Cell separation by immunoaffinity partitioning with polyethylene glycol-modified protein A in aqueous polymer two-phase systems

Past, Present, and Future of Affinity-based Cell Separation Technologies

Progress in cell purification technology is critical to increase the availability of viable cells for therapeutic, diagnostic, and research applications. A variety of techniques are now available for cell separation, ranging from non-affinity methods such as density gradient centrifugation, dielectrophoresis, and filtration, to affinity methods such as chromatography, two-phase partitioning, and magnetic-/fluorescence-assisted cell sorting. For clinical and analytical procedures that require highly purified cells, the choice of cell purification method is crucial, since every method offers a different balance between yield, purity, and bioactivity of the cell product. For most applications, the requisite purity is only achievable through affinity methods, owing to the high target specificity that they grant. In this review, we discuss past and current methods for developing cell-targeting affinity ligands and their application in cell purification, along with the benefits and challenges associated with different purification formats. We further present new technologies, like stimuli-responsive ligands and parallelized microfluidic devices, towards improving the viability and throughput of cell products for tissue engineering and regenerative medicine. Our comparative analysis provides guidance in the multifarious landscape of cell separation techniques and highlights new technologies that are poised to play a key role in the future of cell purification in clinical settings and the biotech industry. STATEMENT OF SIGNIFICANCE: Technologies for cell purification have served science, medicine, and industrial biotechnology and biomanufacturing for decades. This review presents a comprehensive survey of this field by highlighting the scope and relevance of all known methods for cell isolation, old and new alike. The first section covers the main classes of target cells and compares traditional non-affinity and affinity-based purification techniques, focusing on established ligands and chromatographic formats. The second section presents an excursus of affinity-based pseudo-chromatographic and non-chromatographic technologies, especially focusing on magnetic-activated cell sorting (MACS) and fluorescence-activated cell sorting (FACS). Finally, the third section presents an overview of new technologies and emerging trends, highlighting how the progress in chemical, material, and microfluidic sciences has opened new exciting avenues towards high-throughput and high-purity cell isolation processes. This review is designed to guide scientists and engineers in their choice of suitable cell purification techniques for research or bioprocessing needs.

Effects of Coupling Chemistry on the Activity of Poly(ethylene glycol)-Modified Alkaline Phosphatase

Proteins modified by covalent coupling to poly(ethylene glycol) are of interest for several biotechnical applications. In the present work we compare the effects of four commonly used coupling methods on alkaline phosphatase activi ty ; the four methods use the PEG tresylate, succinimidyl succinate, cyanuric chloride derivative, or carbonyl diimidazole derivative. All routes give active enzyme, with only the cyanuric chloride route giving significant deactivation; none the less the cyanuric chloride derivative is useful at lower degrees of modification. Examination of the Michaelis-Menten parameters for the cy anuric chloride coupling suggests that the loss of activity from this route results from intramolecular crosslinking of the protein, which in turn leads to loss of protein conformational flexibility.

Assessment of the Effects of Attaching an Enzyme to Glass by a Poly(ethylene glycol) Tether

Modification of surfaces and proteins by attaching poly(ethylene glycol) (PEG) is an important technique for controlling the properties of these materials. Our goal is to examine the possible combination of these effects by linking proteins to surfaces via a PEG spacer or tether. In the present work we have coupled alkaline phosphatase (as a model protein) to porous glass by means of PEG spacers, and we have compared the activity and operational sta bility of the PEG-bound enzyme to free enzyme and to enzyme immobilized by a conventional, short urea linkage. Significantly, the PEG-bound enzyme differs little in its catalytic properties from free enzyme, indicating that the bound enzyme extends into solution and is in essence free of the surface.

