The Journal of Immunology, 2002, 168: 4756-4763.
Copyright © 2002 by The American Association of Immunologists
IL-16 Promotes Leukotriene C4 and IL-4 Release from Human Eosinophils via CD4- and Autocrine CCR3-Chemokine-Mediated Signaling1
Christianne Bandeira-Melo*,
Kumiya Sugiyama*,
Lesley J. Woods*,
Mojabeng Phoofolo*,
David M. Center
,
William W. Cruikshank
and
Peter F. Weller2,*
* Department of Medicine, Harvard Thorndike Laboratories, Charles A. Dana Research Institute, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA 02215; and
Pulmonary Center, Boston University School of Medicine, Boston, MA 02118
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Abstract
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Human eosinophils are potential sources of inflammatory and
immunomodulatory mediators, including cysteinyl leukotrienes,
chemokines, and cytokines, which are pertinent to allergic
inflammation. We evaluated the means by which IL-16, a recognized
eosinophil chemoattractant, might act on eosinophils to affect their
capacity to release leukotriene C4 (LTC4) or
their preformed stores of chemokines (eotaxin, RANTES) or Th1 (IL-12)
or Th2 (IL-4) cytokines. IL-16 dose dependently (0.01100 nM) elicited
new lipid body formation, intracellular LTC4 formation at
lipid bodies, and priming for enhanced calcium ionophore-activated
LTC4 release. IL-16 also elicited brefeldin A-inhibitable,
vesicular transport-mediated release of preformed IL-4, but not IL-12,
from eosinophils. CD4 is a recognized IL-16R, and accordingly
anti-CD4 Fab, soluble CD4, and a CD4 domain 4-based IL-16 blocking
peptide inhibited the actions of IL-16 on eosinophils. Although CD4 is
not G-protein coupled, pertussis toxin inhibited IL-16-induced
eosinophil activation. IL-16 actions were found to be mediated by the
autocrine activity, not of platelet-activating factor, but rather of
endogenous CCR3-acting chemokines. IL-16 induced the rapid vesicular
transport-mediated release of RANTES. The effects of IL-16 were blocked
by CCR3 inhibitors (met-RANTES, anti-CCR3 mAb) and by neutralizing
anti-eotaxin and anti-RANTES mAbs, but not by
platelet-activating factor receptor antagonists (CV6209, BN52021).
RANTES and eotaxin each enhanced LTC4 and IL-4 (but not
IL-12) release. Therefore, IL-16 activation of eosinophils is
CD4-mediated to elicit the extracellular release of preformed RANTES
and eotaxin, which then in an autocrine fashion act on plasma membrane
CCR3 receptors to stimulate both enhanced LTC4 production
and the preferential release of IL-4, but not IL-12, from within
eosinophils.
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Introduction
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Interleukin-16
is a cytokine initially identified as a chemoattractant for
CD4+ T cells and subsequently shown to be a
chemoattractant for other CD4+ cells, including
eosinophils (1, 2). IL-16 can be formed by varied cell
types including CD4+ and
CD8+ T cells, B cells, mast cells, dendritic
cells, epithelial cells, fibroblasts, and eosinophils (1, 3, 4). IL-16 may have roles in diverse immunologic responses
(1). In forms of allergic inflammation, including asthma,
IL-16 has been found to be prominent. IL-16 was increased in the
bronchial mucosa and sputum of patients with atopic asthma (5, 6) and in the bronchial mucosa and bronchoalveolar lavage fluids
after segmental allergen challenge (7, 8). Likewise, IL-16
increases in the nasal mucosa of patients with allergic rhinitis after
experimental or seasonal allergen exposures (9, 10). In
murine models of asthma, administration of anti-IL-16 mAb prevented
the production of Ag-specific IgE (11), and administration
of an IL-16 blocking peptide inhibited Ag-induced airway
hyperresponsiveness (12).
IL-16 may participate in other forms of inflammation, including
inflammatory bowel disease (13), in which eosinophils may
be a source of IL-16 (14). The functions of IL-16 in these
varied forms of immunologically mediated diseases may be complex. For
instance, IL-16 can inhibit MLR- and anti-CD3-activated lymphocyte
responses (15, 16), can participate in dendritic cell-T
cell interactions (4, 17), can enhance pro-inflammatory
cytokine release from monocytes (18), and can inhibit IL-5
production by Ag-stimulated T cells in atopic subjects
(19).
