BTK and idiopathic pulmonary arterial hypertension: Interestingly, BTK levels in B cells correlated with ICOS expression on cTfh cells in patients with rheumatoid arthritis and with parotid gland T-cell infiltration in patients with Sjögren syndrome.14 In this context, it is of note that we found that proportions of cTfh17 cells were increased in patients with IPAH, similar to findings in systemic sclerosis20 and correlated with BTK expression levels in circulating B cells.