Lack of obesity-related features in adipoctes and inflammatory cells in the infrapatellar fat-pad of osteoarthritis patients (IFP)

Purpose: Obesity is associated with the development and progression of osteoarthritis (OA), both for weight-bearing and non-weight bearing joints. Therefore, it has been proposed that obesity-related systemic factors, such as adipose tissue-derived factors, could be involved in this association. It is currently well-accepted that obesity leads to inflammation in most adipose tissue depots, resulting in release of various of pro-inflammatory soluble factors that can have local and systemic effects. The infrapatellar fat pad (IFP) is an adipose tissue depot localized in the knee joint, in the vicinity of synovium and cartilage. Due to its localization, it has been speculated that IFP could contribute to the OA processes in the joint and could mediate obesity-associated effects. However, it is currently unknown whether and how obesity affects IFP. Therefore, the aim of this study was to investigate the presence of obesity-related features in adipocytes and infiltrating immune cells in the IFP of OA patients. Methods: In total, 155 knee OA patients undergoing joint replacement surgery were recruited in the study (68% women, mean age 65 years, mean (SD) BMI 29.9 (5.7) kg/m2). IFP volume was determined by MRI in 79 knee OA patients, participants in the geMstoan study. The volume of IFP was determined on fat suppressed T1-weighted contrast enhanced (CE)-MR images using the software program OsiriX. IFP and subcutaneous adipose tissue (SCAT) was obtained from 106 patients, who were partially participants in the geMstoan study (53 patients). From the remaining patients, 23 patients were included in the LUMC and 30 patients in Erasmus MC. From these patients, leftover IFP or SCAT was obtained during arthroplasty. Adipocyte volume was determined by histology and, upon isolation, by light microscopy. Likewise, stromal vascular fraction (SVF) cells were isolated and extensively characterized by flow cytometry. Crown-like structures (CLS) were determined using immunohistochemistry. Adipose tissue explants were cultured overnight to obtain fat-conditioned medium (FCM). Cytokine/chemokine secretion in FCM were measured by multiplex ELISA. Results: IFP volume determined by MRI (mean(SD) 23.6(5.4) mm3) was associated with gender and height, but not with BMI. The mean volume of IFP adipocytes was 271 pl and did not correlate with BMI, in contrast to SCAT adipocytes. Few CLS were observed in IFP and their number did not differ between individuals with high (>25 Kg/m2) and low (≤ 25Kg/m2) BMI. Moreover, high BMI was not associated with higher infiltrating immune cell numbers in IFP, nor with changes in immune cell populations. Likewise, no molecular differences were observed in FCM-secreted factors between high and low BMI, except for an increased TNFα secretion in obesity (BMI > 30Kg/m2). Since obesity is usually associated with a shift towards pro-inflammatory macrophages in conventional adipose tissue, we have extensively characterized IFP macrophages. These analyses revealed that CD206 and CD163, usually associated with an anti-inflammatory phenotype were the most abundantly expressed surface markers on macrophages (81% and 41% respectively). In contrast, cytokine profiles revealed a pro-inflammatory phenotype of the total macrophage population, with cells producing predominantly IL-6 and TNFα, but little IL-10. Interestingly, the CD163+ macrophages were bigger and had a more activated and pro-inflammatory phenotype than their CD163- counterparts. However, no association with BMI could be observed for different macrophage populations or their cytokines. Conclusions: Our data indicate that most obesity-related features observed in conventional adipose tissues are not present in IFP of overweight/obese individuals, indicating that IFP is different from the conventional adipose tissues. Moreover, although we did observe an increased TNFα secretion by obese compared to lean IFP, this could not be attributed to changes in immune cell populations or their phenotypes.