Here, we describe a method that combines in situ MHC-tetramer staining with immunohistochemistry to determine localization, phenotype, and quantity of antigen-specific T cells in tissues. This protocol is used to determine the spatial and phenotypic characteristics of antigen-specific CD8 T cells relative to other cell type and structures in tissues.
T cells are critical to many immunological processes, including detecting and eliminating virus-infected cells, preventing autoimmunity, assisting in B-cell and plasma-cell production of antibodies, and detecting and eliminating cancer cells. The development of MHC-tetramer staining of antigen-specific T cells analyzed by flow cytometry has revolutionized our ability to study and understand the immunobiology of T cells. While extremely useful for determining the quantity and phenotype of antigen-specific T cells, flow cytometry cannot determine the spatial localization of antigen-specific T cells to other cells and structures in tissues, and current disaggregation techniques to extract the T cells needed for flow cytometry have limited effectiveness in non-lymphoid tissues. In situ MHC-tetramer staining (IST) is a technique to visualize T cells that are specific for antigens of interest in tissues. In combination with immunohistochemistry (IHC), IST can determine the abundance, location, and phenotype of antigen-specific CD8 and CD4 T cells in tissues. Here, we describe a protocol to stain and enumerate antigen-specific CD8 T cells, with specific phenotypes located within specific tissue compartments. These procedures are the same that we used in our recent publication by Li et al., entitled "Simian Immunodeficiency Virus-Producing Cells in Follicles Are Partially Suppressed by CD8+ Cells In Vivo." The methods described are broadly applicable because they can be used to localize, phenotype, and quantify essentially any antigen-specific CD8 T cell for which MHC tetramers are available, in any tissue.
T cells are critical to many immunological processes, including detecting and eliminating virus-infected cells, preventing autoimmunity, assisting in B-cell and plasma-cell production of antibodies, and detecting and eliminating cancer cells. The development of peptide/MHC class I tetramer staining of antigen-specific CD8 T cells1 and the more recent development of MHC class II tetramer staining of CD4 T cells2 by flow cytometry revolutionized our ability to study and understand the immunobiology of T cells. While extremely useful for determining the quantity and phenotype of antigen-specific T cells, flow cytometry does not allow for the detection of the spatial localization of antigen-specific T cells to other cells and structures in tissues, and current disaggregation techniques to extract the T cells needed for flow cytometry have limited effectiveness in non-lymphoid tissues3.
We and others have developed methods using peptide-loaded MHC class I and class II tetramer or multimer reagents to stain antigen-specific CD8 and CD4 T cells in tissues4,5,6,7,8,9,10,11,12,13. These IST methods allow for the determination of the location, abundance, and phenotype of antigen-specific CD8 and CD4 T cells in tissues and provide a means to detect of these cells relative to other cells and structures in the tissues. Our group has extensively used MHC-I tetramer staining to study human immunodeficiency virus (HIV)- and simian immunodeficiency virus (SIV)-specific CD8 T cells in lymphoid, genital, and rectal tissues to gain an understanding of HIV and SIV immunopathogenesis and to identify correlates of successful vaccination strategies14,15,16,17. In addition, we also developed a technique that combines IST with in situ hybridization (ISH) to localize and quantify virus-specific CD8 T cells and virus-infected cells in tissues and to determine the in vivo effector-to-target levels18,19.
Here, we describe a protocol using peptide-loaded MHC-I tetramers to stain antigen-specific CD8 T cells in fresh tissue sections, to counterstain tissues using IHC, and to quantify cells with specific phenotypes in specific tissue compartments. These procedures are the same as were used in our recent publication by Li et al., in which we determined the location, abundance, and phenotype of SIV-specific T cells in lymphoid tissue during chronic SIV infection in macaques20.
For this procedure, fresh tissues are sectioned and incubated overnight with peptide-loaded MHC-I tetramers conjugated to fluorescein thiocyanate molecules (FITC). They are then fixed in paraformaldehyde. After fixing the tissue, the signal from the MHC tetramers is amplified using rabbit anti-FITC antibodies and incubated with fluorescently tagged anti-rabbit IgG antibodies, which further amplify the signal from the bound tetramers. IHC is used in conjunction with IST to characterize antigen-specific T cells and surrounding cells. Antibodies that recognize epitopes on the surface of cells or in the extracellular space are included in the primary incubation with the tetramers. Antibodies that recognize intracellular epitopes require permeation of the cell wall prior to staining. The stained tissue sections are imaged using a confocal microscope and analyzed using confocal software. Labeled cells are quantified using confocal microscopy software or ImageJ. The described protocol can be used to stain essentially any antigen-specific CD8 T cell in any tissue for which MHC-I tetramers are available.
