Role of microbially-derived metabolites in anti-tumor immunity in breast cancer - Summary Bariatric surgery reduces breast cancer (BC) risk and improves therapeutic outcomes, but the mechanisms underlying these benefits remain undefined. Critical discoveries by our group in the first five years of funding provide support for the premise that microbially derived metabolites (MDM) mediate weight loss-associated protection in BC. First, we demonstrated that MDMs including bile acids and branched chain amino acids (BCAA) are changed by bariatric surgery-mediated weight loss, and these alterations are sustained over time. Second, we published that mimicking primary bile acid signaling by repurposing an FDA-approved drug targeting bile acid receptor FXR demonstrated reduced BC burden in a humanized mouse model. Third, through fecal microbial transplant, we observed that circulating BCAAs were uniquely elevated by the post bariatric surgery gut microbiome and associated with both reduced tumor burden and elevated anti-tumor Natural Killer T (NKT) cell content. In fact, the transfer of gut microbiota from bariatric surgery weight loss mouse and human donors to non-surgical recipient lean mice decreased tumor burden and improved effectiveness of αPD-1 immune checkpoint blockade (ICB), demonstrating conserved benefits. However, the precise interactions between the host-microbiome interactions, MDMs, and immune function in BC remain unclear and will be investigated in this proposal. We posit that MDMs circulate systemically as signaling mediators to modulate immune homeostasis and anti-tumor immunity. Our hypothesis is that improved anti-tumor immunity is mediated by MDM sensing, which shapes anti-tumor immunity. Bile acid G protein coupled receptor TGR5 (GPBAR1) is highly expressed on certain immune cells such as myeloid cells, but the role of immune-specific TGR5 signaling in BC anti-tumor immunity is unclear. Aim 1 will evaluate TGR5 agonism as monotherapy or in combination with ICB therapy to improve immunotherapy effectiveness. Gain and loss of function approaches will be used to investigate this hypothesis, including pharmacologic TGR5 agonism and antagonism, TGR5 null mice, and immune-depletion studies to assess TGR5-dependent signaling in anti-tumor immunity and its role in improved ICB outcomes in complementary models in ER+ and ER- BC, as well as organoid culture. Aim 2 will elucidate the role of BCAA in BC through NKT cell activation. We show that BCAAs correlate with reduced tumor burden and elevated NKT cell content in BC. Primary and metastatic BC models will be used to test BCAA impacts on ICB in multiple BC subtypes, accompanied by in vitro approaches to define BCAA-NKT cell interactions using supplementation, deficiency, and pharmacologic approaches. Effectiveness of the mono/combination therapy will be determined by tumor progression, metastasis, high dimensional flow, and state-of-the-art single cell and spatial transcriptomic analyses. Our team’s expertise in the “immuno-oncology-microbiome” axis will delineate important host-microbe interactions. In sum, this proposal will discern a role for MDMs in BC through repurposing TGR5 agonists for therapy and initiating BCAA supplement to improve ICB effectiveness.
