American Journal of Pathology, Vol. 160, No. 1, January 2002
Copyright © American Society for Investigative Pathology
Expression of the Chemokine Receptors CCR4,
CCR5, and CXCR3 by Human Tissue-Infiltrating
Lymphocytes
Eric J. Kunkel,*† Judie Boisvert,†‡
Kristine Murphy,§ Mark A. Vierra,¶
Mark C. Genovese,储 Andrew J. Wardlaw,**
Harry B. Greenberg,†‡ Martin R. Hodge,§
Lijun Wu,§ Eugene C. Butcher,*† and
James J. Campbell††
From the Laboratory of Immunology and Vascular Biology,*
Department of Pathology, the Department of Microbiology and
Immunology,‡ the Department of Surgery,¶ and the Division of
Immunology and Rheumatology,储 Stanford University School of
Medicine, Stanford, California; Center for Molecular Biology and
Medicine,† Veterans Affairs Palo Alto Health Care System, Palo
Alto, California; Millennium Pharmaceuticals, Inc.,§ Cambridge,
Massachusetts; Division of Respiratory Medicine,** Institute for
Lung Health, Leicester University Medical School, Leicester,
United Kingdom; and the Joint Program in Transfusion
Medicine,†† Children’s Hospital, and the Department of
Pathology, Harvard Medical School, Boston, Massachusetts
Differential expression of adhesion molecules and
chemokine receptors has been useful for identification of peripheral blood memory lymphocyte subsets
with distinct tissue and microenvironmental tropisms. Expression of CCR4 by circulating memory
CD4ⴙ lymphocytes is associated with cutaneous and
other systemic populations while expression of CCR9
is associated with a small intestine-homing subset.
CCR5 and CXCR3 are also expressed by discrete memory CD4ⴙ populations in blood, as well as by tissueinfiltrating lymphocytes from a number of sites. To
characterize the similarities and differences among
tissue-infiltrating lymphocytes, and to shed light on
the specialization of lymphocyte subsets that mediate
inflammation and immune surveillance in particular
tissues, we have examined the expression of CCR4,
CXCR3, and CCR5 on CD4ⴙ lymphocytes directly isolated from a wide variety of normal and inflamed
tissues. Extra-lymphoid tissues contained only memory lymphocytes, many of which were activated
(CD69ⴙ). As predicted by classical studies, skin lymphocytes were enriched in CLA expression whereas
intestinal lymphocytes were enriched in ␣47 expression. CCR4 was expressed at high levels by skin-infiltrating lymphocytes, at lower levels by lung and synovial fluid lymphocytes, but never by intestinal
lymphocytes. Only the high CCR4 levels characteristic
of skin lymphocytes were associated with robust chemotactic and adhesive responses to TARC, consistent
with a selective role for CCR4 in skin lymphocyte
homing. In contrast, CXCR3 and CCR5 were present
on the majority of lymphocytes from each non-lymphoid tissue examined, suggesting that these receptors are unlikely to determine tissue specificity, but
rather, may play a wider role in tissue inflammation.
(Am J Pathol 2002, 160:347–355)
Tissue-specific recruitment and retention of lymphocyte
subsets compartmentalizes the immune system. Such
partitioning increases the likelihood that lymphocytes
bearing rare antigen specificities will encounter their cognate antigen. At the same time, compartmentalization
decreases the likelihood that an expanded lymphocyte
population (specific for an antigen found only within a
given tissue) will be exposed to a potentially cross-reactive host-derived antigen within a different tissue.1 Tissue-targeted recruitment and localization are thought to
be achieved by regulated expression of particular homing receptors on lymphocytes (including adhesion and
chemokine receptors), as well as by regulation of their
counter-receptors expressed by endothelial, epithelial,
inflammatory, and parenchymal cells in the target tissues.2,3
The clearest evidence to support the concept of tissuededicated trafficking of peripheral blood lymphocyte
subsets exists at the level of their expression of adhesion
molecules.1,2 Circulating lymphocytes associated with
skin homing express the E-selectin ligand CLA (cutaneous lymphocyte antigen), whereas those associated with
homing to the gastrointestinal tract express the MAdCAM-1 ligand ␣47 integrin. Although skin- and intestinalspecific lymphocyte populations are the best studied,
there is reason to believe that additional populations
dedicated to other tissues may exist.1,2
Supported by NIH grant AI-46784 to J.J.C. and NIH grants GM-37734,
AI-47822, GM-56527 and AI-37832 and a Merit Award from the Veterans
Administration to E.C.B. E.J.K. is a recipient of an Arthritis Foundation
Postdoctoral Fellowship.
