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Mast Cells Respond to Cell Injury through the Recognition of IL-33


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Mast cells have been attributed several functions in both health and disease. Mast cell activation and release of inflammatory mediators are associated with the pathogenesis of several diseases, in particular that of allergic diseases. While the notion of mast cells as important, protective sentinel cells is old, this feature of the cell is not well recognized outside the mast cell field. The mast cell is a unique, multifunctional cell of our defense system, with characteristics such as wide-spread tissue distribution, expression of receptors capable of recognizing both endogenous and exogenous agents, and a capability to rapidly respond to triggering factors by selective mediator release. In this review, we discuss the function of mast cells as sentinel cells in the context of cell injury, where mast cells respond by initiating an inflammatory response. In this setting, IL-33 has turned out to be of particular interest. IL-33 is released by necrotic structural cells and is recognized by mast cells via the IL-33 receptor ST2. IL-33 and mast cells probably constitute one important link between cell injury and an inflammatory response that can lead to restoration of tissue function and homeostasis, but might under other circumstances contribute to a vicious circle driving chronic inflammation.
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January 2011;
2011;186;2523-2528; Prepublished online 14J Immunol
Carolina Lunderius-Andersson
Möller-Westerberg, Padraic G. Fallon, Gunnar Nilsson and
Mattias Enoksson, Katarina Lyberg, Christine
IL-33 Recognition
Mast Cells as Sensors of Cell Injury through
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The Journal of Immunology
Mast Cells as Sensors of Cell Injury through IL-33
Mattias Enoksson,* Katarina Lyberg,* Christine Mo
¨ller-Westerberg,* Padraic G. Fallon,
Gunnar Nilsson,*
and Carolina Lunderius-Andersson*
In response to cell injury, caused, for example, by trauma, several processes must be initiated simultaneously to achieve an acute
inflammatory response designed to prevent sustained tissue damage and infection and to restore and maintain tissue homeostasis.
Detecting cell injury is facilitated by the fact that damaged cells release intracellular molecules not normally present in the ex-
tracellular space. However, potential underlying mechanisms for the recognition of endogenous danger signals released upon cell
injury have yet to be elucidated. In this study, we demonstrate that mast cells, potent promoters of acute inflammation, play a key
role in responding to cell injury by recognizing IL-33 released from necrotic structural cells. In an in vitro model of cell injury, this
recognition was shown to involve the T1/ST2 receptor and result in the secretion of proinflammatory leukotrienes and cytokines by
mouse mast cells. Remarkably, of all of the components released upon necrosis, our results show that IL-33 alone is a key component
responsible for initiating proinflammatory responses in mast cells reacting to cell injury. Our findings identify IL-33 as a key danger
signal released by necrotic structural cells capable of activating mast cells, thus providing novel insights concerning the role of mast
cells as sensors of cell injury. The Journal of Immunology, 2011, 186: 2523–2528.
The ability to recognize and respond to cell injury is fun-
damental to the survival of all animal species. Upon cell
injury, endogenous danger signals, so-called damage-
associated molecular patterns, are released by necrotic cells, in-
cluding heat shock proteins (1), high-mobility group box 1
(HMGB1) (2, 3), uric acid (4), and cytokines of the IL-1 family:
IL-1a(5, 6) and, as recently suggested, IL-33 (7, 8). Such en-
dogenous danger signals are recognized by various immune cells
that initiate inflammatory processes (9). The requirements made
on the specialized cells that respond to tissue damage are nu-
merous. In the first place, such sentinel cells must be prepositioned
in tissues, allowing a rapid response. Secondly, these cells must
possess the capability to produce and secrete selective mediators
required for the induction of an acute inflammatory response in-
volving vascular changes and the recruitment of leukocytes. Mast
cells possess these important characteristics (10). These long-lived
cells are present in all tissues, especially numerous at sites ex-
posed to the external environment (11, 12), and rapidly produce
and secrete a variety of signal substances upon activation, in-
cluding histamine, proteases, eicosanoids, chemokines, and cyto-
kines (13). Taken together, these properties make mast cells ideal
first-hand responders to tissue damage/cell injury, capable of ini-
tiating and orchestrating an inflammatory response.
As mast cells have previously been shown to recognize and
respond to exogenous danger signals (also called pathogen-
associated molecular patterns) such as LPS, zymosan, and pepti-
doglycan (11, 14), we hypothesized that mast cells also play an
important role in the recognition of endogenous danger signals,
such as IL-33, thereby contributing cell injury responses.
IL-33 is a novel cytokine of the IL-1 family that has previously
been shown to induce Th2-associated cytokines (15), as well as
induce release of proinflammatory mediators in mouse bone
marrow-derived mast cells (BMMCs) (16–20) and human mast
cells (21, 22), in which IL-33 also promoted maturation (21), en-
hanced mast cell survival, and increased mast cell adhesion to fibro-
nectin (22). In addition, IL-33 is involved in tryptase (mouse mast
cell protease-6) regulation in BMMCs (23). Importantly, IL-33
seems to be preferentially released from necrotic cells (24). For
instance, IL-33 has been shown to be released from endothelial cells
following mechanical injury (7). On the contrary, release of bioac-
tive IL-33 from apoptotic cells has not been demonstrated. Instead,
IL-33 has been shown to be inactivated during apoptosis (24).
In this study, we have examined the role of mast cells as sen-
sors of cellular injury. The hypothesis that an important function
for mast cells is to quickly respond to cell injury was proposed al-
ready 50 y ago (25). Despite the fact that mast cells are recog-
nized as important inflammatory cells (10, 13), the mechanism for
this hypothesis has not been deciphered. In this report, by moni-
toring responses of mast cells treated with cell-free supernatant
from necrotic cells, we provide a mechanism for this hypothesis.
Materials and Methods
Experimental animals, isolation, and in vitro stimulation of
Bone marrow was isolated from C57BL/6 wild-type, MyD88
(27), TLR2
(28), TLR4
(29), TLR5
(30), TLR6
(31), TLR7
(32), TLR8
(33) and TLR9
(34), A
*Clinical Immunology and Allergy Unit, Department of Medicine, Karolinska In-
stitute, SE-171 76 Stockholm, Sweden; and
Institute of Molecular Medicine, St.
James’s Hospital, Trinity College Dublin, Dublin 8, Ireland
G.N. and C.L.-A. contributed equally to this work.
