Macrophages originate from distinct developmental pathways during embryogenesis and continue to adapt and renew throughout life (Gautier et al., 2012) as summarized in Figure 1. Macrophage origin, migration, and development have high similarity between humans and mice (Guan et al., 2025). In early embryonic development, macrophages arise from the yolk sac independently of hematopoietic stem cells (HSCs). These yolk sac-derived macrophages, such as microglia in the brain, establish in tissues and persist throughout adulthood without significant input from circulating monocytes. This discovery has overturned the previous dogma of macrophage ontogeny, revealing that tissue-resident macrophages are not exclusively derived from bone marrow progenitors as previously believed (Mosser et al., 2021; Ginhoux and Jung, 2014). A second wave of progenitors is erythro-myeloid precursors (EMPs) originating from the yolk sac. These EMPs subsequently migrate to the developing fetal liver, where they give rise to macrophages that seed tissues such as the liver and spleen (Bian et al., 2020).

In contrast to yolk sac-derived macrophages (like microglia), which are largely self-renewing, these populations have a greater dependence on circulating monocytes for maintenance (Hirayama et al., 2017). Postnatally, HSCs in the bone marrow continuously produce monocytes, which can develop into macrophages in response to local signals, especially in high-turnover tissues like the gut or during inflammatory conditions (Gordon and Plüddemann, 2017). The tri-lineage origin, comprising the yolk sac, EMPs, and HSCs, demonstrates the complexity of macrophage development and emphasizes their specialized roles in tissue homeostasis. Tissue-resident macrophages maintain their populations primarily through self-renewal under steady-state conditions, whereas monocyte-derived macrophages are recruited and contribute to inflammation or injury (Guan et al., 2025).

Macrophages exhibit incredible plasticity and functional diversity, enabling them to adapt to the needs of their microenvironments. This plasticity enables them to perform diverse functions, such as pathogen clearance, tissue repair, immune surveillance, and metabolic regulation (Chi et al., 2025). Macrophages exhibit a range of tissue-specific roles, resulting in a variety of different phenotypes. For instance, the Kupffer cells in the liver regulate lipid metabolism and detoxify harmful substances (Murray and Wynn, 2011). Similarly, alveolar macrophages in the lungs play an important role in maintaining surfactant homeostasis and providing defense against microbes (Hirayama et al., 2017). Macrophages in the bone, known as osteoclasts, are responsible for bone matrix resorption, and microglia in the brain are involved in neural development and maintenance. In general, these tissue-specific functions are driven by local cues, such as cytokines, metabolites, and growth factors, which initiate different transcriptional pathways (Gordon and Plüddemann, 2017). Macrophages demonstrate remarkable adaptability induced by inflammatory stimuli, which dictates their phenotype. For instance, classically activated macrophages (M1) are characterized by the production of pro-inflammatory cytokines and molecules that help combat invading microbial pathogens. On the contrary, alternatively activated macrophages (M2) produce anti-inflammatory cytokines and growth factors that promote the tissue repair process (Luo et al., 2024), as shown in Figure 2, which depicts the polarization of macrophages. However, this traditional paradigm oversimplifies the wide and dynamic phenotypic spectrum of macrophage activation states observed in-vivo, which are influenced by the specific microenvironment (Mosser et al., 2021).

Reducing macrophages to their immune functions alone ignores and overshadows their ancillary biological role. Furthermore, macrophages play a major role in host defense, together maintaining tissue integrity and physiological homeostasis (Mosser et al., 2021). The cellular debris, immune complexes, apoptotic cells, pathogens, and other waste materials are scavenged by macrophages to prevent any disruption in organ function. This proficient phagocytic activity highly facilitates the recycling of cellular components while supporting tissue health (Hirayama et al., 2017). Macrophages also play a significant role in key metabolic pathways, including the turnover of iron, Ca²⁺, lipids, and amino acids, and bilirubin, upholding the maintenance of homeostatic levels of these molecules (Kosteli et al., 2010; Biswas and Mantovani, 2012; Ganz, 2016; Roodman, 1999; Suda et al., 1992). However, the growth factors or signaling molecules secreted by macrophages govern tissue remodeling, wound repair, organogenesis, angiogenesis, and metabolic regulation. Therefore, the multi-angled activities highlight macrophages’ dual role in host defense and tissue homeostasis (Brancewicz et al., 2025; Ferrante and Leibovich, 2012; Guan et al., 2025).

