1. 1. Endometriosis: an
estrogen-dependent, chronic
inflammatory disease
2. Inflammation and EMS:
orchestrated by mast cells?
3. Mast cells in EMS: a painful
liaison?
4. Expert opinion
Editorial
Mast cells in endometriosis: guilty
or innocent bystanders?
Dennis Kirchhoff†
, Stefan Kaulfuss, Ulrike Fuhrmann, Marcus Maurer &
Thomas M Zollner
†
Bayer Pharma AG, GDD - Target Discovery, Berlin, Germany
Endometriosis (EMS) is a chronic, estrogen-dependent inflammatory disease
characterized by growth of endometrial tissue outside the uterine cavity.
Symptoms in EMS patients include severe pelvic pain, dysmenorrhea, dyspar-
eunia and infertility. To date, medical therapies are mostly based on hor-
monal suppressive drugs that induce a hypoestrogenic state. Although
being effective regarding the reduction of endometriotic tissue masses and
pelvic pain, this treatment is accompanied by severe side effects. Since EMS
is associated with chronic inflammation, novel therapeutic strategies also
focus on immune modulating drugs. However, little is known about how
and to what extent immune cell subsets contribute to the network of locally
produced cytokines, chemokines and other mitogenic factors that modulate
the growth of ectopic endometrial implants and the inflammation associated
with them. Mast cells (MCs) are known to be key players of the immune sys-
tem, especially during allergic reactions. However, in recent years MCs have
been identified to exhibit a far broader range of functions and to be involved
in host defense and wound healing responses. Here, recent reports that imply
an involvement of MCs in EMS has been reviewed, while the value of
novel mouse models for clarifying their contribution to the pathology of
this condition has been discussed.
Keywords: endometriosis, inflammation, mast cell, pain
Expert Opin. Ther. Targets (2012) 16(3):237-241
1. Endometriosis: an estrogen-dependent, chronic
inflammatory disease
Endometriosis (EMS) was first described in medical literature more than 300 years
ago and has since been recognized as a chronic, painful disease in women. It is char-
acterized by the presence and growth of endometrial tissues in ectopic sites. EMS is
the most common cause of pelvic pain, dysmenorrhea, dyspareunia and subfertility
in women of reproductive age. Early diagnosis is critical for management but is
still difficult due to the lack of suitable non-invasive tools. In fact, the only reliable
technique is surgical identification by laparoscopy.
Although EMS is one of the most investigated disorders in gynecology, its causes
remain ill-characterized. Among the numerous theories that attempt to explain the
mechanisms involved in the development of EMS, Sampson’s theory of retrograde
menstruation is best supported by evidence [1]. However, retrograde menstruation
occurs in almost all women, but only a small percentage actually develops EMS.
As a consequence, other factors must be involved allowing retrogradely displaced
endometrial tissue to implant and develop into endometriotic lesions. The current
consensus is that EMS is associated with a local pelvic inflammatory process with
altered functions of immune-related cells in the peritoneal environment.
Besides its inflammatory component, EMS is evidently an estrogen-dependent dis-
ease. Therefore, the medical treatment has focused on the use of hormonal
suppression including gonadotropin-releasing hormone (GnRH) analogs, continuous
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2. oral contraceptives, gestrinone, progestins or danazol leading to
a hypoestrogenic state. Although the pharmacological induc-
tion of a hypoestrogenic state reduces EMS-associated pain
temporarily, the disease generally relapses after the termination
of hormonal treatment. As a consequence of the systemically
low levels of estrogen (e.g., after treatment with GnRH analogs),
patients experience a variety of side effects including osteopo-
rosis, headache, nausea and weight gain and menopausal-
type symptoms. Finally, none of these therapies are of any
benefit in resolving the infertility associated with EMS.
These aspects raise the need to develop therapies for EMS
with a better risk:benefit ratio.
