Document Type : Review Article
Authors
1 Department of Immunology, Faculty of Medicine, Isfahan University of Medical Sciences, Isfahan, Iran
2 Department of Anatomy, School of Medicine, Iran University of Medical Sciences, Tehran, Iran
3 Department of Endocrinology and Female Infertility at Reproductive Biomedicine Research Center, Royan Institute for Reproductive Biomedicine, ACECR, Tehran, Iran
4 Department of Anatomy, School of Medicine, Iran University of Medical Sciences, Tehran, Iran;Department of Endocrinology and Female Infertility at Reproductive Biomedicine Research Center, Royan Institute for Reproducti
Abstract
Keywords
All organisms are in a continuous challenge
with the surrounding environment during their
life. The defense mechanism of every organism
which is called the immune system has two main
branches in vertebrates. Innate (natural) immunity, which is the first branch of immune system, is the ancient form of host defense against
infection and plays a critical role in activation
of adaptive (acquired) immunity, the another
branch of the immune system. The adaptive immune system responds to specific 'non-self ' antigens and generates immunologic memory. Innate
immunity comprises different cells and receptors
which provides first line of defense against invading microorganisms (
PRRs have some common characteristics including:
1. They are able to recognize different microbial
components known as pathogen associated molecular patterns (PAMPs). PAMPs are essential for
the survival of the microorganism and therefore it
could not alter them without threatening its life.
2. Their expression is constitutive in the host and
thus could detect the pathogens during their life time.
3. PRRs are germline encoded and nonclonal which
are expressed on all cells of a specific type (
PRRs not only recognize exogenous components
derived from both pathogenic and non pathogenic microorganisms known as PAMPs, but also respond to
endogenous molecules released from dying host cells
upon cellular stress or tissue injury known as damage
associated molecular patterns (DAMPs) (
PRRs exist in every compartment of the body.
Some of them are humoral proteins circulating in the
plasma while endocytic receptors expressed on the
cell surface and signaling receptors can be expressed
either on the cell surface or intracellularly (
One of the main subgroups of PRRs which are
conserved during evolution is TLRs. They are type I
transmembrane glycoproteins which consist of extracellular domains containing varying numbers of
leucine-rich-repeat (LRR) motifs, a trans membrane
portion and a cytoplasmic signaling domain homologous to that of the interleukin-1 receptor (IL-1R),
termed the Toll/ IL-1R homology (TIR) domain (
To date, different TLRs have been identified in
different species. TLRs 1-9 are conserved between
human and mouse. Although TLR10 is not functional in mice, they express TLR11, TLR12 and
TLR13 which are not expressed in humans (
In a host, TLRs are expressed in various cells.
They are not only expressed in immune cells such
as macrophages (MQ), dendritic cells (DCs), B
lymphocytes and specific types of T cells but are
also expressed in non-immune cells including fibroblasts and epithelial cells. In addition, expression of TLRs is dynamic and rapidly changes in
response to pathogens, a variety of cytokines and
environmental stresses (
TLRs are named because of their similarity to a
molecule identified in the fruit fly, Drosophila
melanogaster called 'Toll'. Toll was first identified by Anderson et al. in 1985 as a gene in
Drosophila which its protein product plays an
important role during its embryogenesis in development of dorsal-ventral axis (
Later, receptors were identified which were
similar to Toll so were named "Toll like receptors". The first human TLR was reported by
Nomura et al. in 1994 (
TLRs are classified according to their cellular
localization and respective PAMP ligands. Cell
surface TLRs (TLR1, TLR2, TLR4, TLR5, TLR6
and TLR10) are expressed on the cell membrane
and recognize mainly microbial membrane components such as lipids, proteins and lipoproteins.
