Cancer-testis antigen 7 expression and immune responses following allogeneic stem cell transplantation for multiple myeloma.

Cancer-testis antigen 7 (CT7) is the most frequently and consistently expressed MAGE antigen in multiple myeloma, exhibits tissue-restricted expression, and is an independent negative prognostic factor for multiple myeloma. We sought to characterize CT7 protein expression in the bone marrow of patients with multiple myeloma undergoing allogeneic T cell-depleted hematopoietic stem cell transplantation (alloTCD-HSCT), and to examine the significance of CT7-specific cellular immune responses. We further aimed to determine CT7-derived immunogenic epitopes and their associated allelic restrictions. CT7 protein expression in neoplastic CD138(+) plasma cells was evaluated by immunohistochemistry in bone marrow biopsies from 10 patients. CT7 was present in 8 of 10 patients. Longitudinal analyses of the 10 patients revealed an association between CT7 expression and prognosis. Longitudinal monitoring of CT7-specific T cells revealed an association between increased frequencies of CT7-specific T cells and reductions in specific myeloma markers. Epitope-specific reactivity to the nonamer FLAMLKNTV was detected by intracellular IFNγ assay in peripheral blood (PB) and bone marrow-derived T cells from HLA-A0201(+) patients. Serial monitoring of PB CT7-specific T-cell frequencies in 4 HLA-A0201(+) patients by HLA-A*0201-CT7(1087-1095) tetramer staining revealed an association with disease course. Phenotypic analyses revealed bone marrow enrichment for central memory CT7-specific T cells, while effector memory cells dominated the PB. Together, these findings support the development of immunotherapeutic strategies that aim to enhance CT7-directed immune responses for the treatment of multiple myeloma.