Microbiome-Derived Lactate Promotes Immunosuppression and Radiation Resistance

The cervical microbiome plays a pivotal role in shaping vaginal health and influencing tumor prognosis. Tumor associated microbes secrete bioactive metabolites that modulate both tumor behavior and its surrounding microenvironment. Our lab previously identified a species of L-Lactate producing Lactobacillus (LAB), Lactobacillus iners (L. iners), to be associated with poor chemoradiation response. In vitro experiments revealed that conditioned media from L. iners enhances radiation resistance in cervical cancer cells—a phenomenon replicated by direct lactate supplementation. This study aims to characterize the impact of microbiome-derived lactate on the tumor microenvironment. Hypothesis: We propose that L-lactate produced by cancer-derived LAB supports tumor progression by fostering an immunosuppressive niche. Methods: LAB strains were isolated via targeted culture from cervical, vaginal, vulvar, and oral cavity cancers. These isolates were used in co-culture systems and to generate cell-free bacterial conditioned media (CFS) to assess their capacity to interact with cancer cells and promote tumor-supportive conditions. For initial studies, we focused on cervical cancer isolates. To evaluate LAB-derived lactate’s role in radiation resistance, we performed cell viability assays (CyQUANT) on HeLa, SiHa, and CaSki cervical cancer lines exposed to CFS from patient-derived LAB and radiation. Lactate production was quantified using the Diazyme D-/L-Lactate rapid test kit, enabling precise source attribution between bacterial and tumor origins. We developed a syngeneic lactate exposure model using mEER (mouse oral line) xenografts in C57BL/6 mice, administering weekly intratumoral lactate injections. The tumors were radiated after the tumors reached ∼75mm3. To evaluate the immune landscape, multiplex immunofluorescence using the Lunaphore COMET microfluidic platform was performed on mEER tumors. Results: Cervical cancer cells exhibited increased radiation resistance when co-cultured with L-lactate-producing LAB, the isoform preferentially metabolized by human cells. LAB emerged as the dominant source of lactate in co-culture, confirming its role in driving lactate accumulation within the tumor microenvironment. In vivo, lactate-treated tumors grew significantly larger post-radiation (mean volume 48 mm3 vs. 74 mm3, p < 0.0001). Immune profiling revealed elevated T-cell infiltration, with a skew toward regulatory T cells and reduced CD8+ populations. The lactate-rich microenvironment showed marked upregulation of immune checkpoint proteins CTLA4 and PD-L1, suggesting functional exhaustion of infiltrating immune cells. Citation Format: Christy Charles, David Lo, Jennifer L. Anderson, Ratna Veeramachaneni, Francisco Saenz, Anastasios Maniakas, MIchael T. Spiotto, Andrew G. Sikora, Lauren Elizabeth Colbert. Microbiome derived lactate promotes immunosuppression and radiation resistance [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4900.