A model for the initial phase of cell/surface interactions based on ligand binding phenomena

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.

PREPARATION OF PLLA MEMBRANES WITH DIFFERENT MORPHOLOGIES FOR CULTURE OF LIGAMENT CELLS

Poly (lactic acid) is a biodegradable biomedical material that has been used for connective tissue reconstruction. In this work, poly-L-lactide (PLLA) membranes with different morphologies were prepared by phase separation method. Otherwise, biomaterials coated with various extracellular matrix (ECM) have been shown to promote cell adhesion, proliferation, and differentiation. In addition, the in vitro interaction of medial collateral ligament cells (MCLs) and PLLA membranes with dense, porous and particulate morphologies and with ECM coating was investigated. It was found that the cell compatibility of three types of PLLA membranes almost the same before coating ECM. The results also revealed that collagen type I could improve ligament cells adhesion and fibronectin could improve ligament cells growth, and this effect was most obvious in particulate membrane. Therefore, because the PLLA materials with particulate structure could adsorb more ECM which in turn influenced the cell adhesion and cell growth. The PLLA membrane with the particulate morphology satisfies the biomaterial requirement necessary for temporary scaffold to transplanted ligament cells and provides a means for the architectural design of more complex tissue-engineered systems.

Affinity adsorption of cells to surfaces and strategies for cell detachment

The use of bio-specific interactions for the separation and recovery of bio-molecules is now widely established and in many cases the technique has successfully crossed the divide between bench and process scale operation. Although the major specificity advantage of affinity-based separations also applies to systems intended for cell fractionation, developments in this area have been slower. Many of the problems encountered result from attempts to take techniques developed for molecular systems and, with only minor modification to the conditions used, apply them for the separation of cells. This approach tends to ignore or at least trivialise the problems, which arise from the heterogeneous nature of a cell suspension and the multivalent nature of the cell/surface interaction. To develop viable separation processes on a larger scale, effective contacting strategies are required in separators that also allow detachment or recovery protocols that overcome the enhanced binding strength generated by multivalent interactions. The effects of interaction valency on interaction strength needs to be assessed and approaches developed to allow effective detachment and recovery of adsorbed cells without compromising cell viability. This article considers the influence of operating conditions on cell attachment and the extent to which multivalent interactions determine the strength of cell binding and subsequent detachment.

Folate targeting of drug carriers: A mathematical model

The binding of folate-targeted drug carriers to a receptor-bearing cell surface is modeled using a deterministic approach. The model accounts for the presence of multiple folate ligands on the carrier surface, the anchoring and presentation of the ligands on flexible polymeric tethers, and the combination of both clustered and homogeneous spatial distributions of receptors on the cell surface. The model was validated against an experimental system where folate-bearing liposomes were used as delivery vehicles to deliver drug to tumor cells in vitro. Unknown parameters of the model were then estimated by a least-squares fit to the experimental data. A parametric study systematically varying the estimated parameters around the best-fit values indicated that the model was sensitive to these parameters, lending credence to their estimated values. This study indicates that drug uptake is dependent on several factors including the ligand number, the exposure time, and carrier concentration. For the specific case of folate targeting, the cumulative uptake of folate ligands is important, causing a decrease in the carrier uptake rate once a threshold cumulative uptake is crossed.

Anterior cruciate ligament regeneration using braided biodegradable scaffolds: in vitro optimization studies

The anterior cruciate ligament (ACL) is the most commonly injured intra-articular ligament of the knee, and limitations in existing reconstruction grafts have prompted an interest in tissue engineered solutions. Previously, we reported on a tissue-engineered ACL scaffold fabricated using a novel, three-dimensional braiding technology. A critical factor in determining cellular response to such a graft is material selection. The objective of this in vitro study was to optimize the braided scaffold, focusing on material composition and the identification of an appropriate polymer. The selection criteria are based on cellular response, construct degradation, and the associated mechanical properties. Three compositions of poly-alpha-hydroxyester fibers, namely polyglycolic acid (PGA), poly-L-lactic acid (PLLA), and polylactic-co-glycolic acid 82:18 (PLAGA) were examined. The effects of polymer composition on scaffold mechanical properties and degradation were evaluated in physiologically relevant solutions. Prior to culturing with primary rabbit ACL cells, scaffolds were pre-coated with fibronectin (Fn, PGA-Fn, PLAGA-Fn, PLLA-Fn), an important protein which is upregulated during ligament healing. Cell attachment and growth were examined as a function of time and polymer composition. While PGA scaffolds measured the highest tensile strength followed by PLLA and PLAGA, its rapid degradation in vitro resulted in matrix disruption and cell death over time. PLLA-based scaffolds maintained their structural integrity and exhibited superior mechanical properties over time. The response of ACL cells was found to be dependent on polymer composition, with the highest cell number measured on PLLA-Fn scaffolds. Surface modification of polymer scaffolds with Fn improved cell attachment efficiency and effected the long-term matrix production by ACL cells on PLLA and PLAGA scaffolds. Therefore based on the overall cellular response and its temporal mechanical and degradation properties in vitro, the PLLA braided scaffold pre-coated with Fn was found to be the most suitable substrate for ACL tissue engineering.

Modulating fibroblast adhesion, spreading, and proliferation using self-assembled monolayer films of alkylthiolates on gold

Ultrathin, highly organized functionalized alkylthiol monolayers were applied as model substrates for cell growth and protein adsorption studies. The aim of this approach was to improve the understanding of molecular surface determinants required for adhesion-dependent cell growth and proliferation using well-controlled surface chemistry. Carboxyl- and methyl-terminated alkylthiol monolayers on gold were used to monitor Swiss 3T3 fibroblast adhesion, spreading, and growth. Stress fiber and focal contact formation were determined by immunostaining of actin filaments and paxillin. Fibronectin deposition and conformation on these surface chemistries in the presence and absence of competing proteins were also determined. The relative levels of adsorbed fibronectin were assessed using radiolabeled proteins. Exposure of the 10th type III cell integrin binding domain of fibronectin was assessed using a radiolabeled monoclonal antibody. Distinct alkylthiol substrate chemistry-dependent differences were observed in fibroblast adhesion, spreading, and growth. The formation of focal contacts and stress fibers was enhanced on the carboxyl-terminated surface relative to the methyl surface. Relative deposition and conformations of adsorbed fibronectin were shown to be dependent on surface chemistry in both the presence and absence of competing proteins. The results indicated that well-controlled culture surfaces modulate differential cell adhesion, spreading, and growth through modulations of the amounts and conformations of adsorbed extracellular matrix molecules (e.g., fibronectin).