Application of Natural Killer T-Cells to Posttransplantation Immunotherapy

Graft-versus-host disease (GVHD) and graft-versus leukemia (GVL) effects are closely related to each other after allogeneic stem cell transplantation. This association exists because of the extensive and complicated interaction between cellular donor components and recipient components concomitant with cytokine storms. It has been demonstrated that part of this interaction may be related to the induction of a variety of regulatory cells, such as regulatory T-cells and natural killer T (NKT) cells. A lower number of NKT cells may be found in patients with autoimmune diseases, cancer, viral infection, and severe GVHD. When activated, NKT cells rapidly release suppressive cytokines, such as interleukin 4 (IL-4), IL-10, and IL-13, as well as inflammatory cytokines, such as interferon γ and tumor necrosis factor α. NKT cells therefore act as a double-edged sword in their progressive or suppressive effects on diseases. Such contradictory phenomena may be related to the function or types of antigen-presenting cells (APCs) in response to their ligand. A single-dose injection of a ligand for NKT cells, α-galactosylceramide (α-GalCer), can induce immunity through fully mature dendritic cells in an antigen-specific manner. By contrast, multiple injections of α-GalCer would induce tolerance, which may be caused by immature APCs. This response suggests that the function of NKT cells can be determined by α-GalCer for controlling the immune response. Furthermore, activation of NKT cells followed by activation of APCs and IL-12 production may lead to activation of NK cells and suppress GVHD in mismatched major histocompatibility complex combinations or may induce GVL effects. Control and modification of NKT cell function may play an important role in regulating GVHD/GVL effects.