Immunoglobulin deposition on biomolecule corona determines complement opsonization efficiency of preclinical and clinical nanoparticles

Deposition of complement factors (opsonization) on nanoparticles may promote clearance from the blood by macrophages and trigger proinflammatory responses, but the mechanisms regulating the efficiency of complement activation are poorly understood. We previously demonstrated that opsonization of superparamagnetic iron oxide (SPIO) nanoworms with the third complement protein (C3) was dependent on the biomolecule corona of the nanoparticles. Here we show that natural antibodies play a critical role in C3 opsonization of SPIO nanoworms and a range of clinically approved nanopharmaceuticals. The dependency of C3 opsonization on immunoglobulin binding is almost universal and is observed regardless of the complement activation pathway. Only a few surface-bound immunoglobulin molecules are needed to trigger complement activation and opsonization. Although the total amount of plasma proteins adsorbed on nanoparticles does not determine C3 deposition efficiency, the biomolecule corona per se enhances immunoglobulin binding to all nanoparticle types. We therefore show that natural antibodies represent a link between biomolecule corona and C3 opsonization, and may determine individual complement responses to nanomedicines.

The diagnostic value of iron oxide nanoparticles for imaging of myocardial inflammation--quo vadis?

Cardiovascular magnetic resonance (CMR) is an integral part in the diagnostic work-up of cardiac inflammatory diseases. In this context, superparamagnetic iron oxide-based contrast agents can provide additional diagnostic information regarding the assessment of myocardial infarction and myocarditis. After intravenous administration, these nanoparticles are taken up by activated monocytes and macrophages, which predominantly accumulate in regions associated with inflammation as was successfully shown in recent preclinical studies. Furthermore, first clinical studies with a new iron oxide-complex that was clinically approved for the treatment of iron deficiency anaemia recently demonstrated a superior diagnostic value of iron oxide nanoparticles compared to gadolinium-based compounds for imaging of myocardial inflammation in patients with acute myocardial infarction. In this article, we outline the basic features of superparamagnetic iron oxide-based contrast agents and review recent studies using such nanoparticles for cardiac imaging in case of acute myocardial infarction as well as acute myocarditis. Moreover, we highlight the translational potential of these agents and possible research applications with regard to imaging and therapy.

Effect of MRI tags: SPIO nanoparticles and 19F nanoemulsion on various populations of mouse mesenchymal stem cells

Transplantation of mesenchymal stem cells (MSCs) has emerged as a promising strategy for the treatment of myriad human disorders, including several neurological diseases. Superparamagnetic iron oxide nanoparticles (SPION) and fluorine nanoemulsion (19F) are characterized by low toxicity and good sensitivity, and, as such, are among the most frequently used cell-labeling agents. However, to date, their impact across the various populations of MSCs has not been comprehensively investigated. Thus, the impact of MRI tags (independent variable) has been set as a primary endpoint. The various populations of mouse MSCs in which the effect of tag was investigated consisted of (1) tissue of cell origin: bone marrow vs. Adipose tissue; (2) age of donor: young vs. old; (3) cell culture conditions: hypoxic vs. normal vs. normal + ascorbic acid (AA); (4) exposure to acidosis: yes vs. no. The impact of those populations has been also analyzed and considered as secondary endpoints. The experimental readouts (dependent variables) included: (1) cell viability; (2) cell size; (3) cell doubling time; (4) colony formation; (5) efficiency of labeling; and (6) cell migration. We did not identify any impact of cell labeling for these investigated populations in any of the readouts. In addition, we found that the harsh microenvironment of injured tissue modeled by a culture of cells in a highly acidic environment has a profound effect on all readouts, and both age of donor and cell origin tissue also have a substantial influence on most of the readouts, while oxygen tension in the cell culture conditions has a smaller impact on MSCs. A detailed characterization of the factors that influence the quality of MSCs is vital to the proper pursuit of preclinical and clinical studies.

