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Synthesis1


Departments of
*
Immunology and
Pediatrics and Child Health, University of Manitoba, Winnipeg, Canada
| Abstract |
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and strongly enhanced upon addition of neutralizing
anti-IFN-
mAb. Collectively, the data are consistent with a
negative regulatory role for endogenous IFN-
synthesis in
controlling the intensity of systemic IL-13 responses evoked in both
atopic and nonatopic populations following exposure to common Ags. They
also suggest that the elevated levels of IL-4 and IL-5 characteristic
of type 2-dominated responses in vivo are without detectable impact on
the maintenance of recall Ag-stimulated IL-13
production. | Introduction |
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Although naive and Ag-experienced (CD45RA and RO) CD4 and CD8 T cells,
activated mast cells, and basophils have all been reported to produce
IL-13, the in vitro conditions used to study regulation of this
cytokine have been dominantly polyclonal (5, 6, 7, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 32, 33, 34, 35, 36). Thus,
while anti-CD3, anti-CD28, PMA, calcium ionophore, and other
stimuli elicit intense, readily quantified IL-13 responses, the levels
of IL-13 protein synthesis observed, and their regulation in healthy
and atopic subjects upon Ag-mediated activation, remain ill-defined.
Physiologic T cell activation and different polyclonal activators
frequently trigger different intracellular signaling pathways (37, 38, 39, 40, 41)
and, consequently can evoke responses leading to qualitatively
different conclusions (42). In this study, we investigated the
production of IL-13 in short-term, Ag-driven primary culture of PBMC
obtained from healthy, nonatopic subjects and those with seasonal
allergic rhinitis. Atopic subjects were found to exhibit markedly
stronger IL-13 responses in response to Ag-dependent but not polyclonal
activation. The intensity of IL-13 production was negatively regulated
by endogenous IFN-
synthesis in both atopic and nonatopic subjects.
The data indicate the complexity of the Th2-associated skewing of
responsiveness to environmental Ags seen in atopic individuals and
demonstrate that while both IL-4 and IL-13 are markedly elevated among
atopics, and can be utilized as indicators of excessive type 2
activation, the levels of production of these two cytokines appears
largely independent in Ag-driven responses.
| Materials and Methods |
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This study, approved by the University of Manitoba Faculty Committee on the use of Human Subjects in Research, was conducted in 40 subjects ages 1835 yr who gave prior written informed consent to participation. Twenty subjects had grass pollen allergic rhinitis and 20 were healthy nonatopic controls. Atopic subjects were recruited on the basis of 1) a history of seasonal allergic rhinitis of at least 2 yr duration warranting regular daily treatment with an intranasal glucocorticoid and/or histamine receptor antagonists (except in the 24- to 48-h period immediately before skin testing), and 2) a positive epicutaneous test (wheal diameter >4 mm that of the negative control) to the grass pollen mix used for in vivo and in vitro testing (grass mix 1649, consisting of June/Kentucky blue grass, timothy grass, bromegrass and red-top grass; Bayer, Etobicoke, Ontario, Canada). None of the subjects had previously received allergen immunotherapy. Control subjects had no history of allergic rhinitis, asthma, or other allergic disorders and exhibited negative epicutaneous tests to the grass pollen mixture used in this study as well as to a panel of 12 common aeroallergens.
Cell culture
Whole blood (20 ml) was collected into 1 ml of 2.7% EDTA. PBMC
were obtained by centrifugation with Histopaque-1077 (Sigma, St. Louis,
MO) within 30 min of blood collection. Cells collected from the
interface were washed three times in saline, counted (>99% viability
as determined by trypan blue exclusion), and used for culture
immediately or following storage in liquid N2. Only frozen
samples in which viability exceeded 90%, the great majority, were
used. PBMC were cultured at a final concentration of 2.5 x
106/ml for Ag stimulation or 1.5 x 106/ml
with PHA using 96-well U-bottom plates (Nunc, Naperville, IL) in
RPMI 1640 supplemented with 10% FCS, 10 mM L-glutamine,
2 x 105 M 2-ME, and antibiotic-antimycotic (Life
Technologies, Burlington, Canada). For each subject, a minimum of two
wells were set up for each condition tested. Cells were cultured 1) in
the absence of stimuli; 2) with grass pollen extract (Bayer) at 400
µg/ml alone and in the presence of anti-CD4 (PharMingen,
Mississauga, Canada), anti-DR,DP,DQ (PharMingen), or normal mouse
IgG (Sigma) at 10 µg/ml; and 3) with 1% PHA (Difco, Detroit, MI).