Accepted for publication October 9, 2001.
Address reprint requests to James J. Campbell, Ph.D., Joint Program in
Transfusion Medicine, Children’s Hospital, 300 Longwood Avenue, Room
BD-401, Boston, MA 02115. E-mail: james.campbell@tch.harvard.edu
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In addition to differential expression of adhesion molecules, differential expression of chemokine receptors
may also contribute to tissue-specific homing. This hypothesis would predict the disproportionate expression
of particular chemokine receptors by lymphocytes that
have infiltrated tissues that express the corresponding
chemokine ligand. We have recently found support for
this prediction.4 By carefully isolating T cells from various
human tissues, we demonstrated that the chemokine receptor CCR9 is highly expressed by nearly all T lymphocytes infiltrating the small intestine (the one organ outside
of the thymus where the CCR9 ligand TECK is highly
expressed). In contrast, CCR9 was not expressed by T
lymphocytes isolated from other tissues tested, including
skin, liver, synovium, and lung. In contrast to small intestine-derived lymphocytes, CCR9 was expressed by only
a small percentage of colon lymphocytes, suggesting
that circulating intestinal-specific lymphocytes may be
further subdivided into small intestine-dedicated versus
large intestine-dedicated populations.4
To further examine similarities and differences among
lymphocytes that have infiltrated diverse human tissues,
we have investigated the differential expression of three
more chemokine receptors (CCR4, CCR5, and CXCR3)
by tissue-infiltrating lymphocytes. We previously reported
that circulating skin-associated CLA⫹ lymphocytes uniformly express high levels of CCR45 and that the CCR4
ligand TARC6 is expressed on normal and inflamed cutaneous (but not intestinal) endothelium. Taken together,
these two findings suggest that CCR4 and TARC may
participate in skin-specific lymphocyte recruitment.3,5 Interestingly, CCR4 monoclonal antibodies (mAbs) also
recognized a subset of CLA⫺/␣47⫺ peripheral blood
memory CD4⫹ lymphocytes.5 The role of CCR4 here is
unknown, but these cells may represent another skinassociated subset or a subset dedicated to a systemic
tissue.
CCR5 and CXCR3 have been reported to be tissuespecific receptors in multiple organs. CCR5 is expressed
by lymphocytes in intestinal tissues and was suggested
to be a mucosa-specific receptor.7,8 However, other
groups reported CCR5 expression on lymphocytes in the
brains of multiple sclerosis patients,9 liver,10 and on synovial lymphocytes from arthritis patients.11,12 CXCR3
expression was similarly reported on intestinal,7 inflamed
brain,9 liver,10 and synovial11,12 lymphocytes. Thus, the
roles of CCR4, CCR5, and CXCR3 in tissue-specific lymphocyte homing require further clarification.
By identifying the similarities and differences in expression of chemokine receptors by lymphocytes that
have infiltrated a variety of extralymphoid tissues, we
hope to gain an understanding of the specialized CD4⫹
lymphocyte subsets involved in inflammation and immune surveillance in particular tissue sites. Such knowledge may identify pharmacological targets that could be
used to manipulate tissue-specific homing or other chemokine-mediated inflammatory functions, making possible treatments for regional autoimmune diseases (eg,
psoriasis or Crohn’s disease) as well as tailored treatments to prevent rejection of transplanted tissues.
Materials and Methods
Antibodies and Reagents
Anti-human CCR4 mAbs 1G1 and 2B10 (both mouse
IgG1),5 CLA-FITC mAb HECA-452 (rat IgM),13 and
␣47-PE mAb Act-1 (mouse IgG1)14 have been previously described. Unconjugated mouse anti-human
CD45RO (IgG2a, clone UCHL1), CCR5 (IgG2a, clone
2D7; known to correlate best with genetic expression of
CCR515), CXCR3 (IgG1, clone 1C6), and directly conjugated mouse anti-human CD45RA-FITC (IgG2b, clone
HI100), CD69-PE (IgG1, clone FN50), CD3-FITC (IgG1,
clone UCHT1), TCR␣-FITC (IgM, clone T10B9.1A-31),
and CD4-APC (IgG1, clone RPAT4) were obtained from
PharMingen, Inc. (San Diego, CA). Recombinant human
SDF-1␣ and TARC were purchased from Peprotech
(Rocky Hill, NJ). ICAM-1 was purified from human tonsils
as described.5
Tissue Sources and Lymphocyte Isolation
Human jejunum, ileum, colon, lung, tonsil, and inflamed
liver were obtained from patients undergoing various surgical procedures. Synovial fluid (SF) was obtained from
patients undergoing diagnostic arthrocentesis. Peripheral blood was collected in heparinized tubes from
healthy donors. Bronchoalveolar lavage (BAL) fluid was
obtained by optic bronchoscopy from healthy volunteers.
