Inhibition of immune checkpoints prevents injury-induced heterotopic ossification

Abstract

Heterotopic ossification (HO), true bone formation in soft tissue, is closely associated with abnormal injury/immune responses. We hypothesized that a key underlying mechanism of HO might be injury-induced dysregulation of immune checkpoint proteins (ICs). We found that the earliest stages of HO are characterized by enhanced infiltration of polarized macrophages into sites of minor injuries in an animal model of HO. The non-specific immune suppressants, Rapamycin and Ebselen, prevented HO providing evidence of the central role of the immune responses. We examined the expression pattern of ICs and found that they are dysregulated in HO lesions. More importantly, loss of function of inhibitory ICs (including PD1, PD-L1, and CD152) markedly inhibited HO, whereas loss of function of stimulatory ICs (including CD40L and OX-40L) facilitated HO. These findings suggest that IC inhibitors may provide a therapeutic approach to prevent or limit the extent of HO.

Quercetin Attenuates Trauma-Induced Heterotopic Ossification by Tuning Immune Cell Infiltration and Related Inflammatory Insult

“…In line with this evidence, in this study, we confirmed the highly stimulated monocyte/macrophage and mast cell responses after tendon injury and revealed that inhibition of monocyte/macrophage and mast cell infiltration and ensuing proinflammatory activity by quercetin at an early stage effectively inhibited aberrant HO formation. Macrophages play a fundamental role in the control and remodeling of tissue homeostasis and are primary cellular responders upon injury (35,44). In addition to the tissue in which resident macrophages originally exist preceding injury, macrophages at the injury site also arise from the circulating pool of monocytes that receive stress signals and differentiate into inflammatory macrophages, which finally outnumber the resident macrophages and become the main initiator of inflammatory reactions (41).…”

“…Macrophages play a fundamental role in the control and remodeling of tissue homeostasis and are primary cellular responders upon injury ( 35 , 44 ). In addition to the tissue in which resident macrophages originally exist preceding injury, macrophages at the injury site also arise from the circulating pool of monocytes that receive stress signals and differentiate into inflammatory macrophages, which finally outnumber the resident macrophages and become the main initiator of inflammatory reactions ( 41 ).…”

“…In this regard, the weakened M1 and M2 marker expression at the injury site might also be cautiously regarded as a reflection of the impaired macrophage response. Given the evidence ( 44 ) that immune responses are overactive at early stages and application of immunosuppressants at early stages significantly limits HO development, the dampened initial overactivation of macrophages by quercetin might also constitute an osteogenic-suppressive immune microenvironment that inhibits aberrant HO formation, which was indicated by diminished PDGFRα+ mesenchymal stem cell accumulation. As previous study also indicated that quercetin could dose-dependently promote osteogenic differentiation of stem cells ( 63 – 65 ), the inhibitory effects of quercetin on HO might primarily attribute to its immunomodulatory property.…”

The Survey of Cells Responsible for Heterotopic Ossification Development in Skeletal Muscles—Human and Mouse Models

“…Additionally, the immune system may be a potential target for HO prevention. Neutralizing antibodies against immune checkpoint proteins (ICs) block limit the extent of HO in animal studies [ 9 ].…”

“…Thus, the presence of the cells reflects increased secretion of HO promoting cytokines/chemokines such as interleukin 6 (IL6), IL10, transforming beta-1 growth factor (TGFβ1), and neurotrophin 3 (NT3). A significant dysregulation of macrophage immune checkpoints was proven in HO animal models [ 8 , 9 , 10 ]. Finally, both individual predisposition and risk factors also attribute to HO development [ 11 ].…”