Macrophages make a difference in cholestatic liver diseases – but how?

A+T rich interaction domain protein 3a (Arid3a) impairs Mertk-mediated efferocytosis in cholestasisJournal of HepatologyVol. 79Issue 6PreviewMacrophages are key elements in the pathogenesis of cholestatic liver diseases. Arid3a plays a prominent role in the biologic properties of hematopoietic stem cells, B lymphocytes and tumor cells, but its ability to modulate macrophage function during cholestasis remains unknown. Full-Text PDF See Article, pages 1478–1490 See Article, pages 1478–1490 Primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC) are cholestatic liver diseases targeting the biliary epithelium and are considered immune-mediated liver disorders.[1]Schett G. McInnes I.B. Neurath M.F. Reframing immune-mediated inflammatory diseases through signature cytokine hubs.N Engl J Med. 2021; 385: 628-639Crossref PubMed Scopus (128) Google Scholar Recent work has indicated that macrophages, the most abundant immune cell in the liver, play a role in the progression of PBC and PSC.2Trussoni C.E. O'Hara S.P. LaRusso N.F. Cellular senescence in the cholangiopathies: a driver of immunopathology and a novel therapeutic target.Semin Immunopathol. 2022; 44: 527-544Crossref PubMed Scopus (18) Google Scholar, 3De Muynck K. Heyerick L. De Ponti F.F. Vanderborght B. Meese T. Van Campenhout S. et al.Osteopontin characterizes bile duct associated macrophages and correlates with liver fibrosis severity in primary sclerosing cholangitis.Hepatology. 2023. Aug 2; (Epub ahead of print)https://doi.org/10.1097/HEP.0000000000000557Crossref PubMed Google Scholar, 4Guillot A. Winkler M. Silva Afonso M. Aggarwal A. Lopez D. Berger H. Kohlhepp M.S. et al.Mapping the hepatic immune landscape identifies monocytic macrophages as key drivers of steatohepatitis and cholangiopathy progression.Hepatology. 2023; 78: 150-166Crossref PubMed Scopus (12) Google Scholar However, while the accumulation and persistence of macrophages in the peribiliary area in PBC and PSC is well accepted, what is attracting the macrophages to the bile ducts and what the macrophages are doing once they get there is still not fully elucidated. Studies have shown that proinflammatory, bioactive molecules such as CX3CL1 and CCL2 are highly expressed in biliary epithelial cells (BECs) from patients with PBC and PSC and likely contribute to the recruitment of macrophages to the bile ducts in these cholestatic diseases.[2]Trussoni C.E. O'Hara S.P. LaRusso N.F. Cellular senescence in the cholangiopathies: a driver of immunopathology and a novel therapeutic target.Semin Immunopathol. 2022; 44: 527-544Crossref PubMed Scopus (18) Google Scholar Once there, these macrophages take on a proinflammatory phenotype as indicated by increased expression of iNOS.[5]Guicciardi M.E. Trussoni C.E. Krishnan A. Bronk S.F. Lorenzo Pisarello M.J. O'Hara S.P. Splinter P.L. et al.Macrophages contribute to the pathogenesis of sclerosing cholangitis in mice.J Hepatol. 2018; 69: 676-686Abstract Full Text Full Text PDF PubMed Scopus (95) Google Scholar Additionally, it has been shown that inhibiting macrophage recruitment in cholestatic mouse models diminishes peribiliary fibroinflammation and improves disease outcomes.[5]Guicciardi M.E. Trussoni C.E. Krishnan A. Bronk S.F. Lorenzo Pisarello M.J. O'Hara S.P. Splinter P.L. et al.Macrophages contribute to the pathogenesis of sclerosing cholangitis in mice.J Hepatol. 