By
From the Department of Microbiology and Immunology, University of California at San Francisco, San Francisco, California 94143
A quarter of a century ago, it was proposed that veiled
cells in the lymph were antigen-bearing Langerhans'
cells (LCs) en route to the LN T cell area (1, 2). Extensive
investigations have since established that LCs are immature
dendritic cells (DCs) and that insults to the skin Chemokines are small basic proteins that engage seven
transmembrane receptors on responsive cells and promote
chemotaxis (11). First characterized for their role in attracting cells to sites of inflammation, chemokines have more
recently been found to direct cell movements within lymphoid tissues. Two chemokines that have been suggested
to serve a homing function in the T cell compartment are SLC/6Ckine (12) and EBV-induced molecule 1 ligand chemokine (ELC)/macrophage inflammatory protein (MIP)-3 Mice carrying the paucity of lymph node T cells (plt)
mutation, a spontaneous mutation that arose on the DDD/1
strain background, have a defect in T cell homing into LNs
and splenic white pulp (23, 24). The expression pattern and
properties of SLC led Gunn et al. to consider it a candidate
for the plt gene, and this idea received a boost when mapping studies placed the plt mutation on a region of mouse
chromosome 4 syntenic to the region of human chromosome 9 that contains the linked SLC and ELC genes (12, 17, 24). Gunn et al. have now demonstrated that expression of SLC is defective in plt mice (10). This finding and
the prior T cell trafficking studies by Nakano et al. (23, 24)
together provide strong evidence that SLC is necessary for
homing of naive T cells across HEVs and into lymphoid T
cell areas (10). Expression of the potentially closely linked
ELC gene was also reduced in plt mice, although only partially and possibly as a secondary effect of the defective SLC
expression (10). However, despite the mapping data and
absence of SLC mRNA at levels detectable by Northern
blot, sequence analysis of the SLC gene from plt mice has
failed so far to uncover a mutation that could be responsible for the loss of SLC expression (10). Mutation of a distant regulatory region remains a likely possibility, but until
such a mutation is found one must be cautious in concluding that defects in plt mice reflect solely a deficiency in SLC.
In situ hybridization analysis in wild-type mice demonstrated that lymphatic endothelial cells in many tissues
make SLC (13). Taken together with its expression in LN
T cell areas, this finding suggested that SLC might have a
role in homing of DCs from peripheral tissues to lymphoid
T zones. Support for this possibility came in several important studies over the last year showing that maturing DC
upregulate expression of CCR7 and chemotactically respond to ELC (4, 6). At the time these studies were performed, it had not been reported that SLC was a ligand for
CCR7. In vitro studies have since shown that transfection
of cells with CCR7 is sufficient to confer chemotactic responsiveness to SLC as well as ELC (20, 21), making it
likely that CCR7-expressing DCs migrate towards both
chemokines. By studying DCs in plt mice, Gunn et al. have
provided in vivo evidence of a role for SLC in directing
DC migration (10).
The number of DCs in the LNs of plt mice is reduced
approximately threefold compared with wild-type animals,
consistent with a DC homing defect (10). 1 d after skin
painting with the contact sensitizer FITC, the frequency of
FITC-bearing DCs in plt LNs was fourfold less than in
control LNs, providing evidence that SLC is needed for
DC migration from skin to LNs via afferent lymphatics (10). The frequency of LCs in skin was indistinguishable in plt and wild-type mice, so the next question was whether
SLC was required for DCs to enter lymphatic vessels. The
small size of mice makes it difficult to cannulate afferent
lymphatic vessels and perform the direct measurement of
veiled cell frequencies done so elegantly in larger animals
(1, 2). However, a method of tracking LC migration into
lymphatics has been developed in mice where ears are split
in half and incubated in vitro until many LCs begin to mature and migrate into dermal lymphatic vessels (25, 26).
LCs were found to enter the dermal lymphatic vessels of plt
mice with an efficiency that was indistinguishable from controls (10). These studies provide evidence that SLC is
needed for efficient passage of DCs from lymphatic vessels
into LN T zones, but not for entry into the lymphatic vessels themselves.
