Modified AdCTLA-4 vector blocks alloimmune response in vitro

Background. Gene transfer of the costimulatory blockade molecule CTLA-4Ig into cold-preserved rat liver allografts results in indefinite allograft survival. Despite local delivery, this mode of immunomodula-tion is associated with systemic immunosuppression. In an effort to restrict immunosuppression to the graft, we have constructed a novel adenoviral vector, AdCTLA-4ex-TAG, in which the Ig sequence of CTLA-4Ig DNA has been deleted to destabilize the gene product to promote rapid extrahepatic degradation while maintaining its immunosuppressive activity within the graft milieu. Methods. (1) Vector construction. CTLA-4 extracellular binding domain (CTLA-4ex) was prepared by PCR-based cloning methods and fused in frame to a genetic element encoding an epitope TAG allowing identification of the transgene product CTLA-4exTAG. CTLA-4exTAG was subcloned into a shuttle vector enabling isolation of AdCTLA-4exTAG. (2) In vitro transfection: AdCTLA-4exTAG was transfected into MH₁C₁ cells and then supernatant was recovered for Western blot analysis. (3) In vitro alloimmune response was characterized by CFSE proliferation assay. (4) Extrahepatic effect of AdCTLA-4exTAG was characterized by the ability to control tumor growth after implantation of a regressive, immune sensitive cancer cell line (REGb) in the skin of BDIX rats after liver transduction with AdCTLA-4exTAG. Results. Expression and secretion of the recombinant protein were documented by Western blot after infection of the MH₁C₁ cell line with AdCTLA-4exTAG. Addition of infected MH₁C₁ cell supernatant resulted in abrogation of alloimmune response as shown by markedly diminished division of CD4⁺ T cells in a CFSE proliferation assay. Extrahepatic tumor regressed normally after liver transduction with AdCTLA-4exTAG. Conclusions. These results show efficient in vitro expression of CTLA-4exTAG after transfection with AdCTLA-4exTAG. The modified protein retains its ability to abrogate in vitro alloimmune response. Efficient control of extrahepatic tumor growth after liver-directed delivery of AdCTLA-4exTAG suggests that the immunosuppressive effect of this vector is restricted to the liver. These results set the ground for the utilization of this novel adenoviral vector in the transplant setting.