Aberrant expression of the hematopoietic-restricted minor histocompatibility antigen LRH-1 on solid tumors results in efficient cytotoxic T cell-mediated lysis.

CD8(+) T cells recognizing minor histocompatibility antigens (MiHA) on solid tumor cells may mediate effective graft-versus-tumor (GVT) reactivity after allogeneic stem cell transplantation (SCT). Previously, we identified LRH-1 as a hematopoietic-restricted MiHA encoded by the P2X5 gene. Here, we report that LRH-1 is aberrantly expressed on solid tumor cells. P2X5 mRNA expression is demonstrated in a significant portion of solid tumor cell lines, including renal cell carcinoma (RCC), melanoma, colorectal carcinoma, brain cancer and breast cancer. Importantly, P2X5 gene expression was also detected in a subset of primary solid tumor specimens derived from RCC, brain cancer and breast cancer patients. Furthermore, P2X5 expressing solid tumor cells can be effectively targeted by LRH-1-specific cytotoxic T lymphocytes under inflammatory conditions. The expression of HLA-B7 and CD54 on tumor cells increases upon cytokine stimulation resulting in improved T cell activation as observed by higher levels of degranulation and enhanced tumor cell lysis. Overall, hematopoietic-restricted MiHA LRH-1 is aberrantly expressed on solid tumor cells and may be used as target in GVT-specific immunotherapy after SCT.

Expression of P2X5 in lymphoid malignancies results in LRH-1-specific cytotoxic T-cell-mediated lysis.

Minor histocompatibility antigens (MiHA) selectively expressed by haematopoietic cells are attractive targets for specific immunotherapy after allogeneic stem cell transplantation (SCT). Previously, we described LRH-1 as a haematopoietic-lineage restricted MiHA that is capable of eliciting an allogeneic cytotoxic T-lymphocyte (CTL) response after SCT and donor lymphocyte infusion. Importantly, the gene encoding LRH-1, P2X5, is not expressed in prominent graft-versus-host-disease target tissues such as skin, liver and gut. Here, we investigate whether LRH-1-specific immunotherapy may be exploited for the treatment of lymphoid malignancies. We examined P2X5 mRNA expression in a large panel of patient samples and cell lines from different types of lymphoid malignancies by real-time quantitative reverse transcription polymerase chain reaction. P2X5 mRNA was highly expressed in malignant cells from all stages of lymphoid development. Furthermore, all LRH-1-positive lymphoid tumour cell lines were susceptible to LRH-1 CTL-mediated lysis in flow cytometry-based cytotoxicity assays. However, interferon-gamma production was low or absent after stimulation with some cell lines, possibly due to differences in activation thresholds for CTL effector functions. Importantly, primary cells from patients with lymphoid malignancies were effectively lysed by LRH-1-specific CTL. These findings indicate that MiHA LRH-1 is a potential therapeutic target for cellular immunotherapy of lymphoid malignancies.

Refinement of molecular approaches to improve the chance of identification of hematopoietic-restricted minor histocompatibility antigens.

Minor histocompatibility antigens (mHAgs) constitute the target antigens of the T cell-mediated graft-versus-leukemia response after HLA-identical allogeneic stem cell transplantation (SCT). Several human mHAgs have been identified, but only a few are selectively expressed by hematopoietic cells representing potential targets for specific immunotherapy. Molecular approaches including cDNA library screening and genetic linkage analysis have been successfully applied to identify T cell-defined mHAgs, but each approach has its drawbacks which may lead to mis-identification of the mHAg of interest. We improved both molecular strategies to facilitate more robust identification of hematopoietic-restricted mHAgs. First, we adapted cDNA library cloning by using 293T cells with stable expression of the relevant MHC class I allele, CD80 and CD54. We demonstrated that cDNA library screening using this 293T expression system results in strong activation of cytotoxic T lymphocytes, which significantly contributes to improvement of the assay sensitivity. Second, we refined genetic linkage analysis using single nucleotide polymorphism (SNP) genotyping to narrow down the defined genetic region that holds the mHAg-encoding gene. We showed that SNP marker analysis provides additional information about the genetic position of the antigen-encoding gene. Application of these optimized molecular approaches will lead to more rapid and reliable molecular identification of hematopoietic-restricted mHAgs.

A frameshift polymorphism in P2X5 elicits an allogeneic cytotoxic T lymphocyte response associated with remission of chronic myeloid leukemia.

Minor histocompatibility antigens (mHAgs) constitute the targets of the graft-versus-leukemia response after HLA-identical allogeneic stem cell transplantation. Here, we have used genetic linkage analysis to identify a novel mHAg, designated lymphoid-restricted histocompatibility antigen-1 (LRH-1), which is encoded by the P2X5 gene and elicited an allogeneic CTL response in a patient with chronic myeloid leukemia after donor lymphocyte infusion. We demonstrate that immunogenicity for LRH-1 is due to differential protein expression in recipient and donor cells as a consequence of a homozygous frameshift polymorphism in the donor. Tetramer analysis showed that emergence of LRH-1-specific CD8+ cytotoxic T cells in peripheral blood and bone marrow correlated with complete remission of chronic myeloid leukemia. Furthermore, the restricted expression of LRH-1 in hematopoietic cells including leukemic CD34+ progenitor cells provides evidence of a role for LRH-1-specific CD8+ cytotoxic T cells in selective graft-versus-leukemia reactivity in the absence of severe graft-versus-host disease. These findings illustrate that the P2X5-encoded mHAg LRH-1 could be an attractive target for specific immunotherapy to treat hematological malignancies recurring after allogeneic stem cell transplantation.