The health status of the lung is reflected by the type and number of immune cells that are present in the bronchioles of the lung. We describe a bronchoalveolar lavage technique that allows the isolation and study of nonadherent cells and soluble factors from the lower respiratory tract of mice.
Bronchoalveolar Lavage (BAL) is an experimental procedure that is used to examine the cellular and acellular content of the lung lumen ex vivo to gain insight into an ongoing disease state.
Here, a simple and efficient method is described to perform BAL on murine lungs without the need of special tools or equipment. BAL fluid is isolated by inserting a catheter in the trachea of terminally anesthetized mice, through which a saline solution is instilled into the bronchioles. The instilled fluid is gently retracted to maximize BAL fluid retrieval and to minimize shearing forces. This technique allows the viability, function, and structure of cells within the airways and BAL fluid to be preserved.
Numerous techniques may be applied to gain further understanding of the disease state of the lung. Here, a commonly used technique for the identification and enumeration of different types of immune cells is described, where flow cytometry is combined with a select panel of fluorescently labeled cell surface-specific markers. The BAL procedure presented here can also be used to analyze infectious agents, fluid constituents, or inhaled particles within murine lungs.
The airways encounter numerous insults, which can lead to inflammation, pathogen invasion, or malignant transformation. The epithelial cells that line the lung lumen form one of the major barriers of the mammalian body. Together with alveolar macrophages, they prevent environmental threats from gaining entry to the systemic system via the airways. Examples of such threats include organic and inorganic chemicals, bacteria, and viruses. Likewise, specific immunizations or therapeutic interventions can be designed to target the lungs. In all these instances, an elaborate analysis of the evoked response is important to understand, intervene, or prevent biological processes that take place within the respiratory system.
Bronchoalveolar lavage (BAL) is an invaluable method to analyze such responses, as the resulting samples contain important information about the inflammatory responses, immune mechanisms, and infectious disease progression that can occur in the pulmonary airways1,2. By using BAL, it is possible to study the infiltrating cells. This contrasts with digested lungs, that give a "dirtier" cell population, with many dead and sticky cells. BAL is performed by introducing a saline solution into the terminal bronchioles and subsequently recovering this solution. The retrieved solution can then be used to quantify and phenotypically analyze resident lung and infiltrating inflammatory cells. This method is frequently applied to study cellular influx in disease models of the airways, such as asthma, Chronic Obstructive Pulmonary Disease (COPD), and infectious disease models. Apart from the cellular composition, the molecular composition of the pulmonary airways is also reflected in the BAL fluid. To analyze this, Enzyme-linked Immunosorbent Assay (ELISA), immunoblot, and the simultaneous analysis of multiple cytokines by a cytokine bead array can be performed to assess the presence of cytokines and chemokines.
BAL is a well-established method to study the influx of inflammatory cells in inflammatory respiratory disease animal models. The observation of an altered cellular influx (e.g., increased levels of lymphocytes, eosinophils, or neutrophils) can lead to better insights into the disease and can be an objective parameter to assess the performance of a therapeutic intervention.
The accurate and reproducible interpretation of BAL cellular analysis requires that the BAL is performed correctly and that the collected fluid is handled and processed properly. The term "bronchial lavage" was introduced more than eighty years ago by Stitt3. In 1961, Myrvik obtained alveolar macrophages from the lavage fluid of rabbit lungs3. BAL is now a commonly used method to analyze and monitor the lungs in the mouse model, yet there is still no report of a standardized BAL procedure in the scientific literature4,5. Furthermore, there are probably as many ways to perform BAL as there are research laboratories using the technique3,6,7,8,9,10,11. It is important that the data obtained from the BAL represent the whole murine lung, and not only a part of the lung. This kind of variability complicates the interpretation and comparison of results between different trials.