Stem cell-mediated delivery of SPIO-loaded gold nanoparticles for the theranosis of liver injury and hepatocellular carcinoma

The treatment of liver injuries or hepatocellular carcinoma (HCC) has been hindered by the lack of efficient drug delivery. Even with the help of nanoparticles or other synthetic delivering agents, a large portion of the dose is still sequestered in the reticuloendothelial system. As an alternative, adipose-derived mesenchymal cells (AD-MSCs), which have the capability of homing to the injured liver, can be used as a unique carrier for theranostic agents. Theranostic agents must have the capacity for being non-toxic to host cells during transportation, and for timely activation once they arrive at the injury sites. In this study, we loaded AD-MSCs with superparamagnetic iron oxide-coated gold nanoparticles (SPIO@AuNPs) and tested their effects against liver injury and HCC in cells and in mice. SPIO@AuNP is a non-toxic magnetic resonance (MR)-active contrast agent that can generate heat when irradiated with near-infrared laser. Our results showed that SPIO@AuNPs were successfully transfected into AD-MSCs without compromising either cell viability (P > 0.05) or cell differentiability. In vivo MR imaging and histologic analysis confirmed the active homing of AD-MSCs. Upon laser irradiation, the SPIO@AuNP-loaded AD-MSCs could thermally ablate surrounding HCC tumor cells. SPIO@AuNP-loaded AD-MSCs proved a promising theranostic approach for injured liver and HCC.

Magnetic particle imaging (MPI) for NMR and MRI researchers

Magnetic Particle Imaging (MPI) is a new tracer imaging modality that is gaining significant interest from NMR and MRI researchers. While the physics of MPI differ substantially from MRI, it employs hardware and imaging concepts that are familiar to MRI researchers, such as magnetic excitation and detection, pulse sequences, and relaxation effects. Furthermore, MPI employs the same superparamagnetic iron oxide (SPIO) contrast agents that are sometimes used for MR angiography and are often used for MRI cell tracking studies. These SPIOs are much safer for humans than iodine or gadolinium, especially for Chronic Kidney Disease (CKD) patients. The weak kidneys of CKD patients cannot safely excrete iodine or gadolinium, leading to increased morbidity and mortality after iodinated X-ray or CT angiograms, or after gadolinium-MRA studies. Iron oxides, on the other hand, are processed in the liver, and have been shown to be safe even for CKD patients. Unlike the "black blood" contrast generated by SPIOs in MRI due to increased T2* dephasing, SPIOs in MPI generate positive, "bright blood" contrast. With this ideal contrast, even prototype MPI scanners can already achieve fast, high-sensitivity, and high-contrast angiograms with millimeter-scale resolutions in phantoms and in animals. Moreover, MPI shows great potential for an exciting array of applications, including stem cell tracking in vivo, first-pass contrast studies to diagnose or stage cancer, and inflammation imaging in vivo. So far, only a handful of prototype small-animal MPI scanners have been constructed worldwide. Hence, MPI is open to great advances, especially in hardware, pulse sequence, and nanoparticle improvements, with the potential to revolutionize the biomedical imaging field.

pH-titratable superparamagnetic iron oxide for improved nanoparticle accumulation in acidic tumor microenvironments

A wide variety of nanoparticle platforms are being developed for the diagnosis and treatment of malignancy. While many of these are passively targeted or rely on receptor-ligand interactions, metabolically directed nanoparticles provide a complementary approach. It is known that both primary and secondary events in tumorigenesis alter the metabolic profile of developing and metastatic cancers. One highly conserved metabolic phenotype is a state of up-regulated glycolysis and reduced use of oxidative phosphorylation, even when oxygen tension is not limiting. This metabolic shift, termed the Warburg effect, creates a "hostile" tumor microenvironment with increased levels of lactic acid and low extracellular pH. In order to exploit this phenomenon and improve the delivery of nanoparticle platforms to a wide variety of tumors, a pH-responsive iron oxide nanoparticle was designed. Specifically, glycol chitosan (GC), a water-soluble polymer with pH-titratable charge, was conjugated to the surface of superparamagnetic iron oxide nanoparticles (SPIO) to generate a T(2)*-weighted MR contrast agent that responds to alterations in its surrounding pH. Compared to control nanoparticles that lack pH sensitivity, these GC-SPIO nanoparticles demonstrated potent pH-dependent cellular association and MR contrast in vitro. In murine tumor models, GC-SPIO also generated robust T(2)*-weighted contrast, which correlated with increased delivery of the agent to the tumor site, measured quantitatively by inductively coupled plasma mass spectrometry. Importantly, the increased delivery of GC-SPIO nanoparticles cannot be solely attributed to the commonly observed enhanced permeability and retention effect since these nanoparticles have similar physical properties and blood circulation times as control agents.