We had previously shown that IL-16 was a potent
(ED50
10-12 M)
chemoattractant for human eosinophils and that this activity was
dependent on CD4 expressed by eosinophils (2). IL-16,
however, did not appear to enhance other responses of eosinophils, such
as their capacity for enhanced leukotriene C4
(LTC4)3
formation or their capacity to "degranulate," as assessed by fluid
phase assays of granule-derived arylsulfatase B release
(2). Just as there is an increasing recognition of the
varied activities of IL-16, there has been an increased understanding
of the complexities of eosinophil cell biology and functioning in
varied immunologic responses. Eosinophils may have functions in immune
responses extending beyond their conventional "degranulation"-based
effector responses, and these may include functioning as APCs to
promote Th2 CD4+ responses (20, 21).
Eosinophils are now recognized to contain preformed stores of diverse
cytokines and chemokines within their cytoplasmic granules
(22). Thus, in addition to their distinctive cationic
proteins, eosinophil granules contain chemokines (e.g., eotaxin
(23) and RANTES (24, 25)) and cytokines with
disparate and potentially opposing activities, notably including the
prototypical Th2 cytokine IL-4 (26, 27, 28) and the Th1
cytokine IL-12 (29, 30, 31). The regulated release of these
preformed cytokines occurs not by exocytotic fusion of granules with
the plasma membranes to extrude all granule contents, but rather by
selective processes based on vesicular mobilization and transport of
these granule-derived cytokines (32, 33, 34).
We have now applied more sensitive, recently developed investigative
methods (32, 33) to evaluate whether IL-16 can promote
LTC4 formation by eosinophils and whether IL-16
can participate in the regulated and potentially differential release
of chemokines or cytokines that are preformed within human eosinophils.
Our data demonstrate that IL-16 can activate specific responses of
human eosinophils and in a CD4-dependent manner elicit the release of
RANTES and eotaxin from eosinophils, which then function as
CCR3-mediated autocrine activators to enhance eosinophil eicosanoid
formation, and a selective release of specific cytokines from
eosinophil granule stores.
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Materials and Methods
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Eosinophil purification
Peripheral blood was obtained with informed consent from 16
normal donors, and eosinophils were isolated as described
(32). Briefly, after citrate-anticoagulated blood was
mixed with 6% dextran-saline (MacGaw, Irvine, CA) to facilitate
erythrocyte sedimentation, the leukocyte-enriched plasma was overlaid
onto Ficoll-Paque (Amersham Pharmacia, Uppsala, Sweden) and centrifuged
at 250 x g for 20 min. Granulocyte-enriched cell
pellets were collected, washed at 4°C with calcium- and
magnesium-free HBSS (HBSS-/-), and depleted of
erythrocytes by hypotonic saline lysis. Eosinophils were negatively
selected using the MACS system (Miltenyi Biotec, Auburn, CA) with
anti-CD16 immunomagnetic beads. The viability of freshly isolated
cells was >95% (by trypan blue exclusion) and eosinophil purity was
>99% (by HEMA3 staining, Fisher Scientific, Pittsburgh, PA). Purified
cell suspensions were adjusted to 1 x 106
or 15 x 106 cells/ml in RPMI 1640 medium
containing 0.1% endotoxin-free OVA (Sigma-Aldrich, St. Louis, MO)
for use in fluid- or gel-phase assays, respectively.
Lipid body induction, staining, and enumeration
Eosinophil suspensions (106/ml) were
incubated (37°C) with IL-16 (0.01100 nM; R&D Systems, Minneapolis,
MN) or medium alone for 1 h and then cytocentrifuged (350 rpm, 4
min) onto glass slides. Cytospin slides, while still moist, were fixed
with 2% paraformaldehyde in HBSS-/-, rinsed in
0.1 M cacodylate buffer (pH 7.4), stained in 1.5%
OsO4 (30 min), rinsed in
dH2O, immersed in 1% thiocarbohydrazide (5 min),
rinsed with 0.1 M cacodylate buffer, restained with 1.5%
OsO4 (3 min), rinsed, and then dried and mounted
(35). Lipid bodies were enumerated by light microscopy
with a x100 objective lens in 50 consecutively scanned cells. For each
condition there was a unimodal distribution of lipid body numbers per
eosinophil without wide variations.
