Novel therapy to restore endogenous regulatory T cell function and block disease progression in late stage pre-diabetic NOD mice

Background: We have identified a common defect in the Tregs of patients with certain autoimmune and allergic diseases: diminished inhibition of desensitization of their Treg IL-2 receptors (IL-2R) in response to low dose IL-2. Desensitization, or “turning off the IL-2R” occurs when the SOCS3 cullin ring ligase (CRL) ubiquitinates the IL-2R beta chain associated pJAK1. CRL ubiquitination of pJAK1 leads to its degradation, thus halting IL-2R signaling and pSTAT5 expression. The cullin5 component of the SOCS3 CRL must be post translationally modified (neddylated) at a specific lysine residue to activate its CRL function. The pSTAT5b isoform of STAT5 is required for the transcription of the Treg centric transcriptome including FoxP3 and CD25 (and other genes required for Treg function). The ubiquitin E3 ligase GRAIL is constitutively expressed in normal mouse and human Tregs and inhibits IL-2R desensitization by mono-ubiquitinating cullin5 at the exact lysine residue that must be neddylated to allow the CRL function. Thus, ubiquitination of cullin5 by GRAIL competes with neddylation and delays IL-2R desensitization. This prolongs expression of pSTAT5 in “normal” Tregs in response to low dose IL-2 activation.
Results: We have identified this same defect in inhibition of desensitization of the IL-2R on Tregs of human T1D patients, and have restored function of their Tregs in vitro by inhibition of IL-2R desensitization using a Nedd8-activating-enzyme (NAE) inhibitor to inhibit cullin5 neddylation. Using the same NAE inhibitor in combination with low dose IL-2, we were able to delay the progression to hyperglycemia more efficiently in 12-week old NOD mice that were treated for 21 days with the combination, than with low dose IL-2 as a single agent.