Pulmonary arterial hypertension (PAH) is a cardiopulmonary disease characterized by an incurable condition of the pulmonary vasculature, leading to increased pulmonary vascular resistance, elevated pulmonary arterial pressure resulting in progressive right ventricular failure and ultimately death. PAH has different underlying causes. In approximately 30–40% of the patients no underlying risk factor or cause can be found, so-called idiopathic PAH (IPAH). Patients with an autoimmune connective tissue disease (CTD) can develop PAH [CTD-associated PAH (CTD-PAH)], suggesting a prominent role of immune cell activation in PAH pathophysiology. This is further supported by the presence of tertiary lymphoid organs (TLOs) near pulmonary blood vessels in IPAH and CTD-PAH. TLOs consist of myeloid cells, like monocytes and dendritic cells (DCs), T-cells, and B-cells. Next to their T-cell activating function, DCs are crucial for the preservation of TLOs. Multiple DC subsets can be found in steady state, such as conventional DCs (cDCs), including type 1 cDCs (cDC1s), and type 2 cDCs (cDC2s), AXL⁺Siglec6⁺ DCs (AS-DCs), and plasmacytoid DCs (pDCs). Under inflammatory conditions monocytes can differentiate into monocyte-derived-DCs (mo-DCs). DC subset distribution and activation status play an important role in the pathobiology of autoimmune diseases and most likely in the development of IPAH and CTD-PAH. DCs can contribute to pathology by activating T-cells (production of pro-inflammatory cytokines) and B-cells (pathogenic antibody secretion). In this review we therefore describe the latest knowledge about DC subset distribution, activation status, and effector functions, and polymorphisms involved in DC function in IPAH and CTD-PAH to gain a better understanding of PAH pathology.

Mechanical forces are translated into biochemical stimuli by mechanotransduction channels, such as the mechanically activated cation channel Piezo2. Lung Piezo2 expression has recently been shown to be restricted to endothelial cells. Hence, we aimed to investigate the role of Piezo2 in regulation of pulmonary vascular function and structure, as well as its contribution to development of pulmonary arterial hypertension (PAH). The expression of Piezo2 was significantly reduced in pulmonary microvascular endothelial cells (MVECs) from patients with PAH, in lung tissue from mice with a Bmpr2^+/R899X knock-in mutation commonly found in patients with pulmonary hypertension, and in lung tissue of monocrotaline (MCT) and sugen-hypoxia-induced PH (SuHx) PAH rat models, as well as from a swine model with pulmonary vein banding. In MVECs, Piezo2 expression was reduced in response to abnormal shear stress, hypoxia, and TGFβ stimulation. Functional studies in MVECs exposed to shear stress illustrated that siRNA-mediated Piezo2 knockdown impaired endothelial alignment, calcium influx, phosphorylation of AKT, and nitric oxide production. In addition, siPiezo2 reduced the expression of the endothelial marker PECAM-1 and increased the expression of vascular smooth muscle markers ACTA2, SM22a, and calponin. Thus, Piezo2 acts as a mechanotransduction channel in pulmonary MVECs, stimulating shear-induced production of nitric oxide and is essentially involved in preventing endothelial to mesenchymal transition. Its blunted expression in pulmonary hypertension could impair the vasodilator capacity and stimulate vascular remodeling, indicating that Piezo2 might be an interesting therapeutic target to attenuate progression of the disease. NEW & NOTEWORTHY The mechanosensory ion channel Piezo2 is exclusively expressed in lung microvascular endothelial cells (MVECs). Patient MVECs as well as animal models of pulmonary (arterial) hypertension showed lower expression of Piezo2 in the lung. Mechanistically, Piezo2 is required for calcium influx and NO production in response to shear stress, whereas stimuli known to induce endothelial to mesenchymal transition (EndMT) reduce Piezo2 expression in MVECs, and Piezo2 knockdown induces a gene and protein expression pattern consistent with EndMT.

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.

Background: Conventional type 1 dendritic cells (cDC1s) control anti‐viral and anti‐tumor immunity by inducing antigen‐specific cytotoxic CD8⁺ T‐cell responses. Controversy exists whether cDC1s also control CD4⁺ T helper 2 (Th2) cell responses, since suppressive and activating roles have been reported. DC activation status, controlled by the transcription factor NF‐κB, might determine the precise outcome of Th‐cell differentiation upon encounter with cDC1s. To investigate the role of activated cDC1s in Th2‐driven immune responses, pulmonary cDC1s were activated by targeted deletion of A20/Tnfaip3, a negative regulator of NF‐κB signaling.Methods: To target pulmonary cDC1s, Cd207 (Langerin)‐mediated excision of A20/Tnfaip3 was used, generating Tnfaip3^fl/flxCd207^+/cre (Tnfaip3^Lg‐KO) mice. Mice were exposed to house dust mite (HDM) to provoke Th2‐mediated immune responses.Results: Mice harboring Tnfaip3‐deficient cDC1s did not develop Th2‐driven eosinophilic airway inflammation upon HDM exposure, but rather showed elevated numbers of IFNγ‐expressing CD8⁺ T cells. In addition, Tnfaip3^Lg‐KO mice harbored increased numbers of IL‐12–expressing cDC1s and elevated PD‐L1 expression in all pulmonary DC subsets. Blocking either IL‐12 or IFNγ in Tnfaip3^Lg‐KO mice restored Th2 responses, whereas administration of recombinant IFNγ during HDM sensitization in C57Bl/6 mice blocked Th2 development.Conclusions: These findings indicate that the activation status of cDC1s, shown by their specific expression of co‐inhibitory molecules and cytokines, critically contributes to the development of Th2 cell–mediated disorders, most likely by influencing IFNγ production in CD8⁺ T cells.

Pulmonary arterial hypertension (PAH) is rare disease that is categorized as idiopathic (IPAH) when no underlying cause can be identified. Lungs of most patients with IPAH contain increased numbers of T cells and dendritic cells (DCs), suggesting involvement of the immune system in its pathophysiology. However, our knowledge on circulating immune cells in IPAH is rather limited. We used flow cytometry to characterize peripheral blood DCs and T cells in treatment-naive IPAH patients, compared with connective-tissue disease-PAH (CTD-PAH) patients and healthy controls (HCs). At diagnosis, T-helper (Th) cells of IPAH patients were less capable of producing TNFα, IFNγ, IL-4 and IL-17 compared to HCs. IPAH patients showed a decreased frequency of Th2 cells and significantly enhanced expression of the CTLA4 checkpoint molecule in naive CD4⁺ T cells and both naive and memory CD8⁺ T cells. Frequencies and surface marker expression of circulating DCs and monocytes were essentially comparable between IPAH patients and HCs. Principal component analysis (PCA) separated IPAH patients—but not CTD-PAH patients—from HCs, based on T-cell cytokine profiles. At 1-year follow-up, the frequencies of IL-17⁺ production by memory CD4⁺ T cells were increased in IPAH patients and accompanied by increased proportions of Th17 and Tc17 cells, as well as decreased CTLA4 expression. Treatment-naive IPAH patients displayed a unique T-cell phenotype that was different from CTD-PAH patients and was characterized by reduced cytokine-producing capacity. These findings point to involvement of adaptive immune responses in IPAH, which may have an implication for the development of therapeutic interventions.