Exclusively, the Colony-Stimulating Factors (CSFs) play a pivotal role in macrophage homeostasis, balancing survival, proliferation, and differentiation processes. Macrophage CSF1 and its receptor (CSF1R) are essential for the survival and self-renewal of macrophages. More importantly, brain tissue-resident macrophages, i.e., microglia, possess an alternative ligand for CSF1R (i.e., IL-34), which has been reported to be important to the localized maintenance of tissue-resident macrophages (Xiang et al., 2023). The identity and plasticity of macrophages are additionally confirmed by transcriptional regulators like PU.1, the master transcriptional factor controlling the expression of macrophage-specific genes (Wynn et al., 2013). However, transcription factors such as MafB and GATA6 are reported to be important for the tissue-specific differentiation of macrophages (Hamada et al., 2020). Consequently, the transcriptional networks involving these factors allow macrophages to rapidly respond and adapt to physiological changes. In fact, in the brain and liver tissues with minimal macrophage turnover rate, there exists a self-renewal mechanism to replenish the resident macrophage population (Röszer, 2018). Conversely, in the gut tissues with a high macrophage turnover rate, continuous replenishment of macrophages happens from monocyte-derived precursors to restore their numbers (Viola and Boeckxstaens, 2021). Therefore, this remarkable adaptability of tissue-specific macrophage homeostasis has been lacking in other similar immune cell types.

Macrophage survival is closely associated with their ability to detect and respond to environmental signals. Cytokines like IL-10 and Transforming Growth Factor beta (TGF-β) provide anti-apoptotic signals that promote macrophage survival, particularly during tissue repair and the resolution phase of inflammation. Additionally, these cytokines also skew the macrophages toward anti-inflammatory phenotypes, ensuring that they support tissue homeostasis (Yan et al., 2024). Metabolic reprogramming is important for macrophage survival, where the classically activated (M1) macrophages rely predominantly on glycolysis to maintain their pro-inflammatory nature, while the alternatively activated (M2) macrophages use oxidative phosphorylation and fatty acid oxidation to fulfil their reparative activities. These distinct metabolic changes allow macrophages to meet the energy and biosynthetic needs of their functional roles (Wang et al., 2022). Efferocytosis, the process by which macrophages phagocytize apoptotic cells, also provides survival signals. By removing cellular debris, macrophages not only prevent tissue damage but also obtain nutrients and growth factors that bolster their homeostatic functions. This process is important for maintaining tissue integrity and alleviating inflammation (Cabrera and Makino, 2022).

Calcium signaling has a direct influence on cytokine production in macrophages. The calcium-sensing receptor (CaSR) is widely expressed in tissues such as monocytes and macrophages. IL-1β and IL-6 upregulate the CaSR and also alter systemic calcium concentrations. These cytokines suppress parathyroid hormone secretion and decrease 1,25-dihydroxyvitamin D levels, which contribute to hypocalcemia in critically ill patients, including those with sepsis. The substantial increase in CaSR expression induced by TNF-α may function as a protective response to inflammation. This mechanism has been demonstrated in murine macrophages, where lipopolysaccharide-induced TNF-α release upregulated CaSR and subsequently inhibited TNF-α synthesis. Altogether, these mechanisms demonstrate the bidirectional relationship between inflammatory processes and calcium homeostasis (Hendy and Canaff, 2016).

Furthermore, antigen binding to immunoreceptors, including the TCR and BCR, the Fcγ and Fcϵ receptors, G protein–coupled chemokine receptors, and some innate pattern-recognition receptors such as Dectin-1, results in the depletion of calcium stores that stimulate store-operated calcium entry through CRAC channels (Feske et al., 2015). In macrophages, studies have demonstrated the involvement of calcium/calmodulin-dependent protein kinase in the pyrimidinoceptor-mediated potentiation of iNOS induction in mouse J774 macrophages (Chen et al., 1998). Research indicates that the rise in [Ca²⁺]_i in response to LPS treatment induces ROS generation in murine macrophages (Jin et al., 2006). [Ca²⁺]_i governs various functions, including phagocytosis and perhaps phagosome-lysosome fusion.

