Microgravity culture condition reduces immunogenicity and improves function of pancreatic islets

Background. The failure of pancreatic islet allotransplants observed in almost all clinical attempts is related to poor initial islet function and allograft rejection. To remedy these problems we cultured islets in microgravity conditions to improve their function and to reduce their immunogenicity. Methods. Fresh mouse islets or mouse islets cultured in stationary dishes or microgravity bioreactors were transplanted to streptozotocin-induced diabetic mouse recipients. Results. Both allogeneic dish- or bioreactor-cultured islets survived more than 100 days compared with fresh allogeneic islets, which were rejected in less than 15 days. Islet titration studies revealed that 250 fresh or dish-cultured, but only 30 to 120 bioreactor-cultured, islets were necessary to produce euglycemia. Furthermore, glucose tolerance tests showed that bioreactor-cultured islets functioned better compared with fresh and dish-cultured islets on day 30 postgrafting. Immunostaining and transmission electron microscopy (TEM) analyses showed the gradual disappearance of dendritic cells in cultured islets compared with fresh islets. TEM revealed that the ultrastructure of islets from bioreactor, but not dish, appeared healthy and closely resembled fresh islets. Interestingly, TEM and scanning electron microscopy showed that only bioreactor-cultured islets developed unique and multiple nutritional channels between arrays of islet cells. TEM with colloidal lanthanum tracer revealed that only bioreactor islet cell cultures were devoid of tight junctional complexes, which may facilitate channel formation. Conclusion. Microgravity condition decreases immunogenicity and significantly improves the function of secretory cells.