Image-based cytometry reveals three distinct subsets of activated granulocytes based on phagocytosis and oxidative burst

Granulocytes play a key role in innate immunity and the most common functional assays are phagocytosis and oxidative burst. The purpose of this technical note is to use image-based flow cytometry to divide activated granulocytes into unique subsets based on their degree of phagocytosis and oxidative burst in response to different experimental incubations. Prior to the experiments, all reagents were titered to determine the lowest dose that resulted in an acceptable signal to noise ratio. Heparinized, whole blood (100 μl) was mixed with one of two bioparticles (E. coli and S. aureus) and DHE (10 μg/ml) and incubated for 5, 10, 20, 40, 60, 80, 100, 120, and 140 min in a 37°C water bath. An additional tube kept on ice was used as a negative control. All subsequent processing steps were completed on ice in the dark to minimize additional activation of cells. After the 37°C incubation, N-ethylmaleimide (15 mM) was added to halt phagocytosis, preventing the uptake of additional microparticles. Suspensions were labeled with CD66b-APC and CD45-APCeFluor780 for 60 min and a fix/lyse solution was added. Prior to acquisition, 7AAD was added to stain nuclear DNA. A minimum of 5,000 granulocyte (CD66b+) events were acquired using a Millipore-Amnis FlowSight equipped with blue (488 nm, 60 mW), red (642 nm, 100 mW), and side scatter (785 nm, 12 mW) lasers. Samples were compensated and analyzed using Amnis IDEAS software (v.5.0.983.0). Image-based analysis allowed us to divide activated granulocytes into three distinct subsets, whose relative abundance changed as a function of both bioparticle type and incubation length. The method described in this technical note represents a potential novel adaptation to common methods of assessing granulocyte function. More research is needed to test and validate our image-based method in clinical conditions that impair granulocyte function.