Direct comparison between in vivo and in vitro microsized particle phagocytosis assays in Drosophila melanogaster

•Phagocytosis of polystyrene particles was assessed using Drosophila immune cells.•Phagocytic uptake was quantified using both in vitro and in vivo experiments.•Particle phagocytosis was governed by the total internalized surface area.•Phagocytosis of particles in vitro was reduced compared to phagocytosis in vivo.

The effects of micro and nanoparticles on the innate immune system have been widely investigated and a general lack of agreement between in vivo and in vitro assays has been observed. In order to determine the origin of these discrepancies, there is a need for comparing the results of in vivo and in vitro phagocytosis assays obtained using the same particles and same immune cells. Here, we establish an in vivo polystyrene microsized particle phagocytosis assay in Drosophila melanogaster and compare it with an in vitro assay consisting of exposing the same immune cells in culture to the same particles. The distribution of number of phagocytized beads per cell was shifted to lower numbers of beads per cell in the case of the in vitro assay compared to the in vivo assay, which we suggest is partly due to a reduced amount of membrane available in cultured cells.