue, developed from a template established from the Gdf5-expressing joint interzone during late embryogenesis, and was populated shortly after birth by adipocytes stochastically arising from Pdgfrα-expressing Gdf5-lineage progenitors. While female Bscl2 KO mice with generalised lipodystrophy developed spontaneous knee cartilage damage, Bscl2 cKO mice with intra-articular lipodystrophy did not, despite the presence of synovial hyperplasia and inflammation of the residual IFP. Furthermore, male Bscl2 cKO mice showed no worse cartilage damage after DMM. However, female Bscl2 cKO mice showed increased susceptibility to the cartilage-damaging effects of HFD-induced obesity. CONCLUSION: Our findings emphasise the prevalent role of systemic metabolic and inflammatory effects in impairing cartilage homeostasis, with a modulatory role for intra-articular adipose tissue.

Infrapatellar fat pad size and subcutaneous fat in knee osteoarthritis radiographic progression: data from the osteoarthritis initiative.

OBJECTIVES: Adipose tissue has been associated with knee osteoarthritis (KOA) pathogenesis, but the longitudinal changes in adipose tissue with KOA progression have not been carefully evaluated. This study aimed to determine if longitudinal changes of systemic and local adipose tissue is associated with radiographic progression of KOA. METHODS: This case-control study used data from the Osteoarthritis Initiative (OAI) and included 315 cases (all the right knees with a minimum of Kellgren-Lawrence score (KL) of 0 and an increase of ≥ 1 KL from baseline to 48 months) and 315 controls matched by age, sex, race, and baseline KL. Cross sectional area of IPFP (IPFP CSA) and subcutaneous adipose tissue around the distal thigh (SCATthigh) were measured using MRI images at baseline and 24 months. Conditional logistic regression models were fitted to estimate associations of obesity markers, IPFP CSA, and SCATthigh with radiographic KOA progression. Mediation analysis was used to assess whether IPFP CSA or SCATthigh mediates the relationships between baseline BMI and radiographic KOA progression. RESULTS: 24-month changes of IPFP CSA (ΔIPFP CSA) and SCATthigh (ΔSCATthigh) were significantly greater in cases compared to controls, whereas Δ BMI and Δ abdominal circumference were similar in both groups during follow-up. Adjusted ORs for radiographic KOA progression were 9.299, 95% CI (5.357-16.141) per 1 SD increase of Δ IPFP CSA and 1.646, 95% CI (1.288-2.103) per 1 SD increase of Δ SCATthigh. ΔIPFP CSA mediated the association between baseline BMI and radiographic KOA progression (87%). CONCLUSIONS: Subjects with radiographic progression of KOA, had significant increases in IPFP CSA and subcutaneous adipose tissue while BMI and abdominal circumference remained stable. Additional studies are needed to confirm these associations.

Therapeutic potential of dedifferentiated fat cells in a rat model of osteoarthritis of the knee.

Introduction: Mature adipocyte-derived dedifferentiated fat cells (DFATs) represent a subtype of multipotent cells that exhibit comparable phenotypic and functional characteristics to adipose-derived stem cells (ASCs). In this study, we assessed the chondroprotective properties of intra-articularly administrated DFATs in a rat model of osteoarthritis (OA). We also investigated in vitro the expression of anti-inflammatory and chondroprotective genes in DFATs prepared from the infrapatellar fat pad (IFP) and subcutaneous adipose-tissue (SC) of human origin. Methods: In the cell transplantation experiment, rats were assigned to the DFAT and Control group (n = 10 in each group) and underwent anterior cruciate ligament transection (ACLT) accompanied by medial meniscus resection (MMx) to induce OA. One week later, they received intra-articular injections of 1 × 106 DFATs (DFAT group) or PBS (control group) four times, with a weekly administration frequency. Macroscopic and microscopic evaluations were conducted five weeks post-surgery. In the in vitro experiments. DFATs derived from the IFP (IFP-DFATs) and SC (SC-DFATs) were prepared from donor-matched tissue samples (n = 3). The gene expression of PTGS2, TNFAIP6, PRG4, BMP2, and BMP6 under TNF-α or IFN-γ stimulation in these cells was evaluated using RT-PCR. Furthermore, the effect of co-culturing synovial fibroblasts with DFATs on the gene expression of ADAMTS4 and IL-6 were evaluated. Results: Intra-articular injections of DFATs significantly inhibited cartilage degeneration in the rat OA model induced by ACLT and MMx. RT-PCR analysis revealed that both IFP-DFATs and SC-DFATs upregulated the expression of genes involved in immune regulation, anti-inflammation, and cartilage protection such as PTGS2, TNFAIP6, and BMP2, under stimulation by inflammatory cytokines. Co-culture with DFATs suppressed the expression of ADAMTS4 and IL6 in synovial fibroblasts. Conclusions: The intra-articular injection of DFATs resulted in chondroprotective effects in the rat OA model. Both SC-DFATs and IFP-DFATs induced the expression of anti-inflammatory and chondroprotective genes in vitro. These results indicate that DFATs appear to possess therapeutic potential in inhibiting cartilage degradation and could serve as a promising cellular resource for OA treatment.