Here, a basic, inexpensive, and reproducible BAL procedure is described that allows the collection of the cellular and soluble fraction present in the airway lumen of the mouse. In short, a catheter is placed into the exposed trachea of a terminally anesthetized mouse. A syringe is connected to the catheter, and a buffered saline solution containing ethylenediaminetetraacetic acid (EDTA) is introduced into the lungs. The lung lumen is sampled by gentle repeated flushing of the saline solution using the plunger. The negative pressure applied during this step is minimal to prevent airway collapse. Following collection, the obtained BAL should be processed further to enumerate and identify the cells by flow cytometry.
All animal experiments described in this study were conducted according to the national (Belgian Law 14/08/1986 and 22/12/2003, Belgian Royal Decree 06/04/2010) and European legislation (EU Directives 2010/63/EU and 86/609/EEC). All experiments on mice and all animal protocols were approved by the ethics committee of Ghent University (permit numbers LA1400091 and EC2016-027).
1. Preparation
2. Performing the Bronchoalveolar Lavage (BAL)
3. Collecting the Cellular and Noncellular Components of the BAL Fluid
4. Analysis of the Different Cell Types in the BAL Fluid by Flow Cytometry
NOTE: One possibility is to analyze the absolute and relative cellular composition of the BAL fluid by performing flow cytometry. The goal of this paper is to elaborate the technique of BAL. Flow cytometry is a specialized technique on its own. It is recommended to read specialized papers on the flow cytometry technique13,14,15,16,17. Antibodies coupled to a fluorophore that recognize surface antigens (see Table 1) specific to a particular cell type(s) are used. By using a gating strategy, it is possible to identify T cells, macrophages, dendritic cells, B cells, eosinophils, and neutrophils in the cell fraction of the BAL.
Antigen | Cell type |
Cluster of differentiation 3 (CD3) | Expressed on T cells |
Cluster of differentiation 11c (CD11c) | High expression on most dendritic cells, but also on monocytes, macrophages, neutrophils, and some B cells. |
Cluster of differentiation 11b (CD11b) | Expressed on the surface of many leukocytes including monocytes, neutrophils, natural killer cells, granulocytes and macrophages. |
SiglecF | Alveolar macrophages and eosinophils. |
MHCII | Normally found only on antigen-presenting cells such as dendritic cells, mononuclear phagocytes and B-cells. |
CD19 | B-lymphocyte antigen |
Ly-6G | A marker for monocytes, granulocytes and neutrophils |
Table 1: Selection of Immune Cell Surface Antigens. This table provides a list of surface epitopes used to characterize the different cell types. Combinations of several markers will be required to reliably define a specific cell type.
Samples | ||||
Tube | Antigen-fluorophore to be added to cells | Antibody stock concentration (mg/mL) | Antibody dilution | Total volume (µL) |
Fixable viability dye | 0.2 | 1/1000 | 50 | |
CD11c | 0.2 | 1/800 | 50 | |
SiglecF | 0.2 | 1/100 | 50 | |
sample X | MHCII | 0.2 | 1/200 | 50 |
CD3 | 0.2 | 1/200 | 50 | |
CD19 | 0.2 | 1/200 | 50 | |
CD11b | 0.2 | 1/200 | 50 | |
Ly6G | 0.2 | 1/200 | 50 | |
Voltage controls | ||||
Tube | Antigen-fluorophore to be added to cells | Antibody stock concentration (mg/mL) | Antibody dilution | Total volume (µL) |
Unstained cells | / | / | / | 50 |
Single stained cells | Fixable viability dye | 0.2 | 1/1000 | 50 |
Single stained cells | CD11c | 0.2 | 1/800 | 50 |
Single stained cells | SiglecF | 0.2 | 1/100 | 50 |
Single stained cells | MHCII | 0.2 | 1/200 | 50 |
Single stained cells | CD3 | 0.2 | 1/200 | 50 |
Single stained cells | CD19 | 0.2 | 1/200 | 50 |
Single stained cells | CD11b | 0.2 | 1/200 | 50 |
Single stained cells | Ly6G | 0.2 | 1/200 | 50 |
Compensation controls | ||||
Tube | Antigen-fluorophore to be added to beads | Antibody stock concentration (mg/mL) | Antibody dilution | Total volume (µL) |
Unstained beads | / | / | / | 200 |
Single stained beads | CD11c | 0.2 | 1/2000 | 200 |
Single stained beads | SiglecF | 0.2 | 1/2000 | 200 |
Single stained beads | MHCII | 0.2 | 1/200 | 200 |
Single stained beads | CD3 | 0.2 | 1/2000 | 200 |
Single stained beads | CD19 | 0.2 | 1/2000 | 200 |
Single stained beads | CD11b | 0.2 | 1/400 | 200 |
Single stained beads | Ly6G | 0.2 | 1/200 | 200 |
Table 2. List of Controls to be Included. This table shows all necessary controls for the accurate interpretation of the obtained results.
