The concept of the pre-metastatic niche dates back to Stephen Paget’s 1889 seed and soil hypothesis, which proposed that metastasis depends on compatible interactions between cancer cells and the organ-specific microenvironment they colonize. Modern research has refined this idea considerably. Primary tumors release soluble factors such as TGF-β and VEGF, along with extracellular vesicles carrying molecular cargo, that travel through the bloodstream and condition distant organs. Bone marrow-derived cells are recruited to these sites, local stroma is remodeled, and vasculature is altered. The result is a patch of tissue that is inflamed, immunosuppressed, and metabolically reprogrammed — fertile ground for any tumor cell that arrives. The liver, because of its unique anatomy and immunological profile, is particularly susceptible to this kind of preparation.

What makes the liver special is its blood supply. Approximately 75 percent of hepatic blood arrives via the portal vein, which drains directly from the intestines. This means the liver is continuously bathed in gut-derived nutrients, microbial products, and metabolites. Under normal conditions, this arrangement sustains immune tolerance and metabolic homeostasis. But in tumor-bearing hosts, the gut microbiota undergoes profound compositional shifts, and the signals traveling through the portal vein transition from physiological to pathological. The review identifies this transition as the primary instigator of hepatic pre-metastatic niche formation.

The evidence for tumor-associated gut dysbiosis is now extensive across multiple cancer types. In colorectal cancer, a landmark multi-omics analysis by Yachida and colleagues of 616 individuals undergoing colonoscopy demonstrated that microbial and metabolomic shifts are detectable even at the stage of multiple polypoid adenomas and intramucosal carcinomas. Fusobacterium nucleatum was continuously elevated from intramucosal carcinoma through more advanced stages, while Atopobium parvulum and Actinomyces odontolyticus were enriched specifically in early lesions. Metabolomically, branched-chain amino acids and phenylalanine were elevated in intramucosal carcinoma, and bile acids including deoxycholate accumulated in adenomas. A meta-analysis by Wirbel and colleagues across eight geographically distinct cohorts confirmed consistent enrichment of Fusobacterium nucleatum and Porphyromonas asaccharolytica alongside depletion of butyrate-producing taxa such as Roseburia and Eubacterium. Critically, these dysbiotic profiles differ between non-metastatic and metastatic disease, with metastatic patients showing enrichment of Pyramidobacter, Christensenellaceae_R-7_group, and Romboutsia.

The pattern extends well beyond colorectal cancer. In breast cancer, shotgun metagenomic sequencing of fecal samples from 38 patients and 36 matched controls revealed depletion of beneficial taxa including Limosilactobacillus fermentum and Blautia species, enrichment of Prevotella copri, and reduced short-chain fatty acid and purine metabolism pathways. A machine learning model integrating microbial and metabolic features achieved an area under the curve of 0.78 in the discovery cohort and 0.73 in an independent validation cohort. In non-small cell lung cancer, compositional shifts were pronounced across TNM stages, with Bacteroides predominating in stage T2, Prevotella in stage T4, and Ruminococcus gnavus elevated in stages N3 and M. Gastric cancer displays progressive dysbiosis with pathogenic Fusobacteria and Bacteroides caccae increasing with disease progression while Bifidobacterium longum is depleted. In pancreatic ductal adenocarcinoma, a fecal microbiota signature with high specificity has been defined across multi-center cohorts, and the microbiota-derived tryptophan metabolite indole-3-acetic acid has been shown to influence chemotherapy response through neutrophil myeloperoxidase-dependent reactive oxygen species metabolism.

Once these dysbiotic signals reach the liver, they are sensed and amplified by a complex cellular network. Hepatocytes, which constitute 60 percent of hepatic cells and 80 percent of total liver volume, are the first parenchymal cells exposed to gut-derived factors via the portal vein. Research has shown that fatty liver upregulates hepatocyte Rab27a, promoting secretion of extracellular vesicles enriched with specific microRNAs that suppress LATS2 in colorectal cancer cells, activating YAP-CYR61 signaling and promoting M2-like macrophage infiltration alongside CD8-positive T-cell exhaustion. Colorectal cancer-derived small extracellular vesicles can also induce TGFβ1-mediated epithelial-mesenchymal transition in hepatocytes, generating a pro-fibrotic and pro-tumorigenic hepatic milieu. Loss of hepatocyte metabolic defense, through knockout of prohibitin 1 or methionine adenosyltransferase α1, markedly increases hepatic metastasis in experimental models.

Liver sinusoidal endothelial cells, which comprise approximately 50 percent of liver non-parenchymal cells, serve as the first barrier to gut-derived molecules and circulating tumor cells. During pre-metastatic niche formation, tumor-derived factors and inflammation upregulate adhesion molecules including ICAM-1, promoting tumor cell arrest and transmigration. Blocking ICAM-1 reduced liver tumor retention by 50 percent and metastatic burden by 75 percent in experimental models. Cancer-derived extracellular vesicles carrying AMIGO2 further enhance tumor-endothelial adhesion. Metabolic liver disease and inflammation induce LSEC defenestration and capillarization, and steatohepatitis-associated vascular remodeling has been shown to promote melanoma liver metastasis, indicating that pre-existing endothelial dysfunction creates a pro-metastatic milieu even before tumor cells arrive.

Kupffer cells, the liver’s resident macrophages and the largest tissue-resident macrophage population in the body, are central players in this process. Pancreatic cancer-derived exosomes enriched in macrophage migration inhibitory factor are preferentially taken up by Kupffer cells, triggering TGF-β secretion that activates hepatic stellate cells to produce fibronectin, recruiting bone marrow-derived macrophages and neutrophils and initiating a fibrotic, immunosuppressive niche. Highly metastatic colorectal cancer cells secrete CXCL16, which binds CXCR6 on Kupffer cells and activates PI3K/AKT1/FOXO3a signaling, transcriptionally upregulating CD206 and driving M2 polarization. These M2-like Kupffer cells then secrete CXCL16 in a positive feedback loop, further entrenching the pro-metastatic phenotype. Gut microbiota composition directly influences this process: Bacteroides vulgatus enrichment increases Kupffer cell proliferation and reduces metastasis, whereas Proteus mirabilis expansion decreases Kupffer cell numbers and creates a permissive environment.