LTC4 measurements
After samples were taken for lipid body enumeration, cell
suspensions (106/ml) were washed in
HBSS-/-, resuspended in 1 ml of HBSS containing
calcium and magnesium, and then stimulated with 0.1 µM A23187
(Sigma-Aldrich) for 15 min (37°C). Reactions were stopped on ice,
cell suspensions were centrifuged (500 x g for 10 min;
4°C), and supernatants were assayed for LTC4 by
enzyme immunoassay (EIA) (2) (sensitivity, <7.8 pg/ml)
(Cayman Chemicals, Ann Arbor, MI). Intracellular formation of
LTC4 within eosinophils embedded in an agarose
matrix was evaluated as described previously using carbodiimide
fixation of newly formed LTC4 before its
immunofluorescent localization with an Alexa488-labeled (Molecular
Probes, Eugene, OR) rat
anti-LTC4/LTD4/LTE4
mAb (clone 6E7; Sigma-Aldrich) (35).
EliCell assay for detection of released IL-4, IL-12, and RANTES
The EliCell assay, a gel-phase dual-Ab capture and detection
assay based on microscopic observations of individual viable cells, was
performed as detailed (32) to enumerate the proportions of
eosinophils releasing preformed cytokines or chemokines (IL-4, IL-12,
or RANTES) and to electronically quantitate (in arbitrary units x
106) the average relative amounts of each
cytokine released extracellularly. Biotinylated goat polyclonal Abs
against IL-4, IL-12, and RANTES (each at 20 µg/ml; R&D Systems) were
used as capturing Abs and paired with Alexa546-labeled mouse
anti-IL-4, anti-IL-12, and anti-RANTES mAb (400 µl of 10
µg/ml; R&D Systems) to detect released IL-4, IL-12, and RANTES,
respectively. Alexa546 labeling was performed per a protocol from
Molecular Probes. Controls to ascertain the specificity of
extracellular immunodetection of these three cytokines and to confirm
that the detected cytokines were not from the intracellular pool were
performed. No IL-4, IL-12, or RANTES staining were found either when
Alexa546-labeled mouse IgG1 was used as a nonimmune isotype control or
when the biotinylated capture Abs (necessary to immobilize cytokines at
their extracellular sites of release) was substituted with a
biotinylated irrelevant control Ab. As evaluated by LIVE/DEAD
fluorescent assay (Molecular Probes), eosinophil viability at the end
of assays was >80%.
Stimuli and treatments
Eosinophils were stimulated with IL-16 (0.01100 nM; R&D
Systems), RANTES, or eotaxin (6 nM; R&D Systems) for time periods
ranging from 5 min to 3 h. For inhibitor studies, cells were
pretreated for 30 min with pertussis toxin (PTX) (20 ng/ml; Calbiochem,
La Jolla, CA), recombinant soluble CD4 (sCD4) (50 ng/ml; R&D Systems),
Fab of anti-CD4 (clone OKT4; American Type Culture Collection,
Manassas, VA) and anti-HLA class I (clone W6/32; Sigma-Aldrich)
mAbs (1 µg/ml), anti-CCR3 mAb (clone 61828.111; R&D Systems), or
isotype control rat IgG2a at 10 µg/ml (BD PharMingen, San Diego, CA),
met-RANTES (60 nM; R&D Systems), the platelet-activating factor (PAF)
receptor antagonists CV6209 and BN52021 (10 µM; Biomol, Plymouth
Meeting, PA), brefeldin A (BFA) (0.1 and 1 µg/ml; Biomol), or their
vehicles, as indicated. Alternatively, cells were co-incubated with 10
µg/ml of anti-eotaxin (clone 43911.11) and anti-RANTES
(clone 21445.1) mAb (each from R&D Systems) or with their matching
nonimmune mouse IgG1. Inhibitors were prepared in RPMI 1640 containing
0.1% of endotoxin-free OVA. PTX was prepared in DMSO, and the final
DMSO concentration was <0.01% and had no effect on eosinophils. A
peptide from domain 4 of human CD4, peptide
2:H-Q346CLLSDSG(353)-amide, and a scrambled
control peptide 3:H-DLGQSLSC-amide, were purchased from Research
Genetics (Huntsville, AL) (36).
