Use of Gene-Modified T-Cells as Antigen Presenting Cells (T-APC) for Vaccination Against Myeloma Antigens.

Multiple myeloma is incurable with currently available standard treatments, and novel therapies with lower toxicity, including immunological therapies, are needed. Myeloma cells express class I MHC molecules and several potentially immunogenic proteins. Candidate target antigens, other than the patient-specific immunoglobulin idiotype, include cancer-testis antigens, such as MAGE-C1 (CT-7), and tissue differentiation antigens, such as B-Cell Maturation Antigen (BCMA). We have isolated, from normal donors, CD8⁺ T-cells that are CT-7 and BCMA peptide-specific, suggesting that immunologic tolerance to these proteins is incomplete. A subset of these CD8⁺ T-cells with the highest avidity are capable of recognizing myeloma cells. However, the frequency of T-cells recognizing these peptides is very low. Vaccination offers a potential approach for augmenting the frequency of tumor-reactive T-cells, provided that a vaccine strategy can be developed that elicits high-avidity T-cells. We have previously shown that adoptive transfer of autologous gene-modified T-cells leads to the rapid development of potent CD8⁺ T-cell immunity against transgene products expressed in the transferred T-cells. The aims of this study were to determine if vaccination with T-cells modified to express the CT-7 or BCMA genes (T-APC) could induce anti-myeloma immunity in an HLA-A2 transgenic mouse model, and to map additional immunogenic epitopes of these proteins presented by HLA-A2. To address this we genetically modified HLA-A2 transgenic mouse T-cells with retroviral vectors encoding the candidate myeloma antigens CT-7 and BCMA, or with a control vector encoding green fluorescent protein (GFP). The gene-modified T-APC were injected intravenously into HLA-A2 transgenic mice, and ELISPOT and cytotoxicity assays were performed on splenocytes from immunized mice to evaluate T-cell recognition of target cells expressing CT-7 or BCMA (including human myeloma cell lines). T-cell responses specific for CT-7, BCMA, and control antigens were elicited following gene-modified T-APC immunization, and in some cases these T-cells were capable of recognizing human myeloma cells. Furthermore, responses to CT-7 have been mapped to two different dominant CT-7 peptides. CD8⁺ T-cells from some normal A2⁺ donors also recognized these two epitopes after in vitro peptide stimulation, suggesting that these epitopes may be applicable to human myeloma therapy. Characterization of the BCMA target peptide(s) is ongoing. The ease with which T-cells can be obtained, genetically-modified, and expanded in vitro, together with data showing potent immunogenicity, suggests that T-APC vaccination should be evaluated as an alternative cell based vaccine strategy for human cancer therapy.