EFFECTIVE IMMUNE RECONSTITUTION IN MICE AFTER CO-TRANSPLANTATION OF BONE MARROW AND MARROW STROMA TRANSDUCED WITH THE IL-7 GENE: GENE THERAPY FOR POST-BMT IMMUNE DEFICIENCY. • 40

Bone marrow transplant (BMT) is followed by a period of profound immune deficiency, which is a major problem in clinical BMT and is likely to be important for gene therapy directed at immunologic diseases, e.g., SCID or AIDS. We have previously shown that human IL-7 (huIL-7) induces early thymic reconstitution in syngeneic murine BMT recipients (C57/B6). Another approach to achieve the same effect as multiple IL-7 injections would be to co-transplant bone marrow stroma engineered to produce high amounts of IL-7. Murine bone marrow stromal cultures were transduced with the retroviral vector JZEN hIL-7/tk neo to express huIL-7 (>1000pg/10⁶ cells/24hours). Mice received T cell depleted BMT (TCD-BMT) and 5×10⁶ stromal cells transduced with the human IL-7 gene (IL-7STR) via tail vein injection. Control animals received the same TCD-BMT but were co-transplanted with stromal cells transduced with the LNCX vector, which expresses only the neomycin resistance gene (LN-STR). Co-transplantation of the IL-7STR resulted in enhanced thymic reconstitution by day 28. The mean thymic cell number was 110 ± 7.5×10⁶ in normal mice, 88 ± 21×10⁶ in IL-7STR mice, and 9.8 ± 6.7×10⁶ for the LN-STR mice. Thymic subsets in IL-7STR and normal mice were comparable, but LN-STR mice had a block in early thymic maturation. T and B cell function was tested using the T cell mitogen ConA and the T-dependent antibody response to sheep RBC. Both proliferative responses to ConA and anti-SRBC titers developed sooner and were of greater magnitude in IL-7STR than in LN-STR mice. Transduced bone marrow stromal cells producing huIL-7 (1068-968 pg/10⁶ transduced stromal cells/24hours) could be cultured from the thymus on D28 post-BMT. No toxic effects of the IL-7 STR have been observed in recipient mice, with follow up of 1 year. Thus, bone marrow stroma transduced to overexpress IL-7 has the same effects on post-BMT thymopoiesis as IL-7 injections. Clinical strategies using IL-7STR gene therapy may be useful in enhancing thymopoiesis in BMT, SCID gene therapy, or AIDS patients.