1. Day 1: Fresh Tissue Sectioning and Primary Incubation
2. Day 2: Fixation and Secondary Incubation
3. Day 3: Tertiary Incubation
4. Day 4: Mounting the Sections
5. Acquisition of Confocal Microscope Images
6. Quantitative Image Analysis
NOTE: Quantitative image analysis can be accomplished using confocal microscope analysis and quantification software or by using ImageJ software. Here, ImageJ was used as an example.
Figure 1 shows how to collect confocal images using a confocal microscope. Figure 2 demonstrates quantitative image analysis using ImageJ. Figures 3 and 4 show representative images of lymph node tissues from an SIV infected rhesus macaque stained with MHC tetramers, CD8 antibodies, and CD20 antibodies, and serve to demonstrate the specificity of the MHC-tetramer staining. Figure 3 compares MHC class I tetramers loaded with a peptide from SIV compared to sections from the same tissue stained with MHC class I tetramers loaded with an irrelevant peptide. Figure 4 shows that cells stained with the MHC/SIV-peptide tetramer are co-stained with CD8 antibodies, but not CD20 antibodies, which stain B cells. Figure 5 shows an example of a montage created from multiple confocal z-series fields used for quantification of tetramer stained cells with specific phenotypes in different anatomical compartments on the lymph node. The enlargement shows an area of the lymph node with a B cell follicle delineated by CD20 staining, and surrounding T cell zone, in which MHC-tetramer-stained cells can be detected, some of which co-express Ki67 which is a marker of T cell activation and proliferation.This staining combination allows the determination of the phenotype of SIV-specific CD8 T cells inside and outside of B cell follicles, in relationship to SIV-specific CD8 T cells and to other Ki67 expressing cells, and B cells.
Figure 1: Representative Screenshots Showing How to Collect Confocal Images Using a Confocal Microscope. (A) Acquisition mode used to collect confocal images. (B) Channels used for image collection. (C) 20X objective and 800 x 800 pixel fields were used in image collection. (D) A sequential z-stack was collected at 3 µm intervals. (E) Tiles were adopted to delineate and collect images in multiple fields. Please click here to view a larger version of this figure.
Figure 2: Representative Screenshots Demonstrating Quantitative Image Analysis Using ImageJ. (A) Open a confocal montage by dragging it to the ImageJ window. (B) Duplicate the selected z-scan for analysis. (C) Split the different channels. (D) Draw the ROI for quantitative analysis in the corresponding channel to be objective and add it to the ROI manager by pressing "T." (E) Adjust the fluorescence brightness and contrast of the channel to be analyzed. (F) Flatten the ROI on the image. (G) Count the positive cells in the image. Please click here to view a larger version of this figure.
Figure 3: IST Combined with IHC in Axillary Lymph Node Sections from an SIV-infected Rhesus Macaque. Mamu-A*02 tetramers loaded with SIV Nef YY9 peptides were used to stain antigen-specific CD8 T cells, and similar tetramers loaded with an irrelevant FLP peptide from the hepatitis B virus were used as a negative control (red). Mouse anti-CD20 antibodies were used to stain CD20+ B cells (green), and rat anti-CD8 antibodies were used to stain CD8+ T cells (blue). (A) A representative image from an axillary lymph node section stained with YY9 tetramers, CD20, and CD8 antibodies. (B) The same image as in panel A showing the YY9 tetramer stain alone. (C) Representative image of a section from the same axillary lymph node, stained with FLP tetramers and CD20 and CD8 antibodies. (D) The same image as in panel C with the FLP tetramer stain alone. Scale bars = 100 µm. Please click here to view a larger version of this figure.
