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Division of Allergy and Immunology, Department of Internal Medicine, and Center for Immunology, Washington University School of Medicine, and Howard Hughes Medical Institute, St. Louis, MO 63110
| Abstract |
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| Introduction |
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Using inherited memory in the form of pattern recognition receptors, tissue-resident cells of the innate immune system can recognize pathogens and regulate the induction of an immune response both locally and in secondary lymphoid tissues as part of their subsequent role as APCs (6, 7, 8). Once they have identified a pathogen and responded locally, these sentinels recruit nonresident effector cells to the site of insult. They accomplish this by using soluble mediators such as cytokines and chemokines that act on the local vascular endothelium to enhance its adhesive qualities and to recruit cells directly. Although the cognate Ag plays a central role in activating many functions of T cells, the specific stimulatory activities of Ag in the context of MHC have not been separated from the nonspecific inflammatory signals that local Ag may induce that then may play a role in T cell recruitment to the tissue. Antigenic peptide and MHC can affect the adherence of T cells to an ICAM-1-containing lipid bilayer in vitro (9); however, the effect of specific Ag on recruitment of T cells in vivo is poorly understood.
Leukocyte recruitment is regulated by three main families of molecules: selectins, integrins, and chemokines (10). Expression of the members of each of these families is differentially regulated by Th1 cells and Th2 cells as well as by regulatory T cells (11, 12, 13, 14, 15). The migration patterns of Th1 and Th2 cells are also distinct in vivo, both in recirculation (16) and during inflammatory responses (17). In several experimental model systems, Th1 cells are recruited into effector sites in response to Ag more readily than are Th2 cells. Th1 cells but not Th2 cells have been shown to migrate into inflamed skin, and the migration of Th1 cells was blocked by systemic treatment with Abs to P- and E-selectin (17). Adoptively transferred Th1 cells and Th2 cells also migrate differently into the inflamed peritoneum (3), pancreatic islets (18), and the gastric mucosa, where the data suggest that differential recruitment may genuinely play a role in defining the local milieu. Differential recruitment may even exclude Th2 cells that are produced in the draining LN from the effector site (19).
Studies by Xie et al. (3) demonstrated that the recruitment of Th1 cells to the peritoneal cavity is more efficient than that of Th2 cells, even when recruitment is enhanced by administration of adjuvant or by injection of the chemokine IFN-inducible protein 10 in the absence of specific Ag. This important finding suggested that Th1 cells can be recruited to sites of inflammation in an Ag-independent manner. Xie et al. (3) suggested that because Th1 cells were more readily recruited to the peritoneal cavity, Th2 cells may be particularly adapted for migration to sites such as the lungs that are known to host Th2-type inflammation. Arguing against this broad interpretation, Randolph et al. (20) showed that adoptively transferred Th1 cells are also recruited to the lungs more efficiently than are Th2 cells. Although these studies suggest that Th2 cells are intrinsically less competent for recruitment to peripheral tissues, it is well established that Th2 cells predominate in affected tissues, under conditions of parasitic infection (for example, see Ref. 21). Therefore, questions remain regarding the nature of the stimuli that elicit Th2 cell recruitment.
Our laboratory has previously investigated early events in the challenge phase of Th cell-dependent eosinophilic airway inflammation. Using a short protocol in which in vitro-differentiated Ag-specific transgenic Th cells were adoptively transferred into naive mice and challenged via the airway 1 day later, we simulated the initial phase of T cell recruitment in isolation from the full endogenous response. The unique characteristics of this system allowed us to investigate the regulation of the threshold for T cell recruitment to the lung in vivo. Using this model, we previously observed that adoptively transferred Th2 cells retained their differentiated Th2 phenotype for many days after transfer (22), but when administered in the absence of Th1 cells, they did not enter the lungs in response to airway Ag challenge. Under similar conditions, Th1 cells were efficiently recruited. Importantly, when Th1 and Th2 cells were transferred together, airway challenge led to successful recruitment of both subsets (23). These observations indicated that Ag challenge alone was not sufficient for recruitment of Th2 cells to the airways, but that recruitment of Th1 cells can condition the environment to enable Th2 cell migration. We hypothesized that the recruitment of Th1 cells might influence Th2 cell accumulation in the tissues due to the inflammation they induce independent of their Ag specificity. Interestingly, as shown in the accompanying report (56), when Th2 cell recruitment was elicited by cotransfer of Th1 cells with an antigenic specificity different from that of the Th2 cells, the Th2 cells were recruited even in the absence of Th2-specific Ag. Surprisingly, the recruited Th2 cells showed aspects of an activated phenotype even in the absence of the Th2 Ag. This suggested that recruitment might lead directly to T cell activation. In this report, to investigate the factors influencing Th2 migration into the lungs and to distinguish the effects on recruitment of local inflammation from the effects of Th2 cell activation, we tested the impact of two different types of innate cell stimuli on T cell recruitment both in the presence and absence of the specific Th2 Ag. We showed that mast cell stimulation by cross-linking IgE receptors can induce Th2 cell migration into the lung. Intranasal (i.n.) administration of LPS also induces Th2 cell migration into the airways. Both of these innate cell stimuli were able to affect Th2 cell recruitment in a T cell Ag- and activation-independent manner.
