K. pneumoniae is one of the most common floras causing hospital-acquired infections and lower respiratory tract infections in the intensive care units (Saharman et al., 2020). When K. pneumoniae invades host, the mechanical barrier provides the immediate protection. As the interface between the host and K. pneumoniae, the respiratory tract and its epithelial cells play an active role as a mechanical barrier. Adult microbiota activates the defense of upper respiratory tract through interleukin (IL)-17A, while K. pneumoniae could overcome this obstacle to establish colonization through encapsulation (Sequeira et al., 2020). The adhesion factors with various physiological functions are present in cell wall and other structures of K. pneumoniae (Na et al., 2014). The colonization of K. pneumoniae could damage small airway epithelial cells and increase the level of tumor necrosis factor (TNF)-α in lung, and the upregulation of TNF-α could significantly exacerbate epithelial cell injury (Xu and Xu, 2005).

Additionally, the mucus ciliary layer of respiratory tract can adhere to or remove the bacteria or other particles entering the respiratory tract, while changes in the thickness, properties and cilia clearance of mucus influence the dismissal of K. pneumoniae by respiratory tract. Lung infection with K. pneumoniae could lead to massive infiltration of inflammatory cells, resulting in a progressive decrease in local defenses (Zheng et al., 2014). The outer membrane protein A of Klebsiella pneumoniae (KPOmpA) affects the expression of adhesion molecules and the secretion of cytokine in bronchial epithelial cells (BECs). It has been proved that KPOmpA can bind tightly to human BEC cell line BEAS-2B and primary cultures of BECs, activating the nuclear factor kappa B (NF-κB) signal pathway, thus stimulating the host defense. In addition, BECs exert internalized clearance of K. pneumoniae that invades the respiratory tract (Pichavant et al., 2003).

Therefore, mucus and ciliated epithelial cells in the respiratory system can effectively hinder the invasion of Klebsiella and eliminate it in multiple ways. However, the infection of Klebsiella could trigger an inflammatory response in the respiratory tract, leading to the accumulation of inflammatory cells, which can disrupt the mechanical barrier of the ciliary layer of respiratory tract and disturb the host defense ( Figure 1 ).

Catheter-associated urinary tract infections (CAUTIs) is one of the most common nosocomial infections and complications of indwelling catheters (Maunders et al., 2022). K. pneumoniae is prone to UTIs through catheters, accounting for 2-6% of hospital UTIs (Li et al., 2014; Maunders et al., 2022). However, the mechanical force created by the flow of urine can remove pathogens normally, which acts as an essential barrier for the colonization of bacteria. Furthermore, the pH value of urine is a critical factor in the colonization and proliferation of pathogenic bacteria in the urinary tract, and the alteration of pH value may play an important role in the treatment and prevention of Klebsiella on UTIs (Yang et al., 2014; Wasfi et al., 2020). Meanwhile, the bladder smooth muscle activity could significantly increase the positive rate of Klebsiella in urine, which allows for flushing Klebsiella in vivo (Burnett et al., 2021). It is known that K. pneumoniae may adhere to the host cell surface with the help of various adhesion factors such as the K. pneumoniae MrkD adhesin, colonizing the host and causing infections (Jagnow and Clegg, 2003; Li et al., 2009). Fortunately mechanical forces such as urine activity, bladder contraction, and the alteration of pH value in urine are capable of weakening the colonization of K. pneumoniae ( Figure 2 ).

Studies have shown that the main anti-Klebsiella effect of digestive system comes from gut microbiota. The gut microbiota consists of diverse bacterial communities that perform various functions and influence the host’s overall health, including nutrient metabolism, immune system regulation and natural defense against infection (Al Bander et al., 2020). During the K. pneumoniae infection, there is a complex interaction between the host and gut microbiota.

Researches have shown that in the early stage of K. pneumoniae infection, the richness and composition of gut microbiota changes, especially the numbers of Lactobacillus reuteri and Bifidobacterium pseudolongum decrease significantly (Wu et al., 2020; Wolff et al., 2021). Among the gut microbiota, Bacteroidetes can strengthen the intestinal immune barrier through IL-36 and macrophages to prevent the colonization and transmission of K. pneumoniae (Sequeira et al., 2020).

