Results
After FACS, low cell numbers were obtained ( Figure 1.B ) that subsequently guided further analyses.
Cytometry analyses from all the immune populations showed that CD45 +
cells corresponded to an average of 6.8% of the total sample, in agreement with
other studies wherein leukocytes comprise 10-20% of total endometrial cells
( 33 – 38 ). No significant differences in CD45 +
cells were observed between control and endometriosis groups ( Figure 1.C ). Statistical analyses comparing percentage
of each sub-population between controls and endometriosis were performed, and no
significant differences were observed except for Mφ1, which was
significantly higher in the endometriosis group (p=0.0087) ( Figure 1.D ). Because resident tissue markers were
included in the cytometry panel, contamination of immune populations from the
peripheral circulation could also be calculated. In both the control and
endometriosis groups, uNK (CD16 − ) were significantly higher
compared to blood NK (CD16 + ) ( Figure
1.E ), demonstrating that there was almost no contamination with blood
NK cells. The percentage of Treg coming from blood (CD69) was higher than tissue
Treg (CD69 + ), although it was not significant ( Figure 1.E ). Due to the low number of uNK and Treg
cells obtained, RNA-Seq was only performed in the Mφ populations, that
had significantly greater numbers of cells. Thus, from the 44 original
FACS-sorted immune populations, 22 samples (Mφ populations) from
endometriosis and control were used for the transcriptome study.
RNA concentrations extracted from Mφ ranged between
5-45ng/μl. After RNA-Seq and quality controls, we excluded any FastQ
sequences for which the number of reads did not reach our threshold of five
million reads/sequence. Thus, the populations analyzed were: 5 Mφ1
endometriosis, 3 Mφ1 control, 6 Mφ2 endometriosis and 4 Mφ2
control. After statistical analysis, DEG (FDR<0.05 and LogFC≥2)
were found in all comparisons ( Table 2 ;
Supplemental Table
1 ).
Biological significance of the DEG analyses revealed significant
molecular functions, relevant molecules secreted by Mφ1 and Mφ2,
activation/inhibition of upstream regulators and de-regulated networks in each
comparison (activation Z-score≥2.00) ( Table 3 ). The 25 top de-regulated networks are in Supplemental Table 2 . Increase in
cell-cell contact was observed along with repression of RNA molecular functions,
when comparing Mφ1 endometriosis versus Mφ1 control
( Comparison 4,
Table 2 ). Increased cell-cell contact is
consistent with, e.g., increased adhesion to bacteria to accomplish bacterial
engulfment. Top de-regulated networks showed overexpression of cellular
development, growth and proliferation, and overexpression of immune
response-related networks, such as infectious disease and antimicrobial and
inflammatory responses ( Table 3 ). These
data indicate that Mφ1 in endometriosis have a more extensive
pro-inflammatory phenotype than Mφ1 in the control group.
In contrast, molecular functions upregulated in Mφ2 in
endometriosis ( Comparison 5,
Table 2 ) included an accumulation of
Ca 2+ , increase in carbohydrate transport, and internalization of
bacteria ( Table 3 ). When comparing
Mφ1 of women with versus without endometriosis ( comparison
4,
Table 2 ), the upstream regulator
TNFα was predicted to be increased ( Table
3 ). Increased internalization of bacteria is consistent with
phagocytic properties of the pro-inflammatory Mφ1 phenotype. The top
networks in endometrial Mφ2 from women with endometriosis included
deregulation of connective tissue disorders, endocrine system development and
function, lipid metabolism, inflammatory disease/response, and drug metabolism
( Table 3 ). These data overall
demonstrate that Mφ2 in eutopic endometrium of women with endometriosis
have a pro-inflammatory phenotype, compared to Mφ2 in control women.