LCs are members in a family of tissue DCs, and almost
every tissue contains sentinel DCs (3). Although differences
between immature tissue DCs in different locations have
been reported, most tissue DCs have in common the propensity to emigrate to draining lymphoid tissues in response
to LPS, TNF, or IL-1 (3). All the DC types so far tested
upregulate CCR7 upon stimulation, making it likely that
they all use this receptor in order to migrate to lymphoid T
zones (4, 6). It remains to be investigated whether the
same directional cues are also involved in the homeostatic flux of DCs from tissues to LNs that occurs in the absence
of stimulation (3). A subset of DCs in peripheral lymphoid
tissues, including lymphoid lineage DCs (27), may not derive from peripheral tissues but instead may enter directly
from the blood (3). Some insight into the behavior of these
cells in plt mice is provided by findings in the spleen. Wild-type mouse spleen contains a population of DCs in the T
zone that express high levels of DEC205 and that are
thought to be mostly of lymphoid lineage, and a population of myeloid lineage DCs in the marginal zone that express little DEC205 (3). Exposure to LPS causes marginal
zone DCs to migrate rapidly into the splenic T zone (3). In
plt mouse spleen, the distribution of DCs is altered, with
fewer cells located inside white pulp cords (10). In addition, staining for the T zone DC marker DEC205 is substantially reduced, suggesting either that the number of
lymphoid lineage DCs is reduced or that DEC205 expression is dependent on normal organization of cells in a T
zone. LPS treatment of plt mice failed to cause DCs to congregate in areas thought, by their proximity to arterioles, to
be T zones (10). These findings provide evidence that SLC
is needed for homing of multiple types of DCs to lymphoid
T cell areas.
SLC shares the CCR7 receptor with ELC, and an important issue still to be addressed is the relative contribution
of these two chemokines to DC homing to T cell areas.
ELC expression in plt mice is approximately threefold
lower than in wild-type controls (10). At least a fraction of
the ELC produced in lymphoid tissues comes from T zone
DCs (19), making it possible that the reduced ELC expression in plt LNs and spleen is secondary to lower numbers of
T zone DCs. Normal interactions between T cells and
ELC-producing cells, which are likely to be disrupted in plt
mice, might also be important in maintaining ELC expression. However, the possibility that the plt defect directly affects ELC expression has not been ruled out. Whatever the
explanation, the reduced ELC levels may contribute to the
phenotype of plt mice. Reciprocally, the continued expression of significant amounts of ELC in these animals might account for the incomplete block in DC recruitment to
LNs. Studies in CCR7-deficient mice and SLC- and ELC-deficient mice are likely to help resolve the relative importance of SLC and ELC in DC and T cell homing to lymphoid T zones. It is interesting to consider that as ELC can
be made by T zone DCs and can attract antigen-bearing peripheral DCs, this chemokine may have a novel function
promoting DC-DC encounters, possibly leading to the
passing of antigen between DCs and more efficient presentation to T cells.
A model of the events in LC migration to the LN T
zone that incorporates the recent findings on chemokine
and chemokine receptor expression is presented in Fig. 1.
Immature DCs express a variety of inflammatory chemokine receptors, including CCR1, CCR5, CCR6, and
CXCR1, which may participate in DC recruitment to inflamed tissues (4). Differential chemokine receptor expression may contribute selectivity in the recruitment process, since CCR6, the MIP-3
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References
including
exposure to contact sensitizers, bacteria, or UV light
cause many of these cells to enter lymphatic vessels and
travel to LNs (3). During transit the cells undergo a program of maturation events that take them from being
poorly immunogenic to being the most potent of all APCs
(3). Rapid transit of maturing DCs from the site of infection to the draining lymphoid tissue is likely to be critical
for quick initiation of the adaptive immune response. But how
do these cells migrate to lymphatics and subsequently into
the LN T zone? A flurry of recent studies (4) have implicated chemokines and chemokine receptors in directing DC
migration, and now a study reported in this issue provides
strong evidence that one chemokine, secondary lymphoid
tissue chemokine (SLC), plays an important role in DC migration in vivo to T cell zones of LNs and spleen (10).
(17). SLC and ELC are structurally related chemokines and both bind the receptor CCR7 (20,
21). SLC is expressed at high levels by high endothelial venules (HEVs) in LNs and at lower levels by a poorly defined population of stromal cells in T cell areas of LNs,
spleen, and Peyer's patches (13, 15, 16). ELC is made by a
subset of DCs, and possibly by other nonlymphoid cells, in
T cell areas of lymphoid tissue (19). Both chemokines are
efficacious attractants of T lymphocytes (19, 21) and both
can promote integrin activation on rolling lymphocytes
(13, 22). Together these findings have led to the notion
that SLC functions in recruitment of T cells across HEVs
into LNs and more generally in promoting T cell migration into lymphoid T zones. ELC may work with SLC in recruiting cells into the T zone and in the next step, in promoting encounter between T zone DCs and T cells.