The immune cell landscape of the pre-metastatic liver undergoes equally dramatic remodeling. Natural killer cells, which constitute 30 to 50 percent of hepatic lymphocytes, normally enforce disseminated tumor cell dormancy through IFN-γ signaling. But during niche maturation, their numbers contract and cytotoxicity declines. Colorectal cancer-derived extracellular vesicles downregulate NKG2D on NK cells without altering their numbers, abrogating tumor surveillance. The HLA-E:CD94-NKG2A checkpoint enables circulating tumor cells to escape NK killing by suppressing cytotoxic granule release. Orosomucoid 2, upregulated in the pre-metastatic liver, binds ACVR1 on NK cells and activates Hippo signaling, suppressing the serine and one-carbon metabolism essential for NK cytotoxicity. Meanwhile, myeloid-derived suppressor cells expand under chronic inflammation and are recruited to hepatic sinusoids, where they establish a dominant immunosuppressive compartment through production of iNOS, reactive oxygen species, arginase-1, TGF-β, and IL-10.

Neutrophils add another layer of complexity. Tumor-derived GM-CSF converts antitumor neutrophils to a pro-tumor state via AKT-NF-κB-NFAT5 signaling, and these neutrophils accumulate in hepatic sinusoids and release neutrophil extracellular traps upon CXCR2 ligand stimulation. NET-derived elastase activates ERK in colorectal cancer cells, promoting proliferation and metastatic colonization. Fusobacterium nucleatum triggers NET formation through TLR4-ROS and NOD1/2 signaling, and high Fusobacterium tissues show greater neutrophil infiltration and NET burden. In pancreatic cancer liver metastasis, P2RX1-negative neutrophils with elevated Nrf2 and metabolic reprogramming toward oxidative phosphorylation potently suppress CD8-positive T cells in a PD-L1/PD-1-dependent manner.

The review also surveys emerging diagnostic biomarkers that could detect the pre-metastatic niche before overt metastasis. Exosomal microRNAs including miR-122, miRNA panels, and miR-6803-5p correlate with TNM staging and enable earlier detection than imaging. Elevated circulating neutrophil extracellular traps predict liver metastasis risk in early breast cancer, with NET-DNA binding CCDC25 and activating ILK-β-parvin signaling. Peripheral monocyte count independently predicts liver metastasis in colorectal cancer across a cohort of 7,187 patients. Lipopolysaccharide-binding protein drives intrahepatic fibrotic pre-metastatic niche formation via TLR4/NF-κB and serves as a candidate marker for gastric cancer liver metastasis. A random-forest model combining lactate dehydrogenase, CA19-9, ALT, CEA, total bilirubin, and the albumin-to-globulin ratio predicted liver metastasis in a cohort of 865 colorectal cancer patients.

On the therapeutic front, the review catalogs a range of preclinical interventions targeting the gut-liver axis. The SGK1 inhibitor GSK-650394 reduces IL-6/STAT3-mediated NET formation and polymorphonuclear myeloid-derived suppressor cell infiltration. Galunisertib, a TGF-β receptor I kinase inhibitor, blocks TGF-β/Smad signaling and attenuates hepatic fibrosis in gastric cancer liver metastasis models. The FXR agonist obeticholic acid reduces hepatic inflammation and reverses the immunosuppressive liver microenvironment in colorectal cancer models. A galactose-targeted lipid-calcium-phosphate nanocarrier delivering miR-122 regulates metastasis-associated genes and increases the CD8-positive to CD4-positive T-cell ratio. Dietary interventions including inulin and cellulose expand beneficial taxa and restore short-chain fatty acid production, while sodium butyrate supplementation reduces hepatic regulatory T cells and increases NKT and Th17 cells in colorectal cancer liver metastatic mice.

The authors caution that most mechanistic evidence derives from colorectal cancer, and the dominant taxa identified — Fusobacterium nucleatum, tumor-resident Escherichia coli, Proteus mirabilis, and Bacteroides vulgatus — are specific to colorectal cancer-associated dysbiosis. Features such as gut vascular barrier disruption and secondary bile acid accumulation appear broadly relevant, but the specific microbial signatures and cellular effectors in gastric, pancreatic, breast, and lung cancer are less well defined. Cross-tumor extrapolation should be cautious until equivalent multi-omic and gnotobiotic evidence is available for other primary sites. The review also notes that the pre-metastatic liver may serve as the soil in which dormant tumor cells persist for years before reactivating, raising the possibility that microbiota-targeted interception should be considered not only at the stage of radiologically detectable metastases but also during the dormancy window when disease is clinically occult but the hepatic microenvironment remains conditioned.

Subject of Research: The role of the gut-liver axis, including microbial dysbiosis, metabolite signaling, and hepatic immune cell reprogramming, in the formation of the liver pre-metastatic niche and emerging therapeutic strategies.

Article Title: Gut-liver axis in liver pre-metastatic niche formation: immune networks, microbial, and metabolic and emerging therapeutic opportunities

Article References: Zhao, L., Lin, Q., Zhang, D., Liu, Y., Zong, M., Xi, L., Xie, F., Song, S., Xue, P., Zhu, S., & Chu, X. (2026). Gut-liver axis in liver pre-metastatic niche formation: immune networks, microbial, and metabolic and emerging therapeutic opportunities. Frontiers in Microbiology, 17, Article 1931054. https://doi.org/10.3389/fmicb.2026.1931054

Image Credits: AI Generated

DOI: 10.3389/fmicb.2026.1931054

Keywords: gut-liver axis, pre-metastatic niche, liver metastasis, gut microbiota, bile acids, Kupffer cells, neutrophil extracellular traps, myeloid-derived suppressor cells, colorectal cancer, exosomal microRNAs, immune suppression, microbiota-targeted therapy

The research, led by Wenling Ye, Yang Zhao, and Fengling Wang under the direction of corresponding authors Wei-Dong Chen and Yan-Dong Wang, focused on the farnesoid X receptor, or FXR, a ligand-activated transcription factor expressed abundantly in the liver and intestine. FXR has long been recognized as a master regulator of bile acid homeostasis, lipid metabolism, and glucose control, and several FXR agonists have already entered clinical development for cholestatic liver disease and non-alcoholic steatohepatitis. The new work adds an unexpected dimension to this receptor’s portfolio: it appears to govern how the liver rebuilds its resident population of macrophages, the Kupffer cells, after surgical injury.

