IL33 and obesity due to melanocortin 4 receptor deficiency: Several studies have demonstrated that administration of IL-33 both in vivo and in vitro can induce Treg cell expansion and have highlighted the importance of IL-33 signaling in Treg cells during inflammatory conditions such as obesity-induced insulin resistance, chronic colitis, acutely injured muscles or graft-versus-host disease [22, 23, 45–48].