Controlling the burn and fueling the fire: defining the role for the alarmin interleukin-33 in alloimmunity

Purpose of reviewThe purpose of this review is to provide a general update on recent developments in the immunobiology of IL-33 and IL-33-targeted immune cells. We also discuss emerging concepts regarding the potential role IL-33 appears to play in altering alloimmune responses mediating host-versus-graft and graft-versus-host alloresponses.Recent findingsStromal cells and leukocytes display regulated expression of IL-33 and may actively or passively secrete this pleotropic cytokine. Type 2 innate lymphoid cells and a large proportion of tissue resident regulatory T cells (T-reg) express membrane-bound suppressor of tumorigenicity 2 (ST2), the IL-33 receptor. Although T-reg are appreciated suppressors of the inflammatory function of immune cells, both type 2 innate lymphoid cells and tissue resident T-reg could play key roles in tissue repair and homeostasis. The functions of IL-33 in transplantation are poorly understood. However, like other disease models, the functions of IL-33 in alloimmunity appear to be quite pleiotropic. IL-33 is associated with immune regulation and graft protection in cardiac transplant settings. Yet, it is highly proinflammatory and stimulates lethal graft-versus-host disease through its capacity to stimulate type 1 immunity.SummaryIntensive studies on IL-33/ST2 signaling pathways and ST2(+) cell populations in solid organ and cell transplantation are warranted. A better understanding of this important pathway will provide promising therapeutic targets controlling pathogenic alloimmune responses, as well as potentially facilitating the function of regulatory and reparative immune cells posttransplantation.