LAUSR

As one of the key processes during the immune response, phagocytosis of bacteria plays a significant role in the mammalian immune system. A crucial part of phagocytosis is the transport of phagosomes from the cell periphery to the perinuclear region. This maturation process shows high variations from phagosome to phagosome that are not clearly understood. We hypothesize that the phagosome size has an influence on the maturation process by directly influencing the transport characteristics. We tested this hypothesis by tracking the transport of IgG-coupled microspheres with different diameters ranging from 1 µm to 3 µm inside macrophages. We show that phagosomal transport efficiency increases with increasing phagosome size despite nearly equal instantaneous velocities for all sizes. Furthermore, we show that the bi-directional intracellular motion of phagosomes as well as the transport from perinuclear region back to the periphery decreases with increasing phagosome size. Resting upon the notion that long range phagosomal transport is based on microtuble associated motor proteins, we investigated the spatial distribution of microtubules. We found density differences between the nucleus-facing side of phagosomes and the opposite side, which could explain part of the observed transport characteristics. In addition, we tested the influence of microtubule-associated motor proteins on the transport by labelling or inhibiting motors in vivo. We found that the concentration of dyneins is increased in the perinuclear region compared to the cell periphery. Furthermore, we found that the motility of large phagosomes is strongly decreased in cells without active dynein. We suggest that a size-dependent cellular sorting mechanism might exist, which supports inward transport of large phagosomes containing complete pathogens for enhancing their digestion and which simultaneously supports transport of digested products of phagosomal content back to cell periphery for example for antigen presentation.