Increased engraftment after in utero ACK2 treatment followed by neonatal transplantation. B6.CD45.2 mice treated in utero with ACK2 or control antibody were transplanted on P1-2 with congenic B6.CD45.1/CD45.2 fetal liver mononuclear cells and levels of peripheral blood chimerism (donor CD45 cells/(donor+host CD45 cells) × 100) were determined by flow cytometry starting 3 weeks after transplantation. (A) Rate of engraftment (number of animals with >1% chimerism/surviving animals) at 4 weeks. Control n = 16; 2.5 µg, n = 8; 5 µg, n = 10; 10 µg, n = 9; 20 µg, n = 4. (B) Levels of donor CD45 chimerism at 4 weeks. N, number of chimeric animals reported in panel A, excluding 1 nonchimeric animal. *P < .05 and **P < .01 by ANOVA with Tukey’s multiple comparison test. There is a significant negative correlation between ACK2 dose and level of chimerism: Pearson R = −0.96, P = .04. (C) Levels of donor CD45 chimerism over time. N = 4-9 animals from ≥2 independent experiments for each group. Data presented as percentage or mean ± SEM. *P < .05 by ANOVA with Tukey’s comparison between doses of 2.5 to 10 µg and control.