Adipose Stromal Cells in the Human Rotator Cuff are Resistant to Fibrotic Microenvironmental Cues

Rotator cuff tears are the most common upper extremity orthopaedic injury, causing degenerative changes within the bone, tendon, joint capsule, bursa, and muscle. These degenerative changes are linked to poor rehabilitative and surgical outcomes, which has launched investigations into co-therapeutic biologics. Specifically, Mesenchymal Stem Cells (MSCs) have shown promise in mitigating degenerative changes in animal models of RC tears, however reports of their impact on clinical outcomes remain mixed. Here we describe an alternative source of MSCs in the human shoulder, adipose stromal cells (ASCs) from the subacromial fat (SAF) pad. Compared to the gold-standard subcutaneous (SQ) fat, we show that SAF ASCs are less sensitive to chemical and mechanical fibrotic cues, 1) retaining smaller cell area with reduced actin stress fiber alignment across a range of physiological and pathological stiffnesses, 2) reduced traction forces and ECM production and 3) reduced myofibroblastic conversion in response to cytokine challenge. Furthermore, we show that SAF ASCs enhance fusion of primary human myoblasts via paracrine signaling. Despite a fibrotic signature in SAF from rotator cuffs with tendon tears, SAF ASCs sourced from torn rotator cuffs were equally effective at resisting fibroblastic conversion and promoting myogenesis as those from intact rotator cuffs, further supporting autologous clinical use of these cells. In conclusion, this study describes human SAF ASCs as an alternative, and potentially superior, cell source for rotator cuff therapies.