Abstract
IL-2R signaling is essential for the development and homeostasis of CD4+Foxp3+ regulatory T cells (Tregs). Low-dose IL-2 is being advanced as a therapy for autoimmune diseases by its ability to expand Tregs. Although Treg stability and function is diminished by chronic inflammation, the impact of inflammation on proximal IL-2 receptor (IL-2R) signaling and/or responsiveness to low-dose IL-2 is poorly understood. To explore this question, we examined the effects of acute inflammation on Treg IL-2R signaling using high-dose LPS as well as in experimental autoimmune encephalomyelitis (EAE), as different sources of inflammation may be both qualitatively and quantitatively different. Here, we show that acute inflammation induced by LPS, analogous to responses to acute bacterial infection, led to decreased endogenous STAT5 signaling and proliferative potential as measured by Ki67 in mouse Tregs. This impaired Treg activity was transient, did not lead to a reduction in Treg numbers or function, and was due to Toll-like receptor (TLR) signaling by non-Treg cells. Although acute LPS induced high levels of IL-1 and IL-6, these cytokines did not solely mediate dysregulated Treg activity. Global gene expression analyses demonstrated that acute LPS-induced inflammation substantially and rapidly altered the Treg transcriptome. In the presence of an IL-2R agonist, the mouse IL-2/CD25 (mIL-2/CD25) fusion protein, this type of inflammatory response tempered the transcription of IL-2R-dependent genes in vivo. Gene enrichment and pathway analyses are consistent with LPS attenuating mIL-2/CD25- dependent genes related to the cell cycle, DNA replication, and cholesterol biosynthesis while enhancing mRNAs that mediated Treg suppression in vivo. Acute LPS-induced inflammation diminished some responses by Tregs to mIL- 2/CD25 treatment in vivo.
The impact of EAE-derived inflammation on Treg IL-2R signaling and proliferation was also assessed. While inflammation from EAE does not reduce Treg proliferation as measured by Ki67 or CD69 expression as LPS-derived inflammation does, endogenous STAT5 signaling decreases transiently in splenic Tregs just after induction. The distribution of Tregs also shifts dramatically in the first week after induction, with Tregs migrating from the spleen into the cervical lymph nodes shortly before clinical symptoms appear, followed by an increase in Tregs in the spinal cord after disease is initiated. Administration of low-dose mIL- 2/CD25 shortly before onset and high-dose mIL-2/CD25 pre-immunization ameliorates both weight loss and clinical score and reduces peak clinical scores. mIL-2/CD25 may also play a role in decreasing demyelination and infiltrates into the spinal cord. Together, these results suggest a role for persistent IL-2R signaling in mitigating some, but not all, of the deleterious effects of inflammation on Tregs while supporting their function.