Comparative study of the inhibitory cytokines on human natural killer cell functions

Abstract

Natural killer (NK) cells have potent killing effects and produce several cytokines in response to tumor cells. On the other hand, tumors escape from the host defense system by forming an immunesuppressive network which contains immunosuppressive cytokines. This study focused on the fact that tumor that infiltrated NK cells are inhibited, and the hypothesis that there might be conditions highly related to NK cell inhibition in tumor microenvironment.

In this study, the effect of TGF-β, IL-10, and IL-4, inhibitory cytokines hereafter, on IL-2 activated human NK cells was compared in conditions where the cytokines were put separately or together. The aim of this study is to find out the most potent inhibitory condition of NK cells. The results are as follows. Each of the inhibitory cytokines and the combinations of them inhibited IFN-γ secretion, NK cell proliferation, and the expression of NKp44, NKp46, and NKG2D. Also the inhibitory cytokines affected NK cell degranulation and finally, abrogated the target cell lysis effect of the NK cells. However, they had the little effect on NKp30, TRAIL, Fas L, NKG2A, and 2B4 expression. Among the three cytokines, TGF-β showed the most

potent inhibitory effect. Furthermore, the individual effects of TGF-β were as potent as the combined effects of TGF-β and IL-10/IL-4. Thus, either with or without other cytokines, TGF-β worked as the core cytokine in terms of inhibiting overall NK cell function.

Next, the molecular mechanism of NK cell inhibition caused by TGF-β was examined on NK-92 cell. TGF-β reduced tyrosine phosphorylation of Syk and an expression of c-myc, whereas an expression of GRB2 and the phosphorylation of ZAP70, STAT5, p38, ERK, IKBα, PTEN, AKT, and FAK were not affected by TGF-β. Then specific transcription factors affected by TGF-β were identified through profiling activities of transcription factors in NK-92 cell. It was found that the IL-2- induced c-Myb, AP-1, CREB, and AR activity were completely suppressed by TGF-β treatment. TGF-β also partially reduced the activity of STAT-5 and NFAT. And TGF-β completely suppressed CBF, ER, FAST-1, and MRE, which are constitutively activated regardless of IL-2 stimulation.

In conclusion, This study proved the significance of TGF-β on NK cell inhibition by comparative analysis and suggested biomarkers susceptible to TGF-β-mediated inhibition of NK cells. The goal of our study is to provide an insight into the effects of TGF-β on NK cell immune modulation and an effective therapeutic strategy that enhances NK cell activity.