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2 Immune mechanisms and autoimmunity seem to play a significant role in idiopathic pulmonary arterial hypertension (IPAH) genesis and/or progression but the pathophysiology is still unclear. Recent evidence has demonstrated a detrimental involvement of leptin in promoting various autoimmune diseases by controlling regulatory T lymphocytes. Despite this knowledge, the role of leptin in IPAH is currently unknown. We hypothesized that leptin, synthesized by dysfunctional pulmonary endothelium, might play a role in the immunopathogenesis of IPAH by regulating circulating regulatory T lymphocytes function. First, we collected serum and regulatory T lymphocytes from controls, IPAH and scleroderma-associated (SSc-PAH) patients; secondly, we recovered tissue samples and cultured endothelial cells after surgery or transplantation in controls and IPAH patients, respectively. Our findings indicate that serum leptin was higher in IPAH and SSc-PAH patients compared to controls. Circulating regulatory T lymphocytes number was comparabl