On the other hand, TLR3, TLR7, TLR8 and TLR9
are exclusively expressed on intracellular compartments such as the endoplasmic reticulum
(ER), endosomes, lysosomes and endolysosomes
and recognize nucleic acids including single and
double stranded RNA (ssRNA , dsRNA ) and DNA
(
Distribution and dimerisation of TLRs in different
cellular compartments (
PG; Peptidoglycan, LPS; Lipopolysaccharide, dsRNA; Double stranded RNA and ssRNA; Single stranded RNA.
Although extracellular domain of each TLR (corresponding to ligand recognition) is different from others, all of them have a great similarity in their intracellular domain and respective transduction pathways.
After ligand binding, the intracellular cascades start. As mentioned previously, all TLRs
have cytoplasmic signaling domain homologous to that of IL-1 receptor, known as TIR
domain. Several adaptor molecules containing
TIR domain interact with the TIR domain of
TLRs. These adaptors include myeloid differentiation primary response gene -88 (MyD88),
TIR domain containing adaptor protein/ MyD88
adapter-like protein (TIRAP/Mal), TIR-domain-containing adapter-inducing interferon-β
(TRIF) and TRIF-related adaptor molecule
(TRAM) ( production of inflammatory cytokines and
chemokines. induction of anti viral response by production of
type 1 interferons. maturation of dendritic cells by upregulation of
costimulatory molecules ( Adaptor molecules involved in different TLRs signaling (
Following the engagement of adaptor molecules,
different molecules are recruited including several
IL-1 receptor associated kinases (IRAKs) and TNF
receptor associated factors (TRAFs) and mitogen
activated protein kinases (MAPKs). Afterwards,
inhibitory kappa B kinase (IKK) is engaged and
modulates the activation and translocation of the
transcription factor NF-κB. Another transcription
factor activated by MAPKs is activating protein-1
(AP-1) (
Both TLR3 and TLR4 use this pathway which leads
to the recruitment of TRAFs and IRF3 and production
of both inflammatory cytokines and IFN I (
In particular, anti viral responses are induced by TLRs 3 and 4 activation (using TRIF) or TLRs 7, 8 and 9 activation (using MyD88 pathway).
Signaling pathways of different TLRs (2).
Each TLR has its own distinct PAMP ligands.
The cell surface TLRs recognize different ligands
including components of gram positive and gram
negative bacteria, fungi and parasites such as lipopolysaccharide (LPS), peptidoglycan (PG), lipoteichoic acid (LT), flagellin, mannan, zymosan
and glycoinositolphospholipids. On the other hand,
intracellular TLRs are usually stimulated by nucleic acids of viruses and bacteria including ssRNA,
dsRNA and CpG unmethylated DNA (
As mentioned above, despite relatively limited
types of TLRs known in human (TLR1-10), they
can react with a wide spectrum of PAMPs. This
could be explained by a special characteristic of
TLRs. TLRs usually form dimers (homo or hetero
dimers) which increases the diversity of ligands
recognized by them, for example, TLR2 usually
forms heterodimers with TLR1 or TLR6 with each
dimer having different ligand specificity (
Human TLRs, their respective ligands (
TLRs | Ligands |
---|---|
Triacyl lipopeptides | |
Diacyl lipopeptides, Triacyl lipopeptides, Lipoteichoic acid, Peptidoglycans, Porins, Lipoarabinomannan, Phospholipomannan, Zymosan, Hemagglutinin protein | |
Double stranded RNA | |
LPS, Mannan, Heat-shock protein 60, 70, Fibrinogen | |
Flagellin | |
Diacyl lipopeptides, Lipoteichoic acid, Zymosan | |
ssRNA | |
ssRNA | |
CpG-DNA, Chromatin-IgG complex | |
Unknown | |
Several reports have revealed that TLRs are expressed throughout different parts of the female reproductive tract ( Pro-inflammatory environment during embryo implantation, placentation and early stage of pregnancy. Anti-inflammatory milieu during mid-pregnancy Pro-inflammatory environment at third trimester
and end of pregnancy (
During normal pregnancy, different parts of female reproductive tract including endometrium, myometrium, cervix and vagina undergo histological and functional changes while specific pregnancy related tissues such as amnion, chorion and placenta are created.