Phagocytosis, a primary function of macrophages, depends on precise regulation of calcium signaling. Several transcription factors have been identified whose activity is regulated by calcium-activated signaling pathways (West et al., 2001). A notable example is the adenosine 3′,5′-monophosphate (cAMP) response element-binding protein (CREB), which plays important roles in cell survival, proliferation, and differentiation. Its significance in immune function is increasingly recognized. CREB plays an important role in macrophage homeostasis by limiting proinflammatory response and inducing antiapoptotic survival signals in macrophages (Wen et al., 2010). Activation of CREB restricts the intracellular trafficking of M. tuberculosis to the phagolysosome by inhibiting MLKL phosphorylation. Therefore, CREB activation constitutes a key mechanism by which M. tuberculosis evades the immune response in human macrophages (Leopold Wager et al., 2023).

The P2X4 receptor (P2X4R), another endolysosomal ATP-gated calcium channel, contributes to macrophage activation and inflammatory responses. P2X4R expression increases during early phagocytosis in alveolar macrophages, suggesting a role in initiating calcium-driven phagocytic signaling. It modulates IL-1β and IL-18 release via inflammasome activation, linking lysosomal Ca²⁺ ion fluxes to proinflammatory cytokine production (Alharbi and Parrington, 2021). While there have been studies on calcium and calcium channels in macrophages, limited research has been done regarding its role with respect to bacterial infection. Both TRPML and P2X4 Ca²⁺ ion channels are regulators of calcium signaling in macrophage-mediated immune functions. TRPML1-mediated Ca²⁺ release drives phagosome maturation and phagosome-lysosome fusion, processes that engulf and degrade pathogens in macrophages. Knockout of TRPML1 reduces the ability of bone marrow–derived macrophages (BMDMs) to phagocytose large particles, highlighting its requirement for efficient phagocytosis. TRPML2-dependent calcium signaling regulates the release of CCL2/MCP1, promoting macrophage recruitment and migration during inflammation. Figure 3 gives an overview of calcium homeostasis and reported macrophage effector functions, with persisting gaps in host-pathogen interactions.

Efficient antigen processing and presentation by macrophages require regulated calcium fluxes. Delvig and Robinson demonstrated that the major histocompatibility complex (MHC) class II-restricted antigen processing in murine macrophages depends on both intracellular [Ca²⁺]_i and extracellular Ca²⁺ ion concentration. Their experiments with bacterial epitopes, such as the M5 protein from Streptococcus pyogenes, indicated that the calcium fluxes mediated by thapsigargin-sensitive calcium channels and gadolinium-sensitive pathways are crucial for antigen processing. Disruption of intracellular calcium fluxes impaired this process, thereby underscoring the importance of calcium homeostasis in immune activation (Delvig and Robinson, 1999).

K. pneumoniae takes advantage of calcium signaling pathways to evade immune responses. In a murine peritonitis model, peritoneal macrophages from wild-type mice exhibited enhanced SOCE via TRPC1 channels following infection. This influx activates M1-specific pathways, including STAT1 and NF-κB p65 phosphorylation, while boosting expression of inflammatory mediators (e.g., NOS2, TNF-α, IL-6, IL-23, CXCL9/10) and maturation markers (MHC-II, CD80/86). Conversely, TRPC1 deficiency impairs these responses, resulting in elevated bacterial burdens in the peritoneum, liver, and blood (Chauhan et al., 2018). In another study of K. pneumoniae strains HLJ-D2 and HB-AF5 infection in bovine mammary epithelial cells (bMECs), mitochondrial calcium concentration significantly increased at 1, 3, 6, and 12 hours post-infection when compared to controls contributing to mitochondrial damage and pathogenesis of clinical mastitis (Cheng et al., 2021). However, K. pneumoniae facilitates calcium and magnesium homeostasis with DedA family proteins, including YqjA and YghB. These proteins are essential for membrane potential and cation transport, which help in capsule synthesis and resistance to phagocytosis. Therefore, mutant strains that lack these proteins exhibit reduced calcium uptake and increased susceptibility to immune clearance (Tiwari et al., 2024).

