Identification of transplanted pancreatic islet cells by radioactive dithizone-[131I]-histamine conjugate. Preliminary report

BACKGROUND

The unique mechanism of dithizone action in the interior of the viable pancreatic islet suggests the possible development of a specific radiopharmaceutical that may have a potential clinical application in the diagnosis of the pancreatic organ allografts or islets rejection. The radiodiagnostic properties of the newly developed radioactive analogue of dithizone, i.e. Dithizone-[(131)I]-Histamine conjugate have been evaluated in the present study.

METHODS

The four islet cells transplantation models were chosen for this purpose. The most important feature of the Dithizone-[(131)I]-Histamine conjugate is its possessed ability of zinc chelation. As was presented in the recent study, the conjugate stains pink-reddish the isolated pancreatic islets in vitro. Among the studied transplantation models, only the islets grafting under testis capsule enabled determination of the pancreatic islets in rats by radioactive Dithizone-[(131)I]-Histamine conjugate. The level of the radioactivity in the recipient testis (right) was almost two times higher compared to the controls (0.24 vs. 0.13% ID/g, respectively).

CONCLUSIONS

These preliminary data demonstrate the ability of the developed radioactive analogue of dithizone for in vivo identification of transplanted pancreatic islets, and suggests a potential clinical application of the radiodithizone in the diagnosis of the pancreatic islet rejection.

Monitoring of rat islet allografts with dithizone after induction of donor specific transplant tolerance by intrathymic administration of soluble alloantigens

Transplantation of whole pancreas or pancreatic islets remains a promising approach to treatment of diabetes mellitus. Since there is no efficient method presently known for in vivo detection of pancreatic islet rejection, we have utilized dithizone [DTZ] to monitor the survival of transplanted islet allografts following the induction of tolerance by a new strategy of deliberate introduction of donor antigens into the adult thymus. In this study, we examined the morphology of islet allografts in vivo and in vitro following pretreatment with intrathymic (IT) inoculation of 2 mg soluble Ag obtained from 3M KCl extracts of resting T-cells with or without ALS immunosuppression in the WF-to-Lewis combination. Fresh isolated rat islets stained pink 3-5 minutes following exposure to medium containing 0.12 mM DTZ solution in DMSO. Intravenous (i.v.) injection of DTZ solution into unmodified recipients of islet allografts that had rejected their grafts showed massive degranulation of islets which did not stain pink with DTZ. This was confirmed by microscopic finding of fibrosis and lymphocytic infiltration. In contrast, i.v. injection of DTZ solution into long-term recipients of islet allografts at 50, 100, and 150 days after transplantation showed viable islet cells which stained crimson red with DTZ and the findings were confirmed with microscopic sections. This study demonstrates that DTZ is an effective means of in vivo and in vitro identification of transplanted pancreatic islets and suggests that this strategy may have potential clinical application in the diagnosis of the pancreatic islet rejection.

Localization of endocrine pancreatic islets

Transplantation of whole pancreas or pancreatic islets remains a promising approach to treatment of diabetes mellitus. Because at present there is no efficient method for in vivo early diagnosis of pancreatic islet rejection or for disorders of pancreatic endocrine function, we examined if dithizone (DTZ) and a synthetic iodo-derivative of DTZ (I-DTZ) can be used as a potential radioactive marker for monitoring viable transplanted pancreatic islets. Human pancreatic islets harvested from multiorgan donors were tested ex vivo after intraductal injection of DTZ solution for islet staining. Lewis rats were used in the in vivo experiments for localizing pancreatic islets in situ after intravenous injection of various concentrations of DTZ or I-DTZ. Fresh rat islets were transplanted into streptozotocin-induced diabetic recipients, either underneath the kidney capsule or intraperitoneally. Intravenous DTZ or I-DTZ was then used for macroscopic and microscopic identification of viable transplanted islets. These studies indicate that DTZ and I-DTZ solutions specifically stain pancreatic islets in vivo after intravenous injection without damage to their endocrine function as assessed by plasma insulin and glucose levels. Human islets stain red in in vitro studies similar to the DTZ (I-DTZ) effect in rats. We conclude that DTZ and I-DTZ are effective in the in vivo and in vitro identification of pancreatic islets and may have potential clinical application in the detection of pancreatic islet tumors (insulinomas) and in the diagnosis of rejection of pancreatic organ allografts or islets.