| Materials and Methods |
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Female BALB/c mice (Harlan Sprague Dawley, Indianapolis, IN)
were used between 6 and 8 wk of age. DO11.10 mice (24)
that are transgenic for a TCR that recognizes chicken
OVA323339 in I-Ad were
the generous gift of K. Murphy (Washington University, St. Louis, MO).
Thy1.1+ DO11.10 mice were generated as previously
described (23) and splenocytes were differentiated into
Th1 and Th2 cells as described below. The resulting
Thy1.1+ cells were transferred into
Thy1.2+ BALB/c mice. OT-II.2 mice
(25) are transgenic for a TCR that recognizes the same OVA
peptide in I-Ab. Because the transgene (Tg) is
inserted into the Y chromosome, only males are Tg positive. OT-II.2
mice were a generous gift of Paul Allen (Washington University). Cells
from these mice were transferred into 6- to 8-wk-old male C57BL/6 mice
(The Jackson Laboratory, Bar Harbor, ME) and used in Fig. 6
. P- and
E-selectin double-deficient mice (mixed C57BL/6 and Sv129 background;
The Jackson Laboratory) (26) were maintained by homozygous
mating. All mice were kept in microisolator cages in the specific
pathogen-free Division of Comparative Medicine facility at Washington
University Medical Center. They were fed and watered ad libitum. All
experiments were approved by the Washington University Institutional
Committee for the Humane Use of Laboratory Animals.
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Splenocytes from the DO11.10 or OT-II.2 TCR
Tg+ mice were made into single cell suspensions
with a disposable mesh (BD Falcon, Bedford, MA), cultured at
2.55 x 106/ml with 0.3 µM peptide, and
split as needed with 40 U/ml IL-2 (BD PharMingen, San Diego, CA). T
cells were cultured in 10% FBS (HyClone Laboratories, Logan, UT) in
IMDM with 2 x 10-5 M 2-ME, 1 mM sodium
pyruvate, 1x nonessential amino acids, 10 µg/ml penicillin, 10
µg/ml streptomycin, and 2 mM L-glutamine (all from Life
Technologies, Grand Island, NY). Th2 cells were prepared by culturing
in 40 ng/ml recombinant murine IL-4 (R&D Systems, Minneapolis, MN), 1/8
dilution of anti-IFN-
(clone TOSH; kindly provided by E. Unanue,
Washington University) hybridoma supernatant, and Th1 cells were made
by adding 10 ng/ml IL-12 (BD PharMingen) and 1/8 dilution of anti IL-4
(11B11) hybridoma supernatant. By intracellular staining, 3060% of
the differentiated Th1 cells were potential IFN-
producers and
2030% of the Th2 cells were IL-4 single producers. Cells were used
for adoptive transfer 7 days after the last stimulation. At this time,
these cells did not make detectable IL-4 or IFN-
by intracellular
staining and flow cytometry or by ELISA (data not shown).
Adoptive transfer and Ag challenge
Differentiated, resting DO11.10 Th cells were adoptively transferred i.v. via the retro-orbital plexus. The next day, 1% (w/v) Ag in PBS was administered i.n. or by aerosol with similar results. In the case of i.n. exposure, 40 µl of 1% solution was administered twice 6 h apart to mice anesthetized with Metafane (methoxyflurane; Schering-Plough Animal Health, Union, NJ). Aerosol Ag was administered as described previously (20). Where indicated, 25 µg of Salmonella typhosa LPS (Sigma-Aldrich, St. Louis, MO) was administered i.n. in 20 µl of PBS on the day after T cell transfer and immediately before aerosol Ag.
Immediate type hypersensitivity skin test
The dose of IgE required to produce a detectable immediate type hypersensitivity response was determined by injecting various amounts of monoclonal anti-DNP IgE (clone SPE-7; Sigma-Aldrich) i.v. (modified from Ref. 27). The abdomens of the mice were shaved. The next day, 50 µl of 0.5% Evans blue dye (Sigma-Aldrich) in PBS was given i.v., and then OVA-DNP12 or BSA-DNP22 was injected intraepidermally to form a blister. When injected, Ag causes increased vascular permeability, and Evans blue leaks out of the vasculature and stains the blister blue. At 30 min after challenge, both 0.1 µg and 1.0 µg of IgE were sufficient to allow a blue wheal to form at the site of Ag injection.