Role of joint adipose tissues in osteoarthritis.

Osteoarthritis (OA) is the most common musculoskeletal disease, without any curative treatment. Obesity being the main modifiable risk factor for OA, much attention focused on the role of adipose tissues (AT). In addition to the involvement of visceral and subcutaneous AT via systemic ways, many arguments also highlight the involvement of local AT, present in joint tissues. Local AT include intra-articular AT (IAAT), which border the synovium, and bone marrow AT (BMAT) localized within marrow cavities in the bones. This review describes the known features and involvement of IAAT and BMAT in joint homeostasis and OA. Recent findings evidence that alteration in magnetic resonance imaging signal intensity of infrapatellar fat pad can be predictive of the development and progression of knee OA. IAAT and synovium are partners of the same functional unit; IAAT playing an early and pivotal role in synovial inflammation and fibrosis and OA pain. BMAT, whose functions have only recently begun to be studied, is in close functional interaction with its microenvironment. The volume and molecular profile of BMAT change according to the pathophysiological context, enabling fine regulation of haematopoiesis and bone metabolism. Although its role in OA has not yet been studied, the localization of BMAT, its functions and the importance of the bone remodelling processes that occur in OA argue in favour of a role for BMAT in OA.

Removal of the infrapatellar fat pad and associated synovium benefits female guinea pigs in the Dunkin Hartley model of idiopathic osteoarthritis.

Background: Several tissues contribute to the onset and advancement of knee osteoarthritis (OA). One tissue type that is worthy of closer evaluation, particularly in the context of sex, is the infrapatellar fat pad (IFP). We previously demonstrated that removal of the IFP had short-term beneficial effects for a cohort of male Dunkin-Hartley guinea pigs. The present project was designed to elucidate the influence of IFP removal in females of this OA-prone strain. It was hypothesized that resection of the IFP would reduce the development of OA in knees of a rodent model predisposed to the disease. Methods: Female guinea pigs (n=16) were acquired at an age of 2.5 months. Surgical removal of the IFP and associated synovium complex (IFP/SC) was executed at 3 months of age. One knee had the IFP/SC resected; a comparable sham surgery was performed on the contralateral knee. All animals were subjected to voluntary enclosure monitoring and dynamic weight-bearing, as well as compulsory treadmill-based gait analysis monthly; baseline data was collected prior to surgery. Guinea pigs were euthanized at 7 months. Knees from eight animals were evaluated via histology, mRNA expression, and immunohistochemistry (IHC); knees from the remaining eight animals were allocated to microcomputed tomography (microCT), biomechanical analyses (whole joint testing and indentation relaxation testing), and atomic absorption spectroscopy (AAS). Results: Fibrous connective tissue (FCT) replaced the IFP/SC. Mobility/gait data indicated that unilateral IFP/SC removal did not affect bilateral hindlimb movement. MicroCT demonstrated that osteophytes were not a significant feature of OA in this sex; however, trabecular thickness (TbTh) in medial femorae decreased in knees containing the FCT. Histopathology scores were predominantly influenced by changes in the lateral tibia, which demonstrated that histologic signs of OA were increased in knees containing the native IFP/SC versus those with the FCT. Similarly, indentation testing demonstrated higher instantaneous and equilibrium moduli in the lateral tibial articular cartilage of control knees with native IFPs. AAS of multiple tissue types associated with the knee revealed that zinc was the major trace element influenced by removal of the IFP/SC. Conclusions: Our data suggest that the IFP/SC is a significant component driving knee OA in female guinea pigs and that resection of this tissue prior to disease has short-term benefits. Specifically, the formation of the FCT in place of the native tissue resulted in decreased cartilage-related OA changes, as demonstrated by reduced Osteoarthritis Research Society International (OARSI) histology scores, as well as changes in transcript, protein, and cartilage indentation analyses. Importantly, this model provides evidence that sex needs to be considered when investigating responses and associated mechanisms seen with this intervention.

