We describe here a transgenic reporter mouse model to visualize the IL-22-producing cells inside different mouse tissues. This method can be used to track the location of other cytokines or secretary proteins in the mouse.
Reporter mice have been widely used to observe the localization of expression of targeted genes. This protocol focuses on a strategy to establish a new transgenic reporter mouse model. We chose to visualize interleukin (IL) 22 gene expression because this cytokine has important activities in the intestine, where it contributes to repair tissues damaged by inflammation. Reporter systems offer considerable advantages over other methods of identifying products in vivo. In the case of IL-22, other studies had first isolated cells from tissues and then re-stimulated the cells in vitro. IL-22, which is normally secreted, was trapped inside cells using a drug, and intracellular staining was used to visualize it. This method identifies cells capable of producing IL-22, but it does not determine whether they were doing so in vivo. The reporter design includes inserting a gene for a fluorescent protein (tdTomato) into the IL-22 gene in such a way that the fluorescent protein cannot be secreted and therefore remains trapped inside the producing cells in vivo. Fluorescent producers can then be visualized in tissue sections or by ex vivo analysis through flow cytometry. The actual construction process for the reporter included recombineering a bacterial artificial chromosome that contained the IL-22 gene. This engineered chromosome was then introduced into the mouse genome. Homeostatic IL-22 reporter expression was observed in different mouse tissues, including the spleen, thymus, lymph nodes, Peyer's patch, and intestine, by flow cytometry analysis. Colitis was induced by T-cell (CD4+CD45RBhigh) transfer, and reporter expression was visualized. Positive T cells were first present in the mesenteric lymph nodes, and then they accumulated inside the lamina propria of the distal small intestine and colon tissues. The strategy using BACs gave good-fidelity reporter expression compared to IL-22 expression, and it is simpler than knock-in procedures.
Cell type-specific expression of reporter genes is useful to identify cells actively expressing the target in tissues under homeostatic and perturbed states. It also allows for the purification of these cells, which remain viable, to study their other properties. Reporter mice have been utilized in elucidating the mechanism of action for specific cytokines, transcription factors, and regulatory elements. Previous strategies1,2,3 have largely relied on knocking the reporter into the target locus in the mouse chromosome, a time-consuming and costly procedure. Thus, a simpler method for the generation of reporter mice is desirable.
Cytokines are a broad class of small, secreted proteins/peptides that regulate immune responses through intercellular signaling. Interleukin 22 (IL-22) is a cytokine with many reported activities, including barrier function, tissue repair, and inflammation4. Although IL-22 was initially discovered as a T-cell product5, subsequent reports demonstrated its expression in natural killer (NK) cells in humans6 and mice7 and in other classes of innate lymphocytes8. Despite extensive observation of IL-22-producing cells, visualization of IL-22 previously required ex vivo stimulation and permeabilization to stains with antibodies. Therefore, novel IL-22 reporter mice would be a very useful tool to investigate the function of IL-22 in homeostatic and pathogenic processes.
Here, we developed a simplified transgenic reporter mouse model to observe the IL-22-producing cells in vivo and in vitro. Using a BAC recombineering method9, we inserted the tdTomato cDNA sequence with Poly A signal fragments into the IL-22 locus and replaced exon 1. The other untranslated regions, exons, and regulatory elements were not perturbed, since we would like to mimic the natural regulation of IL-22 as much as possible. The site of reporter insertion disrupts the signal sequence, resulting in the accumulation of the reporter inside the producing cells, unlike IL-22 itself, which is rapidly secreted. This new method can also be applied to the generation of reporter mice for other secreted proteins.
All animals received proper care in accordance with the experimental procedures outlined in the 2011 Guide for Care and Use of Laboratory Animals Committee of Frederick National Laboratory for Cancer Research.
1. Generation of IL-22-tdTomato Reporter Mice by BAC Recombineering
NOTE: The mice should be unconscious and do not move in response to a noxious stimulus. Sterilize the surgical area with 70% ethanol and sterilize all surgical tools using a glass bead sterilizer.
2. Single-cell Preparation from Spleens, Thymus, Lymph Nodes, and Peyer's Patch
NOTE: IL-22-tdTomato reporter mice were maintained at the National Cancer Institute (NCI, Frederick, MD). The euthanasia method conforms to the most recent AVMA Guidelines on Euthanasia. All mice were euthanized using CO2 inhalation14.
