An immunohistochemical study of inflammatory abdominal aortic aneurysms.

Seven cases of inflammatory abdominal aortic aneurysms (IAs) were studied by light microscopy, transmission electron microscopy (TEM) and immunohistochemistry. Microscopically, atherosclerosis coexisted with adventitial fibrosis and inflammation. The inflammatory component showed a follicular and a diffuse pattern. Fibrous entrapment of fatty tissue, adventitial vasculitis, neuritis were also common findings. By TEM, sparse smooth muscle cells having dilated cisternae of rough endoplasmic reticulum, large bundles of collagen fibres and oedematous, amorphous fibrillary elastin were observed. By immunohistochemistry, the follicles mostly contained CD22+ B-cells. T4- (CD2+/CD4+/CD8-), T8-(CD2+/CD4-/CD8+) cells as well as macrophages (CD4+/CD11c+) and follicular dendritic reticulum cells (DRC1+) were also detected. The monoclonal antibody Ki-67 reacted with 2-48% of germinal center cells. In the fibrous extrafollicular adventitia, actively synthesizing plasma cells prevailed over T4-cells, and macrophages. Some of the macrophages were also activated (CD4+/CD11c+/CD25+/CD30-). IgM, IgG and C3c deposits were detected in the fibrous zone, in the germinal centers, within adventitial vessels and nerves. HLA-DR antigen was diffusely expressed in cells populating both the fibrous and the follicular zones as well as in endothelial and Schwann cells. These findings suggest that IAs could develop in some individuals affected by advanced atherosclerosis of the abdominal aorta through a pathogenic B-cell response to locally presented antigens.