In CTEPH patients we observed (1) that the compartments of naive CD4+ and CD8+ T cells, memory CD4+ T cells and Tregs contained increased proportions of CCR6+ cells; (2) that CD4+ and CD8+ T cells displayed reduced production of various cytokines, including TNFα and IFNγ; and (3) that in CD4+ and CD8+ T cells CTLA4 expression was increased. The gene discussed is CD4; the disease is chronic thromboembolic pulmonary hypertension.