(A) Multiplex images showing differences in cortical vessel wall and immune infiltration between cases. CD31 was used to indicate endothelial cells; ACTA2 was used to indicate smooth muscle; CD11c was used to indicate antigen-presenting cells; P2RY12 was used to indicate microglia; LAMP1 was used to indicate activated myeloid cells; Rab5 was used to indicate phagocytosis. Aβ, amyloid β. Scale bars: 100 μm (top); 50 μm (bottom). (B) Multiplex images showing CD11c⁺ APCs infiltrating the cortical vessel wall with cytoplasmic Aβ (red arrows). Scale bar: 50 μm. (C) Multiplex images in the lecanemab-treated case (left), showing fibrinogen and fibronectin within cortical vessels along with hemorrhage. Scale bar: 200 μm. Remnants of the vessel wall and Aβ amyloid within CD11c⁺ APCs (red arrow) are shown (right). Scale bar: 50 μm. (D and E) Representative images of spleen from the lecanemab-treated or untreated case showing expression of Aβ, GFAP, or MBP within CD68⁺ monocytes. Scale bars: 20 μm. Of note, Aβ⁺ macrophages were present in the lecanemab-treated and untreated Alzheimer’s disease spleen (~60%), while GFAP⁺ and MBP⁺ splenic macrophages were noted only in the lecanemab-treated spleen (42% and 20%, respectively).