Direct causal links between specific Ca²⁺ dynamics and macrophage fate outcomes, such as phagocytosis efficiency, intramacrophage bacterial killing, or apoptosis resistance, are largely absent in Klebsiella infection, with existing studies confined to mouse models and lacking human THP-1/PBMC validation, which is needed for clinical translation. Insights into some of these mechanisms can be inferred from studies of other pathogens. For example, S. aureus alters calcium fluxes in macrophages, which disrupts cytokine production and inhibits phagosome-lysosome fusion. Inhibiting extracellular calcium influx or intracellular calcium release changes macrophage response, indicating that different calcium sources distinctly regulate immune cell functions (Verma et al., 2020). Similarly, M. tuberculosis increases CACNA1S expression, an L-type voltage-gated calcium channel, to manipulate host calcium signaling. This channel enables calcium influx via the MyD88-independent TLR pathway involving pCREB, STIM1, and STIM2. Subsequently, the upregulation of CACNA1S results in suppression of protective innate immune responses, such as phagolysosome fusion and apoptosis promoting immune evasion (Antony et al., 2015). However, the disruption of macrophage calcium homeostasis by K. pneumoniae has not yet been experimentally well established. Therefore, investigating the calcium regulation in macrophages upon K. pneumoniae infection might open new horizons in the near future to combat the pathogen by developing next-generation therapeutics.

Upon engagement of TLRs, either of the two signaling pathways-the MyD88-dependent and TRIF-dependent- associates with a characteristic of TLR2 and TLR4 triggering by microbial pathogens. The TLR2/4 triggering recruits MyD88, and associates with IRAK4/IRAK1 (Deguine and Barton, 2014; Pereira et al., 2022), subsequently activating TRAF6 (Pereira et al., 2022). This initiates downstream activation of TAK1, resulting in phosphorylation of MAPKs and activation of NF-κB (Deguine and Barton, 2014). Finally, promoting the transcription of proinflammatory cytokines like IL-1, IL-6, and TNF-α. On the contrary, the TRIF-dependent pathway, activated primarily by TLR3 and TLR4, recruits TRIF, which interacts with TRAF3 and ultimately activates IRF3, leading to the production of type I interferons (IFNs) (Hu et al., 2015). The integration of MyD88 and TRIF pathways, particularly in TLR4 signaling, enables a more comprehensive immune response against diverse pathogens (Pereira et al., 2022).

The MyD88 pathway usually activates NF-κB instantly but only for a short while, whereas the TRIF pathway activates NF-κB and IRF3 more slowly and for longer, leading to the production of both inflammatory cytokines and Type I IFN (Hu et al., 2015). Certainly, the activation of two pathways induces the production of inflammatory cytokines and fights against microbial pathogens, as depicted in Figure 5. Likewise, NLRP3 and NLRC3 of the NLR family of receptors influence inflammatory responses of macrophages through the development of the inflammasome. NLRP3 activation leads to the formation of the NLRP3 inflammasome, which has ASC and caspase-1 within it, which in turn activate IL-1β and IL-18 (Ding et al., 2024; Shao et al., 2015; Yao et al., 2024). This pathway triggers pyroptosis, a type of cell death that causes inflammation and helps eliminate pathogens by destroying their niche (Shao et al., 2015). Conversely, NLRC3 acts as a negative regulator by inhibiting TRAF6-mediated NF-κB activation and inflammasome assembly, thus preserving macrophage homeostasis (Eren et al., 2017; Shao et al., 2015).

The functional responses to the signaling in macrophages are disparate and diverse. The DAMPs and PAMPs, when recognized by the PRRs, lead to the production of antimicrobial peptides (AMPs) like defensins and cathelicidins (Basu et al., 2012). This disrupts the integrity of microbial membranes, thereby eliminates the pathogens. TLR stimulation induces the generation of Reactive Oxygen Species (ROS) and Reactive Nitrogen Species (RNS) through NADPH oxidase and Inducible nitric oxide synthase (iNOS), which have significant efficacy against numerous bacterial species (Canton et al., 2021; Wang et al., 2019). The formation of mitochondrial ROS, enabled by the activation of the TRAF6-ECSIT complex, enhances antimicrobial defenses and functions as a secondary messenger in cytokine signaling (Silwal et al., 2020). The TLR signaling cascade also promotes the release of chemokines and proinflammatory cytokines like TNF-α, IL-6, and IL-1β to the site of infection for effective pathogen clearance. For instance, upon recognizing mycobacteria, TLRs prompt macrophages to produce IL-12 and TNF-alpha. TNF-alpha production during MTB infection has been primarily due to TLR2-triggering (Underhill et al., 1999; Flynn and Chan, 2001).

