Cellular sources of interleukin-6 and associations with clinical phenotypes and outcomes in pulmonary arterial hypertension

Abstract

The pro-inflammatory cytokine interleukin (IL)-6 has been associated with outcomes in small pulmonary arterial hypertension (PAH) cohorts composed largely of patients with severe idiopathic PAH (IPAH). It is unclear whether IL-6 is a marker of critical illness or a mechanistic biomarker of pulmonary vascular remodelling. We hypothesised that IL-6 is produced by pulmonary vascular cells and sought to explore IL-6 associations with phenotypes and outcomes across diverse subtypes in a large PAH cohort.IL-6 protein and gene expression levels were measured in cultured pulmonary artery smooth muscle cells (PASMCs) and endothelial cells (PAECs) from PAH patients and healthy controls. Serum IL-6 was measured in 2017 well-characterised PAH subjects representing each PAH subgroup. Relationships between IL-6 levels, clinical variables, and mortality were analysed using regression models.Significantly higher IL-6 protein and gene expression levels were produced by PASMCs than by PAECs in PAH (p<0.001), while there was no difference in IL-6 between cell types in controls. Serum IL-6 was highest in PAH related to portal hypertension and connective tissue diseases (CTD-PAH). In multivariable modelling, serum IL-6 was associated with survival in the overall cohort (hazard ratio 1.22, 95% CI 1.08-1.38; p<0.01) and in IPAH, but not in CTD-PAH. IL-6 remained associated with survival in low-risk subgroups of subjects with mild disease.IL-6 is released from PASMCs, and circulating IL-6 is associated with specific clinical phenotypes and outcomes in various PAH subgroups, including subjects with less severe disease. IL-6 is a mechanistic biomarker, and thus a potential therapeutic target, in certain PAH subgroups.

FIGURE 1

Comparison of interleukin (IL)-6 levels by pulmonary arterial hypertension (PAH) subtype: connective tissue disease-associated PAH (CTD-PAH), idiopathic PAH (IPAH), familial PAH (FPAH), pulmonary veno-occlusive disease-associated PAH (PVOD-PAH), portopulmonary hypertension-associated PAH (PoPH-PAH), congenital heart disease-associated PAH (CHD-PAH), drug-associated PAH (Drug-PAH), HIV-associated PAH (HIV-PAH) and other unspecified disease-associated forms of PAH (Other-PAH).

FIGURE 2

Kaplan–Meier survival analysis among pulmonary arterial hypertension subjects with available survival data with interleukin (IL)-6 levels above versus below the median (log rank p<0.0001).

FIGURE 3

Kaplan–Meier survival analyses among subjects with a) idiopathic pulmonary arterial hypertension and b) connective tissue disease-associated pulmonary arterial hypertension with interleukin (IL)-6 levels above versus below the median of the respective subgroup (log rank p<0.0001 for each).

FIGURE 4

Comparison of interleukin (IL)-6 levels in conditioned media from a) smooth muscle cells (SMCs) and endothelial cells (EC) from transplanted pulmonary arterial hypertension (PAH) subjects and non-transplanted donor controls (p<0.0001 for SMC-PAH versus EC-PAH); b) SMCs in controls and PAH subtypes; and c) ECs in controls and PAH subtypes. Comparison of IL-6 gene expression levels by fragments per kilobase of exon model per million reads mapped (FPKM) between d) SMCs and ECs from PAH subjects and controls (p<0.0001 for SMC-PAH versus EC-PAH); e) SMCs in controls and PAH subtypes; and f) ECs in controls and PAH subtypes. APAH: disease-associated PAH subtypes; IPAH: idiopathic PAH; FPAH: familial PAH.