Searcy, Madeline B

Abstract Immune checkpoint inhibitors (ICIs) which target immune checkpoint proteins like programmed cell death protein 1 (PD-1) and programmed cell death ligand 1 (PD-L1) have revolutionized cancer therapy. Skeletal toxicities are emerging immune-related adverse events for patients treated with ICIs, and PD-1 blockade in pre-clinical mouse models significantly reduces bone mass. However, the effect of PD-L1 inhibition on the bone throughout development is not well understood. We investigated the role of PD-L1 loss on bone microarchitecture with genetic PD-L1 knockout in myeloid cells using LysM-Cre;PD-L1Flox/Flox mice and pharmacologic inhibition with a PD-L1 neutralizing antibody in adult and aged mice. We report that neither PD-L1 deletion in myeloid cells nor inhibition with α-PD-L1 significantly impacts femoral trabecular bone microarchitecture; however, α-PD-L1 treatment increases vertebral trabecular bone volume. Additionally, PD-L1 blockade influences T cell expansion by decreasing naïve T cells and increasing effector memory T cells in the bone marrow, consistent with previous studies on the effects of PD-1 inhibition. The trabecular bone-sparing effect of PD-L1 inhibition is strikingly different from the bone loss observed with PD-1 blockade. Clinical studies are necessary and justified to determine if PD-L1 may be a less bone destructive alternative for cancer patients treated with ICIs who are at high risk of fracture.

Abstract Immune checkpoint inhibitors (ICIs) block immunoregulatory receptor-ligand interactions and robustly increase survival of cancer patients but frequently result in immune-related adverse events (irAEs). While rheumatologic toxicities are commonly reported as irAEs, the effect of immune checkpoint blockade on the underlying mechanisms of ICI-induced fractures and bone loss is controversial, with reports of both positive and negative effects on bone mass in preclinical models. However, no previous reports have investigated the impact of ICIs on females or aged mice, or on fracture risk in either sex. We report that global deletion of programmed cell death protein 1 (PD-1) broadly results in bone loss in skeletally mature male and female PD-1−/− mice, with a sexually divergent phenotype in adolescent mice, decreased bone strength in adult males and young females, and expansion of multiple T cell subsets in the bone marrow. In a model of pharmacologic PD-1 blockade, administration of α-PD-1 reduced bone mass, expanded multiple T cell subsets in the bone marrow, and increased osteoclast activity and resorptive capacity. T cell deficient mice were resistant to osteoclast-mediated bone loss following α-PD-1 therapy, suggesting that T cells in the bone marrow are necessary for bone loss in the setting of ICI therapy. These findings may be leveraged to identify patients at greater fracture risk following ICI therapy due to enrichment of effector T cell populations in the bone marrow.