Host Immune Responses Against CT Antigens in Multiple Myeloma Patients.

The type I Melanoma Antigen GEne (MAGE) proteins belong to the Cancer-Testis (CT) family of tumor-associated antigens and are widely expressed in solid and hematologic malignancies. They are immunogenic and frequently elicit spontaneous immune responses in patients with CT antigen-expressing tumors, particularly in malignant melanoma. In melanoma patients, there is high concordance between humoral and cellular immunity. Based on these findings, CT antigens are widely investigated as potential antigenic targets for tumor-specific therapeutic vaccines. We previously showed that the type I MAGE proteins CT7 (MAGE-C1), CT10 (MAGE-C2) and MAGE-A3 were commonly detected in primary myeloma specimens, and expression of CT7 and MAGE-A3 was correlated with abnormally elevated plasma cell proliferation. These findings suggest that type I MAGE may be rational targets for vaccine therapy in multiple myeloma. Therefore, it is important to determine if type I MAGE elicit cellular or humoral immune responses in myeloma patients. To investigate this hypothesis, we assessed cellular immunity against CT7 and humoral immunity against a broad panel of CT antigens. To quantify CT7-specific cellular immunity, expanded, polyclonal pools of T cells from the bone marrow, the tumor microenvironment, were co-cultured in interferon gamma (IFNγ) ELISpot assays with autologous antigen-presenting cells (APC) transduced with in vitro transcribed mRNA coding for CT7 or control antigens. CT antigen-specific humoral immunity was examined by ELISA assay using patient serum or plasma and recombinant CT antigens. This analysis demonstrated that 2/9 patients exhibited specific T cell immunity against CT7 in their bone marrow lymphocytes as measured by IFNγ secretion. These same two patients had positive titers for other CT antigens; one for MAGE-A1 (another type I MAGE), the other for SSX-1 (a structurally distinct CT antigen). Interestingly, neither patient had positive serology for CT7. Serum from 16 other myeloma patients did not have detectible antibody titers for a broad panel of CT antigens. These results show that CT antigens are immunogenic in myeloma patients, with cellular responses against CT7 and humoral responses against MAGE-A1 and SSX-1. However, unlike other types of cancer, there appears to be discordance between humoral and cellular immunity against CT7 in multiple myeloma. This may be due in part to the significant derangements of humoral immunity in this disease. These results support further investigation of immunologic therapies targeting type I MAGE in myeloma, especially therapeutic vaccine strategies.