683. Foamy Virus Gene Therapy Significantly Restores Lymphocyte Development and Function in SCID-X1 Mice

X-linked severe-combined immune deficiency (SCID-X1) results from inactivating mutations in the gene encoding the common gamma chain (γc), a cytokine receptor subunit required for lymphoid development and function. In humans, SCID-X1 is characterized by the absence of T cells and natural killer cells, non-functional B cells, and is fatal within the first year of life if left untreated. Although HLA matched allogenic stem cell transplants yield survival rates exceeding 90%, such donors are often unavailable. Thus, gene replacement therapy offers a promising alternative treatment for patients lacking suitable donors. Clinical trials using gamma-retroviral vectors demonstrated efficacy; however, adverse events highlighted the need for improved safety. While adverse events have been eliminated using SIN-gamma-retroviral vectors, we hypothesized that foamy viruses (FV) that lack native pathogenicity and boast an integration site profile much less focused on either active genes or promoter regions than lentiviruses or gamma-retroviruses might provide an improved and safer therapeutic platform. We determined whether a candidate clinical FV vector containing the human phosphoglycerate kinase 1 (PGK) promoter driving human γc expression could safely and effectively rescue lymphocyte development and function in SCID-X1 mice. Our promoter choice was based upon ongoing studies using FV vectors in parallel in a canine model of SCID-X1 where the PGK promoter outperforms the EF1α promoter currently in use in SIN-retro and lentiviral-based SCID-X1 clinical trials. We report here the combined results of 66 primary and 130 secondary transplant mice that demonstrate significant and sustained immune reconstitution. SCID-X1 mice were transplanted with 2×10^6 FV- or LV-transduced, or WT or SCID-X1 lineage negative HSCs. Animals were sacrificed at ~25 weeks post-transplant; bone marrow, spleen and thymus were collected for analysis and secondary recipients were also established. Primary FV gene therapy recipients showed significantly greater B and T lymphoid numbers compared to SCID-X1 controls, with output viral copy numbers of ~1-4 in all tissues. Splenocytes from primary treated animals proliferated in response to CD3/CD28 and also demonstrated γc dependent intracellular pSTAT signaling in response to IL-7 and IL-21. FV and LV gene therapy-treated mice displayed comparable lymphocyte reconstitution and output copy number. Secondary recipients revealed sustained partial rescue of lymphoid compartments and viral marking, indicating the transduction of long-term repopulating HSCs. RIS analysis demonstrated polyclonal marking in splenic lymphoid populations with 3508 unique FV and 2441 unique LV integrations detected (3 experiments/3 animals per experiment). Further, analysis of individual mice from a primary transplant experiment demonstrated polyclonal marking with 588-1060 and 651-895 unique integration sites in FV- and LV-treated animals, respectively. Analysis to determine detailed integration site profile and potential proximity to protooncogenes is ongoing. Together, these data suggest FV gene therapy may provide an effective alternative treatment option for SCID-X1.