Unselected PBMC populations were used in preference to highly purified
CD4 T cells so as to not exclude a priori, allergen-driven cytokine
production by non-CD4 cells. The same Ag preparation was used for in
vivo and in vitro testing. In some experiments, exogenous cytokines or
neutralizing Abs were added: rIL-4 (1.8 x 107 U/mg,
Sterling Winthrop, Collegeville, PA) or polyclonal rabbit
anti-human IL-4 (Sterling Winthrop, at 0.5 µg/ml capable of
neutralizing >10 ng/ml IL-4 in CT.h4S bioassay, 43 ; rIL-5
(PharMingen, sp. act. 6.2 x 107 U/mg) or rat
anti-IL-5 mAb at 2.5 µg/ml (TRFK5, PharMingen,
ND50 60 ng/ml to neutralize the activity of 1 ng/ml
rIL-5); rIFN-
(PharMingen, sp. act. 3 x
106 U/mg) or anti-IFN-
at 1 µg/ml (NIB42,
PharMingen, ND50 0.6 ng/ml to neutralize the
activity of 1 ng/ml rIFN-
).
Cytokine assays
IL-13. A sandwich ELISA using a purified monoclonal rat anti-human IL-13 as capture reagent and a purified, biotinylated polyclonal anti-human IL-13 (PharMingen) in combination with streptavidin-alkaline phosphatase (Jackson ImmunoResearch, West Grove, PA) was used. Internal IL-13 standards of sp. act. 106 U/mg (PharMingen) were included on each plate. The lower limit of detection was typically 8.4 pg/ml.
IL-5. A sandwich ELISA was also used for IL-5 detection with a purified anti-human IL-5 mAb (TRFK4, PharMingen) for Ag capture and biotinylated JES15A10 (PharMingen). IL-5 levels in each sample were calculated against a standard (PharMingen) set in each plate. Assay sensitivity was typically 8.4 pg/ml.
IL-4. Levels were evaluated using CT.h4S cells in an MTS assay (43) that detected 0.251.0 pg/ml of IL-4 as calibrated against WHO IL-4 interim standard 88/656 run on each assay plate.
IFN-
.
IFN-
was determined in ELISA using mAbs obtained from PharMingen.
Each assay was conducted using internal standards of an
IFN-
-containing, PHA-stimulated (Difco) PBMC supernatant, calibrated
against human IFN-
reference reagent Gg 23-901-530 (1
National Institutes of Health unit = 115 pg, provided by Dr. C.
Laughlin, National Institute of Allergy and Infectious Diseases,
National Institutes of Health, Bethesda, MD). All samples were
evaluated in at least two assays, with the concentration of cytokine in
each supernatant calculated from a minimum of three points falling on
the linear portion of titration curves calibrated against recombinant
cytokine standards serially diluted on each plate. SE typically ranged
from 3 to 10%.
Statistical analysis
For individual subjects, the mean (±SEM) is shown. Median values for each group, indicated by a bar, and range are shown for each population. Statistical significance was evaluated using the Mann-Whitney U test. Correlations were determined using nonparametric statistics (Spearmans rank sum).
| Results |
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We first examined the time course of IL-13 production in response
to Ag-specific and polyclonal stimulation. Culture supernatants were
harvested at day 1, 2, 3, 5, and 7. PHA-stimulated IL-13 was readily
detectable by 24 h and increased for the duration of the
experiment (not shown), whereas grass pollen-stimulated IL-13 (Fig. 1
) was undetectable until 4872 h,
peaking at 57 days of culture (Fig. 1
). Data shown are from four of
eight subjects for whom extensive time course analyses were conducted.
The kinetics of IL-13 responses to Ag did not differ between atopic and
nonatopic subjects. Similarly, Ag-stimulated IL-5 production was first
detectable by day 35, peaking at days 57 (Fig. 1
). As
previously reported for IL-10 (42, 44), cultures conducted in the
absence of antigenic stimulation exhibited low but detectable cytokine
production that increased over the duration of culture with peak median
responses of 13 and 21 pg/ml for IL-5 and IL-13, respectively, in these
experiments. This is in marked contrast to a report that atopic
dermatitis subjects spontaneously release large amounts of IL-13
(median >4000 pg/ml at 24 h) and IL-4 (>100 pg/ml) in the absence of
in vitro activation while nonatopic subjects had responses that
were lower but still well above that seen here (45).