Ags used for airway challenges
OVA (grade V) and Salmonella typhosa LPS were
purchased from Sigma-Aldrich. OVA-DNP was initially prepared according
to the method of Eisen et al. (28) as follows. OVA-DNP
(Fig. 1
) was made by mixing 1 g of
OVA and 1 g of KCO3 in 20 ml of water
together with 2 g of dinitrochlorobenzene (either liquid or
crystalline) in 30 ml of water. The reaction was mixed for 4 h or
more at room temperature in the dark. The color changed from yellow to
orange as the conjugate formed. The product was dialyzed exhaustively
against PBS.
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of DNP), (OVA)(moles/liter) =
A595/45,000 (molecular mass of OVA),
OVA-DNPn = (DNP)/(OVA). n was always
found to be 10 for both OVA-DNP and BSA-DNP22. For some
experiments, OVA-DNP and BSA-DNP22 were purchased from
Biosearch Technologies (Novato, CA). Analysis of airway and lung inflammation
Three days after challenge, mice were sacrificed by lethal
injection using 50 µl of a mixture of ketamine (100 mg/ml) and
xylazine (20 mg/ml), and bisection of the descending vena cava and
their airways were assessed for inflammation. The trachea was
cannulated and airway inflammatory cells were obtained by
bronchoalveolar lavage (BAL) with four 0.8-ml aliquots of ice-cold 2%
FBS in PBS. In some experiments, the left upper lobe was tied off after
lavage and kept at 4°C in T cell medium until it was dissociated
through a 40-µm nylon mesh (BD Falcon). This fraction was designated
"lung cells." The numbers of live cells counted in this fraction
were proportional to the total number of cells in the BAL. RBCs were
lysed using 8.3 g/L ammonium chloride, 10 mM Tris base. Nucleated cells
were counted in a hemacytometer. Eosinophil counts were determined in
cytospin preparations of the BAL cells using Wrights stain. Other BAL
cells were identified by Ab staining and FACS analysis. The transferred
Th2 cells were followed using the clonotypic Ab for the DO11.10 TCR,
KJ1-26-tricolor (29), or anti-V
8.1, 8.2-tricolor
(both purchased from Caltag, Burlingame, CA, and used at 1/25
dilution). Other Abs used were anti-Thy1.1-biotin,
anti-CD25-PE, anti-CD4-cyc, anti-CD4-APC, anti-TCR
V
2 and V
5.1/2, and strepdavidin-APC at 1/50 (BD PharMingen). The
analysis was done using a FACSCalibur (BD Immunocytometry Systems, San
Jose, CA) and Cell Quest (version 3.3; BD Immunocytometry Systems).
Staining for carbohydrate-modified mucins was performed by the
Barnes-Jewish Hospital Morphology Core Facility (St. Louis, MO) using
periodic acid Schiffs (PAS) and Mucicarmine on
formaldehyde-fixed and paraffin-embedded sections.
| Results |
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IgE that is bound by the Fc
Ig receptors on mast cells and
basophils leads to cross-linking of these receptors after challenge
with multivalent Ag and induces activation of the mast cell, one facet
of a typical Th2-type inflammatory response (30). To
simulate the potential effect of Fc
R cross-linking-induced mast cell
activation on recruitment of Th2 cells, IgE anti-DNP Abs and
DO11.10 OVA-specific Th2 cells were passively transferred into naive
mice and the mice were challenged with DNP-haptenated protein Ags. The
dose of IgE used was determined by titrating IgE and testing
epicutaneously for an immediate type hypersensitivity response. We used
a dose of IgE sufficient to activate mast cells after cross-linking
with 1% Ag. We used the allotypic marker Thy1.1 to track the
transferred T cells from Thy1.1+ DO11.10 mice in
Thy1.2+ BALB/c recipients. A total of
107 Thy1.1+ DO11.10 Th2
cells and 1.0 µg IgE anti-DNP Abs were transferred i.v. into
naive BALB/c recipient mice whose T cells express Thy1.2 (Fig. 1
A). On the next day, the mice were challenged by i.n.
administration of 1% OVA or OVA-DNP12 solution.
