A transient and quantitatively reduced lymphoid development occurs in Pbx^−/− fetal liver transplant recipients. (A) Wild-type or Pbx1^−/− fetal liver cells (1 × 10⁶) were transferred to lethally irradiated Rag2^−/− recipients. At the indicated time points, FACS analysis was performed to assess B-cell reconstitution in peripheral blood from chimeric mice using fluorochrome-conjugated B cell-specific (B220 and CD19) antibodies. Very few B220⁺CD19⁺ cells were observed in Pbx1^−/− as well as Pbx1^+/+ fetal liver recipients at 4 weeks after transplantation. The numbers of B cells steadily increased at later time points in Pbx1^+/+Rag2^−/− fetal liver chimeras (▴), but not in the Pbx1^−/− chimeras (•). Median values (of 20 recipients) are represented by a solid horizontal line at each time point. (B) FACS analysis was performed to monitor T- cell reconstitution in the peripheral blood of chimeric mice using fluorochrome-conjugated anti-CD4 and CD8 antibodies. Reconstitution of CD4⁺CD8⁻ T cells in the Pbx1⁺⁺ and Pbx1^−/− fetal liver chimeras followed similar patterns as B cells. (C) The kinetics of reconstitution of CD8⁺CD4⁻ was also similar to B cells indicating significantly reduced lymphopoiesis in the Pbx1^−/−Rag2^−/− fetal liver chimeras. (D) Chimeric mice were immunized with KLH-DNP (100 μg, intraperitoneally) and serum anti-DNP IgG antibodies were measured by ELISA. Pbx1^−/− chimeras mounted comparable DNP-specific IgG response with wild-type chimeras. Error bars represent SEMs for triplicate analyses.