Chemokines in the immunopathogenesis of hepatitis C infection

Abstract

Chronic infection with the hepatitis C virus, a noncytopathic hepatotropic RNA virus, affects over 170 million people worldwide. In the majority of cases, neither the early innate immune response nor the later adaptive immune response succeeds in clearing the virus, and the infection becomes chronic. Furthermore, in many patients, the ineffective inflammatory response drives fibrogenesis and the development of cirrhosis. It is critical to understand this immune pathology if preventative and curative therapies are to be developed. Chemokines are a superfamily of small proteins that promote leukocyte migration and orchestrate the immune response to viruses, including hepatitis C virus. Chemokines are crucial for viral elimination, but inappropriate persistence of expression in chronic hepatitis C infection can drive tissue damage and inflammation. Here we review the role of chemokines and their receptors in hepatitis C virus infection.

Figure 1

Important major human chemokines that act on cells of the lymphocyte and monocyte lineages are shown. 50 human chemokines and 18 chemokine receptors have been identified which can be divided into subfamilies based on the position of conserved cysteine residues within a conserved tetra-cysteine motif. In CC chemokines the first two consensus cysteines are next to each other; in CXC chemokines they are separated by a non-conserved amino acid. These two subfamilies account for all but three of the known chemokines the other being CX3CL1 (three intervening amino acids between the first cysteines) and XCL1 and XCL2, which lack two out of four canonical cysteines. Ligands and their receptors are connected by lines and the main leukocytes expressing receptors are shown by crosses. Receptors that are important for leukocyte recruitment to the liver are highlighted in yellow and the intrahepatic cell types expressing the receptors shown in the far right hand column. PMN: polymorphs; Mo: monocyte; mDC: myeloid DC (imm: immature and mat: mature); pDC: plasmacytoid DC; NK: Natural killer cell; NKT: Natural killer T cell; T naïve: naïve T cell; T mem: memory T cell; Th: T helper and T_reg: regulatory T cells. Data are drawn from many sources including references,, and our own unpublished data on liver-infiltrating lymphocytes.

Figure 2. Chemokines are critical factors in regulating lymphocytes recruitment from blood into the liver

A) The process of leukocyte extravasation into tissue involves at least four stages. Cells are captured by carbohydrate-dependent ‘tethering’ which brings the flowing cell into contact with the vessel wall allowing interactions with the endothelium. In the subsequent ‘triggering’ stage chemokines (immobiised on endothelial proteoglycans activate chemokine receptors on leukocytes resulting in activation of integrin binding to endothelial ligands such as ICAM-1 and VCAM-1. During this stage the leukocyte arrests on the vessel wall and then migrates on the endothelium looking for the signals that drive transendothelial migration into tissue. Chemokines are also involved in transmigration during which leukocytes migrate across endothelium and enter tissue. Once in tissue, the cell follows chemokine gradients to sites of infection using chemokine-mediated changes in the actin cytoskeleton to propel migration. B) Chemokines are immobilized on the endothelium by proteoglycans in the glycocalyx that present chemokines to flowing leukocytes. The endothelium can secrete chemokines itself or present chemokines secreted by underlying cells in tissue including hepatocytes, stellate cells, Kupffer cells and infiltrating leukocytes; or capture chemokines secreted upstream by other structures such as cholangiocytes. C) The CXCR3 ligands CXCL9-11 dominate in the recruitment of CXCR3 expressing effector lymphocytes via sinusoidal endothelium whereas CCR5 ligands are more important for recruitment via portal vascular endothelium. The critical trigger for CXCR3 ligand induction in the infected liver is IFNγ acting synergistically with proinflammatory cytokines such as TNFα secreted by multiple cells in response to viral infection and injury.