Most recent developments of better EMS therapies are tar-
geting immune cells and their mediators. For example, it has
been claimed that, in affected women, refluxed endometrial
cells are able to escape T-cell-driven immune surveillance [2]
and that T-cell activating drugs should ameliorate the disease.
However, in two small randomized clinical trials, the
immune-stimulatory cytokines IL-2 and IFN-a had no effect
on lesion size or pain [3,4]. These results indicate that these two
cytokines cannot repair defects in immune surveillance or that
impaired T-cell function is not critical to EMS pathogenesis.
Most recently, the contribution of immune cells to the
chronic inflammation associated with the ectopic lesions in
EMS has evolved as a promising novel therapeutic target.
Many previous reports have demonstrated elevated levels of
various pro-inflammatory cytokines (e.g., IL-4, IL-6, IL-8,
IL-12, TNF, MCP-1 and others [5]) in the peritoneal fluid
and serum of EMS patients, and EMS lesions are characteris-
tically infiltrated by large numbers of different activated
leukocytes. Although the contribution of specific immune
cell subsets and their mediators to the onset and the course
of the inflammatory process in endometrial lesions is still
poorly understood, some evidence exists that mast cells
(MCs) are crucially involved.
2. Inflammation and EMS: orchestrated by
mast cells?
Traditionally, MCs have been most extensively studied for
their role as early effector cells of allergic disease but their
additional roles such killing of pathogens, degrading of toxic
endogenous peptides, regulating the number, viability, distri-
bution, phenotype and ‘non-immune’ functions of structural
cells, such as fibroblasts and vascular endothelial cells become
more and more evident [6].
More recently, MCs have also been reported to be involved
in complex biological functions and pathologies (e.g., wound
healing [7] or autoimmune diseases such as rheumatoid
arthritis and multiple sclerosis) [8] and peripheral tolerance [9].
MCs exhibit several exclusive characteristics that discrimi-
nate them from other leukocytes. First of all, their maturation
and differentiation occurs locally, after migration of their
precursors to the vascularized tissues in which they will finally
reside. Here, they can exert their effector functions through
the direct or indirect actions of a wide variety of preformed
or newly synthesized and selectively released mediators includ-
ing histamine, proteases (e.g., tryptase, chymase), leukotrienes,
prostaglandins as well as numerous cytokines (e.g., TNF, IL-1,
-3, -4, -5, -6, -8, -9, -13), neurotransmitters and growth factors.
This unique mediator profile enables MCs to initiate an
inflammatory cascade leading to the observed symptoms of
EMS, for example, by modulating the recruitment, survival,
development, phenotype or function of other immune cells
described to be involved in EMS pathology, including
monocytes/macrophages, granulocytes, dendritic cells (DCs),
T and B cells [5,10-14].
Increasing evidence supports an involvement of MCs also
in the inflammatory process of EMS. High numbers of degra-
nulated MCs have been found in endometriotic lesions [15].
Of note, this was not the case at unaffected sites of the perito-
neum or eutopic endometrial tissue from EMS patients or
healthy controls. Additionally, it has been shown that stem
cell factor (SCF), the major growth differentiation and
chemoattractant factor for MCs, is found in higher concentra-
tions in the peritoneal fluid of EMS patients, especially in the
early stages of the condition [16].
The production of IgE in allergic diseases typically requires
the release of Th2 cytokines (e.g., IL-4, IL-5 and IL-13) by T-
helper cells. Indeed, in previous reports, the peritoneal fluid
and sera of patients with EMS showed enhanced expression
of IL-4 [17]. However, whether or not the activation of MCs
in EMS lesions is IgE-dependent is yet unclear. The activation
of MCs during the early onset of EMS and the tissue damage/
remodeling associated with EMS lesions may also occur by
multiple other mechanisms. For example, MCs express Toll-
like receptors (e.g., TLR2 and TLR4 [18]), that is, pattern
recognition receptors (PRRs) that sense damage-associated
molecular patterns (DAMPs). DAMPs and so-called
‘hidden-self’ molecules are constituents of menstrual
debris [19], which become displaced by retrograde menstrua-
tion and may cause MC activation and subsequent ‘sterile’
inflammatory responses through TLR activation [20].
