Csf1r or Mer inhibition delays liver regeneration via suppression of Kupffer cells

GAS6/TAM Axis as Therapeutic Target in Liver Diseases

TAM (TYRO3, AXL, and MERTK) protein tyrosine kinase membrane receptors and their vitamin K-dependent ligands GAS6 and protein S (PROS) are well-known players in tumor biology and autoimmune diseases. In contrast, TAM regulation of fibrogenesis and the inflammation mechanisms underlying metabolic dysfunction-associated steatohepatitis (MASH), cirrhosis, and, ultimately, liver cancer has recently been revealed. GAS6 and PROS binding to phosphatidylserine exposed in outer membranes of apoptotic cells links TAMs, particularly MERTK, with hepatocellular damage. In addition, AXL and MERTK regulate the development of liver fibrosis and inflammation in chronic liver diseases. Acute hepatic injury is also mediated by the TAM system, as recent data regarding acetaminophen toxicity and acute-on-chronic liver failure have uncovered. Soluble TAM-related proteins, mainly released from activated macrophages and hepatic stellate cells after hepatic deterioration, are proposed as early serum markers for disease progression. In conclusion, the TAM system is becoming an interesting pharmacological target in liver pathology and a focus of future biomedical research in this field.

Limitations of PLX3397 as a microglial investigational tool: peripheral and off-target effects dictate the response to inflammation

Microglia, the resident macrophages of the central nervous system (CNS), play a critical role in CNS homeostasis and neuroinflammation. Pexidartinib (PLX3397), a colony-stimulating factor 1 (CSF1) receptor inhibitor, is widely used to deplete microglia, offering flexible options for both long-term depletion and highly versatile depletion-repopulation cycles. However, the potential impact of PLX3397 on peripheral (immune) cells remains controversial. Until now, the microglia-specificity of this type of compounds has not been thoroughly evaluated, particularly in the context of peripherally derived neuroinflammation. Our study addresses this gap by examining the effects of PLX3397 on immune cells in the brain, liver, circulation and bone marrow, both in homeostasis and systemic inflammation models. Intriguingly, we demonstrate that PLX3397 treatment not only influences the levels of tissue-resident macrophages, but also affects circulating and bone marrow immune cells beyond the mononuclear phagocyte system (MPS). These alterations in peripheral immune cells disrupt the response to systemic inflammation, consequently impacting the phenotype irrespective of microglial depletion. Furthermore, we observed that a lower dose of PLX3397, which does not deplete microglia, demonstrates similar (non-)MPS effects, both in the periphery and the brain, but fails to fully replicate the peripheral alterations seen in the higher doses, questioning lower doses as a 'peripheral control' strategy. Overall, our data highlight the need for caution when interpreting studies employing this compound, as it may not be suitable for specific investigation of microglial function in the presence of systemic inflammation.

Macrophage Depletion Protects Against Cisplatin-Induced Ototoxicity and Nephrotoxicity

Cisplatin is a widely used and highly effective anti-cancer drug with significant side effects including ototoxicity and nephrotoxicity. Macrophages, the major resident immune cells in the cochlea and kidney, are important drivers of both inflammatory and tissue repair responses. To investigate the roles of macrophages in cisplatin-induced ototoxicity and nephrotoxicity, we used PLX3397, an FDA-approved inhibitor of the colony-stimulating factor 1 receptor (CSF1R), to eliminate tissue-resident macrophages during the course of cisplatin administration. Mice treated with cisplatin alone (cisplatin/vehicle) had significant hearing loss (ototoxicity) as well as kidney injury (nephrotoxicity). Macrophage ablation using PLX3397 resulted in significantly reduced hearing loss measured by auditory brainstem responses (ABR) and distortion-product otoacoustic emissions (DPOAE). Sensory hair cells in the cochlea were protected against cisplatin-induced death in mice treated with PLX3397. Macrophage ablation also protected against cisplatin-induced nephrotoxicity, as evidenced by markedly reduced tubular injury and fibrosis as well as reduced plasma blood urea nitrogen (BUN) and neutrophil gelatinase-associated lipocalin (NGAL) levels. Mechanistically, our data suggest that the protective effect of macrophage ablation against cisplatin-induced ototoxicity and nephrotoxicity is mediated by reduced platinum accumulation in both the inner ear and the kidney. Together our data indicate that ablation of tissue-resident macrophages represents a novel strategy for mitigating cisplatin-induced ototoxicity and nephrotoxicity.

