Hypomethylating drugs convert HA-1-negative solid tumors into targets for stem cell-based immunotherapy.

Clinical responses of solid tumors after allogeneic human leukocyte antigen-matched stem cell transplantation (SCT) often coincide with severe graft-versus-host disease (GVHD). Targeting minor histocompatibility antigens (mHags) with hematopoiesis- and cancer-restricted expression, for example, HA-1, may allow boosting the antitumor effect of allogeneic SCT without risking severe GVHD. The mHag HA-1 is aberrantly expressed in cancers of most entities. However, an estimated 30% to 40% of solid tumors do not express HA-1 (ie, are HA-1(neg)) and cannot be targeted by HA-1-specific immunotherapy. Here, we investigated the transcriptional regulation of HA-1 gene expression in cancer. We found that DNA hypermethylation in the HA-1 promoter region is closely associated with the absence of HA-1 gene expression in solid tumor cell lines. Moreover, we detected HA-1 promoter hypermethylation in primary cancers. The hypomethylating agent 5-aza-2'-deoxycytidine induced HA-1 expression only in HA-1(neg) tumor cells and sensitized them for recognition by HA-1-specific cytotoxic T lymphocytes. Contrarily, the histone deacetylation inhibitor trichostatin A induced HA-1 expression both in some HA-1(neg) tumor cell lines and in normal nonhematopoietic cells. Our data suggest that promoter hypermethylation contributes to the HA-1 gene regulation in tumors. Hypomethylating drugs might extend the safe applicability of HA-1 as an immunotherapeutic target on solid tumors after allogeneic SCT.