Study of function and regulation of Klebsiella Pneumoniae virulence factors and their effects on host

Abstract

As a notorious bacterial pathogen, Klebsiella pneumoniae (Kp) is responsible for causing a spectrum of diseases in the host organism and is considered a major pathogen for nosocomial pneumonia and sepsis. The recent emergence of a single Kp strain carrying genetic elements encoding both phenotypic carbapenem-resistance and hypervirulence poses a major public threat. To identify novel clinical targets for Kp infections, it is vital to gain a deeper understanding of its virulence factors and how they contribute to the pathogenesis of Kp.

A phenotypic characteristic of hvKp is hypermucoviscosity (HMV), and the expression of the HMV phenotype and the biosynthesis of capsular polysaccharide (CPS), which is composed of repeating glycan polymers and formed a dense layer surrounding the bacterial cell in Kp were reported to be encoded by genes located in the chromosomal rmp locus. Both rmpD and rmpD2 contributed to the HMV phenotype, while rmpD2 is more adapted among various capsule types. RmpC contributed to CPS overproduction in K1 and K2 strains, while not in ST11/KL64 strain. Furthermore, we proposed a logistic molecular basis of the HMV phenotype of Kp that prmpD2-mediated HMV is attributed to the increase of cell-free CPS production. Our data confirmed that the rmp homologues carried by the virulence plasmid play a key role in the virulence of Kp, but the phenotype is highly dependent on the genetic background of the host strain. We also showed that the HMV and overproduction of CPS contribute to the immune evasion of Kp. Our results indicated that pRmpD/D2, which regulates HMV is the major contributor of Kp escaping from being captured by macrophages.

CPS is among the most important virulence factors of Kp. Here, we demonstrated that CPS is a major chromosomal determinant of bacterial virulence, and the inability to produce CPS reduces the virulence potential of Kp of different capsule types to the same level, regardless of the effect of the genetic background and other chromosomal virulence factors. We further demonstrated that CPS is the main cellular component that elicits the host immune response to Kp as CPS enables this pathogen to survive for a prolonged period under adverse environmental conditions. We found that CPS prevents Kp from phagocytosis but is indispensable for the intracellular survival of Kp. We also found that phagocytosis of Kp is partially mediated by LOX-1, a scavenger receptor of the host, and that CPS may impede interaction between LOX-1 and the pathogenic bacteria, therefore impairing the phagocytosis process. These findings provided insights into the pathogenesis mechanisms of the important clinical pathogen and should facilitate the design of new strategies to combat Kp infections.

Evidence gathered by our group illustrates that the high mortality of hvKp infections is attributable to cytokine storm evoking in a STAT1-dependent way. We first consolidated that hvKp indeed induces robust cell death, including pyroptosis, apoptosis, and necroptosis (PANoptosis). Such cell death was compromised in Stat1^-/- cells, indicating the overall regulatory role of this gene in Kp-induced PANoptosis. Consistently, the high lethality and pathogenic disorders observed during hvKp infections were attenuated in Stat1^-/- mice, which further proved that STAT1-dependent PANopsome activation contributed to the high mortality of hvKp.

Colibactin, which was first reported in Escherichia coli, was also identified in Kp, which belongs to the K1 type. We found that colibactin contributes to the genotoxicity of Kp and is required for the full virulence and invasiveness of Kp. The absence of colibactin impaired the ability of Kp to colonize the gut and compete with other microbes.

Collectively, this study provided a novel understanding of virulence factors and pathogenic phenotypes of Kp, deciphering their contribution in escaping and defending against host immune responses. We also demonstrated the participation of the JAK-STAT pathway in combating Kp infections, as a detrimental regulatory factor in programmed cell death pathways. Altogether, our data extended the knowledge boundary of Kp pathogenesis and provided new insights into the therapeutic intervention of this infection.

Date of Award	3 Jul 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Guan YANG (Supervisor) & Sheng CHEN (External Co-Supervisor)