Skin lymphocytes were obtained from suction blisters in
sensitized volunteers. All human subject protocols were
approved by the Institutional Review Boards at Stanford
University or Leicester University.
Peripheral blood lymphocytes were isolated as described.5 Briefly, after dextran sedimentation and separation of the mononuclear fraction over Ficoll (Amersham
Pharmacia Biotech, Piscataway, NJ), monocytes were
removed by two 30 minute rounds of adherence to plastic
culture flasks in an incubator. Tonsil lymphocytes were
obtained by dispersal of fresh tonsils through a stainless
steel mesh followed by incubation in a plastic flask to
deplete adherent cells. Bronchoalveolar lavage subjects
were premedicated with nebulized salbutamol, lightly sedated with midazolam and the upper airway was anesthetized with 2% lignocaine. 180 ml of normal saline were
inserted through the bronchoscope into the middle lobe
of the right lung and the lymphocyte-containing fluid was
aspirated using gentle suction (fluid recovery was 20 to
46%). Lymphocytes from the epithelium and lamina propria of human intestine were isolated as described previously.16 Briefly, the epithelium and any resident lymphocytes were removed from the lamina propria by
gentle stirring in the presence of 1 mmol/L EDTA. The
epithelium-free lamina propria was then dispersed
through wire mesh. Lymphocytes were isolated from inflamed human skin as described previously.17 Briefly,
skin lymphocytes were isolated from the fluid drained
from suction blisters raised over the site of an epidermal
delayed-type hypersensitivity reaction (to poison oak
leaves or Candida albicans extract) elicited in allergic
volunteers. Lymphocytes were isolated from normal lung,
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inflamed liver, and inflamed synovial fluid as described.4
Briefly, lung lymphocytes were obtained by mincing the
tissue with fine scissors and passing the supernatant
through gauze; liver lymphocytes were obtained after
mechanical dispersal of liver pieces followed by Ficoll
separation, and synovial fluid mononuclear cells were
isolated by Ficoll separation.
We performed confirmatory studies to investigate
whether chemokine receptor expression on extracted
lymphocytes might have been altered by the tissue-chelation procedure. In these parallel experiments, purified
lymphocytes (from peripheral blood) were treated by
chelation in exactly the same manner as the tissue samples. Donor-matched, untreated and treated lymphocytes were then stained with the same mAbs to chemokine receptors and adhesion molecules presented in the
text. We found no staining differences between the
treated and untreated cells (Kunkel and Campbell, unpublished findings).
Adhesion and Chemotaxis Assays
Chemotaxis assays were performed using 24-well Transwell plates (Corning Costar, Cambridge, MA; 5 m
pores) in RPMI 1640 supplemented with 0.5% bovine
serum albumin for 2 hours in an incubator as described.5
Migrated cells were stained with CD4-APC, CD45RACyChrome, CLA-FITC, and ␣47-PE to analyze T cell
subsets (Pharmingen). Chemokine-triggered adhesion
assays were performed as described.5 Briefly, sorted
lymphocytes were allowed to settle on glass slides
coated with purified human ICAM-1 and stimulated to
adhere with chemokines, after which the slides were
gently washed and bound lymphocytes were fixed with
gluteraldehyde and counted using NIH Image software
(version 1.62b).
FACS Analysis
Tissue or blood lymphocytes were stained and gated for
the populations of interest using markers labeled with
fluorescein isothyocyanate (FITC), phycoerythrim (PE),
or allophycocyanin (APC). Unconjugated (or isotypematched control mAbs) were detected using a biotinylated horse anti-mouse IgG secondary antibody (Vector
Laboratories, Burlingame, CA) and streptavidin-PerCP
(Pharmingen). In some experiments, blood lymphocytes
were stained for CCR4 and CD45RO and sorted on
a FACSVantage SE (Becton-Dickinson). Four-color flow
cytometry was carried out on a FACSCalibur (BectonDickinson) using CellQuest software, version 3.1 (BectonDickinson).