Negative controls received PBS alone. On the third day after challenge,
BAL cells were analyzed for the percentage of
CD4+ Thy1.1+ transferred
Th2 cells by flow cytometry and the number of Th2 cells calculated as
the product of this percentage multiplied by the total number of
nucleated cells in the airway. Challenge with OVA induced a small
increase in CD4+ lymphocytes that were
Thy1.1+ Th2 cells (Fig. 1
) (23). In
contrast, challenge with OVA-DNP, which both cross-links the IgE and
stimulates the Th2 cells, as well as with BSA-DNP22, which
triggers the mast cells without activating the transgenic T cells,
resulted in a greater than sixfold increase in the percent BAL Th2
cells compared with PBS-challenged mice. The percentage of Th2 cells in
the BAL is even larger than that seen in the OVA-DNP challenge, due to
the lack of eosinophils in the BSA-DNP22 BAL (Fig. 2
B). Although a change in the
percent of BAL cells that are Th2 cells is observed in Figs. 1
B and 2A, this difference is more striking when
measured as the change in the actual number of BAL Th2 cells seen in
Fig. 2
. Cross-linking IgE using either OVA-DNP or BSA-DNP22
induced a dramatic increase in the total number of recruited Th2 cells
compared with OVA alone (Fig. 2
). The increase in recruitment of Th2
cells was greater at 1 µg (0.72% transferred cells/total cells or
2 x 105) than at 0.1 µg (0.46%, 1.1
x 105) of IgE, suggesting that Th2 cell
recruitment is proportional to the dose of IgE used for sensitization.
In this experiment, mice that received i.n. PBS without Ag had BAL with
0.12% Th2 cells (total 1.1 x 104 recruited
T cells). These data suggest that IgE-dependent stimulation, presumably
via high-affinity Fc
Rs on the surfaces of mast cells and basophils,
is capable of stimulating production of inflammatory mediators that
increase either the migration potential of the Th2 cells or the
recruitment potential of the lung microenvironment.
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As we have shown previously (23), when Th2 cells are
adoptively transferred to naive mice, Ag challenge results in
insufficient Th2 recruitment to induce eosinophilic airway
inflammation. Increasing the number of repetitions of Ag challenge
leads to increased recruitment of Th2 cells (our unpublished data and
Ref. 31). Having shown that an innate stimulus could
recruit DO11.10 Th2 cells to the airways in the presence of OVA, we
tested whether T cell-specific Ag was required for this recruitment.
Using an Ag, BSA-DNP22, which can cross-link
anti-DNP IgE but which is not an Ag for the OVA-specific T cells,
we could examine the role of specificity in Ag-mediated recruitment.
Intranasal administration of BSA-DNP22 to mice
that had been passively sensitized with anti-DNP IgE increased
recruitment to the airway of transferred OVA-specific Th2 cells in
numbers similar to those recruited after challenge with OVA-DNP (Fig. 2
). This indicates that the T cell-specific Ag is not required for
recruitment of Th2 cells to the airway. Importantly, although
BSA-DNP22 elicited recruitment of OVA-specific
Th2 cells, it did not elicit recruitment of eosinophils, whereas the
Th2-activating Ag OVA-DNP did. These data indicate that mast cell
inflammatory stimuli induced the recruitment of Th2 cells to the
airway, but did not activate these cells to perform their full spectrum
of effector functions. Additionally, this experiment showed no evidence
of direct recruitment of eosinophils by mast cells. On the contrary, in
this system, unless the Th2 cells have been activated by their cognate
Ag, eosinophil recruitment does not occur. The total number of cells in
the BAL of mice challenged with BSA-DNP22 (Fig. 2
B) increased only modestly over background, and the
increase in total cells in the airways of OVA-DNP-challenged mice can
be accounted for entirely by the increase in eosinophils in the
infiltrate. These data suggest that stimulation of mast cells through
the IgE receptor Fc
RI does not lead to additional monocyte or
macrophage infiltration. Furthermore, the effect of IgE on the
recruitment of Th2 cells in these experiments is not dependent on the
presence of T cell Ag.
Recruitment of Th2 cells to the airway is not inherently associated with T cell activation
In most systems it is difficult to separate T cell recruitment
from T cell activation because the presence of T cells in a tissue is
generally a consequence of effector processes for which both activation
and recruitment are required. Consequently, the role of Ag in the
recruitment phase of the Th2 cell response remains unknown. Here,
challenge with BSA-DNP22 resulted in recruitment
of OVA-specific Th2 cells without eosinophils, suggesting that the Th2
cells that had been recruited to the lungs did not express an activated
phenotype. These findings highlight questions regarding how closely
linked Th cell activation and Th cell recruitment are. For example, Th
cells might become "recruitable" due to experiences in the LN
during activation. To test whether the cells recruited to the lung were
also present in the local LN and to determine whether the transferred
CD4+ cells were activated, we extended our
analysis to evaluate expression of IL-2R
(CD25), which is
transiently up-regulated on activated T cells. High expression of CD25
is observed on transferred cells that have migrated to the lungs and
the regional LNs of mice treated with airway OVA or OVA-DNP (Fig. 3
). When passively sensitized mice were
challenged with OVA-DNP, transferred Th2 cells positive for the DO11.10
clonotype KJ1-26 were recruited to both the airway and the paratracheal
LNs. Few KJ1-26+ cells were detected in
nondraining LNs (data not shown). Because the proportion of
CD25+ transgenic cells is lower in the lung
compared with the LNs, it would appear that activation is not a
prerequisite for lung migration. These findings are underscored by the
results of airway challenge with BSA-DNP22. Here,
in the absence of the T cell Ag, although there was substantial
recruitment of the transgenic cells to the airway, the Th2 cells do not
accumulate in the regional LN. This is consistent with previous studies
from our lab and others indicating that Ag-experienced cells are not
recruited to the LNs in the absence of Ag (32). In
contrast, although BSA-DNP22 led to the
recruitment of large numbers of Th2 cells to the airway, this
Ag-independent T cell recruitment was not associated with activation of
the recruited cells, as evidenced by the absence of
CD25high Th2 cells in the lung in this setting
(Fig. 3
). Thus, in contrast to recruitment of Th2 cells by Th1 cells
(56), local cross-linking of IgE leads to recruitment to
the lung of Th2 cells that are not activated.
