Background.Localized delivery of immunosuppressive molecules, limited to the graft site, may allow transplantation of tissue in the absence of systemic immunosuppressive agents. We tested whether purified mouse islets that had been engineered to produce human CTLA4-Ig locally at the graft site could survive in allogeneic recipients receiving no systemic immunosuppression.Methods.CBA (H2k) islets were subjected to biolistic (gene gun) transfection with a cDNA encoding human CTLA4-Ig under control of the human cytomegalovirus immediate early promoter. After 40-48 hr of culture, the transfected islets (500 per recipient) were transplanted beneath the renal capsule of alloxan-induced diabetic BALB/c (H2d) recipients.Results.Control grafts (n=10) consisting of islets biolistically transfected with the expression plasmid alone (i.e., no gene inserted) survived for 12.8±3.6 (mean ± SD) days. In contrast, islets transfected with CTLA4-Ig (n=12) survived 66.8±61.5 days (P=0.01), with 50% demonstrating functional survival until follow-up was concluded at 50 (n=2), 130 (n=2), or 165 (n=2) days. Immunohistochemistry on grafts that survived long term showed well-granulated, insulin-positive islets lying adjacent to, but not infiltrated by, dense aggregates of mononuclear cells.Conclusions.Transfection of allogeneic mouse islets with human CTLA4-Ig results in prolonged allograft survival. Although on histology mononuclear cells are present in the area of the transfected graft, they do not appear to infiltrate or destroy the islet graft.