Blockade of host megakaryocyte function post-TBI decreases BM engraftment and expansion in primary transplant recipients. (A) Baseline LT-HSC (c-kit⁺Lin⁻SCA-1⁺CD135^lo), ST-HSC (c-kit⁺Lin⁻SCA-1⁺CD135⁺CD34⁺), common lymphoid progenitors (CLP, Lin⁻CD127⁺), and common myeloid progenitors (CMP, c-kit⁺Lin⁻SCA-1⁻CD34⁺CD127⁻) in WT and mpl^−/− BM, expressed as the percentage of all live BM cells (mean ± SD, n = 3 each). *P < .05 vs percentage in WT mice. (B) Representative dot plot and histogram demonstrating method of determining percentages of live, GFP⁺ donor cells within irradiated recipient BM at 24 hours after BMT, used with total BM cell counts to determine absolute donor GFP⁺ BM cells. (C) Absolute live BM GFP⁺ donor cells (individual mice and/or mean per group) shown at 24 hours (left), 3 to 7 days (middle), and 3 weeks (right) post-BMT of 5 × 10⁶ (24 hours to 7 days), or 2 × 10⁵ (3 weeks) GFP⁺ whole BM cells into post-TBI primary recipient WT or mpl^−/− mice ± recipient treatment with anti-CD41 at the time of TBI (n = 3-5 mice per group). *P < .05 vs WT recipients. (D) H&E-stained histologic sections showing decreased BM cellularity in anti-CD41–treated mpl^−/− (bottom) vs WT (top) recipients of GFP⁺ donor BM at 3 weeks post-BMT. (E) Flow cytometry dot plots of c-Kit vs SCA-1 expression on live GFP⁺Lin⁻ gated whole BM cells from H2K-GFP donor BM before transplant (used to establish the HSC/progenitor c-Kit⁺lin⁻Sca-1⁺ [KLS] gate), and from either WT (top, right) or anti-CD41–treated mpl^−/− (bottom, right) primary recipients (pooled BM from 2-4 mice per time point), at 3, 5, and 7 days post-BMT with GFP⁺ whole BM cells (3-5 × 10⁶ cells per mouse). Rectangle in each plot represents KLS population with frequency expressed as a percentage of live GFP⁺Lin⁻ cells and of total live cells (in parentheses).