Although microglia are the major phagocytic cells in the brain, astrocytes mediate microglial response to inflammatory stimuli (Krencik et al., 2011, Lee and Landreth, 2010, Skripuletz et al., 2013). We repeated the pHrodo red zymosan assay using microglia-astrocyte co-cultures; both hiPSC-astrocytes and primary human fetal astrocytes enhanced phagocytic capacity of BV2 mouse microglial cells (Figures 4C and S4D). We discriminated microglia and astrocyte phagocytic activity by labeling CD11b- and GFAP-positive cells, respectively (Figure S4E). After 3 hr of labeling, only the CD11b+ BV2 microglial cells phagocytosed the pHrodo red zymosan bioparticles (Figure S4E) Finally, we treated BV2 microglial cells with astrocyte conditioned medium (ACM) for 24 hr prior to analyzing their phagocytic capacity; pretreatment of BV2 microglial cells with ACM from either hiPSC-astrocytes or primary human fetal astrocytes significantly increased microglial phagocytic capacity (Figures 4D and S4F). Taken together, these findings show that both hiPSC-astrocytes and primary human fetal astrocytes secrete factors that increase the capacity of microglia to phagocytose zymosan bioparticles.