Figure 4: IST Combined with IHC Showing the Specificity of MHC-tetramer Staining. Representative axillary lymph node section stained with Mamu-A*02 tetramers loaded with Nef YY9 peptides to label SIV-specific CD8 T cells (red), mouse anti-CD20 antibodies to label B cells (green), and rat anti-CD8 antibodies to label CD8 T cells (blue) (A). An SIV-specific CD8 T cell is tetramer+ (B), CD20– (C), and CD8+ (D). Scale bars = 10 µm. Please click here to view a larger version of this figure.
Figure 5: IST Combined with IHC to Show the Location, Abundance, and Phenotype of Antigen-specific CD8 T Cells. (A) Representative axillary lymph node section stained with Mamu-A*02 tetramers loaded with Nef YY9 peptides to label SIV-specific CD8 T cells (red), Ki67 antibodies to label proliferating cells (green), and IgM antibodies to label B cells (blue). Scale bar = 100 µm. (B) Enlargement of a selected region in panel A. IgM staining defines the follicular area, which is marked as "F;" the extrafollicular area is marked as "EF." Tetramer+ cells are indicated with arrows. Scale bar = 100 µm. Representative tetramer+ Ki67– cell (C, D, E) and tetramer+ Ki67+ cell (F, G, H). Scale bar = 10 µm. Please click here to view a larger version of this figure.
IST combined with IHC provides an essential tool for detecting, characterizing, and quantifying antigen-specific CD8 T cells in native environments with the context of other cells and tissue structures. Here, we described detailed procedures for IST combined with IHC, followed by quantitative image analysis, to determine the location, abundance, and phenotype of antigen-specific CD8 T cells in lymph nodes from rhesus macaques. Similar staining can be applied to human, mouse, or other species tissues for which MHC-I tetramers are available. In addition, peptide MHC Class II tetramer or dextramer staining can be performed using relatively similar methodologies to label antigen-specific CD4 T cells in tissues4,5,6,7,8,9,10,11,12,13. IST can also be combined with ISH to determine, for example, in vivo effector-to-target cell levels18,19. In the future, it will be interesting to carry IST/IHC further by combining IST/IHC with advanced in situ RNA and DNA detection methodologies. Recent advancements in in situ hybridization assays include the development of RNAscope and DNAscope24. These techniques allow for the detection of target RNA and DNA in tissues. It will be exciting to combine these methodologies with IST and IHC to simultaneously detect virus-specific-CD8 T cells, viral RNA, viral DNA, and antibody-labeled antigens of interest.
While we originally described IST methods with fresh tissues, tissues that were fixed for a short duration, and frozen tissues4, in recent years, we have exclusively used fresh tissue sections, as they consistently produce the highest-quality stains and allow for the examination of cells in thick tissue sections. As an alternative to the procedures presented here, one can apply tetramers to tissues, incubate overnight, fix, embed, and cryopreserve in freezing medium (e.g., OCT), produce frozen thin sections, and perform IHC later22. Similarly, we routinely stain a subset of tissue sections with tetramers alone and then freeze the sections in OCT to allow for additional counterstaining combinations in the future. In addition, Qdot 655-conjugated peptide-MHC multimers can be used to directly visualize antigen-specific T cells in cryopreserved tissue sections13.
We have described here indirect tetramer staining. Direct staining using APC- or PE-conjugated tetramers has also been shown to work4,22. In this case, the concentration of MHC tetramer required is higher than that used in indirect tetramer staining. In our hands, a concentration of 20 µg/mL of APC-labeled tetramer was effective at detecting antigen-specific cells. However, the staining intensity was much lower than that obtained with indirect labeling, which includes amplification with anti-FITC antibodies.
We found that the use of a compression-based microtome (see the Table of Materials) for fresh tissue cutting eased the process of cutting fresh tissue sections as compared to using a vibrating microtome23. However, in instances where a compression-based microtome is not available, a vibrating microtome or scalpel can be used for fresh tissue sectioning.
A major limitation of this technique is the use of fresh tissues. Using fresh tissues is much more difficult than fixed or frozen tissues because they require immediate attention and processing. We have successfully shipped fresh tissues overnight on ice in tissue culture medium or PBS-H. However, there have been occasional issues with shipping; for example, snow storms have delayed the shipment of tissues for 48 h or more. In these instances, we found that fresh tissues sectioned and those stained 48 h post-extraction generally show specific staining, with signs of some tissue degradation; tissues stained 72 h post-extraction are too degraded for staining. We also found that the shipment of fresh tissues with ice blocks that are too cold or too close to the tissues can freeze the tissues during shipping; this freezing generally destroys the tissue for staining. Thus, it is extremely important to ship fresh tissues chilled, but not frozen, and to initiate IST staining within 24 h. Fresh tissue processing also requires a great deal of student and staff time, as tissues from multiple animals or study participants cannot be collected and stained together on a later date. Despite these difficulties, we find that fresh tissue sections are the best choice for beautiful, specific IST/IHC staining.
