Departments of Molecular Physiology and Biological Physics,
Biomedical Engineering and Cardiovascular Research Center, University
of Virginia, Charlottesville, Virginia 22908
Chemokines regulate inflammation,
leukocyte trafficking, and immune cell differentiation. The role of
chemokines in homing of naive T lymphocytes to secondary lymphatic
organs is probably the best understood of these processes, and
information on chemokines in inflammation, asthma, and neurological
diseases is rapidly increasing. Over the past 15 years, understanding
of the size and functional complexity of the chemokine family of
peptide chemoattractants has grown substantially. In this review, we
first present information regarding the structure, expression, and
signaling properties of chemokines and their receptors. The second part
is a systems physiology-based overview of the roles that chemokines
play in tissue-specific homing of lymphocyte subsets and in trafficking of inflammatory cells. This review draws on recent experimental findings as well as current models proposed by experts in the chemokine field.
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INTRODUCTION |
CHEMOKINES ARE SMALL CHEMOATTRACTANT
peptides that are structurally very similar (189), as are
their cognate receptors (130). Many of the proximal signal
transduction pathways that are activated after receptor ligation are
also very much alike (9, 12). However, the specific
expression, regulation, and receptor binding patterns of each chemokine
create a functional diversity that allows chemokines to play roles in
such disparate processes as organogenesis (120),
hematopoiesis (94), neuronal communication with microglia
(76), and leukocyte trafficking (41).
Chemokines may have originated from proteins with essential
intracellular functions (186) through gene duplication and
selected mutation at a relatively recent evolutionary stage
(203). Some chemokines and their receptors may have
evolved to fight specific infections (130). Conversely,
some microorganisms such as the human immunodeficiency virus-1
(112) or herpesviruses (184) have evolved
mechanisms to exploit the chemokine system to promote their survival in
the host. Dysregulation of chemokines and chemokine receptors has also
been implicated in various autoimmune conditions (7),
further emphasizing the importance of understanding the physiological
roles of this complex network of molecules.
The chemoattractant property of chemokines was first demonstrated in a
chemotaxis assay for neutrophils using interleukin-8 (IL-8)
(200), and pertussis toxin-sensitive G protein-coupled receptor signaling was later shown to be required for this effect (129, 191). Soon after their discovery, some chemokines
were found to be induced at sites of inflammation and required for proper recruitment of leukocytes to various tissues (10,
11). The goal of this review is to provide a current
understanding regarding the roles of chemokines and their receptors in
leukocyte recruitment to specific tissues under normal physiological
conditions as well as in models of inflammation and disease.
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STRUCTURE |
Chemokines (Table
1) are highly basic
proteins of 70-125 amino acids with molecular masses
ranging from 6 to 14 kDa (94). Most are secreted, although
some, such as fractalkine, are expressed on the cell surface
(86). Although sequence identity (Fig.
1) among chemokines can be quite low
(189), the overall tertiary structure is strikingly
similar (34). In most situations, chemokines are thought
to act as monomers (12). Most chemokines contain at least
four cysteines that form two disulfide bonds, one between the first and
the third and one between the second and the fourth cysteine (Fig.
2). The resulting structure contains
three 
-sheets with short loops in a Greek key formation. Chemokines
are subdivided into CC, CXC, or CX3C groups based on the
number of amino acids between the first two cysteines. Lymphotactin
(93) is the only known chemokine that contains only two
cysteines (C chemokine), corresponding to the second and fourth
cysteines of other classes. The two regions of each chemokine that
interact with the receptor are an exposed loop in the backbone between
the second and third cysteine, believed to be required for low-affinity
binding of chemokines to their receptors, and the
NH2-terminal portion before the first cysteine, which
represents the region of most variability. The NH2-terminal
binding site is required for receptor signaling upon ligation, and the
length and amino acid composition of the NH2 terminus
determines whether a chemokine will bind with high affinity to a
receptor and whether binding has agonistic vs. antagonistic effects
(34). CXC chemokines are further classified according to
the presence of the tripeptide motif glutamic acid-leucine-arginine (ELR) in the NH2-terminal region. ELR+ chemokines are
specific for myeloid cells, whereas ELR
chemokines attract a variety
of leukocytes.
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Fig. 1.
Dendrogram showing the amount of protein sequence similarity among
all known human chemokines. Protein sequences were obtained from the
National Center for Biotechnology Information protein database. The
phylogenetic tree was constructed using the Clustalw program provided
by the European Bioinformatics Institute and analyzed using TreeView
(139). The scale bar reflects the horizontal distance at
which sequences diverge by 10% (90% identity). Amino acid identity
between a pair of chemokines is given by 1 x, where
x is the sum of the 2 horizontal distances to the right of
the pair's vertical branch point. For example, the horizontal
distances before the vertical branch point of monocyte chemoattractant
protein (MCP)-1 and MCP-3 are 13.6 and 12.4%, respectively. Therefore,
the amino acid identity between these chemokines is 100 (13.6 + 12.4)% or 74%. MDC, macrophage-derived chemokine; SDF,
stromal cell derived factor; BCA, B cell-activating chemokine; IL,
interleukin; NAP, neutrophil-activating protein; GRO, growth-related
oncogene; ENA, Epithelial cell-derived neutrophil-activating factor;
I-TAC, interferon-inducible T cell chemoattractant; Mig, monokine
induced by  -interferon; IP, inducible protein; CTACK, cutaneous T
cell attracting chemokine; LARC, liver- and activation-induced
chemokine; RANTES, regulated on activation normal T cell expressed and
secreted; MIP, macrophage inflammatory protein; DC, dendritic cell;
HCC, hemofiltrate cc chemokine; MPIF, myeloid progenitor inhibitory
factor; TARC, thymus- and activation-related chemokine; TECK,
thymus-expressed chemokine; ELC, Epstein-Barr virus-induced receptor
ligand chemokine; SLC, secondary lymphoid tissue chemokine.
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Fig. 2.
Comparison of the 3-dimensional structures of human CXC homeostatic
[stromal cell-derived factor (SDF)-1], CXC inflammatory (GRO ), and
CC inflammatory (RANTES) chemokines. Each chemokine is displayed with
NH2 terminus on the right and COOH-terminal
-helix to the left. The 2 disulfide bonds are
also shown. Protein Data Bank files for SDF-1 (ID# 1QG7), GRO (ID#
1MGS), and RANTES (ID# 1RTO) were obtained from the National Center for
Biotechnology Information structure database. Files were analyzed using
RasWin 2.6-ucb (Roger Sayle, Glaxo Wellcome, Hertfordshire,
UK).
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Chemokine receptors (Table 2) are
heptahelical G protein-coupled receptors, typically
340-370 amino acids in length with 25-80% amino acid
identity (Fig. 3), and common features
including an acidic NH2 terminus, a conserved 10-amino acid
sequence in the second intracellular loop, and one cysteine in each of
the four extracellular domains (130). Structures of
chemokine receptors have yet to be solved, although their transmembrane
domains are likely similar to rhodopsin (115). Homodimers
may be the functional form of at least some chemokine receptors
(151). The chemokine binding site is complex, involving
several noncontiguous sites, including the NH2-terminal
segment (130).
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Fig. 3.
