Image Recognition Cell Dispensing System for Plating of Single Cells and Monodisperse Spheroids

Immunotherapy is recently drawing many attentions in cancer therapy. This technique focuses on expression analysis of the cells, wherethe activities of antibodies must be carefully monitored. For this purpose, there is a growing need for single-cell dispensing system in order toprevent mixing in the expression profile analysis.

In cell-based drug assays, the assays incorporating spheroids are considered more suitable for evaluating drug efficacy than single cells,because spheroid is a better representation of cell microenvironment in biological systems. For accurate evaluation in spheroid-based assays,(1) preparation of monodisperse spheroid population and (2) plating of specific number of spheroids into reaction wells are necessary, due tothe spheroid size-dependent nature of the assays. However, there were not any instrument on the market that fulfilled such requirements.

For this reason, we have developed a system “On-chip SPiS” that allows for damage-less plating of single cells and size spheroids. Thisinstrument draws sample solution in a disposable transparent pipette tip for particle count and size image-recognition, then dispenses outspecified number of particles into multiple wells. On-chip SPiS can fit inside an biosafety cabinet. First, On-chip Sort, a cell sorter withdisposable microfluidic chip at its core, was used to sort a group of spheroids based on the size information from forward scattered lightintensity. On-chip Sort can recover spheroids of size up to 150µm in diameter. Then, the collected spheroids were plated one by one inmultititer plate. We anticipate that the size selection of spheroids by On-chip Sort and single cell dispensing by On-chip SPiS together willbecome a useful tool for efficacy evaluation of drug candidates especially for anticancer drugs.

Introduction Spheroid Sorting and ATP Assay

Single Particle isolation System “On-chip SPiS”On-chip SPiS is a tool for plating of single cells by image

recognition using our accumulated experience of celldetection technology (Fig.1). To ensure only one cell isdispensed at a time, the sample pipetted up in thetransparent polymeric pipette tip is photographed fordifferentiation of cells from debris and scratches on thepipette tip. The entire sample can be captured by takingadvantage of the depth of field and field of view of thecamera. Cells and spheroids of size ranging from 10µm up to200µm can be plated without any damage at accuracy of 90%and above. Although the pipette tips are custom made,standard sample tubes and microtiter plates (both 96 wellsand 386 wells) can be used. This instrument can alsodetermine the size of the cells, and thus we can selectivelydeposit the cells of particular size.

Single cell and Spheroid DispensingPC-9 cell line was plated in 96 well plate using On-chip SPiS. The cells

were initially stained with Hoechst, and they were evaluated under afluorescence microscope (Fig. 2(A)). The observation data showed 92%of the wells contained 1 cells, and 8% of the wells had no cells (Fig. 2(B)).Cell spheroids (cultured using NanoCulture Plate MS pattern, lowbinding, 96 wells, NPC-LS96, SCIVAX Life Sciences, Ltd.) were also platedin 384 well. Microscopy results confirmed that 95% of the wellscontained single spheroid, and 5% of the wells contained 2 or morespheroids (Fig. 2(C)). These results show significantly greater single celldispensing accuracy than that of a conventional dispenser, which platessingle cells at a likelihood of 37% according to Poisson Distribution.

Single cell dispensing results

Single spheroid dispensing results

Spheroid Viability after Dispensing

Hoechst-stained PC-9 cell in a well

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Yuu Fujimura*, Tomiko Tanaka, Jin Akagi, Yohsuke Bansho, Ryoko Watanabe, Masayuki Ishige, Kazuo TakedaOn-chip Biotechnologies Co., Ltd.

Correspondence: Yuu Fujimura

y-fujimura@on-chip.co.jp

Viability of spheroids that have been dispensed using On-chipSPiS inside a biosafety cabinet was evaluated. 7 spheroids wereplated individually, and they were cultured for 4 days. All 7spheroids survived and became about 1.5 times larger than whenthey were just plated (Fig. 3). This data indicates that On-chip SPiScan dispense spheroids aseptically and without any damage.

Acknowledgement

Dead/Alive Determination of Spheroids using On-chip SpheroStainWe used a newly developed dye, On-chip SpheroStain, to observe the distribution of

dead cells present in spheroids. Fig. 6 shows the results of single cells stained using On-chip SpheroStain, where live cells are not stained by the dye, while almost 100% of thedead cells are stained. In the case of spheroid, spheroids of size 150µm and belowshow almost no dead cells present inside (Fig. 7). However, the fraction of dead cellspresent is higher for spheroids 200µm and above . The ordinary evaluation using ATP asan index only provides information for the live cells, but On-chip SpheroStain hasproven it can provide information on the dead cells within spheroids. Consequently, weconclude that use of spheroids 150µm and below is recommended for drug evaluation.

MG132: Cell permeable proteasome inhibitor. It inhibits activation of NF-κB, and leads to apoptosis of various cancer cell lines. ATP measurement was done using Infinite® 200 PRO, Tecan Group Ltd.

Fixed cell after staining for 10minAlmost all cells were stained.

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Drug sensitivity evaluation using spheroids are commonlyundertaken using ATP as an index, but variations in the spheroid sizewould hugely affect the experimental outcomes. This is considereddue to the difference in the extent of drug permeation to the core ofthe cell spheroid. From this hypothesis, we collected a group ofspheroids of a similar size from a heterogeneous sample using ourmicrofluidic chip based cell sorter, On-chip Sort (On-chipBiotechnologies Co., Ltd.). This instrument can recover particles ofsize up to 150µm.

The spheroids were prepared using dedicated plate (NanoCulturePlate MS pattern, low binding, 96 wells, NPC-LS96, SCIVAX LifeSciences, Ltd.). Fig. 4(B) shows (in blue) the distribution of spheroidsafter 3 days of culture. The initial sample contained mostly singularcells (over 1000 cells indicated in light blue), while the spheroid sizevaried from 30µm to 240µm. From this sample, On-chip Sort wasused to collect spheroids of size around 100µm based on the sizeinformation from forward scattered light intensity (Fig. 4(C)). Thedistribution of the sorted spheroids are plotted in red in Fig. 4(B),which suggest that the size distribution have narrowed downsignificantly (between 80-140µm).

These sorted spheroids and unsorted spheroid sample were usedfor single spheroid dispensing using On-chip SPiS. After plating,MG132 was added to each well at various concentration (0, 0.01,0.1, 1, and 10µM). Then, these spheroids were cultured over 4 daysand the concentration of ATP was measured. Fig. 5 shows theaverage concentration of ATP present in each spheroid, where theblue bars indicate the unsorted spheroid sample, whereas theorange bars represent the sorted spheroids. As the concentration ofMG132 increases, the ATP concentration decreases due to the dyingspheroid cells. Note that the concentration of ATP varies significantlymore for the unsorted spheroid samples than for the sortedspheroids, shown by the error bars. Therefore, preparation formonodisperse spheroids are crucial for an accurate drug evaluation.

Fig. 1 On-chip SPiS and its dispensing method

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Fig. 2 Results of single cell and single spheroid plating

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An image of the pipettetip by built-in camera

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Fig. 3 Spheroid growth over 4 days of culture

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Fig. 4 Spheroid sorting

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Fig. 5 ATP concentration vs MG132 concentration for sorted and unsorted spheroid samples

Fig. 6 Single cells stained by On-chip SpheroStain

Many of the data and results presented are provided by the courtesy of SCIVAS Life Sciences. Ltd. as part of collaborative work.

On-chip Sort

Fig. 7 Distribution of dead cells within spheroids