Multiplexed Fluorescent Immunohistochemical Staining of Four Endometrial Immune Cell Types in Recurrent Miscarriage

Summary

Despite the advancements in multiplex immunohistochemistry and multispectral imaging, characterizing the density and clustering of major immune cells simultaneously in the endometrium remains a challenge. This paper describes a detailed multiplex staining protocol and imaging for the simultaneous localization of four immune cell types in the endometrium.

Abstract

Immunohistochemistry is the most commonly used method for the identification and visualization of tissue antigens in biological research and clinical diagnostics. It can be used to characterize various biological processes or pathologies, such as wound-healing, immune response, tissue rejection, and tissue-biomaterial interactions. However, the visualization and quantification of multiple antigens (especially for immune cells) in a single tissue section using conventional immunohistochemical (IHC) staining remains unsatisfactory. Hence, multiplexed technologies were introduced in recent years to identify multiple biological markers in a single tissue sample or an ensemble of different tissue samples.

These technologies can be especially useful in differentiating the changes in immune cell-to-cell interactions within the endometrium between fertile women and women with recurrent miscarriages during implantation. This paper describes a detailed protocol for multiplexed fluorescence IHC staining to investigate the density and clustering of four major immune cell types simultaneously in precisely timed endometrial specimens during embryo implantation. The method includes sample preparation, multiplex optimization with markers for immune cell subtypes, and the scanning of the slides, followed by data analysis, with specific reference to detecting endometrial immune cells.

Using this method, the density and clustering of four major immune cell types in the endometrium can be simultaneously analyzed in a single tissue section. In addition, this paper will discuss the critical factors and troubleshooting to overcome possible fluorophore interference between the fluorescent probes being applied. Importantly, the results from this multiplex staining technique can help provide an in-depth understanding of the immunologic interaction and regulation during embryo implantation.

Introduction

Recurrent miscarriage (RM) can be defined as the loss of two or more pregnancies before 24 weeks of gestation¹. This frequent reproductive condition affects up to 1% of couples worldwide^2,3. The pathophysiology is multifactorial and can be divided into embryologically driven causes (mainly due to an abnormal embryonic karyotype) and maternally driven causes that affect the endometrium and/or placental development. This manifestation can result from parental genetic abnormalities, uterine anomalies, prothrombotic conditions, endocrinology factors, and immunological disorders⁴.

In recent years, immune effector cell dysfunction has been implicated in the pathogenesis of early pregnancy loss⁵. This has inspired many investigations into elucidating the specific populations of immune cells in the endometrium during the menstrual cycle, implantation, and early pregnancy, with specific roles in early pregnancy. Among these immune cells, uterine natural killer (uNK) cells play a critical role during embryo implantation and pregnancy, particularly in the processes of trophoblastic invasion and angiogenesis⁶. Studies have shown an increased uNK cell density in the endometrium of women with RM^7,8, although this finding was not associated with an increased risk of miscarriage⁹. However, this stimulated research evaluating the density of other immune cell types (such as macrophages, uterine dendritic cells) in the endometrium in women with RM^{10, 11}. Nevertheless, it remains uncertain whether there is a significant alteration in the immune cell density in the peri-implantation endometrium in women with RM.

One possible explanation for the uncertainty is that evaluation of the endometrial immune cell density might be difficult due to the rapid changes in the endometrium during the window of implantation. During the 24 h timeframe, significant changes in the endometrium alter immune cell density and cytokine secretion, introducing a source of variation in these results¹². In addition, most reports mainly rely on the use of single-cell staining (e.g., traditional IHC methods) that could not examine multiple markers on the same tissue section. Although flow cytometry can be used for detecting multiple cell populations in a single sample, the large amounts of cells required and the time-consuming optimization hinder the popularity and efficiency of this method. Hence, the recent advancement in multiplex IHC staining could solve this problem by immunostaining multiple markers on the same slide to evaluate multiple parameters, including cell lineage and histological localization of individual immune subpopulations. Further, this technology can maximize the information obtained in case of limited tissue availability. Ultimately, this technique can help elucidate the differences in immune cell interactions in the endometrium between fertile women and women with RM.

Two groups of women were recruited from the Prince of Wales Hospital, including fertile control women (FC) and women with unexplained recurrent miscarriage (RM). Fertile control was defined as women who had at least one live birth without any history of spontaneous miscarriage, and RM women were defined as those who had a history of ≥2 consecutive miscarriages before 20 weeks gestation. The subjects from the two groups met the following inclusion criteria: (a) age between 20 to 42 years old, (b) non-smoker, (c) regular menstrual cycle (25-35 days) and normal uterine structure, (d) no use of any hormonal regimen for at least 3 months before the endometrial biopsy, (e) no hydrosalpinx by hystero-salpingogram. In addition, all the subjects recruited had normal karyotyping, normal 3-dimensional ultrasonography hysterosalpingogram, day 2 follicle-stimulating hormone < 10 IU/L, mid-luteal progesterone > 30 nmol/L, normal thyroid function, and tested negative for lupus anticoagulant and anticardiolipin IgG and IgM antibodies.