Dendrogram showing the amount of protein sequence similarity among
all known human chemokine receptors. Protein sequences were obtained
from the National Center for Biotechnology Information protein
database. The phylogenetic tree was constructed using the Clustalw
program provided by the European Bioinformatics Institute and analyzed
using TreeView (139). The scale bar reflects the
horizontal distance at which sequences diverge by 10% (90% identity).
Amino acid identity between a pair of chemokine receptors is given by
1 x, where x is the sum of the 2 horizontal distances to the right of the pair's vertical branch point.
For example, the horizontal distances before the vertical branch point
of CXCR1 and CXCR2 are 12.9 and 11.8%, respectively. Therefore, the
amino acid identity between these chemokine receptors is 100 (12.9 + 11.8)% or 75.3%.
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SIGNAL TRANSDUCTION AND CONSEQUENCES OF RECEPTOR LIGATION |
The consequences of chemokines binding their receptors have been
studied extensively both in vivo and in vitro (12). One effect brought about by all chemokines involves the chemotaxis of the
cell expressing the receptor toward areas with higher concentrations of
the chemokine. The receptor for chemokines with transmembrane domains
like fractalkine may also induce adhesion and migration in a manner
analogous to adhesion molecules (86). However, most chemokines are secreted, and to elicit chemotaxis in vivo, these highly
basic proteins must be immobilized on cell or extracellular matrix
surfaces by interacting with negatively charged glycosaminoglycans. Interestingly, specific chemokines bind different types of
glycosaminoglycans with divergent affinities (101).
Glycosaminoglycan type can vary with cell type, location, and
inflammatory status. Therefore, selective immobilization at a given
site may be a regulatory step that determines chemokine function in
certain tissues or inflammatory states. Furthermore, oligomerization of
chemokine occurs on glycosaminoglycans and may provide a mechanism for
gradient formation (81). Chemokines near their sites of
production may form higher order oligomers on endothelial or
extracellular matrix glycosaminoglycans, thereby creating and
preserving higher concentrations of chemokine near the initiating
inflammatory or trafficking stimulus that cause the leukocyte to move
up the chemokine gradient and toward the relevant site.
Other effects are more specific to certain chemokines and include
cellular shape changes (Fig. 4),
extension of lamellipodia through cytoskeletal restructuring, and
release of oxygen radicals, histamine, and cytotoxic proteins from
neutrophils, basophils, and eosinophils, respectively (9).
Certain chemokines can trigger integrin-dependent firm adhesion of
rolling cells, an important step in the trafficking of leukocytes to
sites of inflammation (107). IL-8 and monocyte
chemoattractant protien (MCP)-1 can trigger
2-integrin-mediated firm adhesion of monocytes
(57) on intracellular adhesion molecule-1
(ICAM-1)-expressing cells in flow chambers. KC, a mouse chemokine
related to IL-8, but not JE, mouse MCP-1, triggers arrest of monocytes
via 41-integrin in atherosclerotic
arteries (84). Secondary lymphoid tissue chemokine (SLC),
liver- and activation-induced chemokine (LARC), Epstein-Barr
virus-induced receptor ligand chemokine (ELC), and stromal cell derived
factor (SDF)-1, but not IL-8, MCP-1, regulated on activation normal T
cell expressed and secreted (RANTES), eotaxin, macrophage inflammatory
protein-1 (MIP-1), or thymus-and activation-related chemokine
(TARC), also trigger firm adhesion of lymphocytes on ICAM-1
(31). SLC is the only known chemokine that can trigger 47-integrin-mediated firm adhesion of
lymphocytes to mucosal addressin cellular adhesion molecule-1
(MAdCAM-1) (138). Mice lacking CXCR2 show elevated
leukocyte rolling velocity (128), suggesting potential
involvement of this receptor in slowing down rolling leukocytes via
engagement of 2-integrins before arrest (100).
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Fig. 4.
Electron micrograph showing the shape of human neutrophils before
(left) and 5 s after (right) chemoattractant
stimulation. [Reprinted with permission (9)].
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Most work pertaining to signal transduction events (Fig.
5) downstream from
chemokine receptor ligation thus far has focused on neutrophils and
interactions between CXCR1, CXCR2, and their respective ligands
(9, 12). Except for the receptor desensitization pathways
discussed below, most signaling depends on coupling through Bordatella
pertussis toxin-sensitive G proteins (20). CXCR1 and CXCR2
couple most commonly through Gi2, but also through G14, G15, and G16, but not
Gq or G11 (191). Downstream
of G proteins, receptor ligation leads to activation of many
intracellular pathways. One consequence of ligation shared by all
chemokine receptors with the exception of low affinity receptors such
as the Duffy antigen receptor complex (33) is activation
of phosphotidylinositol-specific phospholipase C, which through
inositol triphosphate and diacylglycerol leads to transient
increases in cytosolic free Ca2+ through mobilization of
intracellular stores of Ca2+ and activation of protein
kinase C. In neutrophils, both of these steps are required for granule
release and superoxide production but not for migration
(11). Cytoskeletal restructuring leading to shape changes,
firm adhesion, and chemotaxis results from the activation of small
GTPases such as Rho (20, 104). CXCR1, but not CXCR2,
activates phospholipase D, leading to superoxide formation by human
neutrophils (91, 102). MAP kinases such as ERK2
(90), protein kinase B (179), and numerous
transcription factors are also upregulated by receptor ligation.
Phosphotidylinositol-3-OH kinases (PI3K) are activated by
G
, small GTPases, SRC-related tyrosine kinase and
phosphotyrosines binding the SH2 domain of PI3K (12) and
may play key roles in chemokine signal transduction (178,
180), including chemotactic migration (11) and
homologous and heterologous desensitization (Fig. 5).
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Fig. 5.
Overview of the major intracellular signaling events
induced by a chemokine binding to its receptor and activating responses
in a neutrophil (top). In response to strong or prolonged
chemokine stimulus, the receptor undergoes homologous desensitization
(middle), in which phosphorylation by a G protein-coupled
receptor kinase leads to (thick arrow) uncoupling of G proteins,
binding of arrestins, and subsequent internalization of the receptor.
Heterologous desensitization (bottom) occurs through
activation of protein kinase C and protein kinase A by another
chemoattractant, leading to (thick arrow) phosphorylation of the
receptor and phospholipase C2, respectively, and the
subsequent inactivation, but not internalization, of the receptor.
i2, G-protein subunits; PLD, phospholipase D;
PLC2, phospholipase C isoform 2; STK, soluble
tyrosine kinase; PA, phosphatidic acid; DAG, diacylglycerol; PKC,
protein kinase C; IP3, inositol triphosphate; PI3K,
phosphotidylinositol 3-kinase; Akt, protein kinase B; GPCRK,
G-protein-coupled receptor kinase; fMLP,
formylmethionyl-leucyl-phenylalanine; cAMP, cyclic adenine
monophosphate, MAP kinase, mitogen-activated protein kinase; Ras and
Rho, small G proteins; P, phosphate.
