Single-cell measurement of the uptake, intratumoral distribution and cell cycle effects of cisplatin using mass cytometry.

Although of fundamental importance to the treatment of cancer patients, the quantitative study of drug distribution and action in vivo at the single cell level is challenging. We used the recently-developed technique of mass cytometry to measure cisplatin uptake into individual tumor cells (Pt atoms/cell), combined with measurement of the rate of IdU incorporation into DNA (I(127) atoms/cell/min) and tumor hypoxia identified by the 2-nitroimidazole EF5 in cisplatin-treated BxPC-3 and ME-180 xenografts. Pt levels of 10(5) to 10(6) atoms/cell were obtained following a single cisplatin treatment using clinically relevant doses. Cisplatin caused cell cycle arrest in a dose- and time-dependent manner that paralleled effects in vitro, and it readily penetrated into hypoxic tumor regions. Similar levels of Pt/cell were found in xenografts treated with oxaliplatin. Mass cytometry offers the unique capability to study the cellular uptake and anticancer effects of platinum-containing drugs at the single cell level in animal models, and it has the potential for application to samples obtained from cancer patients during treatment.

Single-cell mass cytometry reveals intracellular survival/proliferative signaling in FLT3-ITD-mutated AML stem/progenitor cells.

Understanding the unique phenotypes and complex signaling pathways of leukemia stem cells (LSCs) will provide insights and druggable targets that can be used to eradicate acute myeloid leukemia (AML). Current work on AML LSCs is limited by the number of parameters that conventional flow cytometry (FCM) can analyze because of cell autofluorescence and fluorescent dye spectral overlap. Single-cell mass cytometry (CyTOF) substitutes rare earth elements for fluorophores to label antibodies, which allows measurements of up to 120 parameters in single cells without correction for spectral overlap. The aim of this study was the evaluation of intracellular signaling in antigen-defined stem/progenitor cell subsets in primary AML. CyTOF and conventional FCM yielded comparable results on LSC phenotypes defined by CD45, CD34, CD38, CD123, and CD99. Intracellular phosphoprotein responses to ex vivo cell signaling inhibitors and cytokine stimulation were assessed in myeloid leukemia cell lines and one primary AML sample. CyTOF and conventional FCM results were confirmed by western blotting. In the primary AML sample, we investigated the cell responses to ex vivo stimulation with stem cell factor and BEZ235-induced inhibition of PI3K and identified activation patterns in multiple PI3K downstream signaling pathways including p-4EBP1, p-AKT, and p-S6, particularly in CD34(+) subsets. We evaluated multiple signaling pathways in antigen-defined subpopulations in primary AML cells with FLT3-ITD mutations. The data demonstrated the heterogeneity of cell phenotype distribution and distinct patterns of signaling activation across AML samples and between AML and normal samples. The mTOR targets p-4EBP1 and p-S6 were exclusively found in FLT3-ITD stem/progenitor cells, but not in their normal counterparts, suggesting both as novel targets in FLT3 mutated AML. Our data suggest that CyTOF can identify functional signaling pathways in antigen-defined subpopulations in primary AML, which may provide a rationale for designing therapeutics targeting LSC-enriched cell populations.

An introduction to mass cytometry: fundamentals and applications.

Mass cytometry addresses the analytical challenges of polychromatic flow cytometry by using metal atoms as tags rather than fluorophores and atomic mass spectrometry as the detector rather than photon optics. The many available enriched stable isotopes of the transition elements can provide up to 100 distinguishable reporting tags, which can be measured simultaneously because of the essential independence of detection provided by the mass spectrometer. We discuss the adaptation of traditional inductively coupled plasma mass spectrometry to cytometry applications. We focus on the generation of cytometry-compatible data and on approaches to unsupervised multivariate clustering analysis. Finally, we provide a high-level review of some recent benchmark reports that highlight the potential for massively multi-parameter mass cytometry.

Mass cytometry: technique for real time single cell multitarget immunoassay based on inductively coupled plasma time-of-flight mass spectrometry.

A novel instrument for real time analysis of individual biological cells or other microparticles is described. The instrument is based on inductively coupled plasma time-of-flight mass spectrometry and comprises a three-aperture plasma-vacuum interface, a dc quadrupole turning optics for decoupling ions from neutral components, an rf quadrupole ion guide discriminating against low-mass dominant plasma ions, a point-to-parallel focusing dc quadrupole doublet, an orthogonal acceleration reflectron analyzer, a discrete dynode fast ion detector, and an 8-bit 1 GHz digitizer. A high spectrum generation frequency of 76.8 kHz provides capability for collecting multiple spectra from each particle-induced transient ion cloud, typically of 200-300 micros duration. It is shown that the transients can be resolved and characterized individually at a peak frequency of 1100 particles per second. Design considerations and optimization data are presented. The figures of merit of the instrument are measured under standard inductively coupled plasma (ICP) operating conditions (<3% cerium oxide ratio). At mass resolution (full width at half-maximum) M/DeltaM > 900 for m/z = 159, the sensitivity with a standard sample introduction system of >1.4 x 10(8) ion counts per second per mg L(-1) of Tb and an abundance sensitivity of (6 x 10(-4))-(1.4 x 10(-3)) (trailing and leading masses, respectively) are shown. The mass range (m/z = 125-215) and abundance sensitivity are sufficient for elemental immunoassay with up to 60 distinct available elemental tags. When <15 elemental tags are used, a higher sensitivity mode at lower resolution (M/DeltaM > 500) can be used, which provides >2.4 x 10(8) cps per mg L(-1) of Tb, at (1.5 x 10(-3))-(5.0 x 10(-3)) abundance sensitivity. The real-time simultaneous detection of multiple isotopes from individual 1.8 microm polystyrene beads labeled with lanthanides is shown. A real time single cell 20 antigen expression assay of model cell lines and leukemia patient samples immuno-labeled with lanthanide-tagged antibodies is presented.