Kupffer cells are the liver’s resident sentinels, embedded in the sinusoidal blood vessels that perfuse the organ. They clear circulating debris and pathogens, but during regeneration they do something equally important: they broadcast the inflammatory and growth-promoting signals that push hepatocytes out of their quiescent state and into the cell cycle. Because these cells are consumed and reprogrammed as regeneration proceeds, the liver must continuously replenish its Kupffer cell pool from two sources. The first is local, the proliferation of resident Kupffer cells that survived the initial insult. The second is systemic, the recruitment of monocytes circulating in the bloodstream, which home to the injured liver and differentiate into macrophage-like cells that can take up residence there.

To test whether FXR controls this replenishment program, the team used a mouse model of two-thirds partial hepatectomy, the standard experimental system for studying liver regeneration. In wild-type mice, the regenerating lobes showed robust activation of both T cells and Kupffer cells, with increased infiltration of these immune populations into the remnant tissue, exactly the kind of coordinated immune response that regeneration theory predicts. The picture in mice lacking FXR was strikingly different. Although these animals started with elevated baseline numbers of both T cells and Kupffer cells in their livers, once surgery removed two-thirds of the organ, their regenerating livers displayed markedly attenuated immune activation and reduced immune cell infiltration compared with normal mice.

The consequence was a measurable delay in regeneration. The researchers traced this defect to failures in both arms of the Kupffer cell replenishment pathway. In FXR-deficient mice, circulating monocytes were less able to migrate into the regenerating liver, starving the organ of fresh macrophage precursors. At the same time, the resident Kupffer cells that remained proliferated poorly, so the local self-renewal route was also compromised. With both supply lines disrupted, the Kupffer cell pool failed to recover its normal size and composition, and the polarization balance between classically activated M1 macrophages and alternatively activated M2 macrophages was altered, further disturbing the cytokine environment that regenerating hepatocytes require.

Mechanistically, the study identified two molecular pathways through which FXR exerts this control. The first operates on monocyte trafficking. FXR, acting as a transcription factor, directly targets the gene encoding CCR2, the chemokine receptor that serves as the homing beacon for inflammatory monocytes. CCR2 allows circulating monocytes to sense chemokines released by injured tissue and migrate along the gradient toward the wound. By promoting CCR2 expression, FXR effectively keeps the recruitment pipeline open during regeneration. When FXR is absent, this beacon dims, monocytes fail to arrive in sufficient numbers, and the liver loses its most important source of replacement macrophages.

The second pathway governs local proliferation. The researchers found that FXR enhances the expansion of resident Kupffer cells through the CSF1/PI3K/Akt signaling cascade. Colony-stimulating factor 1, or CSF1, is the essential survival and growth factor for cells of the mononuclear phagocyte lineage, and signaling through its receptor activates phosphoinositide 3-kinase and the Akt kinase, a canonical pathway that drives cell growth, survival, and division. By potentiating this cascade in Kupffer cells, FXR ensures that the resident population can double and repopulate the regenerating tissue. The two mechanisms work synergistically: one replenishes the pool from the blood, the other from within, and together they restore Kupffer cell homeostasis at the pace that regeneration demands.

The study also offers a possible explanation for a clinical observation that has long frustrated transplant surgeons and hepatologists: regenerative capacity declines with age. In aged mice, the researchers observed impaired monocyte recruitment to the regenerating liver, mirroring the defect seen in FXR-deficient animals. If the same age-related erosion of the FXR-monocyte axis occurs in humans, it could help explain why elderly patients recover more slowly from major hepatic resection and why organs from older donors regenerate less vigorously after transplantation. This finding reframes aging of the liver not simply as a decline in hepatocyte proliferative potential but as a failure of the immune support system that scaffolds that proliferation.

The clinical implications are considerable. Liver transplantation and partial hepatectomy remain the most effective therapies for end-stage liver disease, yet the persistent shortage of donor organs has intensified interest in interventions that could enhance regeneration, expand the pool of usable grafts, and improve post-surgical outcomes. The identification of the FXR-Kupffer cell axis as a central regulator of liver repair suggests a concrete therapeutic target. Small-molecule FXR agonists, several of which are already approved or in late-stage clinical trials for other liver conditions, could in principle be repurposed or optimized to boost monocyte recruitment and Kupffer cell expansion in patients undergoing hepatic resection or receiving partial grafts from living donors.

As with any mouse study, important questions remain before these findings can be translated. The work was conducted in genetically engineered mice lacking FXR entirely, and the timing, dose, and cell-type specificity of any future pharmacological intervention will need careful optimization. It will also be essential to determine whether activating FXR in humans produces the same immune cell effects without exacerbating the inflammatory components of chronic liver disease, since macrophage polarization is a double-edged sword in conditions such as fibrosis and cirrhosis. Nevertheless, by connecting a well-characterized metabolic receptor to the cellular logistics of immune replenishment, the study adds a compelling new layer to the biology of liver regeneration. It suggests that the liver’s famous ability to rebuild itself rests not only on the proliferative drive of its hepatocytes but on a receptor-controlled program that recruits, renews, and reactivates the macrophages those hepatocytes depend on, and that keeping this program running may one day mean the difference between a graft that thrives and one that fails.

Subject of Research: FXR regulation of Kupffer cell pool restoration during liver regeneration

Article Title: An FXR-dependent cellular program for Kupffer cell pool restoration fuels liver regeneration

Article References: Ye, W., Zhao, Y., Wang, F., Wang, Y., Qu, Y., Li, Y., Wang, Y.-D., & Chen, W.-D. (2026). An FXR-dependent cellular program for Kupffer cell pool restoration fuels liver regeneration. Cell Death Discovery. https://doi.org/10.1038/s41420-026-03398-z

Image Credits: AI Generated

DOI: 10.1038/s41420-026-03398-z

Keywords: liver regeneration, FXR, Kupffer cells, monocyte recruitment, CCR2, CSF1, PI3K/Akt signaling, partial hepatectomy, macrophage polarization, liver transplantation, aging, nuclear receptor

The research, led by Nikola Makdissi, Maria Francesca Viola and Elvira Mass at the University of Bonn, together with a large interdisciplinary team across Germany, Austria and Sweden, set out to trace where ingested plastic goes and what it does once it arrives. The team used carboxylated polystyrene particles of defined sizes, ranging from 50 to 500 nanometres, quality-controlled by dynamic light scattering, and administered them to young male mice either as a single oral dose or through a chronic twelve-week feeding regimen designed to mimic continuous human exposure. Fluorescent labelling allowed the researchers to follow the particles through the body with flow cytometry and microscopy, while gold-core-labelled particles enabled visualisation at the ultrastructural level by electron microscopy.

