Molecular Probes Tutorial Series—Introduction to Flow Cytometry

0:03 Flowcytometry is a powerful technique for the analysis of multiple parameters of individual cells within heterogeneous populations. Flowcytometers are used in a range of applications from imuno phenotyping to ployy analysis to cell counting and GFP expression analysis. The flowcytometer performs this analysis by passing thousands of cells per second through a laser beam and capturing the light that emerges from each cell as it passes through. The data gathered can be analyzed statistically by flowcytometry software to report cellular characteristics such as size, complexity, phenotype and health. In this tutorial, we will look at how a flowcytometer works, how scattered light and fluoresence are detected by a

0:49 flowcytometer and how the resulting data can be analyzed. This view shows the primary systems of the flowcytometer schematically. These are the fluidic system which presents samples to the interrogation point and takes away the waste. The lasers which are the light source for scatter and fluoresence. The optics which gather and direct the light, the detectors which receive the light and the electronics and the peripheral computer system which convert the signals from the detectors into digital data and perform the necessary analyses. The interrogation point is the heart of the system. This is where the laser and the sample intersect and the optics collect the resulting scatter and

1:35 fluoresence. First, let's talk about how the sample is delivered to the laser. Here we see how the sample is transported through the interrogation point. For accurate data collection, it is important that particles or cells are passed through the laser beam one at a time. Most flowcytometers accomplish this by injecting the sample stream containing the cells into a flowing stream of sheath fluid or saline solution. As you can see, the sample stream becomes compressed to roughly one cell in diameter. This is called hydrodnamic focusing. In fact, flowcytometers can accommodate cells that span roughly 3 orders of magnitude in size. In most cases, seytometers will be detecting cells between 1 and 15

2:21 microns in diameter. Although through the use of specialized systems, it is possible to detect particles outside this range. Now let's see how laser light is used to detect individual cells in the stream. As a cell passes through the laser, it will refract or scatter light at all angles. Forward scatter or low angle light scatter is the amount of light that's scattered in the forward direction as laser light strikes the cell. The magnitude of forward scatter is roughly proportional to the size of the cell. And this data can be used to quantify that parameter. But how can we record this scattered light? Light is quantified by a detector that converts intensity into voltage. In most cytometers, a blocking bar called an

3:07 obscuration bar is placed in front of the forward scatter detector. The obscuration bar prevents any of the intense laser light from reaching the detector. As a cell crosses the laser, light is scattered around the obscuration bar and is collected by the detector. The scattered light received by the detector is translated into a voltage pulse. Because small cells produce a small amount of forward scatter and large cells produce a large amount of forward scatter, the magnitude of the voltage pulse recorded for each cell is proportional to the cell size. If we plot a histogram of these data, smaller cells appear toward the left and larger cells appear toward the right.

3:53 A histogram of forward scatter data is a graphical representation of the size distribution within the population. But such a graph only presents one-dimensional data. Next, let's look at side scatter. As we have already seen, a cell traveling through the laser beam will scatter light at all angles. Light scattering at larger angles, for example, to the side, is caused by granularity and structural complexity inside the cell. This side scattered light is focused through a lens system and is collected by a separate detector, usually located 90° from the laser's path. The signals collected by the side scatter detector can be plotted on one-dimensional histograms like we saw for forward scatter.

4:40 The one-dimensional histograms we have seen so far do not necessarily show the complexity of the cell populations. For example, what appears to be a single population in the forward scatter histogram is in reality multiple populations that can only be discerned by looking at the data in a second dimension. This is done through the use of two-dimensional dot or scatter plots. You can see that the peaks from the forward and side scatter histograms correlate with the colored dots in the scatter plot. Now we can view the results obtained when we create a scatter plot using forward and side scatter data from a typical peripheral blood cell run. The populations that emerge include lymphosytes which are small cells possessing low internal

5:25 complexity, monocytes, which are medium-sized cells with slightly more internal complexity, and neutrfils and other granular sites, which are large cells that have a lot of internal complexity. This multiparametric analysis is the real power of flowcytometry. Now, let's take a look at another parameter that can tell us more about cell structure and function. Fluoresence. As a brief review, fluoresence is a term used to describe the excitation of a flororahore to a higher energy level followed by the return of that flororahore to its ground state with the emission of light. The energy in the emitted light is dependent on the energy level to which the floror is excited and that light has a specific wavelength and consequently a specific

