Intratumour microbiome associated with the infiltration of cytotoxic CD8+ T cells and patient survival in cutaneous melanoma

The gut microbiome plays an important role in systemic inflammation and immune response. Microbes can translocate and reside in tumour niches. However, it is unclear how the intratumour microbiome affects immunity in human cancer. The purpose of this study was to investigate the association between intratumour bacteria, infiltrating CD8+ T cells and patient survival in cutaneous melanoma.Using The Cancer Genome Altas's cutaneous melanoma RNA sequencing data, levels of intratumour bacteria and infiltrating CD8+ T cells were determined. Correlation between intratumour bacteria and infiltrating CD8+ T cells or chemokine gene expression and survival analysis of infiltrating CD8+ T cells and Lachnoclostridium in cutaneous melanoma were performed.Patients with low levels of CD8+ T cells have significantly shorter survival than those with high levels. The adjusted hazard ratio was 1.57 (low vs high) (95% confidence interval: 1.17-2.10, p = 0.002). Intratumour bacteria of the Lachnoclostridium genus ranked top in a positive association with infiltrating CD8+ T cells (correlation coefficient = 0.38, p = 9.4 × 10-14), followed by Gelidibacter (0.31, p = 1.13 × 10-9), Flammeovirga (0.29, p = 1.96 × 10-8) and Acinetobacter (0.28, p = 8.94 × 10-8). These intratumour genera positively correlated with chemokine CXCL9, CXCL10 and CCL5 expression. The high Lachnoclostridium load significantly reduced the mortality risk (p = 0.0003). However, no statistically significant correlation was observed between intratumour Lachnoclostridium abundance and the levels of either NK, B or CD4+ T cells.Intratumour-residing gut microbiota could modulate chemokine levels and affect CD8+ T-cell infiltration, consequently influencing patient survival in cutaneous melanoma. Manipulating the intratumour gut microbiome may benefit patient outcomes for those undergoing immunotherapy.