Figure 2. Chemokines are critical factors in regulating lymphocytes recruitment from blood into the liver

A) The process of leukocyte extravasation into tissue involves at least four stages. Cells are captured by carbohydrate-dependent ‘tethering’ which brings the flowing cell into contact with the vessel wall allowing interactions with the endothelium. In the subsequent ‘triggering’ stage chemokines (immobiised on endothelial proteoglycans activate chemokine receptors on leukocytes resulting in activation of integrin binding to endothelial ligands such as ICAM-1 and VCAM-1. During this stage the leukocyte arrests on the vessel wall and then migrates on the endothelium looking for the signals that drive transendothelial migration into tissue. Chemokines are also involved in transmigration during which leukocytes migrate across endothelium and enter tissue. Once in tissue, the cell follows chemokine gradients to sites of infection using chemokine-mediated changes in the actin cytoskeleton to propel migration. B) Chemokines are immobilized on the endothelium by proteoglycans in the glycocalyx that present chemokines to flowing leukocytes. The endothelium can secrete chemokines itself or present chemokines secreted by underlying cells in tissue including hepatocytes, stellate cells, Kupffer cells and infiltrating leukocytes; or capture chemokines secreted upstream by other structures such as cholangiocytes. C) The CXCR3 ligands CXCL9-11 dominate in the recruitment of CXCR3 expressing effector lymphocytes via sinusoidal endothelium whereas CCR5 ligands are more important for recruitment via portal vascular endothelium. The critical trigger for CXCR3 ligand induction in the infected liver is IFNγ acting synergistically with proinflammatory cytokines such as TNFα secreted by multiple cells in response to viral infection and injury.

Figure 3

Complex networks of autocrine and paracrine interactions involving many cell types determine the chemokine milieu within the HCV infected liver and thereby determine which leukocytes are recruited. Early events are a consequence of viral infection of hepatocytes and activation of PRR on resident liver cells. This triggers the secretion of chemokines including CCL2; CXCL10; CCL3 and CCL5 which recruit the first wave in innate immune cells including NK cells, NK T cells, monocytes (mono) and plasmacytoid DCs (pDC). These infiltrating cells amplify and broaden chemokine secretion by secreting interferons. Infiltrating monocytes and NKT cells secrete large amounts of IFNγ which further stimulates resident liver cells to secrete chemokines including CXCL10 as well as providing autocrine stimulation of monocytes to secrete CXCL10 and CCL5. pDCs secrete CCL3 and also large amounts of IFNα which acts on monocytes to induce them to secrete CCL2, CCL4 and CXCL10. Effector T cells including Th1 cells are recruited in response to CXCL10 and secrete more IFNγ helping to sustain the Th1 polarized local environment.

Figure 4

Distinct chemokines are involved in the recruitment and positioning of effectors cells in the HCV infected liver. Recruitment via portal vascular endothelium involved CCR5 ligands and the adhesion molecules ICAM-1, VCAM-1 and VAP-on liver endothelium which together promote adhesion and transmigration into tissue. Recruitment via the sinusoids appears to be dependent on CXCR3 rather than CCR5. The chemokine signals are displayed on the endothelial glycocalyx as described above. The mechanisms of migration through the subendothelial tissues are poorly understood but probably involve interactions with underlying matrix and fibroblasts. Localization at infected hepatocytes involves integrin-mediated adhesion to the hepatocytes triggered by chemokines upregulated on infected cells including the transmembrane chemokines CXCL16, shown in this example, and fractalkine.

Figure 5. A sketch of the liver micro-architecture from sinusoidal space (top) to matrix between hepatocytes (bottom) is shown

Direct effects of viral infection of hepatocytes or interactions between viral particles or proteins and liver cells may trigger chemokine secretion (yellow and green stars) and leukocyte recruitment. Lymphocytes are seen crossing the sinsuoidal endothelium using the adhesion molecules VAP-1, ICAM-1 and VCAM-1 and then interacting with hepatic stellate cells (purple) in the Space of Disse using VCAM-1 to promote motility into the underlying tissue. Multiple chemokines are involved at each stage secreted as part of local paracrine and autocrine networks induced by viral infection. Chemokines are retained in the infected liver by binding to the endothelial glycocalyx and cell matrix (pink filaments). Viral proteins and the effects of PAMPs during viral infection can activate endothelium and induce local chemokine secretion; viral proteins also activate stellate cells to secrete chemokines which can then be retained in matrix or transcytosed to the endothelial surface to promote recruitment. Infection of hepatocytes themselves leads to activation of chemokine transcription and secretion; finally viral proteins or particles can interact with infiltrating leukocytes to modulate the secretion of and response to chemokines.