Sotatercept is an activin receptor type IIA-Fc (ActRIIA-Fc) fusion protein that improves cardiopulmonary function in patients with pulmonary arterial hypertension (PAH) by selectively trapping activins and growth differentiation factors. However, the cellular and molecular mechanisms of ActRIIA-Fc action are incompletely understood. Here, we determined through genome-wide expression profiling that inflammatory and immune responses are prominently upregulated in the lungs of a Sugen-hypoxia rat model of severe angio-obliterative PAH, concordant with profiles observed in PAH patients. Therapeutic treatment with ActRIIA-Fc—but not with a vasodilator—strikingly reversed proinflammatory and proliferative gene expression profiles and normalized macrophage infiltration in diseased rodent lungs. Furthermore, ActRIIA-Fc normalized pulmonary macrophage infiltration and corrected cardiopulmonary structure and function in Bmpr2 haploinsufficient mice subjected to hypoxia, a model of heritable PAH. Three high-affinity ligands of ActRIIA-Fc each induced macrophage activation in vitro, and their combined immunoneutralization in PAH rats produced cardiopulmonary benefits comparable to those elicited by ActRIIA-Fc. Our results in complementary experimental and genetic models of PAH reveal therapeutic anti-inflammatory activities of ActRIIA-Fc that, together with its known anti-proliferative effects on vascular cell types, could underlie clinical activity of sotatercept as either monotherapy or add-on to current PAH therapies.

Patients with neuromuscular disorders suffer from a lack of treatment options for skeletal muscle weakness and disease comorbidities. Here, we introduce as a potential therapeutic agent a heterodimeric ligand-trapping fusion protein, ActRIIB:ALK4-Fc, which comprises extracellular domains of activin-like kinase 4 (ALK4) and activin receptor type IIB (ActRIIB), a naturally occurring pair of type I and II receptors belonging to the TGF-β superfamily. By surface plasmon resonance (SPR), ActRIIB:ALK4-Fc exhibited a ligand binding profile distinctly different from that of its homodimeric variant ActRIIB-Fc, sequestering ActRIIB ligands known to inhibit muscle growth but not trapping the vascular regulatory ligand bone morphogenetic protein 9 (BMP9). ActRIIB:ALK4-Fc and ActRIIB-Fc administered to mice exerted differential effectsconcordant with SPR results -on vessel outgrowth in a retinal explant assay. ActRIIB:ALK4-Fc induced a systemic increase in muscle mass and function in wild-type mice and in murine models of Duchenne muscular dystrophy (DMD), amyotrophic lateral sclerosis (ALS), and disuse atrophy. Importantly, ActRIIB:ALK4-Fc improved neuromuscular junction abnormalities in murine models of DMD and presymptomatic ALS and alleviated acute muscle fibrosis in a DMD model. Furthermore, in combination therapy ActRIIB:ALK4-Fc increased the efficacy of antisense oligonucleotide M12-PMO on dystrophin expression and skeletal muscle endurance in an aged DMD model. ActRIIB:ALK4-Fc shows promise as a therapeutic agent, alone or in combination with dystrophin rescue therapy, to alleviate muscle weakness and comorbidities of neuromuscular disorders.

The random amino acid copolymer poly(Y,E,A,K)n (Copaxone®) is widely used in multiple sclerosis treatment and a second generation copolymer poly(Y,F,A,K)n with enhanced efficacy in experimental autoimmune encephalomyelitis in mice has been described. A major mechanism through which copolymers function to ameliorate disease is the generation of immunosuppressive IL-10-secreting regulatory T cells entering the CNS. In addition, the antigen presenting cell to which these copolymers bind through MHC Class II proteins may have an important role. Here, both CCL22 (a Th2 cell chemoattractant) in large amounts and CXCL13 in much smaller amounts are shown to be secreted after administration of YFAK to mice and to a smaller extent by YEAK parallel to their serum concentrations. Moreover, bone marrow-derived macrophages secrete CCL22 in vitro in response to YFAK and to higher concentrations of YEAK. Strikingly, these chemokines are also secreted into serum of MHC Class II −/− mice, indicating that an innate immune receptor on these cells also has an important role. Thus, both the innate and the adaptive immune systems are involved in the mechanism of EAE amelioration by YFAK. The enhanced ability of YFAK to stimulate the innate immune system may account for its enhanced efficacy in EAE treatment.