Phagocytosis of poly(L-lysine)-graft-poly(ethylene glycol) coated microspheres by antigen presenting cells: Impact of grafting ratio and poly(ethylene glycol) chain length on cellular recognition

Microparticulate carrier systems have significant potential for antigen delivery. The authors studied how microspheres coated with the polycationic copolymer poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) can be protected against unspecific phagocytosis by antigen presenting cells, a prerequisite for selective targeting of phagocytic receptors. For this aim the authors explored the influence of PLL-g-PEG architecture on recognition of coated microspheres by antigen presenting cells with regard to both grafting ratio and molecular weight of the grafted PEG chains. Carboxylated polystyrene microspheres (5 microm) were coated with a small library of PLL-g-PEG polymers with PLL backbones of 20 kDa, grafting ratios from 2 to 20, and PEG side chains of 1-5 kDa. The coated microspheres were characterized by their zeta-potential and resistance to IgG adsorption. Phagocytosis of these microspheres by human monocyte derived dendritic cells (DCs) and macrophages (MPhi) was quantified by phase contrast microscopy and by analysis of the cells' side scattering in a flow cytometer. Generally, increasing grafting ratios impaired the protein resistance of coated microspheres, leading to higher phagocytosis rates. For DC, long PEG chains of 5 kDa decreased the phagocytosis of coated microspheres even in the case of considerable IgG adsorption. In addition, preferential adsorption of dysopsonins is discussed as another factor for decreased phagocytosis rates. For comparison, the authors studied the cellular adhesion of DC and MPhi to PLL-g-PEG coated microscopy slides. Remarkably, DC and MPhi were found to adhere to relatively protein-resistant PLL-g-PEG adlayers, whereas phagocytosis of microspheres coated with the same copolymers was inefficient. Overall, PLL(20)-[3.5]-PEG(2) was identified as the optimal copolymer to ensure resistance to both phagocytosis and cell adhesion. Finally, the authors studied coatings made from binary mixtures of PLL-g-PEG type copolymers that led to microspheres with combined properties. This enables future studies on cell targeting with ligand modified copolymers.