FAS and idiopathic pulmonary arterial hypertension: However, enhanced BCR signalling due to BTK overexpression in mice is sufficient to induce resistance to Fas-mediated apoptosis and development of B-cell mediated autoimmunity.13 Parallel to our recent observation of increased BTK expression and phosphorylation in patients with anticitrullinated protein antibody-positive rheumatoid arthritis, Sjogren’s syndrome and autoimmune vasculitis,14 29 we found that BTK expression was also increased in naive B cells in patients with IPAH.