Evaluation of particle uptake in human blood monocyte-derived cells in vitro. Does phagocytosis activity of dendritic cells measure up with macrophages?

This work focuses on microparticles as potential antigen delivery systems t o target professional antigen-presenting cells. Surface modified polystyren e microparticles were administered to human-derived macrophages (M Phis) an d dendritic cells (DCs) in vitro to evaluate the phagocytosis activity of e ach cell type. To discriminate between internalised particles and those clo sely attached to the outside of the cells, particle internalisation was ver ified by confocal laser scanning microscopy. Especially positively charged particles tend to stick to the outer cell membrane and may lead to false po sitive results when measured by conventional microscopy. In contrast, fluor escence microscopy in combination with an extracellular fluorescence quench ing agent (trypan blue) allows the unequivocal assessment of particle uptak e for screening purposes. For this assay, the fluorescent label needs to be in direct contact to the quenching agent and cannot be localised inside th e particle core. Different types of microparticles varying in size, surface -material and zeta potential resulted in vast differences regarding their u ptake by M Phis and DCs as well as the maturation of DCs. Negatively-charge d carboxylated and bovine serum albumin-coated particles were phagocytosed by M Phis to a relatively small extent. Interestingly, phagocytosis of thes e particles was still significantly lower in DCs while positively charged p oly-L-lysine (PLL) coated particles induced high phagocytosis activity in b oth cell types. By comparing our results with literature data, we conclude that phagocytosis activity of DCs and M Phis largely depends on particle si ze and surface charge and is also influenced by the character of bulk and c oating material. PLL can be directed to DCs and M Phis with comparable effi ciency and, in addition, induce maturation of DCs. (C) 2001 Elsevier Scienc e B.V. All rights reserved.