Chronic thromboembolic pulmonary hypertension (CTEPH) is a progressive pulmonary vascular disease caused by single or recurrent pulmonary thromboembolism, leading to elevated pulmonary vascular resistance and maladaptive right ventricular failure (1–3). Removal of organized thrombotic materials and neointima by pulmonary endarterectomy (PEA) is the gold standard therapy for operable CTEPH (4), conferring a 3-year survival rate of up to 89% in operated patients (5). Despite significant improvement in hemodynamic parameters and cardiac function, owing to the frequent occurrence of residual pulmonary hypertension (PH), combined implementation of medical therapy and/or intervention may become necessary after the PEA operation (6).

The characteristics of major vascular remodeling can be revealed by studying tissues removed by PEA. Varied pathological features have been reported, probably reflecting the different stages in the development of CTEPH (7, 8). In general, four major pathological alterations are observed within the specimens from PEA, including neointima formation, atherosclerotic lesions, thrombosis, and recanalization (7). Macrophage-derived foam cells and cholesterol clefts are found in the atherosclerotic lesions in CTEPH (7, 9). However, it remains uncertain whether the traditional cardiovascular risk factors and the thromboembolus-derived atherosclerotic lesions in CTEPH are clinically related (10, 11).

Atherosclerotic lesions do not develop in pulmonary arteries under the normal environment of low pressure and high flow, but they may form during the development of PH (9, 12). Atherosclerotic lesions were found in approximately 30% of CTEPH cases (7, 13). Arbustini et al. reported that atherosclerotic plaques with glycophorin-rich pultaceous cores were more common in CTEPH than those in plexogenic PH (14). Nevertheless, the predisposing factors and clinical relevance of these atherosclerotic lesions in CTEPH have rarely been discussed.

It has been speculated that the atherosclerotic lesion formation in CTEPH may be associated with local metabolic changes (15). To address this question, we have adopted a metabolomic approach to examine the metabolite alterations and define the endophenotypes of the disease. This study aimed to investigate the clinical characteristics of CTEPH patients with atherosclerotic lesions and further explore the relevant metabolic pathways associated with atherosclerosis in the pulmonary artery.

In the present study, we described the clinical characteristics of CTEPH patients with atherosclerotic lesions and discovered the underlying metabolic alterations of these lesions. Our data indicated that atherosclerotic lesions in CTEPH were likely to be associated with a history of symptomatic embolism and a prolonged timespan between thromboembolism and PEA surgery, instead of the common risk factors of atherosclerosis in the systemic and coronary circulation. In line with these clinical findings, the formation of atherosclerotic lesion was related to aberrant amino acid metabolism, possibly responsible for cellular senescence and abnormal energy metabolism. The clinicopathological description of atherosclerosis and its associated changes in metabolites revealed by this study contribute to our understanding of an important subgroup of CTEPH and provide novel insight into the pathogenesis of this pulmonary vascular disease.

The proportion of atherosclerotic lesions in patients with CTEPH population was 27.2% in our patient population, which is consistent with previous pathological studies (7, 13). In our study, patients with atherosclerotic lesions were more likely to have had a history of symptomatic embolism. Patients with a prolonged timespan between thromboembolism and PEA surgery had a significantly higher incidence of atherosclerotic lesions, possibly due to persistent damage of the pulmonary artery by the fibrotic clots over time. However, in terms of this timespan, as nearly a quarter of the population did not have a history of thromboembolic events, and given that the initial episode of thromboembolism may not even be the first thromboembolic event in patients with a history of thromboembolism, the age of the lesion may be undetermined or underestimated for all patients in this study. As for the implication of atherosclerosis of the outcome of surgical treatment, Corsico et al. have demonstrated that after the perioperative period, the early hemodynamic benefits of PEA remain largely stable over 5 years (20). We therefore concluded that at an early time point, patients with and without atherosclerotic lesion had no difference in risk of persistent or residual PH. A possible reason for this might be that atherosclerotic lesions were accompanied by organized fibrotic materials that could be removed more completely by surgery. Taken together, we have characterized this unique subset of CTEPH patients in detail, revealing the clinical features of patients with CTEPH and an atherosclerotic lesion in the pulmonary artery.

