Applications - Application Notes
High Throughput, Multiplexed Detection of Inflammatory Cytokines in an Astrocyte and Monocyte Co-culture Model
18-Feb-16Cytation 3, Cytation 5, Synergy Neo2
Inflammatory cytokine levels are reported to undergo a significant increase in multiple neurodegenerative disorders, including Alzheimer’s disease (AD) and multiple sclerosis1. It has been repeatedly documented that astrocytes are a critical source of IL-6, while glial cells are an important producer of IL-1 and TNFα2. Research also reveals that under certain conditions, astrocytes and neurons can express multiple different cytokines that are normally undetectable. These three cell types, glial, astrocyte and neuron, commonly interact to produce an inflammatory response. For example, in AD, activated astrocytes and microglia are characteristically found in abundance near neurons and plaques3. Therefore, it is imperative that assessment of pertinent cytokine levels be performed with relevant cell types in isolation as well as in a co-culture model.
Previously, cytokine levels were assessed using assays that measured protein concentrations on an individual basis. As cytokines were secreted into the surrounding media, multiple analyses were performed from a single well. The procedure is labor intensive, and performing individual assessments in separate wells may increase variability within the collective data set. By incorporating an assay technology capable of assessing multiple cytokines in the same well, variability can be eliminated, thereby creating a robust detection method.
Here we describe a homogeneous, beadbased immunoassay technology which offers multiplexed detection of several cytokines in a single well. Streptavidin-coated donor beads are bound to biotinylated antibodies specific for one of the target analytes being assessed. Acceptor beads, providing disparate emission profiles upon excitation, are also directly conjugated to a specific anti-analyte antibody. In the presence of the individual analyte, the beads come into close proximity to each other. When excited using the 680 nm laser on a novel high-throughput multimode microplate reader, donor beads convert dissolved oxygen to singlet oxygen molecules, causing a cascade of energy transfer in the acceptor beads and a light emission increase at the appropriate wavelength. Therefore, high signal values are indicative of greater cytokine emission within the well. Cryopreserved primary astrocytes and peripheral blood CD14+ monocytes are incorporated independently, and in co-culture, to increase the relevance of generated results. The combination of multiplexed assay technology, cell model and laser-based detection provide a rapid, yet highly sensitive method to assess important inflammatory cytokine levels.
AlphaPlex™ is a homogeneous, all-in-one-well multiplexing reagent technology that provides highly sensitive detection of a wide range of analytes from large proteins to small molecules and scarce biological samples such as primary cells and stem cells. The protocol is a simple nowash assay format based on Alpha Technology. By using multiple Acceptor beads which emit different wavelengths (AlphaLISA Europium (Eu):615 nm; AlphaPlex Terbium (Tb):545 nm; AlphaPlex Samarium (Sm):645 nm), multiple analytes can be simultaneously detected.
Figure 1. AlphaPlex Assay Principle. Biotinylated anti-analyte antibodies bind to Streptavidin-coated Alpha Donor beads, while other anti-analyte antibodies are conjugated directly to AlphaLISA, AlphaPlex 545, or AlphaPlex 645 Acceptor beads. In the presence of specific analyte(s), the beads come into close proximity.
Materials and Methods
Assay and Experimental Components
AlphaPlex 545 (Tb) IL-6 Detection Kit (Catalog No. AP223TB-C), AlphaPlex 645 (Sm) Human IL1β Detection Kit (Catalog No. AP220SM-C), and Human TNFα Immunoassay Kit (Catalog No. AL208C) were donated by PerkinElmer (Waltham, MA). Lipopolysaccharides from E. coli 055:B5 (LPS) (Catalog No. L2880) and Human Interferon-γ (hIFNγ) (Catalog No. 11040596001) were purchased from Sigma-Aldrich (Saint Louis, MO).
Cryopreserved Normal Human Astrocytes (Catalog No. CC-2565), AGM™ Bullet Kit™ (CC-3186) containing Basal Medium and Supplements to support astrocyte culture, and Cryopreserved Peripheral Blood CD14+ Monocytes (2W-400C) were donated by Lonza Group, Ltd. (Basel, Switzerland).
Synergy Neo2 Multi-Mode Reader is designed for speed and ultra-high performance, incorporating BioTek’s patented Hybrid Technology™. Independent optical paths accommodate diverse assay requirements with variable bandwidth quadruple monochromators, sensitive filter-based optics, laser-based excitation for Alpha assays and up to 4 PMTs for ultrafast measurements. Integrated Gen5™ Data Analysis Software controls the Synergy Neo2. Advanced environment controls, including CO2 /O2 control, incubation to 65 °C and variable shaking are ideal for live cell assays, and cell-based detection is optimized with direct bottom illumination. Barcode-labeled filter cubes help to streamline workflows and limit errors.
