Abstract: The invention relates to the use of microvesicles derived from cells of the endothelial cell lineage, preferably from endothelial progenitor cells, as an adjuvant agent in endothelial cell-pancreatic islet transplantation, particularly in the treatment of type I or type II diabetes by pancreatic islet transplantation. The adjuvant effect of the microvesicles consists in the improvement of survival and functionality of the transplanted islets. The adjuvant effect of the microvesicles is maintained even when the recipient is subjected to rapamycin-based immunosuppression.