Porcine pancreatic icosapeptide as a marker of graft survival and rejection in xenotransplantation

The previous study showed that it is possible to monitor the viability of xenografted fetal pig pancreas in the first 3 weeks after transplantation in a normoglycemic immunoincompetent mouse [1]. This is achieved by measuring serum levels of porcine pancreatic icosapeptide (PI) in the host using a specific immunoassay. PI is secreted from the pancreatic polypeptide (PP) cell, the first mature endocrine cell of the fetal pig pancreas. Insufficient insulin is produced by the graft at this time to use it as a marker of graft viability. We have now examined the usefulness of PI to monitor graft survival in diabetic immunoincompetent mice and in immunosuppressed mice. Fetal porcine pancreatic tissue was transplanted beneath the renal capsule of streptozotocin-diabetic non-obese diabetic severe combined immunodeficient mice (NOD-SCID) and BALB/c mice given cyclosporin (CsA) for 21 or 10 days, respectively. Blood and grafts were analyzed periodically over 15 and 3 weeks, respectively, for porcine PI and C-peptide/insulin. In the diabetic mice, porcine PI was detected in sera up to 2 weeks following transplantation and was absent thereafter. Serum porcine C-peptide was detectable at 3 weeks and reached maximum levels at 12 to 15 weeks. PI content of the grafts was highest at day 4 and 3 weeks and lowest from 9 weeks onwards while insulin levels in the graft were highest from 9 weeks onwards. In BALB/c mice immunosuppressed with CsA, serum PI was detectable for 21 days. In the mice given CsA only for 10 days, serum PI was detectable for that time, but not on day 14 although PI was detectable in the graft. In mice given no CsA, PI was undetectable in serum at any time although PI was present in the graft at day 4. The presence of porcine PI in sera is a marker of graft survival of fetal porcine pancreatic tissue in diabetic immunocompetent mice in the first 2 weeks after transplantation. Its absence in immunosuppressed mice in this period is an early indicator of graft rejection.