A Novel Infrapatellar Fat Pad Preservation Technique in Total Knee Arthroplasty Reduced Postoperative Pain and Wound Complications.

OBJECTIVE: The management of the infrapatellar fat pad (IPFP) during total knee arthroplasty (TKA) remains controversial. This study aimed to evaluate a novel IPFP preservation technique-"the medially pedicled IPFP flap"-for reducing postoperative pain, wound complications, and improving functional recovery after TKA. METHODS: A retrospective analysis was conducted on TKA cases at our institution from 2018 to 2021, including those with IPFP preservation (medially pedicled flap) versus IPFP complete resection. Patient demographics, perioperative parameters (blood loss, operative time, length of hospital stay, visual analogue scale [VAS] score, white cell count [WBC], C-reactive protein [CRP], erythrocyte sedimentation rate [ESR], and wound oozing), and postoperative follow-up data (VAS, Knee Society [KSS], or Knee Society functional assessment [KSFA] scores) were compared between groups. Independent sample t-tests were used to compare continuous data and chi-squared tests were used to compare categorical data between groups. RESULTS: Six hundred thirty patients were included, with 278 in the medial pedicled IPFP flap group (preservation group) and 352 in the IPFP resection group (resection group). The operative time was significantly shorter in the preservation versus resection group (125.5 ± 23.2 vs 130.3 ± 28.7 mins, p = 0.03), as was the length of hospital stay (8.4 ± 2.7 vs 9.2 ± 2.3 days, p < 0.01). Regarding pain, the preservation group had significantly lower VAS scores on postoperative day 2 (2.0 ± 0.8 vs 2.4 ± 1.2, p < 0.001) and day 3 (1.5 ± 0.5 vs 1.8 ± 1.0, p < 0.001). CRP and ESR levels on postoperative day 5 were also significantly lower in the preservation group. Wound oozing rates were significantly lower in the preservation versus resection group (0.7% vs 2.8%, p = 0.04). No significant differences existed in VAS, KSS, or KSFA scores at the last follow-up. CONCLUSION: The novel IPFP preservation technique significantly improved surgical exposure, shortened operative time and length of hospital stay. It also reduced wound pain and oozing compared to IPFP resection.

The thickness change ratio and preservation ratio of the infrapatellar fat pad are related to anterior knee pain in patients following medial patellofemoral ligament reconstruction.

BACKGROUND: The infrapatellar fat pad (IPFP) lies extrasynovial and intracapsular, preserving the joint cavity and serving as a biochemical regulator of inflammatory reactions. However, there is a lack of research on the relationship between anterior knee pain (AKP) and the IPFP after medial patellofemoral ligament reconstruction (MPFLR). Pinpointing the source of pain enables clinicians to promptly manage and intervene, facilitating personalized rehabilitation and improving patient prognosis. METHODS: A total of 181 patients were included in the study. These patients were divided into the AKP group (n = 37) and the control group (n = 144). Clinical outcomes included three pain-related scores, Tegner activity score, patient satisfaction, etc. Imaging outcomes included the IPFP thickness, IPFP fibrosis, and the IPFP thickness change and preservation ratio. Multivariate analysis was used to determine the independent factors associated with AKP. Finally, the correlation between independent factors and three pain-related scores was analyzed to verify the results. RESULTS: The control group had better postoperative pain-related scores and Tegner activity score than the AKP group (P < 0.01). The AKP group had lower IPFP thickness change ratio and preservation ratio (P < 0.001), and smaller IPFP thickness (P < 0.05). The multivariate analysis revealed that the IPFP thickness change ratio [OR = 0.895, P < 0.001] and the IPFP preservation ratio [OR = 0.389, P < 0.001] were independent factors related to AKP, with a significant correlation between these factors and pain-related scores [|r| > 0.50, P < 0.01]. CONCLUSIONS: This study showed the lower IPFP change ratio and preservation ratio may be independent factors associated with AKP after MPFLR. Early detection and targeted intervention of the underlying pain sources can pave the way for tailored rehabilitation programs and improved surgical outcomes. LEVEL OF EVIDENCE LEVEL III.