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To date, IL-13 synthesis by fresh PBMC, T cell or basophil
populations has primarily been studied in response to polyclonal
activators such as various combinations of anti-CD3, anti-CD28,
Con A, PHA, PMA, and/or Ca2+ ionophore. IL-13 production to
PHA is of more rapid onset, is markedly more intense than that seen
following Ag stimulation, and, most importantly, is of equivalent
intensity in atopic and nonatopic groups (Fig. 2
, Mann-Whitney p >
0.05). Similar data were obtained at day 7 (medians: atopic 26,233 vs
nonatopic 16,751 pg/ml, n = 20/group, Mann-Whitney
p > 0.05). A statistically significant but weak
elevation of polyclonally evoked IL-5 synthesis was observed in the
allergic population at day 5 (medians 227 vs 107 pg/ml,
p = 0.013). We previously reported that polyclonally
elicited IL-4 responses in a similar comparison did not differ
significantly between atopics and nonatopics (42).
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Reasoning that Ag-mediated activation would provide a more
biologically relevant comparison of responses in atopic and normal
subjects, we evaluated IL-13 and IL-5 synthesis in grass
pollen-stimulated primary culture (Fig. 3
). Ag-elicited IL-5 responses were
markedly elevated among individuals with seasonal allergic rhinitis
(p = 0.0008). In marked contrast to IL-4 (data
not shown and 46), IL-13 was produced at detectable levels by every
individual in both atopic and nonatopic groups upon Ag-mediated
stimulation. Median IL-13 responses were 609 pg/ml vs 238 pg/ml at day
5 and 874 pg/ml vs 406 pg/ml at day 7 for atopic and nonatopic
subjects, respectively. Thus, environmental Ag-driven IL-13 production
was consistently higher among atopic subjects
(p = 0.0031 and 0.0087 on day 5 and day 7,
respectively). The median cytokine responses in the absence of Ag
ranged from 3284 pg/ml for IL-13 and 1739 pg/ml for IL-5. They did
not differ significantly between atopic and nonatopic subjects for
either cytokine at either time point examined.
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We previously demonstrated that the allergen-specific cytokine
response in grass pollen allergic subjects is characterized by markedly
elevated IL-4 synthesis and reduced IFN-
synthesis relative to
normal, nonatopic subjects (46, 47). In marked contrast to IL-13 or
IL-5 production, where the difference between these populations is
quantitative (Fig. 3
), IL-4 responses were undetectable in virtually
all healthy subjects and ranged from 2 to 30 pg/ml in atopics (data not
shown). Statistical analysis of the relationship between inhalant
Ag-dependent IL-4, IL-5, and IL-13 synthesis in primary culture
revealed that while IL-5 and IL-13 are highly correlated, the intensity
of the IL-4 response observed was not correlated with either IL-13
(Fig. 5
) or IL-5 (data not shown). If
subjects are stratified for clinical phenotype (atopic vs nonatopic),
the intensity of the Ag-driven IL-4 responses remains independent of
IL-13 levels (Spearmans p > 0.05) in both instances.
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production but are independent of IL-4 and IL-5 levels
In light of the role that IL-4 plays in shaping initial commitment
to the Th1/Th2 phenotypes, we subsequently investigated the impact on
recall IL-13 responses of 1) the addition of physiologically relevant
levels of rIL-4 and 2) blocking endogenous IL-4 activity in vitro
during grass pollen-mediated activation. Thus, PBMC were stimulated
with grass pollen Ag in the presence of rIL-4 vs neutralizing
anti-IL-4 Ab or, in subsequent experiments, rIL-5/anti-IL5 mAb.
Atopic and nonatopic subjects were selected to represent the range from
those yielding virtually undetectable (<8 pg/ml) to very high levels
of Ag-dependent IL-13 production. As detailed in Fig. 6
for six representative subjects of the
16 examined, Ag-driven IL-13 production was not affected either by
blocking endogenous IL-4 activity nor by supplementing cultures with
rIL-4, even at levels greatly in excess of those seen upon Ag
stimulation (i.e., 500 pg/ml). In independent experiments, rIL-5 added
at
20-fold the median Ag-driven response or neutralizing mAb to IL-5
also did not affect recall Ag-driven IL-13 responses in the six
subjects examined (Table I
).
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at low levels (i.e.,
levels similar to and those 5- to 10-fold above those seen in Ag-driven
culture (46)) markedly inhibited expression of IL-13 responses (Fig. 7
mAb to block the activity of endogenous
IFN-
enhanced IL-13 synthesis in most subjects by several fold.
Collectively, these data are consistent with a role for endogenous
IFN-
synthesis in controlling expression of environmental Ag-driven
IL-13 synthesis but argue that the elevated levels of IL-4 and IL-5
characteristic of type 2-dominated responses in vivo are without
detectable impact on the maintenance of recall Ag-stimulated IL-13
production.
