(A) PsA mice were injected with anti-CD8 or isotype control antibodies at the time of PBMC injection. At day 30 after transfer, mouse PBMCs were stained with a DEAD/LIVE marker and nonspecific binding blocked with human TruStain FcX Fc receptor blocking solution. The percentage of CD19^–CD3⁺CD8⁺ T cells was defined with FlowJo to determine the efficiency of CD8⁺ T cell depletion. (B and C) hu-PsA mice injected with CD8 antibodies did not develop psoriasiform lesions or ankle swelling, compared with mice injected with isotype control antibodies. (D) Epidermal hypertrophy and numerous dermal infiltrating cells were observed in the skin of hu-PsA mice injected with isotype control antibodies. (E) Infiltrating CD3⁺ T cells (red) and Ki67⁺ proliferating cells in skin of humanized mice. (F) CD8⁺ T cell depletion significantly reduced epidermal thickness in hu-PsA mice. (G) Pannus areas were visualized in hu-PsA treated with isotype control or CD8⁺ T cell–depleting antibodies. The dashed line outlines pannus areas. (H) Proliferative type 1 T cells (CD3, red; T-bet, green; Ki67, white) and proliferating T cells (CD3, red; Ki67, white) were decreased in CD8⁺ T cell–depleted mice. Representative images of Alcian Blue and H&E stain were taken with an Olympus VS120 scanner. Scale bars: 1,000 μm. IF 3 × 3 mosaic images were taken (original magnification, ×200) with a Zeiss Axioplan microscope and recorded with a Hamamatsu camera. (I) Quantitation of CD3⁺ T cells, CD3⁺Ki67⁺ proliferating T cells, and CD3⁺T-bet⁺ type 1 T cells in ankle synovial tissue of hu-PsA mice. Scale bar: 200 μm. n = 5 mice reconstituted with PBMCs and sera from different patients with PsA in each group. Significance was calculated with 2-way ANOVA and Tukey’s multiple comparison test. *P ≤ 0.05; **P ≤ 0.005; ****P ≤ 0.0001.