Cytation 5 is a modular multi-mode microplate reader combined with automated digital microscopy. Filter- and monochromator-based microplate reading are available, and the microscopy module provides up to 60x magnification in fluorescence, brightfield, color brightfield and phase contrast. Integrated Gen5™ Data Analysis Software controls Cytation 5. The instrument was used to capture brightfield images of the cell cultures and co-cultures.
Cryopreserved astrocytes were thawed and added to 96-well plates, using vendor recommendations, at a concentration of 1.5x104 cells/cm2 in a volume of 100 μL. For the 0.32 cm2 wells, a total of 5000 cells were added per well and incubated at 37 ºC/5% CO2 for 72 hours. Media exchanges were performed after 24 hours, and every 48 hours thereafter. Astrocyte doubling time was predetermined as approximately 24 hours, creating approximately 40,000 cells/well after 72 hours. Media was removed, and an equivalent number of monocytes was then added in astrocyte complete media, to empty wells or wells containing astrocytes in a 50 μL volume. Media was added to other wells containing astrocytes. Ultimately, wells contained either astrocytes, monocytes, or co-cultures.
LPS+ IFNy Addition
LPS and IFNγ were diluted in media to create 2X LPS concentrations ranging from 2000-0 ng/ mL in a constant 300 U/mL IFNγ concentration. Fifty microliters of the titration were added to designated wells containing each cell model.
AlphaPlex Assay Performance
Five microliter aliquots were removed from each well after 8, 24, and 48 hour incubations for analysis using either the AlphaPlex Tb IL-6/AlphaLISA TNFα or AlphaPlex Sm IL-1β/AlphaLISA TNFα duplex assays. Laser-based microplate reading was then performed to detect and quantify secreted cytokine levels from each well. The Gen5 Data Analysis Software protocol included initial simultaneous reading of terbium and europium signals using universal Alpha excitation and Tb/Eu emission cube. Tb/Eu cube was then replaced with the Sm emission cube to complete the protocol.
Results and Discussion
Duplex Assay Signal Optimization
Prior to performing the cell-based AlphaPlex assay, reader settings and assay component concentrations were optimized to limit signal crossover during multiplex detection. Narrow filter bandwidths (Table 1) were used to minimize the capture of unwanted signal from each read. Variable acceptor bead concentrations were then tested to further minimize crosstalk.
Per Figure 2, two duplexes were created to demonstrate the ability to combine and effectively detect the signals from multiplexed Tb/Eu and Sm/Eu assays. Upon testing, the starting Tb concentration of 25 μg/mL minimized crosstalk to an acceptable level of 1.8%, while a concentration of 2.5 μg/mL of Eu AlphaLISA beads was necessary to minimize crosstalk to the same level (1%) (Figure 2A). This same Eu bead concentration also yielded minimal crosstalk in the Sm channel (0.85%), whereas a 2.5 μg/mL concentration of Sm beads was again necessary to minimize crosstalk in the Eu channel (1.95%) (Figure 2B).
Table 1. Optimized filters and mirrors for AlphaPlex reads using Synergy Neo2.
Figure 2. Percent signal crosstalk shown for (A) Eu/Tb duplex assay and (B) Eu/Sm duplex assay optimization using reader settings and protocol.
Complete analyte standard curves were then created and tested with assay acceptor bead concentrations of 25 μg/ mL Tb:IL-6; 2.5 μg/mL Eu:TNFα; and 2.5 μg/mL Sm:IL-1β. The signal was captured using the two reader channels for the multiplexed assays (IL-6 and TNFα or IL-1β and TNFα). Figure 3 graphs illustrate that broad assay windows and accurate standard curves can be maintained while effectively minimizing crosstalk using optimized settings.
Figure 3. Analyte standard curve detection using target and duplex assay reader channels. (A) IL-6 Tb assay read using Tb and Eu channels; (B) TNFα Eu assay read using Eu and Tb channels; (C) IL-1β Sm assay read using Sm and Eu channels; (D) TNFα Eu assay read using Eu and Sm channels.
Final Duplex Assay Validation
Comparisons were then made between analyte titrations added to assay wells containing either single or dual assay acceptor beads to assess whether assay quality was affected in the multiplexed setting. Analyte concentrations ranging from 100,000-0 pg/mL were assayed in the presence of target assay acceptor beads only, or duplex assay acceptor beads.
Per Figure 4, the similarity in alpha signal and curve shape from analyte standard curves run in a single or duplex assay format validates the finding of no negative impact on assay quality by the addition of a second set of acceptor beads to assay wells.
Figure 4. Single and duplex assay analyte standard curves. (A) IL-6 plus IL-6 Tb acceptor beads only, or plus IL-6 Tb acceptor beads and TNFα Eu acceptor beads; (B) TNFα plus TNFα Eu acceptor beads only, or plus TNFα Eu acceptor beads and IL-6 Tb acceptor beads; (C) IL-1β plus IL-1β Sm acceptor beads only, or plus IL-1β Sm acceptor beads and TNFα Eu acceptor beads; (D) TNFα plus TNFα Eu acceptor beads only, or plus TNFα Eu acceptor beads and IL-1β Sm acceptor beads.