Results
Memory and Activation Status of TissueInfiltrating Lymphocytes
In an attempt to more thoroughly understand the phenotype of tissue-infiltrating CD4⫹ lymphocytes, we have
Figure 1. Phenotype of blood and tissue CD4⫹ lymphocytes. CD4⫹ lymphocytes in various tissues were isolated and stained for expression of
CD45RA, CD69, CLA, and ␣47. Circulating blood CD4⫹ lymphocytes (A) are
evenly divided into memory (CD45RA⫺ ) and naive (CD45RA⫹ ) populations,
contain virtually no activated (CD69⫹ ) cells, and contain CLA⫹, ␣47hi, and
CLA⫺␣47lo/⫺ memory populations. Tonsils (B) contain memory and naı̈ve
lymphocytes of which about half are activated, and very few lymphocytes
with skin (CLA⫹ ) or gut ( ␣47⫹ ) homing specificity. Tissue sites such as
inflamed skin (C), jejunum (D), BAL (E), and arthritic synovial fluid (F)
contain only memory lymphocytes, including many activated cells. In addition, all skin lymphocytes are CLA⫹ (C), all gut lymphocytes are ␣47⫹ (D),
BAL lymphocytes are mostly CLA⫺␣47⫺ (E), and SF contains many subsets
of lymphocytes (F). Data are representative of three or more different
samples of each tissue.
undertaken to gently isolate lymphocytes from surgical
specimens for direct examination. Our lymphocyte isolation techniques involved chelation at 4°C as previously
described.4 This technique allowed us to avoid prolonged (potentially destructive) exposure of lymphocytes
to digestive enzymes at 37°C as entailed by conventional
protocols. We first examined the activation and naive/
memory status of tissue infiltrating cells. CD4⫹ lymphocytes from non-lymphoid tissues (eg, inflamed skin, normal jejunum, lung BAL fluid, and psoriatic arthritis SF in
Figure 1, C–F) were overwhelmingly of the memory phenotype (CD45RA⫺). This is a dramatic enrichment of
memory cells over their representation in blood, where
they usually comprise less than 50% of CD4⫹ T cells
(Figure 1A). In contrast, CD4⫹ T cells isolated from tonsil
(a representative secondary lymphoid organ) maintained
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a naı̈ve to memory ratio similar to that of blood (Figure
1B). Both lymphoid and non-lymphoid tissues contained
large numbers of cells of activated phenotype (as assessed by the activation marker CD69, Figure 1). Activated
cells were exceedingly rare in the blood (Figure 1A).
Adhesion Molecule Expression on TissueInfiltrating Lymphocytes
To further characterize the tissue-derived lymphocytes,
we examined their expression of the two best-studied
adhesion molecules associated with tissue-specific homing: CLA and ␣47 integrin. CLA is a ligand for E-selectin,
which allows CLA⫹ cells to interact with E-selectin in
cutaneous venules. CLA⫹ cells home preferentially to
cutaneous sites in vivo, and T cells specific for cutaneous
antigens reside within this population.18 The ␣47 integrin
is a ligand for MAdCAM-1, which allows ␣47⫹ cells to
interact with MAdCAM-1⫹ intestinal venules. ␣47- memory cells home preferentially to intestinal sites in vivo,19
and cells specific for intestinal antigens reside within this
population.20
Memory CD4⫹ cells from blood contained a mixture of
CLA-expressing, ␣47-expressing, and other less-well
studied subsets (Figure 1, A; far right panel). However,
when we examined cells isolated directly from skin or
from jejunal lamina propria we found essentially pure
CLA⫹ or ␣47⫹ populations, respectively (Figure 1, C and
D; far right panels). In contrast, CD4⫹ populations from
the lung or tonsil, where these molecules are proposed to
play little or no role in homing, contained very few cells
with high levels of these molecules (Figure 1, B and E; far
right panels). Interestingly, lymphocytes from the synovial
fluid of autoimmune arthritis patients appeared to be an
exception to the rule of tissue-specific lymphocyte homing (eg, psoriatic arthritis in Figure 1E; far right panel).
Although naı̈ve cells were excluded from this extra-lymphoid site, the representation of memory cells expressing
CLA or ␣47 was similar to that of blood.
