Sorting minor subpopulations of cells: use of fluorescence as the triggering signal

Digital data acquisition and processing

A flow cytometer is made up of many different subsystems that work together to measure the optical properties of individual cells within a sample. The data acquisition system (also called the data system) is one of these subsystems, and it is responsible for converting the electrical signals from the optical detectors into list-mode data. This unit describes the inner workings of the data system, and provides insight into how the instrument functions as a whole. Some of the information provided in this unit is applicable to everyday use of these instruments, and, at minimum, should make it easier for the reader to assemble a specific data system. With the considerable advancement of electronics technology, it becomes possible to build an entirely functional data system using inexpensive hobbyist-level electronics. This unit covers both analog and digital data systems, but the primary focus is on the more prevalent digital data systems of modern flow cytometric instrumentation.

High-speed cell sorting

A flow cytometer has the ability to analyze several thousand cells per second. This is particularly valuable for the detection of rare events. For many biological and clinical applications, cells must be isolated on the basis of multiple quantitative properties. This need is ideally met by the analytical power of flow cytometry, which provides the front-end selection mechanism for cell sorting. In essence, cell sorting represents real-time data classification coupled with actual physical isolation of cells with the properties of interest. This unit discusses the principles and practices of operating a cell sorter to maximize the sorting rate, including the trade-offs between speed and resolution.

Principles of gating

Few aspects of flow cytometry are common to every application. Gating principles is one. This unit provides an excellent overview as well as detailed conceptual material on how gating works, when certain types of gating should or should not be applied, and the advantages and disadvantages of various techniques. This unit should be required reading for every technician using a flow cytometer and every graduate student starting out in the area, and is an excellent refresher for those more advanced in the technology. It brings out the historical development of our understanding of gating applications as well as providing a high level of technical accuracy.

Photon-burst analysis in two-photon fluorescence excitation flow cytometry

We studied the use of a dramatically reduced testing zone in combination with two-photon excitation and photon-burst analysis in high-throughput rare-event detection simulation using a modified flow cytometer. Two-photon excitation measurements were performed with a mode-locked titanium:sapphire laser. Fluorescence emission was measured with a photon-counting avalanche photodiode. Measured signal was analysed offline by autocorrelation and burst detection methods. Test samples were composed of full blood and orange fluorescent polystyrene nanospheres mixed in full blood. Results show that two-photon fluorescence excitation and time-correlation analysis provide a good signal-to-noise ratio for rare-event particle detection in a turbid sample environment.

Efficient flow cytometric assay for platelet-leukocyte aggregates in whole blood using fluorescence signal triggering

BACKGROUND

Platelet-leukocyte aggregates (PLAs) may be important in thrombotic and inflammatory disease states, but accurate assessment of PLA formation in vivo is hampered by the propensity for in vitro artefacts caused by sample manipulation. A whole blood flow cytometric assay for circulating PLAs, based on minimal sample manipulation, was thus developed.

METHODS

Citrated whole blood was labeled with a RPE-CD45 MAb (leukocyte marker) and an FITC-CD42a (GPIX) MAb (platelet marker). The latter was used to avoid possible influences of platelet glycoprotein proteolysis by neutrophil-derived proteases. The samples were mildly fixed with 0.5% formaldehyde saline. The cytometer was triggered by RPE-CD45 fluorescence. Leukocyte subpopulations were separated according to their typical light scattering and CD45 expression.

RESULTS

Minimal sample manipulation and mild sample fixation resulted in minor in vitro artefacts and good sample stability. Fluorescence triggering increased the efficiency of the flow cytometric analysis approximately 5-fold compared with triggering with light scatter, and allowed discrimination of leukocyte subpopulations. The majority of PLAs involved monocytes and neutrophils, rather than lymphocytes, both without and with in vitro stimulation by ADP or thrombin. A cocktail of blocking MAbs to CD62P, CD15, GPIIb/IIIa and the CD11b/CD18 complex had no effect on unstimulated samples, whilst totally inhibiting aggregation induced by 10(-5) M ADP, suggesting that the PLAs in unstimulated blood were preformed in vivo.

CONCLUSIONS

This whole blood flow cytometric assay for PLAs is simple and efficient, and appears to reflect closely platelet-leukocyte aggregates in circulating blood in vivo.

