Immunotherapy aims to engage the host immune system against malignant tumors. The authors build on prior observations that bacterial RNA can modulate antigen-presenting and immune-related pathways: specifically, Brucella abortus RNA was previously shown by this group to downregulate IFN-γ–induced MHC-I surface expression in human monocytes/macrophages via a TLR8-dependent mechanism. Given the established association between intratumoral natural killer (NK) cell presence and improved patient outcomes, the investigators hypothesized that bacterial RNA (bacRNA) could be used to modulate tumor MHC-I expression to favor NK-mediated anti-tumor responses.
The study examined a panel of human and murine tumor cell lines. Human lines included glioblastoma U251 and LN-229, colorectal adenocarcinoma HT-29, and breast cancer MCF-7. A murine melanoma line, B16-OVA, was included for in vitro and in vivo experiments. Tumor cells were stimulated with bacRNA in the presence of IFN-γ to evaluate effects on IFN-γ–induced MHC-I surface expression. The authors also tested a synthetic human TLR8 agonist (ORN06 formulated with LyoVec) to probe receptor involvement. Downstream functional consequences for immune recognition were assessed by measuring NK cytotoxicity in vitro and immune infiltrates and tumor growth in vivo.
Across all tumor cell lines studied — U251, LN-229, HT-29, MCF-7, and B16-OVA — exposure to bacRNA diminished the increase in surface MHC-I that was induced by IFN-γ. The reduction in IFN-γ–driven surface antigen presentation was consistent across the human tumor lines and the murine melanoma line tested, indicating a broadly reproducible effect of bacterial RNA on tumor cell MHC-I expression under the conditions reported.
To investigate the signaling mechanism, the investigators used the human TLR8 agonist ORN06/LyoVec. ORN06/LyoVec mimicked the effect of bacRNA on IFN-γ–induced MHC-I reduction, supporting the interpretation that TLR8 engagement mediates the observed downregulation of surface class I molecules. The parallel activity of the synthetic hTLR8 agonist strengthens the mechanistic link between bacRNA sensing and altered tumor antigen presentation reported in the study.
Functionally, the authors report that the bacRNA-mediated decrease in tumor cell MHC-I correlated with increased NK cell cytotoxicity. Reduced class I expression on target cells is a known trigger for NK activation due to loss of inhibitory signals, and the experimental data in this study showed enhanced NK-mediated killing associated with bacRNA treatment in the cell lines evaluated.
In mouse tumor models, treatment with either bacRNA or the ORN06 synthetic agonist produced greater immune cell infiltration and activation within tumors compared with untreated controls. In the B16 melanoma model, administration of bacRNA was associated with a delay in tumor growth. These in vivo effects align with the in vitro observations linking MHC-I downregulation to enhanced NK activity and suggest that modulation of tumor MHC-I by bacRNA or TLR8 agonists can alter the intratumoral immune milieu and slow tumor progression in this model.
The authors propose that controlled downregulation of tumor MHC-I using bacRNA or synthetic hTLR8 agonists could be explored as a therapeutic strategy to promote anti-tumor immune responses, particularly by enhancing NK cell activity. The study demonstrates proof-of-concept across several human tumor cell lines and a murine melanoma model.
Details on experimental parameters (doses, treatment schedules, quantitative effect sizes, and formal statistical analyses) are reported in the full manuscript; the abstract summarizes outcomes but does not provide exhaustive methodological or numerical data. Readers should consult the full article for granular experimental methods and results.
The authors declared that no competing interests exist.
(Reference: Serafino A, Vermeulen M, Bertinat YA, et al. PLoS One. 2026;21(9):e0357999. PMID: 42743356; DOI: 10.1371/journal.pone.0357999.)