Novel approaches toward early diagnosis of islet allograft rejection

Background. The inability to diagnose early rejection of an islet allograft has previously proved to be a major impediment to progress in clinical isl et transplantation. The need to detect early rejection will become even mor e relevant as new tolerance-inducing protocols are evaluated in the clinic. We explored three novel approaches toward development of early diagnostic markers of islet rejection after islet allotransplantation. Methods. (a) Canine islet allograft transplant recipients were immunosuppre ssed for 1 month, then therapy was withdrawn, Serum glutamic acid decarboxy lase antigen (GAD(65)), an endogenous islet protein, was monitored daily wi th a CO2 release assay. (b) Rodent islets were genetically engineered to ex press a unique foreign protein (beta -galactosidase) by using adenoviral ve ctors, and after allograft transplantation, the viral-specific protein was measured in serum using optical luminescence. (c) Rodents receiving islet a llografts were immunosuppressed temporarily, and daily glucose tolerance te sts were followed until graft failure occurred. Results. (a) Although serum monitoring of GAD(65) antigen demonstrated elev ated levels preceding loss of graft function in preliminary studies, the ef fect was not reproducible in all animals. (b) Genetically engineered rodent islets demonstrated normal insulin kinetics in vitro (insulin stimulation index 2.57+/-0.2 vs. 2.95+/-0.3 for control islets, P=ns), and purified vir al protein products had a stable half-life of 8 hr in vivo. After islet all otransplantation, there were two peak elevations in serum viral proteins, c onfirming that an intra-islet "sentinel signal" could be detected serologic ally during acute rejection. There was no lead-time ahead of hyperglycemia, however. (c) Daily sequential intravenous glucose tolerance (IVGT) tests d emonstrated evidence of allograft dysfunction (decline in K-G) with a 2-day lead time to hyperglycemia (2.58+/-0.3 vs. 1.63+/-0.2%/min, respectively, P<0.001), with an accuracy of 89%, sensitivity of 78%, and specificity of 9 5%. Conclusions. Of the three diagnostic tests, metabolic assessment with an ab breviated IVGT was the most effective method of demonstrating early islet d ysfunction due to rejection.