Purpose: Novel partners for established immune checkpoint inhibitors in the treatment of cancer are needed to address the problems of primary and acquired resistance. The efficacy of combination RANKL and CTLA4 blockade in anti-tumor immunity has been suggested by recent case reports in melanoma. Here we provide a rationale for this combination in mouse models of cancer.
Experimental Design: The efficacy and mechanism of a combination of RANKL and CTLA4 blockade was examined by tumor infiltrating lymphocyte analysis, tumor growth and metastasis using a variety of neutralizing antibodies and gene-targeted mice.
Results: RANKL blockade improved the efficacy of anti-CTLA4 mAbs against solid tumors and experimental metastases, with regulatory T cell (Treg) depleting anti-CTLA4 mAbs of the mouse IgG2a isotype showing greatest combinatorial activity. The optimal combination depended on the presence of activating Fc receptors and lymphocytes (NK cells for metastatic disease and predominantly CD8+ T cells for subcutaneous tumor control), while anti-RANKL alone did not require FcR. The significantly higher T-cell infiltration into solid tumors post anti-RANKL and anti-CTLA-4 was accompanied by increased T-cell effector function (cytokine polyfunctionality), and anti-RANKL activity occurred independently of Treg depletion. The majority of RANKL expression in tumors was on T cells whereas RANK-expressing cells were mostly tumor-associated macrophages (TAMs), with some expression also observed on dendritic cells (DC) and myeloid derived suppressor cells (MDSCs).
Conclusions: These results provide a rationale for the further investigation of RANKL-RANK interactions in tumor immunity and a basis for development of translational markers of interest in human clinical trials.