Short-chain fatty acids (SCFA), fermentation products of intestinal flora, including acetic acid, butyric acid and propionic acid, play a pivotal role in resisting the colonization and inflammation of K. pneumoniae. Vornhagen et al. observed that SCFA could directly inhibit bacterial growth through intracellular acidification in a dose-dependent manner. SCFA also reduces epithelial oxygenation and stimulate the expression of antimicrobial peptides in the gut microbiota, thus weakening pathogen colonization. Further, SCFA affects intestinal homeostasis to induce gut microbiota to produce metabolites, thereby decreasing the fitness of K. pneumoniae lacking functional plasmid encoding tellurite TeO3-2-resistance (Ter) operons in the intestinal tract (Vornhagen et al., 2021). Another research also showed that the G protein-coupled receptor 43 (GPR43) combined with acetate could upregulate the activity of neutrophils and alveolar macrophages, which reduce the number of bacteria in the airway in the early stage of infection, and promote inflammation regression to reduce lung injury in the late stage of infection. These results indicate that GPR43 plays a significant role in the “gut–lung axis” as a sensor of the host gut microbiota activity. Increasing SCFA will probably be a new way to promote inflammation resolution in clinical practice (Galvão et al., 2018). Aside from that, butyrate and tryptophan decomposition metabolites are able to enhance gut integrity and stimulate innate lymphoid cells group 3 (ILC3) to produce IL-22. Gut microbiota also could reduce intestinal permeability and increase the epithelial defense mechanism to form a mucosal barrier. Therefore, they maintain the stability of the intestinal environment (Shi et al., 2017). In the case of liver abscess induced by K. pneumoniae, relevant studies have discovered that antibiotic treatment before K. pneumoniae infection weakens the protective effect of intestinal flora in mice. Surprisingly, after fecal transplantation, the concentrations of chemokine (C-X-C motif) ligand 1 protein (CXCL1), TNF-α, monocyte chemoattractant protein-1 (MCP-1), IL-1β, IL-6 and IL-17 in mice serum were recovered and liver injury was alleviated (Zheng et al., 2021).

In a word, gut microbiota and its metabolites is essential in K. pneumoniae colonization and inflammatory response ( Figure 3 ). Administration of exogenous SCFA could be sufficient to reduce fitness of K. pneumoniae. However, whether other substances also have impacts and how these microorganisms and metabolites interact with the host remains need to be further explored.

Dendritic cells (DCs) of the lung are situated in close proximity to alveolar epithelium and resident alveolar macrophages, playing a specific role as antigen-presenting cells (APCs) (Von Wulffen et al., 2007). There are several subtypes of DCs. Plasmacytoid DCs (pDCs) can produce interferon (IFN)-α and sense the damaged skin to heal wounds. CD103⁺ DCs, CD11b^hi DCs and monocyte-derived DCs (MoDCs) can act as migratory DCs to promote the activation of naïve CD4⁺ and CD8⁺ T cells in lymph nodes (Hackstein et al., 2012; Plantinga et al., 2013). Hackstein et al. discovered a rapid increase of activated CD103⁺ DC, CD11b⁺ DC and MoDC within 48 h post infection of K. pneumoniae. The K. pneumoniae-infected animals showed that in respiratory DC subpopulations there were elevated IFN-α in pDC, elevated IFN-γ, IL-4 and IL-13 in CD103⁺ DC and IL-19 and IL-12p35 in CD11b⁺ DC subsets in comparison to CD11c⁺ MHC-class II^low cells indicating distinct functional roles. CD103⁺ DC and CD11b⁺ DC subsets represented the most potent naïve CD4⁺ T helper cell activators in the infection model of K. pneumoniae (Hackstein et al., 2013) ( Figure 4 ). Therefore, the novel insight into the activation of respiratory DC subsets during K. pneumonia infection is provided.