Furthermore, CXCL1 and CCL2 two important cytokines, attract neutrophils and monocytes to the site of infection for an effective immune response against the invading pathogens (T. Liu et al., 2017; Boro and Balaji, 2017). Actively withholding the nutrients is an alternative mechanism employed by macrophages to effectively eliminate the invading pathogens. This process is termed nutrient sequestration, also known as nutritional immunity. Restricting the availability of iron for bacterial growth and virulence, is one such mechanism. Similarly, manganese sequestration has been achieved through the action of natural resistance-associated macrophage protein 1 (NRAMP1), which pumps manganese out of the phagosome (Kehl-Fie and Skaar, 2010; Wessling-Resnick, 2015). In conclusion, though macrophages possess multiple mechanisms to eliminate pathogens, many intracellular pathogens have evolved sophisticated immune evasion strategies that inhibit macrophage cell death, thereby sustaining the macrophages as a niche for their survival and replication. Therefore, a complete understanding of macrophage cell death and survival mechanisms upon bacterial infection appears to be more essential.

Necrosis, an unregulated form of cell death, occurs due to acute injury. Numerous changes occur during necrosis, such as cellular swelling, membrane rupture, and discharge of intracellular contents, which activate an immune response (Tonnus et al., 2019). In contrast, necrosis is not caspase-mediated; rather, it is frequently reported in instances involving inflammation beyond the boundaries of the tissue area. K. pneumoniae causes necrosis of HepG2 cells through ROS generation, membrane rupture, and secretion of endogenous ligands, i.e., DAMPs, that exacerbate tissue damage. By destabilizing cellular integrity and promoting lysosomal enzyme release, K. pneumoniae amplifies inflammation, disrupts hepatocyte function, and facilitates bacterial spread (Yang et al., 2012).

Pyroptosis is an inflammatory form of RCD that occurs due to infection or cellular stress, which leads to the formation of pores in the plasma membrane by proteins known as gasdermins (Yu et al., 2021). It is characterized by the production of pro-inflammatory cytokines, such as IL-1β and IL-18, that boost immunity. K. pneumoniae activates the NLRP3 and NLRC4 inflammasomes in bone-marrow-derived macrophage cells, leading to caspase-1 activation and IL-1β production. K. pneumoniae strains like A28006 induce robust pyroptosis, releasing lactate dehydrogenase (LDH) and pro-inflammatory mediators that promote bacterial clearance. In contrast, the A54970 strain of K. pneumoniae suppresses inflammasome activation and pyroptosis by enhancing IL-10 production, which inhibits caspase-1 activity and IL-1β secretion, facilitating bacterial survival and dissemination. Consequently, pyroptosis raises the bacteria’s vulnerability to microbicidal agents such as hydrogen peroxide, underscoring its protective role in host defense (Codo et al., 2018).

Apoptosis is a programmed form of cell death for maintaining tissue homeostasis and involves the systematic degradation of cellular components through caspase activation. It is characterized by cell shrinkage, chromatin condensation, and apoptotic body formation, occurring through intrinsic or extrinsic pathways. Neisseria gonorrhoeae induces macrophage apoptosis primarily via the mitochondrial pathway, modulating pro-apoptotic and anti-apoptotic factors such as Bax and Bcl-xL (Dell’Annunziata et al., 2024). Additionally, K. pneumoniae-derived Outer membrane vesicles (OMVs) strengthen this pro-apoptotic effect by increasing the expression of Bax and Bim, while decreasing Bcl-xL, as well as disrupting the mitochondrial membrane and releasing cytochrome c during infection of HepG2 cells. The entire process involves the activation of Apaf-1, followed by the activation of caspase-9. However, K. pneumoniae has the ability to delay apoptosis of immune cells like neutrophils by modulating the Bax/Bcl-2 ratio and increasing Mcl-1 expression, which inhibits caspase-3 activation. Therefore, the pathogen exists inside cells over an extended period of time, avoiding immune defense mechanisms, whereas exacerbation of apoptosis has been reported under certain circumstances, such as with hypervirulent strains. Thus, K. pneumoniae inhibits efferocytosis, thereby escalating secondary necrosis, inflammation, and bacterial dissemination (Yang et al., 2012).

