Molecular Mechanisms of Co-Stimulatory Molecules and Their Application for Tumor Immunotherapy

Abstract

Costimulatory molecules function to stimulate or inhibit T cell activation. A balance between activating and inhibiting receptors determines the functional state of a T cell. PD1 and CTLA4 are the two most well studied inhibitory costimulatory molecules, sometimes termed coinhibitory. Antibodies blocking their inhibitory function have revolutionized the field of tumor immunotherapy. Understanding the pathways governing the coinhibitory molecules and the mechanisms behind antibody action is important for their application in cancer treatment. Though PD1 and CTLA4 have given good clinical benefit in about 20% of patients, there is a high percentage of non responders. Finding better combination strategies for these two molecules is an active field of research. My thesis focuses on the PDL1-B7-1 pathway. I find that a particular structural orientation is needed for the binding of PD-L1 to B7-1. PDL1 and B7-1 have a strong binding interaction when the PDL1 molecule is accessible and flexible but not when constrained. The native conformations of B7-1 and PD-L1 on the cell surface are too constrained to allow binding of B7-1 on one cell to PD-L1 on another cell. My data suggests that, cell surface PD-L1 may interact with cell surface PD-1 in trans but not with cell surface B7-1 in trans. Instead, interaction of cell surface PD-L1 with cell surface B7-1 in cis is possible. In this study I also investigate strategies for PD1 combination tumor immunotherapy. I develop and test a novel mAb and find that treatment with the mAb in combination with PD-1 mAb prolongs the survival of tumor bearing mice better than PD-1 alone. This suggests the combination may be an effective tumor immunotherapy. I further explore the mechanism of action of CTLA4 mAb in cancer immunotherapy. I test a novel CTLA4 mAb that is a non blocker of B7 interaction with CTLA4 but depletes CTLA4 positive cells. I find that this antibody fails to show any therapeutic efficacy. This supports the idea that both blockade and cell depletion are important for therapeutic efficacy by CTLA4 mAb. Together, my findings suggest ways of enhancing cancer immunotherapy.