Rationale: In obesity the fine-tuned balance of macrophage phenotypes is disturbed towards a dominance of pro-inflammatory macrophages resulting in exacerbation and persistence of inflammation and impaired tissue repair. However, the underlying mechanisms are still poorly understood.

Obesity is associated with body fat gain and impaired glucose metabolism. Here, we identified both body fat gain in obesity and impaired glucose metabolism as two independent risk factors for increased serum levels of free fatty acids (FFAs). Since obesity is associated with increased and/or delayed resolution of inflammation observed in various chronic inflammatory diseases such as psoriasis, we investigated the impact of FFAs on human monocyte‐derived and mouse bone marrow‐derived dendritic cell (DCs) functions relevant for the pathogenesis of chronic inflammation. FFAs such as palmitic acid (PA) and oleic acid (OA) did not affect the pro‐inflammatory immune response of DCs. In contrast, PA and OA sensitize DCs resulting in augmented secretion of TH1/TH17‐instructive cytokines upon pro‐inflammatory stimulation. Interestingly, obesity in mice worsened a TH1/TH17‐driven psoriasis‐like skin inflammation. Strong correlation of the amount of total FFA, PA, and OA in serum with the severity of skin inflammation points to a critical role of FFA in obesity‐mediated exacerbation of skin inflammation. Our data suggest that increased levels of FFAs might be a predisposing factor promoting a TH1/TH17‐mediated inflammation such as psoriasis in response to an inflammatory danger signal.

Balanced differentiation of mesenchymal stromal cells (MSCs) into osteoblasts and adipocytes is essential for healthy bone and fat homeostasis. A disturbed balance of MSC differentiation results in bone loss and increased adipogenesis as observed in several human conditions, such as osteoporosis, obesity, and aging. This reflects the importance of MSC fate decision. Therefore, it is important to identify factors that regulate the balance between osteogenic and adipogenic differentiation of MSCs so as to identify new targets to improve bone formation in osteoporosis and control adipose tissue development. There is accumulating evidence that thymus cell antigen 1 (Thy‐1), also known as CD90, expressed on MSCs, plays a critical role in the fate decision of MSCs. Recently, Thy‐1 has been shown to promote osteogenic differentiation of MSCs and thus bone formation while inhibiting adipogenic differentiation and restricting adipose tissue accumulation. The clinical importance of these findings was validated by the detection of reduced serum soluble Thy‐1 in patients with disturbed bone remodeling. In this review, we aim to summarize data on the impact of Thy‐1 on the control of MSC differentiation and its consequences for bone and fat formation.—Saalbach, A., Anderegg, U. Thy‐1: more than a marker for mesenchymal stromal cells. FASEB J. 33, 6689–6696 (2019). www.fasebj.org

Thy-1 on mesenchymal stem cells promotes osteogenesis while inhibiting adipogenesis and obesity.