Genetic engineering of T cells to target HERV-K, an ancient retrovirus on melanoma

Purpose: The human endogenous retrovirus (HERV-K) envelope (env) protein is a tumor-associated antigen (TAA) expressed on melanoma but not normal cells. This study was designed to engineer a chimeric antigen receptor (CAR) on T-cell surface, such that they target tumors in advanced stages of melanoma. Experimental Design: Expression of HERV-K protein was analyzed in 220 melanoma samples (with various stages of disease) and 139 normal organ donor tissues using immunohistochemical (IHC) analysis. HERV-K env-specific CAR derived from mouse monoclonal antibody was introduced into T cells using the transposon-based Sleeping Beauty (SB) system. HERV-K env-specific CAR⁺ T cells were expanded ex vivo on activating and propagating cells (AaPC) and characterized for CAR expression and specificity. This includes evaluating the HERV-K-specific CAR⁺ T cells for their ability to kill A375-SM metastasized tumors in a mouse xenograft model. Results: We detected HERV-K env protein on melanoma but not in normal tissues. After electroporation of T cells and selection on HERV-K⁺AaPC, more than 95%ofgenetically modified T cells expressed the CAR with an effector memory phenotype and lysed HERV-K env⁺ tumor targets in an antigen-specific manner. Even though there is apparent shedding of this TAA from tumor cells that can be recognized by HERV-K env-specific CAR⁺ T cells, we observed a significant antitumor effect. Conclusions: Adoptive cellular immunotherapy with HERV-K env-specific CAR⁺ T cells represents a clinically appealing treatment strategy for advanced-stage melanoma and provides an approach for targeting this TAA on other solid tumors.