Host Treg cells compose the predominant component of the Treg compartment during the first month after lethal TBI and autologous donor TCD-BM transplant. (A) B6-wt (CD45.2) recipient mice were conditioned with 9.5 Gy TBI and were transplanted with 5 × 10⁶ TCD-BM from B6-CD45.1 donors. Five weeks post-HCT (day 35), the spleen and lymph nodes were analyzed for the indicated populations (CD4⁺CD25⁺ FoxP3⁺ [left panel], CD4⁺ [middle panel], and CD19⁺ [right panel]) and the origin (donor, host) of each according to CD45.2 expression. The total cell numbers of each population were obtained using the percentage of positive staining from flow cytometric analyses and the total splenic mononuclear cell number. Results represent staining of indicated tissues from 2 individual B6-wt transplanted mice. (B) Kinetic analyses of host and donor Treg cells during the 2-month period after ablative conditioning and syngeneic TCD-BM transplant. B6-wt (Thy1.2) recipient mice were conditioned with 9.5 Gy TBI and transplanted with 5 × 10⁶ TCD-BM from B6-Thy1.1 congenic donors. Spleens (B, left panel) and lymph nodes (axillary, cervical, brachial, and inguinal; B, right panel) were collected from 3 mice at each of the time points indicated, and the numbers of CD4⁺Foxp3⁺ T cells in each compartment of individual mice were determined by the percentage of positive staining and total numbers of mononuclear cells. The total cell numbers of each population were obtained using the percentage of positive staining for CD4, Foxp3, and Thy1.1 from flow cytometric analyses and the total splenic or lymph node cell mononuclear cell number. Results represent mean percentage positively stained cells plus or minus SEM (note, where error bars not apparent, SEM values did not extend outside of symbol) from groups of mice (n = 3/group) assessed from the indicated time points after HCT. Dotted lines represent the total numbers of donor plus host CD4⁺Foxp3⁺ T cells. There are typically 1 to 2 × 10⁶ Treg cells in the splenic compartment of normal, adult B6 mice.