Splenocytes from nonobese diabetic mice overexpressing murine IL (mIL)-4 upon recombinant retrovirus infection lose their capacity to transfer diabetes to nonobese diabetic-scid recipients. Diabetes appeared in 0-20% of mice injected with mIL-4-transduced cells vs 80-100% of controls injected with beta-galactosidase-transduced cells. Protected mice showed a majority of islets (60%) presenting with noninvasive peri-insulitis at variance with beta-galactosidase controls that exhibited invasive/destructive insulitis. Importantly, in all recipients, the transduced proteins were detected within islet infiltrates. Infiltrating lymphocytes from recipients of mIL-4-transduced cells produced high levels of mIL-4, as assessed by ELISA. In recipients of beta-galactosidase-transduced cells, approximately 60% of TCRalphabeta(+) islet-infiltrating cells expressed beta-galactosidase, as assessed by flow cytometry. The protection from disease transfer is due to a direct effect of mIL-4 gene therapy on immunoregulatory T cells rather than on diabetogenic cells. mIL-4-transduced purified CD62L(-) effector cells or transgenic BDC2.5 diabetogenic T cells still transferred disease efficiently. Conversely, mIL-4 transduction up-regulated the capacity of purified immunoregulatory CD62L(+) cells to inhibit disease transfer. These data open new perspectives for gene therapy in insulin-dependent diabetes using T cells devoid of any intrinsic diabetogenic potential.