Intratumoral Bacteria as Mediators of Cancer Immunotherapy Response

Abstract Multiple lines of evidence spanning from animal models to human clinical trials indicate that the microbiome influences cancer immunotherapy response. Whereas initial studies focused exclusively on the gastrointestinal (gut) microbiota–tumor axis, more recent studies have examined the possibility that bacteria located within tumor cells or within the tumor microenvironment mediate cancer treatment response. Strikingly, this phenomenon has been demonstrated in cancers that arise in anatomic locations that are traditionally thought to be devoid of resident microbiota. In this issue of Cancer Research, Wu and colleagues examine the effects of intratumoral bacterial signatures on treatment response in the setting of neoadjuvant chemotherapy combined with immunotherapy (NACI) in the treatment of esophageal squamous cell carcinoma (ESCC). The study reports that intratumoral Streptococcus, presumably due to bacterial translocation from the gut, predicts the treatment efficacy of NACI in murine models as well as individuals with ESCC. These new findings further highlight the possibility that the presence of intratumoral microbes as well as their associated metabolites influence both the tumor immune microenvironment and immunotherapy efficacy. These findings also raise the intriguing possibility of cross-reactivity between tumor and bacterial antigens. Given that the gut microbiome is potentially a modifiable factor via diet, prebiotics/probiotics, and fecal microbiota transplantation, among other strategies, further exploration into the mechanisms by which gut and/or intratumoral bacteria influence antitumor immunity is certainly warranted. See related article by Wu et al., p. 3131