Therapeutic targeting of tumour myeloid cells

Myeloid cells are pivotal within the immunosuppressive tumour microenvironment. The accumulation of tumour-modified myeloid cells derived from monocytes or neutrophils - termed 'myeloid-derived suppressor cells' - and tumour-associated macrophages is associated with poor outcome and resistance to treatments such as chemotherapy and immune checkpoint inhibitors. Unfortunately, there has been little success in large-scale clinical trials of myeloid cell modulators, and only a few distinct strategies have been used to target suppressive myeloid cells clinically so far. Preclinical and translational studies have now elucidated specific functions for different myeloid cell subpopulations within the tumour microenvironment, revealing context-specific roles of different myeloid cell populations in disease progression and influencing response to therapy. To improve the success of myeloid cell-targeted therapies, it will be important to target tumour types and patient subsets in which myeloid cells represent the dominant driver of therapy resistance, as well as to determine the most efficacious treatment regimens and combination partners. This Review discusses what we can learn from work with the first generation of myeloid modulators and highlights recent developments in modelling context-specific roles for different myeloid cell subtypes, which can ultimately inform how to drive more successful clinical trials.

A new liver regeneration molecular mechanism involving hepatic stellate cells, Kupffer cells, and glucose-regulated protein 78 as a new hepatotrophic factor

BACKGROUND/PURPOSE

To overcome liver failure, we focused on liver regeneration mechanisms by the activation of hepatic stellate cells (HSCs) and Kupffer cells (KCs). It is known that the HSC-secreted Mac-2-binding protein glycan isomer (M2BPGi) activates KC in the fibrotic liver. However, its importance for liver regeneration of the HSCs/M2BPGi/KCs axis after hepatectomy is still unknown. The aim of this study was to clarify whether the HSC-derived M2BPGi can activate KCs after hepatectomy, and elucidate the new molecular mechanism of liver regeneration.

METHODS

We examined the effect of M2BPGi on human hepatocytes and KCs, and explored secretory factors from M2BPGi-activated KCs using proteomics. Furthermore, the effect on liver regeneration of glucose-regulated protein 78 (GRP78) as one of the M2BPGi-related secreted proteins was examined in vitro and in murine hepatectomy models.

RESULTS

Although M2BPGi had no hepatocyte-promoting effect, M2BPGi promoted the production of GRP78 in KCs. The KC-driven GRP78 promoted hepatocyte proliferation. GRP78 administration facilitated liver regeneration after 70% hepatectomy and increased the survival rate after 90% hepatectomy in mice.

CONCLUSIONS

The M2BPGi-activated KCs secrete GRP78, which facilitates liver regeneration and improves the survival in a lethal mice model. Our data suggest that the new hepatotrophic factor GRP78 may be a promising therapeutic tool for lethal liver failure.

Eph/Ephrin Promotes the Adhesion of Liver Tissue-Resident Macrophages to a Mimicked Surface of Liver Sinusoidal Endothelial Cells

Kupffer cells are maintained via self-renewal in specific microenvironmental niches, primarily the liver sinusoidal endothelial cells (LSECs). In this study, we propagated tissue-resident macrophages (Mø) from mouse liver using mixed culture with hepatic fibroblastic cells. Propagated liver Mø express Id3, Lxra and Spic transcription factors, which are required for Kupffer cell characterization. Thus, Kupffer cell properties are likely to be maintained in liver Mø propagated using mixed culture with fibroblastic cells. We revealed (i) gene expression of certain Eph receptors and ephrin ligands including EphA2, ephrin-A1, EphB4, and ephrin-B1 in propagated liver Mø and primary LSECs, (ii) immunohistochemical localization of these Eph/ephrin member molecules indicating common expression in Kupffer cells and LSECs, and (iii) surface expression of several integrin α and β subunits, including α4β1, αLβ2, αMβ2, and αXβ2 integrin in propagated liver Mø and that of the corresponding ligands ICAM-1 and VCAM-1 in primary LSECs. Moreover, EphA/ephrin-A and EphB/ephrin-B interactions promoted liver Mø adhesion to the ICAM-1-adsorbed surface, which mimicked that of LSECs and may be implicated in the residence of Kupffer cells in the liver sinusoid. Further studies on regulating the residence and regeneration of Kupffer cells in related hepatic disorders are required to validate our findings.

Methyl 6-O-cinnamoyl-α-d-glucopyranoside Ameliorates Acute Liver Injury by Inhibiting Oxidative Stress Through the Activation of Nrf2 Signaling Pathway

Excessive stimulation of hepatotoxins and drugs often lead to acute liver injury, while treatment strategies for acute liver injury have been limited. Methyl 6-O-cinnamoyl-α-d-glucopyranoside (MCGP) is a structure modified compound from cinnamic acid, a key chemical found in plants with significant antioxidant, anti-inflammatory, and antidiabetic effects. In this study, we investigated the effects and underlying mechanisms of MCGP on acetaminophen (APAP)- or carbon tetrachloride (CCl₄)-induced acute liver injury. As a result, MCGP inhibited cell death and apoptosis induced by APAP or CCl₄, and suppressed the reactive oxygen species (ROS) generation stimulated by H₂O₂ in liver AML12 cells. In vivo, MCGP alleviated APAP/CCl₄-induced hepatic necrosis and resumed abnormal aminotransferase activities and liver antioxidase activities. In addition, MCGP depressed APAP- or CCl₄-induced oxidative stress through the suppression of CYP2E1 and activation of nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway. MCGP also enhanced the number of PCNA-positive hepatocytes, increased hepatic PCNA and Bcl-XL, and decreased BAX expression in APAP-/CCl₄-intoxicated mice. Furthermore, MCGP activated the GSDMD-N/cleaved caspase 1 pathway. In summary, MCGP might act as a potential therapeutic drug against drug-induced and chemical-induced acute liver injuries, and its underlying mechanisms might engage on the pressing of oxidative stress, refraining of hepatocyte apoptosis, and facilitating of liver regeneration.