Materials
Eleven endometrial biopsies in the secretory phase were collected: 6
from women with endometriosis (stage I-IV) and 5 from women with no evidence of
endometriosis at the time of the surgery for benign gynecologic disorders. The
mean age was 37 and 42 (23-49) years old, respectively. In order to evaluate if
the age could confound the results, a non-parametric t-test with a subsequent
Mann-Whitney test (p<0.05) was performed and no significant differences
between groups were found (p=0.2641). Patients had not used hormonal therapy for
at least three months prior to the study. Endometrial samples were obtained
through the University of California San Francisco (UCSF) NIH Human Endometrial
Tissue and DNA Bank under approval of the UCSF Committee on Human Research
(IRB#10-02786), and written informed consent was obtained from all participants.
Endometrial tissue was digested as previously described ( 21 ). Briefly, it was minced mechanically and
incubated for one hour at 37ºC in digestion media, which contained
collagenase type I and hyaluronidase ( 21 ). Subsequently, the single cell suspension was filtered using a
40μm mesh to discard cell clumps, and single cells were cryopreserved in
liquid nitrogen until use.
A cytometry panel of 10 conjugated antibodies able to separate the
immune populations of interest (Mφ1, Mφ2, Treg and uNK) was
designed. Specific membrane markers of resident and blood infiltrating immune
cells were included to avoid sorting cells derived from the peripheral
circulation. The brightest colors were used for the markers with the lowest
antigen density. Minimum overlapping of 11 colors (10 antibodies plus the
live/dead dye) was achieved. The markers and lasers used for each population are
in Table 1 and the gating strategy is in
Figure 1.A . First, the cells were gated
with CD45 (leukocyte marker), conjugated with brilliant violet 605 (BV605). In
the case of Mφ, usually this population is a resident tissue population,
thus no specific tissue markers were used. It is challenging to differentiate
between the Mφ1 and Mφ2 subpopulations, as they have some common
markers and have the ability to polarize from one type to the other. Our
strategy was as follows: for both types, CD14 marker conjugated with
phycoerythrin (PE) was used. For Mφ2, CD163, a specific marker for this
population, was used conjugated with PE-cyanin7 (PE-Cy7). As there are no
specific markers for Mφ1 and it was suspected that the concentration of
activated Mφ1 would be low in the endometrium (as Mφ2 are higher
than Mφ1 in normal endometrium ( 19 )), CD80 (activation marker) conjugated with peridinin chlorophyll
protein complex-cyanin 5.5. (PerCP-Cy5.5) antibody, a bright dye, was used. With
regard to Treg, they express CD3 and CD4 markers. The most accepted specific
marker for Treg is Foxp3. This, is an intracellular marker and thus could not be
used for sorting. However, they also express CD25. To be able to discern between
tissue Treg and Treg deriving from circulation, CD69, an activation marker for
Treg that is expressed in tissue, was included. Thus,
CD3 + CD4 + CD25 + CD69 + cells (tissue
Treg) were isolated. The CD3 antibody was conjugated with ultraviolet B 737
(BUV737), as was the CD4 antibody with ultraviolet B 395 (BUV395). CD25 has a
low antigen density, therefore, brilliant blue 515 (BB515) was used, which is
one of the brightest dyes. Then, CD69, a tissue activation Treg marker, was
conjugated with allophycocyanin-cyanin7 (APC-Cy7), assuring that only resident
Treg were isolated. For uNK cells, which are CD56 + , as it is known
that blood NK are CD16 + ; whereas, uNK are CD16 Low/−
( 22 – 25 ), CD56 + CD16 − were
collected. For CD56, brilliant violet 421 (BV421), was used and in the case of
CD16, allophycocyanin (APC) conjugated antibody was used. Finally, to separate
between live and dead cells, Aqua dye was used (sources of all antibodies are
listed in Table 1 ).
