Background: Regulatory T cells (Tregs) are crucial mediators of immune homeostasis, with the ability to modulate alloreactive T cell responses and control transplant rejection. Interleukin-33 (IL-33) and its receptor ST2 have recently been demonstrated to have a role in the modulation of Tregs. In this study we investigated the effect of IL-33 treatment in vivo on immune phenotype, with a specific focus on Tregs.
Materials and Methods: Wild type mice were treated with recombinant IL-33 (1µg for 6 consecutive days) or saline (control). and their splenocytes and Tregs were isolated and analysed using a nanostring multiplexed gene immune panel. To assess Treg function, CD4⁺FoxP3⁺ Tregs were flow sorted from both groups of mice and adoptively transferred together with effector T cells (Teffs) into syngeneic immunodeficient mice (n=8 for Teff, n= 11 naive Treg, n=11 for IL-33 Treg). Mice were then transplanted with an allogeneic skin graft which was monitored for survival.
Results:  IL-33 treatment in vivo resulted in an increase in the relative abundance of Tregs, together with a reduced relative abundance of DCs, CD8⁺ T cells, NK cells and Th1 cells, and reduced adaptive, cell cycle, chemokine and cytokine receptors, among others.  IL-33 administration also expanded a CD4⁺FoxP3⁺ Treg population with an effector phenotype (CD44^hiCD62L^neg) and upregulated Treg-associated markers. Mice treated with sorted Tregs from IL-33-treated mice had an enhanced ability to modulate Teff responses and suppress allograft rejection in vivo. While control Tregs extended allograft median survival time (MST) from 14 days to 40 days, Tregs from IL-33-treated mice promoted long term survival (>100 days, p= 0.03). Further transcriptomic and phenotypic analysis revealed an upregulation of critical graft-homing chemokine receptor molecules (CCR2, CCR4, CCR5) in IL-33-expanded Tregs.
Conclusion: Administration of IL-33 in vivo can significantly expand suppressive Tregs with graft-homing potential that demonstrate an enhanced ability to prolong allogeneic skin graft survival. These data highlight an important pathway for the modulation of Tregs which may be incorporated into future therapeutic regimens.