TLR4 and pulmonary arterial hypertension: Considering the uniform upregulation of S100A9 and its classical receptors, RAGE and TLR4, in PAH, coupled with their ability to trigger inflammatory responses and perturb immune system homeostasis upon heightened expression, we speculate that the interplay between S100A9 and RAGE or TLR4 might constitute one of the underlying mechanisms driving the pathogenesis of PAH (Figure 5).