Autophagy is a major cellular pathway that is fundamentally based on the degradation of cytosolic components packed into autophagosomes, which in turn fuse with lysosomes for their degradation. Autophagy activation by the PI3K-AKT-mTOR pathway is enabled by bacterial degradation during K. pneumoniae infection of Caenorhabditis elegans (Kamaladevi and Balamurugan, 2017). Although the pathogen subverts this process by recruiting Rab14 to block phagosome maturation and Atg7 to prevent inflammation. These strategies are well documented, but the molecular details of how K. pneumoniae exploits host autophagy machinery remain incomplete. However, not much is known about how specific virulence factors and host receptors regulate autophagy modulation during infection. Furthermore, there is an important need to connect autophagy with other immune responses, such as cytokine production and cell death. These gaps confirm the need for further detailed investigation studies to unravel the manipulation of autophagy by K. pneumoniae (Wei et al., 2023). Autophagy is a mechanism used by macrophage for resistance against K. pneumoniae infection. Research shows that K. pneumoniae can activate ATG7, which is in turn involved in autophagosomal membrane elongation, triggering autophagy. The atg7 KO mice are more susceptible to K. pneumoniae infection, have reduced bacterial clearance, enhanced lung injury, and decreased survival rates. The mechanism involves ATG7 competitively inhibiting the interaction between p-IκBα and ubiquitin, reducing p-IκBα ubiquitination, which promotes NF-κB translocation to the nucleus, and increasing inflammatory damage (Li et al., 2025).

Ferroptosis is a distinct form of iron-dependent regulated cell death driven by the accumulation of lipid peroxides to lethal levels within the cells (Dixon et al., 2012). This phenomenon has been associated with disrupted iron homeostasis, oxidative stress due to increased ROS, and suppression of antioxidant status, especially through glutathione peroxidase 4 (GPX4) (Xie et al., 2016). As an emerging area of study, ferroptosis has been investigated for its role in various diseases, offering potential therapeutic avenues. In bacterial and viral infections, ferroptosis displays diverse interactions. For example, an opportunistic pathogen, Pseudomonas aeruginosa causes ferroptosis in bronchial epithelial cells through the production of 15-hydroperoxy-AA-PE via its lipoxygenase enzyme, pLoxA (Sadikot et al., 2005). The RNS produced by host macrophages can, in turn, neutralize this process and inhibit phospholipid peroxidation.

M. tuberculosis infection induces ferroptosis in host macrophages by decreasing GPX4 levels and increasing lipid peroxidation, ROS, and free iron concentrations. Although iron chelation and ferroptosis inhibitors such as ferrostatin-1 can mitigate these effects, Mycobacterium employs secreted proteins to neutralize ROS, thereby complicating its interaction with ferroptosis pathways (Amaral et al., 2019). In viral infections, particularly those caused by SARS-CoV-2, ferroptosis is an important factor in disease progression (Xia et al., 2021). SARS-CoV-2 disrupts iron metabolism, resulting in iron overload and excessive ROS production, while also suppressing GPX4 expression. These alterations accelerate lipid peroxidation and cell death. Therapeutic strategies, including selenium supplementation and iron chelation, have demonstrated potential to reduce the damage associated with SARS-CoV-2-induced ferroptosis (Wang et al., 2021).

