Background/Purpose: Clinically-isolated aortitis (CIA), defined as inflammation of the aortic wall in the absence of extra-aortic arterial involvement, systemic vasculitis, or other associated autoimmune disease, is often discovered incidentally on surgical pathology after resection of an aortic aneurysm. The pathogenesis of CIA remains largely unknown, complicating both diagnosis and management. Neutrophil extracellular traps (NETs) have been implicated in the pathophysiology of various forms of vasculitis and vascular injury. Calprotectin, a circulating marker of NET formation, is a promising mechanistic biomarker in various NET-driven diseases. This study aimed to investigate the role of calprotectin in CIA pathogenesis, focusing on endothelial activation, vascular remodeling, and calcification (Fig. 1A).

Methods: A multicenter CIA cohort (n=28) with pathology- or imaging-confirmed cases was assembled, along with healthy controls (n=34) without systemic autoimmune disease. Plasma calprotectin and endothelial activation markers (ICAM-1, VCAM-1) were measured. In vitro, plasma-derived and recombinant calprotectin were tested on human aortic endothelial (HAOEC) and smooth muscle cells (HASMC) to assess activation, proliferation, migration, endothelial-to-mesenchymal transition (EndoMT), and calcification.

Results: Circulating calprotectin levels were significantly higher in CIA patients compared to controls (Fig. 1B). Using a threshold defined by the 99th percentile of control values, 68% of patients with CIA were classified as calprotectin-positive. Among 14 patients with longitudinal imaging, 3 had progression of aortic disease (i.e., worsening aortic dilatation or new aneurysm), 2 of whom were previously calprotectin-positive. Patients with CIA showed higher levels of ICAM-1 and VCAM-1 (Fig. 1C-D). Calprotectin levels correlated with CRP (r=0.63, p< 0.001). Mechanistic studies revealed that plasma from calprotectin-high CIA patients activated HAOECs, significantly increasing surface expression of ICAM-1 (p< 0.01) and VCAM-1 (p< 0.05). Similarly, exposure to recombinant calprotectin upregulated expression of ICAM-1 and VCAM-1 transcripts (p < 0.05 for both). Calprotectin exposure resulted in i) increased expression of mesenchymal markers, including α-SMA and Transgelin (Fig. 2A–E), consistent with the induction of EndoMT and ii) promotion of endothelial cell proliferation and migration (Fig. 2F–I). In HASMCs, the addition of recombinant calprotectin significantly increased intracellular calcium deposition (Fig. 3), suggesting a role in promoting microcalcification and vascular stiffening.

Conclusion: Calprotectin is elevated in most patients with CIA and may contribute to disease pathogenesis by activating aortic endothelial cells, triggering proliferation, migration, and EndoMT, while also driving microcalcification in smooth muscle cells. Together, these changes suggest a role for calprotectin in vascular remodeling. Our findings suggest a neutrophil-driven pathway of aortic injury in CIA, supporting further investigation of calprotectin as a potential biomarker and therapeutic target.

Figure 1: Circulating calprotectin and endothelial activation in patients with clinically-isolated aortitis. (A) Study design. Plasma samples were obtained from 28 patients with clinically-isolated aortitis and 34 healthy controls. Circulating levels of (B) calprotectin, (C) ICAM-1, and (D) VCAM-1 were quantified by ELISA. Data presented with median value. Statistical comparisons were performed using the Kruskal-Wallis test. *** – p < 0.001;

Figure 2: Calprotectin triggers endothelial-to-mesenchymal transition (EndoMT) in human aortic endothelial cells (HAOECs). (A) HAOECs were cultured with 10 µg/mL recombinant calprotectin for 24 h. (B, C) Calprotectin-treated cells showed an increased expression of markers of EndoMT: ACTA2 and TAGLN (D, E) further validated by flow cytometry. Calprotectin also altered behavior of HAOECs, promoting (F, H) proliferation and (G, I) migration. Images were captured and analyzed using the Incucyte® Live-Cell Analysis System. Data presented with median ± 95% CI. Statistical analysis was performed using the Kruskal-Wallis test. * – p < 0.05; ** - p < 0.01

Figure 3: Calprotectin-induced microvascular Ca2+ accumulation in human aortic smooth muscle cells (HASMC). (A) HASMCs were treated with 10 µg/mL calprotectin for 9 days. (B) Following treatment, cells were washed and incubated with 1 M HCl to dissolve intracellular calcium. Calcium content was quantified using the QuantiChrom™ Calcium Assay Kit. (C) Calcium accumulation was visualized by staining with 1% Alizarin Red for 45 min, with calcified areas appearing as red deposits. Data presented with median ± 95% CI. Statistical analysis was performed using the Kruskal-Wallis test. **** – p < 0.0001

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-association-of-calprotectin-with-vascular-injury-and-remodeling-in-clinically-isolated-aortitis/