Aberrant Expression in Human Epithelial Cancers of the P2X5-Encoded Minor Histocompatibility Antigen LRH-1: Implications for Graft-Versus-Tumor Immunity Against Solid Tumors.

Allogeneic stem cell transplantation (SCT) in combination with donor lymphocyte infusion (DLI) is an experimental treatment for patients with metastatic solid tumors. The therapeutic efficacy is attributed to the graft-versus-tumor (GVT) response during which donor-derived T cells eliminate malignant cells via recognition of minor histocompatibility antigens (MiHA). To reduce accompanying GVHD, it is crucial to identify MiHA which are selectively expressed on hematopoietic cells and solid tumor cells. Previously, we identified a hematopoietic cell-restricted MiHA, designated LRH-1, which is presented by HLA-B7 and encoded by the P2X5 purinergic receptor gene (J. Clin. Invest.2005:115:3506–3516). Here, we report that LRH-1, in addition to its hematopoietic cell-restricted expression, is aberrantly expressed on epithelial tumor cell lines. We observed that P2X5 mRNA is significantly expressed in 14 out of 42 (33%) solid tumor cell lines tested by real-time quantitative RT-PCR. We detected P2X5 transcripts in 3 out of 11 renal cell carcinoma cell lines, 2 out of 4 melanoma cell lines, 3 out of 7 colorectal carcinoma cell lines, 4 out of 10 brain tumor cell lines and 2 out of 10 breast cancer cell lines. To determine whether P2X5 mRNA expression in solid tumor cell lines results in susceptibility to lysis by LRH-1-specific CTL, we performed flow cytometry-based cytotoxicity assays using P2X5-expressing tumor cell lines. Based on LRH-1 genotyping analysis, we selected six solid tumor cell lines for the cytotoxicity studies. Remarkably, LRH-1-specific CTL efficiently lysed and inhibited the growth of DAOY brain tumor cells up to 3 days of co-culture. The renal cell carcinoma cell lines SKRC-33 and SKRC-18 and the melanoma cell line BLM were also susceptible to LRH-1 CTL-mediated lysis, although less effectively. However, pre-incubation of these tumor cell lines with IFNγ and TNFα significantly increased the susceptibility to LRH-1-specific CTL and resulted in complete target cell lysis. Furthermore, these cytokine-stimulated cell lines induced higher levels of CTL degranulation as determined by CD107a staining. No cytotoxicity was observed against LRH-1-negative FM3 melanoma and SKRC-24 renal cell carcinoma cell lines. These findings illustrate that the GVT reactivity observed in solid tumors after allogeneic SCT may be selectively enhanced by LRH-1-specific immunotherapy.