Role of CD137/4-1BB and Its Ligand in the NK Cell Mediated Immune Surveillance of Acute Myeloid Leukemia.

NK cells play an important role in the reciprocal interaction of tumor cells with the immune system and participate in the surveillance and eradication of hematological malignancies including acute myeloid leukemia (AML). NK cell reactivity is governed by a balance of activating and inhibitory receptors including various members of the TNF receptor (TNFR) superfamily. The TNFR superfamily member CD137/4-1BB has been shown to stimulate proliferation and IFN-γ production, but not cytotoxicity of NK cells in mice. Surprisingly, yet nothing is known regarding the consequences of CD137-CD137 ligand (CD137L) interaction for NK cell reactivity in humans. In this study we demonstrate that CD56^dimCD16⁺ but not CD56^brightCD16⁻ NK cells express CD137 upon stimulation with the activating cytokines IL-2 and IL-15 with peak expression between 48 and 60h. Furthermore, we found that 5 of 7 investigated AML cell lines and 16 of 51 (33%) primary AML cells of patients expressed substantial CD137L levels, while no CD137L expression was detected on CD34⁺ cells of healthy donors (n=5). CD137L expression was not restricted to a specific French-American-British (FAB) subtype, but was significantly (p<0.05, one-way ANOVA) associated with monocytic (FAB M4, M5) differentiation. In addition, no association with a particular cytogenetic abnormality or with expression of MHC class I was observed. Reverse signaling via CD137L into AML cells (n=10) significantly induced the release of the immunoregulatory cytokines IL-10 and TNF (both p<0.05, Mann-Whitney U-test). Surprisingly and in contrast to available data regarding the function of murine CD137, we found that in humans blocking CD137-CD137L interaction caused a significant increase in NK cell cytotoxicity and IFN-γ production about 50% (both p<0.05, Mann-Whitney U-test) in coculture assays with CD137L-expressing patient AML cells and AML cell lines. The inhibitory effect of CD137 on NK cell reactivity was further confirmed in cocultures of NK cells with CD137L-transfectants and by triggering CD137 with an agonistic monoclonal antibody. This indicates that CD137 mediates opposite effects in murine compared to human NK cells. Furthermore we conclude that CD137L expression substantially influences tumor immunoediting by AML cells and diminishes NK cell reactivity against AML.