Pulmonary macrophages are derived from monocytes, which mainly stimulate other immune cells acting as APCs and secreting immune molecules. The latest experiments showed that capsular polysaccharide (CPS) derived from carbapenem-resistant K. pneumoniae KN2 serotype can stimulate J774A.1 mouse macrophage to release TNF-α and IL-6 in vitro. The CPS also exerts an immune response through TLR4 in human embryonic kidney-293 (HEK-293) cells (Lee et al., 2022). Melissa and Kovach observed that the clearance rate of K. pneumoniae in IL-36γ-deficient mice was decreased and the mortality of the mouse was increased, which confirmed that IL-36γ is related to the anti-Klebsiella effect. Further, it is proved that pulmonary macrophages secreted IL-36γ in a non-Golgi-dependent manner, playing a critical role in innate mucosal immunity of lung (Kovach et al., 2016; Kovach et al., 2017). The chemokine-mediated transportation of mononuclear phagocytes also is essential in the defense against bacterial pneumonia. In the K. pneumoniae-infected mouse model, the deletion of chemotactic cytokines receptor 2 (CCR2) could reduce all the monocyte phagocyte subsets and change the phenotype of pulmonary macrophages, reducing the amount of M1 macrophages and TNF in lung (Chen et al., 2013). Myeloid but not neutrophil-specific hypoxia-inducible factor (HIF)-1α-deficient mice increased bacterial loads in the lungs and distant organs after infection of K. pneumoniae as compared to control mice, pointing to a role of HIF-1α in macrophages. What’s more, alveolar and lung interstitial macrophages from myeloid-specific HIF-1α-deficient mice produced a lower level of immunity, suggesting the importance of HIF-1α expressed in lung macrophages in protective innate immunity during pneumonia caused by K. pneumoniae (Otto et al., 2021).

Collectively, the researches above indicate that pulmonary macrophage is essential in innate immunity by secreting cytokine such as IL-36, TNF-α and IL-6 after K. pneumoniae infection, while CCR2 and HIF-1α play an auxiliary role in the anti-K. pneumoniae activity of macrophages ( Figure 4 ). These discoveries provide new ideas for the clinical treatment of pneumonia caused by Klebsiella infection.

Neutrophils are the first line against a variety of infectious pathogens. Neutrophils could kill pathogens by phagocytosis and neutrophil extracellular traps (NETs). NETs is one of the primary defensive mechanisms of neutrophils against carbapenemase resistant hypervirulent K. pneumoniae (CR-hvKp). Through the scanning electron microscope test, Jin et al. found that the NETs in type 2 diabetes patients had lost its smooth and regular shape, which may lead to the defection of congenital immune response for the patients against CR-hvKp. The study confirmed the direct killing effect of NETs to CR-hvKp (Jin et al., 2020). More recently researchers compared the concentration of cytokines in regular diet group of mice and high-fat diet group infected with K. pneumoniae, discovering that the concentrations of IL-1β, IL-6, IL-17, IFN- γ, CXCL2 and TNF-α were much lower in the high-fat diet group, meanwhile, the number of neutrophils was reduced, and the functions including the phagocytosis, killing ability and production of the reactive oxygen intermediates (ROI) were impaired significantly, which proved the critical role of neutrophils in anti-K. pneumoniae effect (Mancuso et al., 2022).

Further studies found that the expression of CXCL5 in IL-17-deficient epithelium decreased, while intranasal injection of recombinant CXCL5 in mice could restore neutrophils’ recruitment and bacterial clearance (Chen et al., 2016). It was also discovered that the number of neutrophils decreased and the production of leukotriene B₄ (LTB₄), reactive oxygen species (ROS) and reactive nitrogen species (RNS) decreased in CXCL1-/- mice infected with K. pneumoniae (Batra et al., 2012). Meanwhile, subsequent experiments on depleted neutrophils showed that neutrophils were the main source of LTB₄ in the lungs after infection (Batra et al., 2012). The above researches reveal the important role of CXCL1 in the expression of ROS and RNS produced by neutrophils, the regulation of host immunity to K. pneumoniae infection, and the curative effect of LTB₄ on the recruitment of neutrophils. In addition, more studies have shown that IL-33 can enhance host defense during bacterial pneumonia through the combined function of neutrophils and inflammatory monocytes (Ramirez-Moral et al., 2021).

Evidences in vitro and in vivo above indicate that neutrophils play an anti-K. pneumonia’s role mainly through NETs and secreting corresponding molecules such as LTB₄, ROS and RNS ( Figure 4 ). What’s more, the administration of CXCL5 and LTB₄ can restore the activity of neutrophils, providing a new direction for the clinical treatment of K. pneumoniae.

