Author ORCID Identifier
Date of Graduation
8-2023
Document Type
Dissertation (PhD)
Program Affiliation
Immunology
Degree Name
Doctor of Philosophy (PhD)
Advisor/Committee Chair
Dr. Michael Curran
Committee Member
Dr. Stephanie Watowich
Committee Member
Dr. Pamela Wenzel
Committee Member
Dr. Michael Davies
Committee Member
Dr. Matthew Gubin
Abstract
INVESTIGATING THE DISTINCT ROLE OF PD-L2 IN MODULATING HUMAN T CELL RESPONSES
Anupallavi Srinivasamani, M.S.
Advisory Professor: Michael A. Curran, Ph.D.
Therapeutic blockade of the Programmed cell death-1 (PD-1) receptor and its ligand Programmed death ligand-1 (PD-L1) has revolutionized the treatment of multiple cancers and made durable tumor regression a possibility in the clinic. PD-1 blockade prolonged progression-free survival and overall survival with lesser high-grade toxicity in patients with advanced melanoma when compared to the other FDA-approved checkpoint blockade target, CTLA-4. Unlike CTLA-4, PD-1 is unique in its ability to regulate T cell functions in lymphoid tissues as well as in non-lymphoid tissues like the tumor microenvironment. The comparatively lower incidence of immune-related adverse events associated with PD-1 blockade and its wide-ranging immune-modulatory functions makes PD-1 signaling axis especially favorable for next-generation of combination therapies aimed at increasing the clinical efficacy of immune checkpoint blockade beyond the current 20% of patients who are expected to positively respond to PD-1/PD-L1 blockade. The current and future development of novel therapeutics targeting the PD-1 signaling axis should be rationally guided by a comprehensive understanding of the immunobiology underlying PD-1 signaling. However, the highest affinity ligand of PD-1, PD-L2 remains understudied and has yet to be directly interrogated for therapeutic potential. Recent reports have begun to reveal the relevance of PD-L2 in tumor immunology, however, critical gaps in our knowledge about the function of PD-L2, particularly in humans, limit efforts to rationally harness its potential clinical benefit. We therefore investigated the distinct function of PD-L2 in the PD-1/PD-L1/PD-L2 signaling axis in human T cells. To this end, we established an in vitro experimental system using human Jurkat T cells that allowed us to study the T cell regulatory functions of PD-1/PD-L1 and PD-1/PD-L2 complexes, exclusive of other protein-protein interactions on the surface of T cells. We resolved the contention over the nature of T cell regulation mediated by PD-L2 in preclinical murine models and conclusively showed that PD-L2 is solely a co-inhibitory ligand of human T cells. We also discovered the capacity of PD-L1 and PD-L2 to heterodimerize and studied the functional consequence of this interaction on T cell signaling. Despite binding to the same receptor, we demonstrated that PD-L1 and PD-L2 differentially engage PD-1 to induce a distinct inhibitory signaling cascade downstream of PD-1 in human T cells. We characterized the differences in the molecular mechanisms of PD-1/PD-L1 and PD-1/PD-L2 mediated inhibition of primary human T cells and elucidated the physiological impact of the differential signaling induced by PD-ligands through PD-1 on T cell function. Taken together, our work provides a more solid immunobiological foundation for clinical translation of PD-L2 and defines its non-redundant role in regulating T cell immunity in humans.
Keywords
Immune checkpoint inhibitors, PD-L2, T cell signaling, PD-L1, Immunotherapy