Endometrial samples were thawed at 37ºC. After centrifugation at
1,300rpm for five minutes, the supernatant was discarded and the pellet was
washed with 1X PBS. After another centrifugation, cells were resuspended with 1X
PBS + 5% bovine serum albumin (BSA) and incubated at room temperature for 30
minutes. A minimum of 100,000 unlabeled cells were separated as a negative
control. Ten conjugated antibodies were used to label the samples ( Table 1 ). 1μl of antibody per
million cells was used in all cases except for CD45 and CD4, where 2μl
per million cells were needed for an optimal cell labelling. A solution of all
Fluorochromes Minus One (FMO) was prepared for each antibody to assess any
overlap among the channels in the FACS instrument. After one hour of incubation
at 4ºC in 1X PBS + 3% BSA and in the dark, cells were washed with 1X PBS
and centrifuged for 5 minutes at 1,300 rpm. The pellet was resuspended with
500μl of 1X PBS and labelled with 1μl of LIVE/DEAD™ Fixable
Aqua Dead cell labelling dye (ThermoFisher, Waltham, MA). On the other hand,
UltraComp eBeads compensation magnetic beads (ThermoFisher, Waltham, MA, USA)
were labelled with each of the 10 antibodies following the manufacturer’s
instructions, to allow the correction of the spectral overlap between
fluorochromes. Using the gating strategy ( Figure
1.A ), each population was sorted in the FACS Aria Jabba the Hutt (BD
Biosciences, East Rutherford, NJ, USA) instrument and collected in 1X PBS. Flow
cytometry analysis of the sorted cells was performed using FlowJo.v10 software
(FlowJo LLC, Ashland, OR, USA), and statistical analyses (Mann-Whitney test,
p-value<0.05) were conducted using GraphPad software (GraphPad Software
Inc, San Diego, CA, USA).
As low yields of cells were obtained after FACS ( Figure 1.B ), RNeasy micro kit (Qiagen, Hilden,
Germany) was used to isolate RNA from Mφ populations (note yields from
uNK and Treg cells were too low, and were not used for RNA-Seq, see Methods ). From the 22-sorted Mφ
samples (Mφ1 and Mφ2 populations of each of the 11 endometrial
samples), RNA was extracted following the manufacturer’s instructions to
perform the total RNA-seq library prep from samples containing >900 cells
(9 samples). The library preparation from the remaining samples, which contained
at least 20 cells, was performed directly from cells in 1X PBS. RNA was eluted
in 10μl of RNase free-water, and the quality (RNA integrity numbers
(RIN)) and concentration were measured using a Tapestation4200 System (Agilent,
Santa Clara, CA, USA).
SMART-Seq™ v4 Ultra™ low input RNA kit for sequencing
(Clontech, Mountain View, CA, USA) was used to perform the RNA-seq library
preparation. It allows RNA-Seq to be performed with very low concentrations of
RNA or to use whole cells to preserve sample integrity. In total, library
preparations for 22 samples (10 from control and 12 from endometriosis) were
performed. The quality of fastq files was tested using the FastQC (v0.11.5)
( 26 ) and the Qualimap (rnaseq module
– v2.2.1) software ( 27 ). Reads
were aligned with the STAR mapper (v2.5.2a) ( 28 ) to release 88 of the Homo sapiens ENSEMBL version of the genome
(GRCh38/hg38 assembly) ( 29 ). A raw count
of reads per gene was also obtained with STAR ( 28 ) . In order to overcome the heterogeneity between samples, first,
samples were removed from the analysis if they had <5 million uniquely
mapped reads, and the remaining samples were downsampled to 30 million mapped
reads when needed. The data have been deposited in NCBI GEO database (accession
number GSE130435 ). The R/Bioconductor package DESeq2 (v1.20.0) ( 30 – 32 )
was used to assess differential expression between experimental groups (Wald
statistical test + false discovery rate (FDR) correction). Statistically
significant differentially expressed genes (DEG) were considered when
FDR2 (LogFC>2). Different
comparisons performed using Mφ populations are shown in Table 2 . Biological significance analyses were
conducted using Ingenuity Pathway Analyses (IPA) software (Ingenuity®
Systems, Redwood City, CA, USA), and significant molecular functions were
established with an activation Z-score> |2.00|.
