B cells in pulmonary arterial hypertension: friend, foe or bystander?

There is an unmet need for new therapeutic strategies that target alternative pathways to improve the prognosis of patients with pulmonary arterial hypertension (PAH). As immunity has been involved in the development and progression of vascular lesions in PAH, we review the potential contribution of B cells in its pathogenesis and evaluate the relevance of B cell-targeted therapies. Circulating B cell homeostasis is altered in PAH patients, with total B-cell lymphopenia, abnormal subset distribution (expansion of naïve and antibody-secreting cells, reduction of memory B cells) and chronic activation. B cells are recruited to the lungs through local chemokine secretion, and activated by several mechanisms: 1) interaction with lung vascular auto-antigens through cognate B cell receptors; 2) co-stimulatory signals provided by T follicular helper (Tfh) cells (IL-21), T helper 2 (Th2) cells and mast cells (IL-4, IL-6 and IL-13); and 3) increased survival signals provided by B cell activating factor (BAFF) pathways. This activity results in the formation of germinal centres within perivascular tertiary lymphoid organs and in the local production of pathogenic autoantibodies that target the pulmonary vasculature and vascular stabilization factors (including angiotensin-II/endothelin-1 receptors and bone morphogenetic protein receptors). B cells also mediate their effects through enhanced production of pro-inflammatory cytokines, reduced anti-inflammatory properties by regulatory B cells, IgG-induced complement activation, and IgE-induced mast cell activation. Precision-medicine approaches targeting B cell immunity are a promising direction for select PAH conditions, as suggested by the efficacy of anti-CD20 therapy in experimental models and a trial of rituximab in systemic sclerosis-associated PAH.