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Homologous receptor desensitization occurs when a receptor binds
chemokine and is phosphorylated by a G protein receptor-coupled kinase
(4, 150). The receptor is subsequently internalized. This
mechanism plays a major role in determining the duration of leukocyte
trafficking, migration, or sequestration in certain situations
(146). Heterologous desensitization occurs without receptor ligation and results from serine phosphorylation of the receptor by a kinase activated by a different signaling cascade (105). One example is the desensitization of CXCR1, CXCR2,
CXCR4, and CCR5 through activation of formyl peptide receptors by the tripeptide formylmethionyl-leucyl-phenylalanine (fMLP)
(4, 43). Only in some instances is this desensitization
accompanied by internalization of the receptor. Unlike homologous
desensitization, this pathway is completely inhibited by blocking
protein kinase C. The importance of this phenomenon in inflammation and
leukocyte trafficking is not completely clear (105), but
the capacity for heterologous desensitization appears to be different
among chemoattractant receptors, suggesting a hierarchy of
chemoattractants (54).
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SPECIFICITY AND INTERSPECIES VARIATION |
Chemokine and chemokine receptor pairs vary widely in terms of
selectivity (Tables 1 and 2). Certain chemokines bind only one receptor
and vice versa, such as the exclusive interactions of CXCR4 with SDF-1
(42), CXCR5 with B cell-activating chemokine-1 (BCA-1)
(106), CCR6 with LARC (199), CCR9 with
thymus-expressed chemokine (TECK) (99), CCR10 with
cutaneous T cell attracting chemokine (CTACK) (79), and
CXCR6 with CXCL16 (124). Another pattern of pairing
involves chemokine receptors that exclusively bind two or three
chemokines, as illustrated by CCR7 binding SLC and ELC (197,
198), CXCR3 binding -interferon-inducible protein (IP-10),
monokine induced by -interferon (Mig), and interferon-inducible T
cell chemoattractant I-TAC (35, 110), CCR4 binding TARC
and macrophage-derived chemokine (MDC) (85), and CCR8
binding TARC and T cell-activation protein-3 (TCA-3)
(202). Many other receptors and chemokines are far more
promiscuous. CCR3 for instance has been shown to bind eotaxin,
eotaxin-2, eotaxin-3, MCP-2, MCP-3, MCP-4, and RANTES (9).
RANTES has been shown to bind at least CCR1, CCR3, and CCR5 with high
affinity (130). There are also chemokines such as lungkine
(152) and dendritic cell chemokine-1 (DC-CK1)
(1) that exert their effects through as yet unidentified receptors. In general, chemokines and receptors that are involved in
inflammatory trafficking and activation of cells tend to
participate in overlapping and redundant pairing, whereas those
involved in homeostatic homing tend to show more exclusive
interactions. Chemokines from the same gene cluster all tend to bind
the same or similar receptors. The genes for many inducible
inflammatory CC chemokines are on human chromosome 17, all of the ELR+
CXC chemokines are on chromosome 4, ELC and SLC are on chromosome 9, and MDC and TARC are on chromosome 16 (130).
The relatively late evolutionary appearance of chemokines resulted in a
large amount of chemokine and receptor variation among species.
Although CXCR1 is thought to play a pivotal role in neutrophil recruitment in humans (11), it does not appear to have a
homolog in mice, and whereas a homologous protein is present in rats, it is expressed on macrophages and not neutrophils (130).
Human chemokines for which a mouse homolog has not yet been discovered include IL-8, neutrophil-activating protein-2 (NAP-2),
I-TAC, MCP-4, hemofiltrate CC chemokine-1 (HCC-1),
myeloid progenitor inhibitory factor-1 (MPIF-1) and -2, and eotaxin-2
and -3, whereas mouse chemokines with no human counterpart include
MIP-related protein-1 (MRP-1) and -2, lungkine, and MCP-5
(203). Findings in animal studies, though often very
beneficial to understanding human chemokine and receptor actions,
should therefore be interpreted cautiously.
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EXPRESSION PATTERNS |
Chemokines are typically expressed in one of two characteristic
patterns (Table 1). Chemokines involved in homeostatic trafficking such
as SDF-1, BCA-1, SLC, ELC, CTACK, and TECK are expressed constitutively
by many cell types in tissue-specific sites and contribute to
homeostatic homing in these areas as discussed in detail below. The
expression of inflammatory chemokines, in contrast, is induced only
under specific conditions, typically by inflammatory cytokines. In
general, lipopolysaccharide (LPS), IL-1, and tumor necrosis factor
(TNF)- induce broad expression of inflammatory chemokines by a
variety of cell types, whereas other inflammatory mediators induce more
specific responses (172). T-helper 1 (Th1)-like responses
produce inflammatory reactions characterized by interactions between
macrophages, neutrophils, and type-1 helper T cells that produce the
cytokines IFN- and IL-12. Th2 responses produce allergic reactions
involving eosinophils, mast cells, basophils, and type-2 helper T cells
that produce the cytokines IL-4 and IL-13 (184). IFN-
induces expression of a number of chemokines that act to recruit
monocytes, neutrophils, and Th1 lymphocytes, while IL-4 and IL-13
induce MCP-1, eotaxin, TCA-3, TARC, and MDC, which lead to a Th2
pattern of cell recruitment. IL-4 and IFN- can antagonize each
other's chemokine induction (23). Only activated effector lymphocytes usually respond to inflammatory chemokines, because naive
cells typically do not express receptors for these chemokines (172).
Chemokine receptors fall loosely into two categories of expression,
those expressed exclusively on a small number of leukocyte types and
those that are more broadly expressed. CXCR4, present on T cells, B
cells, monocytes, neutrophils, blood-derived dendritic cells, and
others, is the most widely expressed chemokine receptor. CXCR1 and
CXCR2 are expressed on most leukocytes but appear to be functionally
significant only for neutrophils, monocytes/macrophages, and mast cells
(130, 135). CXCR3, CXCR5, and CXCR6 are expressed exclusively on cells of lymphoid lineage (51, 147, 182).
CCR1, CCR2, and CCR4-CCR10 are expressed mainly on lymphocytes,
monocytes, and monocyte-derived dendritic cells (130).
CCR3 has a unique expression pattern, as it is found on eosinophils,
mast cells, basophils, Th2 lymphocytes, and certain dendritic cell
populations (155, 161).
Chemokine receptor expression is also regulated by a variety of
inflammatory stimuli (172). T cell expression of CCR1,
CCR2, and CXCR3 is induced and maintained by IL-2, but inhibited by activation through the CD3 complex (111, 113), whereas
CCR3 expression requires the synergistic effects of IL-2 and IL-4
(88). CCR5 is upregulated by Th1 cytokines, but can be
suppressed by IL-10 (140), whereas CXCR4 can be
upregulated or downregulated by IL-4 or IFN-, respectively
(5). Transforming growth factor- decreases CCR1, CCR2,
CCR3, and CCR5, whereas it upregulates CCR7. Interferon-, which
induces expression of CCR1 and decreases CCR4, can either increase or
decrease CCR3 and CXCR3 expression depending on T cell polarization
(160). Activation of cells can also change chemokine
expression. For T cells, activation through T cell receptor stimulation
triggers a decrease in expression of CCR1, CCR2, CCR3, and CCR5;
increased expression of CCR7, CCR8, and CXCR5; and experimental condition-specific changes in levels of CXCR3, CCR4, and CXCR4 (119, 172). Activation leads to decreased CXCR5 on B cells
(51), whereas maturing monocyte-derived dendritic cells
switch off expression of receptors for inflammatory chemokines and
increase expression of lymph node-homing chemokines
(162). Thus expression of chemokine receptors is an axis
of regulation that can greatly influence leukocyte trafficking patterns.
