NK cells play an important role in tumor immunosurveillance, especially of leukemia. Their reactivity is governed by various activating and inhibitory molecules expressed by their targets including multiple members of the TNF family. The TNF family member Receptor Activator of NF-κB ligand (RANKL) and its receptors RANK and osteoprotegerin (OPG) are key regulators of bone remodelling, but recently have also been shown to influence progression of hematopoetic malignancies. Here we studied the yet unkown role of the RANK/RANKL molecule system in NK cells and their reactivity against acute myeloid leukemia (AML). Primary leukemia cells from AML patients were found to substantially express RANKL mRNA and surface protein in 75% of the investigated cases (n=40). Reverse signaling via surface-expressed RANKL into AML blasts induced the release of soluble factors including the immunoregulatory cytokines TNF and IL-10, which impaired NK cell anti-tumor reactivity. Moreover, we observed upregulation of RANK on NK cells among PBMC of healthy donors upon exposure to IL-10. This was not caused by direct effects on NK cells, but was rather due to yet unidentified factors released by monocytes among the PBMC upon IL-10 exposure and could be prevented by the activating cytokine IL-2. Furthermore, functional experiments with NK cells and RANKL transfectants or RANKL-negative controls revealed that forward signaling into RANK-expressing NK cells by tumor-expressed RANKL also directly impaired NK cytotoxicity and IFN-γ production. In line, blocking RANK-RANKL interaction using anti-RANKL antibodies or RANK-Fc fusion protein increased cytotoxicity and cytokine production of allogenic NK cells in cultures with RANKL-positive primary AML cells. Our data indicate that RANKL expression enables immune evasion of leukemia cells both by directly inhibiting reactivity of RANK-expressing NK cells and by orchestrating a reciprocal interplay between AML cells, monocytes and NK cells resulting in an immunosuppressive cytokine milieu. Thus, therapeutic modulation of the RANK/RANKL system, e.g. with Denosumab/AMG162, which is presently being evaluated for treatment of both non-malignant and malignant osteolysis, holds promise to reinforce NK reactivity against hematopoietic malignancies.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.