Tregs in wild-type chimeras after in utero transplantation. (A) Experimental design and (B) gating strategy to enumerate donor-specific Tregs in chimeric mice. BALB/c fetuses underwent IUHCTx with B6 donor cells. Two to three weeks after IUHCTx, lymphocytes were harvested from chimeras and nonchimeras, labeled with CFSE, and transplanted into F1 (B6 × BALB/c) recipients. Primary host (B6) lymphocytes were identified as CD4⁺H-2Kb⁻ and proliferating donor-specific CD4 T cells and Tregs (CD4⁺Foxp3⁺CFSE^low) were (C) quantified in chimeras and nonchimeras. Chimera (n > 5), nonchimera (n > 2). *P < .05 by Student t test.