Element-tagged immunoassay with ICP-MS detection: evaluation and comparison to conventional immunoassays.

We have investigated the possibility of using element-tagged antibodies for protein detection and quantification in microplate format using Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and compared the results to conventional immunoassays, such as Enzyme-Linked Immunosorbent Assay (ELISA) and Western blotting. The technique was further employed to detect low levels and measure DNA-binding activity of transcription factor p53 in leukemia cell lysates through its interaction with immobilized oligonucleotides and recognition by element-tagged antibodies. The advantages of ICP-MS detection for routine performance of immunoassays include increased sensitivity, wide dynamic range, minimal interference from complex matrices, and high throughput. Our approach advances the ICP-MS technology and demonstrates its applicability to proteomic studies through the use of antibodies directly labeled with polymer tags bearing multiple atoms of lanthanides. Development of this novel methodology will enable fast and quantitative identification of multiple analytes in a single well.

Development of analytical methods for multiplex bio-assay with inductively coupled plasma mass spectrometry.

Advances in the development of highly multiplexed bio-analytical assays with inductively coupled plasma mass spectrometry (ICP-MS) detection are discussed. Use of novel reagents specifically designed for immunological methods utilizing elemental analysis is presented. The major steps of method development, including selection of elements for tags, validation of tagged reagents, and examples of multiplexed assays, are considered in detail. The paper further describes experimental protocols for elemental tagging of antibodies, immunostaining of live and fixed human leukemia cells, and preparation of samples for ICP-MS analysis. Quantitative analysis of surface antigens on model cell lines using a cocktail of seven lanthanide labeled antibodies demonstrated high specificity and concordance with conventional immunophenotyping.

Inductively coupled plasma mass spectrometer with axial field in a quadrupole reaction cell.

A novel reaction cell for ICP-MS with an electric field provided inside the quadrupole along its axis is described. The field is implemented via a DC bias applied to additional auxiliary electrodes inserted between the rods of the quadrupole. The field reduces the settling time of the pressurized quadrupole when its mass bandpass is dynamically tuned. It also improves the transmission of analyte ions. It is shown that for the pressurized cell with the field activated, the recovery time for a change in quadrupole operating parameters is reduced to <4 ms, which allows fast tuning of the mass bandpass in concert with and at the speed of the analyzing quadrupole. When the cell is operated with ammonia, the field reduces ion-ammonia cluster formation, further enhancing the transmission of atomic ions that have a high cluster formation rate. Ni x (NH3)n+ cluster formation in a cell operated with a wide bandpass (i.e., Ni+ precursors are stable in the cell) is shown to be dependent on the axial field strength. Clusters at n = 2-4 can be suppressed by 9, 1200, and >610 times, respectively. The use of a retarding axial field for in-situ energy discrimination against cluster and polyatomic ions is shown. When the cell is pressurized with O2 for suppression of 129Xe+, the formation of 127IH2+ by reactions with gas impurities limits the detection of 129I to isotopic abundance of approximately 10(-6). In-cell energy discrimination against 127IH2+ utilizing a retarding axial field is shown to reduce the abundance of the background at m/z = 129 to ca. 3 x 10(-8) of the 127I+ signal. In-cell energy discrimination against 127IH2+ is shown to cause less I+ loss than a post-cell potential energy barrier for the same degree of 127IH2+ suppression.

Single-cell mass cytometry of differential immune and drug responses across a human hematopoietic continuum.

Flow cytometry is an essential tool for dissecting the functional complexity of hematopoiesis. We used single-cell "mass cytometry" to examine healthy human bone marrow, measuring 34 parameters simultaneously in single cells (binding of 31 antibodies, viability, DNA content, and relative cell size). The signaling behavior of cell subsets spanning a defined hematopoietic hierarchy was monitored with 18 simultaneous markers of functional signaling states perturbed by a set of ex vivo stimuli and inhibitors. The data set allowed for an algorithmically driven assembly of related cell types defined by surface antigen expression, providing a superimposable map of cell signaling responses in combination with drug inhibition. Visualized in this manner, the analysis revealed previously unappreciated instances of both precise signaling responses that were bounded within conventionally defined cell subsets and more continuous phosphorylation responses that crossed cell population boundaries in unexpected manners yet tracked closely with cellular phenotype. Collectively, such single-cell analyses provide system-wide views of immune signaling in healthy human hematopoiesis, against which drug action and disease can be compared for mechanistic studies and pharmacologic intervention.