Chemokine Receptor Expression of TissueInfiltrating Lymphocytes
The above data suggested that the gentle method we
used for isolating lymphocytes from tissues (see Materials and Methods)4 preserved the predicted homing phenotypes of the isolated cells. We then examined the expression of chemokine receptors by these cells, focusing
on CCR4, CCR5, and CXCR3.
The vast majority of CD4⫹ cells isolated from the 10
different non-lymphoid tissues examined (Figure 2) expressed CCR5 and CXCR3 (Figure 2, C-L; middle and
right panels). This is in dramatic contrast to tonsil, where
only very small numbers of CD4⫹ lymphocytes expressed CCR5 or CXCR3 (Figure 2B; far right panel).
Nearly all of the CD4⫹ populations infiltrating non-lymphoid tissues contained far more CCR5⫹ and CXCR3⫹
cells than did memory CD4⫹ cells from the blood (with
the exception of CXCR3 in the skin, where the pattern
was similar to that on CLA⫹ cells in the blood11) (Figure
2A; middle and right panels). The ubiquity of CCR5⫹ and
CXCR3⫹ T cells within so many diverse tissues suggests
that CCR5 and CXCR3 are not involved in determining
the tissue-specificity of lymphocyte homing, and that
CCR5 and CXCR3 expression may be a general phenotype of tissue-infiltrating lymphocytes. CCR5 and CXCR3
may therefore be more important with respect to positioning or retention of lymphocytes after tissue entry.21 Indeed, it is possible that these receptors may actually be
induced after tissue entry.
Unlike the ubiquitous expression pattern of CCR5 and
CXCR3, lymphocyte CCR4 expression was variable
among the tissues studied (Figure 2, C–L). CCR4 was not
seen on lamina propria CD4⫹ cells from any of the intestinal segments (including jejunum, ileum and colon, Figure 2, D–F), or on liver-infiltrating CD4⫹ cells (Figure 2G).
CCR4 was expressed at very high levels only in the skin
(Figure 2C). CCR4 was expressed at lower levels by
synovial fluid CD4⫹ cells from autoimmune arthritis patients (rheumatoid, Figure 2K and psoriatic, Figure 2L)
but not from osteoarthritis patients (Figure 2J). CCR4 was
also expressed at low levels on bronchial CD4⫹ cells
(Figure 2I) and even more rarely by lung interstitial CD4⫹
cells (Figure 2H). Like CCR5 and CXCR3, there was little
CCR4 expression in the tonsil (Figure 2B).
Relative CCR4 Expression Levels on CD4 Cells
from Systemic Tissues
The expression of CCR4 by skin-derived lymphocytes
appeared significantly higher than its expression by
bronchial or synovial cells. This higher expression was
confirmed by additional experiments using blood CLA⫹
lymphocytes from tissue donors as an internal standard
(Figure 3). Flow cytometry of CCR4 on tissue-infiltrating
lymphocytes was overlaid directly on that of CD4⫹CLA⫹
blood lymphocytes from the same patient (Figure 3,
A–D). Only skin-derived cells expressed CCR4 at a level
matching that of CD4⫹CLA⫹ cells from donor-matched
blood (Figure 3A). The median CCR4 expression by
bronchial or synovial cells was at least an order of magnitude less that of donor-matched CD4⫹CLA⫹ blood cells
(Figure 3, B–D).
Functional Significance of CCR4 Expression
Levels
We hypothesized that differences in CCR4 expression
levels may be functionally relevant in lymphocyte trafficking. We therefore sorted peripheral blood memory CD4⫹
lymphocytes into four categories of CCR4 expression
(Figure 4A): CCR4neg cells were sorted to match the level
of staining of the isotype control antibody, CCR4lo cells
were sorted to have a level of expression similar to that of
synovial fluid or BAL cells (Figure 2), CCR4hi cells were
sorted to have a level of expression similar to that of
skin-homing cells (Figure 2), and CCR4int cells were
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AJP January 2002, Vol. 160, No. 1
Figure 2. CCR4 expression is restricted to lymphocytes in skin (but not intestines) and certain other systemic sites while virtually all tissue lymphocytes express
CXCR3 and CCR5. Memory lymphocytes from a variety of tissues were stained for CCR4, CXCR3, and CCR5 expression. CCR4 is expressed on a large fraction of
blood (A) and a small fraction of tonsil (B) lymphocytes, but at a high level on virtually all inflamed skin (C) lymphocytes. Intestinal segments including the
jejunum (D), ileum (E) and colon (F) contain virtually no lymphocytes expressing CCR4. Similarly, systemic tissues such as inflamed liver (G), peripheral lung
tissue (H), and osteoarthritis synovial fluid (J) also contain few CCR4-expressing lymphocytes. Other systemic tissues such as BAL (I) and the synovial fluid from
autoimmune arthropathies including rheumatoid arthritis (K), and psoriatic arthritis (L), do contain many lymphocytes that express lower levels of CCR4. Virtually
all lymphocytes in normal or inflamed tissues examined express both CXCR3 and CCR5 (C–L) compared to practically none in tonsil (B) and only 30 to 50% on
blood lymphocytes (A). Number of samples for each tissue (n) shown.
