Loss of Function of the VHL Tumor Suppressor Gene Resurrects An Endogenous Retrovirus in Kidney Cancer That Encodes a Tumor Antigen Targeted by Allogeneic T-Cells Following Hematopoietic Cell Transplantation (HCT).

Abstract 467

Renal cell carcinoma (RCC) has recently been identified as a target for a graft-vs-tumor (GVT) effect following allogeneic hematopoietic cell transplantation (HCT). At the NHLBI, 30 of 76 patients (39%) with metastatic RCC were observed to have delayed tumor regression following a reduced intensity allogeneic HCT consistent with a GVT effect. A better understanding of the tumor antigens targeted by donor immune cells could lead to the development of transplant approaches that enhance GVT effects while avoiding GVHD. We detected RCC-reactive CD8⁺ T-cells by ELISPOT analysis in the blood of several responding patients with metastatic RCC following HCT that were absent before transplantation. Using PBMC obtained from a patient who had a graft-vs-RCC effect associated with prolonged disease free survival, we isolated CD8+ T-cell clones with RCC-specific tumor cytotoxicity. Utilizing cDNA expression cloning, we identified a 10 amino acid peptide to be the target antigen of RCC-specific T-cells. The genes encoding this antigen were found to be derived from a human endogenous retrovirus type E (named CT-RCC HERV-E) previously unknown to be expressed in any human tissues. Remarkably, RT-PCR showed this endogenous RV was expressed in the majority (13/17) of clear cell tumors but was not expressed in normal kidney tissues, or the non-clear cell histological subtypes of kidney cancer (0/16). A number of different endogenous retroviruses have recently been shown to be expressed in both solid tumors and hematological malignancies and can induce cytotoxic T-cell responses in vivo. However, the factors regulating transcriptional activity of most endogenous retrovirus that lead to tumor-restricted expression are poorly understood. Because the clear-cell histological subtype of RCC is characterized by inactivation of the VHL tumor-suppressor, we hypothesized VHL might regulate expression of CT-RCC HERV-E. RT-PCR showed VHL expression was absent in all clear cell tumors expressing the CT-RCC HERV-E. The HERV-E 5' LTR has a promoter with HIF response elements (HRE) that were found by bisulfite PCR sequencing to be extensively methylated in all normal tissues and non-clear cell RCC tumors but de-methylated in all HERV-E expressing clear cell kidney tumors. Transfection of wild type VHL into VHL mutated clear cell carcinomas dramatically reduced expression of HERV-E derived transcripts. In VHL mutant RCC cells, HIF-2α over-expression was required but alone was insufficient to induce HERV-E expression; SiRNA inhibition of HIF-2α silenced HERV-E expression, although the provirus was only expressed in clear cell tumors that expressed HIF-2α and had de-methylated proviral HREs.