Proteins bind the surfaces of nanoparticles, and biological materials in general, immediately upon introduction of the materials into a physiological environment. The further biological response of the body is influenced by the nanoparticle–protein complex. The nanoparticle's composition and surface chemistry dictate the extent and specificity of protein binding. Protein binding is one of the key elements that affects biodistribution of the nanoparticles throughout the body. Here we review recent research on nanoparticle physicochemical properties important for protein binding, techniques for isolation and identification of nanoparticle-bound proteins, and how these proteins can influence particle biodistribution and biocompatibility. Understanding the nanoparticle-protein complex is necessary for control and manipulation of protein binding, and allows for improved engineering of nanoparticles with favorable bioavailability and biodistribution.

Nanoparticles have unique physicochemical properties which make them promising platforms for drug delivery. However, immune cells in the bloodstream (such as monocytes, platelets, leukocytes, and dendritic cells) and in tissues (such as resident phagocytes) have a propensity to engulf and eliminate certain nanoparticles. A nanoparticle's interaction with plasma proteins (opsonins) and blood components (via hemolysis, thrombogenicity and complement activation) may influence uptake and clearance and hence potentially affect distribution and delivery to the intended target sites. Nanoparticle uptake by the immune cells is influenced by many factors. Different nanoparticles have been shown to act on different pathways, while various characteristics/properties also affect which pathway is employed for particle internalization. Nanoparticle protein binding occurs almost instantaneously once the particle enters biological medium, and the physical properties of such a particle-protein complex are often different than those of the formulated particle. These new properties can contribute to different biological responses and change nanoparticle biodistribution. Therefore, in the situation when specific delivery to immune cells is not desired, the ideal nanoparticle platform is the one whose integrity is not disturbed in the complex biological environment, which provides extended circulation in the blood to maximize delivery to the target site, is not toxic to blood cellular components, and is "invisible" to the immune cells which can remove it from circulation. This review discusses the most recent data on nanoparticle interactions with blood components and how particle size and surface charge define their hematocompatibility. This includes properties which determine particle interaction with plasma proteins and uptake by macrophages. We will also provide an overview of in vitro methods useful in identifying interactions with components of the immune system and the potential effects of such interaction on particle distribution to tissues.

Nanoparticle size and plasma binding profile contribute to a particle’s longevity in the bloodstream, which can have important consequences for therapeutic efficacy. In this study an approximate doubling in nanoparticle hydrodynamic size was observed upon in vitro incubation of 30- and 50-nm colloidal gold in human plasma. Plasma proteins that bind the surface of citrate-stabilized gold colloids have been identified. Effects of protein binding on the nanoparticle hydrodynamic size, elements of coagulation, and the complement system have been investigated. The difference in size measurements obtained from dynamic light scattering, electron microscopy, and scanning probe microscopy are also discussed.

Gram-negative bacteria possess an asymmetric lipid bilayer surrounding the cell wall, the outer membrane (OM). The OM inner leaflet is primarily composed of various glycerophospholipids, whereas the outer leaflet predominantly contains the unique amphiphilic macromolecule, lipopolysaccharide (LPS or endotoxin). The majority of all gram-negative bacteria elaborate LPS containing at least one 2-keto 3-deoxy-D-manno-octulosonate (Kdo) molecule. The minimal LPS structure required for growth of Escherichia coli has long been recognized as two Kdo residues attached to lipid A, inextricably linking viability to toxicity. Here we report the construction and characterization of the nonconditional E. coli K-12 suppressor strain KPM22 that lacks Kdo and is viable despite predominantly elaborating the endotoxically inactive LPS precursor lipid IV(A). Our results challenge the established E. coli Kdo2-lipid A dogma, indicating that the previously observed and well-documented dependence of cell viability on the synthesis of Kdo stems from a lethal pleiotropy precipitated after the depletion of the carbohydrate, rather than an inherent need for the Kdo molecule itself as an indispensable structural component of the OM LPS layer. Inclusion of the inner membrane LPS transporter MsbA on a multicopy plasmid partially suppresses the lethal deltaKdo phenotype directly in the auxotrophic parent strain, suggesting increased rates of nonglycosylated lipid A transport can, in part, compensate for Kdo depletion. The unprecedented nature of a lipid IV(A) OM redefines the requisite LPS structure for viability in E. coli.

Aims While numerous studies have reported on nanoparticle uptake by phagocytic cells, the mechanisms of this uptake are poorly understood. A metastudy of research focusing on biological particulate matter has postulated that nanoparticles cannot be phagocytosed and therefore must enter cells via pinocytosis. The purpose of this study was to identify the route(s) of uptake of gold nanoparticles in vitro and to determine if these route(s) depend on particle size. Materials & Methods The parent RAW264.7 cell line and its derivatives, transduced with a virus carrying siRNA to macrophage scavenger receptor A, were used as model phagocytes. Citrate-stabilized gold colloids were used as model nanoparticles. We used chemical inhibitors known to interfere with specific routes of particulate uptake. We developed multifocal light microscopy methods including multifocal stack analysis with NIH ImageJ software to analyze cell uptake. Results Irrespective of size, gold nanoparticles are internalized by macrophages via multiple routes, including both phagocytosis and pinocytosis. If either route was blocked, the particles entered cells via the other route. Conclusion Gold nanoparticles with hydrodynamic sizes below 100 nm can be phagocytosed. Phagocytosis of anionic gold colloids by RAW264.7 cells is mediated by macrophage scavenger receptor A.