Now researchers at the University of California and Yale have shown that the 'T-regs' can be removed from the body, increased by 1,500x in the laboratory and infused back into the bloodstream to restore normal function.
An initial trial of 14 people has shown that the therapy is safe, and can last up to a year.
"This could be a game-changer," said Dr Jeffrey Bluestone, Professor in Metabolism and Endocrinology at the University of California, San Francisco (UCSF).
"By using T-regs to 're-educate' the immune system, we may be able to really change the course of this disease.
"We expect T-regs to be an important part of diabetes therapy in the future."