Kupffer cell

Kupffer cells are specialized, tissue-resident macrophages that line the sinusoids of the liver, serving as the body's primary fixed phagocytic cells and comprising approximately 80–90% of all tissue macrophages while making up about 20% of the liver's non-parenchymal cell population.^[1]^[2] First described in 1876 by Karl Wilhelm von Kupffer and later confirmed as macrophages by Tadeusz Browicz, these stellate or amoeboid-shaped cells are characterized by their elongated processes, microvilli, and indented nuclei, enabling them to adhere to the endothelial lining of hepatic sinusoids.^[1] Primarily originating from embryonic yolk sac progenitors during fetal development, Kupffer cells exhibit long-term self-renewal potential and are maintained independently of bone marrow-derived monocytes under steady-state conditions, though the latter can replenish them during injury or depletion.^[2]^[3] In healthy livers, Kupffer cells play essential roles in immune surveillance and homeostasis by phagocytosing pathogens, gut-derived endotoxins, damaged erythrocytes, and cellular debris from the portal bloodstream, thereby preventing systemic spread of microbes and maintaining hepatic integrity.^[1]^[2] They also contribute to antigen presentation and the release of cytokines, chemokines, and growth factors, balancing pro-inflammatory (M1-like) and anti-inflammatory (M2-like) responses to support tissue repair and lipid metabolism.^[3] Heterogeneity among Kupffer cells, including subsets like CD206^loESAM^- (immune-focused) and CD206^hiESAM⁺ (metabolism-focused) populations, allows them to adapt to zonal differences within the liver lobule, with higher concentrations in periportal areas.^[3] In disease states, Kupffer cells exhibit dual functions: they exacerbate liver injury through reactive oxygen species (ROS) production, tumor necrosis factor-α (TNF-α) secretion, and promotion of fibrosis in conditions like non-alcoholic steatohepatitis (NASH), viral hepatitis, and ischemia-reperfusion injury, yet they also facilitate resolution by secreting interleukin-10 (IL-10) and matrix metalloproteinases.^[2]^[3] Their activation is implicated in alcoholic liver disease, where they process endotoxins via Toll-like receptor 4 (TLR4), and in hepatocellular carcinoma (HCC), where they can either suppress tumor growth through immune modulation or promote progression via angiogenesis factors.^[1]^[2] Therapeutically, targeting Kupffer cells—such as through depletion or repolarization—holds promise for treating liver fibrosis, transplantation rejection, and sepsis, underscoring their pivotal role in hepatic immunity.^[3]

Anatomy and Morphology

Location in the Liver

Kupffer cells are specialized macrophages that primarily reside within the lumen of the hepatic sinusoids, lining the vascular walls and extending pseudopodial processes into the bloodstream to monitor and interact with circulating elements.^[1] They adhere firmly to the underlying sinusoidal endothelial cells, which form the discontinuous barrier of these low-pressure vessels in the liver lobules, allowing direct exposure to portal and systemic blood without impeding overall flow.^[4] Within the classic hepatic lobule architecture, Kupffer cells exhibit zonal heterogeneity along the porto-central axis. In the periportal zone (zone 1, near the portal triads), they are more abundant, larger in size, and display enhanced phagocytic capacity, reflecting their role in initial filtration of nutrient-rich portal blood.^[1]^[4] In contrast, centrilobular Kupffer cells (zone 3, adjacent to the central vein) are smaller and produce higher levels of superoxide anion, adapting to the oxygen-poor environment closer to hepatic venous outflow.^[1] This distribution contributes to a graded functional specialization across zones, with overall density approximately twofold higher in periportal regions.^[5] Kupffer cells are evenly distributed across the liver's anatomical lobes but collectively form a dynamic, selective barrier in the sinusoids, efficiently clearing particulate matter, bacteria, and debris from approximately 25% of the cardiac output passing through the liver daily while preserving sinusoidal patency.^[4] This positioning ensures comprehensive blood filtration without vascular obstruction, leveraging their strategic attachment and extensions for targeted surveillance.^[1]