The first striking finding concerned geography. Within twenty-four hours of a single oral dose, plastic of every size tested had crossed the intestinal barrier and accumulated in the liver and spleen, where it was taken up almost exclusively by macrophages, the resident scavenger cells of the immune system. Particles of 100 nanometres or smaller also reached the brain, preferentially lodging in microglia, the brain’s own macrophage population, whereas the larger 300 to 500 nanometre particles were undetectable there. During chronic feeding, the particles concentrated in the liver, spleen and brown adipose tissue but not in blood, bone marrow, lymph nodes or the small intestine. Within the liver, the primary reservoir was unambiguous: Kupffer cells, the liver-resident macrophages that line the sinusoidal blood vessels and stand guard against everything the gut sends into circulation.

Quantifying exactly how much plastic accumulates in tissues proved unexpectedly difficult, and the team was candid about the methodological obstacles. Europium-spiked particles lost their signal within hours of oral administration, and pyrolysis gas chromatography–mass spectrometry, a widely used technique for detecting plastics in biological samples, suffered from substantial overlap between tissue-derived lipid signals and the spectral markers of polystyrene. In some cases, samples from exposed animals yielded values comparable to or lower than untreated controls. These results align with recent critiques of pyrolysis-based quantification and underscore a broader problem for the field: without reliable measurement tools, assessing human risk remains an inexact science.

Despite the dosing challenges, the biological consequences of chronic exposure were clear and, crucially, size-dependent. After twelve weeks of weekly gavage with 500 nanometre microplastics, mouse livers showed impaired glycogen storage, increased neutral lipid deposition and elevated levels of specific lipid species, including lysophosphatidylethanolamines, lysophosphatidylcholines and monoglycerides. Nanoplastics of 50 nanometres produced none of these hepatic changes. Single-nucleus RNA sequencing revealed that hepatocytes, the liver’s metabolic workhorses, were by far the most transcriptionally responsive cell type, despite not accumulating detectable plastic themselves. Both particle sizes suppressed de novo lipogenesis while increasing fatty acid uptake, but only microplastics downregulated the very-low-density lipoprotein secretory machinery, impairing the cell’s ability to export lipid and tipping the balance toward net retention.

The mechanism appears to run through the macrophages. When Kupffer cells isolated from microplastic-exposed mice were co-cultured overnight with healthy hepatocytes, the hepatocytes accumulated lipid droplets, demonstrating that signals from plastic-laden macrophages are sufficient to induce fatty changes in otherwise normal liver cells. Transcriptomic and proteomic profiling of the exposed Kupffer cells revealed a coordinated reprogramming centred on increased HIF-1α activity and upregulation of apolipoproteins including APOA1, APOA2 and APOC3, proteins known to regulate lipid delivery to hepatocytes. Notably, this reprogramming occurred without classical inflammation: immune cell infiltration, serum cytokines and fibrosis markers were all unremarkable, suggesting the plastic quietly rewires macrophage function rather than triggering an overt immune alarm.

Perhaps the most concerning finding was functional. Microplastic exposure, but not nanoplastic exposure, impaired the phagocytic capacity of Kupffer cells, their fundamental ability to engulf and clear targets from the bloodstream. After just four weeks of exposure, before any lipid accumulation was visible, mice given microplastics cleared antibody-opsonised platelets less efficiently than controls. The defect extended to host defence: when mice were intravenously challenged with Escherichia coli, the blood of microplastic-exposed animals contained significantly more surviving bacteria, indicating compromised systemic bacterial clearance. Because Kupffer cells are central to clearing infected red blood cells in diseases such as malaria, the authors suggest that chronic plastic exposure could have broader implications for blood-borne infections.

Lineage-tracing experiments added another layer of complexity. After twelve weeks of exposure, nearly all Kupffer cells remained of yolk-sac embryonic origin, showing that plastic accumulation does not kill the cells or trigger replacement by bone-marrow-derived monocytes. But when the researchers deliberately depleted Kupffer cells with diphtheria toxin in microplastic-exposed mice, the monocyte-derived replacements failed to fully repopulate the empty niche even twelve weeks later, despite normal monocyte recruitment and differentiation. Plastic exposure therefore appears to impair the long-term maintenance of macrophages within the liver niche, possibly by altering signals from neighbouring endothelial or stellate cells, compounding the functional deficits of the original cells.

The systemic picture grew more intricate when the researchers combined plastic exposure with a high-fat diet. Under metabolic challenge, nanoplastic-exposed mice developed impaired glucose tolerance and elevated fasting glucose, yet paradoxically showed reduced hepatic lipid content, increased oxygen consumption even during cold exposure, reduced fat mass and preserved lean mass, without any change in food intake or activity. This suggests nanoplastics shift whole-body energy balance toward enhanced peripheral lipid burning and thermogenesis. Microplastic-exposed mice, by contrast, showed exacerbated hepatic lipid accumulation under the high-fat diet. The two particle sizes thus produce distinct metabolic syndromes: microplastics clog the liver’s lipid handling through macrophage dysfunction, while nanoplastics perturb systemic glucose and energy regulation.

There was one note of cautious reassurance. After a six-month washout period following the twelve-week exposure, metabolic alterations in the liver had largely resolved, even though plastic particles remained detectable within Kupffer cells. This indicates that unoccupied macrophages can resume their housekeeping functions once exposure ceases, though the persistence of the particles themselves raises unresolved questions about long-term consequences. The authors also emphasise important limitations: the study was conducted mainly in young male mice, which develop diet-induced steatosis faster than females, and the doses used, while lower than in many prior studies, still exceed estimated human exposure. Whether similar size-dependent effects occur in humans, and whether chronic plastic exposure accelerates progression from simple fatty liver to inflammatory liver disease, remain open questions. What the study establishes firmly is that plastic is not biologically inert once inside the body, that macrophages are its first and most burdened hosts, and that particle size is a critical determinant of the damage it does.