sorted from the region between the CCR4lo and CCR4hi
cells and contained some overlap with both populations.
We then tested the ability of the CCR4hi, CCR4int,
CCR4lo and CCR4neg lymphocytes to rapidly adhere to
ICAM-1 or migrate in response to CCR4 ligands. In
TARC-induced rapid adhesion to ICAM-1,5 the CCR4hi
cells adhered most efficiently while the populations expressing lower levels of CCR4 showed a concomitant
reduction in firm adhesion consistent with the level of
CCR4 expression (Figure 4B). Interestingly, the small
number of cells becoming adherent in the CCR4lo sorted
population could be accounted for largely by overlap with
the CCR4int sorted population. All populations adhered
similarly in response to SDF-1␣ (⬃90 to 95% of input, not
shown), and TARC responses were normalized to the
SDF-1␣ response for each population. As seen for adhesion, all four populations responded similarly to SDF-1␣ in
chemotaxis assays a (⬃68 to 74% of input, not shown),
and the TARC responses were also normalized to the
SDF-1a response for each population. CCR4hi cells showed
the best TARC response (Figure 4C). Surprisingly, lower
levels of CCR4 expression corresponded with reduced
chemotaxis to TARC (Figure 4C), similar to the pattern seen
in the adhesion assay. To rule out the possibility that the
optimal chemotactic concentration of TARC was different
for cells bearing different levels of CCR4, we titered TARC
on sorted CCR4lo and CCR4hi cells in the chemotaxis assay
(Figure 4D). We found that the difference in migratory performance between the two sorted populations could not be
accounted for by a difference in relative sensitivity to TARC.
Similar chemotaxis results were obtained with MDC, another ligand for CCR4 (data not shown).
Discussion
In an attempt to more fully characterize tissue-infiltrating
lymphocytes in a search for tissue-specific lymphocyte
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Figure 3. Skin-infiltrating and blood CLA⫹ lymphocytes express higher levels of CCR4 than lymphocytes in non-cutaneous tissues. CCR4 intensity on
lymphocytes from tissues that contained CCR4⫹ cells was compared to the
level of CCR4 expression on matched donor blood CLA⫹ lymphocytes.
Lymphocytes in inflamed skin (A) expressed levels of CCR4 comparable to
circulating CLA⫹ skin-homing lymphocytes, whereas lymphocytes in systemic sites such as rheumatoid (B) and psoriatic (C) synovial fluid or BAL
(D) expressed lower levels of CCR4 comparable to that found on the
systemic and mucosae-homing populations in the blood. Data are representative of three or more independent matched samples for each tissue. The
median fluorescence channel is shown for each staining panel.
specialization, we have examined the cell surface phenotype of lymphocytes directly isolated from a wide variety of tissues for comparison with their circulating counterparts. One striking finding from this work was the
enormous enrichment of memory cells in non-lymphoid
versus lymphoid tissues. These data strongly support
classical in vivo animal work22 indicating that naive cells
have a very restricted pattern of recirculation (only secondary lymphoid organs), whereas memory cell subsets
can traffic to any soft tissue in the body. A corollary to this
theory is the notion (recently demonstrated in mice23,24),
that lymphocytes gain the ability to home to a broader
array of sites after encountering their cognate ligands
and differentiating into memory lymphocytes. Another important finding was that most tissue infiltrating lymphocytes display an “activated” phenotype (as assessed by
expression of CD69), whereas circulating populations
contained essentially no activated cells.
