Background and Purpose Cerebral amyloid angiopathy (CAA) occasionally coexists with cerebral vasculitis. An immune system may influence deposition or degradation of the amyloid in cerebral blood vessels. The purpose of this study was to elucidate immune reactions associated with CAA.

Methods In 11 elderly patients with sporadic CAA, 2 patients with Icelandic familial CAA, and 2 patients with CAA and granulomatous angiitis, the cerebrovascular amyloid proteins and infiltrating inflammatory cells were analyzed immunohistochemically.

Results In both sporadic CAA (β-protein amyloid angiopathy) and Icelandic familial CAA (cystatin C amyloid angiopathy), leptomeningeal and cortical vessels were associated with an increase or activation of monocyte/macrophage lineage cells. In the cases of CAA with granulomatous angiitis, the vascular amyloid was of β-protein and associated with infiltration of many monocyte/macrophage lineage cells, which included multinucleated giant cells containing the amyloid in the cytoplasm as well as T cells composed of CD4⁺ and CD8⁺ subsets. Amyloid P component, which was reported to be a common component of amyloid deposits and to prevent phagocytic proteolysis of amyloid fibrils of β-protein, was negative for the vascular amyloid in a case of CAA with granulomatous angiitis but positive in the others.

Conclusions In both the β-protein and cystatin C amyloid angiopathies, cerebrovascular amyloid deposition was associated with an increase or activation of monocyte/macrophage lineage cells. Prominent reactions of monocyte/macrophage lineage cells admixed with CD4⁺ and CD8⁺ T cells (granulomatous angiitis) were occasionally associated with β-protein angiopathy. In some of these cases, the absence of amyloid P component might be related to pathogenesis of the granulomatous reaction.