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The Th2-specific cytokines IL-9 and IL-13 are excellent promoters
of airway mucus secretion. Sensitized mice carrying targeted null
mutations of the genes encoding either these molecules or their
receptors have decreased mucus production in the lungs after Ag
challenge (33, 34). Conversely, transgenic mice producing
these cytokines under the lung-specific rat Clara cell 10-kDa protein
promoter have severe goblet cell hyperplasia and increased production
of mucins (35, 36). These data suggest that up-regulation
of airway epithelial mucus production is a process driven by Th2 cells.
Enhanced mucus production, therefore, can be used as a marker of the
action of activated Th2 cells. This is consistent with our observation
here that there is a lack of increased epithelial mucus production
after administration of BSA-DNP22, which induces
recruitment of Th2 cells but does not activate them (Fig. 4
). Although Th2 cells accumulate in the
lungs of passively sensitized mice in response to airway challenge with
BSA-DNP22, they localize there without expression
of their effector functions. There were no eosinophils and little
epithelial mucus production in the
BSA-DNP22-challenged lungs. This indicated that
recruitment of Th2 cells into the lung and activation of these Th cells
are separable events.
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Our initial experiments using IgE cross-linking showed that this
kind of Th2-associated inflammation could elicit Ag-independent
recruitment of Th2 cells to the airways. To test the generalized role
of inflammation for recruitment of Th2 cells and to explore the range
of inflammatory signals that could elicit Th2 cell recruitment, we
tested whether LPS could modulate Th2 cell recruitment. LPS induces
activation of macrophages, dendritic cells, and endothelial cells by
binding the LPS receptor (CD14) and signaling through the Toll-like
receptor 4 and MD2, which activates NF-
B. This activated
NF-
B, together with other signals, can lead to IL-12 production.
Because IL-12 drives the differentiation of Th1 cells and the secretion
of IFN-
by NK cells, this kind of stimulus leads to inflammation
with features quite distinct from the inflammation that occurs after
mast cell activation by cross-linking surface IgE.
To test the hypothesis that signals common to many types of
inflammatory responses are the primary determinant of Th2 cell
recruitment to the lung, we instilled Salmonella typhosa LPS
i.n. into recipients of adoptively transferred DO11.10 Th2 cells.
Recruitment of Th2 cells was observed in both the lungs and the airways
of recipient mice receiving LPS (Figs. 5
, A and B). The LPS-induced inflammation was
associated with recruitment of Th2 cells and other cell types yielding
a substantial increase in the total BAL cell numbers (Fig. 5
C). These cells included macrophages and monocytes as well
as lymphocytes and eosinophils.
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Endothelial selectins contribute to the recruitment of Th2 cells to the lung
Several elegant studies that have investigated the usage of P- and
E-selectins by Th1 and Th2 cells have indicated that Th1 cells use
selectins for recruitment to sites of inflammation, but have suggested
that Th2 cells do not. Th2 cells neither bind P- or E-selectin Ig
fusion proteins (3, 38) nor express high levels of the
fucosylating enzyme
(1, 3)-fucosyltransferase VII
(39) necessary to generate functional P-selectin
glycoprotein ligand (40), although they do express the
P-selectin glycoprotein ligand protein backbone, CD34
(38). In contrast, it is clear that recruitment of Th1
cells into the skin and the peritoneal cavity is dependent on the
endothelial selectins. Systemic administration of blocking Abs to P-
and E-selectin blocks Th1 cell recruitment (17, 41, 42).
We have extended these studies to investigate the role of P- and
E-selectin in the recruitment of Th2 cells to the lungs and airways.
