A novel technique for the fluorometric assessment of T lymphocyte antigen specific lysis

Carboxyfluorescein diacetate succinimidyl ester fluorescent dye for cell labeling

Our objective was to study the properties of the carboxyfluorescein diacetate succinimidyl ester (CFDA-SE) and the methodology of cell labeling using CFDA-SE fluorescent dye. First, we analyzed the kinetics of CFDA-SE fluorescent dye intensity over time. Second, we determined the optimal concentration of CFDA-SE fluorescent dye for cell labeling. Third, we tested the toxicity of CFDA-SE fluorescent dye on labeled cells. Finally, we determined the optimal staining time of CFDA-SE fluorescent dye for cell labeling. The results show that the optimal concentration of CFDA-SE fluorescent dye for cell labeling varies according to different cell types. CFDA-SE fluorescent dye is non-toxic to cells as the cell death rate caused by CFDA-SE labeling is below 5%. The optimal cell labeling time was determined to be 8 min of incubation with CFDA-SE fluorescent dye. We concluded that the advantages of using CFDA-SE fluorescent dye for cell labeling are as follows: (1) the binding of CFDA-SE fluorescent dye to cells is stable; (2) CFDA-SE fluorescent dye is not toxic and does not modify the viability of labeled cells; and (3) CFDA-SE fluorescent dye is a suitable fluorochrome for cell labeling.

CD56dimCD16neg cells are responsible for natural cytotoxicity against tumor targets

The activation of natural killer (NK) cells leads to degranulation and secretion of cytotoxic granula. During this process, the lytic granule membrane protein CD107a becomes detectable at the cell surface. Based on this phenomenon, we have analyzed by a novel flow cytometry-based assay, the number and phenotype of NK cells responding to tumor targets. Using human leukemia and lymphoma cell lines, we observed a close correlation between CD107a surface expression and target cell lysis, indicating that NK cell cytotoxicity can be assessed by this method. The number of degranulating NK cells was closely related to the ratio of effector and target cells and showed a maximum at a ratio of 1:1. Moreover, we were able to show that the population of CD56(dim)/CD16(neg) NK cells is primarily responsible for the cytotoxic activity against tumor targets whereas neither CD56(dim)/CD16(pos) nor CD56(bright) NK cells degranulated in response to the cell lines. Our results indicate that the CD107a assay represents a promising new method for the quantification and characterization of cells exhibiting natural cytotoxicity.

Targeted lysis of HIV-infected cells by natural killer cells armed and triggered by a recombinant immunoglobulin fusion protein: implications for immunotherapy

Natural killer (NK) cells play an important role in both innate and adaptive antiviral immune responses. The adaptive response typically requires that virus-specific antibodies decorate infected cells which then direct NK cell lysis through a CD16 mediated process termed antibody-dependent cellular cytotoxicity (ADCC). In this report, we employ a highly polymerized chimeric IgG1/IgA immunoglobulin (Ig) fusion protein that, by virtue of its capacity to extensively crosslink CD16, activates NK cells while directing the lysis of infected target cells. We employ HIV as a model system, and demonstrate that freshly isolated NK cells preloaded with an HIV gp120-specific chimeric IgG1/IgA fusion protein efficiently lyse HIV-infected target cells at picomolar concentrations. NK cells pre-armed in this manner retain the capacity to kill targets over an extended period of time. This strategy may have application to other disease states including various viral infections and cancers.

Vaccine-elicited antibodies mediate antibody-dependent cellular cytotoxicity correlated with significantly reduced acute viremia in rhesus macaques challenged with SIVmac251

Effector cells armed with Abs can eliminate virus-infected target cells by Ab-dependent cellular cytotoxicity (ADCC), an immune mechanism that has been largely overlooked in HIV vaccine development. Here, we show that a prime/boost AIDS vaccine approach elicits potent ADCC activity correlating with protection against SIV in rhesus macaques (Macacca mulatta). Priming with replicating adenovirus type 5 host range mutant-SIV recombinants, followed by boosting with SIV gp120, elicited Abs with ADCC activity against SIV(mac251)-infected cells. In vitro ADCC activity correlated with in vivo reduced acute viremia after a mucosal challenge with pathogenic SIV. Our findings expose ADCC activity as an immune correlate that may be relevant in the rational design of an efficacious vaccine against HIV.

Biophotonic cytotoxicity assay for high-throughput screening of cytolytic killing

We have developed a highly sensitive biophotonic luciferase assay as an alternative to (51)Cr-release for assessment of cell-mediated cytotoxicity. The luciferin/ATP-dependent luminescent signal of target cells stably or transiently transfected with a firefly luciferase reporter gene (fLuc:Zeo) linearly correlates with viable target cell number. Upon incubation of fLuc:Zeo(+) target cells with CD8(+) CTLs, a rapid decrease in bioluminescence was detected that correlated with antigen-specific target cell lysis. The levels of specific lysis measured by (51)Cr-release assays correlated with the attenuation in biophotonic target cell signal, thus validating this approach as a sensitive and accurate method for the measurement of cytolysis. We show that this luminescent-based cytolytic assay (LCA) is amenable for high-throughput screening of effector cell cytolytic activity, allows for the rate of cytolysis to be measured in a single micro-plate, and permits the multiplexing of cytolytic killing with other lymphocyte functional assays such as cytokine release. Importantly, this method accurately measures the cytolytic killing of target cells that are either stably or transiently transfected with a fLuc reporter gene, and thus is ideal for monitoring cytolysis of both primary autologous and immortalized target cell lines. The versatility of the non-radioactive, high-throughput, biophotonic cytolytic assay should make this method an attractive alternative to chromium-release for quantifying effector cell cytolytic activity.

