Patients in the iPAH group had, by significant distance, the highest percentage of CD4+CTLA-4+ T lymphocytes among all PAH groups, as compared to those in the control group (p < 0.001), patients with CTEPH (p < 0.001), CTD-PAH (p < 0.001) and CHD-PAH (p < 0.01). Here, CTLA4 is linked to chronic thromboembolic pulmonary hypertension.