Received for publication October 12, 2010. Accepted for publication December 10,
This work was supported by the Swedish Research Council, the Swedish Cancer
Foundation, the Consul Th C Bergh Foundation, the Ellen, Walter, and Lennart
Hesselman Foundation, the Ollie and Elof Ericsson Foundation, the King Gustaf
V’s 80 Years Foundation, the Hans von Kantzow Foundation, the A
˚ke Wiberg Foun-
dation, the Magnus Bergvall Foundation, and the Karolinska Institute.
Address correspondence and reprint requests to Dr. Gunnar Nilsson, Clinical Immu-
nology and Allergy Unit, Department of Medicine, Karolinska Institute, KS L2:04,
SE-171 76 Stockholm, Sweden. E-mail address:
Abbreviations used in this article: BMMC, bone marrow-derived mast cell; HMGB1,
high-mobility group box 1; MEF, mouse embryonal fibroblast; qPCR, quantitative
PCR; siRNA, small interfering RNA.
Copyright Ó2011 by The American Association of Immunologists, Inc. 0022-1767/11/$16.00
on June 8, 2011www.jimmunol.orgDownloaded from
(36), and T1/ST2
(37) mice, and BMMCs were prepared as
described earlier (38). The purity of the BMMCs obtained was routinely
.95%, as assessed by toluidine blue staining. In a typical experiment,
/ml BMMCs were seeded into each well of 48-well plates and exposed to
various amounts of necrotic cell supernatant (see further below) for 0.5 or
24 h, after which the supernatants from each culture were collected. The
total volume per well was 200 ml. As a positive control for mast cell acti-
vation, ionomycin (Sigma-Aldrich) was used. For inhibition studies, cells
were pretreated with 10 mM SB203580 (a p38 inhibitor) for 30 min prior to
the addition of necrotic cell supernatant or rIL-33 (Alexis Biochemicals).
Preparation of necrotic cell supernatant
Supernatant was collected from mouse embryonal fibroblasts (MEFs),
MEFs, smooth muscle cells, keratinocytes, monocytes, spleno-
cytes, mixed neuronal cells, and BMMCs rendered necrotic by repeated
freeze-thawing. In brief, the cells were resuspended in PBS at a density
of 20 310
, subjected to repeated freeze-thawing (cycling between 280
and 37˚C four times), and then centrifuged at 13,000 rpm for 10 min and
the supernatant collected.
Monitoring the responses of mast cells
Histamine release was determined using an immunoassay kit (Immunotech;
Beckman Coulter). Secreted cys-leukotrienes were quantified using an en-
zyme immunoassay kit (Amersham Biosciences) and cytokines/chemokines
with commercial ELISA kits (Biosource International and R&D Systems)
and Luminex (Millipore).
Knockdown of IL-33 expression by MEFs with small
interfering RNA
To transiently silence IL-33 expression by MEFs, these cells were trans-
fected with IL-33 small interfering RNA (siRNA) or, as a control, non-
specific siRNA (Dharmacon), harvested 24 h later, and rendered necrotic for
the preparation of necrotic supernatant as described above. To validate IL-
33 knockdown, expression of the corresponding mRNA was analyzed by
quantitative PCR (qPCR) and supernatant levels of the protein itself by
ELISA. qPCR was performed in an iCycler (Bio-Rad) using the following
primers: IL-33 sense: 59-TCC TTG CTT GGC AGT ATC CA-39and IL-33
antisense: 59-TGC TCA ATG TGT CAA CAG ACG-39.
Western blotting
Following treatment of BMMCs with 10 ng/ml rIL-33 or necrotic cell
supernatant, levels of phosphorylated p38 and total p38 protein were de-
termined by standard procedures (38), utilizing Abs directed against
phospho-p38 MAPK (Thr
) and p38 MAPK, respectively (both
from Cell Signaling Technology). IL-33 was detected in necrotic cell su-
pernatant utilizing an anti-mouse IL-33 Ab (AF3626; R&D Systems), fol-
lowed by a donkey anti-goat IgG-HRP secondary Ab (Santa Cruz Biotec-
Statistical analysis
The Mann–Whitney and one-way ANOVA (Kruskal-Wallis test) tests were
employed for statistical analyses, with a pvalue ,0.05 being considered
statistically significant.
BMMCs exhibit a potent proinflammatory response to
treatment with necrotic cell supernatant
When cells die in a necrotic fashion following cell injury, cell
content is released into the extracellular space. Certain intracellular
molecules released in this way function as endogenous danger
signals, alerting the immune system, causing it to mount an in-
flammatory response. We hypothesized that mast cells can sense
such warning signals, thereby participating in the initiation of an
inflammatory response through release of proinflammatory medi-
ators. To test this hypothesis, we rendered fibroblasts necrotic by
repeated freeze-thawing cycles. The cell-free necrotic supernatant
isolated from MEFs was subsequently transferred to a culture of
C57BL/6 BMMCs and the responses of the latter monitored. When
incubated in this manner with supernatant from cultures containing
as many or twice as many necrotic MEFs, the BMMCs did not
degranulate (i.e., did not release histamine) (Fig. 1A), but released
leukotrienes (Fig. 1B) and secreted the proinflammatory cytokines
IL-6 (Fig. 1C)andTNF-a(Fig. 1D), thus exhibiting a potent in-
flammatory response. In contrast, supernatants from MEFs rendered
apoptotic through gamma-radiation did not generate IL-6 secretion
by BMMCs (data not shown). Similar results were obtained with
BMMCs prepared from BALB/c mice (data not shown). IL-4, IL-10,
IL-13, and MCP-1 were not secreted in this system (data not shown).
No IL-6 secretion was detected in supernatants from cells treated for
0.5 h with necrotic cell supernatant (data not shown). As a positive
control, cells were treated with 1 mM ionomycin, which induced
histamine release (45.5 68.2% of total, n= 5, measured 30 min
post-treatment) and IL-6 release (39,980 62,836 pg/ml, n=5,
measured 24 h post-treatment) (data not shown).
Our next attempt aimed at identifying the molecular signals
responsible for activating the mast cells. Our initial results led us to
suspect the involvement of HMGB1, a well-characterized danger
signal known to be released from necrotic cells (3) and activate
macrophages (39). To investigate the role of HMGB1 in our
system, BMMCs were subsequently exposed to the necrotic su-
pernatant from MEFs isolated from HMGB1
mice (40). Under
these conditions, the BMMCs secreted levels of IL-6 that did not
differ significantly from the response of wild-type MEFs to ne-
crotic cell supernatant (Fig. 2A). Next, to examine the possible
involvement of uric acid, another danger signal released from
damaged cells (4), the MEFs were rendered necrotic after culture
in the presence of allopurinol (4), an inhibitor of uric acid pro-
duction. Once again, no difference in BMMC activation compared
with untreated necrotic supernatant was observed (Fig. 2A).