Tumor-associated macrophages modulate angiogenesis and tumor growth in a xenograft mouse model of multiple myeloma

Tumor-associated macrophages (TAMs) are closely associated with poor multiple myeloma (MM) prognosis. Therefore, in-depth understanding of the mechanism by which TAM supports MM progression may lead to its effective treatment. We used the MM nude mouse subcutaneous xenograft model to evaluate the efficacy of the macrophage-depleting agent clodronate liposome (Clo) against MM and elucidate the mode of action of this therapy. At the same time, observe whether the elimination of TAM in vivo while silencing the expression of VEGFA has the same effect as in vitro experiments. We also used Clo to eliminate macrophages and reinjected M1 or M2 TAM through mouse tail veins to investigate the effects of various macrophage subtypes on MM xenograft tumor growth. We applied qRT-PCR, immunohistochemistry, and enzyme-linked immunosorbent assay to quantify VEGFA, CD31, and CD163 expression in tumor tissues and sera. Removal of TAMs from the tumor microenvironment impeded tumor growth. The combination of Clo plus VEGFA siRNA had a stronger inhibitory effect on tumor growth than Clo alone, and M2 and M1 macrophages promoted and inhibited tumor growth, respectively. Macrophage depletion combined with cytokine blocking is a promising MM treatment. Targeted M2 macrophage elimination together with cytokine block may be more effective at inhibiting MM growth than either treatment alone. The results of the present study lay an empirical foundation for the development of novel therapeutic strategies for MM.

Liver-specific T regulatory type-1 cells program local neutrophils to suppress hepatic autoimmunity via CRAMP

Neutrophils with immunoregulatory properties, also referred to as type-2 neutrophils (N2), myeloid-derived suppressor cells (MDSCs), or tumor-associated neutrophils (TANs), comprise a heterogeneous subset of cells that arise from unknown precursors in response to poorly understood cues. Here, we find that, in several models of liver autoimmunity, pharmacologically induced, autoantigen-specific T regulatory type-1 (TR1) cells and TR1-cell-induced B regulatory (Breg) cells use five immunoregulatory cytokines to coordinately recruit neutrophils into the liver and program their transcriptome to generate regulatory neutrophils. The liver-associated neutrophils from the treated mice, unlike their circulating counterparts or the liver neutrophils of sick mice lacking antigen-specific TR1 cells, are proliferative, can transfer disease protection to immunocompromised hosts engrafted with pathogenic effectors, and blunt antigen-presentation and local autoimmune responses via cathelin-related anti-microbial peptide (CRAMP), a cathelicidin, in a CRAMP-receptor-dependent manner. These results, thus, identify antigen-specific regulatory T cells as drivers of tissue-restricted regulatory neutrophil formation and CRAMP as an effector of regulatory neutrophil-mediated immunoregulation.

Resident CD34-positive cells contribute to peri-endothelial cells and vascular morphogenesis in salivary gland after irradiation

Salivary gland (SG) hypofunction is a common post-radiotherapy complication. Besides the parenchymal damage after irradiation (IR), there are also effects on mesenchymal stem cells (MSCs) which were shown to contribute to regeneration and repair of damaged tissues by differentiating into stromal cell types or releasing vesicles and soluble factors supporting the healing processes. However, there are no adequate reports about their roles during SG damage and regeneration so far. Using an irradiated SG mouse model, we performed certain immunostainings on tissue sections of submandibular glands at different time points after IR. Immunostaining for CD31 revealed that already one day after IR, vascular impairment was induced at the level of capillaries. In addition, the expression of CD44—a marker of acinar cells—diminished gradually after IR and, by 20 weeks, almost disappeared. In contrast, the number of CD34-positive cells significantly increased 4 weeks after IR and some of the CD34-positive cells were found to reside within the adventitia of arteries and veins. Laser confocal microscopic analyses revealed an accumulation of CD34-positive cells within the area of damaged capillaries where they were in close contact to the CD31-positive endothelial cells. At 4 weeks after IR, a fraction of the CD34-positive cells underwent differentiation into α-SMA-positive cells, which suggests that they may contribute to regeneration of smooth muscle cells and/or pericytes covering the small vessels from the outside. In conclusion, SG-resident CD34-positive cells represent a population of progenitors that could contribute to new vessel formation and/or remodeling of the pre-existing vessels after IR and thus, might be an important player during SG tissue healing.