The possibility of transplanting insulin-producing cells from pig embryos into diabetics who need regular injections of insulin has moved a step closer, thanks to new research at the Weizmann Institute of Science in Rehovot. Until now, attempts to transplant animal tissues into monkeys have triggered an aggressive immune response. However, embryonic tissues, such as those from pigs - in which the insulin-producing cells are similar to those of humans - were thought by immunology Prof. Yair Reisner not to result in strong rejection by the body's immune system. In their previous work, Reisner and his team showed that each embryonic organ has its own "time window" during which the chances for successful transplantation are optimal. Before this window opens, the early tissue's cells, which are still largely undifferentiated, can give rise to tumors; past the window, however, they may be too well-developed. The host identifies these cells as foreign, causing the body to reject them. By transplanting tissues from pig embryos into mice lacking proper immune systems, they determined that the best time frame for pancreatic tissue was about a third of the way through gestation (from 42 to 56 days). In the new study, published in the June issue of PLoS (Public Library of Science) Medicine, Reisner and his team examined the possibility of such tissues functioning in the body. They first implanted embryonic pancreatic tissue from pigs into mice that lacked an immune system of their own, but had human immune cells injected into them. From this experiment they learned that tissues taken at 42 days (within the time frame they had previously determined) exhibited a markedly reduced immune response. Next, they experimented on mice with fully functioning immune systems but destroyed the insulin-producing cells in their pancreases before proceeding with the transplant. With the aid of relatively mild immune suppression protocols, the implanted tissues were fully functional over time, producing insulin and maintaining the mice's blood sugar at normal levels. "The results of this study," said Reisner, "warrant further, pre-clinical research on primate models." The Weizmann team hope their research could eventually lead to an unlimited source of pancreatic cells to replace human organ donations, which are in very short supply.