Toxicity associated with in utero ACK2 administration in mice. (A) Survival to birth after in utero injection of ACK2 at various concentrations. Control (isotype control antibody or PBS, n = 115); 2.5 µg, n = 44; 5 µg, n = 123; 10 µg, n = 45; 20 µg, n = 134; 50 µg, n = 70. ****P ≤ .0001 by χ-square test). (B) Survival to 1 month after neonatal transplantation in mice treated in utero with ACK2 antibody. Control, n = 15; 2.5 µg, n = 8; 5 µg, n = 13; 10 µg, n = 9; 20 µg, n = 17; 50 µg, n = 6. *P < .05 in both groups by χ-square test. (C) Hypopigmentation after in utero ACK2 administration. (D) Frequency of coat discoloration in 2-week-old mice after in utero injection of ACK2. Normal coat, n = 59; ≤50% of total body surface area (TBSA), n = 13; and >50% TBSA coat discoloration, n = 8. (E) Weight of control and treated (5 µg per fetus) mice at various time points. N ≥ 5 per group. (F) RBC concentration (hematocrit) and (G) leukocyte count of peripheral blood collected 7 days after ACK2 administration in control and treated (5 µg per fetus) animals. Control, n = 7; 2.5 µg, n = 5; 5 µg, n = 17; *P < .05, **P < .01 by ANOVA with Tukey’s multiple comparison test. (H) Hematocrit over time in control and treated (5 µg per fetus) pups (n ≥ 4 for each time point per group). (I) Leukocyte count over time in control and treated (5 µg per fetus) pups (n ≥ 4 for each time point per group). Data presented as percentage or mean ± SEM. n.s., no significance.