One important feature of EMS is the estrogen-
dependent progression of the disease. In EMS patients, the
aberrantly high expression of aromatase in ectopic lesions con-
tributes to the increased local concentration of estrogen [21].
Recently, MCs have been reported to express estrogen recep-
tors, and it is generally accepted that estrogen treatment stim-
ulates MCs to release mediators [22]. Importantly, this effect
does not require IgE cross-linking and was measurable at
physiological estrogen (10-11
to 10-10
M) concentrations.
Thus, high levels of locally produced endogenous estrogens
may induce and/or facilitate MC activation and MC-
driven inflammation by binding to MC estrogen a-receptors.
Interestingly, estrogens have attracted significant interest as
potential modulators of immune responses that contribute
to chronic inflammatory conditions including autoimmune
and hypersensitivity diseases, that is, conditions which
are widely held to be, at least in part, MC-mediated.
Mast cells in endometriosis: guilty or innocent bystanders?
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3. 3. Mast cells in EMS: a painful liaison?
EMS is the most common cause of chronic pelvic pain in
women. The correlation between pain intensity and the anat-
omy and biochemistry of the ectopic implants is still poorly
understood. However, parallels have been found between
pain severity and both the depth of infiltration into the peri-
toneum or pelvic organs. Anaf et al. found increased numbers
of activated MCs near endometriotic lesions, often close to
nerve fibers [23]. Importantly, the pain intensity in EMS
patients with increased lesional MC numbers was higher as
compared with patients with normal lesional MC numbers.
MCs are likely to sensitize primary nociceptive neurons by
the release of nerve growth factor (NGF) and pro-
inflammatory cytokines. Lesional NGF can, in turn, attract
MCs and trigger their degranulation [24].
With regard to endometriotic pain, the pro-inflammatory
cytokine IL-1 has some important properties. IL-1, which
can be released by MCs on TLR triggering induces the syn-
thesis of prostaglandins, important pain mediators and stimu-
lates fibroblasts activation and proliferation, which contribute
to fibrosis and adhesion formation by fibrinogen and subse-
quent collagen depositions [18,25]. Adhesions are frequently
found in the area of endometriotic lesions [26,27]. Depending
on the severity of adhesion formation, chronic pain arises
from organ immobilizations and dysfunctions [28].
MCs can release preformed and de novo synthesized TNF, a
pain-mediating pro-inflammatory cytokine which can activate
nociceptive primary afferent nerve fibers by eliciting a dose-
dependent rapid-onset increase in c-fiber discharge indepen-
dent of peripheral receptor involvement [29]. Moreover, MCs
can secrete the pro-inflammatory chemokine IL-8, which is
also known to induce prostaglandin-independent hyperalgesia
in vivo [30].
The stimulation of sensory nerve fibers by inflammatory
substances can lead to the release of neurotransmitters such
as substance P (SP), endothelin, histamine, glutamate, prosta-
glandins and vasointestinal peptide (VIP). SP is known to
provide a positive feedback on MCs by causing degranulation
and the release of pro-inflammatory mediators that, in turn,
can attract other inflammatory cells [31-33].
Thus, MCs may contribute directly to the development of
pain in EMS by releasing mediators that can sensitize/
activate sensory nerve fibers, or indirectly by recruitment of
other pro-inflammatory leukocytes (Figure 1).
4. Expert opinion
MCs are known potent mediators of inflammatory immune
responses, yet their precise role in inflammation and pain
induction in EMS has not been fully elucidated. However, their
role in several other pathologies associated with chronic pelvic
Inflammatory
response
TLR
MC
NC
ERα
Hyperalgesia/Pain
Degranulation:
• Substance P
• Histamine
• Prostaglandins
• VIP
Positive feedback
Activation/
Maturation
Menstrual debris/
DAMPs:
• Necrotic cells
• “Hidden-self” molecules
Lesion
activity:
Debris/
Estrogen
(local)
Retrograde menstruation
Activation/Recruitment:
• Histamine
• Leukotrienes
• Prostaglandins
• PAF
• TNF
• IL-1b
• Neurotransmitters
• NGF
• Serotonin
• IL-8
• Monocytes/macrophages
• Granulocytes
• Dendritic cells
• B-/T-cells
Figure 1. Mast cells as key players in endometriosis pathology.
