Abstract 4977: Isolation and characterization of a PRAME-specific TCR with high avidity, potent antitumor efficacy and a favorable preclinical safety profile
Authors: Weis Manon , Wehner Carina , Ellinger Christian , Wilde Susanne , Schendel Dolores J. 
Published: July 2017

Some of the challenges regarding adoptive T cell transfer as an immunotherapeutic approach for the treatment of cancer patients include the initial induction, isolation and later characterization of suitable TCR candidates. In addition, the selection of appropriate target antigens is important and complex as such antigens need to meet some critical demands. Published data as well as in depth in silico analyses revealed the tumor associated antigen PRAME a suitable candidate. Classified as cancer/testis antigen, PRAME is highly expressed in tumors with only limited or no expression in normal tissues, other than testis.

Here we describe the induction of PRAME-specific, HLA-A*02:01-restricted CD8-positive T cells by using an in vitro priming approach. Mature dendritic cells transfected with ivtRNA encoding full-length PRAME were cocultured with autologous CD8-enriched PBL from a healthy, HLA-A*02:01-positive blood donor. Initially primed T cells were expanded for 2 weeks and T cells bearing TCRs specific for PRAME-derived epitopes were identified by multimer staining and FACS analysis. Single cells were sorted into 96 well plates and after an expansion period of another 2 weeks, growing clones were screened for specificity. TCR sequences of specific clones were identified by using NGS and reconstructed in a retroviral vector system enabling stable transduction of effector cells. The resulting TCR-modified T cells were thoroughly analyzed and characterized using a dedicated set of assays to evaluate multiple parameters concerning safety and efficacy.

This characterization revealed TCR T4.8-1-29 to be highly specific for a PRAME-derived epitope presented on HLA-A*02:01. Only epitope-positive target cells expressing the appropriate HLA-restriction element induced cytokine secretion by T4.8-1-29 expressing effector cells, leading to recognition as well as specific lysis of these target cells. Recognition and lysis, not just of peptide-loaded T2 cells, but also of PRAME-transfected APC, transfected tumor cells as well as endogenously PRAME-positive tumor cells was mediated by this TCR. T4.8-1-29-expressing effectors show a high natural avidity for the target epitope without the need of further manipulation, e.g. functional efficacy enhancement by affinity maturation. In addition, the toxicity assessment using various in vitro and in silico tools revealed a favorable preclinical safety profile for this TCR.

In summary, by using an in vitro priming approach utilizing cells from healthy donors, it was possible to generate a TCR recognizing a PRAME-derived epitope with high natural avidity and specificity. The TCR shows a favorable safety profile, demonstrating that this approach is useful to generate TCRs potentially qualifying for evaluation in clinical trials.

Citation Format: Manon Weis, Carina Wehner, Christian Ellinger, Susanne Wilde, Dolores J. Schendel. Isolation and characterization of a PRAME-specific TCR with high avidity, potent antitumor efficacy and a favorable preclinical safety profile [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4977. doi:10.1158/1538-7445.AM2017-4977