Cellular Structure

Kupffer cells display an amoeboid, irregular stellate morphology characterized by elongated or bean-shaped nuclei and abundant cytoplasm, enabling their phagocytic capabilities within the hepatic sinusoids.^[1] Their plasma membrane features extensive surface projections, including microvilli, pseudopodia, filopodia, and lamellipodia, which extend into the sinusoidal lumen or the space of Disse to promote adhesion, motility, and particle capture.^[6] These structural adaptations allow Kupffer cells to bulge into the bloodstream while remaining anchored to the endothelial lining.^[7] At the ultrastructural level, Kupffer cells are equipped with a rich array of organelles essential for engulfing and degrading foreign materials. Prominent lysosomes, often comprising a significant portion of the cell's volume and staining positive for acid phosphatase, fuse with phagosomes to form phagolysosomes for enzymatic breakdown.^[1] A well-developed Golgi apparatus, along with rough endoplasmic reticulum, ribosomes, mitochondria, microtubules, and microfilaments, supports protein synthesis, energy production, and cytoskeletal dynamics.^[6] Additionally, endocytotic vesicles and worm-like structures contribute to the intracellular processing of captured particles.^[7] Surface receptor expression further defines their specialized structure, with scavenger receptors such as SR-AI/II prominently displayed on the plasma membrane to recognize and bind ligands like lipopolysaccharide (LPS) and lipoteichoic acid from bacterial sources.^[8] These receptors, present from early developmental stages, facilitate receptor-mediated endocytosis of non-opsonized particles.^[1] Ultrastructural features like ruffled membranes, sinuous invaginations, and bristle-coated pits enhance the efficiency of both phagocytosis and micropinocytosis, allowing rapid uptake of solutes and particulates from the blood.^[6]

Origin and Development

Embryonic Origin

Kupffer cells originate from erythro-myeloid progenitors (EMPs) in the yolk sac during early embryogenesis, representing the primitive wave of hematopoiesis that precedes definitive hematopoiesis in the fetal liver or bone marrow. These yolk sac-derived progenitors give rise to primitive macrophages that are the foundational population for tissue-resident macrophages, including those in the liver.^[9]^[10] These primitive macrophages differentiate independently of bone marrow contributions, relying on local embryonic cues for maturation into fetal liver macrophages. Differentiation occurs without input from hematopoietic stem cells (HSCs), which emerge later, ensuring the initial establishment of a self-sustaining macrophage network in the developing liver.^[11]^[12] The initial population of the liver sinusoids happens through bloodstream colonization, with progenitors migrating via the vitelline and umbilical veins to reach the fetal liver around the seventh to eighth week of gestation in humans. This trafficking peaks in a defined embryonic window, allowing EMPs and pre-macrophages to infiltrate and adhere to the sinusoidal endothelium, where they expand locally to form the nascent Kupffer cell population.^[13]^[12]^[9] Key transcription factors, such as PU.1 (encoded by Spi1) and C/EBPα, along with signaling pathways like CSF1/CSF1R, drive the early specification and commitment of these progenitors toward the macrophage lineage during this prenatal phase. PU.1 initiates myeloid priming, while C/EBPα supports granulocyte-monocyte differentiation, enabling the adaptation of yolk sac cells to the hepatic microenvironment.^[14]^[15]

Adult Maintenance and Renewal

In adult livers, Kupffer cells sustain their population predominantly through local self-renewal via proliferation, exhibiting minimal reliance on recruitment from bone marrow-derived circulating monocytes under steady-state conditions.^[16] This process ensures long-term tissue residency, with fate-mapping studies in mice revealing that over 90% of Kupffer cells maintain their embryonic origin without significant monocyte contribution even after extended periods.^[17] Such local proliferation is stochastic yet sufficient to replace dying cells, preserving the Kupffer cell pool without disrupting liver homeostasis. The renewal of Kupffer cells is tightly regulated by key growth factors, including colony-stimulating factor 1 (CSF1), which acts as the primary driver of their survival, differentiation, and proliferative capacity.^[16] Blockade of CSF1 receptor signaling impairs repopulation following depletion, underscoring its essential role in maintenance.^[16] Interleukin-4 (IL-4) provides an additional regulatory layer, promoting Kupffer cell expansion beyond baseline levels through direct signaling on resident macrophages, particularly in contexts requiring heightened numbers, while operating independently of CSF1 in some scenarios.^[18] Early estimates placed the average Kupffer cell lifespan at approximately 3.8 days based on thymidine labeling, but contemporary analyses indicate a much longer duration—potentially spanning months—consistent with their self-renewing nature.^[12] In scenarios of acute depletion, such as pharmacological elimination using clodronate-laden liposomes, Kupffer cell populations can recover through infiltration and differentiation of circulating monocytes, though this monocyte-dependent pathway is secondary to resident proliferation in non-pathological states.^[17] Renewal dynamics also display zonation-specific heterogeneity within the liver lobule, with Kupffer cells more abundant and potentially exhibiting varied proliferative responses in periportal zones (zone 1) compared to pericentral regions (zone 3), reflecting microenvironmental influences on local maintenance.^[19]

