A closed-loop cell therapy engineered to autonomously secrete Activin A inhibitor protects from fibrodysplasia ossificans progressiva

Engineered cell therapies present an opportunity for endogenous, site-specific production of therapeutic agents. Here we describe a closed-loop cell therapy which secretes an inhibitor of Activin A, ActR2A-Fc, upon exposure to Activin A. We demonstrate in vivo therapeutic efficacy of this approach in a mouse model of fibrodysplasia ossificans progressiva (FOP), a morbid condition in which patients develop extensive heterotopic bony lesions in response to aberrant sensitivity to Activin A through a mutation in the type I BMP receptor ACVR1 (ACVR1 R206H). To blunt Activin A activity, we designed a transposon plasmid containing the transgene encoding ActR2A-Fc, with expression controlled by the BMP-responsive element (BRE). In cells containing the causative mutation, the BRE is pathologically activated upon exposure to Activin A. FOP-derived marrow cells modified with the BRE-ActR2AFc plasmid exhibited the desired closed-loop functionality, with increased ActR2A-Fc expression upon exposure to Activin A and reduced expression upon withdrawal of Activin A. Engineered marrow cells secreted bioactive ActR2A-Fc, and bone marrow transplantation of FOP marrow cells engineered with the BRE-ActR2AFc transposon into same-sex FOP mice resulted in absence of heterotopic bony lesions. Experiments with labeled, engineered FOP marrow cells verified trafficking of the therapeutic cells to sites at risk for FOP. These data provide proof-of-concept for the therapeutic utility of engineered cell therapy for the treatment of FOP. Significance statementIn this study, we describe our development of an autologous, closed-loop cell therapy which can migrate to sites of tissue injury and locally secreting an inhibitor of Activin A. Through our use of an Activin A-responsive promoter to drive expression of the recombinant Activin A inhibitor, this engineered cell therapy exhibits closed-loop behavior and effectively prevents heterotopic bone formation in a mouse model of fibrodysplasia ossificans progressiva (FOP). We believe that the findings in this manuscript impactful beyond FOP, and provide a blueprint for the development of marrow-derived cell therapies across the disease spectrum.