Previous work from our laboratory has indicated that the transplantation of pancreatic islets is a feasible approach to the problem of diabetes. A major obstacle to transplantation is presented by passenger leucocytes, which contaminate the preparations and can lead to the prompt rejection of fresh islets. We have extended our previous studies on the rejection of islet allografts by challenging transplanted animals with enriched lymphoid cell populations prepared from animals both syngeneic to the transplanted islets and third party. Rapid and complete rejection was observed when the challenge peritoneal exudate cell population was syngeneic with the transplanted islets; rejection was determined by both functional and histologic criteria. Peritoneal exudate cells from a thirdparty rat strain induced delayed and variable effects upon the function of the transplant. In contrast, splenic T-cells were capable of inducing rejection, regardless of the strain of origin, though the time course of T-cell-induced rejection was slower than that observed by syngeneic peritoneal exudate cells. Finally, splenic B-cells completely failed to induce rejection.

Our data indicate that at least two mechanisms exist by which the rejection of islet allografts may be triggered. The first is a haplotype-specific mechanism initiated by a cell type present at high frequency in peritoneal exudate cells; these are probably macrophages. The second mechanism is initiated by immunocompetent T-cells; this mechanism shows no haplotype specificity. We suggest that both macrophages and T-cells must be considered when devising protocols for the removal of passenger leucocytes from allografts.

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