DAMP: Damage-associated molecular pattern; ER: Estrogen receptor; MC: Mast cell; NC: Nerve cell; NGF: Nerve growth factor; PAF: Platelet-activating factor; TLR:
Toll-like receptor; VIP: Vasoactive intestinal peptide.
Kirchhoff, Kaulfuss, Fuhrmann, Maurer & Zollner
Expert Opin. Ther. Targets (2012) Early Online 239
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4. pain (e.g., irritable bowel syndrome, interstitial cystitis, chronic
pelvic pain syndrome and chronic prostatitis) [34,35], especially
their role as pain mediators, has been analyzed in more detail.
For example, in a murine model of interstitial cystitis it has
been shown that the disease-associated pain is MC-depen-
dent [36]. This study took advantage of the so-called MC
‘knock-in’ mouse model. Here, C57BL/6-KitW-sh/W-sh
mice,
which are genetically MC-deficient due to a mutation in the
Kit gene, were used. To exclude the differences between Kit-
mutant mice and wild-type mice due to MC-independent
abnormalities in these animals, the C57BL/6-KitW-sh/W-sh
mice
were selectively ‘repaired’ by the adoptive transfer of in vitro-
derived MCs [37]. Using this mouse model could clarify the
contribution of MCs to the establishment of EMS-associated
inflammation and pain.
Furthermore, the therapeutic potential of targeting MCs in
murine EMS can be analyzed in a novel transgenic mouse
expressing Cre recombinase under the control of the MC pro-
tease (Mcpt) 5 promoter [38]. Crossing of these mice with the
recently generated iDTR mice [39] generates a mouse in which
MCs exclusively express a high-affinity diphtheria toxin (DT)
receptor. The subsequent application of DT then leads to the
selective depletion of these cells. This conditional MC-
ablated mouse allows the analysis of the role of MCs in ther-
apeutic disease models without affecting other cell subsets.
Guilty or not -- many new tools are available that will help
to better characterize the role of MCs in murine EMS models
and potentially open new therapeutic avenues for the treat-
ment of EMS.
Declaration of interest
All authors except M Maurer are employees of Bayer Pharma
AG. The work has been supported by Bayer Pharma AG.
Bibliography
Papers of special note have been highlighted as
either of interest () or of considerable interest
() to readers.
1. Sampson JA. Metastatic or embolic
endometriosis, due to the menstrual
dissemination of endometrial tissue into
the venous circulation. Am J Pathol
1927;3:93-110; 43
2. Christodoulakos G, Augoulea A,
Lambrinoudaki I, et al. Pathogenesis of
endometriosis: the role of defective
’immunosurveillance’. Eur J Contracept
Reprod Health Care 2007;12:194-202
3. Acien P, Quereda F, Campos A, et al.
Use of intraperitoneal interferon alpha-2b
therapy after conservative surgery for
endometriosis and postoperative medical
treatment with depot
gonadotropin-releasing hormone analog:
a randomized clinical trial. Fertil Steril
2002;78:705-11
4. Acien P, Quereda FJ, Gomez-Torres MJ,
et al. GnRH analogues, transvaginal
ultrasound-guided drainage and
intracystic injection of recombinant
interleukin-2 in the treatment of
endometriosis. Gynecol Obstet Invest
2003;55:96-104
5. Herington JL, Bruner-Tran KL,
Lucas JA, et al. Immune interactions in
endometriosis. Expert Rev Clin Immunol
2011;7:611-26
. Comprehensive overview of the
immunological aspects of EMS.