Another limitation of the IST/IHC method described here is the indirect staining approach. Due to limitations on the number of distinct species of animals available to generate secondary antibody combinations, we are limited by indirect antibody staining methods to only three or four fluorescent antibody staining combinations at a time. This limits the amount of information that can be collected on one tissue slab. Direct IHC staining overcomes this limitation and can expand the capabilities, detecting eight or more antibody-tagged antigens simultaneously, albeit with each antibody producing a much weaker fluorescent signal compared to indirect methods. Thus, indirect IHC might be used as an alternative to indirect IHC for counterstaining IST-stained tissues, allowing for the detection of increased numbers of cellular antigens when combined with the IST detection of antigen-specific CD8 T cells.
In some instances, substantial autofluorescence and/or non-specific tetramer or antibody binding may occur with IST/ISH. Because of this, good positive and negative controls are necessary to discern specific stains from background and autofluorescent staining. Good negative controls for MHC I tetramers include negative control tissues (e.g., non-infected tissues; tissues stained with MHC I tetramers loaded with irrelevant peptides or irrelevant MHC tetramers; or, in a pinch, tissues stained with no tetramers but with amplifying antibodies).
In summary, MHC I IST combined with IHC is a valuable tool to determine the location, abundance, and phenotype of antigen-specific CD8 T cells in tissues. This methodology allows for the detection of antigen-specific CD8 T cells in native environments, with the relative localization to other cell types and tissue structures maintained. This method is broadly applicable because it can be used to localize, phenotype, and quantify essentially any antigen-specific CD8 T cell for which MHC tetramers are available, in any tissue.
The authors have nothing to disclose.
This work was supported by Public Health Service grants from the National Institutes of Health (T32 DA007097, R01AI096966, andUM1AI26617).
MHC-I monomer | NIH tetramer core facility | Materials for MHC-tetramer preparation | |
ExtrAvidin-FITC | Sigma-Aldrich | E2716 | Materials for MHC-tetramer preparation |
Normal goat serum | Jackson Immunoresearch | 005-000-121 | |
Low melt agarose | Promega | V3121 | |
Heparin | Sigma-Aldrich | SLBL6391V | |
Triton X-100 | Sigma-Aldrich | T-6878 | |
Urea | J.T.Baker | 4204-05 | |
Glycerol/gelatin | Sigma-Aldrich | SLBH2672V | |
n-propyl gallate | Sigma-Aldrich | P3130 | |
rat-a-h-CD8 (1:500) | Acris | 0714 | Antibody unstable, use single use frozen aliquot |
m-a-h-CD20 (1:500) | NOVOCASTRA | 6026819 | |
m-a-h-Ki67 (1:500) | Vector | 6022201 | |
goat-a-m-A488 (1:2000) | Jackson Immunoresearch | 124083 | |
goat-a-rb-Cy3 (1:5000) | Jackson Immunoresearch | 106232 | |
goat-a-rat-Cy5 (1:5000) | Jackson Immunoresearch | 118088 | |
goat-a-h-IgM-Dylight649 (1:5000) | Jackson Immunoresearch | 86579 | |
Compresstome: VF-300 Microtome | Precisionary Instruments, LLC | 1079 | |
Quick Set Instant Adhesive | Loctite | 46551 | |
24-well flat bottomed tissue culture plates | Falcon | 353226 | |
Microscope slide | Globe scienfitic Inc. | #1321 | |
Razor blade | Ted Pella, Inc | 121-6 | |
Feather Disposable Scalpel | FEATHER SAFETY RAZOR CO. LTD. | No. 21 | |
Round paintbrush #2 | PRINCETON ART & BRUSH CO. | 4350R | Can trim as needed with razor |
Confocal Microscope | Olympus | FV1000 | |
FV10-ASW_Viewer4.0 | Olympus |