Because decreased CD4+CTLA-4+ levels in patients with CTD-PAH and CTEPH may correlate with the development of immune toxicity and, therefore, a severe disease course, we suggest further studies of the CD4+CTLA+ group as divided into CD4+ Th, Treg and Tm cells. The gene discussed is CTLA4; the disease is chronic thromboembolic pulmonary hypertension.