Fluorescence-activated Cell Sorting for Purification of Plasmacytoid Dendritic Cells from the Mouse Bone Marrow

The overall goal of this experiment is to isolate plasmacytoid dendritic cells, or PDC, with a high purity from the bone marrow of lupus-prone mice for the study of their function. This method can help answer key questions related to the characteristics of a particular immune cell population, such as, interferon alpha producing PDC during the late stage of lupus development. The main advantage of this technique is that one to several target populations can be enriched to a higher purity than by magnetic bead sorting.

After harvesting, transfer the hind limb bones and four milliliters of complete medium, into a mortar. Add six milliliters of fresh complete medium to bring the total volume to 10 milliliters. And crack the bones gently with a pestle.

Gently stir the pestle to release the bone marrow. Then, transfer all of the medium through a 70 micron strainer into a 50 milliliter conical tube on ice. Add 10 milliliters of fresh complete medium to the mortar.

And filter the supernatant two more times until the bones appear white. Then, centrifuge the bone marrow cells, and re-suspend the pellet in 10 milliliters of fresh complete medium. Next, layer the cells on to five milliliters of freshly prepared density gradient medium in a 15 milliliter conical tube, taking care not to disrupt the layers.

And separate the cells by centrifugation. Then, carefully remove the top eight milliliters of the solution, and collect the two milliliter buffy coat at the interphase. Add 12 milliliters of ice-cold complete medium to the isolated mononuclear cells, and cap the tube to mix by inversion.

Then, pellet the cells by centrifugation. To stain the cells for FACS, re-suspend them in 100 microliters of CD1632FC blocking antibody at four degrees Celsius. After 10 minutes, wash the cells in four milliliters of ice cold sorting buffer and label the cells with 100 microliters of the appropriate cell surface staining antibody cocktail.

For 15 minutes at four degree Celsius in the dark. Wash the cells in four milliliters of ice cold sorting buffer to remove any unbound fluorescence conjugated antibody. Then, re-suspend the pellet in 500 microliters of FACS loading solution and transfer the sample into a FACS tube on ice.

To stain the compensation controls, aliquot one times 10 to sixth splenocytes per tube. Wash the cells with four milliliters of sorting buffer and re-suspend the pellets in 100 microliters of sorting buffer. Then, add the appropriate fluorescence conjugated antibody from the master cell surface antibody cocktail to each corresponding compensation tube and shake the tubes to mix.

After 15 minutes in the dark, at four degree Celsius, wash all of the samples with sorting buffer and re-suspend the pellets in 500 microliters of ice cold sorting buffer. Except for the DAPI tube, which is re-suspended in 500 microliters of FACS loading solution. To purify the target cell population by fluorescence activated cell sorting, load collection tubes containing 500 microliters of FBS supplemented with antibiotics below the sorting valves.

Next, load the unstained tube and collect 10, 000 events to set the negative gate for each fluorescent intensity. And 10, 000 events in the single stain compensation tubes to set the positive gate for each fluorescent intensity. Collect 3, 000 events from the experimental sample to set the gates of the sorted cell population.

Using the fluorescence intensities as the parameters of the X and Y access graph to manually gate the target cell population. Then, sort the target cell population into the collection tube. At the end of the sorting, add up to four milliliters of ice cole complete medium to the cells.

Then, cap the tube and mix the cells by inversion. Pellet the purified cells by centrifugation, aspirating all but the last 200 microliters of the supernatant. Then, re-suspend the pellet in one milliliter of ice cold complete medium and transfer the cells to a 1.5 milliliter micro centrifuge tube for a second centrifugation.

Finally, aspirate all but the last 100 microliters of the supernatant and store the sorted cell population on ice until use. Magnetic activated cell sorting of bone marrow plasmacytoid dendritic cells from lupus prone mice results in only a 7.75%purity after the enrichment. FACS enriched plasmacytoid dendritic cells, however, demonstrate a purity as high as 96.4%post sorting.

To ensure a high purity, the mononuclear bone marrow cells are first gated and separated from debris by forward and side scatter parameters. Using forward scatter width versus forward scatter height and side scatter width versus side scatter height, the single cells are further gated to avoid the false positive signals produced by aggregates. The live single DAPI negative cells are then gated by their CD11c positive, CD11B negative cell surface expression allowing further gating of the B220 positive, PDCA-1 positive target bone marrow plasmacytoid dendritic cell population.

The sorted PDC population is not only of a high purity but also functionally normal with the ability to produce IFNalpha upon CPG stimulation in vitro. While attempting this procedure, it's important to remember to crack the bones and stir the released bone marrow gently to preserve cell viability. Following this procedure, other methods like RNA sequencing or in vitro culture and stimulation, can be performed to answer additional questions about the gene expression or function of enriched cell populations.

After its development, this technique paved the way for PDC researchers to explore the role of this powerful cell in the development of lupus and lupus prone mouse models. After watching this video, you should have a good understanding of how to isolate a highly purified PDC population from the bone marrow of lupus prone mice.