2018; 69: 676-686Abstract Full Text Full Text PDF PubMed Scopus (95) Google Scholar Thus, macrophage accumulation in cholestatic mouse models appears to be more harmful than helpful. Typically, as part of the innate immune response, the major function of a macrophage is phagocytosis, whereby it clears away invading pathogens, dead cells, and other tissue debris. However, the persistence of macrophage accumulation, and the contribution of this increased, persistent accumulation to an exacerbated peribiliary fibroinflammatory response in PBC and PSC, implicates a dysfunctional macrophage response in these cholestatic liver diseases. Whether this dysfunction is due to an impairment in macrophage phagocytosis is unknown. In this issue of the Journal of Hepatology, the study by Chen et al. provides insight into how the transcription factor AT-rich interaction domain 3A (ARID3A) contributes to a deleterious macrophage response. Their data show that ARID3A impairs macrophages’ ability to phagocytose apoptotic cells (efferocytosis) via inhibition of the MER proto-oncogene, tyrosine kinase (MERTK) signaling pathway (Fig. 1).[6]Chen R. Huang B. Lian M. Wei Y. Miao Q. Liang J. et al.A+T rich interaction domain protein 3a (Arid3a) impairs Mertk-mediated efferocytosis in cholestasis.J Hepatol. 2023; 79: 1478-1490Abstract Full Text Full Text PDF Scopus (2) Google Scholar This work nicely builds on the authors’ previous studies in which they defined how a PBC-associated genetic variant is linked to upregulation of ARID3A expression in myeloid cells, leading to differentiation of these cells to a proinflammatory state.[7]Li Y. Li Z. Chen R. Lian M. Wang H. Wei Y. et al.A regulatory variant at 19p13.3 is associated with primary biliary cholangitis risk and ARID3A expression.Nat Commun. 2023; 14: 1732Crossref PubMed Scopus (3) Google Scholar Here, the authors extend their work by demonstrating increased expression of ARID3A in PBC patient livers, particularly in CD33 and CD11b immune cells, and correlated this increased ARID3A expression with increased PBC disease severity. As part of the ARID family of chromatin regulators and transcription factors, ARID3A recognizes AT-rich genomic loci and has been shown to bind gene targets that encode the variable regions of immunoglobulins. As such, aberrant, increased expression of ARID3A is associated with autoreactivity of B cells in systemic lupus erythematosus potentially by driving autoantibody production.[8]Guo C.C. Xu H.E. Ma X. ARID3a from the ARID family: structure, role in autoimmune diseases and drug discovery.Acta Pharmacol Sin. 2023. Jul 24; (Epub ahead of print)https://doi.org/10.1038/s41401-023-01134-2Crossref Scopus (1) Google Scholar In PBC, ARID3A binds to promoter regions of key myeloid regulatory genes, such as SPI1, and knockdown of ARID3A in monocyte-derived macrophages decreases expression of fibroinflammatory molecules.[7]Li Y. Li Z. Chen R. Lian M. Wang H. Wei Y. et al.A regulatory variant at 19p13.3 is associated with primary biliary cholangitis risk and ARID3A expression.Nat Commun. 2023; 14: 1732Crossref PubMed Scopus (3) Google Scholar Thus, the involvement of ARID3A in autoimmune diseases is becoming well established, and further elucidating the role of this transcription factor in cholestatic, immune-mediated diseases such as PBC and PSC, and particularly the effect on macrophage function in these diseases, is important. The findings from Chen et al. demonstrate for the first time that ARID3A negatively regulates transcription of MERTK in monocytes in PBC and PSC. MERTK, along with TYRO3 and AXL, form the “TAM” receptors, a group of tyrosine kinase receptors commonly expressed on phagocytic cells that recognize and bind to the GAS6/PROS1/phosphatidyl serine complex found on the surface of apoptotic cells.[9]Burstyn-Cohen T. Fresia R. TAM receptors in phagocytosis: beyond the mere internalization of particles.Immunol Rev. 2023. Oct; 319 (Epub 2023 Aug 19): 7-26https://doi.org/10.1111/imr.13267Crossref PubMed Scopus (2) Google Scholar Upon binding to apoptotic cell ligands, TAM receptors initiate intracellular signaling pathways involved in actin remodeling and phagocytosis leading to engulfment of the dead cell. This receptor-mediated phagocytic removal of dead cells is known as efferocytosis, a process that is crucial to maintaining tissue homeostasis and an anti-inflammatory immune response.