Advances and trends in the design, analysis, and characterization of polymer-protein conjugates for "PEGylaided" bioprocesses

In addition to their use as therapeutics and because of their enhanced properties, PEGylated proteins have potential application in fields such as bioprocessing. However, the use of PEGylated conjugates to improve the performance of bioprocess has not been widely explored. This limited additional industrial use of PEG-protein conjugates can be attributed to the fact that PEGylation reactions, separation of the products, and final characterization of the structure and activity of the resulting species are not trivial tasks. The development of bioprocessing operations based on PEGylated proteins relies heavily in the use of analytical tools that must sometimes be adapted from the strategies used in pharmaceutical conjugate development. For instance, to evaluate conjugate performance in bioprocessing operations, both chromatographic and non-chromatographic steps must be used to separate and quantify the resulting reaction species. Characterization of the conjugates by mass spectrometry, circular dichroism, and specific activity assays, among other adapted techniques, is then required to evaluate the feasibility of using the conjugates in any operation. Correct selection of the technical and analytical methods in each of the steps from design of the PEGylation reaction to its final engineering application will ensure success in implementing a "PEGylaided" process. In this context, the objective of this review is to describe technological and analytical trends in developing successful applications of PEGylated conjugates in bioprocesses and to describe potential fields in which these proteins can be exploited.

Affinity separation by protein conjugated IgG in aqueous two-phase systems using horseradish peroxidase as a ligand carrier

A novel affinity separation method in an aqueous two-phase system (ATPS) is suggested, using protein conjugated IgG as a ligand. For verification of the proposed approach, horseradish peroxidase (HRP) and human IgG was used as a ligand carrier and affinity ligand, respectively. The partition of the affinity ligand, human IgG, was controlled by the conjugation of HRP. Two ATPSs, one consisting of potassium phosphate (15%, w/w) and polyethylene glycol (PEG, M.W. 1450, 10%, w/w) and the other of dextran T500 (5%, w/w) and PEG (M.W. 8000, 5%, w/w), were used. The conjugated human IgG-HRP favored a PEG-rich top phase, whereas human IgG, rabbit anti-human IgG and goat anti-mouse IgG preferred a salt or dextran-rich bottom phase. Using the conjugated human IgG-HRP, rabbit anti-human IgG was successfully separated into a PEG-rich top phase from the mixture with goat anti-mouse IgG. The appropriate molar ratio between human IgG-HRP and rabbit anti-human IgG was around 3:1 and 1:1 for the salt and dextran-based ATPS, respectively. The dextran-based ATPS showed a better recovery yield and purity than the salt-based ATPS for the range of test conditions employed in this experiment. The yield and purity of the recovered rabbit anti-human IgG were 90.8 and 87.7%, respectively, in the dextran-based ATPS, while those in the salt-based ATPS were 78.2 and 73.2%.

Effects of specific binding reactions on the partitioning behavior of biomaterials

Affinity partitioning is a special branch of biomaterials separations using aqueous two-phase systems. It combines the capability of diverse biomolecules to partition in aqueous two-phase systems using the principle of biorecognition. As a result, the macromolecule exhibiting affinity for a certain ligand is transferred to that phase where the ligand is present. This chapter describes the present status of the theoretical background of this approach and the properties of various natural and artificial compounds which act as affinity ligands in liquid-liquid systems. The affinity partitioning of proteins (enzymes and plasma proteins), cell membranes, cells, and nucleic acids are described as typical examples. The results are discussed in terms of theoretical understanding and practical application.

Analytical partitioning of poly(ethylene glycol)-modified proteins

Covalently grafting proteins with varying numbers (n) of poly(ethylene glycol) molecules (PEGs) often enhances their biomedical and industrial usefulness. Partition between the phases in aqueous polymer two-phase systems can be used to rapidly characterize polymer-protein conjugates in a manner related to various enhancements. The logarithm of the partition coefficient (K) approximates linearity over the range O<n<x. However, x varies with the nature of the conjugate (e.g., protein molecular mass) and such data analysis does not facilitate the comparison of varied conjugates. The known behavior of surface localized PEGs suggests a better correlation should exist between log K and the weight fraction of polymer in PEG-protein conjugates. Data from four independent studies involving three proteins (granulocyte-macrophage colony stimulation factor, bovine serum albumin and immunoglobulin G) has been found to support this hypothesis. Although somewhat simplistic, 'weight fraction' based analysis of partition data appears robust enough to accommodate laboratory to laboratory variation in protein, polymer and phase system type. It also facilitates comparisons between partition data involving disparate polymer-protein conjugates.

