Gi-protein signals in BM homing and short-term engraftment. (A) PTX-sensitive signals guide BM homing of TPO-incubated murine CFU-Cs. Murine WT BM cells, incubated overnight in TPO ± PTX, was transplanted and analyzed as described above. As with SCF plus PTX-treated BM cells, 18-hour homing of TPO plus PTX-incubated CFU-Cs was significantly decreased compared with TPO alone (^*P < .05). BM homing is given as percentage of injected CFU-Cs recovered per femur (mean plus SEM). (B) Effect of PTX on lin^-c-kit⁺ BM-cell homing. BM homing of SCF ± PTX-incubated lin^-c-kit⁺ cells, a population enriched in HSCs, was tested. The efficiency of 18-hour BM homing was significantly reduced by PTX (^*P < .05), similar to identically treated CFU-Cs. The mean of cells recovered from BM of recipients of untreated control cells was considered 100%; BM homing is given as percentage thereof (percentage of mean of control plus SEM). (C) Equal numbers of BM cells, which contained similar numbers of CFU-Cs, either incubated with cytokine-free ± PTX for 4 hours (left) or with SCF ± PTX overnight (right), were injected into lethally irradiated recipients. Mean plus SEM of recovered CFU-C/femur 8 days after transplantation of fresh (left) or SCF-incubated (right) BM cells are depicted. Short-term engraftment of PTX-treated samples was significantly poorer than that of PTX untreated samples (^*P < .05), in excess of the effect of PTX on BM homing under the respective conditions. (D) Schematic representation of the cooperative model of homing molecules between α4β1-integrin/VCAM-1, SDF-1/CXCR4/Gi-proteins, endothelial selectins, and β2-integrins, suggested by the data presented in this study. Cytokine incubation shifts the dominance among the molecular homing pathways from α4β1/VCAM-1 (top) toward SDF-1/CXCR4 (bottom).