OVA-specific Th2 cells were fully differentiated in vitro from male
OT-II.2 mice (H-2b) (25) and were
transferred i.v. into P- and E-selectin double-deficient mice
(26). Intranasal administration of LPS stimulated
recruitment of Th2 cells to the BAL in P- and E-selectin-deficient mice
(Fig. 6
A). As seen in our
other experiments, only small numbers of Th2 cells were recovered in
the BAL of mice that received OVA alone. There were substantial numbers
of eosinophils recovered by BAL from P- and E-selectin-deficient mice
72 h after i.n. challenge with OVA and LPS, suggesting that in
mice, use of P-selectin by eosinophils is not absolutely required for
these cells to pass through the lung vascular endothelium and
epithelium into the airway. Our data do not exclude a requirement for
P- or E-selectin to support the normal rate of eosinophil migration
from the circulation to the airway. Although recruitment of Th2 cells
to the airway of P- and E-selectin double-deficient mice (as assessed
by BAL) was increased in response to LPS plus OVA to levels similar to
those seen in wild-type mice, the numbers of Th2 cells found in the
lung parenchyma itself were reduced (Fig. 6
B). In both
wild-type and selectin-deficient mice, an increase in recruitment of
Th2 cells to the airways was observed in response to LPS alone as well
as in response to LPS plus the specific T cell Ag. However, in
selectin-deficient mice, recruitment and accumulation of Th2 cells to
the lung tissue were diminished when recruited by LPS alone (Fig. 6
, A and B). These observations indicate that an
endothelial selectin contributes to Th2 cell recruitment into the
lungs.
Interestingly, the numbers of total cells in the BAL were dramatically
increased in LPS-treated selectin-deficient animals (Fig. 6
C). These numbers were even higher than those seen in
wild-type mice. This is likely due to the fact that the
double-deficient mice have much higher numbers of circulating
leukocytes than do wild-type animals. This leukocytosis, first observed
by Frenette et al. (26), could result in increased tissue
infiltrates if the magnitude of these infiltrates is in proportion to
the numbers of circulating cells. That P- or E-selectin participates in
Ag-nonspecific recruitment was not predicted from previous experiments
by Xie et al. (3), who showed that following Ag
stimulation in vivo, only Ag-specific CD4+ cells
in the lymph node acquire the ability to bind P-selectin Ig after
stimulation with Ag. This indicated that in the lymph node, only
Ag-specific cells express the selectin ligand. Their observations,
however, do not exclude the possibility that Ag-nonspecific T cells
migrate to the site of inflammation using this receptor without first
proliferating in the LN (3).
| Discussion |
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In previous studies we showed that endogenous Th1 cells precede Th2
cells into the airways in a model that includes both systemic priming
and aerosol challenge with the protein Ag OVA (20). In
another model designed to separate the recruitment process from
sensitization events, we found that adoptively transferred OVA-specific
Th2 cells were not effectively recruited to the lungs and airways of
naive mice by challenge with aerosolized Ag alone. In contrast,
adoptive transfer of Th2 cells together with Th1 cells followed by
airway Ag challenge resulted in dramatically increased Th2 cell
recruitment (23). Our current data, however, demonstrate
clearly that Th1 cells per se are not required for Th2 cell
recruitment. Rather, a diverse collection of inflammatory stimuli can
cooperate in the recruitment of Th2 cells. The role of non-Th1 cell
factors is also supported by recent experiments in animals deficient
for the Th1-promoting transcription factor T-bet (45).
Finotto et al. (45) showed that SCID recipients of
T-bet-deficient CD4+ T cells, which are unable to
differentiate into Th1 cells, develop airway hyperreactivity and have
IL-4 in the airways after aerosol Ag challenge, indicative of the
development of the transferred cells into Th2 cells and their
recruitment into the lungs in the absence of a Th1 response (Fig. 4
).
We have recently shown that Th1 cells provide assistance for Th2 cell recruitment using a mechanism that is not dependent on Th2 cell-specific Ag (56). This is consistent with it being inflammatory signals downstream of Th1 cell activation that increase Th2 cell recruitment to the tissue. Our current study provides evidence that two additional stimulators of inflammation, Ag-specific IgE of the adaptive immune response and LPS, acting via the innate immune system can recruit Th2 cells to the lungs and airway in a T cell Ag-independent fashion. The Th2 cells that are recruited to the lung are not required to traffic through the local LN or to become activated to be recruited. Although Ag may affect the outcome of recruitment in ways not measured here, our experiments suggest a model for recruitment of Ag-experienced Th2 cells that, as in recruitment of innate immune cells, depends solely on the coordinated up-regulation of adhesion molecules and chemoattractants in target tissues to render the tissue receptive for immune cell entry.
