MHC Class II and T Cell Receptor Signals Are Dispensable for IL-2-Induced Regulatory T Cell Proliferation In Vivo

Abstract 2181

Regulatory T cells (Tregs) protect the host from autoimmunity and inappropriate immune activation. Thus, to ensure immune tolerance in the steady state, an adequate number of peripheral Tregs must be constantly maintained. Prior work has suggested that major histocompatibility class II (MHC II) and interleukin-2 (IL-2) are both necessary to maintain peripheral Treg homeostasis and proliferation in vivo. However, we have recently reported that Treg proliferation may not strictly depend on MHC II, as the provision of IL-2 was sufficient to drive proliferation of Tregs in an MHC II-independent manner in vitro, as long as the Tregs interacted with dendritic cells (DC)s.

Here, extending our previous in vitro observations, we tested the dependence of Treg proliferation on IL-2, DCs, and TCR signaling in vivo. Proliferation of adoptively transferred Tregs was detected in wildtype (WT) mice. This proliferation was markedly enhanced when the mice were injected with IL-2 immune complexes (IC)s but not when the IL-2 IC-injected mice lacked DCs, suggesting that IL-2-induced Treg proliferation was dependent on DCs in vivo. As previously reported, adoptively transferred Tregs did not proliferate in MHC II-deficient hosts. However, the injection of IL-2 ICs into these mice induced Treg proliferation comparable to those transferred into IL-2 IC-injected WT mice, suggesting that IL-2 signaling by Tregs obviated the need of MHC II for their proliferation. Furthermore, while the ablation of TCR signaling by conditional deletion of the adaptor protein SH2 domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) rendered Tregs unable to proliferate by themselves, IL-2 IC treatment partially rescued this deficiency.

We next examined the signaling pathways involved in Treg proliferation downstream of the IL-2 receptor. Despite the importance of the Stat5 pathway in IL-2 receptor signaling during Treg development in the thymus, activation of Stat5b alone was insufficient to rescue proliferation of SLP-76-deficient Tregs, indicating that alternative pathways must also be activated for Treg proliferation. Additional studies investigating the role of other signaling molecules downstream of the IL-2 receptor are currently underway.

In summary, we have demonstrated for the first time that Tregs do not require TCR signaling through interaction with MHC II for their proliferation in vivo. We propose that this MHC II-independent mode of Treg proliferation allows Tregs with multiple antigen specificities to proliferate, which ensures that a diverse TCR repertoire is continuously maintained in the Treg pool. Furthermore, we believe that exploitation of these pathways may be therapeutically beneficial in autoimmunity and in transplantation.