Effect of isometric quadriceps exercise on local microcirculation of the infrapatellar fat pad in female patients with knee osteoarthritis.

OBJECTIVE: To elucidate the local microcirculation of the infrapatellar fat pad (IFP) in patients with knee osteoarthritis (KOA) by determining the changes in IFP hardness and hemoglobin concentration during isometric quadriceps exercise (IQE). DESIGN: In this observational cross-sectional study, patients diagnosed with bilateral KOA were included in the KOA group (30 knees), healthy older adults in the control group (20 knees), and younger adults in the young group (20 knees). Ultrasonography was performed at rest and during IQE to measure IFP hardness based on shear wave velocity. Near-infrared spectroscopy was performed to measure oxygenated hemoglobin (O2Hb), deoxygenated hemoglobin (HHb), and total hemoglobin (cHb) in the IFP before (Baseline), during (IQE task), and after IQE (Post). IFP hardness and O2Hb, HHb, and cHb concentration were analyzed using a linear mixed model for the groups and measurement points. RESULTS: During IQE, IFP hardness changes were significantly less in the KOA group than in the other groups (KOA: 95 % confidence intervals (CIs) [-0.854, 0.028]; control: 95 % CI [-0.941, -0.341]; and young: 95 % CI [-2.305, -1.706]). In the KOA group, O2Hb concentration exhibited no significant changes at Post compared with Baseline; however, significant changes were observed in the other groups (KOA: 95 % CI [-1.176, 0.423]; control: 95 % CI [-1.452, -0.276]; and young: 95 % CI [-4.062, -2.102]). CONCLUSIONS: During IQE, changes in hardness and hemoglobin concentration in the IFP were not significant in the KOA group, suggesting impaired local microcirculation of the IFP.

Proteomic and lipidomic landscape of the infrapatellar fat pad and its clinical significance in knee osteoarthritis.

Osteoarthritis (OA) is a prevalent joint disease that can be exacerbated by lipid metabolism disorders. The intra-articular fat pad (IFP) has emerged as an active participant in the pathological changes of knee OA (KOA). However, the proteomic and lipidomic differences between IFP tissues from KOA and control individuals remain unclear. Samples of IFP were collected from individuals with and without OA (n = 6, n = 6). Subsequently, these samples underwent liquid chromatography/mass spectrometry-based label-free quantitative proteomic and lipidomic analysis to identify differentially expressed proteins (DEPs) and lipid metabolites (DELMs). The DEPs were further subjected to enrichment analysis, and hub DEPs were identified using multiple algorithms. Additionally, an OA diagnostic model was constructed based on the identified hub DEPs or DELMs. Furthermore, CIBERSORT was utilized to investigate the correlation between hub protein expression and immune-related modules in IFP of OA. Our results revealed the presence of 315 DEPs and eight DELMs in IFP of OA patients compared to the control group. Enrichment analysis of DEPs highlighted potential alterations in pathways related to coagulation, complement, fatty acid metabolism, and adipogenesis. The diagnostic model incorporating four hub DEPs (AUC = 0.861) or eight DELMs (AUC = 0.917) exhibited excellent clinical validity for diagnosing OA. Furthermore, the hub DEPs were found to be associated with immune dysfunction in IFP of OA. This study presents a distinct proteomic and lipidomic landscape of IFP between individuals with OA and those without. These findings provide valuable insights into the molecular changes associated with potential mechanisms underlying OA.

Effect of Low-Intensity Pulsed Ultrasound on Macrophage Properties and Fibrosis in the Infrapatellar Fat Pad in a Carrageenan-Induced Knee Osteoarthritis Rat Model.