The involvement of ferroptosis in K. pneumoniae infections appears to vary by cell type. In bovine mammary epithelial cells, infection with K. pneumoniae triggers classical ferroptosis, characterized by iron accumulation, increased lipid peroxidation, enhanced ROS levels, and a suppression of the Nrf2/xCT/GPX4 antioxidant pathway, ultimately resulting in epithelial injury and inflammatory responses. The use of ferrostatin-1 to inhibit ferroptosis restores antioxidant signaling and reduces inflammation, emphasizing ferroptosis as a major contributor to tissue damage during mastitis (Mao et al., 2025). Conversely, in a mouse model of pulmonary infection, carbapenem-resistant hypervirulent K. pneumoniae inhibits ferroptosis-related pathways in macrophages by upregulating SLC7A11 and glutathione, which restricts iron-dependent oxidative stress and promotes the survival of bacteria within cells. Interestingly, reactivation of lipid peroxidation related to ferroptosis in macrophages improves bacterial clearance without causing significant macrophage death (Yu et al., 2025). Recent macrophage infection models further support this distinction by demonstrating modulation of iron-dependent oxidative stress pathways. These include alterations in glutathione metabolism, lipid peroxidation, and ferroptosis-associated regulators such as SLC7A11. Such findings suggest the presence of ferroptosis-associated redox signaling rather than the execution of classical ferroptotic cell death. Collectively, these results underscore a context-dependent role for ferroptosis in K. pneumoniae pathogenesis. Epithelial ferroptosis appears to contribute to tissue injury, whereas suppression of ferroptosis-associated oxidative mechanisms in macrophages promotes immune evasion and intracellular persistence. Figure 6 summarizes the inflammatory and non-inflammatory forms of cell death. Although apoptosis, autophagy, necrosis, and pyroptosis are well-established contributors to macrophage responses during K. pneumoniae infection, the precise contribution of ferroptosis in immune cells remains incompletely understood and requires further investigation.

The host cells utilize the mechanism of autophagy for their survival upon infection and manage pathogen elimination, regulate inflammation, and maintain cellular homeostasis (Bah and Vergne, 2017). This mechanism allows macrophages to eliminate intracellular pathogens by a process called xenophagy (Gomes and Dikic, 2014), or discard damaged organelles via mitophagy (Youle et al., 2014), and further modulate immune signaling pathways. Autophagy has been triggered by multiple events that stimulate innate immune signaling to fulfill dual protective functions: eliminating inflammatory causes and curtailing excessive immune activation. The above interaction facilitates efficient pathogen elimination while reducing uncontrolled inflammation (Chauhan et al., 2022). Thus, the interplay between autophagy and inflammation is crucial for maintaining cellular homeostasis during infection and innate immune activation.

Autophagy alleviates inflammasomal activation by eliminating damaged mitochondria through mitophagy (Wang S. et al., 2023). Dysfunctional mitochondria emit ROS and disperse their mitochondrial DNA (mtDNA), which are powerful activators of the NLRP3 inflammasome. NLRP3 inflammasome has been reported for its host defense against multiple bacterial invasions (Kelley et al., 2019). In P. aeruginosa infections, the deficiency of Atg7 in macrophages leads to the buildup of mtDNA and increased release of IL-1β, inducing pyroptosis (Pu et al., 2017). Furthermore, autophagy directly degrades inflammasome components through proteins like TRIM20 and Immunity-Related GTPase family M (IRGM). These proteins interact with inflammasome subunits, such as NLRP3 and AIM2, inducing autophagic destruction (Traver et al., 2011). Additionally, autophagy governs immune responses by influencing cytokine production and macrophage polarization (Wu and Lu, 2019). M2 macrophages lacking Atg5 exhibit a proinflammatory transformation to M1-like phenotype characterized by elevated production of TNF-α and IL-6. These findings highlight autophagy as a pivotal regulator in modulating pro- and anti-inflammatory macrophage states.