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ROLE OF CHEMOKINES IN TISSUE-SPECIFIC LEUKOCYTE TRAFFICKING |
Secondary lymphoid organs.
Secondary lymph organs play critical roles in the initiation of immune
responses by serving as sites where naive T cells and B cells become
activated through interactions with circulating memory cells and
maturing dendritic cells arriving from sites of inflammation. These
interactions serve as the basis for the selection, proliferation, and
reprogramming of antigen-specific B cells and T cells, which enables
homing of the correct subtypes of these cells to sites of inflammation
where the initiating agent can be contained and destroyed
(41). Whether these events occur in lymph nodes, Peyer's
patches, the spleen, or in tonsilar tissue depends on the site of
antigen encounter by dendritic cells and the presence of
region-specific cell surface chemokine receptors and adhesion molecules
on leukocytes (28). For instance, expression of the
intestine-tropic integrin 47 causes naive
cells to home selectively to Peyer's patches (74). The
chemokine-dependent events (Fig. 6) that
mediate cellular interactions may be similar in the various secondary
lymphoid organs (27), but there is also evidence for organ
specificity (99).
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Fig. 6.
Chemokine involvement in dendritic cell and lymphocyte
trafficking to and from secondary lymphoid organs. Dendritic cells
undergoing maturation induced by interferon (IFN)- and tumor
necrosis factor (TNF)- lose expression of inflammatory chemokine
receptors and increase expression of CCR7, leading to their trafficking
to lymphoid organs through afferent lymphatics (bottom
left). Naive T cells require secondary lymphoid tissue chemokine
(SLC) to exit high endothelial venules (top middle) and
follow SLC and Epstein-Barr virus-induced receptor ligand chemokine
(ELC) gradients to T cell zones. At these sites, T cells differentiate
into Th1, Th2, circulating memory, and follicular homing cells and the
differential chemokine receptors expressed on these subsets produces
trafficking to Th1-type inflammation (bottom left), Th2-type
inflammation (bottom right), and B cell follicles,
respectively. Naive B cells inside lymphoid organs traffic toward BLC
expressed in follicles, whereas mature B cells leave the follicles by
migrating toward SDF-1 expressed in the area surrounding the follicles.
Chemokines: IP-10 (pink diamond), SDF-1 (open circle), RANTES (blue
circle), TARC (yellow triangle), IL-8 (black square), eotaxin (red
trapezoid), MCP-1 (red square), MDC (green triangle), SLC (black star),
ELC (black half-circle), BLC (red cross). PMN, neutrophil; M ,
macrophage; DC, dendritic cell; T, naive T cell; Th1, Th1-type T cell;
Th2, Th2-type T cell; TCM, circulating memory T cell;
TFH, follicular homing T cell; EOS, eosinophil;
BAS, basophil; MAST, mast cell; B, naive B cell; mB, mature B cell;
HEV, high endothelial venule; MBP, major basic protein.
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Human immature dendritic cells derived from monocytes express the
inflammatory chemokine receptors CXCR1, CCR1, CCR2, and CCR5
(162, 171), which allow these cells to follow chemotactic gradients to inflammatory sites. Once there, dendritic cells process antigen and become exposed to the maturation-stimulating cytokines TNF- and IFN- (158). Maturing dendritic cells
express large amounts of MIP-1 and MIP-1 through 3 h
postinduction along with IP-10, MCP-2, and RANTES in a more sustained
fashion (162). One consequence of this increased chemokine
expression is the downregulation of inflammatory chemokine receptors on
maturing dendritic cells, particularly CCR1 and CCR5, by homologous
receptor desensitization. Although transcriptional regulation may also
be involved, posttranscriptional events appear to dominate
(163). The second consequence of this upregulation of
chemokines is that it strengthens the original chemotactic gradient,
further boosting recruitment of immature dendritic cells, monocytes,
and T cells and creating a continuous antigen-sampling loop
(157). In addition, maturing dendritic cells increase
CCR4, CXCR4, and CCR7 expression and ELC/SLC responsiveness in a manner
that is resistant to homologous receptor desensitization (158). CCR7 induces dendritic cell migration to secondary
lymphatic organs (53). ELC, TARC, MDC, and DC-CK1 are all
upregulated by maturing dendritic cells (162), leading to
increased interactions with and stimulation of T cells.
Entry of naive lymphocytes into lymph nodes occurs via transmigration
across high endothelial venules beginning with a three-step sequential
process involving L-selectin-mediated rolling, a chemokine-induced signaling event, and integrin-mediated firm adhesion to the endothelium (27, 184). In vitro experiments have shown that the
signaling event required for firm adhesion is mediated by the chemokine SLC binding CCR7 on the surface of naive T lymphocytes
(31). CCR7 / mice show marked T cell and dendritic
cell transmigration defects in lymph nodes and Peyer's patches
(53). Similar defects are seen in plt mice that
lack SLC expression in lymphoid organs (70). Once inside
lymphoid organs, T cells and dendritic cells migrate toward T cell
zones in a process that requires CCR7, SLC, and ELC (41).
SLC is produced by endothelial cells of lymphoid organs and stromal
cells within T cell zones of spleen, lymph nodes, and Peyer's patches
(72). ELC is expressed primarily in T cell areas by
dendritic cells, macrophages, and other cells (41).
B cell trafficking to secondary lymphoid organs is much less
altered by the absence of CCR7 or SLC (53, 70), suggesting other chemokines are involved in triggering firm adhesion of B cells on
high endothelial venules. Naive B cells that recirculate to lymphoid
organs express high levels of CXCR5 (51), which, through
interactions with the chemokine BLC, leads to B cell migration into
follicular areas and their subsequent maturation. BLC is produced
specifically within B cell areas by stromal cells and follicular
dendritic cells (71). CXCR5 / and BLC / mice have
disorganized lymph nodes and decreased antibody responses (6,
52). CXCR5 expression is decreased on activated B cells (51), which may allow their emigration from lymphoid
organs. Interestingly, activation of germinal center B cells makes
these cells temporarily unresponsive to SDF-1 despite maintaining CXCR4 expression (18). Once B cells are fully mature,
responsiveness is regained to SDF-1 expressed in areas surrounding
follicles, and thus this regulation may serve as a mechanism to
block premature exit of maturing B cells from germinal centers
(94).
