Stat5b-mediated thymocyte expansion in MHC class I–deficient (β2m^−/−) mice. (A) Thymic CD4/CD8 profiles of 8-week-old β2m^−/− mice with and without the Stat5b transgene. β2m^+/− littermates served as controls. Numbers indicate the percentage of cells in each quadrant, and total cellularity is indicated above in parentheses. (B) CD5 and sTCRβ expression on CD8⁺ SP cells. Numbers indicate the MFI for the CD8⁺ SP population as well as the percentage within the gated region. The expanded CD8⁺ SP population is immature, as evidenced by fewer sTCRβ^hi and CD5^hi cells when compared with WT heterozygous controls. (C) Thymic CD4/CD8 profiles of 10-week-old β2m^−/− mice expressing the ova-specific MHC class I–restricted OT-I TCR transgene. The Stat5b transgene-mediated expansion of CD8⁺ cells in MHC class I (β2m^−/−)–deficient mice is enhanced by coexpression of the TCR transgene. OT-I TG/ β2m^+/− littermates served as controls. Numbers indicate the percentage of cells in each quadrant. Representative data are shown from 1 of 5 similar experiments involving a total of 27 Stat5b TG/β2m^−/− and 20 β2m^−/− mice.