All T lymphocytes including T_H17 and γδT subsets participating in innate immunity derive from common lymphoid progenitor in bone marrow, differentiate and mature in thymus. γδ17 cell is a subset of γδT cells which produce large quantities of IL-17A in the presence of IL-23 and IL-1β. γδ17 cell expresses the type I IL-4R. And IL-4 signaling increases STAT6 phosphorylation in γδT cells. Whereas IL-4 inhibits the production of IL-17A by γδ17 cell. K. pneumoniae infection of STAT6 knockout mice shows a higher amount of γδ17 cell compared to that of wild-type mice, demonstrating that STAT6 signaling negatively regulates γδ17 cell that play a front-line role in mucosal immunity against K. pneumoniae (Bloodworth et al., 2016). Recently, Mackel et al. evaluated the role of T cells in protection against classical K. pneumoniae reinfection and demonstrated that mice lacking T cells were unable to establish a protective response. However, mice individually deficient in either of the major T cell subsets, γδ or αβ (classical T cells), effectively mounted a protective response, indicating either subset alone was sufficient to mediate protection against the reinfection of K. pneumoniae (Mackel et al., 2022) ( Figure 5 ). The researches above demonstrate the imperative contribution of innate T cells to protective immunity against classical K. pneumoniae and will guide further inquiries into host effector responses required to control K. pneumoniae infection.

Nature killer (NK) cells, which belong to innate lymphoid cells (ILCs), are found to recognize and eliminate “altered self” as cytotoxic lymphocytes, which also take part as the source of early inflammatory cytokines in the innate immune system (Pallmer and Oxenius, 2016; Myers and Miller, 2021). After activation, they secrete perforin and TNF to kill “allosome” substances nonspecifically. It was found that after infection with K. pneumoniae, the survival rate of IL-22^-/- mouse was lowered while the survival rate of Rag2^-/- mouse had no significant changes compared with wild-type mouse. Simultaneously, Rag2^-/-Il2rg^-/- mice failed to produce IL-22. NK cells and T cells may produce IL-22 and have conventional host defense against K. pneumoniae, which were confirmed with Rag2^-/-Il2rg^-/- C57BL/6 mice (Xu et al., 2014). Type I IFN receptor (Ifnar) 1-deficient mice infected with K. pneumoniae failed to activate NK cells to produce IFN-γ, which caused the weakening of NK cell killing effect. Meanwhile, exogenous IFN-γ can recover the level of IFN-γ in Ifnar1 ^fl/fl (Ifnar1^tm1Uka)-CD11c^Cre, Ifnar1 ^fl/fl-LysM^Cre and Ifnar1 ^fl/fl-MRP8^Cre mice on C57BL/6 background (Ivin et al., 2017). These data identify NK cell-intrinsic type I IFN signaling as essential driver of K. pneumoniae clearance, and reveal a specific target for future therapeutic exploitations ( Figure 5 ).

Innate immunity depends on signals produced by pattern recognition receptors (PRRs). Toll-like receptors (TLRs) is the earliest PRRs, which can recognize pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) in microorganisms (O’neill et al., 2013). When the ligand binds to TLRs, myeloid differentiation factor88 (MyD88) and Toll/IL-1R (TIR) domain-containing adaptor protein (TIRAP) are recruited into the TLR complex to activate MAPK and NF-κB signal pathways to produce cytokines and chemokines. This cascade reaction is called MyD88-dependent pathway. In the lung against K. pneumoniae TIRAP is a critical mediator of antibacterial defense (Jeyaseelan et al., 2006). Besides, the activation of TLRs also recruits other adapter proteins, such as TIR domain-containing adaptor-inducing IFN-β (TRIF) and TRIF-related adaptor molecule (TRAM). This pathway activates NF-κB and type I IFN, which is called TRIF-dependent pathway. Both TRIF-dependent and MyD88-dependent signaling contributes to host defense against pulmonary Klebsiella infection (Cai et al., 2009). The TLR-mediated innate immune responses control bacterial growth at the infection site, thus minimizing bacterial transmission. Currently, 12 TLRs from mice and 10 TLRs from human have been identified (Baral et al., 2014).

Among TLRs, TLR2 and TLR4 play important roles in K. pneumoniae infection. TLR2 transmits signal mainly by forming heterodimers with TLR1 or TLR6 to resist external pathogens. Meanwhile, TLR4 could induce host defense against gram-negative bacterial pulmonary infection by sensing bacterial LPS (Baral et al., 2014). Compared with wild type (WT) mice, Jeon et al. found that the survival time of TLR4 knock-out (KO) and TLR2/4 double KO (DKO) mice infected with 5×10³ CFU K. pneumoniae was significantly shortened. The mRNA levels of TNF-α, MCP-1 and inducible nitric oxide synthase (iNOS) in TLR2/4 DKO mice were substantially lower than those in the WT group, indicating that TLR2 and TLR4 play a synergistic role in innate immune response during K. pneumoniae infection (Jeon et al., 2017).

