The hypothesis of a pivotal role of inflammation in PH pathogenesis is underlined by various studies investigating circulating inflammatory markers. i.e. IL-1 and IL-6, perivascular infiltrates and tertiary lymphoid tissues in PAH patients7. Intriguingly, inflammation seems to play a role in CTEPH pathogenesis, too, since markers of inflammatory activity (tumour necrosis factor (TNF)-alpha and monocyte chemoattractant protein-1, MCP-1) were found to be elevated in CTEPH and decreased after PEA2526. Interestingly, MCP-1 concentrations were higher in arterial compared with mixed venous blood suggesting increased production or reduced clearance within the pulmonary vasculature27.