Nanoparticle Uptake: The Phagocyte Problem

Abstract

Phagocytes are key cellular participants determining important aspects of host exposure to nanomaterials, initiating clearance, biodistribution and the tenuous balance between host tolerance and adverse nanotoxicity. Macrophages in particular are believed to be among the first and primary cell types that process nanoparticles, mediating host inflammatory and immunological biological responses. These processes occur ubiquitously throughout tissues where nanomaterials are present, including the host mononuclear phagocytic system (MPS) residents in dedicated host filtration organs (i.e., liver, kidney spleen, and lung). Thus, to understand nanomaterials exposure risks it is critical to understand how nanomaterials are recognized, internalized, trafficked and distributed within diverse types of host macrophages and how possible cell-based reactions resulting from nanomaterial exposures further inflammatory host responses in vivo. This review focuses on describing macrophage-based initiation of downstream hallmark immunological and inflammatory processes resulting from phagocyte exposure to and internalization of nanomaterials.

Keywords: biodistribution; circulation; clearance; drug delivery; imaging; macrophage; toxicity.

Figure 1

(Left) Cartoon depicting examples of the body’s resident tissue macrophages responsible for nanoparticle tissue site clearance and inflammatory activation; (Right) Images of silica nanoparticles internalized within macrophages in the liver and spleen (reprinted with permission from [70]).

Figure 2

A) Macrophage polarization (M1 versus M2 traditional dichotomy) plays an important role in the uptake of nanoparticles [70, 82]. Traditional polarization of macrophages to an M1 phenotype enhances nanoparticle uptake, while M2 polarization appears to reduce particle uptake. B) This cell uptake variation may be explained by variations in functions of these macrophage polarization states outlined briefly in this cartoon (adapted with permission from [70]).

Figure 3

Schematic illustration of the ways that particle physicochemical characteristics can influence degrees and conformation of adsorbed proteins. Surface curvature, topography, hydrophilic/hydrophobic chemistry and polymer coating steric barriers on nanoparticle surfaces are shown to alter amounts and conformations of proteins adsorbed to surfaces. Surface-adsorbed proteins (opsonins) influence macrophage recognition and uptake of nanoparticles [28, 32, 91]. Additionally, conformational protein rearrangements on nanoparticle surfaces alters protein epitope exposures to phagocytes [83, 84]. Certain epitopes have the capacity to activate macrophages[31].

Figure 4

Phagocytic-particle recognition is responsible for nanoparticle clearance and internalization. Macrophage surface receptors are depicted here that could be responsible for nanoparticle recognition. Each receptor that recognizes the nanoparticle will induce a specific internalization mechanism, outlined in the diagram. Downstream inflammatory effects, triggered by receptor engagement, can be induced via these internalization pathways.

Figure 5

Dissecting the various nanoparticle entry mechanisms into cells. (Top): Confocal images depicting actin polymerization staining within RAW 264.7 cells (an adherent murine monocyte/macrophage line), actin is in red, nanoparticles are in green and the nucleus is depicted in blue. The image on the left helps to visualize clathrin-mediated endocytosis of nanoparticles under 200nm through bowl-like invaginations. The image on the right depicts phagocyte-mediated uptake of nanoparticles larger than 200nm through protrusions; (Middle) transmission electron microscopy image depicting nanoparticle cellular entry, left appears to be through invagination-like mechanisms and right through protrusion-like mechanisms; (Bottom): This image displays the various forms of cellular uptake of nanoparticles. Depicted are (A) phagocytosis, (B) macropinocytosis, (C) clathrin-mediated endocytosis, (D) clathrin- and caveolae-independent endocytosis, (E) caveolae-mediated endocytosis, (F) passive membrane movement (adapted with permission from [134, 135]).

Figure 6

Patients (n=22) injected with CKD-602 (PEGylated liposomal camptothecin analog) were monitored for particle clearance and drug release versus decrease in monocytes over the course of treatment. Increased monocytic activities in cancer patients reduced the area under the curve (AUC, bioavailability), or the amount of circulating nanoparticles in plasma. This MPS effect clearly implicates involvement of host macrophages in the clearance of clinical nanoparticle formulations. Reprinted with permission from [3].