6:10 color. One of the most common ways to study cellular characteristics using flowcytometry involves the use of fluorescent molecules such as flororahor labeled antibodies. In these experiments, the labeled antibbody is added to the cell sample. The antibbody then binds to a specific molecule on the cell surface or inside the cell. Finally, when laser light of the right wavelength strikes the flororohor, a fluorescent signal is emitted and detected by the flowcytometer. How is this fluorescent information collected? The fluorescent light coming from labeled cells as they pass through the laser travels along the same path as the side scatter signal. As the light travels along this path, it is directed through a series of filters and mirrors

6:56 so that particular wavelength ranges are delivered to the appropriate detectors. Fluoresence data is collected in generally the same way as forward and side scatter data. In a population of labeled cells, some will be brighter than others. As each cell crosses the path of the laser, a fluorescent signal is generated. The fluorescent light is then directed to the appropriate detector where it is translated into a voltage pulse proportional to the amount of fluoresence emitted. All of the voltage pulses are recorded and can be presented graphically. What if we want to do a twocolor experiment? We need to look at the spectra of the two flororahores to see if they are compatible. Alexa 4 488 and ficoythine or RP are commonly used

7:44 together. For these two flororophores, 488 nanome light is an efficient excitation source. When excited with 488 nanome light, you can see that the emission peaks for these two dyes are far enough apart so that discrete emission data can be collected. Compatible dyes such as these allow scientists to easily detect two colors from a single laser. If we analyze data from the two-color experiment using a scatter plot, four distinct populations emerge. Looking at the dot plot in terms of quadrants, cells with only bright orange fluorescent appear in the upper left quadrant. Cells with only green fluorescents appear in the lower right quadrant. Cells with both bright green

8:30 and bright orange fluorescents appear in the upper right quadrant. And finally, cells with both low green and low orange fluorescent appear in the lower left quadrant. Multiple fluoresence parameters are necessary to dissect complex biological systems. How does the flowcytometer collect discrete fluoresence data for the Alexa floor 488 and RP flororahores? filters are available that can capture the peak fluoresence from each of these molecules. A 530 nanometer bandp pass filter will collect most of the alexafl 488 peak and a 585 nanome band pass filter will collect the bulk of the RP peak. Using these filters, the proportional amounts of alexafl 488 and

9:18 rp fluoresence can be recorded for each cell. You can see that portions of each emission peak overlap one another. This is called spectral overlap and we will talk about how to deal with this in the next tutorial. Let's take a moment to talk about filter nomenclature. Filters are normally defined by one of two parameters. Either the center point for a band pass filter or the cutff point for a long or short pass filter. This is a typical band pass filter specific for RP. It has a center point of 585 nm and a width of 42 nm. So this filter optimally passes light in the wavelength range of 564 to 66 nm which corresponds to the emission peak of RP. Other

10:04 filters used to resolve this peak include a 550 nanome long pass filter and a 610 nm short pass filter. One final important point regarding data collection is the use of a threshold. If every single particle passing through the laser caused the instrument to collect data, the data pool would be dominated by information coming from a very large number of minute particles like platelets and debris. To prevent this, a threshold is set such that a certain forward scatter pulse size must be exceeded for the instrument to collect the data. On the histogram, the blank area represents the small cells and debris that are excluded from analysis by the threshold. This means that the majority of events that the

10:49 seytometer collects are the cells of interest. It is important to realize that the small particles are still passing through the instrument. They are just being ignored. We've come to the end of this tutorial. Our journey began with a look at the systems that make up the flowcytometer and how those systems work together to collect information on cells as they pass through a laser. We went on to examine in more detail how seytometers detect light scatter and fluoresence and how that information can be viewed on various plots. In summary, flowcytometry is a unique tool providing scientists with a way to gather statistical data on large numbers of cells and use that information to correlate multiple parameters within a cell population. Four to six color

11:36 experiments are becoming easier. A few labs in the world are able to distinguish up to 18 colors simultaneously. In the next tutorial, we will talk in more detail about some of the trickier aspects of data analysis that are employed in flows. [Music]