Unexpected expression profile of MAGEA9 gene in diffuse large B-cell lymphoma and in normal germinal center cells
(2018)
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- MAGE genes are expressed in tumors but not in normal somatic tissues. When presented at the cell surface by HLA class I molecules, MAGE peptides can be recognized by cytolytic T lymphocytes. MAGE antigens have therefore gathered interest as cancer-specific immune targets. We observed in the tumor RNA-Seq database TCGA the expression of one MAGE gene, MAGEA9, in half of diffuse large B-cell lymphoma (DLBCL) samples. DLBCL derives from activated B lymphocytes in germinal centers. Other MAGE genes were silent, as opposed to what is seen in solid tumors. None of the MAGE genes were expressed by other types of lymphomas. We investigated the molecular mechanisms accounting for this highly selective expression. We found that, in DLBCL, MAGEA9 gene expression was correlated with demethylation of CpG dinucleotides in its promoter. Other MAGE promoters remained methylated. Thus, a selective mechanism causing MAGEA9 promoter demethylation is present in DLBCL, and accounts for its unique MAGE expression pattern. Using a validated anti-MAGEA9 antibody, we screened a series of DLBCL tumor samples, and confirmed that about half of them contained tumor cells expressing the MAGEA9 protein, often heterogeneously. MAGEA9 expression did not correlate with clinical parameters including patient survival, nor with common biological markers associated with DLBCL. These included defects in HLA class I presentation which, as we confirmed here, are particularly frequent in this type of lymphoma. Unexpectedly, we observed MAGEA9-expressing cells in germinal centers from non-cancerous tonsils. These cells did not express common phenotypic markers of germinal center B and T cells, follicular dendritic cells or macrophages. Thus, their precise nature remains uncertain. Intriguingly, MAGEA9-expressing cells were found in all the tonsil tissues from patients older than 8 years, but almost never in younger patients. We assessed whether DLBCL cells present HLA-bound MAGEA9 peptides that can be recognized by cytolytic T cells. Among peptides eluted from the surface of a MAGEA9-expressing DLBCL cell line and analyzed by mass spectrometry, we were able to identify one HLA-A2 restricted MAGEA9 peptide. However, despite numerous attempts of in vitro stimulation of large numbers of naïve blood CD8+ T cells against this eluted peptide and other MAGEA9 peptides selected for their predicted strong HLA binding, we were unable to derive specific CTL clones. This suggests that immune tolerance against MAGEA9 prevails, in line with our observation of MAGEA9 expression in non-cancerous tonsils. Altogether, we have discovered and characterized the selective MAGEA9 gene and protein expression in a specific type of germinal center-derived B-cell lymphoma, the DLBCL, as well as in cells present in germinal centers from lymphoid organs. We have unraveled parts of the mechanisms accounting for this unique expression. We have identified a MAGEA9 peptide presented by HLA-A2 on DLBCL cells. In sum, MAGEA9 can no longer be considered as a tumor-specific antigen and an attractive target for immune therapy as was thought. Instead, our observations shed new light on the biology of DLBCL and of the germinal center cells.
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Dheur, M.-S. (2018). Unexpected expression profile of MAGEA9 gene in diffuse large B-cell lymphoma and in normal germinal center cells. https://hdl.handle.net/2078.5/61695