(Charles River) Validation of humanized PD-1 knock-in mice as an emerging model to evaluate human specific PD-1 therapeutics

Abstract

Over the past decade there has been an increasing demand in the use of syngeneic models for evaluating the efficacy of checkpoint inhibition-based cancer immunotherapies. During tumor development, immune cells can became unresponsive to the presence of tumor cells due to chronic activation and expression of the programmed cell death protein-1 (PD-1) or the T lymphocyte associated antigen 4 (CTLA4) in T-cells or the presence of FoxP3+ Treg cells resulting in tumor immune-tolerance.

Our previous studies have demonstrated that murine anti-PD-1 and CTLA-4 therapy can effectively re-activate the anti tumor response against multiple syngeneic tumor models.  While these models proved instrumental for evaluating murine immune-checkpoint inhibitors (ICI), there is a clear need for additional mouse models to evaluate the efficacy of ICI specific for human targets. To address this need, we describe the development of a humanized PD-1 knock-in (KI) mouse model. This mouse model has the advantage of expressing the human PD-1 protein in the context of a fully functional immune system. We also show the response to clinically relevant immune checkpoint inhibitors in two preclinical tumor models. We evaluated the anti-tumor activity of pembrolizumab in the MC38 colorectal carcinoma and the GL261 glioblastoma models. We observed significant tumor growth inhibition and growth delay in the MC38 tumor model when treated with pembrolizumab monotherapy, but not when treated with the murine counterpart (anti-PD-1 clone RPM1-14).  To extend our validation studies to other tumor models, we implanted GL261 glioblastoma orthotopically in the brain of PD-1 KI mice and achieved a significant increased life span in the group treated with pembrolizumab compared to both the control group and the group treated with murine anti-PD-1 antibody.  The immune profile from control and treated these animals were also characterized in these studies by flow cytometry. In summary, the results shown here underscore the value of the humanized PD-1 knock-in (KI) mouse model as a tool to evaluate human specific immune-checkpoint based therapeutics alone and in combination with other agents.