Isolation of neuronal precursors by sorting embryonic forebrain transfected with GFP regulated by the T alpha 1 tubulin promoter

Neuronal precursor cells are widespread in the forebrain ventricular/subventricular zone, and may provide a cellular substrate for brain repair. Clonal lines derived from single progenitors can become progressively less representative of their parental precursors with time and passage in vitro. We have developed an alternative strategy for the isolation and enrichment of precursor cells, by fluorescence-activated cell sorting of forebrain cells transfected with the gene for green fluorescent protein, driven by the neuronal T alpha 1 tubulin promoter. Using this approach, neural precursors and young neurons can be identified and selectively harvested from a variety of samples, including both avian and mammalian forebrains at different developmental stages.

Use of fluorescence threshold triggering and high-speed flow cytometry for rare event detection

A simple rare event detection method utilizing dual-parameter flow cytometry is described, which allows quantitation of specific cellular events at the level of two cells in 10(7) total cells. Using a standard unmodified single laser flow cytometer sampling at a rate of 25,000 events/sec and a fluorescence discriminator, 10(7) total cells are processed in 7 min. The assay involves precise characterization of instrument flow rates to calculate total events processed by the cytometer rather than accumulate total events in computer memory. This method of detecting rare events is demonstrated by using a model system of breast cancer cells labeled with a metabolically activated dye and serially diluted into normal peripheral blood. Potential applications include validation of methods to detect minimum residual disease following myeloablative therapy, detection of any remaining tumor cells following purging methods, and validation of methods to detect circulating fetal cells in maternal blood.

Flow karyotyping and sorting of Vicia faba chromosomes

Chromosome suspensions were prepared from formaldehyde-fixed, synchronized Vicia faba root tips. After staining with the DNA intercalating fluorochrome propidium iodide, the suspensions were analysed with a flow cytometer. The resulting histograms of integral fluorescence intensity contained peaks similar to those of theoretical V.faba flow karyotypes. From V. Faba cv 'Inovec' (2n = 12) only one peak, corresponding to a single chromosome type (metacentric chromosome), could be discriminated. However, it was found that the peak also contained doublets of acrocentric chromosomes. Bivariate analysis of fluorescence pulse area (chromosome DNA content) and fluorescence pulse width (chromosome length) was necessary to distinguish the metacentric chromosome. To achieve a high degree of purity, a two-step sorting protocol was adopted. During a working day, more than 25 000 metacentric chromosomes (corresponding to 0.2 μg DNA) were sorted with a purity of more than 90%. Such chromosomes are suitable for physical gene mapping by in situ hybridization or via the polymerase chain reaction (PCR) and allow the construction of chromosome-specific DNA libraries. While it was only possible to distinguish and sort one chromosome type from V. Faba cv 'Inovec' with the standard karyotype, it was possible to sort with a high degree of purity five out of six chromosome types of the line EFK of V. Faba, which has six pairs of morphologically distinct chromosomes. This result confirmed the possibility of using reconstructed karyotypes to overcome existing problems with the discrimination and flow sorting of individual chromosome types in plants.

Fluorescence-activated sorting of totipotent embryonic stem cells expressing developmentally regulated lacZ fusion genes

Murine embryonic stem (ES) cells were infected with a retrovirus promoter trap vector, and clones expressing lacZ fusion genes (LacZ+) were isolated by fluorescence-activated cell sorting (FACS). Of 12 fusion genes tested, 1 was repressed when ES cells were allowed to differentiate in vitro. Two of three lacZ fusion genes tested were passed into the germ line, indicating that FACS does not significantly affect stem cell totipotency. The pattern of lacZ expression observed in vivo was consistent with that seen in vitro. Both fusion genes were expressed in preimplantation blastulas. However, a fusion gene whose expression was unaffected by in vitro differentiation was ubiquitously expressed in day-10 embryos, while the other, which showed regulated expression in vitro, was restricted to cells located along the posterior neural fold, the optic chiasm, and within the fourth ventricle. These results demonstrate the utility of using promoter trap vectors in conjunction with fluorescence sorting to disrupt developmentally regulated genes in mice.