NK cells are cytotoxic lymphocytes that play an important role in anti-tumor immunity. A clinically important feature of NK cells is their ability to mediate antibody-dependent cellular cytotoxicity (ADCC) upon application of anti-tumor antibodies. In acute myeloid leukemia (AML) NK cells largely contribute to the therapeutic efficacy allogenic stem cell transplantation (SCT). Recently we demonstrated that AML cells functionally express the TNF family member RANK ligand (RANKL) which impairs NK cell anti-leukemia reactivity (Schmiedel et al., ASH annual meeting 2011). Here we developed a strategy to combine blocking of the NK inhibitory effects of RANKL with targeting of AML cells for NK cell ADCC. To this end we generated fusion proteins consisting of the extracellular domain of RANK and a human IgG1 Fc part that was modified by amino acid exchange. Compared to wild type RANK-Fc fusion protein (RANK-Fc-WT), our mutant RANK-Fc-ADCC (S239D/I332E) displayed highly enhanced affinity to FcγRIIIa (CD16) on NK cells. Primary AML cells expressed substantial levels of RANKL in 53 of 78 (68%) investigated patient cases, and our RANK-Ig fusion proteins bound to AML cells in a target antigen-specific manner. Treatment with both RANK-Fc-WT and RANK-Fc-ADCC clearly reduced the release of RANKL-induced immunomodulatory factors like TNF, IL-6, IL-8 and IL-10 by AML cells. When the effects of the fusion proteins on NK cell ADCC were studied we found that treatment with RANK-Fc-WT only slightly enhanced NK cell reactivity against RANKL-positive patient AML cells. However, RANK-Fc-ADCC potently induced NK cell ADCC and cytokine production in response to AML targets in a target antigen-dependent manner due to the functional properties of its engineered Fc moiety. Taken together, our Fc-engineered RANK-Fc-ADCC fusion protein may serve to modulate the cytokine milieu involved in AML pathophysiology and target RANKL-expressing leukemia cells for NK anti-tumor reactivity. Thus, RANK-Fc-ADCC constitutes an attractive immunotherapeutic means for the treatment of AML, e.g. for elimination of minimal residual disease after conventional therapy including SCT.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.