Background Obesity is associated with the development and progression of osteoarthritis (OA) and it is believed that obesity-associated adipose tissue inflammation could play a role in this association. The infrapatellar fat pad (IFP) is an adipose tissue organ present in the knee next to the synovium and cartilage, thereby constituting a potential player in the pathological processes in the joint. Obesity-associated changes do occur in IFP and this supports the hypothesis that IFP could mediate the association between obesity and OA. Interestingly, these changes seemed to be present in the stromal vascular fraction (SVF) rather than adipocytes.
Objectives To get insight into the underlying mechanism of the association between obesity and OA we characterized the cells present in SVF of the IFP in knee OA patients.
Methods IFP samples were obtained from knee OA patients (N=43) undergoing joint replacement surgery (58,1% women; mean (SD) age 66.4 years (10.9); mean (SD) BMI 29.2 kg/m2 (5.7)). The SVF was isolated and cells were characterized based on surface markers expression and cytokine production using flow cytometry. Correlations were calculated using Spearman's correlation test.
Results Characterization of the SVF of IFP showed the presence of various immune cells in this tissue, whereby macrophages and T cells were most abundant. Previous research has shown that CD4+ T cells are important in controlling adipose tissue inflammation. Therefore, we further determined the phenotype of CD4+ T cells in IFP. Polyclonal stimulation of SVF cells indicated that CD4+ T cells could secrete primarily IFNγ, TNFα and IL-6, with little IL-10 and IL-4. Surprisingly, CD4+ T cells secreted IL-6 also in the absence of ex vivo stimulation, but no IFNγ and TNFα. This indicated that IL-6 secreting T cells have an activated phenotype. To further expand these findings, we studied the activation markers, CD40L, CD69 and CD25, on freshly isolated SVF. All these markers could be detected at different levels on CD4+ T cells from different patients. Remarkably, approximately 30% of the IL-6-secreting T cells were also positive for CD69, suggesting they were recently activated. Moreover, the frequency of IL6 producing T cells correlated with the frequency of CD69+CD4+ T cells in adipose tissue (r=0.47, p=0.027).
Co-staining for different cytokines upon polyclonal stimulation revealed that IL-6-secreting CD4+ T cells are distinct from the T cells secreting other cytokines such as IFNγ and TNFα. In addition, chemokine receptor expression revealed that these IL6 producing T cells could not be categorized as conventional T helper 1 (Th1), Th2, Th17 or Tfh cells. These data indicate that IL6-secreting T cells are a distinct population of T cells.
Finally, we have also studied whether these IL-6 producing T cells are also present in other tissues. Indeed, we have found these cells also in sc adipose tissues and synovium of OA patients, but only at low frequencies in blood.
Conclusions In conclusion, we have found a novel population of CD4+ T cells which secrete IL-6 directly ex vivo and are in an activated state, indicating that these CD4+ T cells might recognize adipose tissue antigens and could be involved in the inflammatory processes present in human adipose tissue. Moreover, they are a source of IL-6 in the OA joint, thereby potentially contributing to joint inflammation.
Disclosure of Interest None declared
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