GENE THERAPY Nonmyeloablative conditioning is sufficient to allow engraftment of EGFP-expressing bone marrow and subsequent acceptance of EGFP-transgenic skin grafts in mice

Immunologic reactions against gene therapy products may prove to be a frequent problem in clinical gene therapy protocols. Enhanced green fluorescence protein (EGFP) is commonly used as a marker in gene transfer protocols, and immune responses against EGFP-expressing cells have been documented. The present study was designed to investigate the effect of a pharmacologic, nonmyeloablative, conditioning regimen on the development of EGFP donor/recipient mixed bone marrow chimerism and ensuing tolerance to EGFP-expressing transplants. To this end, C57BL/6J (B6) mice were treated with soluble formulations of either busulfan (Busulfex) or the closely related compound treosulfan, followed by transplantation of bone marrow cells from EGFP-transgenic (B6-EGFP.Tg) donor mice. Such conditioning regimens resulted in long-term persistence of donor EGFP cells among various hematopoietic lineages from blood, bone marrow, and thymus. Stable hematopoietic chimeras transplanted at 10 to 17 weeks after bone marrow transplantation (BMT) with B6-EGFP.Tg skin grafts all accepted their transplants, whereas non-EGFP chimeric B6 control animals were able to mount rejection of the EGFP B6 skin grafts. Control third-party grafts from major histocompatibility complex (MHC)– mismatched mice were rejected within 20 days, indicating that acceptance of EGFPexpressing skin grafts was the result of specific immune tolerance induction by the transplantation of EGFP-transgenic bone marrow. Long-term tolerance to EGFP in chimeric recipients was confirmed by the absence of anti-EGFP– reactive T cells and antibodies. These results broaden the therapeutic potential for using hematopoietic molecular chimerism in nonmyeloablated recipients as a means of preventing rejection of genetically modified cells. (Blood. 2003;101: 4305-4312)