The CX3CL1-CX3CR1 chemokine axis contributes to tumor immune evasion and its blockade enhances responses to anti-PD-1 immunotherapy

CX3CL1 secreted in the tumor microenvironment serves as a chemoattractant playing a critical role in metastasis of CX3CR1 expressing cancer cells. While CX3CR1 can be expressed in both cancer and immune-inhibitory myeloid cells to facilitate their migration, the mechanisms employed by this axis on these cells to mediate immune suppression remain poorly understood. Here, we explore the immune evasion strategies implemented by this axis and find that it initiates a resistance program in cancer cells that results in 1) facilitation of tumor cell migration, 2) secretion of soluble mediators to generate a pro-metastatic niche, 3) secretion of mediators to attract myeloid populations, and 4) generation of tumor-inflammasome. This axis is involved in the underlying mechanisms of resistance to anti-PD-1 immunotherapy. We used a novel monoclonal antibody against mouse CX3CR1 that binds to CX3CR1, blocks the CX3CL1-CX3CR1 interaction and antagonizes the pro-tumorigenic outcome of this axis. This monoclonal antibody reduces migration of tumor cells and decreases secretion of immune suppressive soluble mediators from the tumor. In combination with anti-PD-1 immunotherapy, this CX3CR1 monoclonal antibody enhances survival in an immunocompetent mouse colon carcinoma model through a decrease in tumor-associated myeloid populations and their conversion into an anti-tumor phenotype

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

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