Finally, the signal from the hTNFα analyte standard curve, when assayed with the TNFα Eu assay, and duplexed with either the IL-6 Tb or IL-1β Sm assays, was compared. Figure 5 plotted curves demonstrate that equivalent results are attained when the AlphaLISA assay is multiplexed with either terbium or samarium AlphaPlex assays.
Figure 5. hTNFα analyte titrations assayed in the presence of TNFα Eu acceptor beads duplexed with IL-6 Tb or IL-1β Sm acceptor beads.
Inflammatory Cytokine Detection
Following optimization and validation of the two multiplexed assays, a cell-based experiment was used to detect inflammatory cytokine secretion from glial and mononuclear cells. Cryopreserved primary astrocytes and monocytes were added to wells independently (Figures 6A and B) or in co-culture (Figure 6C) as previously described. The images in Figure 6 demonstrate that the two cell types can be successfully co-cultured in a single well and thus used for the subsequent test. Variable concentrations of the known stimulant LPS combined with IFNγ were then added to the wells. Aliquots from the supernatant were removed at regular intervals and analyzed using either the IL-6 Tb AlphaPlex/TNFα AlphaLISA or IL-1β Sm AlphaPlex/TNFα AlphaLISA duplex assays to monitor potential changes in cytokine secretion between basal and stimulated cell conditions.
Figure 6. Glial and monocytic cell cultures. 20x brightfield images showing (A) astrocytes; (B) monocytes; or (C) co-cultured cells in 96- well format.
Next, IL-6 pg/mL concentrations were determined by interpolating the Alpha signal from unknown samples based upon purified analyte standard curves using Graph Pad Prism V. 5.01. When examining the graphs in Figure 7A-C, it is apparent that the combination of LPS and IFNγ stimulate IL-6 secretion with all cell models tested. Furthermore, cytokine secretion is both time and concentration dependent. Figure 7D also illustrates that co-cultured astrocytes and monocytes maintain consistent IL-6 secretion, creating proportionately higher cytokine concentrations when combined, compared to independent cell cultures. This agrees with previously published findings using astrocytic and monocytic cell lines4.
Figure 7. IL-6 secretion from astrocyte and monocyte cell cultures and co-cultures. Secreted pg/mL IL-6 concentrations in media following 8, 24, and 48 hour LPS and IFNγ stimulation of (A) astrocytes; (B) monocytes; or (C) co-cultured cells. (D) 48 hour timepoint IL-6 concentrations in media per cell model in basal and stimulated conditions.
TNFα secretion levels into media, after a 48-hour incubation with LPS and IFNγ (Figure 8A), show little change in wells containing astrocytes only. Concentrations were interpolated as previously described. In comparison, stimulation is seen in wells containing monocytes or co-cultured cells, peaking at 1 ng/mL LPS. However, concentrations do not vary from monocyte only wells, indicating that secretion is due to monocytes in the co-culture. These findings are equivalent to previous literature publications reporting higher TNFα production in LPS stimulated microglia compared to astrocytes5. IL- 1β follows a similar pattern (Figure 8B), with higher levels of cytokine secretion seen from monocytes, compared to astrocytes. Interestingly, IL-1β secretion decreases in the co-cultured cell model in a dose dependent manner, indicating a potential suppressive effect between the two cell types on specific inflammatory pathways, again agreeing with the literature4. This underscores the importance of using correct cell models in experimental procedures.
Figure 8. 48 hour LPS/IFNγ stimulation of TNFα and IL-1β cytokine secretion. Secreted concentrations of (A) TNFα; or (B) IL-1β from astrocytes, monocytes, or co-cultured cells following 48 hour LPS/IFNγ stimulation.
We have shown that the AlphaPlex assays from PerkinElmer represent a simple, sensitive, homogeneous and high-throughput method to monitor secretion of duplex analytes from the same assay well. At the same time, incorporating Lonza’s cryopreserved primary cells simplifies cell-based assay procedures. The Synergy Neo2 Multi-Mode Microplate Reader, with laser-based excitation and dual PMT capability, then allows high density, multiplexed AlphaPlex assays to be performed easily and with a single protocol.
The use of appropriate primary cell models ensures that in vitro analyte expression closely mimics that seen in vivo, opposed to the use of cancerous cell line surrogates, as seen by the work of Oh et al., 19996. Astrocyte and monocyte co-cultures, can also affect the extent and rate of cytokine production and other experimental factors. Finally, the combination of multiplexed assay methods, cell models, and microplate reader create robust in vitro methods to easily and accurately measure inflammatory cytokine secretions.
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