TLR4 and pulmonary arterial hypertension: Previous studies demonstrated that serum endotoxin and sCD14 levels were significantly increased in patients with PAH and MCT-treated rats (Ranchoux et al., 2017), which promoted immune response and pulmonary vascular remodeling via activation of the Toll-like Receptor 4 (TLR4)/nuclear factor kB (NF-kB) inflammatory signaling pathway and confirmed intestinal bacterial translocation and macrophage activation in PAH (Perros et al., 2011; Bauer et al., 2014; Ranchoux et al., 2017).