(A) Frozen sections of the indicated organs from Rag2-deficient mice were stained with serum from 7- to 12-month-old control mice (Prmt5^fl/fl, n = 6) and Prmt5-cKO mice (Prmt5^fl/fl Foxn1-Cre, n = 8). Scale bar: 100 μm. (B) Summary of the integrated density of the fluorescence signals shown in A. Dashed lines indicate the average of control integrated density values plus 2 SDs. Significance was determined using the unpaired, 1-tailed Student’s t test. (C) Summary of autoantibodies in Prm5-cKO mice. Each pentagon represents an individual mouse. (D and E) Lung sections from 7- to 12-month-old control mice (n = 5) or Prmt5-cKO mice (n = 4) were stained with H&E (D) or CD3 and DAPI or DAPI, CD4, and CD8 (E). Original magnification, ×4 (D). Scale bar: 100 μm (E). Representative data from 2 independent experiments are shown. (F) Relative expression of melanoma antigens in mTEC^hi cells. (G and H) B16F10 Red-FLuc melanoma cells (5 × 10⁵) were injected into the tail vein of 6-month-old mice. Representative ex vivo bioluminescence images of the lungs 14 days after tumor injection (G). Quantification of the bioluminescence (arbitrary unit minus background: AU–BG) (H). (I–K) Flow cytometric analysis to determine CD44 and CD62L expression in left lung CD8⁺ T cells from 6-month-old control mice (n = 8) and Prmt5-cKO mice (n = 7) 14 days after tumor injection and the frequency of naive, effector, and central memory of CD8⁺ T cells (I). Intracellular staining for IFN-γ after PMA plus ionomycin stimulation in lung CD8⁺ T cells from the indicated mice and the frequency of lung IFN-γ⁺CD8⁺ T cells (J). Frequency and number of lung Tregs from the indicated mice (K). *P < 0.5 and **P < 0.01, by 2-tailed Student’s t test (H–K).