Laser type | Filter setup | |
505 LP | 525/50 | |
Blue (488 nm)Â | 550 LP | 575/26 |
100 mW | 670 LP | 685/35 |
750 LP | 780/60 | |
violet 405 nm | 450/50 | |
100 mW | ||
red 633 nm | 660/20 | |
70 mW | 750 LP | 780/60 |
Table 3: Overview of the Lasers and Filters of the Flow Cytometer used in This Study.
After performing BAL with 3x 1 mL of buffered salt solution, a volume between 2 and 3 mL should be recovered. This BAL fluid can be analyzed further to characterize the cellular and noncellular content. To investigate the presence of cytokines and chemokines, ELISA19, immunoblot20, and the simultaneous analysis of multiple cytokines by a cytokine bead array21 can be performed. In addition, the albumin and total protein content of this fluid can be determined22.
As an example, this manuscript describes how to analyze the cellular content of the BAL fluid by flow cytometry. The analyzed BAL fluid was collected from female Balb/cAnNCrl mice (age: 7 weeks) 24 h after they were intratracheally instilled with lipopolysaccharide. The following antibodies, coupled to a fluorophore, were used to identify the different cell types: CD11c, SiglecF, MHCII, CD3ε, CD19, Ly6g, and CD11b (see Table 1 and the Table of Materials). The fixable viability dye was also used. By using a gating strategy based on the differential expression of antigens on the surfaces of the different cell populations (Figure 1), it was possible to identify macrophages, dendritic cells, B cells, T cells, neutrophils, and eosinophils.
First, debris and doublets were gated out based on forward- and side-scatter parameters. A viability dye facilitated gating on the live cells. Next, CD11c high cells and CD11c low cells were identified. In the CD11c high population, macrophages and dendritic cells were identified based on MHCII and SiglecF expression, respectively. In the CD11c low population, T cells and B cells were identified based on CD3ε and CD19 expression, respectively. In the remaining cell population, neutrophils and eosinophils were identified based on the CD11b and Ly-6G marker expression, respectively.
Counting beads were added to determine the absolute cell numbers of the different cell populations by comparing the ratio of bead events to cell events23. These counting beads were identified based on their forward- and side-scatter properties (Figure 1). Table 4 gives an overview of the absolute cell numbers of the different cell populations in the BAL fluid of a naïve mouse and a mouse that was stimulated for 24 h with 5 µg of lipopolysaccharide.
Figure 1: Gating Strategy for the Flow Cytometric Detection of Macrophages, Dendritic Cells, T Cells, B Cells, Neutrophils, and Eosinophils in BAL Fluid. BAL cells were isolated using the described BAL protocol. Cells were isolated from mice 24 h after intratracheal instillation of lipopolysaccharide. Counting beads and cells were identified based on forward- and side-scatter properties. In the cell gate, single cells were identified using forward and side scatter. In this last population, cells that were alive were identified. CD11chigh cells and CD11clow cells were then identified. In the CD11chigh population, macrophages and dendritic cells were identified based on MHCII and SiglecF expression, respectively. In the CD11clow population, T cells and B cells were identified based on CD3ε and CD19 expression, respectively. In the remaining cell population, neutrophils and eosinophils were identified based on CD11b and Ly-6G expression, respectively. Please click here to view a larger version of this figure.
cell population | absolute number of cells in naive mice | absolute number of cells in LPS stimulated mice |
macrophages | 79,612 | 25,439 |
dendritic cells | 495 | 671 |
T cells | 45,271 | 28,089 |
B cells | 4,164 | 2,926 |
neutrophils | 632 | 566,716 |
eosinophils | 3,483 | 4,332 |
Table 4: Representative Results of the Flow Cytometry Analysis on the BAL Fluid of Naïve and LPS-stimulated Mice.
