Idiopathic and connective tissue disease-associated pulmonary arterial hypertension (PAH): Similarities, differences and the role of autoimmunity

Pre-capillary pulmonary arterial hypertension (PAH) is hemodynamically characterized by a mean pulmonary arterial pressure (mPAP) ≥ 20 mmHg, pulmonary capillary wedge pressure (PAWP) ≤15 mmHg and pulmonary vascular resistance (PVR) > 2. PAH is classified in six clinical subgroups, including idiopathic PAH (IPAH) and PAH associated to connective tissue diseases (CTD-PAH), that will be the main object of this review. The aim is to compare these two PAH subgroups in terms of epidemiology, histological and pathogenic findings in an attempt to define disease-specific features, including autoimmunity, that may explain the heterogeneity of response to therapy between IPAH and CTD-PAH.

Central Role of Dendritic Cells in Pulmonary Arterial Hypertension in Human and Mice

The pathogenesis of idiopathic pulmonary arterial hypertension (IPAH) is not fully understood, but evidence is accumulating that immune dysfunction plays a significant role. We previously reported that 31-week-old Tnfaip3^DNGR1-KO mice develop pulmonary hypertension (PH) symptoms. These mice harbor a targeted deletion of the TNFα-induced protein-3 (Tnfaip3) gene, encoding the NF-κB regulatory protein A20, specifically in type I conventional dendritic cells (cDC1s). Here, we studied the involvement of dendritic cells (DCs) in PH in more detail. We found various immune cells, including DCs, in the hearts of Tnfaip3^DNGR1-KO mice, particularly in the right ventricle (RV). Secondly, in young Tnfaip3^DNGR1-KO mice, innate immune activation through airway exposure to toll-like receptor ligands essentially did not result in elevated RV pressures, although we did observe significant RV hypertrophy. Thirdly, PH symptoms in Tnfaip3^DNGR1-KO mice were not enhanced by concomitant mutation of bone morphogenetic protein receptor type 2 (Bmpr2), which is the most affected gene in PAH patients. Finally, in human IPAH lung tissue we found co-localization of DCs and CD8+ T cells, representing the main cell type activated by cDC1s. Taken together, these findings support a unique role of cDC1s in PAH pathogenesis, independent of general immune activation or a mutation in the Bmpr2 gene.

Identification of Potential Risk Genes and the Immune Landscape of Idiopathic Pulmonary Arterial Hypertension via Microarray Gene Expression Dataset Reanalysis

Gene dysfunction and immune cell infiltration play an essential role in the pathogenesis of idiopathic pulmonary arterial hypertension (IPAH). We aimed to investigate the immune landscape and novel differentially expressed genes (DEGs) of IPAH. In addition, potential druggable molecular targets for IPAH were also explored. In this study, the GSE117261 dataset was reanalyzed to explore the immune landscape and hub DEGs of IPAH. Lasso Cox regression analysis and receiver operating characteristic curve analysis were performed to detect the predictive value of IPAH. Additionally, the underlying drug targets for IPAH treatment were determined by drug-gene analysis. IPAH was significantly associated with the transforming growth factor-β (TGF-β) signaling pathway and Wnt signaling pathway as well as energetic metabolism dysfunction. We identified 31 upregulated and 39 downregulated DEGs in IPAH patients. Six hub genes, namely, SAA1, CCL5, CXCR1, CXCR2, CCR1, and ADORA3, were related to IPAH pathogenesis regardless of sex differences. Prediction model analysis showed that the area under the curve values of the hub DEGs except CXCR2 were all above 0.9 for distinguishing IPAH patients. In addition, the relative proportions of 5 subtypes of immune cells, namely, CD₈ ⁺ T cells, CD₄ ⁺ memory resting T cells, γ delta T cells, M1 macrophages, and resting mast cells, were significantly upregulated in the IPAH samples, while 6 subtypes of immune cells, namely, CD₄ ⁺ naive T cells, resting NK cells, monocytes, M0 macrophages, activated mast cells, and neutrophils, were downregulated. Additionally, a total of 17 intersecting drugs targeting 5 genes, CCL5, CXCR1, CXCR2, CCR1, and ADORA3, were generated as potential druggable molecular targets for IPAH. Our study revealed the underlying correlations between genes and immune cells in IPAH and demonstrated for the first time that SAA1, CCL5, CXCR1, CCR1, and ADORA3 may be novel genetic targets for IPAH.

Circulating plasmablasts are elevated and produce pathogenic anti-endothelial cell autoantibodies in idiopathic pulmonary arterial hypertension

Idiopathic pulmonary arterial hypertension (IPAH) is a devastating pulmonary vascular disease in which autoimmune and inflammatory phenomena are implicated. B cells and autoantibodies have been associated with IPAH and identified as potential therapeutic targets. However, the specific populations of B cells involved and their roles in disease pathogenesis are not clearly defined. We aimed to assess the levels of activated B cells (plasmablasts) in IPAH, and to characterize recombinant antibodies derived from these plasmablasts. Blood plasmablasts are elevated in IPAH, remain elevated over time, and produce IgA autoantibodies. Single-cell sequencing of plasmablasts in IPAH revealed repertoires of affinity-matured antibodies with small clonal expansions, consistent with an ongoing autoimmune response. Recombinant antibodies representative of these clonal lineages bound known autoantigen targets and displayed an unexpectedly high degree of polyreactivity. Representative IPAH plasmablast recombinant antibodies stimulated human umbilical vein endothelial cells to produce cytokines and overexpress the adhesion molecule ICAM-1. Together, our results demonstrate an ongoing adaptive autoimmune response involving IgA plasmablasts that produce anti-endothelial cell autoantibodies in IPAH. These antibodies stimulate endothelial cell production of cytokines and adhesion molecules, which may contribute to disease pathogenesis. These findings suggest a role for mucosally-driven autoimmunity and autoimmune injury in the pathogenesis of IPAH.