1016P ImmTAC redirect exhausted tumor-infiltrating T-cells: An effect enhanced by pembrolizumab against PD-L1+ tumors

Abstract

ImmTAC molecules are TCR-anti-CD3 bispecific fusion proteins that can redirect polyclonal T-cell activation. Tebentafusp, a gp100-directed ImmTAC, has demonstrated survival benefit in metastatic uveal melanoma (1). ImmTAC activate TILs or recruit circulating T-cells into tumors before redirecting them to kill cancer cells. Here, we examined in vitro whether ImmTAC can redirect exhausted (defined as PD1+) TILs or circulating T-cells to kill tumor cells and assessed the effect of anti-PD1 combination. TILs and circulating T-cells were isolated from melanoma patients’ biopsies and blood respectively. Chronic T-cell activation was modelled in vitro by repeated stimulations. The capacity of purified PD1+ TILs and T-cells to be redirected by ImmTAC was tested in vitro with or without pembrolizumab. The target melanoma cell line Mel624 was untransduced or transduced to overexpress PD-L1. T-cells chronically stimulated in vitro express high levels of PD1. While ImmTAC can redirect both PD1+ and PD1- T-cells to kill PD-L1- tumor cells, chronically stimulated PD1+ T-cells have up to two-fold reduced capacity to kill PD-L1+ tumors compared to non-chronically stimulated T-cells. PD1+ T-cell killing activity against PD-L1+, but not PD-L1-, tumors was improved by pembrolizumab (from 40±8 % to 69±4 % maximum killing) at clinically relevant ImmTAC concentrations. Ex vivo purified PD1+ TILs and PD1+ circulating T-cells were as efficiently redirected by ImmTAC as their PD1- counterparts to kill tumor cells lacking PD-L1 expression; addition of pembrolizumab had no effect. However, ImmTAC-mediated killing by PD1+ TILs was reduced from 78±17 % killing without PD-L1 to 30±3 % with PD-L1 expressed on tumor cells, which was reversed to 68±4 % in the presence of pembrolizumab. ImmTAC mediated killing by exhausted PD1+ TILs was efficient against PD-L1- but reduced against PD-L1+ tumor cells. This reduced killing activity was reversed by pembrolizumab. These data provide the rationale for clinical investigation of ImmTAC combinations with anti-PD1/L1 in tumors where both PD1 and PD-L1 are expressed on TILs and tumor cells respectively. (Piperno-Neumann et al. AACR 2021 Abstract # 5342).