Within T cell zones of lymph nodes and Peyer's patches, activation
leads to the production of polarized sets of T cells. Th1 cells
selectively express high levels of CCR5 and CXCR3, although functional
CXCR3 is also expressed at low levels on Th2 cells (22,
114). These receptors, along with decreased levels of CCR7, are
thought to allow migration of Th1 cells to inflammatory sites where
IFN- has induced the production of IP-10, Mig, and MIP-1
(119). All Th2 lymphocytes express CCR4 and CCR8, and a
subset in humans, but not mice (69), expresses CCR3
(5, 22). IL-4-induced MDC, TARC, TCA-3, and eotaxin can
then recruit these cells to sites of allergic inflammation (22,
202). Activation of Th2 cells is also accompanied by markedly
decreased CCR7 expression allowing for migration out of T cell zones
and into B cell areas where these cells may participate in antibody
production (121). Two nonpolarized subsets of T cells,
tonsilar and blood follicular homing (Tfh) T cells, are
also produced on T cell activation in T cell zones. These cells express
high levels of CXCR5 while losing expression of CCR7 (26,
165). Tonsilar Tfh cells (CD4+,
L-selectinlo, CD69hi) are localized within
germinal centers of secondary lymphoid organs and may play a major role
in antibody responses by expressing inducible costimulator (ICOS), a
molecule that binds the signaling protein B7RP on B cells and induces
antibody production (121). Blood Tfh
(CD4+, CD69
, L-selectin+,
ICOS) regain expression of CCR7, do not possess immediate
effector function, and may be analogous to other types of circulating
memory cells (159).
Memory T cells are generally grouped into central memory and effector
memory T cell populations (158). Central memory cells express L-selectin, CCR1, CCR4, CCR6, and CCR7, while lacking CD69 and
immediate effector function. CCR7 allows these cells to migrate through
lymphoid organs to participate in activation of naive B and T cells.
Effector memory cells are able to produce inflammatory cytokines or
exhibit effector function. These cells express combinations of CCR1,
CCR3, CCR4, CCR5, CCR6, and CXCR3. CD4+ memory cells can be
of either central or effector type, whereas most CD8+
memory cells are of the effector type. Within 6 h of T cell
receptor stimulation, effector memory cells lose this chemokine
receptor expression and instead express CXCR5, CCR4, CCR7, and CCR8
(156). Naive cells stimulated in the presence or absence
of IL-4 and IL-12 become effector memory or central memory cells,
respectively (158). Recently, a new chemokine receptor,
CXCR6, that is similar in humans and mice (182), has been
shown to be induced on in vitro T cells primed in the presence of IL-12
but not IL-4 (95, 122). In blood, some CD4+
and CD8+ cells are CXCR6+. Very few CXCR6+ cells are found
in lymph nodes, but large numbers are seen in sites of inflammation.
This evidence suggests that CXCR6 may be a marker of effector Th1 and
Tc1 cells. CXCR6 binds the chemokine CXCL16, a transmembrane or soluble
protein produced by dendritic cells that attracts T cells and natural killer (NK) cells (124).
Lung inflammation.
In asthma and animal models of allergic airway disease, many
leukocytes, including mast cells, basophils, T cells, eosinophils, and
alveolar macrophages, are recruited in tightly controlled spatial and
temporal patterns associated with the pathophysiology of these diseases
(73, 118). The adult respiratory distress syndrome (ARDS)
results from the accumulation of neutrophils within the pulmonary
circulation and alveolar spaces via well-studied adhesion
molecule-dependent and -independent pathways (44). Chemokines and chemokine receptors are thought to play critical roles
in the recruitment, activation, and coordination of leukocytes in both
disease processes.
Much information regarding the expression and function of chemokines in
allergic airway disease (AAD) has been provided by the
ovalbumin-sensitized/challenged AAD model, in which mice are given an
intraperitoneal injection of ovalbumin, followed by intranasal challenge starting ~1 wk later (73). Production of
chemokines has been measured in broncheoalveolar lavage (BAL) specimens
and by immunostaining and in situ hybridization of the interstitium (87). These studies showed chemokine production by a
variety of cell types, including alveolar epithelium, endothelium,
smooth muscle, alveolar macrophages, and, at least for eotaxin,
infiltrating T lymphocytes (73). In this study,
Gutierrez-Ramos et al. (73) place chemokines into three
groups on the basis of temporal expression. Eotaxin, MDC, and MCP-1
levels correlate with the accumulation of eosinophils, T cells, and
monocytes, respectively. RANTES, MCP-5, and MIP-1 levels increase
nonspecifically over the course of inflammation. SDF-1 is expressed
constitutively, and levels do not change over time. Interstitial
eosinophilia and bronchoalveolar lavage (BAL) eosinophilia are blocked
by antagonists to MCP-1, RANTES, and SDF-1 during both sensitization
and challenge periods but not during challenge alone. These findings
imply central roles for these chemokines in initial leukocyte subset
recruitment. Eotaxin blockade decreases all eosinophilia regardless of
time of administration. Blockade of MDC and MCP-5 has no effect on BAL
eosinophilia but significantly blocks interstitial eosinophilia and
consequent airway hyperreactivity and mucous secretion. These findings
suggest that MDC and MCP-5 may be necessary to maintain sequestration
of eosinophils within lung interstitial tissues (73).
The first cells to be recruited to the lungs during allergic
inflammation are mast cells and basophils. MCP-1, MCP-2, MCP-3, RANTES,
MIP-1, and eotaxin, all expressed by various cells in the lung
during early stages of inflammation, can recruit mast cells and
basophils (118). Furthermore, both MCP-1 and RANTES can
activate basophils to release histamine and produce other mediators
(37). For example, intratracheal administration of MCP-1
has been shown to increase leukotriene C4
(LTC4) levels in BAL and leukotriene production in
pulmonary mast cells (29). In the same study, mice
deficient in CCR2 expression were shown to have decreased airway
resistance and histamine release in a murine model of allergen airway
inflammation induced by cockroach antigen. MCP-1 may be critical in the
formation of allergic lung inflammation even in the absence of CCR2
(98). Furthermore, eotaxin can interact with
MCP-1-activated basophils to increase their expression of IL-4,
ensuring that a Th2-type environment is created and enabling subsequent
migration of eosinophils and Th2 lymphocytes and the progression of the
allergic response (118).
On the basis of studies using adoptive transfer of labeled cells along
with neutralizing antibodies to various chemokines, Th2 lymphocyte
homing to lungs during acute airway disease appears to sequentially
rely on SDF-1, eotaxin, and MDC (73). SDF-1 is expressed
constitutively in the lung, and accumulation of CXCR4+ cells
always accompanies inflammatory changes within pulmonary tissue
(64). Whether these cells represent critical precursors that recruit other leukocytes to produce inflammation or whether these
cells home to the lung constitutively is unclear (73). The
next phase of the response involves the upregulation of eotaxin by many
cells including CXCR4+ Th2 cells and early expression of CCR3 by newly
differentiated Th2 cells (108). The influx of more Th2
cells peaks around 4 days after the initial insult, CCR3 is
subsequently downregulated, and CCR4 expression by Th2 lymphocytes and
MDC expression throughout the lung are increased. CCR4 and MDC are
required for maintenance of interstitial Th2 lymphocytes for the
duration of the response (108).
Eosinophil accumulation and activation directly leads to allergic
airway inflammation, as these cells release a variety of toxic granular
proteins and lipid mediators that cause direct damage to alveolar cells
and endothelium as well as indirect damage through influencing other
physiological regulators (9). The end results of this
damage include bronchial smooth muscle contraction, airway
hyperreactivity, increased vascular permeability, and mucous hypersecretion. Initial recruitment of eosinophils is thought to be
mediated primarily through CCR1/MIP-1 (153) and CCR3
and its many ligands expressed in the lung, including eotaxin,
eotaxin-2, RANTES, MCP-3, and MCP-4 (118). Once fully
activated and in tissues, however, eosinophils can migrate in response
to MIP-1 via CCR5, TCA-3 via CCR8, and MDC in a process that does
not involve CCR4 or CCR3 (19, 66).