[9]Burstyn-Cohen T. Fresia R. TAM receptors in phagocytosis: beyond the mere internalization of particles.Immunol Rev. 2023. Oct; 319 (Epub 2023 Aug 19): 7-26https://doi.org/10.1111/imr.13267Crossref PubMed Scopus (2) Google Scholar Moreover, it has been shown that transcription factor KLF4-driven upregulation of the MERTK receptor was required for the switch of alveolar macrophages to a pro-restorative phenotype, resulting in an increased capacity for efferocytosis and a quicker injury resolution response.[10]Chakraborty S. Singh A. Wang L. Wang X. Sanborn M.A. Ye Z. Maienschein-Cline M. et al.Trained immunity of alveolar macrophages enhances injury resolution via KLF4-MERTK-mediated efferocytosis.J Exp Med. 2023; : 220Google Scholar These studies highlight the importance of MERTK-mediated efferocytosis in tissue injury resolution, underscoring a key role for macrophage efferocytosis in health and disease. Chen et al. also noted that mouse monocyte-derived macrophages with increased Arid3a expression had decreased Mertk expression and impaired efferocytosis. However, if they blocked Arid3a expression in myeloid cells, they could reverse this phenomenon in three mouse models of biliary injury. They accomplished this by creating a myeloid-specific Arid3a conditional knockout mouse (Arid3aΔLysm) which they: (i) treated with DDC (3,5-diethoxycarbonyl-1,4-dihydrocollidine), which induces cholestatic liver injury, (ii) immunized with 2OA-BSA (2-octynoic acid coupled to BSA) to induce autoimmune cholangitis, or (iii) crossed with the Mdr2-/- mouse, a cholestatic mouse model.[6]Chen R. Huang B. Lian M. Wei Y. Miao Q. Liang J. et al.A+T rich interaction domain protein 3a (Arid3a) impairs Mertk-mediated efferocytosis in cholestasis.J Hepatol. 2023; 79: 1478-1490Abstract Full Text Full Text PDF Scopus (2) Google Scholar In all three models, depletion of Arid3a decreased liver-infiltrating macrophages, reduced hepatic inflammation and fibrosis, enhanced macrophage efferocytosis, and increased macrophage expression of reparative polarization marker, CD206. Moreover, the authors showed that this amelioration of cholestatic liver injury in the Arid3aΔLysm mouse model was due to an increase in macrophage Mertk expression as treatment with a Mertk-specific inhibitor reversed the increased ability of Arid3aΔLysm macrophages to efferocytose apoptotic BECs.[6]Chen R. Huang B. Lian M. Wei Y. Miao Q. Liang J. et al.A+T rich interaction domain protein 3a (Arid3a) impairs Mertk-mediated efferocytosis in cholestasis.J Hepatol. 2023; 79: 1478-1490Abstract Full Text Full Text PDF Scopus (2) Google Scholar In addition to the findings in their mouse models, Chen et al. also showed that circulating monocytes and hepatic macrophages in patients with PBC and PSC have increased ARID3A expression, linking this to a potential deficiency in MERTK-mediated macrophage clearance of apoptotic BECs and poor disease outcomes.[6]Chen R. Huang B. Lian M. Wei Y. Miao Q. Liang J. et al.A+T rich interaction domain protein 3a (Arid3a) impairs Mertk-mediated efferocytosis in cholestasis.J Hepatol. 2023; 79: 1478-1490Abstract Full Text Full Text PDF Scopus (2) Google Scholar This study supports previous findings that implicate proinflammatory macrophages as key drivers of pathogenesis in cholestatic liver diseases and provides further evidence that a uniquely distributed subset of macrophages localized to the bile ducts may be involved in peribiliary fibroinflammation and disease progression.[3]De Muynck K. Heyerick L. De Ponti F.F. Vanderborght B. Meese T. Van Campenhout S. et al.Osteopontin characterizes bile duct associated macrophages and correlates with liver fibrosis severity in primary sclerosing cholangitis.Hepatology. 