BACKGROUND: In the progression of knee osteoarthritis (KOA), fibrosis of the infrapatellar fat pad (IFP) is a key pathological change. Low-intensity pulsed ultrasound (LIPUS) inhibits IFP fibrosis by decreasing the gene expression and activity of hypoxia-inducible factor (HIF-1α), which is a protein involved in IFP fibrosis in KOA rat models. On the other hand, macrophages play an important role in the progression of fibrosis in various tissues, and LIPUS irradiation suppresses macrophage infiltration and inflammatory cytokine secretion. However, whether LIPUS suppresses macrophage polarity and IFP fibrosis in KOA remains unclear. Therefore, we investigated the effect of LIPUS on macrophage polarity and IFP fibrosis. MATERIALS AND METHODS: A KOA model was created by injecting carrageenin into the bilateral knee joints of Wistar rats (eight weeks old). Tissues were harvested over time for histological and molecular biological analysis. The KOA model was also subjected to LIPUS irradiation for two weeks following the injection of carrageenin. RESULTS: RM-4-positive cells were widely distributed in IFP two weeks after carrageenin administration, but M2 macrophages were significantly increased, and the Sirius red area was decreased in the LIPUS-irradiated group compared with those in the non-irradiated group. The gene expression of M1 macrophage markers was significantly decreased and that of M2 macrophage markers was significantly increased in the LIPUS-irradiated group. The expression of transforming growth factor-ß (TGF-ß) and type 1 collagen was also significantly decreased. CONCLUSION: These results suggest that LIPUS may serve as a novel approach for the treatment of KOA through its effect on M1 macrophages and suppression of TGF-ß expression.

Comparative characteristics of the stem cells' number in the stromal vascular fraction of infrapatellar fat pad and subcutaneous fat tissue.

OBJECTIVES: The use of infrapatellar fat pad adipose stem cells (IPFP-ASCs) shows an age-independent proliferation and differentiation potential. In addition, the pronounced chondrogenic potential of IPFP-ASCs makes them promising candidates for research for use in other methods of regenerative therapy. The purpose of this study was to ascertain the presence and compare the relative abundance of cells exhibiting an immunohistochemical profile characteristic of adipose-derived mesenchymal stem cells in selected samples of the stromal vascular fraction (SVF) obtained from the IPFP and subcutaneous fat tissue. METHODS: A direct immunohistochemical study was carried out in serial paraffin sections of the SVF of the infrapatellar fat pad (IPFP) and subcutaneous tissue, using monoclonal antibodies. The minimum criteria were established by the International Society for Cell Therapy to ensure the identity of mesenchymal stem cells use CD73, CD90, and CD105 as positive markers and CD34, CD31, and CD45 as a negative. RESULTS: According to the results of histological, immunohistochemical, morphometric, and statistical studies, it was found that in the SVF of IPFP and subcutaneous adipose tissue, the relative number of cells with the profile CD105+, CD73+, CD34+, CD31-, CD45- in the standard field of view (×200), the SVF of IPFP was 1.58%, whereas the SVF of subcutaneous adipose tissue was 6.92 %, which was statistically significantly greater by 4.38 times (p â€‹< â€‹0.05). CONCLUSION: The presence of a sufficient number of mesenchymal stromal cells in IPFP in combination with their topographic relationship with the structures of the joint determines the use of the SVF of the IPFP for the treatment of diseases of the knee joint. LEVEL OF EVIDENCE: III.

PLOD2 gene expression in infrapatellar fat pad is correlated with fat mass in obese patients with end-stage knee osteoarthritis.

Objective: To investigate associations between obesity-linked systemic factors and gene expression indicative for the inflammatory and fibrotic processes in the infrapatellar fat pad (IFP), in a population of obese patients with end-stage knee osteoarthritis (KOA). Methods: We collected human IFPs from 48 patients with a mean body mass index (BMI) of 35.44 â€‹kg/m2 during total knee replacement procedures. These patients were part of a randomized controlled trial and met the criteria of having OA and a BMI of ≥30 â€‹kg/m2. Blood samples were collected to assess serum levels of glucose, total cholesterol, HDL cholesterol, LDL cholesterol, triglycerides, and leptin. Total body composition was measured using dual-energy X-ray absorptiometry. Gene expressions of IL6, TNFA, COL1A1, IL1B, ASMA, PLOD2 in the IFP were analyzed. Results: Univariate analysis resulted in a positive correlation between BMI and procollagen-lysine,2-oxoglutarate 5-dioxygenase 2 (PLOD2) expression (r2 â€‹= â€‹0.13). In univariate analyses of obesity-linked systemic factors and PLOD2, significant correlations were found for lean mass (r2 â€‹= â€‹0.20), fat mass (r2 â€‹= â€‹0.20), serum cholesterol (r2 â€‹= â€‹0.17), serum triglycerides (r2 â€‹= â€‹0.19) and serum leptin (r2 â€‹= â€‹0.10). A multiple linear regression model indicated fat mass to be a strong predictor of PLOD2 production in the IFP (r2 â€‹= â€‹0.22, P â€‹= â€‹0.003). Conclusion: Our study demonstrates the positive association between fat mass and PLOD2 expression in the IFP of obese end-stage knee OA patients. This may indicate that within this patient population the fibrotic process in the IFP is influenced by systemic adipose tissue, next to local inflammatory processes.