BAL is a useful technique to obtain cytological and biochemical information in response to infections or drugs. Initially, BAL was used to manage the excessive mucus production in human patients suffering from phosgene toxicity3. Nowadays, the technique is used in humans to investigate lung pathogenesis, diagnosis, and therapeutic management of diseases3,24. In laboratory animals, BAL is commonly used to monitor inflammatory responses, immune mechanisms, and infectious disease processes that occur in the pulmonary airways1,2.
To study the inflammatory cellular pattern in respiratory disease models, BAL should be followed by absolute and differential cell counting. In addition to the absolute cell number, the relative cell numbers are also of interest. For example, repair and cancer models show very small to no BAL cell count increases. In this model, the assessment of cellular composition is useful. By using cell staining combined with light microscopy, different cell types, such as eosinophils, neutrophils, macrophages, and lymphocytes, can be identified based on morphology25,26,27,28,29,30. Flow cytometry can be used for specific assessments, such as to identify different T-cell phenotypes7,31. In addition to the identification of the different infiltrating cell populations, the non-cellular composition of the lung can be investigated using BAL. Methods such as ELISA, immunoblot, cytokine bead array, immunohistochemistry, and quantitative polymerase chain reaction are performed on BAL fluid to determine cytokines, growth factors, and other inflammatory components. To determine lung damage, total protein and lactate dehydrogenase levels in the BAL fluid can also be measured32,33.
With the development of new diagnostic tools, the genomic and proteomic characterization of BAL components will be possible in the near future. The combination of expanding computational capabilities and high-throughput gene expression technologies will make it possible to define specific gene expression profiles for various disease states. Performing these techniques on BAL fluid can provide gene and protein expression patterns to identify the important molecules involved in the different phases of lung diseases.
The main limitation of data obtained from BAL fluid is the lack of comparability between different research trials3,9. There is a high degree of variability in the lavage technique and the subsequent processing of BAL fluid. To be able to compare each BAL trial, it is necessary to standardize the type of lavage fluid that is instilled, the site of instillation, and the fraction that is to be analyzed for cellular and non-cellular composition. There are significant differences in the number of lavage fractions between different trials, varying from one to 14 times34,35,36. This difference may have an impact on the estimated total cell numbers in the lungs. It is important to know which BAL fluid fraction contains the majority of cells. Song et al. showed that approximately 70% of the total number of cells were retrieved in fraction one to three22. However, other reports suggested that the second lavage contained more cells than the first one37,38. We can conclude from these studies that a lavage with just one fraction does not represent the whole lung, leading to misinterpretation of the results.
The noncellular composition of the BAL fluid contains valuable information on the health status of the lung33,39,40. Variations in the dilution of the BAL fluid contributes to the difference in the quantification of the soluble fraction and, consequently, to differences in the results between trials. Song et al. compared the protein and lactate dehydrogenase levels of each lavage fraction and concluded that the first lavage fraction contained two to three times more than the second fraction.
To retrieve a representative BAL sample for analysis, some technical considerations are crucial. One of them is to perform proper anesthetization. It is very important to check the foot reflex of the mouse to ensure terminal sedation. This is not only important for ethical reasons, but also because it is difficult to place and retain the catheter in the correct position if the mouse is not properly anesthetized.
A second important technical consideration is the position of the catheter in the trachea. When the catheter is inserted too deep, it can damage the lung structure. The distal end of the catheter should not reach the lungs during the BAL procedure. The catheter should also be stabilized and tied off with a cotton thread. If the catheter is not stabilized, the injected saline solution may flow upwards into the nasal cavity instead of down into the lungs. During injection and aspiration of the saline solution, it is important to hold the catheter steady.