CCR3 is involved in the initial lung-specific recruitment of mast
cells, basophils, eosinophils, and Th2 cells, all four of the cell
types that are critical in allergic lung inflammation. Mast cells,
basophils, and eosinophils mediate the effector functions, whereas Th2
cells are required to make IgE antibody via IL-4 production and to
prime and activate basophils and eosinophils via IL-5 (9). Blocking CCR3 using the decoy ligand MetRANTES (189)
can significantly block most aspects of allergic airway inflammation,
including bronchial hyperactivity and CD4+ cell and
eosinophil accumulation in the interstitium and broncheoalveolar lavage
fluid (65, 189). Blocking MIP-1 and RANTES using
antibodies has also been shown to significantly reduce eosinophil
accumulation (117). Alveolar macrophages have also been
shown to play a role in chemokine-dependent leukocyte recruitment
through low affinity IgE receptors (80). In response to
large doses of antigen, IgE-dependent activation of these macrophages
causes production of IL-8, MCP-1, and MIP-1 leading to the
recruitment of neutrophils, eosinophils, and other leukocytes
(67).
Although their role in allergic airway disease remains controversial
(118), neutrophils are critical for producing the
pathophysiology in sudden-onset atypical asthma (103) as
well as in ARDS (45). Intratracheal instillation of IL-8
produces large influxes of neutrophils into the lungs in animal models
(192). IL-8 is substantially elevated in BAL from patients
with acute lung injury, and expression levels are positively correlated
to neutrophil influx (103). Blocking IL-8 has been shown
to ameliorate reperfusion-induced neutrophil influx within the lung
(168). Epithelial cell-derived neutrophil-activating
factor (ENA)-78 is stored in mast cells within the lung, and blockade
with an antibody results in a large decrease in lung neutrophil
accumulation (116). Surprisingly, eotaxin may negatively
regulate neutrophil recruitment to the lung by blocking IL-8-mediated
neutrophil chemotaxis (118). Eotaxin-deficient mice have
increased neutrophil and decreased eosinophil numbers during acute
allergic inflammation. The eosinophil deficit is compensated only much
later in the response (154).
Leukocyte trafficking in the central nervous system.
Leukocyte trafficking to the central nervous system (CNS) during
homeostasis, antigen-specific inflammation, or trauma is complicated by
the blood-brain barrier, which prevents most leukocytes from entering
the CNS. Chemokines play key roles in overcoming this barrier
(59). The CNS is unique in that chemokines are critical
for the proper execution of many nonimmune functions. Knocking out the
genes for SDF-1 or CXCR4 results in mice with an embryonic lethal
phenotype, partly because of grossly abnormal organogenesis of the
murine cerebellum (120). Fractalkine is expressed by
neurons and is believed to bind CX3CR1 expressed by
microglia, although the significance of this binding is unclear (76, 136, 167). Despite the blood-brain barrier, selective subsets of leukocytes can home to the CNS under baseline conditions. CXCR3+ T cells, representing 90% of all CD3+ cells in
cerebrospinal fluid (CSF), are seen in small numbers in the absence of
inflammation (82), suggesting that this chemokine receptor
may be required for T cell passage across the blood-brain barrier.
Multiple sclerosis (MS) is an autoimmune condition in which
antigen-specific Th1 cells cross the blood-brain barrier to mediate focal inflammation of the CNS, resulting in a variety of neurological symptoms and deficits (7). CSF levels of IP-10, Mig, and
RANTES are increased in patients with MS undergoing active inflammatory demyelination compared with control subjects without disease
(170). Consistent with Th1 lymphocytes being a key
pathogenic cell type in this disease, CXCR3 is expressed on 99% of T
cells that home to leukocyte-rich areas within MS lesions. These cells
are attracted by IP-10, which is expressed at high levels in MS lesions
(13, 147), perhaps by astrocytes that form the glial
limitans (170). CD8+ T cells
specific for myelin basic protein (MBP) are recruited by these Th1
cells and subsequently produce more IP-10, along with MIP-1 and
MIP-1, leading to a vicious cycle of T cell recruitment through
CCR1, CCR5, and CXCR3 (17, 94). CCR5 is also expressed on
many T cells, macrophages, monocytes, and microglia in CSF and active
lesions of patients with MS (13, 170). The importance of
CCR5 in recruitment of leukocytes during MS is emphasized by the better
prognosis of patients with MS homozygous for the CCR5
32 mutation, a
population including ~1% of all North American Caucasians (82).
Much has been learned regarding chemokine-dependent leukocyte homing to
the CNS during inflammation through the experimental autoimmune
encephalitis model (EAE), in which mice are given a subcutaneous injection of CNS myelin proteins in complete
Freund's adjuvant followed by doses of pertussis toxin to manipulate
disease severity and frequency (82). Symptoms, visible
inflammation, and CNS-specific chemokine expression are always preceded
by initial antigen-specific T cell infiltration into the CNS (61,
92). During initial inflammation, MCP-1, KC, RANTES, TCA3, and
IP-10 are all expressed, possibly by astrocytes (149), 2 days before clinical symptoms (63), and these chemokines
may play pivotal roles in leukocyte recruitment. In situ hybridization
combined with immunocytochemistry for glial fibrillary acidic protein
(GFAP), an indicator of the astrocyte reactive stress response known as astrogliosis, shows that IP-10 and MCP-1 colocalize with GFAP-producing astrocytes, implying that astrocytes may have to undergo astrogliosis to produce chemokines (175). As in MS, IP-10 and CXCR3
play critical roles in EAE. Antisense oligonuceotides for IP-10
(190) and anti-CXCR3 antibodies (7) can block
mononuclear cell infiltration and disease development. In the relapsing
phase of EAE, RANTES and MIP-1 are produced (60) in a
temporal fashion that correlates with leukocyte infiltration and
disease onset and resolution of the acute attack (175).
In animal models of cortex trauma (82), MCP-1 RNA is
elevated 3 h after trauma, before initial infiltration of
leukocytes (58). MCP-1/GFAP colocalization studies suggest
that this MCP-1 is produced by astrocytes, and in neonatal models where
astrogliosis does not occur, no increase in MCP-1 is seen
(14). Spinal cord trauma models (82) have
shown growth-related oncogene (GRO) to be key in producing
neutrophil recruitment within 24 h (125). In the same
study, MCP-1, MCP-5, MIP-3, and IP-10, but not RANTES or MIP-1,
were elevated at later time points, implying that these chemokines may
recruit cell types other than neutrophils to sites of spinal cord injury.
Lymphocyte trafficking and inflammation in the gastrointestinal
system.