Sex-specific effects of injury and beta-adrenergic activation on metabolic and inflammatory mediators in a murine model of post-traumatic osteoarthritis.

OBJECTIVE: Metabolic processes are intricately linked to the resolution of innate inflammation and tissue repair, two critical steps for treating post-traumatic osteoarthritis (PTOA). Based on lipolytic and immunoregulatory actions of norepinephrine, we hypothesized that intra-articular ß-adrenergic receptor (ßAR) stimulation would suppress PTOA-associated inflammation in the infrapatellar fat pad (IFP) and synovium. DESIGN: We used the ßAR agonist isoproterenol to perturb intra-articular metabolism 3.5 weeks after applying a non-invasive single-load compression injury to knees of 12-week-old male and female mice. We examined the acute effects of intra-articular isoproterenol treatment relative to saline on IFP histology, multiplex gene expression of synovium-IFP tissue, synovial fluid metabolomics, and mechanical allodynia. RESULTS: Injured knees developed PTOA pathology characterized by heterotopic ossification, articular cartilage loss, and IFP atrophy and fibrosis. Isoproterenol suppressed the upregulation of pro-fibrotic genes and downregulated the expression of adipose genes and pro-inflammatory genes (Adam17, Cd14, Icam1, Csf1r, and Casp1) in injured joints of female (but not male) mice. Analysis of published single-cell RNA-seq data identified elevated catecholamine-associated gene expression in resident-like synovial-IFP macrophages after injury. Injury substantially altered synovial fluid metabolites by increasing amino acids, peptides, sphingolipids, phospholipids, bile acids, and dicarboxylic acids, but these changes were not appreciably altered by isoproterenol. Intra-articular injection of either isoproterenol or saline increased mechanical allodynia in female mice, whereas neither substance affected male mice. CONCLUSIONS: Acute ßAR activation altered synovial-IFP transcription in a sex and injury-dependent manner, suggesting that women with PTOA may be more sensitive than men to treatments targeting sympathetic neural signaling pathways.

Single cell transcriptome profiling of infrapatellar fat pad highlights the role of interstitial inflammatory fibroblasts in osteoarthritis.

OBJECTIVES: Osteoarthritis (OA) is a whole-joint disease in which the role of the infrapatellar fat pad (IFP) in its pathogenesis is unclear. Our study explored the cellular heterogeneity of IFP to understand OA and identify therapeutic targets. METHODS: Single-cell and single-nuclei RNA sequencing were used to analyze 10 IFP samples, comprising 5 from OA patients and 5 from healthy controls. Analyses included differential gene expression, enrichment, pseudotime trajectory, and cellular communication, along with comparative studies with visceral and subcutaneous fats. Key subcluster and pathways were validated using multiplex immunohistochemistry. RESULTS: The scRNA-seq performed on the IFPs of the OA and control group profiled the gene expressions of over 49,674 cells belonging to 11 major cell types. We discovered that adipose stem and progenitor cells (ASPCs), contributing to the formation of both adipocytes and synovial-lining fibroblasts (SLF). Interstitial inflammatory fibroblasts (iiFBs) were a subcluster of ASPCs that exhibit notable pro-inflammatory and proliferative characteristics. We identified four adipocyte subtypes, with one subtype showing a reduced lipid synthesis ability. Furthermore, iiFBs modulated the activities of macrophages and T cells in the IFP. Compared to subcutaneous and visceral adipose tissues, iiFBs represented a distinctive subpopulation of ASPCs in IFP that regulated cartilage proliferation through the MK pathway. CONCLUSION: This study presents a comprehensive single-cell transcriptomic atlas of IFP, uncovering its complex cellular landscape and potential impact on OA progression. Our findings highlight the role of iiFBs in OA, especially through MK pathway, opening new avenues for understanding OA pathogenesis and developing novel targeted therapies.

Culture of Hoffa fat pad mesenchymal stem/stromal cells on microcarrier suspension in vertical wheel bioreactor for extracellular vesicle production.

