Abdominal aortic aneurysms and platelets: infiltration, inflammation, and elastin disintegration

This editorial refers to ‘Crosstalk of platelets with macrophages and fibroblasts aggravates inflammation, aortic wall stiffening and osteopontin release in abdominal aortic aneurysm’, by M.U. Wagenhäuser et al., https://doi.org/10.1093/cvr/cvad168.

Abdominal aortic aneurysms (AAAs) have a frequent occurrence that portends death due to rupture-induced exsanguination. The natural history of AAAs has not been elucidated in great depth. Although there is a paucity of data to assign function and relative importance to specific populations of cells, a multitude of cell types are recruited during the evolving pathology. Of all the cell types that have been studied, platelets have been the focus of relatively few studies. Therefore, the study of Wagenhauser et al.¹ is a welcome addition to the literature, particularly since it combines animal and human data (Figure 1).

Figure 1.

Schematic summary of the study by Wagenhauser et al.¹ Crosstalk of activated platelets to macrophages or fibroblasts contributes to development of AAAs. The molecular mechanisms underlying this crosstalk remain undefined.

Evidence for participation of platelets in human AAAs is discernable from the presence of this cell type in intraluminal thrombus (ILT). While the presence of ILT is common in human AAAs, this pathological characteristic is a rare occurrence in animal models of AAAs. Of the most commonly used mouse models for AAAs, thrombus is present in the early phase of angiotensin II (AngII)-induced AAAs, although it is located in the adventitia.² However, thrombus is not commonly described to be present in other mouse models. Despite the lack of ILT in mouse models, there is a potential for platelets to exert activities in AAAs. Previous studies have demonstrated that manipulation of platelet function influenced AAA formation in animals (Table 1). This includes modes of inhibiting platelet function that could reduce disease severity in mice in which dilatation was induced by xenografting,⁷ AngII infusion,^3,5 or topical elastase application⁸: The figure was generated using bioRENDER.

Table 1.

Studies on the impact of platelet-related interventions in mouse models of AAAs

Mice	Sex	Intervention	Manipulation	Aortic phenotype	Reference
LDLR −/−	Male	AngII infusion	Aspirin or clopidogrel	Diameters ↔; rupture and dissection↓	Owens et al.3
ApoE−/−	Male		Platelet transfusion	Diameter↓	Liu et al.4
ApoE−/−	Male		Clopidogrel	Diameter↓	Liu et al.5
C57BL/6 and FVB/N	Male	BAPN + elastase + anti-TGF-β antibody	Carvone or aspirin	Aneurysm growth↓	Morrell et al.6

↔, no difference; ↓, decrease; LDLR, low-density lipoprotein receptor; ApoE, apolipoprotein E; BAPN, beta-aminopropionitrile; TGF-β, transforming growth factor-beta.

Wagenhauser et al.¹ studied the role of platelets using transient intraluminal porcine elastase perfusion to provoke the expansion of the infrarenal aorta, as originally described by the Thompson laboratory.⁹ Unlike the previous studies using pharmacological inhibitors to attenuate platelet activation, in this study, platelets were depleted by two injections of an anti-GP1b alpha antibody that led to a profound decrease in platelet count. Ultrasound-based measurements of aortic diameters showed decreased diameters in platelet-depleted mice. These diameter changes were described as percent increases, rather than as absolute measurements, which hinders evaluation of the robustness of the luminal dilatation. Nevertheless, analysis of aortic tissues revealed enhanced platelet and macrophage accumulation that was attenuated by platelet depletion. Also, elastin fragmentation was reduced by platelet depletion with reduced mRNA abundance of MMP2 and MMP9 that can cleave elastic fibres. A particular focus of this study was the large increase in the abundance of Spp1 mRNA that encodes osteopontin (OPN).

To explore the underlying mechanisms, cell culture studies were performed in which cultured macrophages, fibroblasts, and smooth muscle cells were exposed to a solution derived from platelets incubated with two common platelet activators: adenosine diphosphate and collagen-related peptide. While the content of the platelet-derived materials was not characterized, pharmacological depletion of platelets resulted in an increased mRNA abundance in a human macrophage cell line, THP-1, but not in human primary smooth muscle cells or fibroblasts. Subsequent studies demonstrated that OPN promoted platelet adhesion under flow-based concentrations, but not under static conditions. The inference from these data is that OPN is a critical component in the mechanism of platelet enhancement of AAAs. A confounder of this interpretation is that, while OPN deficiency decreases aortic pathology during AngII infusion, the only study in elastase-induced aortic dilatation, the mouse model used in this study, failed to show any effect.¹⁰

Wagenhauser et al.¹ also performed platelet depletion studies in apolipoprotein E −/− mice infused with AngII. Consistent with the findings in elastase-induced AAAs, platelet depletion attenuated the increase in AngII-induced aortic dilatations in apolipoprotein E −/− mice. Many of the tissue changes caused by platelet depletion were equivalent in both the elastase- and AngII-induced aortic pathologies. A contrast between the two mouse models that was noted was the lack of discernable platelet activation in AngII-infused mice, compared to elastase-induced aortic pathologies.

To enhance the clinical impact of the mouse studies, human AAA tissues were analysed for the colocalization of OPN with platelets and macrophages in both the ILT and aortic wall. OPN was overtly colocalized with platelet staining in the ILT and some colocalization of platelets with smooth muscle cells and fibroblasts. Platelets were also detectable in the aortic wall. In addition to characterization of platelets in AAA tissues, these cells were isolated from blood for comparisons of affiliated individuals with age-matched controls. Consistent with studied in elastase-induced AAAs in mice, platelets from AAA patients had indexes of increased platelet activation in the absence of any agonist. This contrasted with a previous study.⁶ The different results may be potentially attributable to the disparity of ages between the comparison groups in the previous study.⁶ While this conflict will need to be resolved, it illustrates the need to provide a detailed description of comparison groups with inclusion of information such as age, sex, and current pharmaceutical treatments that may impact platelet function.

In summary, the intricacies of platelet biology have been sparsely represented in research on AAA mechanisms. There is a current assumption that platelets are negative contributors to aortic pathology. Indeed, some of the current literature is in agreement with this premise. However, the inconsistencies in the literature also provided a call for a greater focus on performing these studies in multiple laboratories to assess consistency across venues and platforms.

Funding

The authors’ research work was supported by the National Heart, Lung, and Blood Institute of the National Institutes of Health (R35HL155649), the American Heart Association Merit Award (23MERIT1036341), and the Leducq Foundation for the Networks of Excellence Program (Cellular and Molecular Drivers of Acute Aortic Dissections). The content in this editorial is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.