LpIMB19 and its RHP capsule activate TLR2⁺ macrophages to drive lipocalin-2 iron sequestration in tumors, expand cytotoxic CD8⁺ clones, and synergize with PD-L1 blockade, offering certifiable nutritional-immunity biomarkers for HMTC.
Authors
- Sharma G, Sharma A, Kim I, et al. — Nature Immunology. 2024;
What was studied
This original study investigated how a food-derived commensal bacterium, Lactiplantibacillus plantarum IMB19 (LpIMB19), enhances antitumor immunity by reprogramming tumor macrophages toward inflammatory phenotypes that exert lipocalin-2 iron sequestration in tumors. The authors used syngeneic melanoma, breast, and renal mouse models and a humanized xenograft bladder cancer model to test live and heat-killed LpIMB19 as well as its purified capsular rhamnose-rich heteropolysaccharide (RHP). Mechanistically, RHP ligates TLR2 on tumor-associated macrophages, boosting cytotoxic CD8⁺ T-cell responses and triggering nutritional immunity: macrophages secrete lipocalin-2 (LCN2) to capture siderophore-bound iron and upregulate ferritin to store iron intracellularly, collectively depriving nearby tumor cells of bioavailable iron. The work also shows additive benefit with anti-PD-L1 therapy, TCR clonal expansions in CD8⁺ tumor-infiltrating lymphocytes, and iron-handling rewiring in macrophages visualized by gene expression and functional assays.
Who was studied
Multiple murine cancer models were used, including B16F10 melanoma, EMT-6 breast carcinoma, and orthotopic Renca renal tumors, with interventions delivered by oral LpIMB19 or intraperitoneal RHP. The study further humanized NOD-SCID mice with peripheral blood mononuclear cells and implanted patient-derived bladder cancer organoids to assess translation. Genetic tools included macrophage depletion (Cd11bDTR), Tlr2⁻/⁻ and Lcn2⁻/⁻ macrophage transfers, and LysMCre-driven ferroportin ablation to dissect pathway dependence. Across models, macrophages and CD8⁺ T cells in tumors and draining lymph nodes were phenotyped by flow cytometry, single-cell RNA-seq, bulk RNA-seq, and TCRβ sequencing, with iron flux tracked using SiRhoNox and ferritin immunoblotting.
Most Important Findings
| Critical point | Details |
|---|---|
| LpIMB19/RHP suppresses tumor growth and boosts survival | In B16F10 melanoma and other models, both live and heat-killed LpIMB19 reduced tumor burden; RHP alone recapitulated suppression, and co-therapy with anti-PD-L1 showed additive control and increased IFNγ⁺/TNF⁺ CD8⁺ TILs with reduced Lag3 expression. |
| Macrophage TLR2 is the principal sensor | RHP engages TLR2, not TLR4; macrophage activation (↑iNOS2, ↑CD86, ↑MHC I) and CD8⁺ priming were blunted with Tlr2 deficiency, establishing TLR2-dependent innate activation upstream of CD8⁺ responses. |
| Lipocalin-2–driven iron capture and ferritin storage | Gene-set enrichment highlighted iron sequestration pathways; macrophages upregulated Lcn2 and Fth1, secreted LCN2, and increased ferritin. Intratumoral macrophages accumulated iron, while tumor cells were iron-depleted in competition assays. |
| LCN2 is required for antitumor effect | In Cd11bDTR transfer experiments, Lcn2⁻/⁻ macrophages abrogated LpIMB19/RHP tumor control, failed to increase macrophage iron uptake, and reduced tumor apoptosis, establishing LCN2 as an effector of nutritional immunity. |
| Ferroportin loss does not add benefit | Macrophage-specific ferroportin deletion (Fpn^F/F LysMCre) did not enhance LpIMB19 efficacy or change steady-state intratumoral macrophage iron, indicating iron capture via LCN2, not export blockade, is primary. |
| Durable CD8⁺ T-cell cytotoxicity and clonal remodeling | scRNA-seq and TCRβ-seq showed more effector/memory CD8⁺ cells, reduced exhaustion, higher cytotoxic gene expression, and expanded dominant clonotypes, suggesting epitope broadening and clonal replacement dynamics. |
| Human relevance | In a PBMC-humanized bladder organoid xenograft, LpIMB19 and RHP reduced tumor mass and increased human IFNγ⁺ CD8⁺ T cells; macrophage signature orthologs stratified TCGA survival and improved PD-L1 response prediction with TMB. |
Key implications
For primary regulatory impacts, these data position macrophage-mediated nutritional immunity, especially lipocalin-2 iron sequestration in tumors, as an actionable safety and performance endpoint. Certification requirements for HMTC could include validated LCN2 induction, ferritin upregulation, and TLR2-dependent assays under tumor-mimetic conditions. Industry applications span probiotic oncobiotics, iron-chelation adjuncts, and PD-(L)1 combinations. Research gaps include human dose-response, microbiome variability, and off-tumor iron effects. Practical recommendations prioritize standardized iron-flux biomarkers, macrophage activation panels, and co-therapy compatibility testing.
Citation
Sharma G, Sharma A, Kim I, et al. A dietary commensal microbe enhances antitumor immunity by activating tumor macrophages to sequester iron. Nature Immunology. 2024;25:790-801. doi:10.1038/s41590-024-01816-x