BACKGROUND: Mesenchymal stromal cells (MSCs) are increasingly employed in regenerative medicine approaches for their immunomodulatory and anti-inflammatory properties, which are encoded in their secretome including extracellular vesicles (EVs). The Hoffa fat pad (HFP) located infrapatellarly harbours MSCs that could assist in tissue homeostasis in osteoarthritic joints. Intraarticular injection therapies based on blood products could modulate the populations of released HFP-MSC-EVs in a quantitative manner. METHODS: To obtain amounts of HFP-MSC-derived EVs that allow pre-clinical evaluation, suitable EV production systems need to be developed. This work investigates the release of EVs from primary HFP-MSCs cultivated in a 3D environment using microcarrier suspension culture in a vertical wheel bioreactor in comparison to conventional 2D culture. To simulate an intraarticular blood product therapy, cultures were treated with citrate-anticoagulated platelet-rich plasma (CPRP) or hyperacute serum (hypACT) before EV collection. HFP-MSC-EVs are enriched via ultrafiltration and characterised via Western Blot, nanoparticle tracking analysis in scatter as well as fluorescence mode. EV potency was determined via RT-qPCR analysing the expression of type II and X collagen (COL2 and COL10), as well as inducible nitric oxide synthase (iNOS) in primary OA chondrocytes. RESULTS: Blood product supplementation elevated HFP-MSC metabolic activity as determined via XTT assay over the course of 14 days. 3D culture resulted in a roughly 100-fold EV yield compared to 2D culture and elevated number of EVs released per cell. Total protein content correlated with the EV concentration. While typical EV marker proteins such as CD9, CD63 or Alix were detected in total protein extracts, CD9 and CD73 colocalised on individual EVs highlighting their cell origin. The type of blood product treatment did not affect the size or concentration of EVs obtained from HFP-MSCs. Assessing potency of 3D culture EVs in comparison to 2D EVs revealed superior biological activity with regard to inhibition of inflammation, inhibition of chondrocyte hypertrophy and induction of cartilage-specific ECM production. CONCLUSIONS: HFP-MSCs proliferate in presence of human blood products indicating that animal serum in culture media can be avoided in the future. The culture of HFP-MSCs in the employed bioreactor was successfully used to generate quantities of EVs that could allow evaluation of HFP-MSC-EV-mediated effects in pre-clinical settings. In addition, EV potency of 3D EVs is superior to EVs obtained in conventional 2D culture flasks.

Modification of Mesenchymal Stem/Stromal Cell-Derived Small Extracellular Vesicles by Calcitonin Gene Related Peptide (CGRP) Antagonist: Potential Implications for Inflammation and Pain Reversal.

During the progression of knee osteoarthritis (OA), the synovium and infrapatellar fat pad (IFP) can serve as source for Substance P (SP) and calcitonin gene-related peptide (CGRP), two important pain-transmitting, immune, and inflammation modulating neuropeptides. Our previous studies showed that infrapatellar fat pad-derived mesenchymal stem/stromal cells (MSC) acquire a potent immunomodulatory phenotype and actively degrade Substance P via CD10 both in vitro and in vivo. On this basis, our hypothesis is that CD10-bound IFP-MSC sEVs can be engineered to target CGRP while retaining their anti-inflammatory phenotype. Herein, human IFP-MSC cultures were transduced with an adeno-associated virus (AAV) vector carrying a GFP-labelled gene for a CGRP antagonist peptide (aCGRP). The GFP positive aCGRP IFP-MSC were isolated and their sEVs' miRNA and protein cargos were assessed using multiplex methods. Our results showed that purified aCGRP IFP-MSC cultures yielded sEVs with cargo of 147 distinct MSC-related miRNAs. Reactome analysis of miRNAs detected in these sEVs revealed strong involvement in the regulation of target genes involved in pathways that control pain, inflammation and cartilage homeostasis. Protein array of the sEVs cargo demonstrated high presence of key immunomodulatory and reparative proteins. Stimulated macrophages exposed to aCGRP IFP-MSC sEVs demonstrated a switch towards an alternate M2 status. Also, stimulated cortical neurons exposed to aCGRP IFP-MSC sEVs modulate their molecular pain signaling profile. Collectively, our data suggest that yielded sEVs can putatively target CGRP in vivo, while containing potent anti-inflammatory and analgesic cargo, suggesting the promise for novel sEVs-based therapeutic approaches to diseases such as OA.