DKK-1 Is a Widely Expressed, Potent Tumor-Associated Antigen in Multiple Myeloma Recognized by Cytotoxic T Lymphocytes.

Idiotype proteins have been used for immunotherapy in multiple myeloma (MM); and the results thus far have been disappointing. Therefore, the identification and use of novel and more potent tumor-associated antigens are urgently needed to improve the efficacy of the treatment. Dickkopf-1 (DKK1), a secreted protein and Wnt signaling pathway inhibitor, is highly expressed by the tumor cells of most myeloma patients and may be responsible for suppressed osteoblast formation. Based on these data and the fact that DKK1 is not expressed in normal tissues except placenta, prostate, and mesenchymal stem cells, we hypothesized that DKK1 is a novel tumor-associated antigen in MM. In this study we examined the capacity of DKK1 to trigger the induction of antimyeloma cytotoxic T lymphocyte (CTL) responses. Using DKK-1 peptide-pulsed dendritic cells (DCs), we successfully generated DKK-1 peptide (DKK1_20-29 and DKK1_66-74V)-specific CTL lines and clones from HLA-A2⁺ MM patients and healthy individuals. In our experiments, mature DCs obtained from cultures of blood monocytes were pulsed with HLA-A2-restricted DKK1 peptides. Autologous T cells were then stimulated weekly with these DCs, and cytotoxicity was examined against DKK1 peptide-pulsed T2 cells, myeloma cell lines, and primary myeloma cells isolated from patients. After 4 cycles of in vitro stimulation and subsequent T cell cloning and expansion, specific CD8⁺ CTL lines and clones were obtained. These CTLs not only had cytolytic activity against DKK1 peptide-pulsed T2 cells, but also significantly lysed HLA-A2⁺ MM cell line U266 and primary tumor cells in vitro. No killing was observed against HLA-A2⁺ normal lymphocytes including B cells or HLA-A2⁻ myeloma cell lines or primary myeloma cells from patients. The CTL response could be inhibited by anti-HLA-A2 antibody indicating that the response was indeed restricted by HLA-A2 molecules. IFN-γ, but not IL-4, was secreted by the T cells indicating that the effector cells were type-1 CD8⁺ T cells. Further functional studies are underway. Thus, these results demonstrate that DKK1-specific CTLs are able to lyse myeloma tumor cells including primary myeloma cells from patients, and identify DKK1 as a potentially important antigenic target for antimyeloma immunotherapeutic strategies