Mobilized hematopoietic stem cells (HSC) now serve as a major source for transplantation. Better mobilizing approaches are clinically important. AMD3100, a CXCR4 competitive inhibitor, has been found to be a rapid mobilizing agent. We demonstrated that 5.0 mg/kg was the optimal dose of AMD3100 and mobilization peaked at 1 hour. For better mobilization, AMD3100 was co-administered (day 10) with FLT3-ligand (FL, day 1 to 10) and G-CSF (day 4 to 10). The 230.8-fold and 10.1-fold increase of lineage^–/Sca-1⁺/c-kit⁺ HSC and CD8⁺/TCR^– facilitating cells was detected when compared with AMD3100 treatment alone (P < 0.00001), respectively.

To determine the engraftment-potential of mobilized HSC from B6 (H-2^b) mice with FL/G-CSF/AMD3100, MHC disparate BALB/c (H-2^d) mice were lethally irradiated with 950 cGy total body irradiation (TBI) and reconstituted with varying doses of mobilized peripheral blood mononuclear cells (mPBMC) from of 10 × 10⁶ to 1 × 10⁶. All recipients expired by day 40 with manifestation of acute GVHD including weight loss, diarrhea, and hunched posture. When T cells in mPBMC were eliminated by sorting out ΑΒ-TCR⁺ and ΓΔ-TCR⁺ T cells, long-term survival and engraftment were achieved in the majority of recipients.

To evaluate whether outcomes correlated with the intensity of conditioning, recipients were nonmyeloablatively conditioned with anti-CD154 (day 0 and +2) and rapamycin (day 0 to +4) plus 100-300 cGy of TBI and transplanted with 2 x 10⁶ mPBMC. 42.9%, 85.7%, and 100% mice engrafted at 1 month with 100, 200, and 300 cGy, respectively. Chimerism was durable and stable > 6 months and multilineage. Lymphocytes from mixed chimeras showed no response to both host- and donor-antigens, suggesting functional bi-direction T cell tolerance. Humoral tolerance was demonstrated as no antibodies were detected against donor. Most importantly, all engrafted mice were healthy without any GVHD. In mixed chimerism, the allo-activated T cells were deleted by negative selection in the thymus to balance both immune systems without triggering the GVHD. Moreover, we found that anti-CD154 co-stimulation blocking and rapamycin synergistically prevented GVHD.

In summary, FL/G-CSF/AMD3100 is an efficient synergistic treatment to maximally mobilize HSC. Durable engraftment and donor-specific tolerance can be achieved with mPBMC in a nonmyeloablative conditioning strategy without causing GVHD.

Ildstad, S.: Other, Regenerex, LLC, CEO.

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