Tetraacylated lipid A enhancement of checkpoint blocking cancer immunotherapy - PROJECT SUMMARY Immune checkpoint inhibitor (ICI) therapy is a therapeutic approach that blocks pathways that inhibit anti-tumor immunity and activates the immune system to fight cancer. Although ICI therapy has seen significant success in some patients, unfortunately, most eligible patients still fail to respond to treatment. Several studies have highlighted the modulatory capacity of the microbial composition of the gut microbiome as well as certain organisms to direct a patient’s response to ICI. Our preliminary data show that the human gut commensal Bacteroides ovatus 8483 has the capacity to enhance ⍺-PD-L1 (Programmed Death-Ligand 1) therapy in germ-free (GF) and antibiotic-treated mice. The lipooligosaccharide (LOS) of B. ovatus is a major cell surface molecule comprising about 50% of the surface of this microbe’s outer membrane. The LOS exists naturally in several different structural analogs. We have found that among the many analogs of lipid A produced by B. ovatus, an orally delivered tetra-acylated monophosphorylated lipid A (4A-MPLA) can potently enhance the efficacy of ⍺-PD-L1 in mice that would otherwise fail to clear tumors. We propose to determine whether 4A- MPLA could serve as a novel therapeutic for enhancing the current cancer immunotherapy arsenal. Our long- term goal is to use 4A-MPLA in combination with PD-1 pathway blockade to treat cancer patients, particularly non-responders to ⍺-PD-L1 alone. Over the course of the next two years, we aim to understand: 1) the cell types and signaling pathways by which 4A-MPLA influences local and systemic improvement of the ⍺-PD-L1 response, and 2) understand the types of cancers that may respond to this treatment. Given our preliminary studies demonstrating the effect of B. ovatus on colonic regulatory T cells (Tregs), we hypothesize that B. ovatus is affecting peripheral Tregs in the colon and disrupting their ability to restrain antitumor immunity. To test this hypothesis, we will compare the immune profiles of cells in several tissues including the gut lamina propria, mesenteric lymph nodes, tumor draining lymph nodes, and the tumor itself in gnotobiotic, antibiotic-treated, and specific-pathogen-free (SPF) mice treated with 4A-MPLA compared to controls. Tregs as well as other relevant compartments of the tumor microenvironment (TME), such as CD8+ and CD4+ T cells, NK cells, NKT cells, B cells, dendritic cells, M1 and M2 macrophages, and monocytic and granulocytic myeloid-derived suppressor cells, will be analyzed in mouse tumor models for immunologic responses to ⍺-PD-L1 therapy with or without 4A-MPLA. Finally, we will determine which murine tumor models respond to this therapy to better predict its clinical applicability.