Moreover, the use of BMMCs from A
and A
deficient in the adenosine A
and A
receptors, respectively, still
gave the same levels of IL-6 secretion (Fig. 2B).
Together, these findings revealed that the inflammatory response
of mast cells to factors released by necrotic fibroblasts does not
involve the known danger signals HMGB1, uric acid, or adenosine.
Mast cell responses to necrotic cell supernatant are mediated
through the adaptor protein MyD88
Because several exogenous danger signals (including LPS and
zymosan), as well as certain endogenous danger signals (41, 42),
FIGURE 1. Mast cells initiate proinflammatory responses upon expo-
sure to the supernatant of necrotic cell cultures. BMMCs exposed to the
supernatant from necrotic cultures containing as many (1:1) or twice as
many (2:1) cells did not release histamine (A) but released cysteinyl leu-
kotrienes (B) during 0.5 h and IL-6 (C) and TNF-a(D) during 24 h. In all
experiments, unexposed BMMCs (Unstim) served as controls. During
BMMC treatment, 10
cells/ml were used in a total volume of 200 ml. The
values presented are means 6SEM (n= 5). For statistical analysis, one-
way ANOVA test was applied. The experiments were repeated at least
three times. *p,0.05, **p,0.01 compared with unexposed cells.
on June 8, 2011www.jimmunol.orgDownloaded from
act through TLRs, for which MyD88 acts as an adaptor protein,
we reasoned that responses to necrotic supernatant perhaps also
could be mediated through a receptor using MyD88. To investigate
this, BMMCs were prepared from MyD88
mice and treated
with necrotic supernatant. MyD88
BMMCs were found to lack
the normal IL-6 response to necrotic cell supernatant completely
(Fig. 2C), indicating that the observed IL-6 production was MyD88
dependent. To pinpoint the upstream receptor of MyD88 that re-
sponds to necrotic cell supernatant, BMMCs were derived from
, TLR2
, TLR4
, TLR5
, TLR6
, TLR7
, and TLR9
mice. When exposed to necrotic cell su-
pernatant, these cells secreted IL-6 at similar levels as wild-type
cells (Fig. 2D), thus demonstrating that none of these receptors
alone are necessary for the observed response.
IL-33 present in necrotic cell supernatant activates mast cells
through the T1/ST2 receptor
Because members of the IL-1R family share MyD88-dependent
signaling pathways with TLRs, the T1/ST2 receptor (43), a mem-
ber of the IL-1 family of receptors, was examined next.
Interestingly, we found that mast cells lacking the T1/ST2 re-
ceptor failed to secrete IL-6 and released considerably lower lev-
els of TNF-acompared with wild-type cells upon exposure to ne-
crotic cell supernatant (Fig. 3A). In addition, T1/ST2
also produced substantially lower levels of cysteinyl leukotrienes
and LTB
upon exposure to necrotic supernatant (Fig. 3B). As all
observed responses were T1/ST2 dependent, we investigated IL-
33 protein levels in necrotic cell supernatant by Western blotting
(Fig. 3C) and found full-length pro–IL-33 to be present as a band
at 30 kDa, in line with a previous publication (19). In addition,
we observed a band migrating identically to rIL-33, likely repre-
senting cleaved IL-33. We next investigated responses of BMMCs
treated with the T1/ST2 ligand IL-33. As shown in Fig. 3D,
treating BMMCs with IL-33 resulted not only in a dose-dependent
release of IL-6 and TNF-a, but also in the release of cysteinyl
leukotrienes. Thus, treating BMMCs with IL-33 generated similar
responses as BMMCs treated with necrotic supernatant. Taken
together, the observed results imply that IL-33 released by ne-
crotic cells induce the production of IL-6, TNF-a, and leuko-
trienes by mast cells, as production of these mediators was absent
or limited in T1/ST2
mast cells.
In an attempt to detect further similarities in responses of
BMMCs treated with necrotic supernatant and IL-33, we next in-
vestigated signaling pathways downstream of T1/ST2 and MyD88.
This was investigated by monitoring IL-6 secretion by mast
cells preincubated with SB203580, an inhibitor of p38, prior to
treatment with necrotic cell supernatant or IL-33. Preincubation
with SB203580 reduced IL-6 secretion markedly, both in BMMCs
treated with necrotic supernatant and also in BMMCs treated
with IL-33 (Fig. 4A). In agreement with these findings, phos-
phorylation of p38 was detected 5 min after exposure of BMMCs
to necrotic supernatant or IL-33 (Fig. 4B).
To provide definite evidence that IL-33 is the molecular com-
ponent of necrotic cell supernatant that activated the mast cells,
expression of this IL by MEFs was silenced with siRNA, and the
MEFs were rendered necrotic. The effectiveness of this silencing
was validated by qPCR (Fig. 5A) and by determining the levels of
IL-33 in the necrotic supernatant (Fig. 5B). When BMMCs were
exposed to the supernatant from necrotic MEFs lacking IL-33,
secretion of IL-6 was potently attenuated (Fig. 5C), demonstrat-
ing that the observed responses of BMMCs treated with necrotic
supernatant are dependent almost exclusively on IL-33. In con-
clusion, our results demonstrate that IL-33 released during ne-
crosis is an important mast cell activator.
IL-33 is released from necrotic cells of structural but not
hematopoietic origin
We next investigated the possibility that necrotic structural cells
other than fibroblasts, such as keratinocytes, neuronal cells (as-
trocytes), and smooth muscle cells, some of which express IL-33 at
relatively high levels, also release factors that activate mast cells.
The supernatant from cultures of above-mentioned necrotic struc-
tural cells could indeed induce IL-6 secretion by BMMCs (Fig. 5D),
whereas corresponding necrotic supernatant prepared from cells
of hematopoietic origin (splenocytes, monocytes, and BMMCs)
had no effect. Moreover, the necrotic supernatant from structural
cells contained higher levels of IL-33 (Fig. 5E). Taken together, our
data suggest that IL-33 released upon cell injury mainly derives
from damaged structural cells.