Tissue-Specific Adhesion Molecules
Our findings strongly support the importance of adhesion
molecules classically associated with tissue-specific lymphocyte homing.1 As reported previously, CLA⫹ lymphocytes, ⬃20% of circulating memory T cells, were enriched to almost 100% in skin-infiltrating populations, but
virtually absent from the jejunal lamina propria.17 Conversely, ␣47⫹ lymphocytes, also ⬃20% of circulating
memory CD4⫹ cells, were enriched to nearly 100% in
jejunal lamina propria-infiltrating populations, but absent
in the skin. CLA and ␣47 expressing lymphocytes were
rare in memory populations isolated from lung and tonsil,
where these molecules are not thought to play a role in
homing. In contrast to the skin and jejunum, SF contained
Figure 4. High CCR4 expression is required for effective triggered adhesion
and chemotaxis. Peripheral blood lymphocytes were sorted into four CCR4
expression levels: CCR4hi, CCR4int, CCR4lo, and CCR4neg (A) and compared
for their ability to chemotax and adhere in response to chemokine stimulation. Compared to CCR4hi lymphocytes, lower levels of CCR4 concomitantly
reduced the ability of lymphocytes to undergo rapid adhesion to ICAM-1 (B)
and chemotax toward TARC (C). The low chemotactic ability of CCR4lo
lymphocytes was not due to reduced TARC sensitivity as shown by a dose
response (D). Data are representative of three separate donors done in
triplicate.
CLA⫹ and ␣47⫹ memory CD4⫹ subpopulations in proportions similar to the blood. The SF data may reflect a
reduction in homing specificity at sites of intense chronic
inflammation. It is noteworthy that immunophenotypicallydefined skin-homing, gut-homing, and other memory lymphocyte types are found together in synovial fluid from
autoimmune arthritis cases. In contrast, naive lymphocytes are apparently excluded from this compartment.
This finding may imply the existence of a previously unrecognized homing pathway that distinguishes between
memory and naive cells, but does not distinguish among
the various tissue-specialized memory lymphocyte subsets. This may shed light on a previous finding that intestine-derived lymphocytes are able to interact with synovial endothelium.25 Instead of implying that autoimmune
arthritis is mediated by intestine-derived lymphocytes,
the finding may simply demonstrate another manifestation of the notion that any type of memory cell can potentially enter this compartment.
Expression of CCR5 and CXCR3 by TissueInfiltrating Lymphocytes
Our survey of lymphocytes from various human tissues
has shown that the vast majority of resident tissue CD4⫹
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cells express both CXCR3 and CCR5. Expression of
these inflammatory receptors has been previously reported for tissue lymphocytes in the intestines,7 liver,10
brain,9 and synovial fluid,11 and explained as particularly
important for specific homing/localization to each location. The strong expression of CCR5 and CXCR3 by
lymphocytes from the wide variety of tissues shown here
however suggests that these receptors are unlikely to
contribute to the specificity of lymphocyte homing per se.
However, their near ubiquity implies important, albeit not
yet understood, roles in tissue lymphocyte homing and/or
function. We and others have found that only about 25 to
30% of circulating memory CD4⫹ lymphocytes co-express CCR5 and CXCR3 (E.J. Kunkel and J.J. Campbell,
unpublished data).11 It is possible that tissue-infiltrating
lymphocytes are recruited only from this double-positive
population. However, the correlation between CD69 expression (an activation marker) and CXCR3/CCR5 expression raises the possibility that both receptors may be
induced after tissue entry. In either scenario, CCR5 and
CXCR3 likely facilitate the movement of lymphocytes
within target tissues.26,27 The CCR5 ligands, RANTES
and MIP-1␣, are known to be produced by activated
dendritic cells,28 suggesting a potential role for CCR5 in
mediating co-localization of lymphocytes and dendritic
cells within tissues. Alternatively, one or both of these
receptors may contribute to retention of activated cells
within tissues. Finally, CCR5 and/or CXCR3 up-regulation
on lymphocytes in tissues may enhance the ability of
these lymphocytes, once they return to the circulation, to
subsequently be recruited to active sites of subacute
inflammation where CCR5 and CXCR3 ligands may play
a more prominent role.