Physiological Functions

Phagocytosis and Clearance

Kupffer cells serve as the primary phagocytic cells in the liver, efficiently removing bacteria, cellular debris, and senescent red blood cells from the bloodstream through a combination of receptor-mediated endocytosis and macropinocytosis. Receptor-mediated endocytosis involves specific recognition via surface receptors such as Fc receptors, complement receptors (e.g., CR3), and scavenger receptors, which bind opsonized particles or pathogens, leading to their internalization into phagosomes that fuse with lysosomes for degradation. Macropinocytosis, a non-specific fluid-phase uptake mechanism, allows Kupffer cells to engulf larger volumes of extracellular fluid containing soluble debris or unbound microbes, facilitated by actin-driven membrane ruffling and pseudopodia formation. These processes are enhanced by the cells' strategic positioning in the liver sinusoids, where blood flow from the portal vein exposes them to high concentrations of potential threats.^[1]^[20]^[21] A critical aspect of Kupffer cell function is the clearance of senescent erythrocytes, which they achieve through phagocytosis, subsequently recycling the iron from hemoglobin to support systemic homeostasis. Upon engulfing effete red blood cells, Kupffer cells degrade hemoglobin within phagolysosomes using heme oxygenase-1 (HO-1), liberating ferrous iron that is either stored in ferritin or exported via ferroportin for reuse in erythropoiesis, meeting approximately 90% of the body's daily iron requirements under steady-state conditions. This process prevents iron overload and hemolytic complications, with the liver acting as the main site for on-demand erythrocyte disposal during stress.^[22]^[23] Kupffer cells also play a pivotal role in clearing microbial products, such as lipopolysaccharide (LPS) from gram-negative bacteria in portal blood, thereby averting systemic inflammation. LPS is recognized primarily via Toll-like receptor 4 (TLR4) and scavenger receptors, leading to rapid endocytosis and neutralization, with the liver filtering the majority of circulating LPS to maintain endotoxin tolerance. This high-capacity clearance prevents excessive immune activation while allowing controlled responses to infection.^[12]^[24]

Immune Response

Kupffer cells serve as key sentinels in the liver's innate immune system, rapidly detecting pathogen-associated molecular patterns (PAMPs) such as lipopolysaccharide (LPS) from Gram-negative bacteria. Upon recognition, they engage Toll-like receptor 4 (TLR4) to initiate signaling cascades, primarily through the NF-κB pathway, leading to the production of pro-inflammatory cytokines including tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), and interleukin-6 (IL-6).^[25] These cytokines amplify the inflammatory response by recruiting and activating other immune cells, such as neutrophils and monocytes, while also influencing endothelial permeability to facilitate leukocyte extravasation.^[25] This acute phase reaction is essential for containing infections but must be tightly regulated to prevent excessive tissue damage. Kupffer cells exhibit phenotypic plasticity, polarizing into distinct functional states based on environmental cues, which modulates their contribution to inflammation or resolution. Pro-inflammatory M1 polarization is driven by stimuli like LPS and interferon-γ (IFN-γ), activating pathways such as TLR4/NF-κB and JAK/STAT1, resulting in heightened cytokine secretion and antimicrobial activity.^[26] In contrast, anti-inflammatory M2 polarization occurs in response to interleukin-4 (IL-4) or IL-13, engaging JAK/STAT6 and PPARγ signaling to promote tissue repair, phagocytosis of debris, and production of immunosuppressive factors like IL-10 and transforming growth factor-β (TGF-β).^[26] This bidirectional polarization allows Kupffer cells to adapt dynamically, interacting with hepatic stellate cells and lymphocytes to either escalate or dampen immune responses. As professional antigen-presenting cells, Kupffer cells express MHC class II molecules and costimulatory factors, enabling them to process and present antigens to CD4+ T cells within the liver sinusoids. They interact closely with other liver immune cells, including natural killer T cells and dendritic cells, to orchestrate adaptive immunity while often favoring tolerogenic outcomes due to lower expression of costimulatory molecules like B7-1 and B7-2 compared to splenic macrophages.^[27] This presentation can suppress T cell proliferation via prostaglandin E2 (PGE2) secretion, which inhibits calcium mobilization and JAK3 signaling in T cells, thereby limiting effector responses.^[27]