The gastrointestinal system contains four spatially distinct
populations of lymphocytes located within Peyer's patches, mesenteric lymph node, lamina propria, and epithelial tissue. Lymphocytes homing
to any of these compartments express the
47-integrin, which binds to MAdCAM-1, an
immunoglobulin-like molecule expressed exclusively by mucosal and high
endothelial venules (16). Selective localization to one of
the four gastrointestinal lymphatic compartments depends on expression
of other surface markers. Most intraepithelial lymphocytes express the
E7-integrin, which can bind E-cadherin expressed on epithelial cells (3). The role of chemokines
in regulating homeostatic homing and inflammatory trafficking of cells
within this mucosal system is just beginning to be understood (119). SLC is the only chemokine that can trigger
47-mediated firm adhesion to MADCAM-1 in
flow chambers in vitro (138), and thus by analogy to its
role in peripheral lymph nodes, SLC may play a role in homing to
Peyer's patches and mesenteric lymph node. The constitutive expression
of a number of chemokines by intestinal epithelial cells has been noted
(3), although the significance of most of these molecules
is unclear. Positioning of chemokines at specific sites in the
epithelium may be important. LARC is expressed by cells comprising the
villous epithelium, but not by crypt cells, whereas TECK is expressed
in the opposite pattern (99, 174). TECK, constitutively
expressed selectively by the small intestine, has recently been
postulated to play a critical role in homing to the lamina propria and
the epithelium of this organ. All small intestine-homing
CD4+ and CD8+ T cells express CCR9, the
receptor that exclusively binds TECK (99).
Interestingly, there also appear to be inflammation-specific pathways
of T cell trafficking to the gut (119). Under
physiological conditions, virtually all lamina propria and
intraepithelial lymphocytes express CXCR3, and this expression
can be significantly increased in vitro by IFN- (46,
169). In the same studies, CXCR3 ligands, IP-10, and Mig were
shown to be expressed on intestinal cells in vitro after IFN-
treatment, and antibodies to these chemokines blocked intraepithelial
lymphocyte chemotaxis in epithelial cell-conditioned medium. Thus
Th1-type responses leading to the production of IFN- may cause
intestine-specific homing through induction of chemokines and their
respective receptors. This model may be applicable to ulcerative
colitis, as IP-10 is expressed at high levels in focal lesions
associated with this disease (181), although CCR3+ Th2 cells are also present at these sites (56).
Skin-homing lymphocytes and cutaneous allergies.
Skin-homing memory cells express the cutaneous lymphocyte antigen
(CLA), which correlates with E-selectin binding of these cells
(28). Specific responses to skin allergens are restricted to CLA+ cells (164). Two chemokines are also critical for
skin-specific homing. CTACK, a recently discovered chemokine thought to
be expressed exclusively in the skin by keratinocytes, selectively
attracts CLA+ skin-homing T cells, which presumably express the CTACK
receptor CCR10 (127). TARC is expressed in venules in
chronically inflamed skin, and its receptor CCR4 is expressed on
lymphocytes that infiltrate inflamed skin but not intestine
(30). In the same study, TARC was shown to trigger
integrin-dependent firm adhesion of CLA+ cells, but not
47+ cells, in flow chambers coated with
ICAM-1 in vitro, further demonstrating the selective role TARC plays in
recruitment of skin-homing cells. Through CCR6 binding, LARC is able to
direct the constitutive homing of Langerhans-type dendritic cells to
areas of the epidermis, providing resting antigen presenting capability
(32).
In inflamed skin lymphatics, the ability of glycosaminoglycans to bind
RANTES, MCP-1, and MCP-3, but not MIP-1 or IL-8, is thought to play
a role in determining which inflammatory cells are recruited (25,
83). RANTES and MCP-3 recruit CCR3+ Th2 cells in patients with
contact dermatitis (56), and these chemokines also
activate eosinophils and basophils in vitro, causing their chemotaxis
and the release of mediators such as histamine and leukotrienes
(10). RANTES may also recruit and activate cutaneous mast
cells, as a RANTES antagonist is effective in reducing eosinophilia and
swelling by blocking mast cell degranulation (177, 189).
Chemokine-dependent leukocyte recruitment has been studied extensively
in atopic dermatitis. The current model postulates that the initial
phase of the disease is mediated by antigen-specific Th2 cells that
produce local inflammation. These cells use chemokines to subsequently
recruit eosinophils and macrophages, which then produce cytokines such
as IL-12 that induce a switch to a Th1-type response (68).
Eighty-five percent of T cells in skin inflammatory infiltrates are
CLA+ (143). Eotaxin expression by resident mononuclear cells may be the key step in recruiting CCR3+ Th2 cells and later eosinophils (80). T cell expression of eotaxin and CCR3
appears to be greatly elevated in patients with atopic dermatitis vs. normal controls, whereas there are no differences in MCP-3, MIP-1, and IL-8 (195). RANTES is also increased in atopic lesions
(166), although peak expression occurs at 24 h, much
later than the peaks of chemokines that initially attract Th2 cells
(196). Late expression of RANTES may help facilitate the
switch from a Th2- to a Th1-type response, as RANTES can attract Th2
cells via CCR3 and Th1 cells via CCR5 (80).
Bone marrow.
SDF-1 is expressed at high levels by stromal cells in bone marrow
(176). Mice deficient in SDF-1 or its receptor CXCR4 do not survive past the first week in utero and have profound defects in
hematopoiesis, particularly B cell lymphopoiesis and myelopoiesis (120, 131). Whereas in humans SDF-1 is thought to attract
B cells in all stages of development, mouse SDF-1 only attracts pro-
and pre-B cells (42). The current model is that SDF-1 may be important in directing B cells to proper sites of maturation within
the bone marrow microenvironment, and the loss of SDF-1 responsiveness
after maturation allows exit of mature B cells into the circulation
(9). ELC and SLC have recently been shown to attract pre-B
cells after CD34 expression is downregulated and before IgM is
upregulated. CCR7 and its ligands may therefore also be important in
allowing B cell emigration from bone marrow (96).
Thymus.
In the thymus, dendritic cells involved in negative selection express
MDC and TECK, and stromal cells have been shown to express TARC, SDF-1,
LARC, SLC, and ELC (94). TECK attracts thymocytes, macrophages, and dendritic cells in the thymus and may assist in
negative selection by attracting macrophages to destroy double positive
(CD4+, CD8+) thymocytes that are bound by
dendritic cells with high avidity (183). SDF-1 may direct
homing within the thymus of the more immature thymocyte populations, as
it selectively attracts triple negative (CD3,
CD4, CD8), double negative
(CD3+, CD4, CD8), and double
positive thymocytes. In contrast, ELC selectively attracts the more
mature single positive (CD4+ or CD8+) thymocyte
population in the medulla and may aid in their migration into the
circulation (97). MIP-1, which binds CCR5 and CCR8, attracts double positive or single positive thymocytes and may be
involved in differentiation of CD8+ or CD4+
subsets, as double positive and CD8+ thymocytes express the
MIP-1 receptor CCR5, whereas CD4+ thymocytes are
attracted to this chemokine via CCR8 (94). MDC, LARC, and
SLC are expressed in thymus but do not attract immature thymocytes.
These chemokines may be responsible for negative feedback of T cell
production mediated by circulating mature cells. (94)
Liver.