Messenger RNA Detection in Leukemia Cell lines by Novel Metal-Tagged in situ Hybridization using Inductively Coupled Plasma Mass Spectrometry.

Conventional gene expression profiling relies on using fluorescent detection of hybridized probes. Physical characteristics of fluorophores impose limitations on achieving a highly multiplex gene analysis of single cells. Our work demonstrates the feasibility of using metal-tagged in situ hybridization for mRNA detection by inductively coupled plasma mass spectrometry (ICP-MS). ICP-MS as an analytical detector has a number of unique and relevant properties: 1) metals and their stable isotopes generate non-overlapping distinct signals that can be detected simultaneously; 2) these signals can be measured over a wide dynamic range; 3) ICP-MS is quantitative and very sensitive. We used commercial antibodies conjugated to europium (Eu) and gold together with biotinylated oligonucleotide probes reacted with terbium-labeled streptavidin to demonstrate simultaneous mRNA and protein detection by ICP-MS in leukemia cells.

Detection of ultratrace phosphorus and sulfur by quadrupole ICPMS with dynamic reaction cell.

A method of detection of ultratrace phosphorus and sulfur that uses reaction with O2 in a dynamic reaction cell (DRC) to oxidize S+ and P+ to allow their detection as SO+ and PO+ is described. The method reduces the effect of polyatomic isobaric interferences at m/z = 31 and 32 by detecting P+ and S+ as the product oxide ions that are less interfered. Use of an axial field in the DRC improves transmission of the product oxide ions 4-6 times. With no axial field, detection limits (3sigma, 5-s integration) of 0.20 and 0.52 ng/mL, with background equivalent concentrations of 0.53 and 4.8 ng/mL, respectively, are achieved. At an optimum axial field potential (200 V), the detection limits are 0.06 ng/mL for P and 0.2 ng/mL for S, respectively. The method is used for determining the degree of phosphorylation of beta-casein, and regular and dephosphorylated alpha-caseins at 10-1000 fmol/microL concentration, with 5-10% v/v organic sample matrix (acetonitrile, formic acid, ammonium bicarbonate). The measured degree of phosphorylation for beta-casein (4.9 phosphorus atoms/molecule) and regular alpha-casein (8.8 phoshorus atoms/molecule) are in good agreement with the structural data for the proteins. The P/S ratio for regular alpha-casein (1.58) is in good agreement with the ratio of the number of phosphorylation sites to the number of sulfur-containing amino acid residues cysteine and methionine. The P/S ratio for commercially available dephosphorylated alpha-casein is measured at 0.41 (approximately 26% residual phosphate).

Chemical resolution of Pu+ from U+ and Am+ using a band-pass reaction cell inductively coupled plasma mass spectrometer.

Determination of the concentration and distribution of the Pu and Am isotopes is hindered by the isobaric overlaps between the elements themselves and U, generally requiring time-consuming chemical separation of the elements. A method is described in which chemical resolution of the elemental ions is obtained through ion-molecule reactions in a reaction cell of an ICPMS instrument. The reactions of "natural" U(+), (242)Pu(+), and (243)Am(+) with ethylene, carbon dioxide, and nitric oxide are reported. Since the net sensitivities to the isotopes of an element are similar, chemical resolution is inferred when one isobaric element reacts rapidly with a given gas and the isobar (or in this instance surrogate isotope) is unreactive or slowly reactive. Chemical resolution of the m/z 238 isotopes of U and Pu can be obtained using ethylene as a reaction gas, but little improvement in the resolution of the m/z 239 isobars is obtained. However, high efficiency of reaction of U(+) and UH(+) with CO(2), and nonreaction of Pu(+), allows the sub-ppt determination of (239)Pu, (240)Pu, and (242)Pu (single ppt for (238)Pu) in the presence of 7 orders of magnitude excess U matrix without prior chemical separation. Similarly, oxidation of Pu(+) by NO, and nonreaction of Am(+), permit chemical resolution of the isobars of Pu and Am over 2-3 orders of magnitude relative concentration. The method provides the potential for analysis of the actinides with reduced sample matrix separation.

A sensitive and quantitative element-tagged immunoassay with ICPMS detection.

We report a set of novel immunoassays in which proteins of interest can be detected using specific element-tagged antibodies. These immunoassays are directly coupled with an inductively coupled plasma mass spectrometer (ICPMS) to quantify the elemental (in this work, metal) component of the reacted tagged antibodies. It is demonstrated that these methods can detect levels of target proteins as low as 0.1-0.5 ng/mL and yield a linear response to protein concentration over 3 orders of magnitude.