6. Abraham SN, St John AL. Mast
cell-orchestrated immunity to pathogens.
Nat Rev Immunol 2010;10:440-52
7. Weller K, Foitzik K, Paus R, et al. Mast
cells are required for normal healing of
skin wounds in mice. FASEB J
2006;20:2366-8
8. Gregory GD, Brown MA. Mast cells in
allergy and autoimmunity: implications
for adaptive immunity.
Methods Mol Biol 2006;315:35-50
9. Lu LF, Lind EF, Gondek DC, et al.
Mast cells are essential intermediaries in
regulatory T-cell tolerance. Nature
2006;442:997-1002
10. Maurer M, Metz M. The status quo and
quo vadis of mast cells. Exp Dermatol
2005;14:923-9
11. Galli SJ, Grimbaldeston M, Tsai M.
Immunomodulatory mast cells: negative,
as well as positive, regulators of
immunity. Nat Rev Immunol
2008;8:478-86
12. Dudeck A, Suender CA, Kostka SL,
et al. Mast cells promote Th1 and
Th17 responses by modulating dendritic
cell maturation and function.
Eur J Immunol 2011;41:1883-93
13. Osuga Y, Koga K, Hirota Y, et al.
Lymphocytes in endometriosis. Am J
Reprod Immunol 2011;65:1-10
14. Saito A, Osuga Y, Yoshino O, et al.
TGF-beta1 induces proteinase-activated
receptor 2 (PAR2) expression in
endometriotic stromal cells and
stimulates PAR2 activation-induced
secretion of IL-6. Hum Reprod
2011;26:1892-8
15. Sugamata M, Ihara T, Uchiide I.
Increase of activated mast cells in human
endometriosis. Am J Reprod Immunol
2005;53:120-5
16. Osuga Y, Koga K, Tsutsumi O, et al.
Stem cell factor (SCF) concentrations in
peritoneal fluid of women with or
without endometriosis. Am J
Reprod Immunol 2000;44:231-5
17. Hsu CC, Yang BC, Wu MH, et al.
Enhanced interleukin-4 expression in
patients with endometriosis. Fertil Steril
1997;67:1059-64
18. Supajatura V, Ushio H, Nakao A, et al.
Differential responses of mast cell
Toll-like receptors 2 and 4 in allergy and
innate immunity. J Clin Invest
2002;109:1351-9
19. Flowers CE Jr, Wilborn WH. New
observations on the physiology of
menstruation. Obstet Gynecol
1978;51:16-24
20. Mrabet-Dahbi S, Metz M, Dudeck A,
et al. Murine mast cells secrete a unique
profile of cytokines and prostaglandins in
response to distinct TLR2 ligands.
Exp Dermatol 2009;18:437-44
21. Bulun SE, Fang Z, Imir G, et al.
Aromatase and endometriosis.
Semin Reprod Med 2004;22:45-50
22. Zaitsu M, Narita S, Lambert KC, et al.
Estradiol activates mast cells via a
non-genomic estrogen receptor-alpha and
calcium influx. Mol Immunol
2007;44:1977-85
23. Anaf V, Chapron C, El Nakadi I, et al.
Pain, mast cells, and nerves in peritoneal,
ovarian, and deep infiltrating
Mast cells in endometriosis: guilty or innocent bystanders?
240 Expert Opin. Ther. Targets (2012) Early Online
ExpertOpin.Ther.TargetsDownloadedfrominformahealthcare.combyBayerHealthCareon07/05/15
Forpersonaluseonly.

5. endometriosis. Fertil Steril
2006;86:1336-43
24. Anaf V, Simon P, El Nakadi I, et al.
Hyperalgesia, nerve infiltration and nerve
growth factor expression in deep
adenomyotic nodules, peritoneal and
ovarian endometriosis. Hum Reprod
2002;17:1895-900
25. Liu W, Ding I, Chen K, et al.
Interleukin 1beta (IL1B) signaling is a
critical component of radiation-induced
skin fibrosis. Radiat Res
2006;165:181-91
26. Nezhat F, Nezhat C, Allan CJ, et al.
Clinical and histologic classification of
endometriomas. Implications for a
mechanism of pathogenesis.