Statistical analysis
Data were expressed as means ± SD. Percentage inhibition
with antagonists was calculated in comparison with stimulated increases
in lipid body numbers, LTC4 production, or IL-4
release above baselines. Statistical comparisons were done by ANOVA
followed by Newman-Keuls Students test. Differences were considered
significant when p < 0.05. Correlation coefficients
evaluating lipid body numbers vs quantities of
LTC4 released were determined from the means of
four different experiments by linear regression with significance (F
test) at p < 0.05.
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Results
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IL-16 primes for enhanced LTC4 production and elicits
lipid body formation in eosinophils
IL-16 very effectively primed eosinophils for increased
LTC4 release in response to a submaximal 0.1 µM
concentration of calcium ionophore A23187. Prestimulation of
eosinophils for 1 h with IL-16 dose dependently elicited increases
in A23187-induced LTC4 production (Fig. 1
B). At 100 nM, IL-16-primed
eosinophils released
6-fold as much LTC4 as
did eosinophils challenged with A23187 alone. Because the formation of
lipid bodies and LTC4 synthesis at these
organelles provides a basis for this enhanced capacity for
LTC4 release (35), we analyzed the
effect of IL-16 on the number of cytoplasmic lipid bodies. Resting
eosinophils contained
9.4 ± 2.3 lipid bodies (mean ± SD,
n = 10). IL-16 dose dependently induced the
formation of new lipid bodies withineosinophils, doubling their
initial numbers (Fig. 1
A). The increased quantities of
LTC4 generated by eosinophils primed with
increasing concentrations of IL-16 correlated highly with the increased
numbers of lipid bodies (r = 0.89; p <
0.05; n = 4), in accord with similar correlations
observed previously with other specific chemoattractants (i.e, PAF,
eotaxin, eotaxin-2, eotaxin-3, and RANTES) (35, 37, 38, 39).
In the absence of A23187 activation, eosinophils stimulated for 1
h with IL-16 only (100 nM) did not release extracellular quantities of
LTC4 sufficient to be detectable by EIA (data not
shown), but they did synthesize new LTC4
immunodetectable at lipid bodies within 33 ± 7% (mean ±
SD, n = 3) of IL-16-stimulated eosinophils (Fig. 2
D).

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FIGURE 1. Dose responses of IL-16-induced lipid body formation
(A), priming for enhanced LTC4 production
(B), and IL-4 release (C and
D) from human eosinophils at 1 h. Lipid bodies are
expressed as the mean number enumerated in 50 consecutive eosinophils.
LTC4 quantities released by 106 eosinophils in
1 ml after 15-min activation with 0.1 µM A23187 were assayed by EIA
in cell supernatants. IL-4 release, detected in solid-phase EliCell
assays, was expressed both as the percentage of eosinophils exhibiting
extracellularly released immunofluorescent detected IL-4
(D) and as the average of fluorescence intensities (in
arbitrary units x 106) for immunoreactive IL-4 around
50 individual eosinophils (C). Results are means ±
SD from four donors. * and **, p < 0.05 and
p < 0.01, respectively, compared with
nonstimulated eosinophils.
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FIGURE 2. IL-16 elicits release of IL-4 and RANTES, but not of IL-12, from
eosinophils and stimulates LTC4 formation at lipid bodies
within eosinophils. Phase contrast (left) and
fluorescent (right) microscopic images of identical
fields of eosinophils within a solid-phase matrix are shown. Anti-IL-4
(A), anti-IL-12 (B), and
anti-RANTES (C) immunoreactive sites in red and
anti-LTC4 (D) in green are overlaid on
phase-contrast images to facilitate their extracellular localization
for cytokines/chemokines around nonpermeabilized eosinophils or for
intracellular LTC4 within fixed and permeabilized
eosinophils. Images show representative eosinophils stimulated for
3 h with 100 nM IL-16 (bottom) or medium alone
(top). Bar, 5 µm.
