Effect of TCR syngeneic and allogeneic HCT on the recipient's Treg compartment. B6 (CD45.2, Thy1.2) mice were conditioned with 9.5 Gy TBI and transplanted 1 day later with TCD-BM from B6-Thy1.1 congenic mice or TCR with 5 × 10⁶ purified CD4⁺CD8⁻ and CD4⁻CD8⁺ cells from CD45.1⁺Thy1.2⁺ B6 mice. Four weeks later, the CD4⁺FoxP3⁺ T-cell compartment was analyzed in the spleen and lymph nodes from 4 mice. (A) Data from a representative mouse illustrate (1) the residual host (CD4⁺FoxP3⁺Thy1.2⁺) Treg cells in recipients of TCD transplants (panels 2 and 8), as well as in recipients of TCR transplants (panels 5 and 11, top left quadrant by subtraction of percentage donor transplanted CD45.1⁺ Thy1.2⁺ (panels 6 and 12); (2) Treg cells transplanted in the donor inoculum (panels 6 and 12); and (3) donor BM (CD4⁺FoxP3⁺Thy1.1⁺) derived (panels 2, 5, 8, and 11, bottom right quadrants). (B) The summary of the average overall percentage contribution of residual Tregs, donor BM-derived Tregs, and transplanted donor Treg cells from both groups of transplant recipients (n = 4/group). *Significant difference between groups: P < .001, 1-way repeated measure ANOVA followed by Newman-Keuls multiple comparison test compared with BM only in the specified organ. (C) Lack of host CD4⁺FoxP3⁺ T cells in recipients of TCR allogeneic BM transplants. BALB.B (8.25 Gy TBI) recipients of 2 × 10⁶ TCD BM alone (top panels), or together with 7 × 10⁶ B6 lymph node T cells (bottom panels) were examined at 4 (spleen) and 5 (lymph nodes) weeks post-HCT for CD4⁺FoxP3⁺ Treg cells of host (Ly9.1⁺) or donor (Ly9.1⁻) origin. Data from an individual transplant recipient is presented as percentage CD4⁺FoxP3⁺ of host (vs donor) origin (histograms). Identical results were observed from 2 additional recipients at this time point.