Poly(ethylene glycol) amphiphile adsorption and liposome partition

Surface localized poly(ethylene glycol) (PEG) amphiphiles of type C16:0-EO151 and C18:2-EO151 were studied via ellipsometry at macroscopic, flat methylated silica (MeSi), phosphatidic acid (PA), and phosphatidylcholine (PC) surfaces. At these surfaces the amphiphiles adsorb similarly, in a non-cooperative manner, achieving a plateau (approximately 0.1 PEG chains/nm2) well below amphiphile critical micelle concentration (CMC). The resultant PEG-enriched layers were 10-15 nm thick, with a polymer concentration (approximately 0.07 g/cm3) greater than the PEG-enriched phase of many dextran, PEG aqueous two-phase systems. PEG-amphiphile adsorption (mg/m2) at hydrophobic and phospholipid flat surfaces correlated with changes in the partition (log K) of PC liposomes in such two-phase systems. PEG-amphiphile adsorption at macroscopic surfaces appears to represent a balance between hydrophobic attraction and repulsive intra-chain interactions which promote chain elongation normal to the surface.

Polyethylene glycol-modified avidin: a novel agent for the selective extraction of biotinylated immune-complex in an aqueous two-phase system

Chicken avidin was chemically modified with 2,4-bis[O-methoxypoly(ethylene glycol)]-6-chloro-s-triazine (activated PEG2) to form PEG-avidin. The PEG-avidin, in which 78% of the amino groups were modified, retained 49% of the active biotin-binding sites. The modified avidin was partitioned preferentially into the PEG-phase in an aqueous two-phase system (PEG/dextran). Using PEG-avidin, the immune-complex formed between biotinylated anti-mouse IgG and its antigen IgG (mouse) molecules, was successfully transferred into the PEG-phase in an aqueous two-phase system. This finding leads to the effective isolation of a specific antigen among various kinds of antigens by partitioning with a two-phase system using PEG-avidin.

Quantitative analysis of polyethylene glycol (PEG) in PEG-modified proteins/cytokines by aqueous two-phase systems

Covalent attachment of poly(ethylene glycol) (PEG) to proteins produces conjugates with altered/improved physicochemical and biological properties which depend upon the number of PEG chains linked. Quantification of the attached PEG is however not a trivial issue. The partition coefficient, K, of the PEG-protein conjugate in PEG/dextran two-phase systems provides a quantitative measure for the degree of modification. A linear relationship between log K and the number of PEG chains was observed in fractionated PEG-modified-granulocyte-macrophage colony stimulating factor conjugates having 1 to 3 substitutions. Furthermore, in mixtures of PEG-bovine-serum-albumin conjugates with increasing degrees of modification, a linear relationship was found between log K and n, the average substitution. The increment in log K per PEG chain added is protein specific and this suggests that the interactions between the PEG-protein conjugate and the polymers in the phase system are more complex than just a simple affinity of the PEG for the PEG-rich top phase. Increasing the polymer concentration in the phase system produces larger increments in log K per PEG molecule attached and the proportionality between log K and number of PEG molecules is only compromised for conjugates with high degree of substitution when partitioned in biphasic systems of high concentration of polymers.

Fractionation of erythroblasts with affinity-mediated modifications of their electrical properties using counter-current distribution

We have previously reported the possibility of modifying the electrical properties of cells by means of their interaction with a specific ligand carrying a polyelectrolyte (Anal Biochem 200: 280-285). This selective modification of receptor-containing cells changed their partition in a charge-sensitive aqueous two-phase system. We here present the fractionation of electrically modified erythroblasts by the use of an automatic multiple-partition procedure, counter-current distribution. The cells were fractionated according to the degree of differentiation of erythroblasts as evaluated from the hemoglobin content as well as the relative activities of the two enzymes, 3-phosphoglycerate kinase and bisphospho-glycerate mutase.

