The Immunological Contributions to Heterotopic Ossification Disorders - Insights from Fibrodysplasia Ossificans Progressiva

The development of pathological bone outside the skeleton, termed heterotopic ossification (HO), is a significant clinical complication that often greatly reduces mobility and diminishes overall quality of life for affected individuals. Patients with fibrodysplasia ossificans progressiva (FOP; OMIM #135100), a genetic disorder of HO in which most affected individuals express a recurrent heterozygous gain-of-function mutation (R206H) in the bone morphogenetic protein (BMP) type I receptor ACVR1/ALK2, develop episodes of HO formation frequently follow injury. Terminal HO formation in FOP occurs following a series of lesion development stages, of which the first recognized is an inflammatory stage associated with immune cell invasion. Of note, an early inflammatory response is a normal response to tissue injury, however in tissues expressing the FOP mutation, the repair program rapidly diverges from a path leading to tissue repair and instead forms ectopic cartilage and bone. I hypothesized that Acvr1 R206H enhances the early inflammatory response to injury in FOP and that immune cells promote a permissive microenvironment for the downstream anabolic events that result in HO. Using a conditional knock-in Acvr1 R206H mouse model (Acvr1cR206H/+) to investigate the cellular and molecular inflammatory events in FOP Acvr1cR206H/+ and wild-type Acvr1+/+ mice following injury, I determined that the response to tissue injury is similar between cohorts up to 48 hours post-injury, but then diverges toward a prolonged fibroproliferative stage, then to chondrogenic, and osteogenic events in Acvr1cR206H/+ mice. This coincides with a significantly elevated and prolonged pro-inflammatory cytokine expression in vivo and in vitro. I further investigated how modulation of the inflammatory response controls the development of HO in FOP. Induction of the R206H mutation exclusively in immune cells, by whole bone marrow transplant or LysM-Cre-induced myeloid-lineage expression, or selective inhibition of the inflammatory response by Interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) neutralizing agents were insufficient to prevent HO formation. However, depletion of mast cells and macrophages from Acvr1cR206H/+ mice dramatically impaired development of HO, highlighting a direct immune cell contribution to HO formation