Epigenetic therapy induces the expression of Epstein-Barr virus antigens and tumor-associated antigens in HRS cells, which are potential targets for cytotoxic T cells
Epstein-Barr virus (EBV)-associated antigens are known to be expressed by a proportion of HRS cells, however, only the subdominant EBV antigens LMP1 and LMP2 are expressed and provide T-cell target antigens. EBV-specific T cells that are enriched for LMP-specific T-cell clones have been expanded from patients and administered to patients with relapsed disease [
36,
37]. Unfortunately, T cells specific for the viral latent nuclear antigens, EBNAs 3A, 3B and 3 C and the immediate early lytic cycle antigen, BZLF1, BRLF1 and BMLF1 dominate the human immune response to EBV. If these proteins could be reactivated in EBV-positive HRS cells, then their susceptibility to EBV-specific T-cell lines would be increased. Both HDACis and hypomethylating agents induce EBV gene expression. HDACs reactivate EBV from latency to replication, so that the immunogenic lytic cycle antigens are expressed [
38‐
40], while azacytidine can switch latently infected cells from a minimal form of latency (LMP1,LMP2 and EBNA1) to an unrestricted latency in which all EBNAs (2, 3’s and-LP) and LMPs are expressed. Thus, these drugs may transform poorly antigenic tumors into highly immunogenic tumors and increase the number of available target antigens for T-cell recognition immunodominant viral proteins. In addition, combining different agents like chemotherapy and HDAC inhibitors results in synergistic effects on activating lytic viral replication [
40]. Since only about 20% of patients with relapsed HL have EBV-positive tumors, it is also important to determine if other tumor-associated, T-cell target antigens are upregulated by epigenetic therapy. Cancer Testis Antigens (CTAs) are known to comprise at least 44 families of genes or isoforms. Among these, 19 families are purely testis-restricted, while others are expressed in one or more somatic tissues. Some CTAs are expressed in lymphoma [
41,
42], and provide potential T-cell targets. CTA-specific T cells from melanoma patients have demonstrated therapeutic benefits, including complete remissions. CTA-specific CTL could have similar beneficial effects in EBV-negative lymphomas. Recently, hypomethylating agents have been shown to induce the expression of several CTAs in cultured tumor cell lines (including lymphomas) [
43]. Such upregulated expression has been reported for several MAGE members – LAGE-1, SSX-2, CAGE, and NY-ESO-1. Demethylation proved necessary and sufficient for MAGE-A1 gene expression in melanoma cell lines, suggesting that methylation is a primary mechanism of transcription control. Histone deacetylase (HDAC) inhibitors, on their own, or in combination with hypomethylating agents, can also induce CTA expression, including MAGE, SSX, and NY-ESO-1 family members [
43]. Recent data suggested that HDACi may upregulate the expression of CTA in HRS, suggesting HDACis may enhance the activity of EBV-directed cellular therapy in HL [
33]. Since epigenetically enhanced CTA expression may activate endogenous CTA-specific T cells in vivo, this may induce epitope spreading and facilitate the ex vivo activation and expansion of these normally low frequency CTA-specific T cells.