Phagocytosis by Immune Cells of Protein-Modified Polymer Microparticles

The present work was aimed at studying the ability of three model green proteins to covalently bind to microparticles (MPs) based on poly(D,L-lactic acid) (PLA). Green fluorescent protein (sfGFP), the fusion protein of recombinant human β2-microglobulin (β2M) with sfGFP (β2M–sfGFP) and the fusion protein of recombinant human amylin (IAPP) with sfGFP (IAPP–sfGFP) were isolated using affinity chromatography. MP–PLAs were formed by the double-emulsion method. The modification of MP–PLAs by protein was confirmed by laser scanning microscopy (LSM). In addition, LSM was used to study the phagocytosis of MP–PLA modified by different proteins and free model proteins by macrophages. Recombinant sfGFP was shown to binds to the surface of particles at lower amounts compared to β2M–sfGFP and IAPP–sfGFP. This is probably due to the fact that protein amino groups that could potentially react with activated carboxyl groups on the surface of particles are sterically inaccessible for this reaction because of the sfGFP structure. The β2M and IAPP proteins, being components of the respective recombinant fusion proteins, are spacer structures between the surface of spherical particles and sfGFP. It was established that a threefold increase in the protein/particles ratio did not lead to an increase in the bound protein per unit of particle mass, which may indicate the amount of protein that can be bound per unit of particle mass is limited by the capacity of particles themselves. The study of phagocytosis of protein-modified MP–PLAs has shown that MP–PLAs containing model proteins (β2M–sfGFP and IAPP–sfGFP) on their surface are successfully phagocytized by macrophages and, thereby, can contribute to the activation of cell-mediated immune response, which is important for controlling various, including viral, infections. Phagocytosis of model proteins (β2M–sfGFP, IAPP–sfGFP) has also been shown in the present work. This may be due to the fact that both β2M and IAPP are amyloidogenic and aggregation-prone proteins. In all likelihood, the aggregates of these proteins can be absorbed by macrophages due to the increased size compared to their monomeric forms.