Metabolic Roles

Kupffer cells contribute to hepatic lipid metabolism by expressing scavenger receptors, such as SR-A and CD36, that facilitate the uptake of modified lipoproteins, including oxidized low-density lipoprotein (oxLDL) and advanced glycation end-products.^[28] This process allows Kupffer cells to process cholesterol and export it to hepatocytes via high-density lipoprotein (HDL) particles, regulated by transcription factors like liver X receptor alpha (LXRa) and peroxisome proliferator-activated receptor gamma (PPARγ).^[28] In steady-state conditions, this uptake helps maintain lipid homeostasis by preventing excessive accumulation in the sinusoidal space.^[28] Kupffer cells influence glucose homeostasis primarily through cytokine production that modulates hepatic insulin sensitivity. Activated Kupffer cells secrete pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β), which inhibit insulin signaling pathways, including IRS-1/PI3K/Akt, leading to increased hepatic glucose output via gluconeogenic enzymes like PEPCK and G6Pase.^[29] This cytokine-mediated crosstalk underscores their role in linking inflammation to metabolic dysregulation.^[29] In iron metabolism, Kupffer cells coordinate storage and release with hepatocytes by phagocytosing senescent erythrocytes and recycling heme-derived iron through hemoxygenase-1 (HO-1), expressing ferroportin for export under normal conditions.^[28] Hepatocytes regulate this via hepcidin, which binds ferroportin on Kupffer cells to sequester iron during inflammation, preventing overload while maintaining systemic balance.^[28] This interplay ensures efficient iron homeostasis, with Kupffer cells handling approximately 80-90% of daily recycled iron.^[28]

Pathological Roles

Involvement in Liver Diseases

Kupffer cells play a central role in the pathogenesis of alcoholic liver disease (ALD) through their heightened sensitivity to endotoxins derived from gut bacteria, which translocate into the portal circulation due to alcohol-induced intestinal barrier dysfunction. This endotoxin-mediated activation, primarily via Toll-like receptor 4 (TLR4) signaling, triggers Kupffer cells to release pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α) and interleukin-1β (IL-1β), initiating a cascade of hepatic inflammation that progresses to steatosis, necrosis, and fibrosis.^[30] In chronic ALD, persistent Kupffer cell activation sustains oxidative stress and neutrophil recruitment, exacerbating tissue damage and contributing to the transition from alcoholic steatohepatitis to cirrhosis.^[31] In viral hepatitis, particularly hepatitis B and C, Kupffer cells often undergo M1 polarization in response to viral antigens and damage-associated molecular patterns (DAMPs) from infected hepatocytes, leading to amplified production of pro-inflammatory mediators, induction of interferon-gamma (IFN-γ) from NK and T cells, and reactive oxygen species (ROS). This M1-skewed phenotype promotes hepatocyte apoptosis and necrosis by enhancing cytotoxic T-cell responses and direct inflammatory assault, thereby accelerating liver injury in chronic infections.^[32] Similarly, in non-alcoholic steatohepatitis (NASH), lipid accumulation and lipotoxicity in the liver drive Kupffer cells toward M1 polarization through ligands like free fatty acids and TLR activation, resulting in elevated TNF-α and IL-6 secretion that fosters lobular inflammation and hepatocyte ballooning.^[33] This polarization imbalance disrupts the normal M2-mediated resolution, perpetuating a cycle of damage in NASH progression to fibrosis.^[34] In severe cases such as acute alcoholic hepatitis, resident Kupffer cells are often depleted or functionally impaired, leading to their replacement by infiltrating monocyte-derived macrophages recruited from the peripheral blood. These infiltrating cells, expressing markers like CCR2 and Ly6C, dominate the hepatic macrophage population and exhibit heightened pro-inflammatory activity, contributing to massive hepatocyte necrosis and systemic inflammatory response syndrome.^[35] Unlike steady-state Kupffer cells, these replacements amplify cytokine storms and impair tissue repair, underscoring their maladaptive role in acute decompensation.^[36] Kupffer cells contribute to liver fibrosis across various etiologies by producing transforming growth factor-beta (TGF-β), a key profibrogenic cytokine that activates hepatic stellate cells (HSCs) into myofibroblasts capable of excessive extracellular matrix deposition. Activated Kupffer cells, particularly in M1 states, upregulate TGF-β1 expression through pathways involving nuclear factor-kappa B (NF-κB) and ROS, directly stimulating HSC proliferation, migration, and collagen synthesis.^[37] This paracrine signaling forms a vicious cycle, as fibrotic remodeling further sensitizes Kupffer cells to activation signals, promoting advanced fibrosis and cirrhosis.^[38]

