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Lack of complement inhibitors in the outer intracranial artery aneurysm wall associates with complement terminal pathway activation - PubMed

Lack of complement inhibitors in the outer intracranial artery aneurysm wall associates with complement terminal pathway activation

Riikka Tulamo et al. Am J Pathol. 2010 Dec.

Abstract

Inflammation and activation of the complement system predispose to intracranial artery aneurysm (IA) rupture. Because disturbances in complement regulation may lead to increased susceptibility to complement activation and inflammation, we looked for evidence for dysregulation of the complement system in 26 unruptured and 26 ruptured IAs resected intraoperatively. Immunohistochemical and immunofluorescence results of parallel IA sections showed that deposition of the complement activation end-product C5b-9 was lacking from the luminal part of the IA wall that contained complement inhibitors factor H, C4b binding protein, and protectin as well as glycosaminoglycans. In contrast, the outer, less cellular part of the IA wall lacked protectin and had enabled full complement activation and C5b-9 formation. Decay accelerating factor and membrane cofactor protein had less evident roles in complement regulation. The Factor H Y402H variant, studied in 97 IA patients, was seen as often in aneurysm patients with or without aneurysm rupture as in the control population. The regulatory capacity of the complement system thus appears disturbed in the outer part of the IA wall, allowing full proinflammatory complement activation to occur before aneurysm rupture. Insufficient complement control might be due to matrix remodeling and cell loss by mechanical hemodynamics and/or inflammatory stress. Apparently, disturbed complement regulation leads to an increased susceptibility to complement activation, inflammation, and tissue damage in the IA wall.

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Figures

Figure 1
Figure 1

Immunohistochemical and immunofluorescence results for C4b-binding protein (C4bp; brown, A, E, H, and K; green, C and D), protein S (brown, F, I, and L), and C5b-9 (brown, B, G, J, and M; red, D) in parallel sections of two ruptured (A and B and K–M) and three unruptured (C and D, E–G, and H–J) aneurysms. Protein S located to same areas as C4bp (E and F, H and I, K and L). C5b-9 located at subareas of those positive for C4bp and protein S (pointing arrows; B, G, J, and M). The insets show negative controls [irrelevant antibody (A) or primary antibody omitted (C, J)]. The slim arrows indicate the lumen to adventitia direction. Small L indicates lumen. Scale bar corresponds to 200 μm (A–D and K–M) and 100 μm (E–J).

Figure 2
Figure 2

Immunofluorescence microscopy detection of factor H (fH; green; A, C, E, G, and H) and C5b-9 (red; B, C, F, G, and I) in separate channels and as merged images (C and G) and histochemical Alcian Blue staining for glycosaminoglycans (blue; D) in three unruptured IAs (A–D, E–G, and H and I). Pointing arrows (A, D, E, and H) indicate the luminal part that stained for factor H and glycosaminoglycans, but lacked prominent staining for C5b-9. C5b-9 stained intensely in the outer part of the IA wall (pointing arrows; B, F, and I). In the area of strong staining for C5b-9, some staining for factor H was also seen (asterisk; E, F, H, and I). Factor H stained in the extracellular matrix (arrowheads; H). Neg. is negative control (primary antibody omitted). The slim arrows indicate the lumen to adventitia –direction. L indicates lumen. Scale bar corresponds to 200 μm (A–D), 100 μm (E–G, Neg.), and 50 μm (H and I).

Figure 3
Figure 3

Immunofluorescence staining (green) for C5b-9 (A and D), clusterin (B and E), vitronectin (C and F), membrane cofactor protein (G), and decay accelerating factor (H). In parallel sections (A–C and D–F), vitronectin stained similar but slightly restricted areas as C5b-9, whereas only some clusterin was detected (arrowheads, B and E). The membrane cofactor protein stained mainly endothelial cells (arrowheads; G) and few mural cells (arrowheads; G). The decay accelerating factor was detected mainly in the adventitia (arrowheads; H). The insets show negative controls (irrelevant antibody). The slim arrows indicate the lumen to adventitia–direction. L indicates lumen. Scale bar corresponds to 100 μm (A–C and D–F) and 50 μm (G and H).

Figure 4
Figure 4

Immunofluorescence staining for protectin (green; A, C, E, G, and I) and C5b-9 (red; B, D, F, and I) in parallel sections of unruptured IAs (A and B, C and D, and E and F). Pointing arrows (A–F) indicate the band-like area of maximum staining for C5b-9 lacking protectin in the outer part of the IA wall. An arrowhead (C and D) indicates an adventitial capillary characteristically positive for protectin, but negative for C5b-9. The intensity of C5b-9 accumulation in the luminal part (asterisk; C–D) of the IA wall depends reciprocally on the expression of protectin. The cells in the area of strong staining for C5b-9 (single-lined square in E) are decreased in number and nearly lack protectin (arrowheads; G). Faint staining for protectin in the extracellular matrix is shown with a double-arrowhead (G). In contrast, cells in the C5b-9 negative area (double-lined square in E) express protectin intensely (arrowheads; H). The superimposed image (I) shows the reciprocal staining of protectin and C5b-9 at the border site (arrowheads, F and I; dashed lined rectangles in E and F). Neg. is negative control (primary antibody omitted). The slim arrows indicate the lumen to adventitia direction. L indicates lumen. Scale bar corresponds to 200 μm (A and B), 100 μm (C–F, Neg.), and 50 μm (G and H).

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