Macrophages are one of the most important candidates of innate immune response with pronounced phagocytic activity. As a part of the defense mechanism of our system, an increased level of particular chemokines initiate the activation and further differentiation of the macrophages into two major phenotypes, M1 (classically activated) and M2 (alternatively activated). M1 macrophages promote ‘pro-inflammatory’ activities, whereas, M2 macrophages show ‘anti-inflammatory’ activities. Now, normally a certain ratio of M1/M2 macrophages is maintained in healthy individuals. However, at certain disease conditions, a shift in the M1 to M2 or M2 to M1 macrophage population is observed. An assessment of the M1/M2 ratio would readily predict the health condition of the individual. Here, we propose a novel approach to identify M1 and M2 populations using simple flow cytometry and Gold nanorods (GNRs). According to reports, macrophages can readily internalize GNRs by phagocytosis. Now, this internalization of highly scattering GNRs will increase the cellular scattering of macrophages and thus can be identified by the Flow Cytometric technique. For the first time, we are reporting about the differential uptake of polyallylamine hydrochloride (PAH) coated GNRs by M1 and M2 macrophages (differentiated from THP1 cells). A 24 h incubation with the 100μg/ml PAH-GNRs results in a greater intake of PAH-GNRs by M2 cells compared to M1, which leads to an increased side scatter for M2 cells in Flow cytometry. Overall, this study opens a new avenue for simple identification of M1 and M2 cell types.

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Ruchira Chakraborty, Dorit Leshem-Lev, and Dror Fixler, "Differential uptake of gold-nanorods promotes identification of M1/M2 subtype of macrophage by flow cytometry," Proc. SPIE 11254, Nanoscale Imaging, Sensing, and Actuation for Biomedical Applications XVII, 112540N (Presented at SPIE BiOS: February 03, 2020; Published: 21 February 2020); https://doi.org/10.1117/12.2543937.