Cells are promising as carriers that can enhance the delivery of nanomedicines. Cells that carry nanomedicinal cargo, either immobilized on the cell surface or internalized, can allow for highly specific delivery and can enable the transport of nanomedicines across challenging physiological barriers. The effective use of cells as carriers for the transport and delivery of nanomedines requires a careful selection of the chemical strategies that are used to load the cell-based carriers with their cargo. To this end, an in-depth understanding of the impact of various cell-surface modification chemistries on the viability and functional properties of the cells is essential, and techniques are needed that allow characterization of nanoparticle-modified living cells. This article touches upon both of these aspects. The first part of this review will present an overview of contemporary strategies that are available for the cell surface immobilization of nanoparticle cargo. After that, the various techniques that are most frequently used for the characterization of nanoparticle-modified cells will be discussed.

Cells are attractive carriers for the transport and delivery of nanoparticulate cargo. The use of cell-based carriers allows one to enhance control over the biodistribution of drugloaded polymers and polymer nanoparticles. One key element in the development of cell-based delivery systems is the loading of the cell-based carrier with the nanoparticle cargo, which can be achieved either by internalization of the payload or by immobilization on the cell surface. The surface modification of cells with nanoparticles or the internalization of nanoparticles by cells is usually monitored with fluorescence-based techniques, such as flow cytometry and confocal microscopy. In spite of the widespread use of these techniques, the use of fluorescent labels also poses some risks and has several drawbacks. Fluorescent dyes may bleach, or leach from, the nanoparticles or alter the physicochemical properties of nanoparticles and their interactions with and uptake by cells. Using poly(D,L-lactic acid) nanoparticles that are loaded with Coumarin 6, BODIPY 493/503, or DiO dyes as a model system, this paper demonstrates that the use of physically entrapped fluorescent labels can lead to false negative or erroneous results. The use of nanoparticles that contain covalently tethered fluorescent dyes instead was found to provide a robust approach to monitor cell surface conjugation reactions and to quantitatively analyze nanoparticle-decorated cells. Finally, it is shown that optical diffraction tomography is an attractive, alternative technique for the characterization of nanoparticle-decorated cells, which obviates the need for fluorescent labels.

Cells are attractive as carriers that can help to enhance control over the biodistribution of polymer nanomedicines. One strategy to use cells as carriers is based on the cell surface immobilization of the nanoparticle cargo. While a range of strategies can be used to immobilize nanoparticles on cell surfaces, only limited effort has been made to investigate the effect of these surface modification chemistries on cell viability and functional properties. This study has explored seven different approaches for the immobilization of poly(lactic acid) (PLA) nanoparticles on the surface of two different T lymphocyte cell lines. The cell lines used were human Jurkat T cells and CD4 + T EM cells. The latter cells possess blood−brain barrier (BBB) migratory properties and are attractive for the development of cell-based delivery systems to the central nervous system (CNS). PLA nanoparticles were immobilized either via covalent active ester−amine, azide−alkyne cycloaddition, and thiol−maleimide coupling, or via noncovalent approaches that use lectin−carbohydrate, electrostatic, or biotin−NeutrAvidin interactions. The cell surface immobilization of the nanoparticles was monitored with flow cytometry and confocal microscopy. By tuning the initial nanoparticle/cell ratio, T cells can be decorated with up to ∼185 nanoparticles/cell as determined by confocal microscopy. The functional properties of the nanoparticle-decorated cells were assessed by evaluating their binding to ICAM-1, a key protein involved in the adhesion of CD4 + T EM cells to the BBB endothelium, as well as in a two-chamber model in vitro BBB migration assay. It was found that the migratory behavior of CD4 + T EM cells carrying carboxylic acid-, biotin-, or Wheat germ agglutinin (WGA)-functionalized nanoparticles was not affected by the presence of the nanoparticle payload. In contrast, however, for cells decorated with maleimide-functionalized nanoparticles, a reduction in the number of migratory cells compared to the nonmodified control cells was observed. Investigating and understanding the impact of nanoparticle−cell surface conjugation chemistries on the viability and properties of cells is important to further improve the design of cell-based nanoparticle delivery systems. The results of this study present a first step in this direction and provide first guidelines for the surface modification of T cells, in particular in view of their possible use for drug delivery to the CNS.

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