In this study, we analyzed the bronchial microbiome in patients with NSCLC by performing 16S rRNA gene sequencing to analyze the relationship among microbiota composition, response to immunotherapy, and PD-L1 expression level. The ratio of Firmicutes to Bacteroidetes and the proportion of Veillonella were higher in the high-PD-L1 and immunotherapy responder groups than in the low-PD-L1 and non-responder groups.
In healthy individuals, the microbiota contributes to barrier function, communicates with the exterior, plays a role in immune homeostasis, and enhances anticancer immune surveillance via tumor antigenicity.[16] Additionally, the microbiota can trigger systemic innate immune responses via pattern recognition receptors, which further activate host responses against tumor cells.[17]
Recent studies have shown that the gut microbiota affects the responses to immune checkpoint blockade therapy in patients with cancer.[18–20] The transplantation of fecal microbiota has been reported to augment and restore human immune responses, resulting in increased sensitivity to immunotherapy.[21] Recent studies have reported that organ-specific microbiomes, such as the lung microbiome, play an important role in lung cancer development.[22]
Overall, our study did not show significant differences in the abundances of phyla in culture-independent DNA-based molecular assays; however, certain estimates of species richness showed significant differences in the PD-L1 expression groups.
There was a significant difference in culture-based genus types according to the differences in PD-L1 expression levels and immunotherapy responses. DNA sequencing analysis showed that the abundances of the genera Neisseria, Veillonella, and Haemophilus differed significantly between the two groups.
Regarding the LDA score indicating the effect size, the genus with the highest score in the low-PD-L1 group was Neisseria of the Proteobacteria phylum. In the non-responder group, the LDA score of N. perflava was significantly increased. This result suggested that Neisseria may be associated with PD-L1 expression levels and immunotherapy responses. It was previously reported that Proteobacteria abundance markedly increased in an anti-PD-L1 immunotherapy non-responder group of hepatocellular carcinoma.[23] However, the causal relationship of these correlations is not understood, and further research is needed.
V. dispar was dominant in the high-PD-L1 and immunotherapy responder groups. At present, studies show that the genus Veillonella is associated with the development of asthma[24, 25] and acute exacerbation of idiopathic pulmonary fibrosis.[26] Furthermore, Yan et al.[27] reported that Veillonella is abundant in patients with squamous cell carcinoma and adenocarcinoma of lung cancer, compared to healthy patients. They found that Veillonella may be used as a diagnostic marker to evaluate the development of both squamous cell carcinoma and adenocarcinoma.[28] Based on the findings of these studies, Veillonella present in samples collected from the airways may be associated with the development of multiple lung diseases. Our study further revealed that the Veillonella population was additionally associated with the responses to immunotherapy.
Tumor immunotherapy is performed not only to kill cancer cells but also to provide a long-term protective immunity mediated by memory CD8+ T cells.[29, 30] During cancer progression, the dysfunction in CD8+ T cell engagement and exhaustion owing to the tumor microenvironment results in an impairment in their function.[31] Identification of bacteria that directly or indirectly induce antitumor activities is crucial to developing microbiome-based combinatory treatments that can improve the overall response rate of anti-PD-1/PD-L1 treatment,[32] which are targeted for T lymphocytes. We further demonstrated that the genus Veillonella was dominant in both high-PD-L1 and immunotherapy responder groups. This finding may contribute to the development of treatment strategies for increasing the effectiveness of immunotherapy in the future.
In this study, Haemophilus, which belongs to the phylum Proteobacteria, showed the largest LDA score of 5.1 in the non-responder group. It is known that most Haemophilus strains are opportunistic pathogens that coexist with the host without causing diseases and only cause diseases during viral infections or decreased immunity. Furthermore, a study showed that Haemophilus was more abundant in patients with lung cancer than in healthy individuals.[33] Our result showed that Haemophilus was abundant in the non-responder group; therefore, it can be inferred that members of this genus lower the effectiveness of anticancer drugs. Thus, Haemophilus may be considered as a target for disease-modulating drugs in lung cancer treatment.
The lung microbiome affects lung immunity and metabolism; several studies have shown that these microbial niches are associated with the pathogenesis of COPD, asthma, cystic fibrosis, and lung cancer.[34–37] Similarly, our study found that the microbiome was related to lung cancer treatment and showed the possibility of devising a method to increase the therapeutic effect using these microbiomes.
This study has a few limitations. First, the daily diet and antibiotic use in the patients, which could have affected the microbial composition, were not investigated in this study. However, bronchoscopy for obtaining the BAL fluid was performed at the time of first diagnosis; therefore, the effect of antibiotics was considered to be minimal. Second, the microbiome differed from one group to another; however, its role in cancer development is unclear. Third, we could not collect data from healthy controls and follow-up BALF to compare microbial differences after cancer treatment.
Despite these limitations, this study was conducted prospectively using 16S rRNA sequencing analysis, which is more sensitive and more informative than the conventional methods. More accurate and elucidating results can be obtained in large-scale studies.