Abstract 1173: Desaminotyrosine contributes to the anticancer effect of fecal microbial transplantation during immune checkpoint blockade in mice and patients

Abstract Background: Gut dysbiosis is associated with primary resistance to immune checkpoint blockade (ICB). Fecal microbiota transplantation (FMT) is a promising microbiota-centered intervention to circumvent ICB resistance. Mechanisms underlying success of FMT engraftment remain unclear. Here, we describe the impact of the desaminotyrosine in the thymus-gut axis during FMT. Methods: We generated avatar mice with humanized microbiota by orally gavaging animals with various patient stools of defined taxonomic composition categorized as beneficial (FMT1) or harmful (FMT2). We monitored tumor growth kinetics after inoculation of MCA205 sarcoma and treatment with anti-PD1 Abs. To explore the cause-effect relationship between metabolic patterns and tumoricidal effects, we supplemented FMT-recipient tumor bearers with the desaminotyrosine in drinking water for 20 days. To investigate the gut- bone marrow (BM) axis and tracing of myeloid progenitors to the periphery, we used Ms4a3CreERT2 fate-mapping mouse models. At sacrifice, flow cytometry analyses were performed on thymus, spleen, mesenteric lymph node (mLN), BM and lamina propria. Kinetics of plasma cytokine profiles were studied using Olink® 48-plex panel. Plasma metabolomics and 16S-rRNA sequencing on fecal amplicons were performed in mice. Mass spectrometry-based plasma metabolomics were conducted in patients from the FMT trial MIMIc (NCT 03772899) and IMMUNOLIFE1 cohort ((NCT04567446) prospectively. Results: We identified two opposite profiles of FMT in avatar mice, those enhancing ICB anticancer effects (FMT1) and those that failed to do so (FMT2). Plasma metabolomics revealed the flavonoid metabolite desaminotyrosine (DAT) standing out highly significant (x4fold increase) after 2 anti-PD1 Abs, 5 days post-FMT1 but not post-FMT2 (p= 0.0052) accross three independent experiments while DAT levels dropped post-antibiotics (p=0.035). DAT is known as a type 1 IFN inducer and correlated with IFN lambda serum levels (p=0.06). Moreover, we could restore ICB efficacy by passively breeding FMT2 recipients with DAT containing drinking water. DAT reduced the CD45+ CD3+ ileal exfoliome (leukocytes recovered alive in feces) while reducing a4b7+ Tr17 and Treg accumulation in spleen. Surprisingly, DAT promoted a drastic reduction of CCR9+ single positive CD4+ or CD8+ thymocytes, suggesting that those cells are redirected to CCL25 secreting ilei of FMT recipients. Then, myeloid precursor fate mapping revealed an early demargination of neutrophils in mLN only in FMT1 and faster monocytes recirculation in FMT1 than FMT2. Clinically, DAT increased in 72 lung and bladder cancer patients responding to ICB (p= 0.022) and in 14 melanoma patients in MIMIc FMT trial (p= 0.008). Conclusions: Successful FMT reprograms the gut-BM-thymus axis and the metabolic rewiring may involve DAT in mice and cancer patients. Citation Format: Simon Thomas, Déborah Suissa, Cassandra Thélémaque, Marijana Rucevic, Valerio Iebba, Sylvère Durand, Garett Dunsmore, Florent Ginhoux, Lisa Derosa, Jennifer Wargo, Bertrand Routy, Marine Fidelle, Laurence Zitvogel. Desaminotyrosine contributes to the anticancer effect of fecal microbial transplantation during immune checkpoint blockade in mice and patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1173.