Fludarabine phosphate: A DNA synthesis inhibitor with potent immunosuppressive activity and minimal clinical toxicity

Fludarabine phosphate selectively eliminates normal and malignant mononuclear cells in large animals and man through the inhibition of DNA synthesis. The drug depletes mononuclear cells from culture within 24 hours of initial exposure, CD4 and CD8 T cells being more sensitive than either CD20 B cells or CD34 bone marrow precursors. Mitogenic activation of lymphocytes enhances cellular elimination from culture. Fludarabine inhibits PHA-induced T-cell proliferation by >90 per cent and mixed lymphocyte reactions (allogeneic and xenogeneic) by >95 per cent. Fludarabine exerts its cytolytic effects through the induction of endonuclease-independent apoptosis. A 5-day course of fludarabine (50 mg/m2 intravenously once daily) induces both T- and B-cell lymphopenia in Cynomolgus monkeys and Papio baboons. Transient neutropenia was the only side-effect seen in experimental animals. Pretreatment of Cynomolgus monkeys with this regimen of fludarabine causes a prolongation of ABO-compatible skin allograft survival from 8 days (control) to 16 days (drug treated group). Secondary allotransplantation into presensitized recipients showed a similar prolongation of graft survival with fludarabine pretreatment (8 days vs 5 days control). Fludarabine promises to be a potent immunosuppressive agent with low clinical toxicity.

Transplantation tolerance to rat cardiac and islet allografts by posttransplant intrathymic inoculation of soluble alloantigens

The search for strategies for induction of specific tolerance in adult animals that will avoid long-term host immunosuppression with its complications has led to the deliberate introduction of alloantigens (Ag) into the adult thymus. However, pretransplant intrathymic (IT) inoculation of alloantigens (Ag), which has consistently induced tolerance to vascularized and neovascularized allografts in adult rodents, has limited future clinical application. To overcome the practical limitations of pretreatment, we have examined in the Lewis-to-WF combination the effect on graft survival of either simultaneous or posttransplant IT inoculation of soluble Ag obtained from 3M KCl extracts of donor T cells in transiently rabbit antirat lymphocyte serum (ALS) immunosuppressed recipients. While IT injection of 2.0 mg soluble Ag alone on day of cardiac transplantation caused acute graft rejection, IT inoculation of 2.0 mg Ag combined with 1 ml ALS transient immunosuppression of the recipient on day 0 led to long-term graft survival (> 250 days) in 5/6 recipients. Similarly, IT injection of soluble Ag on posttransplant day 3 or day 7 combined with 1 ml ALS on day 0 relative to allografting resulted in permanent graft survival in all recipients. In contrast, intravenous injection of soluble Ag combined with ALS immunosuppression on day 0 led to acute graft rejection that paralleled the rejection seen in ALS treated controls. Third-party Brown Norway (BN) hearts were acutely rejected in similarly prepared recipients of IT-Ag, thus confirming donor specificity. The long-term unresponsive Wistar-Furth (WF) recipients challenged 100 days after cardiac transplantation with a second-set graft specifically and permanently (> 100 days) accepted the second-set donor cardiac allografts, thus demonstrating donor-specific tolerance. In similar experiments, IT inoculation of 2 mg soluble Ag combined with transient ALS immunosuppression resulted in donor-specific unresponsiveness to islets in the same rat combination of Lewis-to-WF. Our findings suggest that this new strategy of immunologic manipulation of the adult thymus offers a safe, effective, and reproducible method of inducing tolerance that may have therapeutic application in cadaveric organ transplantation.