Background

[PubMed]

Magnetic resonance imaging (MRI) maps information about tissues spatially and functionally. Protons (hydrogen nuclei) are widely used to create images because of their abundance in water molecules, which comprise >80% of most soft tissues. The contrast of proton MRI images depends mainly on the density of nuclear proton spins, the relaxation times of the nuclear magnetization (T1, longitudinal; T2, transverse), the magnetic environment of the tissues, and the blood flow to the tissues. However, insufficient contrast between normal and diseased tissues requires the use of contrast agents. Most contrast agents affect the T1 and T2 relaxation times of the surrounding nuclei, mainly the protons of water. T2* is the spin–spin relaxation time composed of variations from molecular interactions and intrinsic magnetic heterogeneities of tissues in the magnetic field (1). Cross-linked iron oxide (CLIO) and other iron oxide formulations affect T2 primarily and lead to a decreased signal. On the other hand, paramagnetic T1 agents, such as gadolinium (Gd³⁺) and manganese (Mn²⁺), accelerate T1 relaxation and lead to brighter contrast images.

The superparamagnetic iron oxide (SPIO) structure is composed of ferric iron (Fe³⁺) and ferrous iron (Fe²⁺). These particles have large combined magnetic moments or spins, which are randomly rotated in the absence of an applied magnetic field. SPIO is used mainly as a T2 contrast agent in MRI, though it can shorten both T1 and T2/T2* relaxation processes. SPIO particle uptake into the reticuloendothelial system (RES) is by endocytosis or phagocytosis. SPIO particles are also taken up by phagocytic cells such as monocytes, macrophages, and oligodendroglial cells. SPIO nanoparticles are sometimes modified with dextran, poly(ethylene glycol) (PEG), or other biomaterials to reduce RES uptake and to prolong blood half-life. A variety of cells can also be labeled with these particles for cell trafficking and tumor-specific molecular imaging studies. SPIO agents are classified by their sizes with coating material (~20–3,500 nm in diameter) as large SPIO (LSPIO) nanoparticles, standard SPIO (SSPIO) nanoparticles, ultrasmall SPIO (USPIO) nanoparticles, and monocrystalline iron oxide nanoparticles (MION) (1).

The complement system consists of >30 proteins and acts as important part of the innate immune system. Complement activation helps eliminate pathogens during infection but also causes injury to host tissues in autoimmune diseases such as systemic lupus erythematosus (2) and allergic asthma (3). Most lupus patients develop renal abnormalities with glomerular deposition of C3 fragments (C3b, iC3b, and C3d) (4). Complement receptor type 2 (CR2, CD21) binds C3 fragments but does not bind C3 (5). Serkova et al. (6) conjugated a recombinant protein containing the C3d-binding region of CR2 to SPIO nanoparticles. CR2-Fc-SPIO nanoparticles were evaluated in a mouse model of lupus nephritis.

Synthesis

[PubMed]

A recombinant protein containing the C3d-binding region of CR2 was linked to the Fc portion of mouse IgG₁ (6). The recombinant protein (6.7 µmol) was incubated with SPIO (10 nm in diameter) containing 1.5 nmol Fe, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, and N-hydroxysulfosuccinimide for 2 h at room temperature. CR2-Fc-SPIO nanoparticles were purified with high-speed centrifugation. The copies of protein per SPIO were not reported.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

Chinese hamster ovary (CHO) cells were incubated with 10% normal mouse serum to opsonize C3 fragments on their cell surface. The binding of serum proteins on the cell surface enhanced phagocytosis of C3 fragments in the serum. Flow cytometry analysis confirmed binding of CR2-Fc-SPIO to the opsonzied CHO cells, whereas SPIO did not bind to the opsonzied CHO cells. Furthermore, CR2-Fc-SPIO did not bind to unopsonized CHO cells. Cy3-Labeled anti-mouse IgG was used to detect CR2-Fc-SPIO.

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

NIH Support

DK076690, AI31105, P30 CA046934, T32 CA079446

References

1.

Wang Y.X., Hussain S.M., Krestin G.P. Superparamagnetic iron oxide contrast agents: physicochemical characteristics and applications in MR imaging. Eur Radiol. 2001;11(11):2319–31. [PubMed: 11702180]

2.

Bao L., Quigg R.J. Complement in lupus nephritis: the good, the bad, and the unknown. Semin Nephrol. 2007;27(1):69–80. [PubMed: 17336690]

3.

Zhang X., Kohl J. A complex role for complement in allergic asthma. Expert Rev Clin Immunol. 2010;6(2):269–77. [PMC free article: PMC2856962] [PubMed: 20402389]

4.

Rovin B.H., McKinley A.M., Birmingham D.J. Can we personalize treatment for kidney diseases? Clin J Am Soc Nephrol. 2009;4(10):1670–6. [PubMed: 19808246]

5.

Kovacs, J.M., J.P. Hannan, E.Z. Eisenmesser, and V.M. Holers, Biophysical investigations of complement receptor 2 (CD21, CR2) ligand Interactions reveal amino acid contacts unique to each receptor-ligand pair. J Biol Chem, 2010. [PMC free article: PMC2930724] [PubMed: 20558730]

6.

Serkova N.J., Renner B., Larsen B.A., Stoldt C.R., Hasebroock K.M., Bradshaw-Pierce E.L., Holers V.M., Thurman J.M. Renal inflammation: targeted iron oxide nanoparticles for molecular MR imaging in mice. Radiology. 2010;255(2):517–26. [PMC free article: PMC2858816] [PubMed: 20332377]