Intra-tumoral bacteria in head and neck cancer: holistic integrative insight

Abstract

Intra-tumoral bacteria are pivotal in the initiation and progression of head and neck squamous cell carcinoma (HNSCC), exerting a significant influence on tumor cell biology, immune responses, and the tumor microenvironment (TME). Different types and distribution of bacteria threaten the balance of metabolism and the immune environment of tumor cells. Taking advantage of this disrupted homeostasis, intra-tumoral bacteria stimulate the secretion of metabolites or influence specific immune cell types to produce inflammatory or chemokines, thereby influencing the anti-tumor immune response while regulating the level of inflammation and immunosuppression within the TME. Some intra-tumoral bacteria are used as diagnostic and prognostic markers in clinical practice. Based on the unique characteristics of bacteria, the use of engineered bacteria and outer membrane vesicles for drug delivery and biological intervention is a promising new therapeutic strategy. The presence of intra-tumoral bacteria also makes chemoradiotherapy tolerable, resulting in a poor treatment effect. However, due to the immune-related complexity of intra-tumoral bacteria, there may be unexpected effects in immunotherapy. In this review the patterns of intra-tumoral bacteria involvement in HNSCC are discussed, elucidating the dual roles, while exploring the relevance to anti-tumor immune responses in the clinical context and the prospects and limitations of the use of bacteria in targeted therapy.

Keywords: Head and neck squamous cell carcinoma; immunity; intra-tumoral bacteria; therapy; tumor progression.

Figure 1

Common intra-tumoral bacteria in each major subtype of HNSCC. (A) OSCC: The common bacterial flora in OSCC is shown in the figure. P. gingivalis and F. nucleatum are the most common and most studied intra-tumoral bacteria in Porphyromonas and Fusobacterium, respectively, while Streptococcus has been observed to have reduced abundance in most studies. (B) LSCC: Fusobacterium is also the most common species in LSCC. Helicobacter pylori is a specific bacterial species found in LSCC. (C) NPC: Fusobacterium and Prevotella are common in NPC and intra-tumoral bacteria in NPC are often associated with prognosis. (D) HPSCC: The intra-tumoral bacteria in HPSCC are like other subtypes. Specifically, the Eubacterium coprostanoligenes group comprises specific bacteria that inhibit the tumor. HNSCC, head and neck squamous cell carcinoma; OSCC, oral squamous cell carcinoma; LSCC, laryngeal squamous cell carcinoma; NPC, nasopharyngeal carcinoma; HPSCC, hypopharyngeal squamous cell carcinoma.

Figure 2

Intra-tumoral bacteria promote tumor initiation and progression. (A) Regulation of DNA damage: ① F. nucleatum may directly cause DSB and disrupt Ku 70/80 complex in inhibiting DNA repair. ② Metabolites of F. nucleatum activate TLR4/MyD88 followed by miR-205-5p upregulation, which inhibits MMR gene expression, including MLH1, MSH2, and MSH6. ③ Intracellular bacteria may promote the formation of JUN and FOS heterodimers and the AP-1 complex for transcriptional activation of DNA damage repair genes, such as ERCC-1 and XPC. (B) Promotion of chronic inflammation: ④ Downregulation of antibacterial HBD1 coupled with IL-1β–mediated induction of oncogenic HBD2 and HBD3 facilitates bacterial persistence and contributes to tumor progression. ⑤ F. nucleatum and P. gingivalis activate the NF-κB and ERK/MYC pathways through the TLR/MyD88 axis, driving tumor progression. ⑥ Intra-tumoral bacteria induce monocyte secretion of IL-6 via metabolites and chemokines to activate IL-6/STAT3 signaling and promote effectors, such as cyclin D1 and MMP9. (C) Regulation of oxidative stress: ⑦ F. nucleatum promotes tumor growth by driving NF-κB activation, which increases ROS production, indirectly activates the AKT/mTOR pathway, and suppresses the p53 pathway. ⑧ F. nucleatum regulates oxidative stress by activating the miR-361-3p/NUDT1 axis via TLR4, relieving miR-361-3p inhibition of NUDT1 to promote autophagy and DDR. Autophagy indirectly activates DDR, while DDR alleviates DNA damage stress through autophagy, forming a feedback loop that reinforces both processes and drives tumor progression. ⑨ F. nucleatum inhibits ROS production by suppressing purine degradation and uric acid production, a key antioxidant. (D) Synergy with viruses: ⑩ Co-culture of F. nucleatum with EBV enhances EBV antigen expression. Periodontal pathogens activate and exacerbate the oncogenic potential of EBV and HPV through their metabolites, synergistically promoting malignant transformation. Streptococcus species contribute to carcinogenesis by inducing inflammation and generating carcinogens, such as acetaldehyde. AMP, adenosine monophosphate; AP-1, activator protein 1; DDR, DNA-damage response; DSB, double-strand break; EBV, Epstein–Barr virus; HBD, human beta-defense; HPV, human papillomavirus; MMP9, matrix metalloprotein 9; MMR, mismatch repair; MyD88, myeloid differentiation primary response gene 88; NUDT1, nudix hydrolase 1; ROS, reactive oxygen species; TLR, Toll-like receptor.