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IL-16 elicits IL-4, and not IL-12, release from eosinophils
Eosinophils contain preformed stores of almost two dozen cytokines
resident largely within eosinophil-specific granules (22),
and these include IL-4 and IL-12, two cytokines with potentially
disparate and opposing activities. As detected with a sensitive
solid-phase dual-Ab capture and detection assay developed to detect the
extracellular release of eosinophil chemokines and cytokines
(32), IL-16 stimulated the release of IL-4 from
eosinophils. IL-16-stimulated eosinophils showed a punctate pattern of
immunoreactive IL-4 released at discrete loci proximate to the cell
surface, compatible with a vesicular transport-mediated process of IL-4
release (Fig. 2
A). No IL-4 staining was found with
nonstimulated eosinophils (Fig. 1
, C and D), when
the Alexa546-labeled anti-IL-4 detection Ab was replaced by an
Alexa546-labeled isotype nonimmune control (data not shown), or when
biotinylated anti-IL-4 capture Ab (which immobilizes IL-4 at its
extracellular sites of release) was substituted with a biotinylated
irrelevant control (data not shown). The last condition assured that
neither intracellular nor membrane-bound IL-4 was being detected in the
nonpermeabilized eosinophils. In contrast, IL-16 did not stimulate the
release of IL-12 from eosinophils (Fig. 2
B), another
cytokine stored preformed within eosinophils (29, 30, 31), nor
did RANTES or eotaxin (data not shown).
CD4 receptors mediate the effects of IL-16 on eosinophils
Because the chemoattractant activity of IL-16 for human
eosinophils is mediated by IL-16 engagement of CD4 (2), we
evaluated whether IL-16 promotion of LTC4 and
IL-4 release was likewise mediated through CD4 expressed on
eosinophils. Treatment of eosinophils with either soluble CD4 or
neutralizing Fab of OKT4 anti-CD4 mAb (but not with a control mAb
to surface-expressed class I MHC protein) blocked lipid body formation,
priming for enhanced LTC4 production and IL-4
release from eosinophils stimulated with IL-16 (Fig. 3
, left panels). Of note, Fab
were used because the binding of CD4 with whole OKT4 mAb mimics IL-16
stimulation of eosinophils (2). Thus, IL-16-elicited
eosinophil activation was mediated via CD4 expressed by
eosinophils.

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FIGURE 3. Receptors involved in IL-16-induced activation of eosinophils.
A, CD4 mediates the activation of eosinophils by IL-16.
As indicated, eosinophils were pretreated for 30 min with inhibitors of
IL-16/CD4 interaction (including sCD4 and Fab of OKT4 anti ( )-CD4 Ab
(or irrelevant control W6/32 anti-HLA Ab)) or with the
G i protein inhibitor PTX and then were incubated with
IL-16 (100 nM) for 1 h for induction of lipid bodies, priming for
enhanced LTC4 production (fluid-phase assays), and IL-4
release (solid-phase matrix assays). B, Involvement of
G i protein-coupled CCR3 receptors. As indicated,
eosinophils were pretreated for 30 min with antagonists of PAF (CV6209
and BN52021) and CCR3 (Met-RANTES (MetR) and anti-CCR3 neutralizing
mAb (or isotype control rat IgG2a)) receptors and then were activated
with IL-16. Results are means ± SD from three independent assays.
+ and *, p < 0.05 compared with nonstimulated
and IL-16-stimulated eosinophils, respectively. B,
Values represent the calculated percentage of inhibition with
antagonists in comparison with stimulated increases in lipid body
numbers, LTC4 production, and IL-4 release above
baselines.
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The amino acid sequence of the extracellular Ig-like region of CD4
contains four domains designated D1 through D4. Recent studies reported
that the binding sites for IL-16 within the CD4 molecule are within the
D4 residues (36, 40). To further investigate the specific
CD4 domain required for IL-16 binding andeosinophil activation,
eosinophils were pretreated with an octapeptide (peptide 2) derived
from the D4 region of CD4 molecule. As shown in Table I
, peptide 2 but not peptide 3 (a control
peptide with a random scrambled amino acid sequence) abolished
IL-16-induced lipid body formation, enhanced LTC4
production, and IL-4 release. It is of note that neither
these peptides nor the Fabof anti-CD4 affected the resting
status of nonstimulated eosinophils or the lipid body formation and
LTC4 and IL-4 release from RANTES-stimulated
eosinophils (data not shown). These data indicate that the effects of
IL-16 on eosinophils depend on IL-16 binding to the D4 domain of CD4
expressed by eosinophils.