We and others have investigated potential mediators and factors
responsible for induction of Th2 cell recruitment in response to tissue
inflammation. Although Th1 cells can stimulate inflammation, leading to
increased Th2 recruitment, experiments using Abs to block the primary
Th1-specific mediators IFN-
, lymphotoxin, and IL-2 proved that the
effects triggered by Th1 cells are not dependent on these mediators
individually. Thus, major mediators that are products of Th1 cells
uniquely do not appear to be the primary agents inducing recruitment.
We anticipate that similar findings will hold for the inflammatory
stimuli tested in the present study. Rather than cell type-specific
mediators driving Th2 cell recruitment, we anticipate that mediators
common to many inflammatory processes drive the Th2 cell response. For
example, neutralization of the broad inflammatory mediator TNF did
dampen Th2 cell recruitment induced by Th1 cells (23). We
have also investigated representatives of each of the main categories
of adhesion receptorsthe integrins, the selectins, and the chemokine
receptors. We have shown that blocking VCAM-1 reduces Th2 cell
recruitment (23), that P- or E-selectin is involved in
this recruitment (this manuscript), and that various chemokines are
up-regulated in the presence of Th1 cells (56). Using a
similar adoptive transfer model with repetitive aerosol Ag challenge as
a stimulus, Mathew et al. (46) have underscored the
potential role of chemokine receptors, showing that blockage of
G-protein-coupled receptors on Th2 cells with pertussis toxin limits
their recruitment. Furthermore, the work of Mathew et al.
(47) has elegantly demonstrated that STAT6 signaling in
the lung parenchyma is necessary for recruitment of wild-type Th2
cells. This work suggests that Th2 cells themselves can also facilitate
their own recruitment via local cytokine production, as first suggested
by Cohn et al. (31) in their studies of IL-4-deficient T
cells.
If Ag-independent T cell recruitment to sites of tissue inflammation were to be obligatorily linked to T cell activation or if only activated cells could be recruited, then Th2 cell recruitment would generally confer a Th2 character on the local inflammatory response by importing Th2 effector cells. If, in contrast, recruitment of Th2 cells could occur without cell activation and inflammation could lead to the influx of primarily resting Th2 cells, then the physiological impact of this recruitment would be expected to be less profound. Other circulating cell types may also be recruited in an Ag-independent manner that could also modify the outcome.
Prior studies using transgenic models of autoimmune diabetes have
investigated this issue but suggest that both inflammation and the
activating Ag are required for the T cell-mediated destruction of
-cells in pancreatic islets. The presence of Ag alone does not lead
to islet destruction in most murine models of insulin-dependent
diabetes mellitus. Inflammatory cytokines are required for
up-regulation of T cell costimulation and for prevention of Ag-induced
anergy (48, 49). These important studies suggest that both
Ag and inflammation are required for the activation of a full immune
response; however, whether it is Ag or inflammation that is
specifically required for the recruitment of the T cells has not been
addressed in these studies. We have shown here that recruitment of
Ag-experienced Th2 cells is not obligatorily linked to cell activation.
In other studies that use autoimmune tissue destruction as a readout,
it is difficult to distinguish recruitment functions from effector
functions. The studies of Xie et al. (3) of adoptively
transferred transgenic cells have indicated that Th1 cells could be
recruited into the peritoneal cavity by the inflammatory stimulus IFA
and the IFN-induced chemokine IFN-inducible protein 10, suggesting that
Ag is not required for recruitment into this tissue compartment either.
In our studies, we are able to separate T cell recruitment from T cell
effector functions. Under some conditions (for example, after
stimulation of IgE-bearing mast cells using the T cell irrelevant Ag
BSA-DNP22; Fig. 3
), large numbers of Th2 cells
are recruited with a resting phenotype similar to their status at the
time of adoptive transfer. This demonstrates clearly that activation
and recruitment are not inextricably linked. Under conditions of
natural exposure to inflammation-inducing pathogens, T cell activation
and recruitment generally occur simultaneously, because pathogens
contain both T cell-activating protein Ags and components that induce
local inflammation. Activation is also important because the low
frequency of naive T cells specific for any individual pathogen
generally necessitates activation and clonal expansion of these cells
to elicit a protective response. Whether T cell recruitment is tied to
T cell activation is particularly critical in the cases of autoimmunity
and allergy in which recruitment of activated T cells may determine
whether pathological tissue injury occurs.
