Defining mechanisms of how immune checkpoint inhibitor-associated bacteria modulate tumor immunity in cancer

Project Summary: Cancer is a global epidemic with rising incidence. Immune checkpoint inhibitors (ICI) which enhance interferon-  producing CD8+ T cell (Tc1) mediated anti-tumor immunity have shown remarkable efficacy in the treatment of cancer, including melanoma. However, the majority of cancer patients are resistant to ICI therapy. Therefore, approaches that further potentiate Tc1 antitumor immunity are needed to boost ICI therapies. The gut microbiome plays a critical role in mediating response to ICI therapy in cancer, and a consortium of ICI-response associated bacteria across multiple melanoma patient cohorts have been identified. Missing is an understanding of whether and if so, by which mechanism(s) ICI-bacteria improve ICI response in melanoma. We recently showed that probiotic Lactobacillus reuteri converts dietary tryptophan (Trp) into the aryl hydrocarbon-receptor (AhR) agonist, indole-3-aldehyde (I3A) that potently restrains growth of multiple cancer models, including melanoma. The ligand-activated transcription factor AhR drives CD4 T cell fate. However, whether microbial AhR ligands modulate systemic CD8 T cell function remained unknown. We discovered that I3A drives antitumor Tc1 fate in AhR-dependent and CD8 T cell-intrinsic manner in vitro and in vivo thereby improving ICI efficacy (Bender et al., Cell 2023). While the exact molecular mechanism remains undefined, we found that I3A promotes activation of cAMP response element-binding protein (CREB), a key transcriptional enhancer of IFN and Tc1 effector function. Missing is whether and which ICI-bacteria-produced AhR ligand(s) induce the AhR-pathway that enhance Tc1 fate thereby improving ICI efficacy in melanoma. Excitingly, by performing an in silico approach, we identified that 18 out of 50 known ICI-bacteria are potential AhR ligand producers. In key preliminary data we show that metabolites produced by ICI-bacteria enhance Tc1 immunity in vitro via AhR dependent and independent mechanisms. Our overarching hypothesis is that ICI-bacteria improve ICI-efficacy in melanoma by their ability to metabolize dietary Trp into AhR ligands(s) driving antitumor Tc1 immunity via AhR dependent and independent mechanisms. We will test this hypothesis in two Specific Aims: Aim 1: Test whether ICI-bacteria metabolize dietary Trp into AhR ligand(s) enhancing Tc1 immunity via AhR-dependent and independent mechanisms in vitro. Aim 2: Test whether AhR ligand-producing ICI-bacteria enhance Tc1 immunity and ICI efficacy via activating AhR-dependent and/or independent mechanisms within CD8 T cells in ICI- sensitive and ICI-resistant melanoma models in vivo. If successful, this proposal will identify the underlying mechanisms of how a specific gut microbial metabolite mediates protective immunity against melanoma, with the ultimate goal of developing novel and rational, clinically relevant approaches to enhance the efficacy of current ICI therapies in both ICI-sensitive and ICI-resistant melanoma patients.