Adoptive transfer of T-cells genetically-modified to express cancer-specific receptors can mediate impressive tumor regression in terminally-ill patients. However, T-cell function and persistence over time could be hampered by the activation of inhibitory costimulatory pathways, such as PD1/PDL1, leading to T-cell exhaustion and providing tumor cells with an escape mechanism from immunosurveillance. In addition, the lack of positive costimulation at the tumor site can further dampen T-cell response. Thus, as T-cell genetic engineering has become clinically-relevant, we aimed at enhancing T-cell anti-tumor activity by genetically diverting T-cell negative costimulatory signals into positive ones using chimeric molecules we termed "costimulatory converters" and which are composed of the PD1 extracellular domain fused to the signaling domains of positive costimulatory molecules such as CD28 and 4-1BB.
After characterizing the optimal PD1 chimera, we designed and optimized a tripartite retroviral vector that enables the simultaneous expression of this chimeric molecule in conjunction with a cancer-specific TCR. Human T-cells, transduced to express a PD1/28 chimeric molecule exhibited enhanced cytokine secretion and upregulation of activation markers upon co-culture with tumor cells. These engineered cells also proliferated better compared to control cells. Finally, we tested the function of these cells in anin ovocytotoxic assay using a chorioallantoic membrane (CAM)-based model of human melanoma tumors and show that PD1/28-engineered human T-cells demonstrated superior anti-tumor function. Overall, we propose that engineering T-cells with a costimulatory converter can enhance their function which bears important implications for the improvement of T-cell immunotherapy.