Subject of Research: Size-dependent effects of microplastic and nanoplastic exposure on Kupffer cell function and liver and systemic metabolism in mice

Article Title: Size-dependent plastic exposure disrupts macrophage function and tissue-specific metabolism

Article References: Makdissi, N., Viola, M. F., Steinheuer, L. M., Blank-Stein, N., Kardinal, R., Musacchio, F., Franco Taveras, E., Aliprandi, V., Sieckmann, K., Abdulkadyrov, A., Mayer, M. L., Diefenbach-Wilke, M.-L., Fink, D., Beckert, H., Böhmdorfer, S., Deveuve, Q., Graelmann, F. J., Mauel, K., Kronau, N., … Mass, E. (2026). Size-dependent plastic exposure disrupts macrophage function and tissue-specific metabolism. Nature Metabolism. https://doi.org/10.1038/s42255-026-01615-8

Image Credits: AI Generated

DOI: 10.1038/s42255-026-01615-8

Keywords: microplastics, nanoplastics, Kupffer cells, macrophages, liver metabolism, phagocytosis, hepatocytes, lipid metabolism, glucose tolerance, plastic pollution, Nature Metabolism, mouse study

The clinical backdrop makes the advance urgent. Gastric cancer remains one of the world’s deadliest malignancies, characterized by molecular heterogeneity, a high propensity for metastatic spread, and frequent therapeutic resistance. Immune checkpoint blockade, particularly antibodies against the PD-1/PD-L1 axis, has transformed outcomes in several tumor types, yet in gastric cancer the response rates stubbornly remain below 30 percent, with a substantial fraction of patients showing intrinsic resistance from the very first dose. The researchers turned to cancer genetics for an explanation. A significant subset of gastric cancers, especially those defined by chromosomal instability, carry two simultaneous alterations: loss of the tumor suppressor TP53 and hyperactivation of YAP, the transcriptional co-activator that serves as the main effector of the Hippo signaling pathway. YAP1 is amplified in roughly 18 percent of human gastric cancers, and the combination of YAP activation with p53 loss correlates clinically with aggressive, refractory disease. Whether this genetic pairing was a mere correlation or a true causal driver of immunotherapy resistance was the central question the study set out to answer.

To probe that question, the investigators engineered a sophisticated mouse model they call AYP. These animals carry a conditional, constitutively active YAP1 mutant, designated Yap1-6A, in which six phosphorylation sites were mutated so the protein resists degradation, together with floxed Tp53 alleles. Both alterations were activated specifically in the Atp4b-expressing parietal cell lineage of the stomach, which encompasses parietal cell progenitors, pre-parietal cells, and mature acid-producing cells. After tamoxifen induction, the mice developed invasive gastric adenocarcinoma within two to three months, with a median survival of 131 days. Crucially, the model demonstrated biological synergy rather than mere additivity: mice with YAP activation alone or p53 loss alone failed to develop gastric cancer even ten months after induction, while the double-hit configuration produced fully penetrant, aggressive malignancy. Histopathological examination revealed the full spectrum of human disease, including well-differentiated intestinal-type tumors, poorly differentiated signet-ring cell carcinomas of the diffuse type, and mixed forms, all accompanied by dense immune infiltration.

The metastatic behavior of the model proved equally faithful to the human condition. By three to four months after induction, the majority of AYP mice had disseminated disease involving the gastric lymph node, diaphragm, liver, pancreas, mediastinal lymph node, and lung, with metastatic incidence rates recorded for each organ across the cohort. Fluorescent labeling confirmed that the metastatic lesions originated from the Atp4b-lineage tumor cells. When the researchers treated these mice with anti-PD-1 antibodies for three weeks, the results were sobering but clinically familiar: stomach weights were unchanged, metastatic incidence and morphology were unaltered, and histopathology confirmed no therapeutic response. Immune profiling showed only a modest reduction in neutrophils, regulatory T cells, and group 3 innate lymphoid cells. In other words, the AYP tumors were intrinsically resistant to PD-1 blockade despite being what immunologists would call hot tumors, densely infiltrated with immune cells that should, in principle, be attackable.

To find the mechanism, the team performed single-cell RNA sequencing on nearly 50,000 high-quality cells harvested from the stomach, gastric lymph node, and visible liver and lung metastases of AYP mice and matched wild-type controls. The analysis identified three distinct tumor cell clusters, and gene set variation analysis revealed strong enrichment of Hippo signaling, YAP/TAZ target gene signatures, epithelial-mesenchymal transition features, and immunosuppression programs. When the researchers compared the mouse tumor transcriptomes with a single-cell dataset from 26 gastric cancer patients, the AYP tumor cells correlated closely with human poorly differentiated gastric adenocarcinoma, including signet-ring cell carcinoma, with high correlation coefficients for signature genes such as OLFM4, PLA2G2A, and ENO1. A dominant feature of the microenvironment, at both primary and metastatic sites, was a pronounced infiltration of neutrophils displaying characteristics of polymorphonuclear myeloid-derived suppressor cells, key systemic mediators of immunosuppression that are clinically associated with poor immunotherapy responses. Depleting neutrophils substantially inhibited tumor development and metastasis, establishing these cells as essential players rather than bystanders.

The search for the molecular bridge between YAP and the suppressive microenvironment converged on BST2, or bone marrow stromal cell antigen 2. Integrated analysis of tumor cell signature genes and genes downregulated after YAP1 knockout identified BST2 as a top candidate. Immunofluorescence confirmed that BST2 was co-expressed with the tumor marker KRT7 specifically within tumor tissue and was undetectable in normal gastric epithelium. Clinically, the pattern was striking: both BST2 and YAP1 transcription were markedly higher in gastric cancer patients whose disease progressed on anti-PD-1 therapy than in those achieving complete or partial responses, mirroring established immunosuppressive markers such as NR4A1 and CD55. Mechanistically, the team showed that deleting YAP1 with CRISPR/Cas9 significantly reduced both BST2 mRNA and surface protein levels, and CUT&RUN assays demonstrated direct binding of the YAP1-TEAD4 transcriptional complex to the Bst2 promoter, formally establishing BST2 as a direct YAP target gene.

Functional experiments then revealed BST2 as a genuine immune checkpoint operating on innate immune cells. Deleting BST2 from AYP tumor cells had only a marginal effect on proliferation in a dish but significantly impaired tumor formation and growth in living mice, accompanied by reduced numbers of tumor-infiltrating neutrophils and lower PD-L1 expression on those cells, decreased exhaustion of natural killer and CD4 T cells, and increased production of the cytotoxic enzyme Granzyme B. Ligand-receptor interaction analysis pointed to BST2 engaging a receptor called PIRA2, the murine ortholog of human leukocyte immunoglobulin-like receptors, which is highly expressed on neutrophils. Co-immunoprecipitation and protein truncation experiments mapped the physical interface: the coiled-coil domain of BST2 directly binds the Ig-like domain of PIRA2 through two distinct contact surfaces. Co-culture assays confirmed that AYP tumor cells drive bone marrow cells toward an immunosuppressive SiglecF-positive, PD-L1-positive neutrophil phenotype, an effect abolished either by BST2 knockout or by a BST2-blocking antibody.