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| Discussion |
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, or neutralization of endogenous IFN-
, substantively alters
allergen-driven IL-13 synthesis. While it needs to be kept in mind that
these conclusions are based on net cytokine protein levels in culture
supernatants, with the value obtained for each cytokine affected by
consumption and cross-regulation, this applies to any system utilizing
in vivo or in vitro restimulation.
The relationship between IL-4 and IL-13 production has been examined by
several groups, with a statistically significant association noted in
some reports (20, 27, 32) and others finding that they are not
correlated (22, 23, 28). Each of the studies cited relied upon
polyclonal activation to elicit IL-4 and IL-13. In one of the few
studies using Ag-driven activation, Kroegel et al. investigated IL-13
secretion into airways of 10 mild allergic asthmatics and 4 controls
following local allergen challenge. While the intensity of IL-13
responses did not correlate with IL-4 levels, it did with eosinophil
numbers in bronchoalveolar lavage, suggesting that IL-13 is actively
secreted during the late asthmatic response (48). Taken together, the
data argue that while the intensity of IL-5 and IL-13 recall responses
in both atopic and nonatopic subjects are tightly correlated, the
intensity of Ag-dependent IL-4 and IL-13 responses are essentially
independent. Endogenous IFN-
production appears to play a negative
regulatory role, dampening the intensity of the IL-13 responses evoked
upon chronic exposure to environmental Ags.
As an alternative approach to investigate the role that IL-4 may play
in modulating IL-13 synthesis in established responses, Huang et al.
used Th2 clones derived from TCR transgenic and, independently, IL-4
knockout mice (49). They found similar IL-13 mRNA levels under neutral
conditions, when endogenous IL-4 production or IL-4R function was
neutralized, and when substantial amounts of rIL-4 were added at
restimulation, leading them to conclude that IL-4 does not play a
demonstrable role in expression of established IL-13 responses.
Collectively, the data suggest that once CD4 T cells are committed to
type 2 patterns of activation, IL-4 and IL-5 may have minimal impact on
Ag-driven expression of IL-4, -5, or -13, whereas IFN-
markedly
reduces the intensity of recall type 2 cytokine responses in both
atopic and nonatopic situations.
Characterization of the relative intensity of IL-13 responses in atopic
and normal subjects has yielded varying conclusions. Freshly obtained
naive and memory CD4 and CD8 T cells (22, 23, 27), as well as T cell
lines and clones (21, 50, 51), from both atopic and control donors
exhibit strong IL-13 synthesis following stimulation with
anti-CD3/anti-CD28, PMA plus A23187, or anti-CD3 plus
rIL-2. Responses to Ag were not examined in any of these studies.
Similarly (24, 26, 36), activated basophils secrete substantial
quantities of IL-13, but only polyclonal activators or anti-IgE
were used to characterize its production. Several studies of PBMC from
atopic and nonatopic subjects indicate similar kinetics and comparable
or marginally different levels of IL-13, as well as IL-4 and IFN-
,
in the two populations following in vitro (polyclonal) activation (22, 27, 52). However, in light of the data above (Fig. 2
, also using
polyclonal activation), comparisons of the relative responsiveness of
atopic and nonatopic subjects to such stimuli are difficult to
interpret.
Looking at mRNA expression, several groups have obtained evidence of elevated IL-13 in atopic dermatitis lesions (45, 53), allergic rhinitis (54), or in the nasal mucosa of atopic subjects after challenge with diesel exhaust particles, an adjuvant believed to promote Th2-associated disease (55, 56). Esnault et al. used RT-PCR and subsequent autoradiography of dot blots following stimulation with PHA plus PMA (57), reporting substantial IL-13 mRNA in both groups. Responses by PBMC from atopics were slightly (p = 0.02) elevated relative to the normal controls.
In Ag-stimulated systems, Essayan et al. (52) evaluated IL-13 production by eight T cell clones derived from one ragweed allergic and one nonatopic control subject. Huang et al. (58) reported increased IL-13 mRNA and protein levels among allergen-challenged BAL of asthmatic and rhinitic subjects compared with saline-challenged healthy controls. Interestingly, no IL-13 synthesis (mRNA or protein) was seen in the two controls before and following challenge with the locally common environmental Ag mixture used (58). Similarly, Ying et al. (59) examined IL-13 gene expression via in situ hybridization and immunohistochemistry subjects with allergic rhinitis or asthma after allergen challenge in vivo. IL-13 mRNA and intracellular protein levels were undetectable prechallenge and in diluent challenged atopics, while atopics exhibited marked increases in IL-13 and IL-4 at skin test sites (i.e., not restricted to nasal or bronchial mucosa) over the course of the late phase reaction. The four controls exhibited IL-13 responses at or below the limits of detection. Humbert (60), comparing IL-13 mRNA levels in atopic and nonatopic asthmatics, nonasthmatic atopics, and healthy controls, found a weak statistically significant increase in IL-13 mRNA levels in mucosal biopsies (p = 0.02) between asthmatics (both atopic and nonatopic) and healthy controls but, surprisingly, no difference between atopic subjects and nonatopic controls. In contrast, we observed readily detectable Ag-dependent IL-13 (protein) responses in all of the 40 subjects examined following short-term primary culture of unselected PBMC populations, with differences in IL-13 expression being quantitative.
