Quantification of Cellular Densities and Antigenic Properties using Magnetic Levitation

Summary

This paper describes a magnetic levitation-based method that can specifically detect the presence of antigens, either soluble or membrane-bound, by quantifying changes in the levitation height of capture beads with fixed densities.

Abstract

The described method was developed based on the principles of magnetic levitation, which separates cells and particles based on their density and magnetic properties. Density is a cell type identifying property, directly related to its metabolic rate, differentiation, and activation status. Magnetic levitation allows a one-step approach to successfully separate, image and characterize circulating blood cells, and to detect anemia, sickle cell disease, and circulating tumor cells based on density and magnetic properties. This approach is also amenable to detecting soluble antigens present in a solution by using sets of low- and high-density beads coated with capture and detection antibodies, respectively. If the antigen is present in solution, it will bridge the two sets of beads, generating a new bead-bead complex, which will levitate in between the rows of antibody-coated beads. Increased concentration of the target antigen in solution will generate a larger number of bead-bead complexes when compared to lower concentrations of antigen, thus allowing for quantitative measurements of the target antigen. Magnetic levitation is advantageous to other methods due to its decreased sample preparation time and lack of dependance on classical readout methods. The image generated is easily captured and analyzed using a standard microscope or mobile device, such as a smartphone or a tablet.

Introduction

Magnetic levitation is a technique developed to separate, analyze, and identify cell types^1,2,3, proteins^4,5 and opioids⁶ based solely on their specific density and paramagnetic properties. Cell density is a unique, intrinsic property of each cell type directly related to its metabolic rate and differentiation status^7,8,9,10,11<....

Protocol

The experimental protocol used in this study was approved by the Beth Israel Deaconess Medical Center Institutional Review Board (IRB).

1. Instrument setup

NOTE: Imaging levitating cells requires two rare earth neodymium magnets magnetized on the z-axis to be placed with the same pole facing each other to generate a magnetic field. The distance between the magnets can be customized depending on the intensity of the magnetic field and the density of the targets. In this case the magnets are separated by a 1mm space sufficient for insertion of a 50 mm long 1x1 mm squared glass capillary tube. The device was 3-D ....

Results

Magnetic levitation focuses objects of different densities at different levitation heights depending on the object's density, its magnetic signature, the concentration of paramagnetic solution, and the strength of the magnetic field created by two strong, rare-earth magnets. As the two magnets are placed on top of each other, levitating samples can only be viewed, while maintaining Köhler illumination, by using a microscope turned on its side (Figure 1). The final levitation height .......

Discussion

Gradient centrifugation is currently the standard technique for isolating subcellular components based on their unique densities. This approach, however, requires the use of specialized gradient media as well as centrifuge equipment. The magnetic levitation approach presented here allows detailed investigation of the morphological and functional properties of circulating cells, with minimum, if any manipulation of the cells, providing a near in vivo access to circulating cells.

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Materials

Name	Company	Catalog Number	Comments
2-(N-Morpholino)ethanesulfonic acid hydrate	Sigma Aldrich	M-2933	(MES); component of activation buffer
50x2.5x1 mm magnets, Nickel (Ni-Cu-Ni) plated, grade N52, magnetized through 5mm (0.197") thickness	K&J Magnetics	Custom	Magnets used for the magnetic levitation device
Capillary Tube Sealant (Critoseal)	Leica Microsystems	267620	Used to cap the ends of the capillary tubes
Centrifuge tube filters (Corning Costar Spin-X)	Sigma Aldrich	CLS8163	Used to wash beads
Compact Lab Jack	Thorlabs	LJ750	Used for adjusting the magnetic levitation device
DPBS, no calcium, no magnesium	Gibco	14190-144	Solution for bead suspensions
Ethanolamine	Sigma Aldrich	E9508-100ML	Used during a wash step for beads
Fluorescent Plasma Membrane Stain (CellMask Green)	Invitrogen	C37608	Used to stain Rh+ cells
Gadoteridol Injection	ProHance	NDC 0270-1111-03	Gadolinium (Gd3+); magnetic solution used to suspend cells
HBSS++	Gibco	14025-092	Solution for sample preparation
Human C5b,6 complex	Complement Technology, Inc	A122	Used to generate RBC Evs
Human C7 protein	Complement Technology, Inc	A124	Used to generate RBC Evs
Human C8 protein	Complement Technology, Inc	A125	Used to generate RBC Evs
Human C9 protein	Complement Technology, Inc	A126	Used to generate RBC Evs
Mini Series Post Collar	Thorlabs	MSR2	Used to secure magnetic levitation device to lab jacks
N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride	Sigma Aldrich	E1769-10G	(EDC); used in antibody coupling reaction
Normal Rabbit IgG Control	R&D Systems	AB-105-C	Used to coat beads as a control condition
Phosphate Buffered Saline (10X Solution, pH 7.4)	Boston Bioproducts	BM-220	Component of coupling buffer, used for washing steps
Polysorbate 20 (Tween 20)	Sigma Aldrich	P7949-500ML	Component of activation buffer
Polystyrene Carboxyl Polymer	Bangs Laboratories	PC06004	Top density beads (1.05 g/mL), used for antibody coupling
Rabbit RhD Polyclonal Antibody	Invitrogen	PA5-112694	Used to coat beads for the dectection of Rh factor in red blood cells
Research Grade Microscope	Olympus	Provis AX-70	Microscoped used to mount magnetic levitation device and view levitating cells
Rubber Dampening Feet	Thorlabs	RDF1	Used to support the breadboard table
Square Boro Tubing	VitroTubes	8100-050	Capillary tube used for loading sample into Maglev
Sulfo-NHS	Thermoscientific	24510	Used in antibody coupling reaction
Translational Stage	Thorlabs	PT1	Used for focusing and for scanning capillary tube