The liver emerged as a particularly instructive battleground. Kupffer cells, the resident macrophages of the liver, also express high levels of PIRA2, and transcriptional profiling showed that AYP mice accumulated immunosuppressive Kupffer cell subsets at the expense of immunostimulatory ones. In co-culture, AYP tumor cells expanded the pool of CD11b-high Kupffer cells and upregulated the suppressive markers Arg1 and PD-L1, effects again dependent on BST2. In a liver metastasis model based on splenic injection of tumor cells, BST2 deficiency significantly prolonged host survival, reduced metastatic tumor burden, and boosted Granzyme B production by liver CD8 T cells, indicating reinvigorated anti-tumor immunity. The therapeutic culmination came in the spontaneous AYP model itself: combining anti-BST2 with anti-PD-1 antibodies reduced stomach weights by nearly 30 percent compared with anti-PD-1 alone, restored glandular tissue architecture, and, most strikingly, completely eradicated visible liver metastases in all treated animals while suppressing lymph node spread beyond what either agent achieved alone.

The implications reach beyond gastric cancer. BST2 was upregulated in tumors from resistant patients across multiple cancer types, suggesting it may function as a pan-tumor marker of anti-PD-1 failure, and its known role in suppressing plasmacytoid dendritic cells offers a plausible explanation for how anti-BST2 therapy also curbed lymph node metastases. The study also raises tantalizing questions about the microbial dimension, since bacteria such as Helicobacter pylori and Streptococcus anginosus can activate YAP signaling in gastric epithelium, potentially sustaining the BST2-driven resistance program. Significant work remains before patients benefit: the downstream signaling events of BST2-PIRA2 engagement are not fully mapped, structural studies of the interaction are needed to design high-affinity blockers, and humanized anti-BST2 antibodies must now prove themselves in patient-derived models. Still, for the large population of patients whose hot, immune-infiltrated gastric tumors inexplicably shrug off PD-1 blockade, the identification of a druggable YAP-BST2 axis offers something they have not had before: a mechanistic explanation and a concrete combination strategy to test in the clinic.

Subject of Research: YAP-BST2-mediated intrinsic resistance to anti-PD-1 immunotherapy in metastatic gastric cancer

Article Title: Targeting the YAP‐BST2 Axis Overcomes Intrinsic Anti‐PD‐1 Resistance in Metastatic Gastric Cancer

Article References: Zhang, W., Wang, S., Wang, M., Yu, R., Yue, J., Shao, L., Zhang, H., Zhu, M., Tian, L., Cheng, S., Qin, W., Tang, Y., Han, Y., Wang, W., An, L., Meng, Y., Jiao, S., & Zhou, Z. (2026). Targeting the YAP‐BST2 Axis Overcomes Intrinsic Anti‐PD‐1 Resistance in Metastatic Gastric Cancer. Advanced Science, Article e77708. https://doi.org/10.1002/advs.77708

Image Credits: AI Generated

DOI: 10.1002/advs.77708

Keywords: gastric cancer, immunotherapy resistance, YAP, TP53, BST2, PD-1, neutrophils, Kupffer cells, PIRA2, Hippo pathway, mouse model, liver metastasis

SALI is defined as an acute, secondary hepatic dysfunction that arises during sepsis and is observed in roughly 34 to 46 percent of patients. At the bedside it announces itself through abnormalities in standard liver tests, which can follow hepatocellular, cholestatic, or mixed patterns and range from modest enzyme elevations to profound liver failure. Crucially, the authors insist that SALI be treated as an operational clinical syndrome rather than the signature of a single underlying mechanism. Abnormal liver biochemistry in a septic patient may reflect inflammatory injury, cholestasis, hypoxic hepatitis caused by insufficient oxygen delivery, right-sided cardiac congestion, drug toxicity, or pre-existing conditions such as metabolic dysfunction-associated steatotic liver disease and occult fibrosis. The question, they argue, is not whether SALI has uniform biology, because it does not, but whether routinely available data can flag the patients most likely to deteriorate.

On the mechanistic side, one of the most striking recent findings concerns the gut. In a mouse model of sepsis, Murao and colleagues identified a pathway in which gut-primed neutrophils drive hepatic injury. Gut intraepithelial lymphocytes interact with neutrophils through the molecule CD112, facilitating the formation of neutrophil extracellular traps, the web-like DNA structures that neutrophils eject to ensnare pathogens. These primed neutrophils migrate through the portal vein into the liver, where they release their traps and activate Kupffer cells, the liver’s resident macrophages, triggering the secretion of interleukin-6 and tumor necrosis factor-alpha. Notably, portal vein neutrophils from septic mice produced significantly more neutrophil extracellular traps and induced greater Kupffer cell activation than systemic neutrophils, an effect that disappeared entirely in mice lacking PAD4, the enzyme essential for trap formation. The implication is provocative: the gut does not merely spill inflammatory mediators into the portal circulation, it actively educates immune cells that then inflict damage on distant organs.

Although the authors caution that translation to human disease requires care, the concept has clear clinical resonance. The liver receives most of its blood supply from the portal circulation and is therefore continuously bathed in gut-derived inflammatory signals. During sepsis, disruption of the intestinal barrier allows bacterial translocation and the spillover of pathogen-associated molecular patterns, which activate hepatic Toll-like receptors. Supporting this mechanistic bridge, human data from Czaikoski and colleagues have shown that neutrophil extracellular traps accumulate in organ tissue during experimental and clinical sepsis and correlate with damage. Together, these findings nominate trap formation and downstream Kupffer cell activation as candidate precision-medicine targets in SALI.

