Klebsiella pneumoniae hijacks the Toll-IL-1R protein SARM1 in a type I IFN-dependent manner to antagonize host immunity

In vivo single-cell high-dimensional mass cytometry analysis to track the interactions between Klebsiella pneumoniae and myeloid cells

In vivo single-cell approaches have transformed our understanding of the immune populations in tissues. Mass cytometry (CyTOF), that combines the resolution of mass spectrometry with the ability to conduct multiplexed measurements of cell molecules at the single cell resolution, has enabled to resolve the diversity of immune cell subsets, and their heterogeneous functionality. Here we assess the feasibility of taking CyTOF one step further to immuno profile cells while tracking their interactions with bacteria, a method we term Bac-CyTOF. We focus on the pathogen Klebsiella pneumoniae interrogating the pneumonia mouse model. Using Bac-CyTOF, we unveil the atlas of immune cells of mice infected with a K. pneumoniae hypervirulent strain. The atlas is characterized by a decrease in the populations of alveolar and monocyte-derived macrophages. Conversely, neutrophils, and inflammatory monocytes are characterized by an increase in the subpopulations expressing markers of less active cells such as the immune checkpoint PD-L1. These are the cells infected. We show that the type VI secretion system (T6SS) contributes to shape the lung immune landscape. The T6SS governs the interaction with monocytes/macrophages by shifting Klebsiella from alveolar macrophages to interstitial macrophages and limiting the infection of inflammatory monocytes. The lack of T6SS results in an increase of cells expressing markers of active cells, and a decrease in the subpopulations expressing PD-L1. By probing Klebsiella, and Acinetobacter baumannii strains with limited ability to survive in vivo, we uncover that a heightened recruitment of neutrophils, and relative high levels of alveolar macrophages and eosinophils and the recruitment of a characteristic subpopulation of neutrophils are features of mice clearing infections. We leverage Bac-CyTOF-generated knowledge platform to investigate the role of the DNA sensor STING in Klebsiella infections. sting-/- infected mice present features consistent with clearing the infection including the reduced levels of PD-L1. STING absence facilitates Klebsiella clearance.

Characteristics of Immunocytes and Cytokines in Patients with Bloodstream Infections Caused by Carbapenem-Resistant Klebsiella pneumoniae in China

Objective

To evaluate the characteristics of immunocytes and cytokines associated with bloodstream infections (BSIs) caused by carbapenem-resistant Klebsiella pneumoniae (CRKP).

Methods

Patients with BSIs K. pneumoniae (BSIs-Kpn) were enrolled in our hospital between 2015 and 2022. Whole blood and serum samples were collected on the first day after diagnosis. Immunocytes and cytokines profiles were assessed using multicolor flow cytometry and multiplex immunoassays, respectively. The test cytokines included interferon-gamma (IFN-γ), tumor necrosis factor-alpha (TNF-α), interleukin (IL)-2, IL-4, IL-6, IL-10, and IL-17A.

Results

A total of 313 patients had BSIs-Kpn, including 145 with CRKP, 43 with extended-spectrum β-lactamases (ESBL) producing Kpn (ESBL-Kpn) and 125 with non-CRKP or non-ESBL-Kpn (susceptible Kpn, S-Kpn). Absolute number of leukomonocyte (CD45+) in CRKP, ESBL-Kpn and S-Kpn were 280.0 (138.0-523.0) cells/μL, 354.5 (150.3-737.3) cells/μL, and 637.0 (245.0-996.5) cells/μL, respectively. Compared with S-Kpn group, the absolute numbers of leukomonocyte (including T lymphocytes, B lymphocytes and natural killer cells) in patients with CRKP were significantly lower than that in patients with S-Kpn (P < 0.01). The levels of cytokines IL-2 and IL-17A were significantly higher in patients with S-Kpn than in those patients with CRKP (P<0.05). The area under receiver operating curve (AUC) of IL-2, IL-4, and IL-17A for S-Kpn was 0.576, 0.513, and 0.561, respectively, whereas that for the combination of these three cytokines with immunocytes was 0.804.

Conclusion

Patients with BSIs-CRKP had lower leukomonocyte counts. High levels of IL-2 and IL-17A combined with immunocyte subpopulations showed relatively high diagnostic value for BSIs-S-Kpn from BSIs-CRKP.

Klebsiella pneumoniae manipulates human macrophages to acquire iron

Background

Klebsiella pneumoniae (KP) is a major cause of hospital-acquired infections, such as pneumonia. Moreover, it is classified as a pathogen of concern due to sprawling anti-microbial resistance. During infection, the gram-negative pathogen is capable of establishing an intracellular niche in macrophages by altering cellular metabolism. One factor critically affecting the host-pathogen interaction is the availability of essential nutrients, like iron, which is required for KP to proliferate but which also modulates anti-microbial immune effector pathways. We hypothesized, that KP manipulates macrophage iron homeostasis to acquire this crucial nutrient for sustained proliferation.

Methods

We applied an in-vitro infection model, in which human macrophage-like PMA-differentiated THP1 cells were infected with KP (strain ATCC 43816). During a 24-h course of infection, we quantified the number of intracellular bacteria via serial plating of cell lysates and evaluated the effects of different stimuli on intracellular bacterial numbers and iron acquisition. Furthermore, we analyzed host and pathogen specific gene and protein expression of key iron metabolism molecules.

Results

Viable bacteria are recovered from macrophage cell lysates during the course of infection, indicative of persistence of bacteria within host cells and inefficient pathogen clearing by macrophages. Strikingly, following KP infection macrophages strongly induce the expression of the main cellular iron importer transferrin-receptor-1 (TFR1). Accordingly, intracellular KP proliferation is further augmented by the addition of iron loaded transferrin. The induction of TFR1 is mediated via the STAT-6-IL-10 axis, and pharmacological inhibition of this pathway reduces macrophage iron uptake, elicits bacterial iron starvation, and decreases bacterial survival.

Conclusion

Our results suggest, that KP manipulates macrophage iron metabolism to acquire iron once confined inside the host cell and enforces intracellular bacterial persistence. This is facilitated by microbial mediated induction of TFR1 via the STAT-6-IL-10 axis. Mechanistic insights into immune metabolism will provide opportunities for the development of novel antimicrobial therapies.

Modulation of innate immunity in airway epithelium for host-directed therapy

Innate immunity of the mucosal surfaces provides the first-line defense from invading pathogens and pollutants conferring protection from the external environment. Innate immune system of the airway epithelium consists of several components including the mucus layer, mucociliary clearance of beating cilia, production of host defense peptides, epithelial barrier integrity provided by tight and adherens junctions, pathogen recognition receptors, receptors for chemokines and cytokines, production of reactive oxygen species, and autophagy. Therefore, multiple components interplay with each other for efficient protection from pathogens that still can subvert host innate immune defenses. Hence, the modulation of innate immune responses with different inducers to boost host endogenous front-line defenses in the lung epithelium to fend off pathogens and to enhance epithelial innate immune responses in the immunocompromised individuals is of interest for host-directed therapy. Herein, we reviewed possibilities of modulation innate immune responses in the airway epithelium for host-directed therapy presenting an alternative approach to standard antibiotics.