Emergence of dendritic cells in rupture-prone regions of vulnerable carotid plaques

Abstract

Dendritic cells (DC), which are critically involved in various immunological disorders, were detected in atherosclerotic plaques in 1995. Since DC might be related to the immunological processes in atherosclerosis (AS), we analyzed the emergence of DC and other inflammatory cells in different stages of AS. Serial cross-sections of 44 carotid specimens were immunohistochemically analyzed for the presence of DC, T cells, macrophages, and HLA-DR. Atherosclerotic specimens were histologically defined as initial lesions, advanced stable, or vulnerable plaques. In initial lesions significantly lower DC numbers were detected than in advanced plaques (P < 0.001). For advanced plaques, DC numbers were significantly higher in vulnerable than in stable plaques (P = 0.005). In contrast to initial lesions, approximately 70% of DC in advanced plaques exhibited a mature phenotype (CD83⁺, DC-LAMP⁺), indicating a functional activity of DC. In plaques of patients with acute ischemic symptoms DC numbers were markedly elevated (P = 0.03), whereas significantly lower DC numbers and more often a stable plaque morphology were detected in statin-treated patients (P = 0.02). DC clusters with a strong HLA-DR expression and frequent DC-T cell contacts were located particularly in the rupture-prone plaque regions and at complications. The results of the present study indicate that DC might contribute to plaque destabilization through an activation of T cells.

Introduction

Atherosclerosis (AS) is a chronic inflammatory disease triggered by various autoantigens such as oxidized low density lipoprotein (oxLDL), heat shock protein 65/60, and β₂-glycoprotein Ib [1]. According to their different morphologies and clinical outcomes, atherosclerotic plaques can be classified as stable (fibrous) or vulnerable (lipid-rich) [2], [3]. Vulnerable plaques, which are more prone to rupture, are characterized by a large lipid core, a thin fibrous cap, and a substantial infiltration with inflammatory cells. A key feature of plaque destabilization is the thinning of the fibrous cap, leading finally to plaque rupture. The importance of inflammatory cells and their products in the process of plaque destabilization has been established [4]. For example, interferon-γ, a cytokine released by T cells, triggers apoptosis of smooth muscle cells, thus reducing the synthesis of the plaque-stabilizing extracellular matrix. Moreover, matrix metalloproteinases (MMP) released by macrophages (M∅), cause the degradation of the extracellular matrix. Different proteases of mast cells (MC) lead to the activation of MMP or degrade the extracellular matrix directly [5], [6]. Plaque rupture in the shoulder region is the most common type of plaque complication and leads to acute ischemic events such as myocardial infarction or stroke [4].

Until now, research on AS has mainly focused on the contribution of T cells and M∅ to the initiation and progression of AS [4]. However, studies about the role of dendritic cells (DC) in AS are rare. DC are “professional” antigen-presenting cells with the unique ability to initiate a primary immune response to certain antigens by the activation of “naive” T cells [7]. The importance of DC in different diseases, including tumors, viral and bacterial infections, autoimmune diseases, and transplant rejection, is well known [8]. So far, the presence of DC was described in atherosclerotic plaques of carotid arteries and aortas [9], [10] as well as stenotic aortocoronary vein bypass grafts [11]. These vascular DC have specific ultrastructural features, including a well-developed tubulovesicular system, dendritic cellular processes, and a lack of secondary lysosomes and phagolysosomes [12], [13]. The co-localization of DC and T cells, as well as the expression of HLA-DR and costimulatory molecules on DC in atherosclerotic plaques [14], suggest that DC initiate an antigen-specific immune response, contributing to the progression of AS. In addition, recent studies showed that in normal intima DC form a network, especially in vascular areas subjected to hemodynamic stress [15], suggesting a role of DC in plaque initiation.

The immune response to autoantigens sustains the inflammation in atherosclerosis [1]. In particular, the importance of oxLDL as an autoantigen in AS has been proven [16], [17]. With regard to DC, it has been recently demonstrated that oxLDL or lysophosphatidylcholine promotes the maturation of cultivated DC, enabling these mature DC to trigger an antigen-specific activation of T cells [18], [19].

The aim of our study was to compare the frequency and distribution of DC, the expression of HLA-DR on DC, and the occurrence of DC-T cell contacts in different stages of AS. In contrast to former studies, the numbers of immunohistochemically stained DC, T cells, M∅, and HLA-DR-expressing cells were exactly quantified and compared between different stages of AS, particularly between stable versus vulnerable plaques, to investigate a potential role of DC in plaque destabilization. Additionally, the clinical data of the patients were related to the immunohistochemical results.