Gut Bacterium Lactobacillus intestinalis Recruits Immune Cells to Fight Colorectal Cancer
A probiotic gut bacterium triggers tumor cells to release CCL5, summoning dendritic cells that suppress colorectal cancer growth.
Summary
Researchers discovered that Lactobacillus intestinalis, a probiotic gut bacterium, suppresses colorectal cancer (CRC) by activating an immune-recruitment cascade. In both chemically induced and spontaneous mouse CRC models, oral supplementation with L. intestinalis significantly reduced tumor burden. The mechanism involves the bacterium stimulating tumor cells—via the NOD1/NF-κB signaling pathway—to secrete the chemokine CCL5, which recruits dendritic cells (DCs) into the tumor microenvironment. When DCs were depleted or CCL5 was knocked down, the anti-tumor effect was abolished, confirming the pathway's necessity. Clinical data from CRC patient samples and public datasets further corroborated positive correlations between L. intestinalis abundance, CCL5 expression, and DC-related gene signatures, suggesting translational relevance.
Detailed Summary
Colorectal cancer is the third most common cancer worldwide and the second leading cause of cancer death. The gut microbiome is increasingly recognized as a key modulator of the tumor immune microenvironment, but the specific mechanisms by which individual bacterial strains influence anti-tumor immunity remain incompletely understood. This study is the first to characterize the anti-tumor role of Lactobacillus intestinalis in CRC and to delineate its molecular mechanism.
Using two established murine CRC models—the AOM/DSS chemically induced model and the ApcMin/+ spontaneous adenoma model—the researchers demonstrated that daily oral gavage of L. intestinalis (10⁹ CFU/mouse) significantly reduced tumor number, size, and burden compared to PBS or E. coli MG1655 controls. Histopathological analysis confirmed reduced proliferation (Ki67) and angiogenesis (CD31) in tumors from L. intestinalis-treated mice. Flow cytometric analysis of tumor-infiltrating immune cells revealed a striking increase in dendritic cell (DC) infiltration, along with downstream activation of CD8+ T cells, in treated animals.
To dissect the mechanism, the team performed co-culture experiments with the murine CRC cell line MC38 and L. intestinalis at MOI 100:1. Cytokine profiling and chemokine arrays of conditioned media identified CCL5 as the primary chemokine upregulated in tumor cells following bacterial exposure. Immunofluorescence and ELISA confirmed that L. intestinalis induced CCL5 secretion from tumor cells, not from the bacteria themselves. Bone marrow-derived DC (BMDC) migration assays demonstrated that CCL5-enriched conditioned media from L. intestinalis-treated tumor cells robustly recruited DCs in vitro. Knockdown of CCL5 using siRNA in MC38 cells abolished DC recruitment and eliminated the protective anti-tumor effect in subcutaneous tumor models, confirming CCL5 as indispensable to the mechanism.
Further investigation revealed that L. intestinalis activates the NOD1/NF-κB signaling pathway in tumor cells to drive CCL5 transcription. NOD1 siRNA knockdown in MC38 cells significantly attenuated L. intestinalis-induced CCL5 upregulation and NF-κB nuclear translocation. To confirm DC necessity in vivo, the team used Itgax-DTR mice in which CD11c+ DCs were depleted via diphtheria toxin. In DC-deficient mice, the tumor-suppressive effect of L. intestinalis was completely abrogated, validating the essential role of DCs in this axis.
Clinical validation was provided through analysis of CRC patient fecal and tumor tissue samples alongside public transcriptomic datasets. L. intestinalis abundance positively correlated with CCL5 expression and DC-related gene signatures in tumor tissue. These concordant findings in human data strengthen the translational potential of the preclinical results. The study positions L. intestinalis as a promising candidate for microbiome-based CRC prevention or adjunct therapy, acting through a previously undescribed tumor cell-intrinsic CCL5/DC recruitment axis.
Key Findings
- L. intestinalis oral supplementation significantly reduced tumor burden in two independent mouse CRC models.
- The bacterium induced tumor cells to secrete CCL5 via NOD1/NF-κB signaling, recruiting dendritic cells into the TME.
- DC depletion or CCL5 knockdown completely abolished L. intestinalis anti-tumor effects, confirming pathway essentiality.
- Clinical CRC datasets showed positive correlations between L. intestinalis abundance, CCL5 levels, and DC-related gene signatures.
- L. intestinalis acts on tumor cells—not immune cells directly—to remodel the immunosuppressive tumor microenvironment.
Methodology
The study used two murine CRC models (AOM/DSS-induced and ApcMin/+ spontaneous) plus subcutaneous MC38 tumor models in C57BL/6 and Itgax-DTR (DC-depleted) mice. In vitro co-culture of CRC cells with L. intestinalis, siRNA knockdown of CCL5 and NOD1, BMDC chemotaxis assays, flow cytometry, immunohistochemistry, and analysis of clinical patient samples and public transcriptomic datasets were employed.
Study Limitations
All mechanistic work was conducted in murine models and cell lines; direct causal evidence in human CRC patients is lacking. The study did not assess whether L. intestinalis colonization is durable or how it interacts with existing microbiome composition or chemotherapy regimens.
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