sdAb19 rescues Nef-mediated thymic CD4 maturation block in vivo. Fetal liver cells from CD4C/HIV^Nef Tg and non-Tg 14.5-day-old embryos were infected in vitro with retrovirus coding sdAb19-IRES-GFP or were uninfected and were injected into syngenic lethally irradiated hosts. Thymuses from host animals were analyzed 1-5 months after reconstitution. (A) Percentage of GFP^hi and GFP^low cells measured by flow cytometry within designated thymocyte subsets. Gating of GFP⁻, GFP^hi, and GFP^low cells was based on distribution of cell populations with distinct GFP intensities as well as levels of CD4 in Tg sdAb19⁺ chimeras (supplemental Figure 6A). (B-C) Thymocytes were labeled with antibodies against TCR, CD4, and CD8, and percentages of SP thymocytes as well as average CD4⁺SP to CD8⁺SP ratios among TcR^hi thymocytes were determined by flow cytometry. (D) Average CD4 MFI on thymocytes was compared with average CD4 MFI found on thymocytes in non-Tg chimeras. CD4 MFI in Tg GFP⁻ cells was determined on CD4^low cells. Data represent ≥ 2 distinct experiments with a total of 3-8 mice per group. Statistical analysis was performed with the Student t test (*P < .5, **P < .01, and ***P < .001; ns indicates not significant). Error bars represent 1 SD from the mean.