In the following, we will overview the researches done on the expression of different TLRs and their function during pregnancy in different tissues of the female reproductive tract which are closely in relation to embryo.
The placenta serves as an active barrier between the embryo and the surrounding environment. Different PRRs are considered to play roles in this interaction including TLRs and NLRs (34).
Various studies have evaluated the presence and
function of TLRs or their related molecules in
placental tissues. By using immunohistochemistry, Kumazaki et al. investigated the expression of
TLR4 protein in human placentas obtained from
normal and complicated pregnancies delivered in
the second and third trimesters. They showed that
TLR4 was found on the extravillous trophoblasts,
intermediate trophoblasts/X cells in the degenerative villi, and villous Hofbauer cells of both preterm and term placentas and on the inflammatory
cells in placentas with chorioamnionitis (CAM).
TLR4 immunoreactivity was increased in the villous Hofbauer cells of preterm CAM placentas
compared with those of preterm placentas without
CAM or those of term placentas with or without
CAM (
Klaffenbach et al. studied the chorioncarcimoma
cell lines by using different techniques including
real time polymerase chain reaction (real time
PCR), fluorescence-activated cell sorter analysis
and immunoblat. They showed that LPS and CpG
DNA increases the expression of TLR2 mRNA
and protein (
Activation of TLRs on trophoblast has various
consequences during pregnancy including immune
cell recruitment, cytokine secretion and protective
responses to invading pathogens (
The expression of TLRs in fetal membrane such
as amnion has been studied but not as extensive
as placenta. Kim et al. showed that the expression
of TLR2 and TLR4 is increased in chorioamnion
membrane at time of labor and in presence of chorioamnionitis. They also showed that TLR2 was
polarized to the basal surface of amniotic epithelial
cells in women without inflammation but this distribution was lost in the presence of chorioamnionitis (
Little data exists in regard to TLRs expression in
human myometrium. Youssef et al. demonstrated
that expression of TLRs 2 and 4 was significantly
higher in pregnant myometrium at term in comparison with preterm. In addition, they showed that
the level of TLR2 protein significantly increased
during labor. The authors suggest that these TLRs
may be important in labor and their function could
be suppressed by progesterone (
For the first time, Young et al. showed that TLRs
1-6 and 9 are expressed in both whole endometrium and separated endometrial epithelial cells
using reverse transcriptase-PCR (RT-PCR). They
also showed that Ishikawa cells expressed TLRs
2 and 5 while RL95-2 cells expressed TLRs 3, 5
and 9 (
Aflatoonian et al. studied endometrium obtained
from normal women at different phases of menstrual cycle (menstruation, proliferative and secretory) and detected TLRs 7-10 proteins in both endometrial epithelium and stroma. The authors also
demonstrated that all ten TLRs were expressed in
human endometrial tissue and most of them had
significantly higher expression during the secretory phase in comparison with other phases of the
menstrual cycle (
expression in endometrium
were not limited to in vivo studies. Aboussahoud
et al. investigated TLRs expression and function in
three human endometrial epithelial cell lines. They
revealed that these established cell lines not only
express TLRs but also respond to their known agonists and could be used as reliable
Decidua is defined as the transformed endometrium during pregnancy, which forms the maternal
part of the placenta. For the first time , Krikun et al.
studied the decidual tissues and cells obtained from
women undergoing first trimester elective terminations or repeat cesarean sections and showed that
human decidua differentially express TLRs (
Pregnancy is a fundamental stage in life of every
woman in reproductive age. The immunologic features of normal pregnancy are unique because the
mother tolerates the semi allogenic embryo. The
immunologic changes during pregnancy are very
important not only in normal tissues of the female
reproductive tract but also in embryo-related tissues created during pregnancy such as fetal membrane and placenta. According to different findings
obtained using several study models including