Previous reports demonstrated that CD4 and CD8 T cells, mast cells, and basophils are capable of substantial (polyclonally driven) IL-13 responses (21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31), with several groups (21, 23, 28) finding CD8 cells to be more potent than CD4 cells. Here, incorporation of anti-CD4 or anti-MHC class II mAbs blocked virtually all IL-13 protein synthesis. Whether this difference is attributable to use of different stimuli or whether it reflects CD4-dependent, CD8-mediated IL-13 synthesis remains to be determined. It is clear that polyclonal and Ag-driven triggering initiate distinct signaling pathways and functional consequences, which can include different patterns of cytokine synthesis (40, 41), and that anti-CD3 can stimulate or inhibit IL-13 production depending on the costimulus used (23). The optimal conditions for (polyclonal) activation of IL-13 are actively inhibitory for IL-4 gene expression (21, 23). In light of the extensive literature indicating that the nature of stimulation conditions plays a pivotal role in influencing cytokine production and that the relationship between these polyclonal stimuli and MHC-dependent Ag processing and presentation remains speculative, these data argue for greater use of Ag as a more physiologic means of activation.
An important caveat to this study is that the cellular profile and
interactions in PBMC may differ from those at the site of allergen
exposure in vivo (i.e., in nasal mucosa). Indeed, one could hypothesize
that the increased IL-4, IL-5, and IL-13 synthesis seen in vitro merely
reflects an increased frequency of total CD4 cells in the circulation
of atopics. We believe this unlikely as extensive studies in the early
1980s, using well in excess of 200 subjects, revealed that there were
no detectable differences in major T cell subsets (i.e., CD3, CD4, and
CD8) between PBMC of atopic and nonatopic individuals (61, 62, 63).
Moreover, the frequency of PBMC exhibiting Ag-driven proliferation (64)
or estimates of the frequency of cytokine-producing PBMC (total of
IFN-
plus IL-4-producing cell populations) (65), as determined by
limiting dilution analysis, is also highly similar in atopic and
nonatopic populations.
Finally, we examined the role of other type 1 and type 2 cytokines in
regulation of Ag-driven IL-13 recall responses. We previously
demonstrated that markedly higher frequencies of atopic individuals are
capable of exhibiting inhalant Ag-driven IL-4 responses in primary
culture (47) and that the levels of IL-4 production are substantially
elevated (Mann-Whitney p = 0.000001), while IFN-
responses are lower (p = 0.008) (42). The role
played by IFN-
in regulation of IL-13 synthesis has also been
controversial, with some reporting it to be without effect (22) and
others demonstrating that addition of exogenous IFN-
inhibits IL-13
production (66). Our data, demonstrating that exogenous rIFN-
inhibits IL-13 while neutralization of endogenous IFN-
activity
increases IL-13 production (Fig. 7
), taken with the known differences
in the capacity of normals and atopics to generate IFN-
responses to
allergen stimulation (1, 2, 3, 4, 46), argues in support of a role for
IFN-
in IL-13 regulation in vivo. Indeed, it raises the possibility
that the elevated IL-13 levels seen in atopic subjects may be secondary
to deficient responses to IFN-
-promoting, Th1-associated cytokines
such as IL-12 and IL-18. Both consequences may be related to the
finding that PBMC of subjects with seasonal allergic rhinitis exhibit
similar levels of IL-12 production but markedly lower responsiveness to
rIL-12 stimulation (as measured by IFN-
synthesis) than do
nonatopics (67) (Y. Li et al., manuscript in preparation).
| Footnotes |
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2 Address correspondence and reprint requests to Dr. Kent HayGlass, Department of Immunology, University of Manitoba, 626730 William Ave., Winnipeg, MB R3E 0W3, Canada; E-mail address: ![]()
Received for publication October 22, 1997. Accepted for publication August 31, 1998.
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