Expression of CCR4 by Tissue-Infiltrating
Lymphocytes
In contrast to CCR5 and CXCR3, lymphocyte expression
of CCR4 was only observed on lymphocytes from a subset of non-intestinal tissues, and only at high levels within
the skin (as will be discussed further below). The enriched expression of CCR4 by skin-infiltrating lymphocytes supports the model that the combined expression
of CLA and CCR4 is necessary for fruitful interaction
between cutaneous venules and circulating skin-homing
lymphocytes.5 Cutaneous venules are known to express
E-selectin, the ligand for CLA,29 and TARC, one of the
chemokine ligands for CCR4.5
Our finding that CCR4 is strongly expressed by lymphocytes isolated directly from DTH- or contact hypersensitivity-inflamed skin in vivo would tend to argue
against the notion that CCR4 is a marker for Th2 cells, as
suggested by in vitro models of polarized T cell development,30 –32 because DTH and contact hypersensitivity
responses are classical, universally accepted models for
Th1-mediated inflammation.
CCR4 Expression Levels and Function
We have previously proposed that high numbers of cellsurface chemokine receptors may be necessary to mediate adhesion triggering under shear, whereas smaller
numbers of receptors per cell may be sufficient to mediate chemotaxis.33 By this argument, only skin-infiltrating
lymphocytes (those with the highest CCR4 expression)
might express sufficiently high levels of CCR4 to mediate
rapid adhesion-triggering under shear.5 Lymphocytes
expressing lower levels of CCR4 (as in BAL fluid) could,
by the same argument, use this receptor for chemotaxis.
We were therefore surprised to find that peripheral blood
lymphocytes sorted for low levels of CCR4 expression (as
determined by two mAbs to CCR4) responded very
poorly to CCR4 ligands in both adhesion and chemotaxis
assays.
There is, however, recent evidence proposing a role
for CCR4 in inflammation at these sites (ie, bronchi and
synovium). Bronchial epithelial cells stimulated with both
tumor necrosis factor-␣ and interferon-␥ produce large
amounts of TARC,34 and activated macrophages (found
in synovial tissue) are extremely potent sources of another CCR4 ligand, MDC.35 If the low levels of CCR4
observed on BAL and synovial fluid lymphocytes are
genuine, this receptor may potentially mediate chemotaxis at these sites. CCR4 on these lymphocytes may
serve to chemotactically guide lung lymphocytes to the
epithelial layer once they have entered peri-bronchial
tissues. Similarly, synovial lymphocytes may be guided
through the tissue by using gradients of MDC secreted
by resident activated macrophages. Expression of CCR4
ligands may also play roles in other aspects of immunity
not related to lymphocyte localization.
Synovial lymphocytes from autoimmune arthritis patients have previously been shown to have some characteristics of skin-homing cells.36 This has led to the proposal (especially in the case of psoriatic arthritis) that
attraction of cutaneous lymphocytes to the synovium may
be a component of the disease mechanism.36 Our findings demonstrate that lymphocytes from synovial fluid do
indeed contain cells of the cutaneous homing phenotype,
but cells of intestinal and other homing phenotypes are
present as well. Thus, cutaneous lymphocytes are not
enriched in the synovial fluid above their relative numbers
in normal peripheral blood. These findings could be explained by the notion that control of memory T cell homing specificity has broken down in such chronic autoimmune disorders.
Conclusions
We have examined tissue-infiltrating lymphocytes from a
variety of human tissues to explore the similarities and
differences among tissue-infiltrating lymphocytes in various extralymphoid sites in the body, and thus shed light
on the specialization of lymphocyte subsets that mediate
inflammation and immune surveillance in particular tissues. We have found that 1) Naı̈ve CD4⫹ T cells are
restricted to blood and lymphoid organs, and are not
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Kunkel et al
AJP January 2002, Vol. 160, No. 1
found in non-lymphoid organs. 2) Activated CD4⫹ lymphocytes (as assessed by CD69 expression) are rare to
non-existent in the blood, but constitute the majority of
tissue-infiltrating lymphocytes. 3) The dramatic enrichment of lymphocytes expressing the adhesion molecules
CLA and ␣47 within skin and intestine, respectively, is
consistent with a role for these molecules in tissue specific homing. 4) CCR5 and CXCR3 are nearly ubiquitously
expressed by lymphocytes that have infiltrated non-lymphoid tissues, and are thus unlikely to contribute to the
determination of tissue specificity. 5) CCR4 is expressed
by CD4⫹ T cells from only a subset of non-intestinal
tissues, and expressed at the highest, most functional
levels only by skin-infiltrating CD4⫹ cells.
14.
15.
16.
17.
Acknowledgment
We thank Maris Handley of the Dana-Farber Flow-Cytometry facility for expert cell sorting.
18.
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