Cellular and molecular mechanisms in abdominal aortic aneurysm growth and rapture

Sammanfattning: Fragmentation of elastin, loss of smooth muscle cells (SMCs) and accumulation of inflammatory cells are the histological landmarks of abdominal aortic aneurysm (AAA). In larger aneurysms, an intraluminal thrombus is almost always present . It has been suggested that the thrombus predisposes for AAA rupture. In order to study the structure and composition of the thrombus-free and the thrombus-covered aneurysm wall, we collected biopsies from patients selected for elective operation, where CT scan showed an eccentrically located thrombus with both thrombus-free and thrombus-covered parts of the AAA wall. In paper I it was shown that there was a significant difference in wall thickness between the two wall segments. The wall underneath the thrombus was thinner compared to the wall without thrombus. Furthermore, the thrombus-covered wall contained less and degraded elastin fibers, less SMCs but more inflammatory T- and B-cells. The decreased levels of SMCs within the thrombus-covered wall may be a result of increased SMC apoptosis, which was supported by the results of electron microscopy analyses and TUNEL assay. In paper II we examined the possibility that the intraluminal thrombus influences expression and activity of matrix-degrading proteases in the AAA wall. Gene arrays and quantitative real-time PCR showed that matrix metalloproteinase (MMP)-1, -7, -9 and -12 mRNA expressions were upregulated in the thrombus-free wall compared with the thrombus-covered wall. Immunohistochemistry confirmed the differential expression of MMP-9 but also localized MMP-9 to the interface between the thrombus and the underlying vessel wall. MMP-9 expression was co-localized with the presence of macrophages. Gelatinase activity was detected in the same regions as MMP-9 protein expression, i.e. within the thrombus-free wall and in the interface between the thrombus and the underlying wall. Neutrophil gelatinase-associated lipocalin (NGAL) has been suggested to influence the activity of MMP-9 by binding to MMP-9 and inhibit its degradation, thus preserving its enzymatic activity. In paper III it was demonstrated that NGAL/MMP-9 complexes were present in different regions of the AAA. NGAL expression was co-localized with CD66b positive neutrophils. Importantly, NGAL/MMP-9 complexes were present in the thrombus itself and in the interface fluid between the thrombus and the underlying vessel wall. These findings support the concept that the wall underlying the thrombus is degraded from within the lumen by enzymatic activity related to the thrombus. This could lead to AAA rupture after blood has entered the thrombus. In paper IV we studied the presence of mast cells (MCs) in AAA. Immunostaining of MC tryptase in AAA specimens showed that MCs, in contrast to normal aorta, were absent in the intimal layer of AAAs, and that MCs associate with medial and adventitial neovessels. Furthermore, MCs reside partly in the same areas with macrophages, CD3 positive T-cells and neutrophils. Cathepsin G and tryptase double immunostaining showed that MCs positive both for cathepsin G and tryptase are more numerous than cells with only tryptase or cathepsin G. Cathepsin G and chymase expressions did not differ significantly between thrombus-covered AAA wall and AAA wall without intraluminal thrombus when evaluated by RT- PCR. In summary, the present thesis provides evidence that the intraluminal thrombus is of importance for the pathogenesis of aneurysm growth and possibly ruptures.