receptor, is expressed at
high levels by lung DCs and DCs derived in vitro from
CD34+ cord blood precursors but not by monocyte-derived
DCs (4, 7, 28). It seems likely that some chemokines often
thought of as inflammatory
as well as others still to be
characterized
help recruit immature DCs to become sentinels in noninflamed tissues, a possibility supported by the
finding of constitutively expressed MIP-3
in liver and
lung (29). Monocyte chemotactic protein 1 (MCP1) is expressed constitutively in many tissues (30), and although its
cognate receptor, CCR2, is not highly expressed by immature DCs, it is strongly expressed by monocytes (31). As recent findings indicate that monocytes may differentiate into DCs during migration into lymphatic vessels (32), MCP1
and CCR2 may make an important contribution to DC
trafficking.
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Fig. 1.
Homing of LCs to LN T zones. Multiple types of stimuli
cause epidermal LCs to downregulate receptors for chemokines produced
locally at the site of inflammation and to upregulate CCR7. One CCR7
ligand, SLC, is made by LN HEVs and stromal cells, and by lymphatic
endothelial cells (reference 13; although dermal lymphatics have not yet
been tested for SLC expression). A second ligand, ELC, is made by T
zone DCs and possibly other T zone stromal cells (reference 19). These
CCR7 ligands help direct migration of LCs to the LN T zone where they
function as immunogenic APCs. Dark shading shows an LC moving from
the epidermis to become a veiled cell in the lymph and subsequently an
interdigitating DC in the T zone. Light shading shows B cell areas, where
B lymphocyte chemoattractant (BLC) is made (reference 34). Not shown
are the many other cell types, including monocyte-derived DCs, that
travel via afferent lymphatics to the LN T zone. Within the T zone,
chemokines (including SLC, ELC, BLC, and SDF), extracellular matrix
components, and adhesion molecules all may influence the final positioning and cell-cell interactions of the different DCs.
The rapid upregulation of chemokine expression that
occurs at sites of inflammation should help recruit more
DC precursors to the site, but might also be expected to interfere with the ability of antigen-bearing DCs to emigrate.
Hence, the recently observed rapid decrease in chemokine
receptor function during DC maturation, either by direct
downregulation (4) or by functional modulation as a result of intrinsic expression of chemokine (7), is likely to be
important in allowing the cells to move from the site (Fig.
1). Adhesion molecule changes may also be important for
emigration, including reduced expression of E-cadherin,
activation of 6 integrins, and switching of CD44 isoforms
(3). In addition to these changes, it would seem likely that
attractant cues are needed to guide DCs to the lymphatic
vessels, and the expression of SLC in many lymphatic vessels (13) suggests it has a role at this site. The failure to detect any effect of the plt mutation on DC entry into dermal
lymphatics does not yet exclude a role for SLC, as the plt
mutation may be in a regulatory region of the SLC gene
and so may not fully disrupt SLC expression at all sites. The
possibility that ELC is made by lymphatic endothelium in
the skin has yet to be explored.
Once DCs have entered lymphatic vessels and become veiled cells, how do they then move out into the T zone parenchyma to become interdigitating DCs (Fig. 1)? CCR7, SLC, and ELC can now be said to have a role and CXCR4/stromal cell factor 1 (SDF1) might also contribute (5), but what other molecules are needed for the cells to move from the subcapsular sinus? Cells lining the sinus are part of a larger network of fibroblastic reticular cells that form cords and channels through the LN parenchyma (33). What role does this network play in presenting chemokines and other guidance cues to the migrating DCs? Within lymphoid tissues, even within the T zone itself, DCs are not evenly dispersed. What additional cues contribute to subcompartmentalization of the cells? Finally, as DCs themselves are being found to express an increasing array of chemokines, to what extent do they contribute to the overall organization of the lymphoid tissue? Could migrating DCs provide a homeostatic link between the severity of a peripheral infection and the magnitude of increased lymphocyte retention that occurs in the draining lymphoid tissue? As an understanding of the factors regulating DC migration to the T cell areas of lymphoid tissues has important implications for many aspects of immunobiology, including the development of new adjuvants and immunosuppressants, we can be sure that answers to many of these questions will soon be unveiled.
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Footnotes |
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Address correspondence to Jason Cyster, Department of Microbiology and Immunology, University of California, San Francisco, 513 Parnassus Ave., San Francisco, CA 94143-0414. E-mail: cyster{at}itsa.ucsf.edu
Received for publication 21 December 1998 and in revised form 22 December 1998.
The author thanks Drs. Sanjiv Luther, Lucy Tang, and Ralph Steinman for helpful comments on the manuscript. ![]() |
References |
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