It is imperative that pathogens develop novel strategies that sabotage host surveillance and elimination mechanisms. The intracellular defense mechanisms, like production of ROS, RNS, phagosome-lysosome fusion and degradation pathways, must be evaded successfully for intracellular survival (Colombo, 2005). To circumvent xenophagy, pathogens have developed strategies to avoid their detection. For example, L. monocytogenes synthesizes ActA and InlK proteins that attract host actin and major vault protein and thus camouflages under the host proteins and thereby inhibits detection by autophagy receptors (Dortet et al., 2012). Francisella tularensis synthesizes O-antigen polysaccharides that protect the bacterium from cytosolic sensors, thus evading autophagic targeting (Jessop et al., 2025). Pathogens can suppress autophagy induction as seen in S. typhimurium infection. The bacteria stimulate the mTOR pathway through FAK/AKT signaling and degrade AMPK, hence inhibiting autophagy (Ganesan et al., 2017). Likewise, M. tuberculosis employs host miRNA miR-30c-1-3p to suppress the production of ATG4B and ATG9B, thereby obstructing autophagosome biogenesis (Peng et al., 2025). Some pathogens utilize the autophagic route as a site for replication. Brucella melitensis stimulates the production of autophagosomes to create Brucella-Containing Vacuoles (BCVs), facilitating bacterial multiplication (Qin et al., 2024). Similarly, Coxiella burnetii utilizes the effector protein Cig2 to facilitate the fusion of autophagosomes with its vacuole, thereby enlarging the replicative compartment and enhancing bacterial viability (Kohler et al., 2016).

Macrophages employ diverse cell death pathways during K. pneumoniae infection, forming a unified model where autophagy serves as the primary defense mechanism, while K. pneumoniae strategically shifts the balance toward inflammatory lysis (pyroptosis/necroptosis) and away from immunologically silent apoptosis to evade clearance, thereby amplifying pathogenesis. Autophagy, driven by ATG7 activation, promotes KP degradation via autophagolysosome formation, but K. pneumoniae evades this by PI3K–AKT–mTOR/Rab14-mediated xenophagy, allowing intracellular replication. K. pneumoniae preferentially induces pyroptosis (NLRP3/NLRC4/caspase-1/11–GSDMD) and necroptosis, which release IL-1β/IL-18 and DAMPs to recruit neutrophils while compromising membrane integrity for bacterial escape; deficiencies in these pathways (NLRP3, caspase-11, NLRC4, IL-1R1) increase mortality by impairing early phagocytic killing, highlighting their net protective role despite pathology. Conversely, K. pneumoniae suppresses apoptosis via CPS-mediated Bax/Bcl-2 modulation and Mcl-1 upregulation (primarily in neutrophils, with macrophage implications), preventing PS exposure and efferocytosis to favor dissemination (Li et al., 2025).

Genetic and functional defects in autophagy compromise the macrophages’ capacity to eliminate intracellular infections. Studies have shown that conditional knockout of Atg5 in myeloid cell lineages resulted in increased M. tuberculosis burden. Similarly, the deficiency of Atg7 in alveolar macrophages worsens P. aeruginosa sepsis due to impaired mitophagy (Wang et al., 2025; Pu et al., 2017). The absence of autophagy leads to unregulated inflammasome activation and increased inflammatory damage. Ultimately, a defect in autophagy leads to increased metabolic depletion of macrophages. Notably, the accumulation of impaired mitochondria and misfolded protein aggregates depletes energy reserves, which further impair phagocytic function and reduce cellular viability upon infection or food deprivation (Tabibzadeh, 2023). Autophagy in macrophages functions at the intersection of antimicrobial defense, inflammatory management, and cellular homeostasis and is involved in eradicating intracellular infections. However, it is often exploited and undermined by many pathogens (Colombo, 2005; Peng et al., 2025; Qin et al., 2024; Kohler et al., 2016).

Intracellular pathogens prevent macrophage death to promote their replication. For example, the obligate intracellular pathogen C. burnetii releases effectors that prevent mitochondria-dependent apoptosis and also inhibit pyroptosis through IcaA-mediated suppression of caspase-11. It is also reported that NLRP3/caspase-1activation is observed due to cytosolic LPS exposure. On the contrary, Legionella pneumophila utilizes its T4SS for prolonged activation of NF-κB signaling to increase pro-survival factors such as BCL-2 and SidF. Likewise, Leishmania subverts macrophage apoptosis by activating the host PI3K/AKT pathways, which leads to the phosphorylation of BAD, further promoting cell survival (Chow et al., 2016). However, pathogens have developed intricate strategies to counteract autophagy, leading to a dynamic interaction that influences infection outcomes. Unraveling how pathogens exploit macrophage survival to ensure their own persistence promises novel avenues.