The thymoproteasome optimizes CD8⁺ T cell production in the absence of additional antigen-presenting cells. (A) MHC-I expression in indicated cell populations from 2-wk-old B6 mice. Histograms show the expression of H-2K^b (blue) and background fluorescence (shaded) in cTECs, mTECs, PDGFRβ⁺CD45⁻EpCAM⁻ fibroblasts, PDGFRβ⁻CD45⁻EpCAM⁻ other nonhematopoietic cells, CD11c⁺ DCs, CD19⁺ B cells, TCRβ^high thymocytes, and CD11c⁻ CD19⁻TCRβ^negative-low other hematopoietic cells. Numbers in histograms indicate median fluorescence intensity (MFI) for H-2K^b (blue) and background (black). Plots (means and SEMs, n = 3–4 in three independent measurements) show net MFI values for H-2K^b and H-2D^b. (B–D) dGuo-treated fetal thymuses from control mice, β5t-KO mice, relB-KO mice, and double KO (DKO) mice were reconstituted with β2m-KO fetal thymocytes cultured for 7 d. (B) Net MFI values (means and SEMs, n = 3–16 in at least two independent measurements) for H-2K^b (top) and H-2D^b (bottom) in indicated cell populations are shown. Heterozygous or homozygous β5t-Venus knock-in knock-out allele was included, so that Venus⁺ TECs represented β5t-expressing cTECs. (C and D) Contour plots for CD8β and CD4 expression in PI⁻ TCRβ^high thymocytes from indicated fetal thymus organ cultures are shown. Numbers in plots indicate the frequency of cells within the indicated area. Cell numbers (means and SEMs; n = 8–12 in C, n = 11–16 in D in four independent measurements) of indicated thymocyte populations measured in four independent experiments are plotted. *, P < 0.05 (by unpaired t test with Welch’s correction).