To better understand the immunological basis of RM, it would be most desirable to simultaneously quantify and localize the major immune cell types present in the endometrium at the time of implantation. This paper describes the entire protocol from sample preparation, the multiplex optimization with markers for immune cell subtypes, and the scanning of the slides, followed by data analysis with specific reference to detecting endometrial immune cells. Moreover, this paper describes how to determine the density and clustering of the immune cell types simultaneously in the endometrium.

Protocol

The study was approved by the Joint Chinese University of Hong Kong-New Territories East Cluster Clinical Research Ethics Committee. Informed consent was obtained from the participants before collecting the endometrial biopsies. Refer to the introduction section for inclusion criteria of the control and RM groups. 1. Sample preparation Ensure that all the women in this study undergo a daily urine dipstick test from day 9 of the menstrual cycle onwards to identify the luteinizing ho…

Representative Results

The overall schematic process of performing a 4-color multiplex assay for the detection of 4 endometrial immune cell types is shown in Figure 1. In brief, the protocol for this multiplex immunofluorescence staining required 8 key steps: 1. Slide preparation, 2. Epitope retrieval, 3. Blocking, 4. Primary antibody application, 5. Secondary antibody application, 6. Signal amplification, 7. Removal of antibody, and 8. Counterstain and mount. Image rendering and analysis were then conducted using…

Discussion

Critical steps within the protocol
It is important to note that multiplex staining requires diligent optimization. Antigen retrieval, using citrate buffer and microwave technology, requires optimization to ensure complete antibody stripping and maintain tissue viability. As TSA reagents covalently bind to sites surrounding the antigen, they can potentially inhibit the binding of a subsequent primary antibody through steric hindrance (also known as the "umbrella effect"). This tends to occur…

Materials

Amplification Diluent	Perkin Elmer	FP1498	Fluorophore diluent buffer
Antibody diluent	Perkin Elmer	ARD1001EA	Diluting the antibody
CD3	Spring Bioscience	M3072	Primary antibody
CD20	Biocare Medical	CM004B	Primary antibody
CD56	Leica	NCL-CD56-504	Primary antibody
CD68	Spring Bioscience	M5510	Primary antibody
Citrate Buffer Solution, pH 6.0 (10x)	Abcam	AB64214	Antigen retrieval solution
EMSURE Xylene (isomeric mixture)	Merck	108297	Dewaxing
Ethanol absolute	Merck	107017	Ethyl alcohol for rehydration
HistoCore BIOCUT Manual Rotary Leica Microtome	Leica	RM2125RTS	Sectioning of paraffin-embedded tissue
inForm Advanced Image Analysis Software	Perkin Elmer	inForm® Tissue Finder Software 2.2.1 (version 14.0)	Data Analysis software
Mantra® Workstation	Akoya Biosciences	CLS140089	Spectral imaging
Microwave	Panasonic	Inverter	Microwave stripping
Opal 520	Perkin Elmer	FP1487A	Appropriate tyramide based fluorescent reagent
Opal 620	Perkin Elmer	FP1495A	Appropriate tyramide based fluorescent reagent
Opal 650	Perkin Elmer	FP1496A	Appropriate tyramide based fluorescent reagent
Opal 690	Perkin Elmer	FP1497A	Appropriate tyramide based fluorescent reagent
Oven	Memmert	U10	Dewaxing
Peroxidase Blocking Solution	DAKO	S2023	Removal of tissue peroxidase activities
Poly-L-lysine coated slide	FISHER SCIENTIFIC	120-550-15	Slide for routine histological use
PolyHRP Broad Spectrum	Perkin Elmer	ARH1001EA	Secondary antibody
ProLong™ Gold Antifade Mountant	ThemoFisher Scientific	P36930	Mounting
Spatstat	/	Version 2.1-0	Spatial point pattern analysis
Spectral DAPI	Perkin Elmer	FP1490A	Nucleic acid staining
Tissue Processor	Thermo Fischer	Excelsior ES	Tissue processing for dehydration and paraffination
Tris Buffer Saline (TBS), 10x	Cell Signaling Technology	12498S	Washing solution
Tween 20	Sigma-Aldrich	P1370-1L	Nonionic detergent