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Table I. Inhibitory effect of peptide 2 derived from the D4
domain of CD4 molecule on IL-16-induced eosinophil
activation1
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Involvement of G protein-coupled CCR3 activation
in IL-16-initiated eosinophil activation
Although membrane-bound CD4 molecules are not G
protein-coupled receptors (GPCRs), we evaluated whether endogenously
generated ligands that signal via GPCR might be downstream mediators of
responses initiated by IL-16 engagement of CD4. Inhibition of G protein
activation by PTX pretreatment blocked IL-16-elicited lipid body
formation, priming for enhanced LTC4 production
and IL-4 release from eosinophils (Fig. 3
, left panels).
PTX-sensitive GPCRs known to elicit lipid body formation and enhanced
eicosanoid synthesis include the PAF receptor (38, 39, 41)
and CCR3 (35, 37). Pretreatment with two specific PAF
receptor antagonists (CV6209 and BN52021) did not inhibit
IL-16-elicited lipid body formation, priming for enhanced
LTC4 production, or IL-4 release from eosinophils
(Fig. 3
, right panels). In contrast, IL-16-mediated
activation of eosinophils was blocked by pretreatment with either the
CCR3 antagonist met-RANTES (42) or a neutralizing
anti-CCR3 receptor mAb (but not by an isotype control Ab) (Fig. 3
, right panels). These findings suggested that actions of
IL-16 to enhance LTC4 and IL-4 release from
eosinophils might be mediated by endogenous CCR3-acting chemokines.
Eosinophils contain two chemokines, eotaxin and RANTES, that act via
CCR3 and are stored preformed within eosinophil granules
(23, 24, 25). We have shown that both RANTES and eotaxin
induce lipid body formation, prime for enhanced
LTC4 production (35), and elicit
vesicular transport-mediated IL-4 release (43) from
eosinophils via CCR3 activation. With the EliCell assay, we evaluated
whether IL-16 elicited release of endogenous, preformed RANTES from
eosinophils. Indeed, IL-16 stimulated the rapid release of RANTES that
temporally preceded IL-16-induced IL-4 release (Fig. 4
). Consistently, in kinetic studies with
three different donors, IL-16 induced RANTES release as early as 5 min,
whereas at least 30 min were needed for detectable IL-4 release, a time
course consistent with autocrine intermediary roles for endogenously
derived CCR3 agonists. With eosinophils incubated for 1 h with 100
nM IL-16 (n = 3), 57.6% exhibited extracellular RANTES
release, and addition of sCD4 or OKT4 Fab anti-CD4 mAb during
incubation with IL-16 inhibited the numbers of eosinophils releasing
RANTES by 91.8 and 87.6%, respectively (Table II
). In a punctate pattern similar to
that for IL-4 release, released RANTES also appeared as extracellular
focal spots close to eosinophil membrane, again indicating a vesicular
transport-dependent secretory process (Fig. 2
C).
Interestingly, exogenous eotaxin also induced the rapid release of
endogenous RANTES (Fig. 4
, left panel). The RANTES-releasing
activity of eotaxin was fully inhibited by PTX and a CCR3-blocking mAb
and was partially inhibited by met-RANTES (Table II
). These inhibitors
of RANTES release, however, did not inhibit IL-16-induced release of
RANTES.

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FIGURE 4. IL-16-induced RANTES release precedes IL-4 release. Time courses of
RANTES (left panel) and IL-4 (right
panel) release from eosinophils stimulated with medium,
eotaxin, RANTES, or IL-16. With EliCell assays, RANTES and IL-4
released extracellularly from eosinophils embedded in an agarose matrix
were captured with anti-RANTES or anti-IL-4 bound to a gel
matrix and detected with Alexa546-labeled anti-RANTES or
anti-IL-4 mAb, respectively. Results were expressed as the
percentages ± SD of triplicates from a single experiment of
eosinophils exhibiting extracellularly released RANTES or IL-4 (300
cells counted). Data are representative of one of three replicate
experiments.