Figure 3

Dual role of intra-tumoral bacteria in regulating the anti-tumor immune response. (A) Enhancement of anti-tumor immunity: ① P. gingivalis downregulates MUC1 to reduce PD-L1 and inhibit MDSC, thereby enhancing anti-tumor immunity. ② F. nucleatum has a negative correlation with M2 macrophages and a positive correlation with M1 macrophages, suggesting involvement in tumor suppression, potentially through activation of the TNFSF9/IL-1β axis. ③ Flammeovirga and Luteibacter are positively associated with chemokines, such as CXCL9, CXCL10, and CCL5, facilitating the recruitment and activation of CD8⁺ T cell. ④ Lachnoclostridium further supports anti-tumor immunity by promoting CD8⁺ T cell activity and producing the anti-inflammatory metabolite, butyrate, which exerts tumor-suppressive effects. (B) Inhibition of anti-tumor immunity: ⑤ P. gingivalis activates NOD1 in tumor cells via teichoic acid carried by OMVs, upregulating PD-L1. OMVs also stimulate monocytes to secrete IL-10 and inhibit TNF production, enhancing P. gingivalis survival and promoting immune evasion. ⑥ F. nucleatum and S. aureus utilize TLR signaling to activate ERK/MYC and NF-κB pathways, driving PD-L1 and suppressing T cell activation to facilitate tumor progression. ⑦ F. nucleatum and Selenomonas are positively correlated with FOXP3, a marker of Treg, suggesting an association with Treg. ⑧ High abundance of Capnocytophaga showed a negative correlation with Tem, further linking them to immunosuppressive effects. ⑨ Roseobacter, Streptococcus, and Clusterobacter are associated with the abundance of TN and Tcm, yet significantly reductions of TN and Tcm in tumors may impair the efficacy of these T cell subsets. ⑩ Corynebacterium, Prevotella, and members of the Peptostreptococcaceae family show positive correlations with the Th2 marker GATA3, suggesting roles as potential immunosuppressive agents in the tumor microenvironment. AI-2, autoinducer-2; CCL, C-C motif chemokine ligand; CXCL, C-X-C motif chemokine ligand; MDSC, myeloid-derived suppressor cell; MUC1, pleomorphic epithelial mucin 1; NOD1, nucleotide-binding and oligomerization domain 1; OMVs, outer membrane vesicles; PD-1, programmed death 1; PD-L1, programmed death-ligand 1; RIP2, receptor-interacting protein kinase 2; Tcm, central memory T cell; Tem, effector memory T cells; Th2, T helper 2 cell; TLR, toll-like receptor; TN, naive T cell; TNF, tumor necrosis factor; TNFSF9, tumor necrosis factor ligand superfamily member 9; TRAF1, tumor necrosis factor receptor-associated factor 1; Treg, regulatory T cell.

Figure 4

Anti-tumor therapy associated with intra-tumoral bacteria. (A) Radiotherapy: Anaerobes associated with vascular distribution can induce radiation resistance of tumor cells in hypoxic areas. (B) Chemotherapy: In traditional chemoradiotherapy, intra-tumoral bacteria induce resistance to cisplatin and gemcitabine through intrinsic components of tumor cells. (C) Immunotherapy: P. gingivalis induces the upregulation of PD-L1 expression in DCs and attenuates ICB efficacy, while the synthetic material containing Peptostreptococcus has a synergistic effect with ICB. (D) Engineering bacteria: The design of engineering bacteria is an effective means of targeted drug delivery in TME, improving the response rate to anti-PD-1 drugs and reversing immunosuppression. (E) BMV therapy: The membrane vesicles of intra-tumoral bacteria are often designed to have anti-tumor effects and positive implications in adjuvant therapy. (F) Injection therapy: Direct injection of bacteria or supernatant into tumor cells tended to inhibit tumor growth. (G) Molecular inhibitors: Bacteria promote the occurrence and progression of tumors through a variety of signaling molecules and the use of corresponding molecular inhibitors is a potential treatment method. BMVs, bacterial membrane vesicles; CARD10, caspase recruitment domain family member 10; DC, dendritic cell; ICB, immune checkpoint blockade; OMVs, outer membrane vesicles; TILs, tumor infiltrating lymphocyte.