Contribution to Systemic Conditions

Kupffer cells play a pivotal role in exacerbating systemic inflammation during sepsis, primarily through the excessive release of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), which can precipitate a cytokine storm and contribute to multi-organ dysfunction.^[39] In experimental models of polymicrobial sepsis, Kupffer cell-derived TNF-α and interleukin-6 (IL-6) levels are markedly elevated, amplifying the systemic inflammatory response and promoting remote organ injury beyond the liver.^[40] Depletion of Kupffer cells in these models has been shown to attenuate hepatic and systemic inflammation, as well as apoptosis in affected tissues, highlighting their central contribution to the dysregulated immune cascade in septic conditions.^[41] In the context of metabolic syndrome and obesity, Kupffer cells contribute to the development of insulin resistance by altering lipid handling and promoting chronic low-grade inflammation. Activated Kupffer cells in obese states secrete factors that directly impair insulin signaling in hepatocytes and peripheral tissues, leading to reduced glucose tolerance and systemic metabolic dysregulation independent of hepatic lipid accumulation.^[42] For instance, pharmacological activation of peroxisome proliferator-activated receptor delta (PPARδ) in Kupffer cells shifts them toward an anti-inflammatory M2 phenotype, which ameliorates obesity-induced insulin resistance and improves lipid metabolism.^[43] Furthermore, selective depletion of Kupffer cells in high-fat diet-fed mice reduces systemic insulin resistance and inflammation while enhancing liver autophagy, underscoring their role in linking hepatic immune activation to broader metabolic dysfunction.^[44] Kupffer cells are integral to the immune defense against bloodstream infections, where hepcidin enhances their phagocytic capacity to clear circulating pathogens and sustain systemic iron homeostasis during infection. Hepcidin, produced primarily by hepatocytes but influencing Kupffer cell function, bolsters their ability to engulf and eliminate bacteria from the bloodstream, preventing dissemination and mitigating the severity of systemic infections.^[45] Additionally, Kupffer cells regulate thrombopoietin (TPO) production through their involvement in platelet clearance, thereby influencing systemic platelet homeostasis and megakaryocyte proliferation. In this process, Kupffer cells facilitate interactions between senescent platelets and hepatocytes, promoting TPO production and release from hepatocytes, maintaining circulating platelet levels and supporting the hematopoietic niche.^[46] This mechanism ensures adaptive responses to thrombocytopenia in systemic conditions like infection or inflammation, where platelet dynamics are critical for hemostasis and immune modulation.

