Functional blood cell analysis by label-free biosensors and single-cell technologies.

In this review we aim to summarize the current state of methods for label-free identification and functional characterization of leukocytes with biosensors and novel single cell techniques. The growing interest in this field is fueled from multiple directions, with the different aspects highlighting benefits of these novel technologies in comparison to classical methods. The advantage of label-free characterization is that labeling the cells might affect their behavior, and therefore lead to a biased description of the investigated biological phenomena. Label-free biosensors can offer the benefit of (i) decreasing processing time and reagent costs, (ii) enable point-of-care diagnostics, and (iii) allow downstream application of the investigated cells. Moreover, (iv) label-free detection allows the monitoring of real-time kinetic processes, opening up new avenues in contrast to traditional structural characterizations. The emphasis in the review will be on techniques on the characterizations of single cells with special attention to surface sensitive technologies. Recent developments highlighted the importance of small cell populations and individual cells both in health and disease. Nonetheless techniques capable of analyzing single cells offer a promising tool for therapeutic approaches where characterization of individual cells is necessary to estimate their clinical therapeutic potential. Most of the approaches discussed here will cover the cellular activation, adhesion as measured on functionalized solid substrates, since this approach offers the most advantages. Analyzing various cells on solid substrates not only allows their individual morphological characterization and therefore a more precise description of their activation, but as well offers an opportunity to design multiplex measurements. With this approach different stimuli can be investigated in parallel and measure cellular avidity to targets, an important aspect of gaining more and more attention recently in characterization of T-cells and antibody effector functions. Finally, novel label-free approaches provide a solution to extracting unlabeled cells for downstream processing (e.g., transcriptome analysis, cloning or the aforementioned clinical potential), where ongoing and potential further applications are discussed.