Ligand-receptor interactions in affinity cell partitioning. Studies with transferrin covalently linked to monomethoxypoly(ethylene glycol) and rat reticulocytes

The partitioning of rat reticulocytes in poly(ethylene glycol) (PEG)-dextran two-phase systems increases into the PEG-rich top phase when the cells are incubated with transferrin covalently modified with monomethoxy-PEG (MPEG-transferrin) prior to partitioning. Two observations support the suggestion that such an increase in top-phase partitioning is due to the specific interaction of the MPEG-transferrin conjugate with the transferrin receptor on the surface of the reticulocyte: first, the MPEG-transferrin conjugate competes with [125I]transferrin for the transferrin receptor on reticulocytes (Ka = 6.28 x 10(6) l mol-1); and second, the MPEG-modified transferrin is unable to change the partitioning of rat erythrocytes, cells lacking the transferrin receptor. This example illustrates the feasibility of manipulating the partitioning of a selected cell population when ligand-receptor interactions are exploited. The increase in the partitioning of the reticulocytes takes place within a narrow range of MPEG-transferrin bound per cell, viz., 10.2-11.3 fg per cell. The latter range corresponds to ca. 80,000-89,000 molecules of MPEG-transferrin bound per cell.

Separation of cell mixtures by immunoaffinity cell partitioning: strategies for low abundance cells

The partitioning of cells in aqueous two-phase systems formed by poly(ethylene glycol) (PEG) and dextran can be changed by incubating the cells with a PEG-modified antibody directed specifically against its surface. We have developed a new approach for immunoaffinity cell partitioning (IACP) in which the antibodies are first reacted with tresylated monomethoxy PEG (TMPEG) in sodium phosphate buffer, pH 7.5, the excess TMPEG is quenched by reaction with bovine serum albumin, and the resulting preparation is used directly for incubation with the cells without any isolation of the monomethoxyPEG (MPEG)-antibody conjugates. We have demonstrated the specificity of this IACP method by showing that MPEG-modified anti-human red blood cell antibody increases the partition of human erythrocytes from the interface to the PEG-rich top phase (up to 100%) but not the partitioning of either neutrophils or HL60 cells. Irrelevant antibodies do not affect the partitioning of red blood cells. The partitioning behaviors of erythrocytes and HL60 cells in mixtures varying from 75 to 10% red blood cells subjected to IACP are similar to those of the pure cell population, i.e., erythrocytes ca. 100% and HL60 cells 3% in top phase. Thus, the population of erythrocytes can be almost completely extracted into the top phase in a single step. The contaminant cells represent only a small percentage (less than 5% in most of the cases) of the cell mixture recovered in top phase. Both cell populations can be completely separated by countercurrent distribution (CCD).(ABSTRACT TRUNCATED AT 250 WORDS)

Polyethylene glycol significantly enhances the transfer of membrane immunoblotting

Poly(ethylene glycol)n is a group of water-soluble, hydrophobic, optically transparent and biomacromolecule-nondenaturing polymers. These properties have caused it be widely used for various purposes in the biological sciences. In this study, the effects of poly(ethylene glycol)n on protein preservation, electrotransferring, and immunoblotting from sodium dodecyl sulfate (SDS)-polyacrylamide gel onto polyvinylidene difluoride (PVDF) membrane have been systematically evaluated. After SDS-polyacrylamide gel electrophoresis, 30% poly(ethylene glycol)n may be applied to reversibly fix proteins within the gel more completely, differing from irreversible fixation produced by solutions such as trichloroacetic acid-sulfosalicylic acid or acetic acid-methanol systems. The intragel proteins, fixed by poly(ethylene glycol)n, can be electroblotted directly onto PVDF membranes in the presence of 30% poly(ethylene glycol)n. We have shown that treatment with poly(ethylene glycol)n may reduce background, raise signal-to-noise ratio, sharpen protein bands, and increase resolution, resulting in enhancement of the immunoblotting transfer. It is possible to visualize a few picograms of a single protein band, increasing the sensitivity of the method by 10- to 100-fold, as compared with standard immunoblotting techniques.