Investigating Microparticles as a Link Between Atherosclerosis, Wegener’s

By Rula Hajj-Ali, MD

Granulomatosis with polyangiitis (GPA; Wegener’s) is characterized by necrotizing granulomatous inflammation and vasculitis of the small and medium-size blood vessels. Although survival of patients with GPA has improved with immunosuppressive therapy, atherosclerosis has emerged as a significant GPA-associated morbidity that is independent of traditional cardiovascular risk factors. Coronary artery disease, stroke and peripheral arterial occlusive disease occur more frequently in patients with GPA than in healthy controls.

The link between GPA and atherosclerosis is not well characterized at a mechanistic level, but it raises the possibility that persistently active vasculitis, or nonspecific inflammation, plays a role in early atherosclerosis.

Microparticles as Markers of Cardiovascular Risk

An increasing body of evidence suggests that, in the course of systemic inflammation, cell-derived microparticles may be prognostic markers for thrombosis, atherosclerotic vascular disease and systemic inflammation. Microparticles are membrane-bound vesicles that bud off normal cells, including leukocytes, platelets and vascular endothelial cells, during activation or apoptosis (Figure 1).

Elevated levels of circulating microparticles have been shown to be associated with cardiovascular risk and are indicative of a poor clinical outcome. Circulating microparticle counts are increased in patients with cardiovascular risk factors. Moreover, microparticle counts are an independent marker of subclinical carotid atherosclerosis in asymptomatic subjects and may be more valuable for mortality prognosis than usual biological markers of myocardial infarction are. In addition, several studies point to microparticles as effectors of vascular wall inflammation.

Microparticles may contain pro-inflammatory mediators, such as IL-1, and can interact directly with activated vascular endothelial cells, triggering monocyte arrest in inflamed and atherosclerotic endothelium. Furthermore, microparticles can interact directly with platelets through the CD36 receptor (Figure 2). This raises the prospect of a novel mechanism for platelet-dependent monocyte recruitment in inflammation and atherosclerosis.

The Link with Rheumatic Diseases

In rheumatic conditions such as rheumatoid arthritis, systemic lupus erythematosus, systemic vasculitides and specifically GPA, microparticle counts potentially can serve as important markers of disease activity.

In keeping with its overall mission, Cleveland Clinic’s Center for Vasculitis Care and Research is studying the relationship between inflammatory disease in GPA and the development of atherosclerosis, with the aim of better understanding the pathogenesis of atherosclerosis in GPA. Our research findings have suggested that (1) the recurrent inflammatory burst that occurs during each relapse in patients with GPA may have a direct role in the pathogenesis of atherosclerosis, and (2) levels of circulating microparticles are elevated during relapse and correlate with platelet reactivity.1

Our Latest Findings ‒ and Implications

We recently further assessed the role of microparticles and their interaction with platelets and vascular endothelial cells in the pathogenesis of atherosclerosis in GPA.2 Microparticles isolated from plasma of GPA patients were added at various ratios to human dermal microvascular endothelial cells (huDMVEC) and incubated for timed periods. Cells were then detached, washed, resuspended in buffer and analyzed by immunofluorescence flow cytometry with anti-ICAM-1 IgG to detect endothelial cell activation. An isotope-matched control IgG was used as control. In addition, fluorescently tagged normal human platelets were incubated with GPA patient-derived microparticles (at a microparticle-to-platelet ratio of 10-to 1), and platelet activation was detected by flow cytometry with PAC-1, an antibody to the activated form of the α2bß3 integrin.

This study found that:

• Microparticles induce ICAM expression on huDMVECs.
• Platelet surface expression of activated α2bß3 integrin was enhanced significantly when platelets from healthy donors were pre-incubated with patient-derived microparticles.

These findings demonstrate that microparticles isolated from the plasma of GPA patients can activate platelets and vascular endothelial cells. This suggests a possible role for microparticles as an interface between inflammation and atherothrombosis in GPA ‒ a possibility that we look forward to evaluating in future investigations.

References

1. Hajj-Ali R, Silverstein R, Hoffman G, Zhang L, Imrey P, Langford CA. The correlation between circulating microparticles and platelet aggregation and disease activity in Wegener granulomatosis. Presented at: American College of Rheumatology Annual Meeting; October 2010; Atlanta, Ga. Arthritis Rheum. 2010;62(10 suppl):S856. Abstract 2047.
2. Hajj-Ali R, Silverstein RL, Hoffman GS, Langford CA. Pathogenesis of atherosclerosis in granulomatosis with polyangiitis. Presented at: American College of Rheumatology Annual Meeting; November 2012; Washington, D.C. Arthritis Rheum. 2012;64(10 suppl):S657. Abstract 1537.

Dr. Hajj-Ali is a staff physician in the Center for Vasculitis Care and Research, Department of Rheumatic and Immunologic Diseases, with a special interest in systemic vasculitides and central nervous system vasculitis