Discussion
In the current study, we developed a cytometry panel that allowed for
separating circulating immune cells and tissue resident cells and different immune
cell types within human endometrium. Thus, the analyzed immune populations were
purely tissue-activated resident cells devoid of contamination by circulating immune
cells. One goal was to develop and optimize this panel for the current study.
However, it will also have value for other researchers aiming to separate these
tissue-specific populations, since it is a challenging panel to design due to the
multiple colors used and the possible overlap between channels.
After cytometry analyses, where Mφ1 were found to be significantly
higher in endometriosis, Mφ were studied in more detail by transcriptomic
analyses. To our knowledge this is the first RNA-Seq dataset of Mφ in eutopic
endometrium of women with endometriosis. Abnormal distribution of Mφ within
eutopic endometrium of women with disease could contribute to the aberrant
distribution of immune cells in the pelvic cavity and the abnormal development and
gene expression of this tissue. While Mφ maintain organ homeostasis and
facilitate host defense and wound healing, they also underlie the pathogenesis of
many chronic inflammatory diseases ( 39 ).
The increased de-regulated molecular functions and networks in Mφ1 in
endometrium of women with endometriosis indicate these cells have a more
pro-inflammatory phenotype than Mφ1 in the control group. In addition, a
significantly higher number of sorted Mφ1 was observed in endometriosis
patients ( Figure 1.D ), confirming a previous
report ( 40 ). Moreover, these results
suggesting that eutopic endometrium of women with endometriosis is more
pro-inflammatory than control endometrium, is consistent with findings from other
groups ( 17 , 41 ).
An unexpected finding herein was the pro-inflammatory phenotype exhibited by
endometrial Mφ2 from women with endometriosis. Mφ2 in other tissues
generally display an anti-inflammatory phenotype ( 39 ), and, importantly, Mφ are phenotypically plastic with regard
to their polarization state depending on their microenvironment ( 42 ). Moreover, Mφ1 and Mφ2 gene expression
signatures often overlap, and the resultant phenotype depends on the tissue
microenvironment ( 40 ). Thus, endometrial
Mφ2 of women with endometriosis could undergo polarization in
situ to Mφ1, adopting a pro-inflammatory phenotype, due to an
altered environment. The paradigm of different subpopulations of Mφ is
controversial in the immunology literature. Specifically, it is unclear whether
there are unique Mφ populations (as Mφ1, Mφ2) or if Mφ
comprise a unique population that alters its phenotype depending on environmental
cues. Herein, we have referred to Mφ as two different subpopulations
(Mφ1 and Mφ2), although the dynamics and mechanisms driving
pro-inflammatory and anti-inflammatory Mφ functional phenotypes remain to be
determined.
Notably, tumors take advantage of macrophage plasticity. For example, in the
early phases of cancer, high production of Mφ1 inflammatory mediators
activates the adaptive immune response capable of eliminating nascent neoplastic
cells, and also support neoplastic transformation ( 40 ). In contrast, once the tumor is stablished, the main population of
Mφ is Mφ2, producing an anti-inflammatory environment, which allows
tumor growth. Endometriosis it is not a malignancy, however, it shares some
characteristics with cancers. In endometriotic lesions and peritoneal fluid of women
with endometriosis, e.g., Mφ2 are increased ( 43 ), indicating that, as in cancer, an anti-inflammatory environment
prevails favoring development and growth of the endometriotic lesions. In addition,
that Mφ2 have a role in angiogenesis further supports this paradigm. Finally,
Mφ2 are also involved in nerve growth, suggesting they may also have a role
in endometriosis-related pain ( 44 ).
The initial pro-inflammatory phenotype of Mφ in cancer increases NFKB
and downstream events and increases transcription of pro-inflammatory cytokines such
as TNFα, IL12, IL23, IL1β, IL6, and ROS. In the current study, the
NFKB pathway was activated in Mφ1 of endometriosis, which does not occur in
Mφ1 of control women ( Table 3 ).
