ENDOTHELIAL BMPR2 LOSS PROMOTES ALTERED IL-15 SIGNALING, CONTRIBUTING TO IMMUNE DYSFUNCTION IN PULMONARY HYPERTENSION

BACKGROUND Natural killer (NK) cell impairment is a feature of pulmonary arterial hypertension (PAH) and contributes to disease development in mouse models of NK cell deficiency (Ormiston et al, Circulation, 2012; Ratsep et al, AJP Lung, 2018). PAH is also strongly associated with mutations in BMPR2, the gene encoding the type-II bone morphogenetic protein receptor. Genetic and experimental evidence point to a loss of BMPR2 in the pulmonary endothelium as a critical contributor to disease development. However, the impact of endothelial BMPR2 loss on NK cell impairment in PAH remains unknown. METHODS AND RESULTS SiRNA-mediated silencing of BMPR2 in human pulmonary artery endothelial cells (HPAEC) significantly reduced the surface presentation and secretion of the α-type receptor for interleukin-15 (IL-15Rα), a major regulator of NK cell survival, proliferation and activity. Assessment of IL-15Rα transcriptional variants by RNA-Seq and glycosylation by immunoblotting demonstrated that this reduction was not due to changes in receptor transcription or processing. Treatment of HPAECs with the matrix metalloproteinase inhibitor Batimastat also ruled out a role for altered IL-15Rα cleavage with BMPR2 silencing. Instead, confocal microscopy identified impaired trafficking of IL-15Rα through the trans-Golgi-network (TGN) in BMPR2-silenced cells. To evaluate the effects of impaired IL-15 mediated signaling and NK cell loss on PAH development, NK cell-deficient Il15-/- rats and immune-competent Il15+/+ littermates were exposed to either the monocrotaline (MCT) or SUGEN/hypoxia models of experimental pulmonary hypertension. While both male and female Il15-/- rats developed more severe disease than their wildtype counterparts in the SUGEN/Hypoxia model, only male Il15-/- rats demonstrated this enhanced severity in the MCT model, which is the only one of the two models to reproduce the sex-based difference in disease severity that is observed in human PAH patients. Assessment of genes associated with sex hormone signaling and metabolism identified a potential increase in pulmonary Cyp1b1 expression (n=5-7, p=0.08) in male MCT-challenged Il15-/- rats relative to Il15+/+ controls. CONCLUSION We have identified the loss of IL-15 signaling as a novel BMPR2-dependent contributor to NK cell impairment and pulmonary arterial hypertension pathogenesis. Ongoing work includes assessing the contribution of sex hormones to immune-mediated disease processes in PAH.