Atherosclerosis is mainly characterized by an accumulation of cholesterol crystals and foam cells derived from macrophages, under the influence of a variety of predisposing factors (21, 22). However, the common risk factors of atherosclerosis in the systemic and coronary circulation, such as smoking, altered blood lipid, and pulmonary emphysema, were not found in CTEPH patients with atherosclerotic lesions in the present study. It is well known that the major pulmonary artery and its branches are rarely affected by atherosclerotic lesions due to their relatively low blood pressure and high flow (12). In contrast, during the development of PH, atherosclerotic lesions may occur in the main pulmonary artery (9, 12, 23). Atherosclerotic lesions in the pulmonary vascular bed have also been noted in pulmonary arterial hypertension, revealed by autopsy, possibly as a result of shear stress-induced endothelial injury and subsequent inflammation (24, 25). During the histopathological examination in our study, typical lipid deposition was found to be located under the thrombi and was suspected to be a secondary lesion caused by chronic fibrotic clots, which is consistent with previous reported findings (26). We also found that atherosclerotic lesions were mostly localized in the proximal region, while angiogenesis was found in the distal region (results not shown), suggesting that atherosclerotic lesions may cause defective angiogenesis in some way, such as a local release of pro-inflammatory cytokines, which deserves further investigation. It has been proposed that “multiple hits” contribute to the pathogenesis of CTEPH following thromboembolism, including local release of inflammatory factors, endothelial dysfunction, and macrophage polarization (27, 28). Intriguingly, the same factors are also known to induce the formation of atherosclerotic lesions. Collectively, this manifestation of atherosclerotic lesions following chronic thromboembolism is distinct from systemic and coronary atherosclerosis but may instead be attributable to a unique local microenvironment that is influenced by a variety of stimuli.

In agreement with this notion of an altered microenvironment, an infiltration of macrophages was found in the atherosclerotic lesions, suggesting that foam cells in the atherosclerotic lesion could be derived from activated macrophages. Consistently, Quarck et al. found that the accumulation of inflammatory cells, represented by macrophages, was found in atherosclerotic and thrombotic lesions (7). The influence of pro-inflammatory mediators on pathophysiological processes such as macrophage polarization is a major underlying cause of many chronic inflammatory diseases (29, 30). Choline, a key metabolite that we identified in the atherosclerotic tissue in CTEPH, has been proven to be involved in the macrophage-foam cell formation and the development of atherosclerotic lesion (31). In addition, serine deprivation inhibits macrophage interleukin-1β (IL-1β) production to modulate macrophage function, resulting in less inflammation upon pro-inflammatory challenge (32). Of note, serine metabolism is necessary for glutathione synthesis to support IL-1β production (33). It has also been proven that the glycine-related metabolic axis is involved in energy metabolism (19, 34). More importantly, previous studies showed that alterations in amino acid metabolism could accelerate the development of coronary atherosclerosis and ischemic heart disease (35, 36). The differential metabolites in atherosclerotic lesions found in this study were significantly related to altered energy metabolism, which were consistent with metabolomic investigations of atherosclerosis using coronary tissues (37, 38). Therefore, we believe that the chronic thromboembolism has a certain impact on the proinflammatory cytokine production and altered energy metabolism of the local pulmonary artery, thereby affecting lipid deposition. The findings of this study may promote further research on the link between metabolic reprogramming, proinflammatory microenvironment, and atherosclerotic lesions in CTEPH. Further studies are required with targeted metabolomics, validation, and mechanistic exploration in order to understand the roles of specific metabolic pathways.

The current European Respiratory Society statements recommend screening patients with persistent or new-onset dyspnea after pulmonary embolism despite standard anticoagulation, indicating that earlier diagnosis and intervention may determine the outcome of the patient (2). In our study, the difference of the age of the lesion between the two groups represented the prolonged disease duration of patients with atherosclerotic lesions. Meanwhile, an atherosclerotic lesion was observed in none of the three chronic thromboembolic disease patients without PH. We thus presumed that the atherosclerotic lesions developed during the late stage of remodeling of the pulmonary arteries. Although the existing evidence does not support a relation between an atherosclerotic lesion and the severity of CTEPH based on preoperative hemodynamics and right heart size and function, the observed correlation with the course of disease still supports the notion of earlier screening patients with acute PE. It is possible that early effective intervention before or during the development of atherosclerotic lesions may arrest the progression of CTEPH.

We acknowledge the limitations of our study. First, this study was derived from a single-center retrospectively observational study and could be better supported with replications from independent centers. The absence of a follow-up assessment has limited us in deducing conclusions related to long-term prognosis. We also acknowledge the potential selection bias that may arise from propensity score matching. Additionally, a quantitative assessment of macrophage activation was not performed and will be explored in future mechanistic studies. However, the evidence of an atherosclerotic lesion in the pulmonary artery in CTEPH and relevant alterations of metabolites were the main focus and findings of the current study.

In summary, we found that CTEPH patients with atherosclerotic lesions generally had a higher proportion of symptomatic embolism and prolonged disease duration. A potential linkage has been suggested between atherosclerotic lesions and an aberrant metabolic program, which has been suggested to play a key role in proinflammatory mediator production and energy metabolism. Collectively, we have characterized the clinical features and the metabolic landscape of this distinct group of CTEPH patients, providing novel insights into the pathophysiology and potential therapeutic strategies for the development of CTEPH.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The project was approved by the Ethics Committee of the China-Japan Friendship Hospital. The patients/participants provided their written informed consent to participate in this study.

JL, ZheZ, and PY conceived the project, interpreted the data, and drafted the manuscript. ZC, BW, and DZ performed morphometric analysis. HT, YL, ZF, KZ, and SM collected patient information. WX, QG, SZ, YZ, and ZhuZ provided technical support and contributed to the discussion of the project. All authors critically reviewed the manuscript and made intellectual contributions to the study.