3. Isolation of Intraepithelial Lymphocytes and Lamina Propria Cells from the Intestines
4. Expression of IL-22-tdTomato in Colitis
A murine IL-22 reporter transgene was created using recombineering to modify a bacterial artificial chromosome carrying the IL-22 locus. Figure 1 shows a diagram of pBACe3.6 vector containing the sacBII gene, a positive-selection marker, and chloramphenicol antibiotic resistance gene11. After introducing tdTomato into exon 1, the signal peptide sequence was disrupted, as shown in Figure 2. Thus, the tdTomato reporter was trapped inside the IL-22-expressing cells, enabling their detection and isolation by flow cytometry. The homozygous mice were bred from founder lines and screened by PCR, and they did not require backcrossing in order to achieve offspring with a genetic identity, as is required in the usual knock-in strategy. To test the fidelity of the IL-22 reporters, in vitro-generated splenocytes were cultured under Th22 or neutral conditions. Figure 3 demonstrates that IL-22-tdtomato was well expressed in Th22 cells in vitro, detected either by flow cytometry or by fluorescent microscopy. In vivo, as shown in Figure 4, the IL-22 reporter was detected in different mouse tissues under homeostatic condition. Most of the tdTomato signal was found in the lamina propria (LP) cells from the gut, but not in the axillary lymph node (ALN), the spleen, or the thymus. To visualize the tdTomato reporter in inflamed tissue, we used a mouse colitis model induced by the transfer of reporter CD4+CD45Rbhi T cells into Rag1-/- mice. Figure 5 demonstrates that the reporters were first present in the mesenteric lymph nodes and then accumulated inside the lamina propria of distal small intestine and colon tissues. Taken together, this novel method for the generation of IL-22 reporter mice is effective and time-saving.
Figure 1: Site map of the pBACe3.6 vector. pBACe3.6 is characterized by containing chloramphenicol antibiotic resistance and a sacBII gene as a positive-selection marker. During recombination, the desired clones are sucrose-resistant through their expressing the sacBII gene and are allowed to grow on sucrose-containing media, while the negative BAC colonies are sucrose-sensitive.
Figure 2: Schematic view of the modified RP23-401E11 BAC clone. A tdTomato reporter gene was introduced into the Il-22 locus, which includes IL-22 and regulatory elements in a murine bacterial artificial chromosome (BAC RP23-401E11), using recombineering technology. By homologous recombination, the sequence of the signal peptide of Il-22 in the BAC was disrupted and exon 1 of Il-22 was replaced by the tdTomato reporter gene. Please click here to view a larger version of this figure.
Figure 3: Identification of IL-22 reporters in splenocytes cultured in vitro. CD4 T cells were purified from the spleen and then stimulated with anti-CD28; anti-CD3 (neutral condition); or anti-CD3, anti-CD28, anti-IFNƳ, anti-IL4, IL-6, TGF-β, and 6-Formylindolo (3,2-b)carbazole (FICZ) for 5 days. IL-22-tdTomato was analyzed by flow cytometry (top two) and fluorescent microscopy (bottom two, scale bar: 100 µm). The numbers in the quadrants indicate the percent of CD45+ cells. Please click here to view a larger version of this figure.
Figure 4: Visualization of IL-22-producing cells in different mouse tissues. Single-cell suspensions were prepared from the spleen, thymus, lymph nodes, intestines (IEL and LP), and Peyer's patch. They were then surface-stained with FITC-anti-CD3 and examined for tdTomato expression. MLN: mesenteric lymph node, ALN: axillary lymph node, PP: Peyer's patch, IEL: intraepithelial cells isolated from the small intestine, LP: lamina propria cells purified from the small intestine. The numbers in the quadrants indicate the percent of CD45+ cells. Please click here to view a larger version of this figure.
Figure 5: Localization of IL-22 reporters during the development of T-cell transfer colitis. CD4CD45RBHigh T cells from IL-22 reporter mice were transferred into Rag1-/- mice, and the tdTomato signals were evaluated by immunohistochemistry in the different tissues at different time points during the development of colitis. The arrows point to the IL-22-tdTomato signals. Scale bars = 25 µm. Please click here to view a larger version of this figure.
IL-22 plays an essential role in innate host defense and tissue remodeling. IL-22-producing cells have been identified ex vivo by intracellular staining. However, it still remains difficult to track IL-22 expression in situ, either in the normal state or in inflammatory conditions. This protocol describes a novel method to develop an IL-22 reporter mouse model, which enables us to localize the reporter-expressing cells in vivo. The reporter gene encoding TdTomato was inserted into the IL-22 locus at a site that disrupts the signal sequence. This strategy results in the reporter being trapped in the producing cell, unlike IL-22 which is rapidly secreted, enabling visualization of the producing cell. The engineered IL-22-reporter was contained within a large BAC, with the aim of preserving the regulatory elements, thus conferring fidelity of expression of the reporter corresponding to that of IL-22. The intestinal tissues of transgenic mice displayed homeostatic reporter expression in CD4 T cells and innate lymphocytes in the lamina propria of the small intestine. In induced colitis, CD4 T cells expressed the reporter in the colon.