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To confirm the involvement of endogenously eosinophil-derived CC
chemokines (RANTES and eotaxin) on IL-16-mediated activation of
eosinophils, neutralizing anti-RANTES and anti-eotaxin mAbs
were co-incubated with IL-16-stimulated eosinophils. Both mAbs
inhibited IL-16-elicited lipid body formation, enhanced
LTC4 production, and IL-4 release from
eosinophils (Fig. 5
), indicating that
released chemokines were acting as autocrine activators. In addition,
IL-16-induced release of RANTES was inhibited 34.3% by
exogenous anti-eotaxin mAb and 0% by a controlmAb (data
not shown). Thus, IL-16 was directly mobilizing both RANTES and eotaxin
from eosinophils for their extracellular release and autocrine
activities. That IL-16 was eliciting the release of these
chemokines by vesicular transport-mediated secretion was supported by
findings with BFA, a vesicle formation inhibitor. Pretreatment with BFA
blocked IL-16-induced RANTES release and enhanced
LTC4 production and IL-4 release (Fig. 6
).

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FIGURE 5. Endogenous eosinophil-derived RANTES and eotaxin mediate IL-16-induced
activation of eosinophils. As indicated, IL-16-stimulated eosinophils
were cotreated with neutralizing anti ( )-eotaxin or anti-RANTES
mAb or with isotype control mouse IgG1. After 1 h, formation of
new lipid bodies, priming for enhanced LTC4 production
(fluid-phase assay), and IL-4 release (gel-phase assay) were analyzed.
Results are means ± SD from three independent assays. + and *,
p < 0.05 compared with nonstimulated and
IL-16-stimulated eosinophils, respectively. Dotted lines represent
basal levels.
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FIGURE 6. Vesicular transport mediates IL-16-induced RANTES and IL-4 release from
eosinophils. Eosinophils were pretreated for 30 min with an inhibitor
of vesicle formation, BFA, and then were stimulated with 100 nM IL-16
or 6 nM RANTES. After 1 h, RANTES release, priming for enhanced
LTC4 production, and IL-4 release were analyzed. Results
are means ± SD from three independent assays. + and *,
p < 0.01 compared with nonstimulated and
IL-16-stimulated eosinophils, respectively. Dotted lines represent
basal levels.
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Discussion
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The capabilities of eosinophils to contribute to varied
immunologic and inflammatory responses within tissues may be dependent
on the activation of mechanisms that enhance their formation of
eicosanoids, notably including LTC4, and their
regulated release of varied granule-stored proteins, which include not
only their cardinal cationic proteins but also varied chemokines and
cytokines with potentially diverse and disparate biologic activities.
Because these responses may not be adequately studied in conventional
fluid-phase assays, we have developed complementary techniques to
evaluate the capacity of individual eosinophils to synthesize
LTC4 and to release specific preformed chemokines
and cytokines by a physiologic process of vesicular mobilization of
granule-derived proteins for their transport and focal release outside
of the plasma membrane of eosinophils. With techniques that can
evaluate the functional responses of individual eosinophils, we have
shown that both RANTES and eotaxin induce lipid body formation
(35), prime for enhanced LTC4
production (35), and elicit vesicular transport-mediated
IL-4 release (43) from eosinophils via CCR3 activation.
Our current results indicate that IL-16 can not only enhance specific
responses of eosinophils, but that it can also identify major roles for
eosinophil-derived eotaxin and RANTES in functioning as important
autocrine intermediators of these IL-16-elicited responses.
IL-16 dose dependently elicited new lipid body formation, intracellular
LTC4 formation at lipid bodies, and priming for
enhanced calcium ionophore-activated LTC4
release. In our earlier studies with IL-16 (2), lesser
concentrations of IL-16, a greater concentration of activating calcium
ionophore, and a lack of methods to detect intracellular
LTC4 formation contributed to an inability to
recognize that IL-16 stimulation of eosinophils could enhance the
formation of the 5-lipoxygenase pathway-derived eicosanoid
LTC4. IL-16 also elicited BFA-inhibitable,
vesicular transport-mediated release of preformed RANTES and IL-4, but
not IL-12, from eosinophils. Although studies in CD4-knockout mice
indicate that IL-16 may act on monocytes and dendritic cells via as yet
unidentified CD4-independent mechanisms (44, 45), with
human and mouse T cells CD4 is the recognized IL-16R (1).
We previously established that IL-16-elicited chemotaxis of human
eosinophils was CD4-dependent (2). In support of a central
role for eosinophil-expressed CD4 in functioning as the
signal-transducing receptor on eosinophils for IL-16, we demonstrated
that anti-CD4 Fab, sCD4, and a CD4 domain 4-based IL-16 blocking
peptide inhibited the actions of IL-16 on eosinophils.
