Inhibition of S-phase kinase-associated protein 2 (Skp2) reprograms and converts diabetogenic T cells to Foxp3+ regulatory T cells

Autoreactive pathogenic T cells (Tpaths) and regulatory T cells (Tregs) express a distinct gene profiles; however, the genes and associated genetic/signaling pathways responsible for the functional determination of Tpaths vs. Tregs remain unknown. Here we show that Skp2, an E3 ubiquitin ligase that affects cell cycle control and death, plays a critical role in the function of diabetogenic Tpaths and Tregs. Down-regulation of Skp2 in diabetogenic Tpaths converts them into Foxp3-expressing Tregs. The suppressive function of the Tpath-converted Tregs is dependent on increased production of TGF-beta/IL-10, and these Tregs are able to inhibit spontaneous diabetes in NOD mice. Like naturally arising Foxp3(+) nTregs, the converted Tregs are anergic cells with decreased proliferation and activation-induced cell death. Skp2 down-regulation leads to Tpath-Treg conversion due at least in part to up-regulation of several genes involved in cell cycle control and genes in the Foxo family. Down-regulation of the cyclin-dependent kinase inhibitor p27 alone significantly attenuates the effect of Skp2 on Tpaths and reduces the suppressive function of converted Tregs; its effect is further improved with concomitant down-regulation of p21, Foxo1, and Foxo3. In comparison, Skp2 overexpression does not change Tpath function, but significantly decreases Foxp3 expression and abrogates the suppressive function of nTregs. These findings support the critical role of Skp2 in functional specification of Tpaths and Tregs, and demonstrate an important molecular mechanism mediating Skp2 function in balancing immune tolerance during autoimmune disease development.