Indeed, it has been described that the NFKB pathway is de-regulated in the eutopic
endometrium of women with endometriosis ( 45 ),
which also indicates that the microenvironment in endometrium of women with disease
is more pro-inflammatory than heathy tissue.
Notably, an increase of transport of carbohydrates was observed in
Mφ2 of women with endometriosis. It is known that glycolysis is high in
Mφ1 and is decreased in Mφ2 and Mφ polarization may derive from
a reprogramming of glucose metabolism ( 46 ).
Several studies have suggested that altering nutrient availability or blocking
specific metabolic pathways skews the Mφ phenotype and alters their effector
functions in chronic inflammatory diseases ( 47 ). In this regard, Mφ metabolism modulation could open a new
therapeutic window for treating inflammatory diseases including endometriosis.
Finally, the upstream regulator TNFα was increased in IPA analysis
when comparing Mφ1 of women with versus without endometriosis
( comparison 4,
Table 2 ), as well as increased
Ca 2+ accumulation was activated in Mφ2 ( Table 3 ). It has been noted that a transient increase of
Ca 2+ plays a role in the expression of TNFα by Mφ1
( 48 ). Intracellular Ca 2+
oscillations are likely to induce permanent changes in Mφ physiology, and a
supra-physiologic elevation of Ca 2+ in mitochondria can be cytotoxic and
induce apoptosis in the long term ( 48 ).
Whether TNFα-mediated events play a role in Mφ function awaits further
studies.
Over the past decade, high-throughput sequencing techniques have challenged
the dogma of the sterility of the uterine endometrium ( 49 – 54 ), and
in particular an altered endometrial microbiome in women with endometriosis has been
proposed ( 55 ). In addition, the endometrial
microbiome also correlates with IVF outcomes ( 52 ), although whether this occurs in women with endometriosis awaits
further study. However, treatment with antibiotics resulted in reduced numbers of
endometriosis lesions in a mouse model, with concomitant alteration of the gut
microbiome ( 56 ), although the endometrial
microbiome was not reported in this study ( 54 ). Interestingly, a recent systematic review supports the use of
antibiotics prior to oocyte retrieval in patients with endometriosis, among other
gynecologic disorders ( 57 ). The presence of
pathogenic, non-commensal bacteria in the endometrium may induce an altered immune
cell profile and activation (increased numbers and activation of Mφ1 and
activation of Mφ2) that could impact the production of cytokines by immune
resident cells that adversely affect embryo implantation ( 58 ). In addition to effects on reproductive outcomes, the
observed greater pro-inflammatory endometrial environment herein could be related to
the pathophysiology of the disease. While attractive, we are aware that the sample
size of the study is small. Therefore, these results should be taken with caution.
Finally, whether the pro-inflammatory phenotype of the Mφ2 population
reported herein is in response to commensal bacteria or pathogens, or if Mφ
populations are implicated in reproductive outcomes, is not clear. However, it is
anticipated that this important area of research could have profound implications
clinically and diagnostically.
Conclusions
Overall, the results of the current study lead to the conclusion that both
Mφ1 and Mφ2 in eutopic endometrium of women with endometriosis display
a higher pro-inflammatory phenotype compared to controls without disease.
Endometrial Mφ2 appear to be predisposed to Mφ1 polarization in women
with endometriosis, thus increasing their inflammatory phenotype. These findings
suggest that eutopic endometrium has different Mφ gene signatures depending
on the presence or absence of disease and that the endometrial environment of women
with endometriosis is more pro-inflammatory than control endometrium. Whether
subtypes of the disease are associated with different subsets of immune and whether
the macrophage pro-inflammatory status is related to bacteria in the endometrium of
women with disease are yet to be determined. Finally, the results herein may have
implications regarding the impact of the macrophage phenotypes on reproductive
outcomes and possible novel therapeutics for microbiome-related symptoms and
response for fertility and pain in women with endometriosis.