In this study, we investigated the hypothesis that mast cells are
important sensors of cell injury, as they display qualities important
FIGURE 2. The activation of mast cells by necrotic cell supernatant involves MyD88. Following 24 h of exposure to supernatant from the same number
(1:1) or twice as many (2:1) necrotic MEFs, secretion of IL-6 by BMMCs exposed to the supernatant of MEFs treated with allopurinol or isolated from
mice (A); A
and A
BMMCs (2:1 ratio) (B); MyD88
BMMCs (C); and TLR1
, TLR2
, TLR4
, TLR5
, TLR6
, TLR8
, and TLR9
BMMCs (2:1 ratio) (D) was determined. During BMMC treatment, 10
cells/ml were used in a total volume of 200 ml.
The values presented are means 6SEM (n= 3–5). The experiments were repeated at least three times. *p,0.05, **p,0.01 in comparison with untreated
control cells (Unstim) (A) or wild-type cells (C). For statistical analyses, one-way ANOVA (A) and Mann–Whitney (BD) test were applied.
The Journal of Immunology 2525
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to first-hand responders to tissue damage. By monitoring the
responses of mast cells treated with cell-free supernatant from
necrotic cells, we demonstrate that IL-33 is a key alarmin released
by necrotic structural cells. We show that mast cells are activated by
IL-33 released from necrotic cells to initiate a potent proin-
flammatory response, suggesting that mast cells make important
contributions to early cell injury responses.
During necrosis, cell integrity is compromised, resulting in the
release of several different danger signals. To investigate whether
mast cells could respond to endogenous danger signals released
from necrotic cells, we rendered MEFs necrotic and treated mast
cells with the supernatant from these cells. The use of necrotic
supernatant from lysed cells rather than purified individual danger
signals better represents physiological conditions, allowing for
a more credible in vitro model of cell injury. Our initial experi-
ments revealed that BMMCs treated with supernatant from necrotic
MEFs exhibited a potent inflammatory response, manifested by IL-
6 and TNF-asecretion and leukotriene release. However, this
response was not accompanied by degranulation, indicating solely
de novo production of the released mediators. It has been shown
earlier that HMGB1 is an important danger signal released by dying
cells and that HMGB1
cells have a greatly reduced capacity to
induce inflammation compared with wild-type necrotic cells (3).
However, in our hands, BMMCs treated with necrotic supernatant
obtained from HMGB1
MEFs secreted IL-6 at similar levels as
BMMCs treated with necrotic supernatant from wild-type cells.
This is in agreement with a study performed by Chen et al. (5), in
FIGURE 4. Mast cell responses to necrotic cell supernatant and IL-33
are mediated through a p38-dependent pathway. A, IL-6 release was de-
termined in wild-type BMMCs pretreated with or without 10 mM selective
p38 inhibitor SB203580 for 30 min prior to treatment with necrotic su-
pernatant (2:1 ratio) or 10 ng/ml IL-33. B, Levels of phosphorylated p38
(p-p38) in wild-type BMMCs following treatment with necrotic cell su-
pernatant (2:1 ratio) or 10 ng/ml IL-33 for the time periods indicated were
examined by Western blotting. During BMMC treatment, 10
were used in a total volume of 200 ml. The values shown are means 6
SEM (n= 5). For statistical analysis, the Mann–Whitney Utest was ap-
plied. The experiments were repeated at least three times. **p,0.01 in
comparison with cells not exposed to SB203580.
FIGURE 5. IL-33 released from necrotic cells is responsible for the
activation of mast cells. siRNA-mediated silencing of the expression of IL-
33 in MEFs was validated by qPCR (A) and an ELISA assay (B). C,
Following exposure of BMMCs for 24 h to supernatant from twice as many
(2:1) MEFs rendered necrotic following treatment with IL-33 or non-
targeting siRNA, IL-6 secretion was assayed. D, Following exposure of
BMMCs for 24 h to supernatant from twice as many (2:1) cells of the types
indicated, IL-6 secretion was determined. E, The levels of IL-33 in the
same necrotic cell supernatants were measured by ELISA. During BMMC
treatment, 10
cells/ml were used in a total volume of 200 ml. In A,
a representative experiment is shown. In BD, the values presented are
means 6SEM (n= 4–5). In E, the values are means 6SEM (n= 2–3). The
experiments were repeated at least three times. *p,0.05 in comparison
with the cells treated with nontargeting siRNA. For statistical analysis, the
Mann–Whitney Utest was applied (B,C). SMC, smooth muscle cell.
FIGURE 3. The proinflammatory responses activated by mast cells upon
exposure to necrotic cell supernatant are mediated by IL-33. After expo-
sure for 24 h to supernatant from the same number (1:1) or twice as many
(2:1) necrotic MEFs, secretion of cytokines IL-6 and TNF-a(A) by wild-
type and T1/ST2
BMMCs were measured. After exposure for 30 min,
release of cysteinyl leukotriene and LTB
(B) by these same BMMCs was
determined. IL-33 protein was investigated in necrotic cell supernatant and
compared with rIL-33 using Western blotting (C). D, Release of IL-6,
TNF-a, and cysteinyl leukotrienes were measured in BMMCs treated with
0, 10, or 100 ng/ml IL-33. During BMMC treatment, 10
cells/ml were
used in a total volume of 200 ml. The values shown are means 6SEM (n=
5–6). For statistical analyses, Mann–Whitney (A,B) and one-way ANOVA
(D) were applied. The experiments were repeated at least three times. *p,
0.05, **p,0.01, ***p,0.001 in comparison with wild-type cells (A,B)
or untreated cells (D).
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which it is was demonstrated that necrotic HMGB1
cells initi-
ated inflammation similarly to necrotic HMGB1
cells in mice.
Additionally, in our experimental system, necrotic supernatant
from MEFs with inhibited uric acid formation did not significantly
reduce the proinflammatory responses in BMMCs, suggesting
other components than HMGB1 or uric acid to be responsible for
the observed mast cell activation.