It has been suggested that activation of mast cells and basophils can lead directly to production of Th2 cytokines and recruitment of eosinophils (50, 51). Studies using anti-IgE blocking Abs have shown a role for IgE and Th2 cytokines in eosinophil recruitment (50). In murine asthma models that use a small number of repetitive airway Ag challenges and in those in which both priming and challenge are via the airway, c-kit-deficient (W/Wv) mice with congenitally low numbers of tissue mast cells have impaired eosinophil recruitment (52). In other models, this contribution of mast cells to eosinophil recruitment has not been observed (53). It appears that stronger priming and more frequent airway Ag challenges can overcome the eosinophil recruitment defect in mast cell-deficient mice (27). Notably, stronger stimuli also increase Th2 cell recruitment to the airway (23, 31).
The present set of experiments was designed in such a way that we could distinguish between direct and indirect effects of mast cells on eosinophil recruitment. Systemic administration of anti-DNP IgE followed by airway challenge with BSA-DNP22, which activates only mast cells and basophils without activating the T cells, does not induce eosinophil recruitment. This indicates that recruitment of eosinophils is not a direct consequence of IgE-stimulated mast cells. This study and others support an indirect model in which mast cell activation can increase Th2 cell recruitment and that activation of these immigrant Th2 cells can in turn recruit eosinophils.
The observation that P- or E-selectin contributes to Th2 cell
recruitment to the lungs is consistent with the observation by Lukacs
et al. (54) and Broide et al. (55) who
observed decreased eosinophilia at early time points after challenge in
models of allergic airway inflammation similar to that reported here.
Lukacs et al. observed similar levels of Th2 cytokines in the lungs of
both E-selectin-deficient and wild-type mice 8 h after aerosol Ag
challenge, suggesting that recruitment of Th2 cells was not dependent
on E-selectin in this model. Together with our data in P- and
E-selectin-deficient mice, this may point to a primarily
P-selectin-mediated event in Th2 recruitment to the lung.
Alternatively, it may suggest that the difference in T cell recruitment
in selectin-deficient mice is not apparent until later in the response
because the peak of T cell recruitment and cytokine production in our
system is
2448 h after challenge.
Of particular interest was the finding that Th2 cell recruitment to the
lung parenchyma and the airway lumen was not completely parallel. One
possible explanation is that the lack of selectins on lung endothelial
cells in P- and E-selectin-deficient mice may limit the rate at which
Th2 cells enter the lung parenchyma from the circulation. The residual
influx of Th2 cells from the blood in the selectin double-deficient
mice may be mediated by other receptors such as
4 integrins
(reviewed in Ref. 44). If the rate of movement of Th2
cells from the parenchyma out into the airway lumen is not affected by
the selectin defect, then the number of cells accumulating in the
parenchyma may be substantially reduced.
Several studies suggest that Th1 cells exhibit a lower threshold for migration into target tissues than do Th2 cells (14). We show here that the barrier to Th2 cell migration can be overcome by induction of a variety of types of inflammation in the tissue target. IgE is a component of Th2-type inflammation, whereas the bacterial component LPS is generally associated with Th1-type responses. Interestingly, both can elicit a permissive environment for recruitment of Th2 cells. Our data imply that Ag-nonspecific inflammatory signals are sufficient for recruitment of Th2 cells to the lung and show that the T cells cognate Ag is not necessary to overcome the threshold for recruitment of Th2 cells. Two important principles emerge from our experiments. First, we suggest that recruitment of Th2 cells in this system is analogous to recruitment of neutrophils and other cells of the innate immune system, depending primarily on local inflammatory signals and independent of specific Ag. Second, we have shown that activation of Ag-experienced Th2 cells and their migration into a site of inflammation are not necessarily linked. In the second observation lies a lesson for future studies of Th2-type responses in that measurable effector functions are the consequence of both recruitment and activation of the effector Th2 cells. Our studies describe a novel mechanism by which a pool of activated Ag irrelevant Th2 cells may impact the quality of a tissue-specific inflammatory response.
| Acknowledgments |
|---|
| Footnotes |
|---|
2 Current address: Division of Parasitology, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, U.K. ![]()
3 Address correspondence and reprint requests to Dr. David D. Chaplin, Department of Microbiology, University of Alabama, 845 19th Street South, Bevill Biomedical Research Building 276/11, Birmingham, AL 35295-2170. E-mail address: dchaplin{at}uab.edu ![]()
4 Abbreviations used in this paper: LN, lymph node; i.n., intranasal; Tg, transgene; BAL, bronchoalveolar lavage; PAS, Periodate Acid Schiffs. ![]()
Received for publication November 30, 2001. Accepted for publication September 13, 2002.
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transgene in murine lung causes lymphocytic and fibrosing alveolitis: a mouse model of progressive pulmonary fibrosis. J. Clin. Invest. 96:250.
1 integrin-dependent adhesion in human Th2 T lymphocytes. J. Immunol. 164:3292.
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3 fucosyltransferases in Th1 and Th2 cells correlates with their ability to bind P-selectin. Biochem. Biophys. Res. Commun. 247:307.[Medline]
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