Introduction
Endometriosis is an estrogen-dependent inflammatory disease that results in
pelvic pain and/or infertility. It affects approximately 10% of reproductive age
women ( 1 – 4 ) and is characterized by the presence of endometrial-like tissue
outside the uterus where it elicits an inflammatory response ( 1 , 3 , 4 ). The eutopic endometrium of women with endometriosis
has been widely studied with regard to dysfunctionality of steroid hormone response,
stem cell populations, and recruitment of immune populations for immune tolerance
and overall tissue homeostasis and pregnancy success versus women without disease
( 1 – 3 ). However, there are scant data about the function and phenotypes of
eutopic endometrial immune cells in women with and without endometriosis. As the
endometrial immune niche involves multiple cell types with varying degrees of
activation and communications among immune and non-immune cells that dictate
functionality of the tissue, characterizing the endometrial immune niche is of great
relevance to understanding endometrial function and dysfunction.
Uterine natural killer cells (uNK) secrete angiogenic factors that
contribute to the maturation of blood vessels having a role in embryo implantation
and successful pregnancy ( 6 , 7 ). In healthy endometrium, their cytotoxic activity
diminishes during the secretory phase of the menstrual cycle which allows embryo
implantation ( 8 – 10 ). However, in infertile endometriosis patients, uNK
have high cytotoxicity in eutopic endometrium that could lead to an inhospitable
environment for embryo implantation ( 11 ).
Other immune cells, such as T regulatory cells (Treg), have been also described to
behave differently in the endometrium of women with endometriosis. In healthy
endometrium, they increase in the proliferative and decrease during the secretory
phase, with the latter creating an immune-tolerant environment allowing embryo
implantation. However, in infertile women with endometriosis, Treg are increased in
the peri-implantation endometrium, leading to an implantation failure ( 12 ).
Endometriosis has been referred to as “a disease of the
macrophage” ( 13 ), based mainly on a
replete literature on the roles and functionality of this cell type in peritoneal
fluid of women with disease and in establishment of endometriosis lesions and
associated processes of angiogenesis and fibrosis. Mφ are key effector cells
in both innate and humoral immunity as they phagocytose pathogens, act as antigen
presenting cells, and have a role in tissue regeneration, angiogenesis and wound
healing ( 14 ). In eutopic endometrium of women
without endometriosis, their numbers vary throughout the menstrual cycle, increasing
in the secretory and menstrual phases ( 15 ).
This increase may be attributed to their phagocytic properties and role in clearing
cell debris and apoptotic cells during endometrial shedding ( 16 ). Cycle variation among endometrial Mφ does not
occur in women with endometriosis ( 17 ),
suggesting that survival of shed and refluxed endometrial cells may be enhanced,
enabling them to migrate to the peritoneal cavity and establish disease. Mφ
are classified as either “classically activated” Mφ
(Mφ1) or “alternatively activated” Mφ (Mφ2)
( 14 ) and, depending on the
microenvironment, they can switch from one type to the other ( 18 ). Mφ1 have a role in pro-inflammatory
responses; whereas, Mφ2 are involved in angiogenesis, anti-inflammatory
reactions, and tissue repair ( 14 , 19 ). In healthy endometrium, the predominant
population is Mφ2 ( 19 , 20 ), suggesting that the normal environment is
anti-inflammatory. Taken together, most of the studies in eutopic endometrium have
focused on the number of immune cells in this tissue and how they fluctuate
throughout the cycle, but little is known about their functionality in women with
endometriosis.
Herein, we designed a novel flow cytometry panel to isolate Mφ1,
Mφ2, Treg and uNK from eutopic endometrium of women with endometriosis and
those with no evidence of disease. RNA High-Sequencing (RNA-Seq) was used to
elucidate Mφ1 and Mφ2 phenotypes and possible functions in disease.
Overall, the data support a phenotypic switch of the common anti-inflammatory
Mφ2 to the pro-inflammatory Mφ1 phenotype and a more exaggerated
pro-inflammatory phenotype of the Mφ1 population in women with
endometriosis.
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