A second biological pillar concerns the loss of hepatic immune tolerance. In health, the liver is a strikingly tolerant organ, and Kupffer cells orchestrate that tolerance through antigen clearance and the induction of regulatory T cells. Recent work shows that during hepatic inflammation this tolerogenic phenotype collapses: Kupffer cells lose their signature tolerance markers, and antigen presentation shifts to infiltrating monocyte-derived macrophages. Activated Kupffer cells then recruit further immune cells to the liver, amplifying injury. Evidence from viral hepatitis research suggests that the transition from tolerance to inflammation involves dysregulation of inhibitory pathways, such as the Toll-like receptor pathway inhibitor SHIP, that normally restrain receptor signaling and keep Kupffer cells quiescent. Hepatic stellate cells, likewise, depend on inhibitory signals to remain dormant; when stimulated by microbial products or damage-associated molecular patterns, they produce extracellular matrix proteins and profibrogenic cytokines, and their contractile activation can raise sinusoidal resistance and portal pressure. Toll-like receptor 4-dependent crosstalk between Kupffer cells and stellate cells converts inflammatory signals into profibrogenic activation.

Within sepsis specifically, the inflammatory polarization of Kupffer cells toward the M1 phenotype has emerged as a hallmark of SALI. Extracellular cold-inducible RNA-binding protein, a damage-associated molecular pattern released during stress, promotes this M1 polarization through Toll-like receptor 4 signaling, driving overproduction of inflammatory cytokines. In mouse sepsis models the ratio of M1 to M2 Kupffer cells rises sharply, indicating a decisive shift toward proinflammatory function, and this polarization is not merely a byproduct of inflammation but an active driver of hepatocyte injury through reactive oxygen species, cytokines, and the recruitment of more neutrophils. In parallel, regulated forms of cell death, including apoptosis, necroptosis, pyroptosis, and ferroptosis, appear to contribute to hepatocyte dysfunction. These converging mechanisms point toward the restoration of hepatic immune tolerance as a promising future therapeutic strategy, though no SALI-targeted therapy has yet been established.

It is on the clinical side that the commentary delivers its most immediately practical message. In a retrospective cohort study spanning two large intensive care cohorts, Palmowski and colleagues examined how well routine biomarkers could stratify mortality risk among patients meeting operational criteria for SALI, defined as sepsis-associated liver-test abnormalities within seven days of sepsis onset, excluding pre-existing chronic liver disease. The criteria included alanine aminotransferase at five or more times the upper limit of normal, alkaline phosphatase at twice the upper limit, or elevated bilirubin combined with enzyme elevations. Their central finding was that the De Ritis ratio, the simple ratio of aspartate to alanine aminotransferase, outperformed both the conventional R-factor and alanine aminotransferase alone in predicting thirty-day mortality. A ratio of one or below indicated low risk, values between one and two indicated intermediate risk, and values of two or above flagged the highest risk, a pattern consistent across infection sources and admission types.

The authors of the commentary are careful to frame these strata correctly. The De Ritis ratio is not a liver-specific diagnostic marker or a mechanistic endotype, and elevated aspartate aminotransferase can also signal hypoxic hepatitis, shock, right-sided congestion, systemic inflammation, chronic kidney disease, alcohol-related injury, or cardiometabolic comorbidity. Its pragmatic value lies in risk enrichment among patients who already meet operational SALI criteria, complementing rather than replacing SOFA-bilirubin scoring. Interpreted alongside the SOFA score, lactate, hemodynamic status, cardiac context, and comorbidities, a rising ratio should trigger a structured reassessment: is infection control optimized, are hemodynamics adequate, is the lactate trajectory improving, is there occult congestion or biliary obstruction, are hepatotoxic drugs on board, and does the patient carry underlying fibrosis risk? In this framework, routine liver tests define the dominant biochemical injury pattern, whether hepatocellular, cholestatic, or mixed, and link prediction to the prevention of further hepatic and systemic deterioration and of iatrogenic harm.

The translational pathway forward, the authors suggest, will require prospective studies testing whether serial liver tests, the De Ritis ratio, SOFA scores, lactate, hemodynamic data, and immune readouts such as monocyte HLA-DR expression or ex vivo monocyte cytokine responses can identify reproducible SALI trajectories and clinically actionable phenotypes. Preclinical work has already nominated an unusually rich set of therapeutic targets, including neutrophil extracellular trap formation, Kupffer cell polarization, inflammasome activation, ferroptosis, necroptosis, and the restoration of hepatic immune tolerance. Until such approaches are validated, however, current clinical utility remains deliberately pragmatic: recognize SALI early, classify the dominant biochemical pattern, stratify mortality risk with the De Ritis ratio, hunt actively for reversible contributors, and intensify surveillance in high-risk patients.

For the authors, the larger significance of this work lies in adding an organ-specific decision layer to the 2026 Surviving Sepsis Campaign framework. New-onset liver-test abnormalities in septic adults, they argue, should no longer be treated as incidental laboratory noise. When detected, the humble ratio of two transaminases, a calculation older than modern critical care and available in every hospital on earth, may identify the patients who need intensified monitoring and protection from modifiable second hits, while the expanding immunobiology of the gut-liver axis steadily maps the routes toward genuine precision medicine for a complication that affects as many as one in two patients with sepsis.

Subject of Research: Sepsis-associated liver injury: immunobiology and bedside risk stratification with routine liver tests

Article Title: Sepsis-associated liver injury: from liver-test risk signals to immunobiology-guided precision medicine

Article References: Katsounas, A., Tsochatzis, E., & Rello, J. (2026). Sepsis-associated liver injury: from liver-test risk signals to immunobiology-guided precision medicine. Intensive Care Medicine. https://doi.org/10.1007/s00134-026-08593-1

Image Credits: AI Generated

DOI: 10.1007/s00134-026-08593-1

Keywords: sepsis-associated liver injury, De Ritis ratio, Kupffer cells, neutrophil extracellular traps, gut-liver crosstalk, hepatic immune tolerance, risk stratification, intensive care, Toll-like receptors, precision medicine, Sepsis-associated, liver

Kupffer cells have long been viewed primarily as scavengers, clearing debris and worn-out blood cells from the hepatic circulation. The new work, led by Zhuoyu Jia, Xinqiang Li, and Jinzhen Cai of Qingdao University and collaborators, demonstrates that they are far more sophisticated. Using single-cell RNA sequencing of liver graft biopsies and peripheral blood from transplant patients, the team mapped the full diversity of myeloid cells within the graft and isolated a cluster defined by exceptionally high APOE expression alongside canonical Kupffer cell markers such as CD5L, VSIG4, and MARCO. Functional enrichment analysis showed that this APOE-positive subset was strongly enriched in pathways governing receptor-mediated endocytosis, antigen processing and presentation, and efferocytosis, the engulfment of dying cells, pointing to a cell primed for both scavenging and immune regulation.