Specialized resident NK 1.1+ T cells and Kupffer cells home to the
liver constitutively by as yet unknown mechanisms. In
acetaminophen-induced liver toxicity (APAP), MCP-1 is expressed at high
levels, potentially by Kupffer cells (21). CCR2 / mice
develop substantially more injury than do wild-type mice, and this
susceptibility can be ameliorated by blocking TNF- or IFN-
(78), suggesting MCP-1 and CCR2 may have protective
functions during this type of oxidative injury. Fractalkine is also
expressed by Kupffer cells in APAP, and fractalkine / mice show
decreased neutrophil infiltration, serum aspartate aminotransferase
(AST), and overall injury (21). ELR+ CXC chemokine
administration usually leads to increased infiltration of inflammatory
cells but decreases APAP injury due to direct proliferative effects on
hepatocytes (77). These effects may also be required for
hepatic regrowth in partial resection models (36).
Ischemia/reperfusion models, unlike APAP models, have shown
that MCP-1 is increased by reactive oxygen species and may produce
injury by increasing ICAM-1 expression on hepatic endothelium (194). Reducing this chemokine decreases subsequent injury
(193), suggesting that reperfusion-induced inflammation
and subsequent oxygen radical production may occur by a mechanism
distinct from that of APAP. In chronic alcoholic liver disease and
cirrhosis (21), MCP-1 is required for infiltration of
monocytes and subsequent inflammation and fibrosis, and expression
levels correlate with AST levels and severity of injury (2, 50,
123). IL-8, MCP-1, and MIP-2 are produced by ethanol-challenged
hepatocytes in rats and are linked to increased ICAM-1 and vascular
cellular adhesion molecule-1 (VCAM-1) expression, both of which are
required for neutrophil infiltration (132, 142). T cell
migration during chronic hepatitis appears to be selectively controlled
by liver expression of IP-10 (173), suggesting that the
infiltrating cells may have a Th1 phenotype.
Atherosclerosis and vasculitis.
Chemokines are also involved in site-specific homing of leukocytes to
atherosclerotic plaques. RANTES, released by thrombin-stimulated platelets, is present on the luminal surface of carotid arteries of
apolipoprotein E-deficient mice with early atherosclerotic lesions, and
this chemokine along with KC, but not MCP-1, can trigger monocyte
arrest on atherosclerotic endothelium in an ex vivo-perfused carotid
artery model (84, 185). The formation of a plaque is
believed to begin through accumulation of minimally modified
low-density lipoproteins (MM-LDL), which become trapped in the
extracellular matrix of subendothelial spaces and stimulate endothelial
cells and smooth muscle cells to produce MCP-1 and IL-8
(40). KC, a murine chemokine related to human IL-8 and GRO-, triggers arrest of rolling monocytes in atherosclerotic arteries (84). Subsequent transmigration of large numbers
of monocytes into the subendothelial space can be blocked by
high-density lipoproteins (HDL), antioxidants, or anti-MCP-1 antibodies
(133). Apolipoprotein E-deficient mice lacking CCR2 show
smaller lesion sites than apolipoprotein E-deficient control mice
(24). Once monocytes are present, MM-LDL are oxidized and
taken up by monocytes, creating foam cells that produce MCP-1, IL-8,
MIP-1, MIP-1, and RANTES (187). These chemokines
promote migration of CCR2+ and CCR5+ macrophages and lymphocytes into
the plaque. Many of these chemokines have other, pleiotropic effects
within plaques (187). MCP-1 induces proliferation of
smooth muscle cells (145), whereas IL-8 is mitogenic and
chemotactic for smooth muscle cells (201). IP-10, also
expressed at high levels in plaques, attracts lymphocytes via CXCR3,
but also attracts and leads to the proliferation of smooth muscle cells
(188). RANTES, thought to be important in typical plaque
formation by recruiting lymphocytes, has been shown to be required in
variants such as transplantation-associated accelerated atherosclerosis
(141).
Wegener's granulomatosis is a disease characterized by a systemic
necrotizing vasculitis likely caused by cytoplasmic antineutrophil cytoplasmic antibodies (cANCA) that recognize a proteinase expressed on
the surface of TNF--primed neutrophils and monocytes
(15). IL-8 may play a key role in the pathogenisis of this
vasculitis, as cANCA has been shown to greatly increase IL-8 expression
by TNF-primed peripheral blood monocytes (148). RANTES has
been localized to Wegener's lesions and therefore may also play a role (39). IL-8 expression is also significantly increased by
neutrophils and mononuclear cells isolated from patients with systemic
vasculitis due to Kawasaki disease (8). Serum RANTES
levels correlate closely with disease activity in Takayasu arteritis
(137). MCP-1 is present in the vessel wall and plasma
MCP-1 levels are significantly elevated in patients with temporal
arteritis and polymyalgia rheumatica (47). In a rat model
of chronic adjuvant-induced vasculitis, infusion of MCP-1 produces a
large increase in neutrophil transendothelial migration
(89), suggesting that MCP-1 may play a critical role in
the development of vascular lesions.
Other diseases.
Chemokine-dependent inflammation in other tissues is just beginning to
be understood. Fractalkine has been shown to induce glomerulonephritis
(48), whereas MetRANTES and MIP-2 antibodies have been
shown to be effective in treating this disease (49, 109).
Interestingly, RANTES expression in the kidney is downregulated by
activation of NO production pathways (94). MetRANTES can also block inflammation in collagen-induced arthritis, a murine model
of rheumatoid arthritis (144). IP-10 levels are high in rheumatic fluid, as are the levels of CXCR3+ and CCR5+ Th1 cells, but
not CCR3+ Th2 cells (56). Coxsackie virus-induced
myocarditis appears to involve MIP-1 (38), implying
that there are important roles for chemokines in heart pathology as
well. Chemokines are also thought to play critical roles in acute
allograft rejection (134). Expression of CXCR3, CXCR4, and
CCR5 was upregulated on circulating and graft-infiltrating lymphocytes
after liver transplantation (62). The presence of CXCR3
and IP-10 correlated strongly with acute rejection of human cardiac
allografts (126). CXCR3 / and CCR5 / mice or mice
treated with neutralizing antibodies to either of these receptors
appear to be resistant to the development of cardiac allograft
rejection (55, 75).
| |
FUTURE DIRECTIONS |
Because of the structural and functional differences of chemokines
between humans and rodents (203), the development of
animal models that are true representations of human conditions is
challenging. Another challenge is the production of antibodies and
small molecule inhibitors of chemokine receptors and chemokines
(146). Data obtained with these reagents would complement
data from knockout animals to help determine causal relationships
between chemokines and certain homing patterns in homeostasis and
inflammation. Much more work needs to be done to understand
constitutive lymphocyte homing pathways, particularly for lung, liver,
spleen, the gastrointestinal system, and the CNS. Due to the redundancy
of the chemokine system, further understanding of general intracellular
events produced by chemokine receptor ligation is necessary to
understand how cell movement toward a chemotactic gradient is regulated
and how chemokines interact with other classes of chemoattractants and their receptors. This knowledge would allow development of therapeutic interventions at the signaling level that may be broadly applicable to
large sets of chemokines and their receptors, thereby producing better
clinical outcomes in complex inflammatory diseases.
Address for reprint requests and other correspondence: K. Ley, Health Science Center Box 800759, Charlottesville, VA 22908 (E-mail: klausley{at}virginia.edu).
TOP
ABSTRACT
INTRODUCTION
