HPC engraftment restores splenic follicular structure in Rag2^−/−γ_c^−/− mice. (A) Histologic splenic sections of WT B6, chimeric, and nonchimeric Rag2^−/−γ_c^−/− mice were examined. The spleen from a nonchimeric Rag2^−/−γ_c^−/− mouse shows complete loss of follicular structure and hypocellularity in the periarteriolar region. The chimeric Rag2^−/−γ_c^−/− mouse (56 days after transplantation) shows follicular reconstitution as well as hypercellularity around the periarteriolar regions, comparable to the spleen of the mouse reconstituted with WT B6 bone marrow cells. Control mice were nontransplanted B6 mice. The bottom panels show the histologic sections at higher magnification. (B) Similarly, GFP staining in the HPC-induced chimeric Rag2^−/−γ_c^−/− mouse showed that GFP-positive donor-derived cells clustered in follicular form within the spleen. The bottom panel shows the same section at higher magnification. Scale bars represent 50 μm. (C) To further demonstrate the phenotype of the transplant-derived cells, splenic tissues of recipient Rag2^−/−γ_c^−/− mice were stained for Gr-1 and B220. Mice that received the defective Rag2^−/−γ_c^−/− bone marrow showed poor recovery of the follicles and a higher percentage of Gr-1–expressing cells but no B220-positive cells. However, both mice receiving either HPCs or WT bone marrow showed restoration of the follicular structure. The HPC-derived cells were predominantly Gr-1–positive cells, in contrast to the high levels of B220-positive cells in the mouse engrafted with B6 bone marrow cells. Both Gr-1–positive and B220-positive cells are stained red. Scale bars represent 50 μm.