Computational modeling of intercellular communication using the CellChat algorithm revealed that these APOE-positive Kupffer cells behave as major signal senders within the graft, engaging T cells through a battery of co-stimulatory and co-inhibitory ligand-receptor pairs. Among the most prominent were CD86 engaging CTLA4 and CD28, LGALS9 binding the inhibitory receptor TIM-3, and NECTIN2 pairing with TIGIT. The strength and pattern of these interactions differed dramatically between patients whose grafts were tolerated and those experiencing rejection, suggesting that the APOE-positive macrophages help determine whether the local immune response escalates or winds down. Multiplex immunohistochemistry of patient biopsies confirmed that the proportions of these cells shift measurably as rejection develops.

To establish causality rather than mere correlation, the researchers built a technically demanding murine model of orthotopic liver transplantation, transplanting livers from C57BL/6 donors into C3H/He recipients and tracking the immune environment across four post-operative weeks. Histology and Banff rejection scoring documented the expected trajectory: severe acute rejection at one week, followed by spontaneous resolution and immune tolerance by week four. Flow cytometry with rigorous fluorescence-minus-one controls then revealed a striking biphasic dynamic. During acute rejection, the proportion of APOE-expressing Kupffer cells in the graft plummeted, likely reflecting ischemia-reperfusion injury and cellular death, while macrophages surged systemically across the spleen, blood, lymph nodes, and bone marrow.

As tolerance took hold, the picture reversed. Systemic myeloid expansion contracted, but the fraction of APOE-positive Kupffer cells within the graft climbed steadily, peaking at four weeks. These accumulating cells increasingly co-expressed CD206, a hallmark of alternatively activated, inflammation-resolving macrophages, and multiplex imaging showed extensive in-situ co-localization of F4/80, CD206, and APOE exclusively in tolerated grafts. A parallel enrichment of APOE-positive macrophages appeared in the spleen, lymph nodes, and bone marrow, hinting at a coordinated systemic regulatory program rather than a purely local phenomenon.

The target of this regulatory activity emerged as a specific population of tissue-resident memory CD8-positive T cells. These cells, marked by CD69 but lacking CD103, reside permanently within the graft and act as rapid-response effectors of localized rejection. The team showed that during acute rejection, this CD69-positive CD103-negative subset expanded robustly, depressing the CD4-to-CD8 ratio within the graft. As tolerance developed, however, the pool contracted and progressively upregulated the inhibitory checkpoints PD-1 and TIGIT, the classic signature of T cell exhaustion, a hyporesponsive state that limits immune-mediated tissue damage without requiring systemic T cell depletion.

Genetic proof came from transgenic experiments. When the researchers transplanted livers from APOE-knockout donors into allogeneic recipients, rejection exploded in severity. Grafts showed dense inflammatory infiltrates and structural destruction with markedly elevated Banff scores, and serum alanine and aspartate aminotransferase levels surged, reflecting profound liver injury. Flow cytometry revealed unchecked expansion of the CD69-positive CD103-negative CD8-positive tissue-resident memory population, and, crucially, the exhausted PD-1-positive and TIGIT-positive phenotype failed to appear. Without APOE, the molecular brake on alloreactivity was effectively dismantled.

In vitro co-culture experiments reinforced the causal chain. Kupffer cells harvested from wild-type mice upregulated APOE when stimulated with allogeneic T cells over 72 hours, while cells from APOE-knockout mice could not mount this response and cells engineered to overexpress APOE amplified it. When these macrophages were paired with responder splenic T cells, APOE deficiency accelerated CD8-positive T cell proliferation, whereas forced APOE overexpression blunted expansion to near baseline levels. Notably, Transwell experiments that physically separated the two cell populations showed that the suppressive effect persisted without direct contact, implying that APOE acts as a secreted immunomodulator bathing neighboring T cells in co-inhibitory signals, potentially through lipid receptors such as LRP1 or other LDL receptor family members on the T cell surface.

The study also delivered a clinically actionable finding. Analyzing preoperative serum from 31 liver transplant donors, the researchers found that higher donor APOE levels correlated negatively with recipient MELD scores and with post-operative monocyte counts, and tracked consistently with lower bilirubin and ALT levels during the first five days after surgery. Donor APOE, the authors propose, may reflect an intrinsic tolerogenic reserve of the graft, a liver inherently predisposed to a smoother immunological recovery. This positions a simple blood measurement as a potential tool for stratifying donor organs, guiding the use of marginal grafts, or identifying recipients in whom immunosuppression might be safely tapered earlier.

The work is the first to systematically assign a tolerogenic role to the APOE-positive Kupffer cell subset in transplantation, and it reframes a protein best known for cholesterol transport and Alzheimer’s disease risk as a central player in graft acceptance. The authors acknowledge limitations: mRNA abundance does not always mirror protein levels, which they addressed by anchoring key conclusions in flow cytometry and multiplex imaging, and the precise receptor that binds Kupffer-cell-derived APOE on T cells remains to be identified. Even so, the mechanistic axis they describe, in which APOE-positive macrophages recruit and exhaust pathogenic CD8-positive tissue-resident memory cells through chemokine-guided proximity and checkpoint signaling, offers a blueprint for therapies that could coax the liver’s own immune circuitry toward tolerance, potentially freeing transplant recipients from a lifetime of systemic immunosuppression.

Subject of Research: The role of APOE-positive Kupffer cells in inducing CD8-positive T cell exhaustion and immune tolerance after liver transplantation

Article Title: ApolipoproteinE + Kupffer cells maintain immune homeostasis following liver transplantation by inducing CD8 + T cell exhaustion

Article References: Jia, Z., Li, X., Wang, X., Zhou, X., Cai, H., Jiang, P., Ding, R., Liu, H., Wang, Y., Li, S., & Cai, J. (2026). ApolipoproteinE+ Kupffer cells maintain immune homeostasis following liver transplantation by inducing CD8+ T cell exhaustion. iScience, 29(10), Article 117501. https://doi.org/10.1016/j.isci.2026.117501

Image Credits: AI Generated

DOI: 10.1016/j.isci.2026.117501

Keywords: liver transplantation, Kupffer cells, apolipoprotein E, immune tolerance, T cell exhaustion, tissue-resident memory T cells, single-cell RNA sequencing, PD-1, TIGIT, macrophages, allograft rejection, transplant immunology