Background Obesity is associated with the development and progression of osteoarthritis (OA). The infrapatellar fat pad (IFP) could contribute to this association due to its localization in the knee joint and secretion of inflammatory mediators. However, little is known about the effects of obesity on the 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.
Materials and methods IFP volume was determined by MRI in 79 knee OA patients. IFP and subcutaneous adipose tissue (SCAT) were obtained from 106 knee OA patients (total n=155: 68% women, mean age 65 years, mean (SD) body mass index (BMI) 29.9 kg/m2 (5.7)) undergoing joint replacement surgery. Crown-like structures (CLS) were determined using immunohistochemistry. Adipocyte size was determined by light microscopy and histology. Stromal vascular fraction (SVF) cells were characterised by flow cytometry.
Results IFP volume (mean(SD) 23.6 (5.4) mm3) associated with gender and height, but not with BMI. Likewise, volume and size of IFP adipocytes (mean 271 pl, mean 1933 μm) was not correlated with BMI. Few CLS were observed in IFP and the number did not correlate with BMI. Moreover, high BMI was not associated with higher SVF immune cell numbers in IFP, nor with changes in their phenotype. No molecular differences were observed with BMI, besides an increase in TNFα expression. Extensive characterisation of IFP macrophages 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), while macrophages produced predominantly IL-6 and TNFα, but little IL-10. Interestingly, surface marker and cytokine expression revealed that CD163+ macrophages had an activated and pro-inflammatory phenotype.
Conclusions Obesity-related differences usually observed in SCAT and visceral adipose tissue could not be detected in IFP of OA patients, a fat depot implicated in OA pathogenesis.