That IL-16 was not directly acting to enhance eosinophil
LTC4 formation and IL-4 release was indicated by
the inhibition found with PTX, which catalyzes the ADP ribosylation of
certain G protein
-subunits and uncouples PTX-sensitive G proteins
from cell surface receptors (46). Potentially, CD4 on
eosinophils might be interacting with the PTX-sensitive CXCR4, which is
expressed on eosinophils (47). Alternatively,
PTX-sensitive receptors for PAF or for chemokines acting via CCR3 might
be involved. Bartemes et al. (41) recently documented a
central role for endogenous PAF and its receptor in the augmented
functional responses of eosinophils elicited by IL-5 or IgG, and these
included enhanced LTC4 formation and correlative
increases in lipid body formation. Endogenous PAF generation and
signaling through its receptor were not involved in IL-16-mediated
signaling, because two PAF receptor antagonists (CV6209 and BN52021)
were without effect. Instead, IL-16 actions were mediated by the
autocrine activities of endogenous CCR3-acting chemokines. IL-16
induced the rapid vesicular transport-mediated release of RANTES, which
temporally preceded later IL-4 release (Fig. 4
). The effects of IL-16
were blocked by CCR3 inhibitors (met-RANTES and anti-CCR3 mAb) and
by neutralizing anti-eotaxin and anti-RANTES mAbs. Both RANTES
and eotaxin each enhanced LTC4 and IL-4 (but not
IL-12) release.
Thus, IL-16 acts to initiate the release of specific chemokines and
cytokines that are stored preformed within eosinophils. Our findings
provide insights into the mechanisms that regulate this mobilization
and release process. First, as indicated by the finding that exogenous
eotaxin also induced the rapid release of endogenous RANTES (Fig. 4
, left panel; Table II
), chemokines acting via CCR3 can
enhance the release of additional CCR3 active chemokines from within
eosinophils. This could provide a positive feedback loop amplifying the
initial IL-16-elicited chemokine release and consequent eosinophil
activation. Second, the vesicular transport-mediated process of
eosinophil "degranulation" was highly selective. Notably, the
IL-16-initiated transport and release of RANTES occurred in vesicles
that were not also loaded with IL-4, whose release occurred only later
in vesicles formed in response to the CCR3-active chemokines.
Additional selectivity in the regulated release of eosinophil-derived
cytokines was evident in the release of IL-4, but not of IL-12, by the
IL-16- or CCR3 chemokine-activated eosinophils. Because eosinophils
also contain preformed IL-16 (3), an ability for further
autocrine activation of eosinophils exists, but at present the
mechanisms that physiologically elicit IL-16 release from eosinophils
have not been defined.
Thus, the capacity of IL-16 to activate specific functional responses
of eosinophils has the potential to influence the nature of immunologic
responses. IL-16 can direct eosinophils to make new
LTC4 and release the chemokines, RANTES, and
eotaxin. These chemokines can act in an autocrine fashion on
eosinophils via CCR3 and potentially on other cells that express CCR3,
including basophils (48), mast cells (49),
airway epithelial cells (50), and Th2 cells
(51). Moreover, the differential release of IL-4, which
can contribute to the polarization toward Th2 differentiation, promote
IgE class switching, and stimulate other cellular responses pertinent
to allergic inflammation (52) (in contrast to IL-12, which
can suppress Th2-type responses and the manifestations of allergic
inflammation (53, 54)), provides a means by which IL-16
and CCR3 chemokines may promote specific eosinophil contributions to
allergic and other types of inflammation.
 |
Footnotes
|
|---|
1 This work was supported by National Institutes of Health Grants AI20241, AI22571, AI51645, HL56386, AI35680, and HL32802. 
2 Address correspondence and reprint requests to Dr. Peter F. Weller, Beth Israel Deaconess Medical Center, DA-617, 330 Brookline Avenue, Boston, MA 02215. E-mail address: pweller{at}caregroup.harvard.edu 
3 Abbreviations used in this paper: LTC4, leukotriene C4; EIA, enzyme immunoassay; PTX, pertussis toxin; sCD4, soluble CD4; PAF, platelet-activating factor; BFA, brefeldin A; GPCR, G protein-coupled receptor. 
Received for publication December 20, 2001.
Accepted for publication February 15, 2002.
 |
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