Compatibility of superparamagnetic iron oxide nanoparticle labeling for ¹H MRI cell tracking with ³¹P MRS for bioenergetic measurements

Labeling of cells with superparamagnetic iron oxide nanoparticles permits cell tracking by (1)H MRI while (31)P MRS allows non-invasive evaluation of cellular bioenergetics. We evaluated the compatibility of these two techniques by obtaining (31)P NMR spectra of iron-labeled and unlabeled immobilized C2C12 myoblast cells in vitro. Broadened but usable (31)P spectra were obtained and peak area ratios of resonances corresponding to intracellular metabolites showed no significant differences between labeled and unlabeled cell populations. We conclude that (31)P NMR spectra can be obtained from cells labeled with sufficient iron to permit visualization by (1)H imaging protocols and that these spectra have sufficient quality to be used to assess metabolic status. This result introduces the possibility of using localized (31)P MRS to evaluate the viability of iron-labeled therapeutic cells as well as surrounding host tissue in vivo.

Coating optimization of superparamagnetic iron oxide nanoparticles for high T2 relaxivity

We describe a new method for coating superparamagnetic iron oxide nanoparticles (SPIOs) and demonstrate that, by fine-tuning the core size and PEG coating of SPIOs, the T2 relaxivity per particle can be increased by >200-fold. With 14 nm core and PEG1000 coating, SPIOs can have T2 relaxivity of 385 s-1 mM-1, which is among the highest per-Fe atom relaxivities. In vivo tumor imaging results demonstrated the potential of the SPIOs for clinical applications.

Scaling of transverse nuclear magnetic relaxation due to magnetic nanoparticle aggregation

The aggregation of superparamagnetic iron oxide (SPIO) nanoparticles decreases the transverse nuclear magnetic resonance (NMR) relaxation time T2CP of adjacent water molecules measured by a Carr-Purcell-Meiboom-Gill (CPMG) pulse-echo sequence. This effect is commonly used to measure the concentrations of a variety of small molecules. We perform extensive Monte Carlo simulations of water diffusing around SPIO nanoparticle aggregates to determine the relationship between T2CP and details of the aggregate. We find that in the motional averaging regime T2CP scales as a power law with the number N of nanoparticles in an aggregate. The specific scaling is dependent on the fractal dimension d of the aggregates. We find T2CP∝N-0.44 for aggregates with d = 2.2, a value typical of diffusion limited aggregation. We also find that in two-nanoparticle systems, T2CP is strongly dependent on the orientation of the two nanoparticles relative to the external magnetic field, which implies that it may be possible to sense the orientation of a two-nanoparticle aggregate. To optimize the sensitivity of SPIO nanoparticle sensors, we propose that it is best to have aggregates with few nanoparticles, close together, measured with long pulse-echo times.

Caught in the act: in vivo mapping of macrophage infiltration in nerve injury by magnetic resonance imaging

In vivo tracking of hematogenous macrophages has been a major challenge because these cells are key players in nerve injury and repair. We visualized the spatiotemporal course of macrophage infiltration after acute peripheral nerve injury in living rats by using superparamagnetic iron oxide (SPIO) particles and magnetic resonance imaging (MRI). A signal loss on MR images indicating iron accumulation was present in degenerating sciatic nerves between days 1 and 8 after a crush lesion, ceased thereafter, and corresponded to the transient presence of iron-labeled ED1-positive macrophages in tissue sections. In contrast, no SPIO accumulation was seen after optic nerve crush, which revealed microglial activation but lacked macrophage infiltration. SPIO-enhanced MRI provides a new tool to selectively visualize active periods of macrophage transmigration into the nervous system, thus enabling dynamic views on a fundamental process in a multitude of nerve disorders.