Supported by the finding that MyD88
BMMCs were unable
to secrete IL-6 in response to necrotic supernatant, we also in-
vestigated responses to necrotic supernatant by TLR-deficient
mast cells. Although TLRs are best known for recognizing ex-
ogenous danger signals like LPS, some members of the TLR
family have previously been reported to respond to endogenous
ligands. For instance, recognition of heat shock protein 60 has
been suggested to be TLR4 dependent (44). However, TLR-
deficient BMMCs secreted IL-6 at similar levels as wild-type
BMMCs when exposed to necrotic cell supernatant, indicating
another receptor upstream of MyD88 to be involved. This does
not, however, exclude the possibility that the necrotic cell super-
natant affects multiple TLR signaling pathways. However, as
responses to necrotic supernatant were completely abrogated in
BMMCs, we consider it unlikely that necrotic super-
natant might affect mast cells by activating a combination of
The discovery that T1/ST2
BMMCs displayed abrogated
IL-6, TNF-a, cysteinyl leukotrienes, and LTB
secretion in re-
sponse to necrotic supernatant (Fig. 3A,3B) led us to speculate
that IL-33, the only known T1/ST2 ligand, might be involved in
the mast cell’s ability to respond to necrotic supernatant. In agree-
ment with our hypothesis that IL-33 is a principal danger signal
released from necrotic cells, it has previously been shown that
IL-33 is released upon necrosis rather than during apoptosis (7,
24). Importantly, IL-33 does not seem to require proteolysis for
activation, a favorable feature indeed for an alarmin. Lu
¨thi et al.
(24) also showed that IL-33 was proteolytically cleaved during
apoptosis, thus diminishing its bioactivity. Most importantly, only
small amounts of IL-33 were released during apoptosis, whereas
induction of necrosis led to IL-33 release (24). In line with this,
we show in Fig. 3Cthat necrotic MEFs release full-length pro–
IL-33 as well as cleaved IL-33. The fact that a much higher
concentration of rIL-33 was required to elicit IL-6 secretion
by BMMCs compared with the concentration we detected in
necrotic supernatant by ELISA might thus suggest that pro–IL-33
is more active compared with rIL-33.
It has been shown that endothelial cells subjected to mechanical
wounding by cell scraping or freeze-thawing also release IL-33 (7).
To confirm that IL-33 was the chief component of the necrotic
supernatant activating mast cells in our system, we generated
necrotic supernatant from MEFs with siRNA-silenced IL-33 ex-
pression. In line with our earlier results suggesting IL-33 to be
responsible for activating the BMMCs, necrotic supernatant gen-
erated from MEFs with silenced IL-33 expression failed to induce
IL-6 secretion by BMMCs. Amazingly, IL-33 alone, in a soup of
components released by necrotic cells, thus has the capability to
alone potently activate mast cells. In support of these results, as
shown by us in this paper and also by several earlier studies, IL-33
can induce secretion of IL-6 as well as other cytokines in mast
cells (17–20). In addition to confirming that IL-33 can induce
cytokine release by BMMCs, we also show, to our knowledge, for
the first time that IL-33 can induce release of cysteinyl leuko-
trienes by mast cells.
Taken together, our findings provide an important link between
studies revealing that IL-33 is released upon necrosis and studies
demonstrating that mast cells are activated by IL-33 to secrete
cytokines. Hence, to our knowledge, our findings provide, for the
first time, a plausible mechanism for how mast cells might function
as sensors of cell injury. Most remarkably, of all potential acti-
vators present in the necrotic cell supernatant, our results show IL-
33 to be the sole inducer of cytokine secretion by mast cells, in-
disputably demonstrating that IL-33 is an important alarmin.
Expression of IL-33 has been described in a variety of tissues
(15) and cell types; for instance, in fibroblasts (45), epithelial
cells, and endothelial cells (8). Therefore, we addressed the ques-
tion whether necrotic supernatant generated from other cell types
than MEFs would elicit similar responses in BMMCs. These
experiments revealed that necrotic supernatant generated from
structural cell types (keratinocytes, smooth muscle cells, and
mixed neuronal cells) induced IL-6 secretion in BMMCs, whereas
necrotic supernatant from cells of hematopoietic origin (BMMCs,
monocytes or splenocytes) did not. Moreover, we show that ne-
crotic supernatant from the former cell types correspondingly also
contain higher IL-33 levels, in agreement with previous reports
(15, 46). Theoretically, should a structural cell die by necrosis,
adjacent mast cells will respond to released IL-33 and initiate an
inflammatory response.
In summary, using an in vitro model of cell injury, we provide
a mechanism for the hypothesis that mast cells are important
sensors of cell injury. The present findings indicate that structural
cells, such as fibroblasts and keratinocytes, release IL-33 upon
injury and that adjacent mast cells respond by producing proin-
flammatory factors, including IL-6, TNF-a, and leukotrienes. Sub-
sequently, these signals can induce vascular changes, includ-
ing vasodilatation, increased permeability of the microvasculature,
and recruitment of inflammatory cells to the site of injury. Thus,
our investigation provides support for the relatively early hy-
pothesis that an important physiological function for mast cells is
to act as key sensors of endogenous danger signals released upon
cell injury. Moreover, the present demonstration that this response
involves recognition of IL-33 by T1/ST2 receptors on the surface
of mast cells describes a novel mechanism underlying the role of
mast cells as sensors. At the same time, new evidence com-
plementing earlier studies suggesting that IL-33 is a key alarmin
released by dying cells is also provided. The responses of mast
cells to cell injury are likely to play a highly important role in the
initiation of acute inflammation and subsequent healing and,
thereby, in the maintenance of tissue homeostasis.
We thank Drs. Shizuo Akira (Department of Host Defense, Osaka Univer-
sity, Japan), Andrew McKenzie (Medical Research Council, Laboratory of
Molecular Biology, Cambridge, U.K.), Marco Bianchi (Department of Bio-
technology, San Raffaele Scientific Institute, Milano, Italy), Bertil Fred-
holm (Department of Physiology and Pharmacology, Karolinska Institute,
Stockholm, Sweden), Tobias Bergstro
¨m (Department of Medical Biochem-
istry and Microbiology, Uppsala University, Sweden), and Mikael Adner
(Department of Environmental Medicine, Karolinska Institute) for provid-
ing reagents necessary for these experiments. We also thank Dr. Sara Lind
(Department of Medicine, Karolinska Institute) for technical assistance.
The authors have no financial conflicts of interest.
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... Multiple proteins control mast cell activation or have been linked to the function of this cell type, including the RAS oncogene family member RAB27A and interleukin 33 (IL33). IL33 is an alarmin cytokine that is released in response to cell injury, infection, or mechanical damage, to induce inflammation [11] through the promotion of mast cells granulation and recruitment to the site of injury [12]. RAB27A inhibits mast cell degranulation and activation [13]. ...
... We further found that NFE2L3 is required for the induction of Il33 transcripts and the reduction of mRNA levels of members of the RAB pathway. These results are relevant as the RAB pathway differentially controls mast secretory granules and IL33 activates mast cells by promoting their granulation and recruitment to the site of injury (Fig. 6a) [12,13]. ...
