Preclinical studies to understand nanoparticle interaction with the immune system and its potential effects on nanoparticle biodistribution

Abstract

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.

Figure 1

Complement activation: friend or foe? The complement system serves as a nonspecific pathogen clearance aid, which complements humoral and cell-mediated immunity. Undesirable activation of the complement system results in hypersensitivity reactions, and overwhelming activation may even lead to anaphylactic shock. For this reason, it is frequently desirable to design nanoparticles intended for systemic administration such as to avoid complement activation. However, in addition to its primary role in pathogen clearance, the complement system also promotes humoral and cell-mediated immunity. Local activation of complement by nanoparticles administered via subcutaneous and intradermal routes may benefit vaccine efficacy. Nanotechnology-based pharmaceuticals are frequently composed of multiple components with varying compositions, size, and surface properties which may be engineered to achieve desirable properties.

Figure 2

Role of protein binding on nanoparticle distribution to immune cells. Nanoparticle surface properties largely determine particle interaction with plasma proteins. Unprotected nanoparticles, such as citrate-stabilized colloidal gold, rapidly bind plasma proteins (this can be analyzed by particle separation from bulk plasma, removal of surface bound proteins and analysis by 2D PAGE). After opsonization with human plasma, these particles are taken up by macrophages. Here, transmission electron microscopy (TEM) micrographs of macrophages demonstrate particle accumulation inside cells. In contrast, PEGylated gold particles bind less protein (as can be seen by 2D PAGE) and are not taken up by macrophages (as can be seen from TEM images).

Figure 3

Particle fate inside the immune cell is largely determined by its composition. Biodegradable particles are digested and cleared from the body, while nonbiodegradable particles accumulate in cells for extended periods. Processing of multicomponent, multifunctional nanoparticles is more complex, and more studies are required to answer questions regarding the fates of the individual components of these particles.

Figure 4

Points to consider during assay development. Nanoparticles differ in size, composition, and surface characteristics. These differences may cause spurious experimental outcomes if the properties of the particle are poorly understood. The main challenge in nanoparticle characterization using traditional techniques is identifying potential nanoparticle interferences (and then overcoming them) to avoid false-positive and false-negative results. The particle properties responsible for false-positive and false-negative results are summarized in this diagram. Examples are given as type of nanoparticle - assay in which they cause interference. Abbreviations: PSN, polystyrene nanoparticle; cAU-NP, citrate-stabilized gold nanoparticles (colloidal gold); Dox, doxorubicin; QD, quantum dots; LAL, limulus amoebocyte lysate; ELISA, enzyme linked immunosorbent assay; NP, nanoparticle; NE, nanoemulsion.