Intestinal Klebsiella pneumoniae infection expansion strategy clarified

It is becoming clear that intestinal bacteria are causative factors for various systemic diseases, including cancer and metabolic disorders, and that they also influence treatment responsiveness. In particular, remote inter-organ networks centered on intestinal bacteria are thought to contribute significantly to biological homeostasis. However, the formation of these networks is poorly understood. Research and development of disease prevention and therapeutic interventions targeting intestinal bacteria has not sufficiently progressed.

A joint research team, including Associate Professor Hitoshi Tsugawa of the Division of Host Defense Mechanism at the Tokai University School of Medicine and Professor Juntaro Matsuzaki of the Research Center for Drug Discovery at the Keio University Faculty of Pharmacy, has discovered that extracellular vesicles (KpEVs) secreted by the intestinal bacterium Klebsiella pneumoniae migrate from the digestive tract to the liver. There, they alter the phenotype of macrophages into an inflammatory, immunosuppressive type that has lost its ability to eliminate foreign substances. This promotes the infection of the liver by the bacteria and is also associated with liver cancer.

In other words, KpEVs form an immunosuppressive liver microenvironment that promotes both the colonization of K. pneumoniae in remote organs and liver carcinogenesis, revealing that they are promising therapeutic targets and diagnostic biomarkers for liver cancer and related liver diseases. The findings were published in the online edition of the Journal of Extracellular Vesicles.

The research team found that bacterial-derived tRNA fragments (tsRNA) exist in human serum, and their abundance is significantly higher in liver cancer patients compared to healthy individuals. They clarified that this tsRNA is encapsulated in KpEVs and efficiently delivered into host cells such as macrophages. This inhibits the production of nitric oxide (NO) by macrophages, reducing their ability to eliminate pathogens. Since NO production by macrophages is also a critical cancer immune response involved in eliminating cancer cells, the suppression of NO production via KpEV-mediated tsRNA delivery leads to a breakdown in cancer immunity.

As a result, it has become clear that serum tsRNA can serve as a new type of liver cancer biomarker that cannot be captured by conventional blood tests, which is expected to lead to the development of new diagnostic methods for earlier assessment of liver cancer risk. Furthermore, the researchers revealed that because KpEVs switch the macrophage phenotype to an immunosuppressive state favorable to the bacteria, K. pneumoniae evades the elimination response and establishes survival within the macrophages.

In addition to suppressing NO production through tsRNA delivery, KpEVs induce macrophages toward an M2-type with enhanced phagocytic capacity and increased production of inflammatory cytokines (IL-1β), while suppressing inflammatory programmed cell death (pyroptosis). These KpEV-induced inflammatory M2-type macrophages, which have lost their foreign-substance elimination capabilities, are a novel phenotype never seen before. The research term termed them "zombie macrophages."

The formation of zombie macrophages by KpEVs represents a collapse of cancer immunity and is thought to work in favor of the generation and progression of cancer cells. Furthermore, the team confirmed that KpEVs in the digestive tract accumulate in the liver via the bloodstream. In livers where KpEVs accumulated, M2-type macrophages increased, and the expression of the enzyme for producing NO (inducible nitric oxide synthase) was suppressed. In these livers, the transmission of K. pneumoniae from the intestinal tract to the liver increased significantly.

The research team clarified that K. pneumoniae sends KpEVs to the liver to turn liver macrophages into "zombies," remotely forming a microenvironment that facilitates its own expansion of infection. This KpEVs-based strategy of infection spread by K. pneumoniae is also associated with the development of liver cancer.

In this study, the team elucidated the mechanism by which K. pneumoniae-derived extracellular vesicles (KpEVs) hijack macrophages to form a pro-inflammatory, immunosuppressive communication link between the gut and the liver, allowing the bacteria to successfully expand its infection to the liver.

The strategy of spreading infection using KpEVs of K. pneumoniae is positioned as a gut-liver network of intestinal bacteria that can involve not only liver abscesses but also the development of liver cancer. This is a significant discovery that concretely links the biological behavior of intestinal bacteria, which has been overlooked until now, with disease onset. Furthermore, as the tsRNA encapsulated in KpEVs is detectable in the blood, there are high expectations for its application as a non-invasive biomarker for liver cancer risk and as an indicator of the bacterial-involved carcinogenic process.

Journal Information
Publication: Journal of Extracellular Vesicles
Title: Gut Commensal Klebsiella pneumoniae Extracellular Vesicles Shape a Liver Microenvironment Conducive to Gut-Liver Bacterial Translocation and Pro-Tumorigenic Processes
DOI: 10.1002/jev2.70262