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We investigated the role of the NFE2L3 transcription factor in inflammation-induced colorectal cancer. Our studies revealed that Nfe2l3−/− mice exhibit significantly less inflammation in the colon, reduced tumor size and numbers, and skewed localization of tumors with a more pronounced decrease of tumors in the distal colon. CIBERSORT analysis of RNA-seq data from normal and tumor tissue predicted a reduction in mast cells in Nfe2l3−/− animals, which was confirmed by toluidine blue staining. Concomitantly, the transcript levels of Il33 and Rab27a, both important regulators of mast cells, were reduced and increased, respectively, in the colorectal tumors of Nfe2l3−/− mice. Furthermore, we validated NFE2L3 binding to the regulatory sequences of the IL33 and RAB27A loci in human colorectal carcinoma cells. Using digital spatial profiling, we found that Nfe2l3−/− mice presented elevated FOXP3 and immune checkpoint markers CTLA4, TIM3, and LAG3, suggesting an increase in Treg counts. Staining for CD3 and FOXP3 confirmed a significant increase in immunosuppressive Tregs in the colon of Nfe2l3−/− animals. Also, Human Microbiome Project (HMP2) data showed that NFE2L3 transcript levels are higher in the rectum of ulcerative colitis patients. The observed changes in the tumor microenvironment provide new insights into the molecular differences regarding colon cancer sidedness. This may be exploited for the treatment of early-onset colorectal cancer as this emerging subtype primarily displays distal/left-sided tumors.
... assay ( Fig. 3b) and inhibited recombinant IL-33 red driven IL-8 release (Fig. 3c) in HUVECs with a potency approximately tenfold greater than sST2 (Table 1). Mast cells are an important responding cell type of IL-33 and a major source of ST2 in human lung tissue 42,43 . In human blood-derived mast cells, tozorakimab demonstrated potent inhibition of IL-33-driven inflammatory mediators (IL-13, IL-6, IL-8, granulocyte-macrophage colony-stimulating factor and tumour necrosis factor α; Fig. 3d and Table 1; Supplementary Fig. S2a-e). ...
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Interleukin (IL)-33 is a broad-acting alarmin cytokine that can drive inflammatory responses following tissue damage or infection and is a promising target for treatment of inflammatory disease. Here, we describe the identification of tozorakimab (MEDI3506), a potent, human anti-IL-33 monoclonal antibody, which can inhibit reduced IL-33 (IL-33red) and oxidized IL-33 (IL-33ox) activities through distinct serum-stimulated 2 (ST2) and receptor for advanced glycation end products/epidermal growth factor receptor (RAGE/EGFR complex) signalling pathways. We hypothesized that a therapeutic antibody would require an affinity higher than that of ST2 for IL-33, with an association rate greater than 10⁷ M⁻¹ s⁻¹, to effectively neutralize IL-33 following rapid release from damaged tissue. An innovative antibody generation campaign identified tozorakimab, an antibody with a femtomolar affinity for IL-33red and a fast association rate (8.5 × 10⁷ M⁻¹ s⁻¹), which was comparable to soluble ST2. Tozorakimab potently inhibited ST2-dependent inflammatory responses driven by IL-33 in primary human cells and in a murine model of lung epithelial injury. Additionally, tozorakimab prevented the oxidation of IL-33 and its activity via the RAGE/EGFR signalling pathway, thus increasing in vitro epithelial cell migration and repair. Tozorakimab is a novel therapeutic agent with a dual mechanism of action that blocks IL-33red and IL-33ox signalling, offering potential to reduce inflammation and epithelial dysfunction in human disease.
... Expressed constitutively by many tissues, including blood vessels, lymphoid tissues, and epithelial cells, IL-33 acts as a ligand for IL-1 receptor-related protein ST2, found on the surface of immune cells such as mast cells and Th2 cells [269][270][271]. Additionally, IL-33 functions as an alarmin, helping mast cells recognize cellular injury and acting as one of the first responders to parasitic invasion [272][273][274]. IL-33 is inherently linked to the hypoxic response via mTOR, a serine/threonine protein kinase that regulates cell growth, proliferation, and metabolism [275,276]. ...
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Body tissues are subjected to various oxygenic gradients and fluctuations and hence can become transiently hypoxic. Hypoxia-inducible factor (HIF) is the master transcriptional regulator of the cellular hypoxic response and is capable of modulating cellular metabolism, immune responses, epithelial barrier integrity, and local microbiota. Recent reports have characterized the hypoxic response to various infections. However, little is known about the role of HIF activation in the context of protozoan parasitic infections. Growing evidence suggests that tissue and blood protozoa can activate HIF and subsequent HIF target genes in the host, helping or hindering their pathogenicity. In the gut, enteric protozoa are adapted to steep longitudinal and radial oxygen gradients to complete their life cycle, yet the role of HIF during these protozoan infections remains unclear. This review focuses on the hypoxic response to protozoa and its role in the pathophysiology of parasitic infections. We also discuss how hypoxia modulates host immune responses in the context of protozoan infections.
... As innate immune cells, MCs possess toll-like receptors (TLRs), which can be activated by pathogen-or damage-associated molecular pattern molecules (25) to effect certain MC functions like mediator release, their antigen-presenting cell (APC) capabilities and interaction with dendritic cells (DCs) (34,35) or interaction with other immune cells (18,19). They respond to cell injury independently of TLRs through IL-33 activation and many other mediators (36). As an 'unprofessional' APC they can, in conjunction with DCs, fine-tune a type 2 immune response through promoting DC migration to draining lymph nodes, thereby priming an adequate T helper 2 (Th2) cell response. ...
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Mast cells (MCs) are innate immune cells with a versatile set of functionalities, enabling them to orchestrate immune responses in various ways. Aside from their known role in allergy, they also partake in both allograft tolerance and rejection through interaction with regulatory T cells, effector T cells, B cells and degranulation of cytokines and other mediators. MC mediators have both pro- and anti-inflammatory actions, but overall lean towards pro-fibrotic pathways. Paradoxically, they are also seen as having potential protective effects in tissue remodeling post-injury. This manuscript elaborates on current knowledge of the functional diversity of mast cells in kidney transplants, combining theory and practice into a MC model stipulating both protective and harmful capabilities in the kidney transplant setting.
