Pituitary cotransplantation significantly improves the performance, insulin content, and vascularization of renal subcapsular islet grafts

Because the pituitary contains hormones with β-cell trophic activity, we evaluated whether cotransplantation of pituitary tissue with pancreatic islets might be beneficial for islet graft function and survival. Streptozotocin diabetic nude mice were transplanted under the kidney capsule with 150 handpicked islets alone or mixed with two diced pituitaries and were then followed for 4 weeks. Mice transplanted with mixed islet/pituitary grafts had higher levels of circulating prolactin (PRL) than mice transplanted with islets only, while serum cortisol, growth hormone, and follicle-stimulating hormone were similar in the two groups. After transplantation, recipients of mixed islet/pituitary grafts showed a more pronounced decrease in glycemic levels and higher systemic insulin levels than mice transplanted only with islets. Mixed islet/pituitary grafts were macroscopically characterized by an excellent vascularization and were biochemically characterized by higher insulin and PRL content than pure islet grafts. Histologically, posttransplantation remodeling originated a hybrid organ in which healthy, well-vascularized islets were adjacent to pituitary cell clusters. Transplantations performed to address the specific effect of the anterior versus the intermediate pituitary lobes indicated the former as responsible for the improved function of cotransplanted islets. Mixed islet/pituitary grafts composed of anterior lobes were also the best vascularized and were histologically characterized by the presence of many folliculo-stellate cells. In conclusion, we obtained evidence that pituitary cotransplantation significantly improves the function, insulin content, and vascularization of suboptimal islet grafts. Evidence suggesting that ectopically produced PRL and/or locally released angiogenic peptides might play a causal role is provided.