PD-1/PD-L1 signaling at the interface between immune and stem cell crosstalk

This project proposal is designed to elucidate the nature of immune-stem cell interactions that are inadvertently targeted by immunotherapies. Specifically, we focus on signaling between the immune checkpoint proteins programmed cell death 1 (PD-1) and programmed cell death ligand (PD-L1) as one mechanism by which immune cells and stem cells coordinate tissue immunity with tissue regeneration. We use melanocyte stem cells (McSCs) and hair repigmentation as the model. Our rationale for this proposal stems from our discoveries that 1) PD-L1 is expressed at the right time (during hair dormancy) and the right place (by quiescent McSCs in the hair follicle) to regulate both McSC immune privilege and quiescence; 2) blocking PD- L1 in this cell lineage increases cell cycling in vitro, and 3) blocking PD-L1 provides resistance to stress- associated hair graying in vivo. Further rational derives from the literature— PD-1/PD-L1 immune checkpoint inhibitor therapy for cancer causes two opposite pigmentation/immune-related adverse events (irAEs). On one hand we see vitiligo-like hypopigmentation in melanoma patients but on the other hand we see reversal of gray hair in non-melanoma patients. The objectives of our study are to decipher how PD-L1 enables crosstalk between McSCs and neighboring immune cells to regulate McSC immune privilege, maintain McSC quiescence, and balance the immunogenic and tolerogenic processes within the skin to enable pigment regeneration. To complete these objectives, we have assembled a team of experts in stem cells, pigmentation and hair biology, mouse transgenics, inflammatory disease models and immunology. With this combined expertise, we will investigate the regenerative capacity of McSCs in combination with adoptive transfer of Jedi T cells, anti- PD-1 and anti-PD-L1 antibodies that mimic human immunotherapies, and in the context of a novel mouse model of post-inflammatory skin hypopigmentation. We will also employ McSC lineage tracing and our novel in vitro quiescence assay to further interrogate PD-L1 function in cell cycling. The results of these studies are the foundation to addressing a critical question in the field of immunotherapy. How do we do we tip the balance towards immunotherapy’s undeniable benefits and away from its irAEs? The diversity of irAEs, the organs they affect, and their onset and severity suggest immunotherapy has numerous targets that do not all represent the same basic pathology. Thus, revealing the underlying biology behind individual immune-related toxicities is key to harnessing the full potential of immunotherapy treatment. This study will also provide insight into the immune-stem cell dialog enabled by PD-1/PD-L1 signaling. More broadly, this study contributes to the growing paradigm that immune cells and stem cells mutually regulate one another’s behavior to orchestrate regenerative tissue homeostasis and repair in the context of tissue immunity. Project Number: 1R56AI177511-01A1 | Fiscal Year: 2025 | NIH Institute/Center: National Institute of Allergy and Infectious Diseases (NIAID) | Principal Investigator: Melissa Harris | Institution: UNIVERSITY OF ALABAMA AT BIRMINGHAM, BIRMINGHAM, AL | Award Amount: $384,250 | Activity Code: R56 | Study Section: Arthritis, Connective Tissue and Skin Study Section[ACTS] View on NIH RePORTER: https://reporter.nih.gov/project-details/1R56AI17751101A1