J Reprod Med 1992;37:771-6
27. Fauconnier A, Chapron C.
Endometriosis and pelvic pain:
epidemiological evidence of the
relationship and implications.
Hum Reprod Update 2005;11:595-606
28. Liakakos T, Thomakos N, Fine PM,
et al. Peritoneal adhesions: etiology,
pathophysiology, and clinical significance.
Recent advances in prevention and
management. Dig Surg 2001;18:260-73
29. Sorkin LS, Xiao WH, Wagner R, et al.
Tumour necrosis factor-alpha induces
ectopic activity in nociceptive primary
afferent fibres. Neuroscience
1997;81:255-62
30. Cunha FQ, Lorenzetti BB, Poole S, et al.
Interleukin-8 as a mediator of
sympathetic pain. Br J Pharmacol
1991;104:765-7
31. Kulka M, Sheen CH, Tancowny BP,
et al. Neuropeptides activate human mast
cell degranulation and chemokine
production. Immunology
2008;123:398-410
32. Tancowny BP, Karpov V, Schleimer RP,
et al. Substance P primes lipoteichoic
acid- and Pam3CysSerLys4-mediated
activation of human mast cells by
up-regulating Toll-like receptor 2.
Immunology 2010;131:220-30
33. Siebenhaar F, Magerl M, Peters EM,
et al. Mast cell-driven skin inflammation
is impaired in the absence of sensory
nerves. J Allergy Clin Immunol
2008;121:955-61
34. Mehik A, Leskinen MJ, Hellstrom P.
Mechanisms of pain in chronic pelvic
pain syndrome: influence of prostatic
inflammation. World J Urol
2003;21:90-4
35. O’Sullivan M, Clayton N, Breslin NP,
et al. Increased mast cells in the irritable
bowel syndrome.
Neurogastroenterol Motil
2000;12:449-57
36. Rudick CN, Bryce PJ, Guichelaar LA,
et al. Mast cell-derived histamine
mediates cystitis pain. PLoS One
2008;3:e2096
37. Grimbaldeston MA, Chen CC,
Piliponsky AM, et al. Mast cell-deficient
W-sash c-kit mutant Kit W-sh/W-sh
mice as a model for investigating mast
cell biology in vivo. Am J Pathol
2005;167:835-48
. Important study demonstrating the
advantages of the KitW-sh/W-sh
MC
deficient mouse model over the other
known kit mutant strains.
38. Scholten J, Hartmann K, Gerbaulet A,
et al. Mast cell-specific Cre/loxP-
mediated recombination in vivo.
Transgenic Res 2008;17:307-15
.. Description of a novel transgenic
mouse which potentially allows
therapeutic MC ablation.
39. Buch T, Heppner FL, Tertilt C, et al.
A Cre-inducible diphtheria toxin receptor
mediates cell lineage ablation after toxin
administration. Nat Methods
2005;2:419-26
Affiliation
Dennis Kirchhoff†1
PhD, Stefan Kaulfuss2
,
Ulrike Fuhrmann2
, Marcus Maurer3
Thomas M Zollner1
†
Author for correspondence
1
Bayer Pharma AG, GDD-Target Discovery,
Muellerstr. 178, 13342 Berlin, Germany
Tel: +49 30 468 193479; Fax: +49 30 468 92581;
E-mail: dennis.kirchhoff@bayer.com
2
Bayer Pharma AG, GDD-TRG Women’s
Healthcare, Berlin, Germany
3
Department of Dermatology and Allergy,
Allergie-Centrum-Charite´, Charite´-
Universita¨tsmedizin Berlin, Berlin, Germany
Kirchhoff, Kaulfuss, Fuhrmann, Maurer Zollner
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