... IgE is thought to have a central role in the activation of mast cells through cross-linking of its high-affinity receptors (FceIRs), whereas non-IgE-mediated activation of mast cells has been regarded as potentially important factor in the initation and amplification of acute inflammatory responses induced by tissue injury (53)(54)(55). DAMPs released from injured tissues, such as ATP (56) and IL-33 (45,57), are recognized by mast cells via their receptors (P2X and P2Y receptors for ATP, ST2 receptor for IL-33), and then recognized DAMPs increase intracellular Ca 2+ and activate mast cell degranulation. C3a and C5a, two complement components, can stimulate mast cell migration and degranulation via C3aRs and C5aRs (58,59). ...
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Responding to tissue injury, skeletal muscles undergo the tissue destruction and reconstruction accompanied with inflammation. The immune system recognizes the molecules released from or exposed on the damaged tissue. In the local minor tissue damage, tissue-resident macrophages sequester pro-inflammatory debris to prevent initiation of inflammation. In most cases of the skeletal muscle injury, however, a cascade of inflammation will be initiated through activation of local macrophages and mast cells and recruitment of immune cells from blood circulation to the injured site by recongnization of damage-associated molecular patterns (DAMPs) and activated complement system. During the inflammation, macrophages and neutrophils scavenge the tissue debris to release inflammatory cytokines and the latter stimulates myoblast fusion and vascularization to promote injured muscle repair. On the other hand, an abundance of released inflammatory cytokines and chemokines causes the profound hyper-inflammation and mobilization of immune cells to trigger a vicious cycle and lead to the cytokine storm. The cytokine storm results in the elevation of cytolytic and cytotoxic molecules and reactive oxygen species (ROS) in the damaged muscle to aggravates the tissue injury, including the healthy bystander tissue. Severe inflammation in the skeletal muscle can lead to rhabdomyolysis and cause sepsis-like systemic inflammation response syndrome (SIRS) and remote organ damage. Therefore, understanding more details on the involvement of inflammatory factors and immune cells in the skeletal muscle damage and repair can provide the new precise therapeutic strategies, including attenuation of the muscle damage and promotion of the muscle repair.
... Irrespective of the cause of inflammation (physical, chemical, or biological), tissue-resident cells are the first ones to sense the abnormality in the microenvironment and signals for generating appropriate responses. Neighboring mast cells are the first cells to sense injury and necrotic tissue (Lunderius-Andersson et al, 2012). The role of mast cells in inflammation has been studied extensively (Krystel-Whittemore et al, 2015). ...
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There is an increasing need to develop biological anti-inflammatory agents that are more targeted, effective, and with lesser side effects as compared to conventional chemical drugs. In the present study, we found that Mycobacterium tuberculosis protein PPE2 and a synthetic derivative peptide can suppress the mast cell population and inhibit several vasoactive and fibrogenic mediators and pro-inflammatory cytokines induced by mast cells in formalin-induced tissue injury. PPE2 was found to inhibit transcription from the promoter of stem cell factor, important for mast cell maintenance and migration. Thus, PPE2/peptide can be used as a potent nonsteroidal therapeutic agent for the treatment of inflammation and tissue injury.
... The IL-33/ST2 pathway is involved in CNS homeostasis and its pathologies, including neurodegenerative diseases (Sun et al., 2021). Mast cells are a population of IL-33 targeting cells, recognizing it by IL-33 receptor, ST2 (Lunderius-Andersson et al., 2012). Mast cells activation is observed in PD brains and may play role in neuroinflammation in this disease (Kempuraj et al., 2015(Kempuraj et al., , 2019. ...
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Parkinson's disease (PD), the second most common neurodegenerative disorder, is characterized by neuroinflammation, formation of Lewy bodies, and progressive loss of dopaminergic neurons in the substantia nigra of the brain. In this review, we summarize evidence obtained by animal studies demonstrating neuroinflammation as one of the central pathogenetic mechanisms of PD. We also focus on the protein factors that initiate the development of PD and other neurodegenerative diseases. Our targeted literature search identified 40 pre-clinical in vivo and in vitro studies written in English. Nuclear factor kappa B (NF-kB) pathway is demonstrated as a common mechanism engaged by neurotoxins such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA), as well as the bacterial lipopolysaccharide (LPS). The α-synuclein protein, which plays a prominent role in PD neuropathology, may also contribute to neuroinflammation by activating mast cells. Meanwhile, 6-OHDA models of PD identify microsomal prostaglandin E synthase-1 (mPGES-1) as one of the contributors to neuroinflammatory processes in this model. Immune responses are used by the central nervous system to fight and remove pathogens; however, hyperactivated and prolonged immune responses can lead to a harmful neuroinflammatory state, which is one of the key mechanisms in the pathogenesis of PD.
... In response to several viral infections the production of chemokines, along with type 1 interferons (IFN) represent the predominant mast cell response and leads to the recruitment of NK cells and CD56+ T cells (11)(12)(13)(14)(15)(16)(17)(18)(19)(20). Mast cells also respond to tissue damage via responses to alarmins, such as IL-33, subsequently giving rise to a further unique pattern of mediators including IL-13 and IL-5 (21)(22)(23). While degranulation is induced by certain stimuli, such as nematode parasites and select bacteria, mediator production often occurs in its absence. ...
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Mast cells are well known to be activated via cross-linking of immunoglobulins bound to surface receptors. They are also recognized as key initiators and regulators of both innate and adaptive immune responses against pathogens, especially in the skin and mucosal surfaces. Substantial attention has been given to the role of mast cells in regulating T cell function either directly or indirectly through actions on dendritic cells. In contrast, the ability of mast cells to modify B cell responses has been less explored. Several lines of evidence suggest that mast cells can greatly modify B cell generation and activities. Mast cells co-localise with B cells in many tissue settings and produce substantial amounts of cytokines, such as IL-6, with profound impacts on B cell development, class-switch recombination events, and subsequent antibody production. Mast cells have also been suggested to modulate the development and functions of regulatory B cells. In this review, we discuss the critical impacts of mast cells on B cells using information from both clinical and laboratory studies and consider the implications of these findings on the host response to infections.
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The skin is the body's largest organ. It serves as a barrier to pathogen entry and the first site of immune defense. In the event of a skin injury, a cascade of events including inflammation, new tissue formation and tissue remodeling contributes to wound repair. Skin-resident and recruited immune cells work together with non-immune cells to clear invading pathogens and debris, and guide the regeneration of damaged host tissues. Disruption to the wound repair process can lead to chronic inflammation and non-healing wounds. This, in turn, can promote skin tumorigenesis. Tumors appropriate the wound healing response as a way of enhancing their survival and growth. Here we review the role of resident and skin-infiltrating immune cells in wound repair and discuss their functions in regulating both inflammation and development of skin cancers.
