Exogenous interleukin-33 targets myeloid-derived suppressor cells and generates periphery-induced Foxp3⁺ regulatory T cells in skin-transplanted mice.

Interleukin-33 (IL-33) has been a focus of study because of its variety of functions shaping CD4(+) T-cell biology. In the present work, we evaluated the modulatory effect of IL-33 on suppressor cells in an in vivo transplantation model. C57BL/6 wild-type mice were grafted with syngeneic or allogeneic skin transplants and treated with exogenous IL-33 daily. After 10 days of treatment, we analysed draining lymph node cellularity and found in allogeneic animals an increment in myeloid-derived suppressor cells, which co-express MHC-II, and become enriched upon IL-33 treatment. In line with this observation, inducible nitric oxide synthase and arginase 1 expression were also increased in allogeneic animals upon IL-33 administration. In addition, IL-33 treatment up-regulated the number of Foxp3(+) regulatory T (Treg) cells in the allogeneic group, complementing the healthier integrity of the allografts and the increased allograft survival. Moreover, we demonstrate that IL-33 promotes CD4(+) T-cell expansion and conversion of CD4(+)  Foxp3(-) T cells into CD4(+)  Foxp3(+) Treg cells in the periphery. Lastly, the cytokine pattern of ex vivo-stimulated draining lymph nodes indicates that IL-33 dampens interferon-γ and IL-17 production, stimulating IL-10 secretion. Altogether, our work complements previous studies on the immune-modulatory activity of IL-33, showing that this cytokine affects myeloid-derived suppressor cells at the cell number and gene expression levels. More importantly, our research demonstrates for the first time that IL-33 allows for in vivo Foxp3(+) Treg cell conversion and favours an anti-inflammatory or tolerogenic state by skewing cytokine production. Therefore, our data suggest a potential use of IL-33 to prevent allograft rejection, bringing new therapeutics to the transplantation field.