Allogeneic Cytokine-Induced Killer Cells Eliminate Host Dendritic Cells Due To The Enhanced Killing Activity By IFN-Gamma, Leading To Less Graft-Versus-Host Disease

Cytokine-induced killer (CIK) cells are ex vivo–expanded T lymphocytes expressing both natural killer (NK)– and T-cell markers. We have reported that adoptive transfer of allogeneic CIK cells in a murine model caused minimal graft-versus-host disease (GVHD) with retention of antitumor activity mediated by NKG2D, which is an activating receptor expressed on NK cells. It was reported previously that IFN-gamma was a critical cytokine for preventing GVHD because CIK cells generated from IFN-gamma knock out mice caused lethal GVHD. One possible mechanism of protective effect against GVHD was that a large amount of IFN-gamma secreted by CIK cells could promote Fas-mediated apoptosis of recipient antigen activated donor T cells. However the mechanism of suppression of GVHD with CIK cells is not fully understood. Host residual dendritic cells (DCs) are most important cells in initiating GVHD reaction. Therefore, we hypothesize that CIK cells eliminate host DCs as same as kill tumor cells, and then reduce GVHD severity. To test this, 51Cr release assay was performed using BM derived DCs as target cells. We showed that allogeneic CIK cells had cytolytic activity against DCs, in marked contrast killing activity of IFN-gamma deficient CIK cells was much less compared to that of WT CIK cells (p< 0.05). Expression level of surface markers including NKG2D and CD8/CD4 ratio was not different between WT and IFN-gamma KO CIK cells. To further explore the mechanism why IFN-gamma deficient CIK cells reduced killing activity against DCs, IFN-gamma neutralizing antibody was added into killing mixture with WT CIK cells. However killing activity of WT CIK cells against DCs with IFN-gamma antibody did not changed compared to that of WT CIK alone. These results suggested that IFN-gamma itself did not affect killing activity directly, but IFN-gamma was essential cytokine to enhance killing activity during the culture of CIK cells. Next concern was whether CIK cells eliminated host DCs in vivo or not, we compared the residual number of host-type splenic DCs between the mice receiving bone marrow (BM) cells plus WT CIK cells and IFN-gamma deficient CIK cells. As shown in the figure below, the number of host DCs in the spleen in the mice receiving WT CIK cells was significantly lower compared to those of the mice receiving IFN-gamma KO CIK cells (p<0.05). In conclusion, allogeneic CIK cells eliminated host DCs due to the enhanced killing activity by IFN-gamma, leading to much less GVHD.