Meanwhile, by analyzing the gene expression profiles in the lung of C57BL/6 mice (resistant to bacterial transmission), 129/SVJ mice (susceptible), C3H/HeJ mice (susceptible and TLR4 signal deficient) and their respective control strains C3H/HeN mice (moderately resistant), it was found that the most significant number of TLR4-dependent induced genes were expressed in C57BL/6 and C3H/HeN mice after infection with K. pneumoniae. These genes include cytokines and chemokine genes needed for neutrophil activation or recruitment, growth factor receptors, MyD88 and adhesion molecules. The results indicated that in the early stage of infection, the TLR4 signal controlled the expression of most genes in lung to cope with gram-negative bacterial infection (Schurr et al., 2005).

At the same time a variety of TLRs expressed on DCs, such as TLR9, can trigger the cascade signal response of proinflammatory cytokines, leading to the production of TNF-α, IL-12 and other proinflammatory cytokines in large quantities to resist the invasion of K. pneumoniae (Bhan et al., 2007; Von Wulffen et al., 2007). Interestingly, though the initiation of most TLRs depends on MyD88, Adam et al. discovered that the inflammatory response induced by K. pneumoniae does not depend on MyD88 in lung epithelial cells and platelets (De Stoppelaar et al., 2015; Anas et al., 2017).

Collectively, TLR2 and TLR4 signaling could improve the levels of TNF-α, MCP-1, iNOS and other proinflammatory cytokines to indirectly eliminate the bacteria during the early stage of K. pneumoniae infection ( Figure 6 ).

Nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) are classical pattern recognition receptors highly expressed in first-reactive cells such as neutrophils, macrophages and DC cells. They can regulate various signal pathways, including MyD88 and TLRs containing adaptor molecular 1-dependent pathways. Besides these regulatory effects, some NLRs also are assembled into polymer protein complexes called inflammasome (Ravi Kumar et al., 2018; Ghimire et al., 2020).

Nucleotide-binding oligomerization domain, leucine- rich repeat and pyrin domain-containing (NLRP) is one category of NLRs. Studies have shown that NLRP6 and NLRP12 can act as a negative regulator of the NF-κB and mitogen-activated protein kinase (MAPK) signal pathways to attenuate intestinal inflammation during the infection of K. pneumoniae (Ghimire et al., 2020). NLRP6 and the adaptor protein, apoptosis-associated speck-like protein (ASC) -mediated inflammasome activation are thought to shape the composition of the commensal gut microbiota, controlling the gut microbiota and the immune response to systemic and intestinal infections (Elinav et al., 2011). However, the finding was challenged by later research (Mamantopoulos et al., 2017). It is meaningful that during the infection of K. pneumoniae, NLRP6 gene-deficient mice show the low levels of neutrophil recruitment, CXC chemokine and granulocyte factor (Cai et al., 2021).

Furthermore, bone marrow-derived DCs (BMDCs) lacking NLRP12 could induce the production of TNF-α and IL-6 (Allen et al., 2013; Tuladhar and Kanneganti, 2020). It was found that intratracheal injection of IL-17A⁺ CD4 T cells or CXCL1⁺ macrophages could prolong the survival of Nlrp12 ^-/- mice and recruit neutrophils to eliminate K. pneumoniae. It was revealed that the NLRP12-IL-17A-CXCL1 axis in vivo could play a vital role in removing extracellular bacteria by recruiting neutrophils (Cai et al., 2016). And another study found that IL-17 can upregulate the expression of CCR2 ligands CCL2 and CCL7, promoting the recruitment of neutrophils and enhancing the anti-bacterial activity in C57BL/6 mice (Xiong et al., 2015).

NLRC4, another inflammasome, is also essential for the clearance of K. pneumoniae and neutrophil-mediated lung inflammation (Xiong et al., 2015; Xiong et al., 2016). Macrophage-derived NLRC4 can induce the production of IL-1 and IL-17A from NK cells and γδT cells in lung, activating NF-κB and MAPK signal pathways to regulate the production of neutrophil chemokine, TNF-α, the expression of intercellular cell adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) in lung homogenates, which could recruit neutrophils and hinder the colonization of K. pneumoniae (Cai et al., 2012).

These evidences revealed that NLRP6 and NLRP12 could act as negative regulators of NF-κB and MAPK signal pathways to deregulate the inflammation caused by K. pneumoniae. Meanwhile, NLRs prevent the colonization of K. pneumoniae by recruiting neutrophils ( Figure 6 ).