Therapeutic Potential

Targeting Strategies

Kupffer cells, as resident macrophages in the liver, express specific surface receptors that enable selective targeting for therapeutic modulation of their activity. Nanoparticles designed to exploit these receptors, such as mannose-modified lipid nanoparticles, facilitate the delivery of anti-inflammatory agents like siRNA against HMGB1 directly to Kupffer cells, enhancing nucleic acid uptake and reducing inflammatory responses in liver diseases.^[47] Similarly, poly(lactic-co-glycolic acid) nanoparticles conjugated with Kupffer cell-targeting ligands, such as INT-777, a selective FXR agonist, encapsulate dexamethasone to achieve pH-responsive release in inflammatory environments, thereby suppressing cytokine production in models of alcohol-associated liver injury.^[48] These approaches leverage the phagocytic nature of Kupffer cells while minimizing off-target effects on other hepatic cells, as demonstrated by improved drug accumulation and reduced systemic toxicity in preclinical studies.^[49] Inhibition of intracellular pathways in Kupffer cells represents another precise strategy to dampen excessive inflammatory signaling. Enolase 1 (ENO1), a glycolytic enzyme upregulated in activated Kupffer cells, promotes proinflammatory cytokine production, such as TNF-α and IL-6, during liver inflammation. Blockade of ENO1 using small-molecule inhibitors or targeted siRNA attenuates this cytokine release, alleviating liver inflammation in experimental models such as hemorrhagic shock by disrupting glycolytic metabolism essential for macrophage polarization toward a proinflammatory phenotype.^[50] Liposome-based depletion offers a method to transiently eliminate or suppress hyperactive Kupffer cells, aiding both research and therapeutic applications. Clodronate encapsulated in liposomes is phagocytosed by Kupffer cells, leading to intracellular release of the bisphosphonate and subsequent apoptosis, which depletes up to 80-90% of these cells within 24-48 hours post-administration. This technique has been widely used to study Kupffer cell contributions to pathology and to enhance nanoparticle drug delivery by reducing clearance, with applications in mitigating hepatic inflammation during sepsis or ischemia-reperfusion injury.^[51]^[52] Receptor-targeted therapies focus on modulating Kupffer cell scavenger receptors to neutralize endotoxins like lipopolysaccharide (LPS). Scavenger receptor A (SR-A), highly expressed on Kupffer cells, binds LPS and initiates inflammatory cascades via TLR4 signaling; antagonists such as polyinosinic acid or neutralizing antibodies block this interaction, reducing NF-κB activation, cytokine secretion, and endotoxin-induced liver damage in rodent models.^[53]^[54]

Clinical Applications

Kupffer cells play a pivotal role in inducing immune tolerance during liver transplantation through their polarization to an M2 phenotype, which promotes anti-inflammatory responses and reduces acute rejection. In experimental models, inducing M2 polarization of Kupffer cells has been shown to ameliorate rejection by suppressing proinflammatory cytokine production and enhancing regulatory T-cell activity, suggesting potential for clinical strategies to improve graft survival and minimize immunosuppression needs.^[55] For instance, administration of interleukin-34 (IL-34) has demonstrated efficacy in shifting Kupffer cells toward M2 polarization, leading to prolonged allograft survival in rat models, with implications for human transplantation protocols. In the management of alcohol-induced liver injury, targeting Kupffer cells with antioxidants aims to mitigate oxidative stress and inflammation triggered by ethanol exposure. Nanoparticle-based delivery systems designed to specifically target Kupffer cells and deliver anti-inflammatory agents have shown promise in preclinical studies by reducing reactive oxygen species production and hepatic inflammation, offering a foundation for future clinical interventions in alcoholic liver disease.^[48] These approaches leverage the cells' phagocytic activity to localize therapy, potentially enhancing efficacy while minimizing systemic side effects in patients with chronic alcohol consumption.^[48] Kupffer cell modulation via anti-tumor necrosis factor (TNF) agents holds potential for reversing liver fibrosis by inhibiting proinflammatory signaling that drives extracellular matrix deposition. Agents like infliximab, which block TNF-α produced by activated Kupffer cells, have been investigated in preclinical models for fibrotic liver conditions, demonstrating reductions in inflammatory markers and improvements in steatosis and fibrosis.^[56] This targeted inhibition helps shift Kupffer cells away from a profibrogenic M1 state, supporting fibrosis regression as part of broader antifibrotic regimens.^[57] Kupffer cells enhance diagnostic imaging of the liver through their uptake of specific contrast agents in magnetic resonance imaging (MRI), enabling differentiation of benign and malignant lesions. Superparamagnetic iron oxide (SPIO) nanoparticles are phagocytosed by Kupffer cells in healthy liver tissue, causing signal loss on T2-weighted images and making non-Kupffer cell-containing tumors, such as metastases or hepatocellular carcinoma, appear bright for improved detection.^[58] This technique is routinely applied in clinical practice to characterize focal liver lesions, assess resectability in oncology patients, and monitor disease progression with high sensitivity.^[59] As of 2025, emerging strategies include targeting the C/EBPβ–VCAM1 axis in Kupffer cells to reduce hepatic inflammation in metabolic dysfunction-associated steatotic liver disease (MASLD).^[60]