The data obtained from the BAL fluid must represent the whole murine lung. Therefore, it is important to instill an adequate volume of saline buffer (i.e. 3 mL, divided in 3 aliquots of 1 mL each). There is no linear relationship between the cell yield and the BAL fluid yield. It is important to collect the solution gently while massaging the thorax of the mouse. If shearing forces are too strong, the viability, function, and structure of cells within the airways and BAL fluid may be compromised. If the aspirated fluid is not visible in the syringe, carefully move the catheter deeper or higher in the trachea.
Special notice should be given to specific aspects of BAL processing and analysis. This will maximize the information retained from BAL samples. After BAL, the cells are in a nutrient-poor saline medium. It is therefore very important to process the samples within 1 h after BAL sampling. If prolonged storage is necessary, the use of a nutrient-supplemented medium is required.
To preserve cell viability, avoid tubes that promote cell adherence to the surface. Avoid centrifugation of cell suspensions at speeds that are likely to compromise cellular integrity or to prevent uniform resuspension of the retrieved BAL cells. BAL fluid containing cells should be centrifuged at 400 x g and 4 °C for 7 min. It is important to keep in mind that cell suspensions should be held at 4 °C during processing.
L.V.H. is a research assistant at the Department of Biomedical Molecular Biology of Ghent University. E.R.J. is supported by UniVacFlu, grant number 607690. K.R. is supported by EC-FP7 project FLUNIVAC.
Name | Company | Catalog Number | Comments |
balanced salt solution | Thermo Fisher Scientific | 14175-129 | |
ethyleendiaminetetra acetic acid | Sigma-Aldrich | E6511 | irritating |
23G x 1 1/4 needle | Henke Sass Wolf | 4710006030 | size: 0,60 x 30 mm |
26G x 1/2 neelde | Henke Sass Wolf | 4710004512 | size: 0,45*12 mm |
plastic tubing | BD medical technology | 427411 | Polyethylene Tubing, I.D 0.58 mm (.023") O.D. .965 mm (0.38") 30.5m (100') |
sodium pentobarbital | Kela NV | 514 | |
phosphate buffered saline | Lonza | BE17-516F | PBS without Ca++ Mg++ or phenol red; sterile filtered |
1ml syringes | Henke Sass Wolf | 5010.200V0 | non pyrogenic and non toxic |
Forceps | Fine Science Tools GmbHÂ | 91197-00 | |
Surgical scissors | Fine Science Tools GmbHÂ | 91460-11 | |
centrifuge tube 50 ml | TH.Geyer | 7696705 | Free from Rnase/Dnase/endotoxin |
centrifuge tube 15 ml | TH.Geyer | 7696702 | Free from Rnase/Dnase/endotoxin |
microcentrifuge tube 1,5 ml | Sigma-Aldrich | 0030 120.094 | polypropylene |
Microcentrifuge | Sigma-Aldrich | 5415R | |
Centrifuge | Thermo Fisher Scientific | 75004030 | |
ammonium-chloride-potassium (ACK) lysing buffer | Lonza | 10-548E | sterile filtered |
live/dead -efluor506 | ebioscience | 65-0866-18 Â | fixable viability dye |
CD11c-PE-cy7 | eBiosciences | 25-0114-81 | |
SiglecF-PE | BD Pharmingen | 552126 | |
MHCII-APCefluor780 | Biolegend | 107628Â | |
CD3-PE-cy5 | VWR | 55-0031-U100Â | |
CD19-PE-cy5 | eBiosciences | 15-0193-83Â | |
CD11b-V450 | BD Pharmingen | 560455 | |
Ly6G-AF700 | Biolegend | 127621 | |
Absolute Counting Beads | Life Technologies Europe B.V. | C36950 | |
anti-CD16/CD32 | BD Pharmingen | 553142 | |
96-well 340 µl storage plate plate | Falcon | 353263 | V-bottom, natural polypropylene |
flow cytometer | BD Biosciences | ||
catheter | BD Biosciences | 393202 |
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