Abstract Body (Do not enter title and authors here): Background: Pulmonary arterial hypertension (PAH) is a severe condition characterized by pulmonary vascular remodelling leading to right ventricle failure. Emerging evidence highlights inflammation as crucial pathogenic factor of PAH. Members of the complement pathway have been identified as dysregulated in multiple studies. We aimed to investigate the relative contribution of different complement pathway components in PAH pathogenesis using pre-clinical rodent models.

Methods and Results: The Sugen 5416 (20 mg/kg, i.p.) plus hypoxia (SuHx) model in both male C57Bl/6 mice (weekly Sugen + 3 weeks hypoxia) and Wistar rats (single Sugen, 3 weeks hypoxia, 6 weeks in room air) were used to assess the effect of complement inhibition on PAH. Targeting the terminal complement pathway with TPP-903, an anti-C5 antibody administered at 3- or 6 weeks, failed to demonstrate any beneficial effects on the PAH phenotype in SuHx rats. To investigate the alternative pathway, we first utilised the mouse SuHx model, targeting hepatic C3 knockdown using anti-C3 siRNA administered 1 week before PAH induction. There was a significant decrease in both the percentage of remodelled vessels and the degree of remodelling but no significant effect on right ventricular systolic pressure (RVSP). To further assess hemodynamic outcomes alongside vascular remodelling, we repeated the use of anti-C3 siRNA (from week 5) in the more severe rat SuHx model. Treatment with anti-C3 siRNA was associated with more than 90% reduction in C3 RNA expression in the liver, as well as around 91% reduction in circulating C3 levels. Considerable improvement in the PAH phenotype, as evidenced by reduction in RVSP (94.63 vs. 69.71 mmHg, p<0.001, n=8), improvement RV cardiac function (RV-fractional area change (RV-FAC%): 33.54 vs. 43.86%, p<0.05, n=8) were associated with reduced pulmonary vascular remodelling (75 vs. 57%, n=4) compared to vehicle treated rats. Furthermore, there was a reduction in cardiac fibrosis in anti-C3 siRNA treated rats, as assessed by Sirius red staining.

Conclusion: The therapeutic benefits observed in both right ventricular function and pulmonary vasculature emphasize the potential of complement pathway modulation, particularly through hepatic targeting of C3 as an approach for the treatment of PAH. Ongoing investigations aim to uncover the localized mechanisms driving this therapeutic effect, providing valuable insights for future clinical applications.