PD-L1 Positive Hepatic Macrophages in Cholangiocarcinoma
The liver has a unique immune microenvironment with many resident immune cells (macrophages called Kupffer cells) that contribute to the liver's ability to induce immune tolerance. The liver's Kupffer cells are the single largest population of resident macrophages (80% to 90% of total body population). Hepatic macrophages are key mediators of immunological tolerance in the liver. This capability is particularly important in cancer biology, because cholangiocarcinoma may use this machinery to promote pro-tumor liver immune tolerance. Pro-tumor macrophages, often referred to as tumor-associated macrophages, are abundant in the cholangiocarcinoma microenvironment.
Preliminary data from our work in the Hepatobiliary SPORE's Career Enhancement Program suggest that tumor-associated macrophages expressing programmed cell death 1 ligand 1 (PD-L1) play an essential role in mediating liver tolerance to cholangiocarcinoma. Targeting these cells or altering them to an anti-tumor phenotype is a promising anticancer therapeutic approach. This technically and conceptually innovative proposal is highly significant because it identifies new mechanisms for therapeutically targeting cholangiocarcinoma.
The tolerogenic capability of liver macrophages is especially important in tumor biology because it may promote tumor progression in the liver by mediating cytotoxic T lymphocyte (CTL) exhaustion, a state of reduced T cell effector function. CTLs express multiple inhibitory receptors, such as programmed cell death 1 (PD-1), and lose their effector functions when these receptors are stimulated by such ligands as programmed cell death 1 ligand 1 (PD-L1).
Cholangiocarcinoma (CCA) is a prototypic model to explore the liver's immunologic response to tumors given its rich multicellular, stromal interactions. Although the role of PD-L1 in tumor biology is under intense investigation given the therapeutic success of checkpoint blockade, the majority of the existing body of work has focused on PD-L1 expression on cancer cells and its role in CTL exhaustion.
However, our preliminary data unexpectedly demonstrated that PD-L1 knockout animals have a significant reduction in tumor burden despite expression of PD-L1 by implanted cancer cells. This suggests that PD-L1 expression on host cells in the tumor microenvironment (for example, macrophages) is more potent in facilitating CTL exhaustion than PD-L1 expression on cancer cells.
To this end, we have generated considerable preliminary data. First, we established a unique syngeneic, orthotopic murine transplantation model to study cholangiocarcinoma progression to permit examination of the immune response. Second, we identified PD-L1 expression by hepatic tumor-associated macrophages (TAMs) in CCA; interestingly, a subset of these macrophages also express Clec4f, a marker specific for murine Kupffer cells. Moreover, we demonstrated increased expression of PD-L1 on TAMs compared to tumor cells in resected human CCA specimens. Finally, our data indicate that macrophage-targeted therapy with colony-stimulating factor 1 receptor (CSF1R) inhibition or checkpoint inhibition with PD-L1 blockade is tumor suppressive in our model of tumor progression.
Based on these preliminary data, we propose the central hypothesis that the liver manifests a pro-tumor, immune tolerogenic niche mediated by PD-L1 positive hepatic macrophages, which can be overcome by immune-directed therapies reprogramming macrophages, leading to robust tumor suppression.