The role of IL-22 in inflammatory bowel disease has been unclear. By participating in the intestinal mucosal defense and tissue regeneration, IL-22 is capable of eliciting the production of both protective20,21,22 and proinflammatory mediators (e.g., IL-8)23,24. Two models of T cell-driven colitis showed increases in IL-22 in the colon, including TCR α -/- mice25,26 and the CD45RBhi transfer model27. Comparing inflammatory bowel disease models, IL-22 expression in the intestine was higher in a Crohn's disease modelthan in a model of ulcerative colitis21,24.Here, we transfer CD4CD45RBhi T cells from the reporter mice into Rag1-/- recipients and mimic the pathogenic process of Crohn's disease. We found that, preceding colitis, IL-22 reporters were visualized in intestinal draining lymph nodes (MLN). The signals then diminished in the MLN and moved to the intestines, especially the distal small intestines and colons, with the development of colitis. Most tdTomato reporters were localized inside the lamina propria mucosa of the intestine.
A different type of reporter for IL-22 has been previously described, in which cells are permanently marked such that they and their daughters continue to express the reporter, whether or not transcription of IL-22 has continued28. As in our study, gut innate lymphocytes were marked under homeostatic conditions, and in colitis, the reporter was localized in T cells from the mesenteric lymph nodes to the intestinal tissue.
The procedures described in this protocol would be applicable for the establishment of other transgene reporter mice, such as IL-17, IL-25, and so on. However, it is crucial to choose suitable BAC clones carrying distal regulatory elements to assure the fidelity of reporter gene expression, since the transgenes are randomly integrated into the mouse genome. The reporter mouse allows for the identification of cells expressing in vivo, as well as for the purification and characterization of live cells. We used tdTomato, an exceptionally bright red fluorescent protein, as a fluorescent reporter gene to make the reporter suitable for live animal imaging studies.
The authors have nothing to disclose.
We thank Kelli Czarra and Megan Karwan for animal technical assistance, Kathleen Noer and Roberta Matthai for flow cytometry assistance, and Donna Butcher andMiriam R. Anver for pathology analysis. This project was supported by a grant from the Ely and Edythe Broad Foundation (to Scott Durum) and has been funded in whole or in part with federal funds from the National Cancer Institute, National Institutes of Health, under Contract No. HHSN261200800001E (MRA).
RP23-401E11 BAC | Thermo Fisher Scientific | RPCI23.C | Need gene ID: 50929 |
NucleoBond BAC 100 | Takara Clontech | 740579 | |
PCR SuperMix High Fidelity | Thermo Fisher Scientific | 10790020 | |
PI-SceI | New England Biolabs | R0696S | |
SpeI | New England Biolabs | R0133S | |
LB Broth | Thermo Fisher Scientific | 10855-001 | 1L: 10 g SELECT Peptone 140, 5 g SELECT Yeast Extract, 5 g sodium chloride |
Anti-mouse CD3 | eBioscience | 11-0031 | |
Anti-mouse CD4 | eBioscience | 17-0041 | |
Anti-mouse CD45 | Thermo Fisher Scientific | MCD4530 | |
Anti-mouse CD45RB | eBioscience | 11-0455 | |
Anti-mouse RFP | Abcam | Ab62341 | |
HBSS, no calcium, no magnesium, no phenol red | Thermo Fisher Scientific | 14175145 | KCl, KH2PO4, Na2HPO4, NaHCO3, NaCl, D-Glucose |
Dnase I | Roche | 10104159001 | |
ACK lysing buffer | Thermo Fisher Scientific | A1049201 | |
Percoll | GE healthcare life sciences | 17-0891-01 | |
Collagenase D | Roche | 11088858001 | |
Dispase II (neutral protease, grade II) | Roche | 4942078001 | |
IX70 inverted fluorescence microscope | Olympus | Ask for quote | |
Nikon Eclipse 80i microscope | Nikon | Ask for quote | |
Dynal shaker | Electron Microscopy Science | 61050-10 | |
FACSAria | BD Bioscience | Ask for quote | |
LSRII SORP/flow cytometry | Becton, Dickinson and Company | Ask for quote |