Differential Impact of Host Derived Interleukin-18 on Regulatory and Conventional T Cell Expansion Following Murine Allogeneic Hematopoietic Cell Transplantation.

Allografting as a curative approach for many hematological malignancies is hampered by the occurrence of acute graft-versus-host disease (aGvHD). Interleukin (IL)-18 stimulates T helper 1 (Th1) and Th2-mediated immune responses and has been shown to modulate aGvHD. It is still unknown whether increased IL-18 levels during aGvHD are of host or donor origin and how the absence of IL-18 impacts migration and expansion of conventional CD4⁺CD25⁻ (Tconv) and CD4⁺CD25⁺ regulatory (Treg) T cells in vivo. By utilizing IL-18 gene deficient donor versus recipient animals we found that the major cytokine production during the early phase of aGVHD induction was recipient derived, while donor hematopoietic cells contributed significantly less. By generating IL-18^{−/ −} luciferase transgenic mice we were able to investigate the impact of IL-18 on Tconv and Treg expansion and trafficking with in vivo bioluminescence imaging. While migration to secondary lymphoid organs was not significantly impacted by the absence of host IL-18, Tconv but not Treg expansion increased significantly. Absence of host IL-18 production translated into lower IFN-γ levels in the early phase after transplantation. We conclude that host derived IL-18 is a major factor for IFN-γ production that may have a protective effect on CD4⁺ mediated aGvHD, but is non-essential for Treg expansion in an allogeneic environment.