Introduction
Primary biliary cirrhosis (PBC) and primary sclerosing cholangitis (PSC) are chronic cholestatic liver diseases characterized by progressive destruction of bile ducts. Whereas PBC affects the small intrahepatic bile ducts, PSC involves the intra- and/or extrahepatic bile ducts. PBC is an organ-specific autoimmune disease with female predominance, genetic predisposition, high titer serum anti-mitochondrial autoantibodies, disease-specific antinuclear autoantibodies, frequent association with other autoimmune diseases, and strong indications that the destruction of small bile ducts is autoimmune-mediated [
1]. PSC shares many of these features including
p-ANCA autoantibodies in up to 80% of patients, genetic predisposition, and association with other autoimmune diseases. In particular, there is a link between inflammatory bowel disease (IBD), with >70% of PSC patients having IBD, most frequently ulcerative colitis, at some point during their lives. Nonetheless, PSC differs from classical autoimmune diseases in male rather than female predominance and is poorly responsive to immunosuppressive treatment. In addition, there is only circumstantial evidence that bile duct destruction in PSC is an immune-mediated response to an autoantigen [
2,
3].
Both PBC and PSC are characterized by massive infiltration of T cells into the portal tract, the infiltrate consisting predominantly of T cells [
4-
7]. For reviews and discussion on the immunobiology of PBC, including generic mechanisms of autoimmunity, we refer to several recent and key publications [
8-
22].
In PBC liver compared to peripheral blood, there is a 100-fold enrichment of CD4+ T cells and a 10-fold enrichment of CD8+ T cells specific for the major PBC autoantigen, strongly suggesting that both populations participate in the bile duct injury seen in PBC [
1]. In keeping with a major role for cellular immune mechanisms in PBC, there is type-1 cytokine predominance in serum and liver of PBC patients [
23,
24]. Very little is known about the contribution of T cells to the lesions of PSC and the cytokine milieu in PSC liver, except that liver-infiltrating lymphocytes (LIL) from PSC patients produce significantly increased amounts of TNFα and IL-1β but decreased levels of IFNγ [
7].
According to the general paradigm, leukocyte extravasation is a complex multistep process that begins with “tethering” and rolling of blood-born leukocytes, i.e., reversible and transient interactions mediated mainly by selectins. This slows down the immune cells sufficiently to allow them to interact with chemokines presented by endothelial cells. These can then increase the affinity and avidity of leukocyte integrins for their endothelial adhesion molecule receptors, thereby allowing firm adhesion to occur. The adhesion molecules involved in firm attachment frequently belong to the immunoglobulin superfamily, which includes intercellular adhesion molecules (ICAM)-1–5, vascular cell adhesion molecule 1 (VCAM-1), and mucosal addressin cellular adhesion molecule 1 (MAdCAM-1). Firm adhesion is a prerequisite for subsequent transendothelial migration into the tissue. Once lymphocytes have entered a specific tissue, they follow sequential and combinatorial chemotactic gradients to their final destination.
Chemokines play an essential role at every step of this adhesion cascade and subsequent migration. They do so by increasing the affinity and avidity of integrins for their adhesion molecules, providing chemotactic signals for transendothelial migration and the subsequent migration to specific compartments within a tissue and by triggering integrin activation required for the binding to adhesion and cell-matrix molecules involved in tissue retention. They are divided into four subfamilies (CC, CXC, C, and CX3C, with CC and CXC chemokines representing the largest group) based on the arrangement of the first two of four conserved cysteine residues. They can be further subdivided functionally into homeostatic and inflammatory chemokines (see Table
1 for the nomenclature and receptor usage of inflammatory cytokines). Inflammatory chemokines are induced or upregulated during inflammation and mediate leukocyte recruitment; homeostatic chemokines are constitutively expressed and regulate navigation of leukocyte precursors during hematopoiesis and of mature lymphocytes in secondary lymphoid tissue [
25]. Note, however, that some chemokines (e.g., CCL21) do not neatly fit into one of these categories.
Table 1
Chemokines, their official and common names, and their receptors
CCL2 | Monocyte chemoattractant protein-1 | MCP-1 | CCR2, CCR4, CCR9 |
CCL3 | Macrophage inflammatory protein-1α | MIP-1α | CCR1, CCR5 |
CCL4 | Macrophage inflammatory protein-1β | MIP-1β | CCR5, CCR9 |
CCL5 | Regulated upon activation normal T l | | CCR1, CCR3, CCR5, CCR9 |
CCL20 | Macrophage inflammatory protein-3α, Liver and activation regulated chemokine | MIP-3α, | |
LARC | CCR6 | | |
CCL21 | Secondary lymphoid chemokine | SLC | CCR7 |
CCL25 | Thymus-expressed chemokine | TECK | CCR9 |
CCL28 | | | CCR10 |
CXCL9 | Monokine induced by IFNγ | MIG | CXCR3 |
CXCL10 | | IP-10 | CXCR3 |
CXCL11 | IFN-inducible T cell a chemoattractant | I-TAC | CXCR3 |
CXCL12 | Stromal (cell)-derived factor-1 | SDF-1 | CXCR4 |
CXCL16 | | | CXCR6 |
CX3CL1 | Fractalkine | | CX3CR1 |
There is increasing evidence that chemokines do not establish soluble chemokine gradients at the luminal endothelial surface but instead are immobilized via attachment to specific glycosaminoglycans [
26,
27]. Therefore, the production of specific glycosaminoglycans by endothelial cells may represent mechanisms to provide tissue-specific recruitment signals [
26,
27]. In addition, although endothelial cells can produce many chemokines, themselves, they are also able to take up, transcytose and then present chemokines produced by surrounding cells [
27,
28].
