Immunomagnetic isolation of magnetoferritin-labeled cells in a modified ferrograph.

Pan T, helper, and cytotoxic lymphocytes were isolated from the human peripheral blood mononuclear cell fraction by antibody staining, ferritin labeling, and deposition on glass slides. Two distinct forms of ferritin were used: one was native horse spleen ferritin, and the other was magnetoferritin. Magnetoferritin was obtained by reconstituting the horse spleen ferritin iron core with superparamagnetic magnetite instead of the usual paramagnetic ferrihydrite crystal. The cell deposition on microscopic glass slides in the magnetic field was obtained by an instrument that was adapted from an industrial magnetic deposition analyzer, the ferrograph. The identity of cells in the magnetic deposits was confirmed by comparing the cell fractions in the feed and in the eluate with the use of flow cytometry. The immunostaining protocol amplified the number of ferritin molecules per cell surface antigen 20-70 times. Magnetoferritin, but not native ferritin, imparted a sufficient magnetic moment to cells to deplete the labeled cell population between 67 and 88% of its initial concentration in a magnetic field of 1.67 Tesla (T), a field gradient of 2.57 T/mm, and a flow rate of 0.01 ml/min. This study showed that the magnetic moment of magnetoferritin was sufficient for immunomagnetic isolation of lymphocytes from mononuclear cell preparations in the modified ferrograph.

Initial assessment of magnetoferritin biokinetics and proton relaxation enhancement in rats.

RATIONALE AND OBJECTIVES: We evaluated the biokinetics and proton relaxation enhancement of magnetoferritin, a recently developed class of superparamagnetic iron oxides, in rats. METHODS: "Equine" magnetoferritin was administered intravenously at 5 mg protein and 1.4 mg Fe/kg in nude rats carrying subcutaneous xenografted human small-cell lung carcinoma with and without preinjection of 100 mg/kg equine apoferritin. Blood clearance, in vivo biodistribution, and proton relaxation enhancement were assessed by variable field relaxometry, immunohistochemistry, and magnetic resonance (MR) imaging at 1.5 T. RESULTS: Magnetoferritin clearance from blood followed biexponential kinetics, with a short initial half-life of 1.4-1.7 min. A second, longer component lasted for several hours. Histochemical staining, MR imaging, and ex vivo relaxometry revealed rapid uptake of magnetoferritin in the liver, spleen, and lymph nodes. There was no difference in biodistribution after apoferritin preinjection. CONCLUSION: In the rat, equine magnetoferritin is rapidly sequestered by cells of the reticuloendothelial system, with no direct involvement of ferritin receptors. These properties may allow the use of magnetoferritin as an MR contrast agent for the liver and spleen.

Classical and quantum magnetic phenomena in natural and artificial ferritin proteins.

Artificial ferritin has been synthesized with control of both the magnetic state (antiferromagnetic or ferrimagnetic) and the particle size over an ofer of magnitude in the number of iron atoms. The magnetic properties of the artificial ferritin were compared with those of natural horse spleen ferritin in a range of temperatures (20 millikelvin to 300 kelvin) and fields (1 nanotesla to 27 tesla). In the classical regime, the blocking temperature was found to correlate with the average particle size. A correlation was also observed in the quantum regime between the resonance frequency of macroscopic quantum tunneling of the Néel vector and the particle size. At high magnetic fields (to 27 tesla), a spin flop transition with a strong dependence on orientation was seen in the natural ferritin, providing evidence of antiferromagnetism in this system.

Magnetoferritin: characterization of a novel superparamagnetic MR contrast agent.

A protein-encaged superparamagnetic iron oxide has been developed and characterized by using horse spleen apoferritin as a novel bioreactive environment. The roughly spherical magnetoferritin molecules, 120 A in diameter, are composed of a monocrystalline maghemite or magnetite core 73 A +/- 14 in diameter. Except for the additional presence of iron-rich molecules of higher molecular weight, the appearance and molecular weight (450 kd) of magnetoferritin are identical to that of natural ferritin; the molecules are externally indistinguishable from their precursor, with a pI (isoelectric point) in the range 4.3-4.6. The measured magnetic moment of the superparamagnetic cores is 13,200 Bohr magnetons per molecule, with T1 and T2 relaxivities (r1 and r2) of 8 and 175 L.mmol-1 (Fe).sec-1, respectively, at body temperature and clinical field strengths. The unusually high r2/r1 ratio of 22 is thought to arise from ideal core composition, with no evidence of crystalline paramagnetic inclusions. T2 relaxation enhancement can be well correlated to the field-dependent molecular magnetization, as given by the Langevin magnetization function, raised to a power in the range 1.4-1.6. With its nanodimensional biomimetic protein cage as a rigid, convenient matrix for complexing a plethora of bioactive substances, magnetoferritin may provide a novel template for specific targeting of selected cellular sites.