History and Research

Discovery

Kupffer cells were first described in 1876 by the German anatomist Karl Wilhelm von Kupffer, who observed star-shaped "Sternzellen" (stellate cells) in the sinusoids of rabbit livers using a gold chloride staining technique under light microscopy.^[12] Initially, von Kupffer classified these cells as specialized endothelial cells lining the liver sinusoids.^[61] In 1898, von Kupffer refined his description, acknowledging the phagocytic properties of these stellate cells after further observations.^[62] However, it was the Polish pathologist Tadeusz Browicz who, in 1899, reclassified them as distinct macrophages based on their ability to engulf particulate matter, distinguishing them from endothelial cells.^[63] This reclassification highlighted their role in phagocytosis within the liver.^[64] The identification sparked naming debates, with the cells occasionally referred to as Kupffer-Browicz cells in recognition of both contributors, particularly in Polish literature.^[65] Early 20th-century investigations, notably by Karl Albert Ludwig Aschoff in 1924, incorporated Kupffer cells into the reticuloendothelial system as fixed macrophages resident in the liver sinusoids, though observations suggested limited migratory behavior along sinusoidal walls.^[66] These studies confirmed their status as a stable, tissue-specific population distinct from circulating monocytes.^[63]

Recent Advances

Recent studies from 2023 to 2025 have elucidated the dynamic replacement of Kupffer cells by infiltrating monocyte-derived macrophages (MoMFs) in acute liver injuries, such as severe alcohol-associated hepatitis (sAH). In sAH patients, single-cell RNA sequencing revealed that approximately 90% of liver macrophages are replaced by MoMFs, with Kupffer cell markers like MARCO, CD5L, and TIMD4 significantly downregulated, while MoMF markers such as TREM2, GPNMB, and SPP1 are upregulated, correlating with disease severity as measured by MELD scores.^[35] This replacement is driven by hepatocyte damage-induced CCL2 release, which recruits monocytes that displace resident Kupffer cells, leading to transitional C1Q+ macrophage subtypes exhibiting both inflammatory and phagocytic properties.^[35] Such shifts may exacerbate immunosuppression and infection risk, highlighting the adaptive yet maladaptive plasticity of the liver macrophage niche in acute hepatitis.^[35] In liver regeneration, Kupffer cells are regulated by factors including IL-4 and Wnt signaling, as detailed in 2025 reviews and studies. Interleukin-4 (IL-4) polarizes Kupffer cells toward an anti-inflammatory, reparative M2-like phenotype, upregulating Arg1 expression and reducing pro-inflammatory markers, which enhances hepatocyte proliferation through activation of the Wnt/β-catenin pathway in three-dimensional liver culture models.^[67] This IL-4-mediated reprogramming improves cellular viability and ATP production in precision-cut liver slices, from 340 nM/slice to 930 nM/slice in healthy tissue over five days, supporting tissue repair post-injury.^[67] Concurrently, Wnt signaling, particularly Wnt5a from monocytes and Wnt2/Wnt9b ligands from macrophages, facilitates neovascularization and Kupffer cell redistribution, orchestrating hepatocyte proliferation via interactions with endothelial cells during post-hepatectomy regeneration.^[62]^[68] Discoveries in 2025 have advanced understanding of Kupffer cell metabolic plasticity, linking their adaptations to non-alcoholic fatty liver disease (NAFLD) and obesity. In metabolic dysfunction-associated steatotic liver disease (MASLD), a KC2 subset (CD206high ESAM+) upregulates genes like CD36 for enhanced fatty acid uptake, contributing to steatosis and oxidative stress under high-fat diet conditions.^[69] Lipid overload depletes embryonic-origin Kupffer cells, prompting monocyte recruitment and differentiation via the Notch-RBPJ pathway, which alters cholesterol homeostasis and promotes pro-inflammatory responses that drive NAFLD progression to fibrosis.^[69] Suppressing KC2 activity in obesity models reduced weight gain and improved glucose tolerance, underscoring their role in inter-organ metabolic communication.^[69] Despite these advances, key unresolved gaps persist in Kupffer cell research, particularly regarding in vivo fate mapping and the complete monocyte-to-Kupffer cell differentiation process. In vitro models mimicking the liver microenvironment with factors like macrophage colony-stimulating factor and hepatocyte growth factor induce partial expression of Kupffer markers (e.g., Id3, Spic, Marco, Timd4) in monocyte-derived macrophages but fail to replicate the full phenotypic profile, even after three days of coculture.^[70] This indicates limitations in recapitulating niche-specific cues, leaving uncertainties about the precise ontogenic contributions and long-term maintenance of Kupffer cells in vivo.^[70] Future studies must address these through advanced lineage-tracing techniques to clarify differentiation trajectories and therapeutic targeting opportunities.^[69]