Pancreatic islet xenograft tolerance after short-term costimulation blockade is associated with increased CD4(+) T cell apoptosis but not immune deviation

Background. Our purpose was to determine if shortterm inhibition of the CD4 0/CD40L and CD28/B7 costimulatory pathways was capable of inducing specific unresponsiveness to pancreatic islet xenografts and to ascertain the mecha nism of tolerance induction. Methods. Diabetic B6AF1 mice were transplanted with Wister or DA rat islets and were treated short term with CTLA4-Fc and anti-CD40L mAb (MR1). Results. Coadministrationo of CTLA4-Fc with MR1, resulted in indefinite rat islet xenograft survival in mice. Tolerance was species but not strain spe cific as long-term surviving recipients rejected third party BALB/c islet a llografts but accepted a second rat islet xenograft from the same or differ ent donor strain. Tolerance induction was associated with a large leukocyte infiltrate that did not exhibit features of immune deviation as intragraft T cell-specific cytokine gene expression was globally reduced. In particul ar, interleukin-4 gene expression was markedly suppressed. There was a comp lete inhibition of anti-donor IgG, IgG1, and IgM antibody in the serum of C TLA4-Fc/MR1- treated animals. Tolerance induction was associated with incre ased CD4(+) T cell apoptosis as there was an increased proportion of annexi n-V staining and Fas expressing CD4(+) T cells and a decrease in CD4(+) T c ell Bcl-2 expression in the grafts and draining lymph nodes of CTLA4-Fc/MR1 -treated recipients. Conclusion. Combined costimulatory blockade was capable of producing tolera nce to pancreatic islet xenografts. The induction of this tolerant state wa s associated with increased T cell apoptosis, whereas the maintenance phase of tolerance was associated with the accumulation of a large number of ina ctive lymphocytes within the graft.