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Interleukin-33 (IL-33), a member of the IL-1 cytokine family and a multifunctional cyto-kine, plays critical roles in maintaining host homeostasis and in pathological conditions, such as allergy, infectious diseases, and cancer, by acting on multiple types of immune cells and promoting type 1 and 2 immune responses. IL-33 is rapidly released by immune and non-immune cells upon stimulation by stress, acting as an "alarmin" by binding to its receptor, suppression of tumorigen-icity 2 (ST2), to trigger downstream signaling pathways and activate inflammatory and immune responses. It has been recognized that IL-33 displays dual-functioning immune regulatory effects in many diseases and has both pro-and anti-tumorigenic effects, likely depending on its primary target cells, IL-33/sST2 expression levels, cellular context, and the cytokine microenvironment. Herein, we summarize our current understanding of the biological functions of IL-33 and its roles in the pathogenesis of various conditions, including inflammatory and autoimmune diseases, infections, cancers, and cases of organ transplantation. We emphasize the nature of context-dependent dual immune regulatory functions of IL-33 in many cells and diseases and review systemic studies to understand the distinct roles of IL-33 in different cells, which is essential to the development of more effective diagnoses and therapeutic approaches for IL-33-related diseases.
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RA is a debilitating disorder that manifests as chronic localized synovial and systemic inflammation leading to progressive joint destruction. Recent advances in the molecular basis of RA highlight the role of both the innate and adaptive immune system in disease pathogenesis. Specifically, data obtained from in vivo animal models and ex vivo human tissue explants models has confirmed the central role of Toll-like receptors (TLRs) in RA. TLRs are pattern recognition receptors (PRRs) that constitute one of the primary host defence mechanisms against infectious and non-infectious insult. This receptor family is activated by pathogen-associated molecular patterns (PAMPs) and by damage-associated molecular patterns (DAMPs). DAMPs are host-encoded proteins released during tissue injury and cell death that activate TLRs during sterile inflammation. DAMPs are also proposed to drive aberrant stimulation of TLRs in the RA joint resulting in increased expression of cytokines, chemokines and proteases, perpetuating a vicious inflammatory cycle that constitutes the hallmark chronic inflammation of RA. In this review, we discuss the signalling mechanisms of TLRs, the central function of TLRs in the pathogenesis of RA, the role of endogenous danger signals in driving TLR activation within the context of RA and the current preclinical and clinical strategies available to date in therapeutic targeting of TLRs in RA.
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Mast cells are known to be the effector cells of immediate-type allergy, but experimental evidence obtained during the last decade has revealed their role in innate and acquired immunity. Upon activation mast cells can undergo an anaphylactic or piecemeal degranulation or degranulation-independent mediator secretion, resulting in rapid or slow release of soluble mediators, such as serine proteinases, histamine, lipid-derived mediators, cytokines, chemokines and growth factors. Mast cells can express different receptors and ligands on the cell surface, molecules that can activate the cells of the immune system, such as different subsets of T cells. All these mediators and cell surface molecules can promote inflammation in the skin. During the last years, a new role for mast cells has emerged; induction of tolerance or immunosuppression and interaction with regulatory T cells. However, the mechanisms that switch the proinflammatory function of mast cells to an immunosuppressive one are unknown. In this review, the immunoregulatory function of mast cells and its relation to skin inflammation are discussed.
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Interleukin (IL)-33 is a new member of the IL-1 superfamily of cytokines that is expressed by mainly stromal cells, such as epithelial and endothelial cells, and its expression is upregulated following pro-inflammatory stimulation. IL-33 can function both as a traditional cytokine and as a nuclear factor regulating gene transcription. It is thought to function as an 'alarmin' released following cell necrosis to alerting the immune system to tissue damage or stress. It mediates its biological effects via interaction with the receptors ST2 (IL-1RL1) and IL-1 receptor accessory protein (IL-1RAcP), both of which are widely expressed, particularly by innate immune cells and T helper 2 (Th2) cells. IL-33 strongly induces Th2 cytokine production from these cells and can promote the pathogenesis of Th2-related disease such as asthma, atopic dermatitis and anaphylaxis. However, IL-33 has shown various protective effects in cardiovascular diseases such as atherosclerosis, obesity, type 2 diabetes and cardiac remodeling. Thus, the effects of IL-33 are either pro- or anti-inflammatory depending on the disease and the model. In this review the role of IL-33 in the inflammation of several disease pathologies will be discussed, with particular emphasis on recent advances.
Human heat shock protein 60 (hsp60) elicits a potent proinflammatory response in cells of the innate immune system and therefore has been proposed as a danger signal of stressed or damaged cells. We report here that macrophages of C3H/HeJ mice, carrying a mutant Toll-like-receptor (Tlr) 4 are nonresponsive to hsp60. Both the induction of TNF-α and NO formation were found dependent a functional Tlr4 whereas stimulation of macrophages by CpG DNA was Tlr4 independent. We conclude that Tlr4 mediates hsp60 signaling. This is the first report of a putative endogenous ligand of the Tlr4 complex.
Psoriasis is a common chronic autoimmune condition of the skin characterized by hyperplasia of epidermal keratinocytes associated with pro-inflammatory cytokines. IL-33 is a new member of the IL-1 superfamily that signals through the ST2 receptor and was originally defined as an inducer of T helper 2 (Th2) cytokines. Recently, broader immune activatory potential has been defined for IL-33 particularly via mast cell activation and neutrophil migration. Here, we show that ST2−/− mice exhibit reduced cutaneous inflammatory responses compared with WT mice in a phorbol ester-induced model of skin inflammation. Furthermore, injections of IL-33 into the ears of mice induce an inflammatory skin lesion. This inflammatory response was partially dependent on mast cells as mast cell-deficient mice (KitW-sh/W-sh) showed delayed responses to IL-33. IL-33 also recruited neutrophils to the ear, an effect mediated in part by increased production of the chemokine KC (CXCL1). Finally, we show that IL-33 expression is up-regulated in the epidermis of clinical psoriatic lesions, compared with healthy skin. These results therefore demonstrate that IL-33 may play a role in psoriasis-like plaque inflammation. IL-33 targeting may provide a new treatment strategy for psoriasis.