Levels of spontaneous apoptosis in HUVEC control cultures varied between 10·00 and 12·50%. The mean percentage of EC apoptosis induced by cell starvation and staurosporine was 55·46 and 66·80%, respectively. As demonstrated by the enumeration of hypoploid Gefitinib chemical structure cells, purified IgG from the AECA-positive SLE patients induced a significantly higher percentage of apoptosis of HUVECs in comparison to AECA-negative SLE patients (P = 0·001) and healthy controls (P < 0·0001) (Fig. 2). Purified

IgG from the AECA-positive PAH patients did not induce a higher percentage of apoptosis of HUVECs compared to the AECA-negative PAH patients (P = 0·92) and healthy controls (P = 0·08), as assessed by the enumeration of hypoploid cells (Fig. 2). Also in the SSc cohort, no induction of apoptosis was observed (Fig. 2). Further analysis of the PAH cohort demonstrated that IgG from the AECA-positive IPAH patients did not induce a significantly higher percentage of apoptosis of HUVECs compared to the AECA-negative PAH patients (P = 0·94) and healthy controls (P = 0·09), as assessed by the enumeration of hypoploid cells. Incubation with IgG from AECA-positive SLE (n = 3) patients induced a significant decrease in the CI value compared to IgG from AECA-negative SLE (n = 3) patients (P = 0·050) and healthy controls (P = 0·020) (Fig. 3). In fact,

IgG from AECA-positive SLE patients induced a decrease in CI value of 79%, which was comparable with the decrease in CI values induced by cell starvation YAP-TEAD Inhibitor 1 mw (82%) and incubation with 5 nmol/ml staurosporine (93%). Incubation of HUVECs with IgG from the AECA-positive PAH (n = 8) and SSc (n = 6) patients, however, did not alter the CI value significantly compared to IgG from the

AECA-negative PAH (n = 8) and SSc (n = 6) patients (P = 0·248 and P = 0·749, respectively) and healthy controls (P = 0·121 and P = 0·337, respectively). The aetiology of PAH is still poorly understood, and it is postulated that dysfunction of pulmonary ECs plays next an important role in the pathophysiology of PAH [5]. EC dysfunction may lead to pulmonary vascular remodelling and ultimately to the development of PAH [4, 5]. Mounting evidence suggests an important role for EC apoptosis in this process. Taraseviciene-Stewart et al. demonstrated that selective blockade of the vascular endothelial growth factor receptor 2 (VEGFR-2) resulted in severe irreversible pulmonary hypertension associated with precapillary arterial endothelial cell proliferation in chronically hypoxic rats [7]. EC apoptosis following VEGFR-2 blockade was a prerequisite for endothelial proliferation, because caspase inhibition throughout the course of chronic hypoxia and VEGFR-2 blockade prevented EC proliferation and the development of severe pulmonary hypertension [7].