Can indoleacetic acid help overcome P. vulgatus–induced IO resistance in HCC?

Researchers from the University of Hong Kong (HKU) have identified enrichment in Phocaeicola vulgatus as a potential cause for immunotherapy (IO) resistance in patients with hepatocellular carcinoma (HCC) and shown Sthat supplementation with indoleacetic acid (IAA) can overcome P. vulgatus–induced immunosuppression, thereby establishing a causal relationship between P. vulgatus and anti–PD-1 resistance in HCC.

“IO, particularly immune checkpoint inhibitors [ICIs], has emerged as a key treatment for unresectable HCC … However, response rates remain highly variable among patients, and mechanisms underlying resistance are not fully understood,” wrote the researchers. “In this study, we aimed to address this knowledge gap by investigating the relationship between gut microbiota composition and IO outcomes in HCC patients.” [Cell Rep Med 2025;6:102370]

The study analyzed a cohort of 65 HCC patients who received first-time IO at Queen Mary Hospital and University of Hong Kong–Shenzhen Hospital between August 2020 and December 2022. Their stool samples were collected for metagenomics sequencing. No antibiotic use within 30 days prior to stool sample collection was permitted to avoid interference with gut microbiota.

Of the 65 patients, 50 were classified as IO responders (complete response, 8 percent; partial response, 38 percent; stable disease, 54 percent), while the remaining 15 patients were nonresponders. Responders had significantly longer mean progression-free survival (mPFS; 30.8 vs 3.8 months; p<001) and overall survival (mOS; 94.8 vs 15.0 months; p<001) vs nonresponders.

IO responders had significant faecal enrichment of Clostridiaceae family, Clostridium and Citrobacter genera, while P. vulgatus was the most enriched species among nonresponders and was also more abundant in nonresponders vs responders according to quantitative PCR. In addition, Kaplan–Meier plots showed that patients with higher vs lower P. vulgatus abundance had poorer survival outcomes (mPFS: 11.1 vs 30.8 months; p=0.047) (mOS: 22.9 vs 63.2 months; p=0.037).

To determine whether P. vulgatus impairs the efficacy of anti–PD-1 treatment, P. vulgatus was gavaged to syngeneic and orthotopic mouse tumour models. “Although P. vulgatus alone did not affect tumour growth, its presence significantly impaired the antitumour efficacy of anti–PD-1,” reported the researchers.

To investigate the underlying mechanisms by which gut microbiota influence immune response, the researchers examined the main lymphoid and myeloid cell subtypes in tumour-bearing mice using flow cytometry. The frequency of IFN-γ+ and GzmB+CD8+ T cells was significantly reduced in tumours of mice that that received an anti–PD-1 anatibodyand P. vulgatus transplant vs tumours of mice that were not exposed to P. vulgatus.

Next, targeted metabolic profiling of faeces from P. vulgatus–gavaged mice revealed a significant decrease in IAA and indolelactic acid (ILA), which was also confirmed by plasma metabolomics findings. However, in plasma samples of IO nonresponders, only IAA levels were significantly lowered, while ILA levels remained unchanged.

Therefore, the researchers conducted an IAA rescue experiment to determine whether IAA supplementation could overcome P. vulgatus–mediated resistance. They found that IAA supplementation fully restored anti–PD-1 efficacy in mice colonized with P. vulgatus, supporting a causal role for IAA depletion in impairing CD8+ T cell function and anti–PD-1 efficacy.

“By measuring levels of P. vulgatus in the gut, clinicians could anticipate an HCC patient’s response to IO,” commented first author, Dr Caining Zhao of the Department of Clinical Oncology, HKU. “This discovery could enable doctors to formulate more effective treatment strategies tailored to a patient’s gut microbiota profile. Approaches such as IAA supplementation or gut microbiota modulation may enhance IO efficacy, marking a significant step towards precise, personalized treatment in the future.”