Chemokines exert their effects through binding to seven-membrane-spanning G-protein coupled receptors expressed on leukocyte and lymphocyte subsets. Frequently, a certain chemokine can bind to several different receptors; conversely, a receptor can be shared by several chemokines. There is extensive cross-talk between chemokines on numerous levels, e.g., through downregulation of shared receptors [
29], enhancing the surface density of unrelated receptors [
30] or through blocking of signaling events downstream from receptor activation [
31].
Naïve lymphocytes recirculate between blood and peripheral lymph nodes. During this stage, they express L-selectin and CCR7, which allow them to enter lymph nodes which express the L-selectin ligand peripheral lymph node addressin (PNad) and the CCR7 ligands, CCL19 and CCL21. Once lymphocytes encounter their cognate antigen, they generally downregulate CCR7 and L-selectin expression and instead acquire a different set of adhesion receptors that allow them to be recruited to sites of inflammation and/or to “home,” i.e., to preferentially return to the tissue in which they were originally activated. Recruitment and homing to certain tissues has been shown to involve the recognition of specific “addressins,” i.e., tissue-specific ligands expressed by vascular endothelial cells, by homing receptors expressed on the lymphocyte. A further level of specificity is provided by chemokine receptor pairings. For example, homing to the gut is mediated by α4β7 integrin on lymphocytes binding to mucosal MAdCAM-1. The chemokine CCL25, which is capable of activating α4β7, is expressed preferentially in the small intestine, where it interacts with lymphocytes expressing its cognate receptor CCR9. Skin homing involves interactions between cutaneous lymphocyte antigen with E-selectin and CCL17 with CCR4. No specific addressin has been identified for the liver.
A unique feature of the liver is its dual blood supply, with arterial blood entering via the hepatic arteries and venous blood coming from the gut via the portal veins. Hepatic arterioles and portal venules transport arterial and venous blood into the sinusoids, which then take it through the lobule to the central vein. Lymphocytes can enter the liver at several sites, the most important probably being the portal vessels and the sinusoids. Portal veins constitutively express low levels of ICAM-1 and occasionally ICAM-2 and VCAM-1 [
32-
35]. E-selectin is generally not detectable [
32,
36], whereas P-selectin may be weakly expressed on some portal vein endothelia [
32]. Lymphocyte extravasation at this site is thought to proceed according to the general paradigm.
Sinusoidal endothelium constitutively expresses ICAM-1 [
32,
37] and ICAM-2 [
32], whereas levels of VCAM-1 and E-selectin are minimal or undetectable [
32,
36]. Note, however, that constitutive expression of E-selectin has been reported on cultured hepatic sinusoidal endothelial cells (HSEC) [
38]. Lymphocyte recruitment in the sinusoids does not involve the classical tethering and rolling step in cultured human [
39] or mice HSEC [
40]. Instead, the vast majority of lymphocytes immediately arrest. This is thought to be due to the low to undetectable levels of sinusoidal selectin expression, which seems to be dispensable in the low-shear environment of the sinusoids. There are indications that vascular adhesion protein 1 (VAP-1) plays a particularly important role in lymphocyte extravasation in the sinusoids.
The existing data on T lymphocyte recruitment to PSC and PBC liver suggest the following scenario: inflammatory signals in both PBC and PSC liver induce or enhance the expression of adhesion molecules such as ICAM-1, VCAM-1, and MAdCAM-1, whereas VAP-1 expression is not altered. At the same time, a variety of chemokines are also upregulated. In PSC, expression of CCL25, CCL21, and CCL28 all are implicated in activating α4β7 integrins and thereby enhancing lymphocyte binding to MAdCAM-1. In addition, CCL21and CCL28 could promote adhesion to VCAM-1 by activating α4β1 integrin. The same holds true in PBC, except that CCL25 does not participate. Several of these chemokines have also been shown to enhance transendothelial migration. Data on other chemokines are largely confined to PBC. They indicate that induced or upregulated expression of MIG and IP-10 in portal tracts may also contribute to enhanced lymphocyte recruitment into PBC liver. Once lymphocytes have entered the portal tract tissue, they are recruited to, and retained around, the bile ducts by the combinatorial or sequential action of CXCL12 (SDF-1), CXCL16, fractalkine (CX3CL1), CCL28, and possibly MIG and IP-10. At this point, the relative importance of each of these chemokines in the recruitment or the retention of lymphocytes around the bile ducts remains unclear. These limited data underscore the complexity of lymphocyte recruitment and homing to the liver. The data also suggest that there is no liver addressin, but instead, liver homing is likely to require complex combinations of adhesion molecule ligands and chemokine receptors that provide not only entry into the liver but also localization to specific liver compartments.