While additional studies are needed to clarify the mechanisms by which IL9 promotes amelioration of PH and to investigate the potential of F8IL9F8 to modify the pathological course of PH disease, the results presented in this study provide the first evidence that targeted delivery of IL9, mediated by the F8 antibody, to pulmonary hypertension lesions may represent a potential new treatment for PH. This evidence concerns the gene IL9 and pulmonary arterial hypertension.