2023. Aug 2; (Epub ahead of print)https://doi.org/10.1097/HEP.0000000000000557Crossref PubMed Google Scholar,[4]Guillot A. Winkler M. Silva Afonso M. Aggarwal A. Lopez D. Berger H. Kohlhepp M.S. et al.Mapping the hepatic immune landscape identifies monocytic macrophages as key drivers of steatohepatitis and cholangiopathy progression.Hepatology. 2023; 78: 150-166Crossref PubMed Scopus (12) Google Scholar,[11]Liang W. Huang X. Shi J. Macrophages serve as bidirectional regulators and potential therapeutic targets for liver fibrosis.Cell Biochem Biophys. 2023. Dec; 81 (Epub 2023 Sep 11): 659-671https://doi.org/10.1007/s12013-023-01173-wCrossref PubMed Scopus (2) Google Scholar While more work is needed to confirm if increased ARID3A expression inhibits MERTK-mediated efferocytosis of apoptotic cells in PBC and PSC, and if a lack of apoptotic BEC clearance contributes to PBC and PSC disease progression, it does highlight how targeting macrophage efferocytic function could be a novel and important therapeutic intervention in these liver diseases. Reprogramming macrophages to become more phagocytically adept could overcome limitations associated with drug studies that only target chemokines and cytokines involved in macrophage recruitment and retention at sites of injury. Here, functional redundancy in macrophage chemokine and cytokine signaling pathways, in addition to the heterogeneity of macrophage populations, has reduced the effectiveness of treatment strategies that only target cellular homing via these pathways.[12]Mantovani A. Allavena P. Marchesi F. Garlanda C. Macrophages as tools and targets in cancer therapy.Nat Rev Drug Discov. 2022; 21: 799-820Crossref PubMed Scopus (253) Google Scholar,[13]Bart V.M.T. Pickering R.J. Taylor P.R. Ipseiz N. Macrophage reprogramming for therapy.Immunology. 2021; 163: 128-144Crossref PubMed Scopus (24) Google Scholar Reprogramming macrophages to target apoptotic or injured (e.g., senescent) BECs by modulating key molecules in the MERTK pathway (e.g., ARID3A) could help ameliorate liver injury and promote liver repair and regeneration in cholestatic liver diseases. In the end, we know macrophages make a difference in health and disease by maintaining tissue homeostasis and balancing the need for a targeted attack on invading pathogens and injured cells with the need for injury resolution and tissue restoration. They do this by adapting to local, environmental cues and shifting their polarization state based on how they need to function in response to a given stimuli. In cholestatic diseases such as PSC and PBC, macrophages not only accumulate around the bile ducts, but their increased presence is also associated with disease progression. Therapeutically targeting macrophages; more specifically, the subset of macrophages located near bile ducts, may ameliorate peribiliary fibroinflammation in PSC and PBC. While additional studies are needed to better understand this peribiliary subset of macrophages and their functional profile, the important work by Chen et al. indicates that macrophage efferocytosis of apoptotic cholangiocytes is crucial for liver tissue repair and restoration. In circumstances where macrophages become dysfunctional due to impaired efferocytosis, possibly through aberrant ARID3A/MERTK signaling and sustained tissue injury, modulating efferocytosis by targeting ARID3a expression may be a novel therapeutic strategy. The authors received no financial support to produce this manuscript. The authors have no conflict of interest related to the manuscript. Please refer to the accompanying ICMJE disclosure forms for further details. CT and NFL contributed to the writing of this editorial. 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