Restoring the endothelium of cryopreserved arterial grafts: co-culture of venous and arterial endothelial cells

The use of arterial homografts in clinical practice is becoming increasingly common, yet there is an urgent need to address one of the most well-established problems associated with their use: the loss of integrity of the endothelium following cryopreservation. The partial lack of endothelium causes contact between the extracellular matrix and blood flow, which, in turn, often gives rise to thrombosis and/or restenosis. Our objective was first to attempt to replace the arterial endothelial cells lost during the cryopreservation process by seeding autologous venous endothelial cells, and to evaluate the behaviour of venous and arterial endothelial cells in co-culture. The idea was to establish whether venous endothelial cells would be accepted by arterial endothelial cells and could therefore be used to restore the endothelial lining for the subsequent use of these vessels in in vivo grafting procedures. For the co-culture experiments, endothelial cells were obtained from the jugular vein and both iliac arteries of the minipig by treatment with 0.1% type I collagenase. The venous endothelial cells were fluorescently labelled with the membrane intercalating dye PKH26. Equal numbers of venous and arterial endothelial cells were mixed and co-cultured for 24h, 48h or 4 days. Cell viability, determined by 2% trypan blue staining and the TUNEL method, was established before and after fluorescence labelling. Cellular activity was determined by estimating PGI2 levels in the cultures. The proliferation index was established by [H(3)]thymidine (1muCi/ml) in the cell culture medium. For the in vivo tests, 5 cm length segments of minipig iliac artery were used to establish the groups: control (n = 6), fresh arterial segments; group I (n = 16), cryopreserved arterial segments and group II (n = 16), cryopreserved arterial segments seeded with autologous venous endothelial cells. The cryopreserved vessels in group II were seeded by flooding with a labelled venous endothelial cell suspension. Once seeded, the arterial segments were included in an in vitro flow circuit. All the specimens were processed for fluorescence and light microscopy, and scanning electron microscopy. The denuded endothelial surface was determined in each group. Cell death was evaluated by the TUNEL method. We confirmed the existence of intercellular PECAM1-type junctions between venous (PKH26+) and arterial cells in co-culture and the functional activity of the cells. The cryopreserved arterial segments showed a well-preserved wall structure. However, different size areas of marked endothelial denudation were detected. After seeding with labelled cells (PKH26+), these denuded areas of the cryopreserved artery were entirely covered by fluorescent cells. After seeding, a drop in the proportion of damaged endothelial cells was recorded. Despite some loss of seeded cells after inclusion in the in vitro flow circuit, the endothelial cell count was not significantly different to those recorded for control, non-cryopreserved specimens. In conclusion arterial and venous endothelial cells growing in co-culture modify their behaviour to form multilayers. The two cell populations form normal PECAM1 junctions and preserve their functional properties. Seeding autologous venous endothelial cells on the luminal surface of cryopreserved arterial segments serves to restore the integrity of the endothelial layer.

Cutting edge: identification of hepatitis C virus-specific CD8+ T cells restricted by donor HLA alleles following liver transplantation

By necessity, human liver transplantation is performed across HLA barriers. As a result, intracellular infection of the allograft presents a unique immunologic challenge for the recipient's immune system. In this study, we describe the presence of HLA-A2-restricted, hepatitis C virus (HCV)-specific CD8+ T cells in liver transplant recipients in whom the allograft is HLA-A2 positive and the recipient is HLA-A2 negative. These memory-effector T cells are recipient derived and recognize HCV peptide uniquely in the context of HLA-A2. Furthermore, these cells were absent before the transplant, suggesting that the allograft is capable of selectively expanding naive CD8+ T cells. The in vitro specificity to donor HLA allele-restricted CD8+ T cells suggests that these cells may function to control HCV spread in the allograft.

Beyond help: direct effector functions of human immunodeficiency virus type 1-specific CD4(+) T cells

The immune correlates of protection in human immunodeficiency virus type 1 (HIV-1) infection remain poorly defined, particularly the contribution of CD4(+) T cells. Here we explore the effector functions of HIV-1-specific CD4(+) T cells. We demonstrate HIV-1 p24-specific CD4(+)-T-cell cytolytic activity in peripheral blood mononuclear cells directly ex vivo and after enrichment by antigen-specific stimulation. We further show that in a rare long-term nonprogressor, both an HIV-1-specific CD4(+)-T-cell clone and CD4(+) T cells directly ex vivo exert potent suppression of HIV-1 replication. Suppression of viral replication was dependent on cell-cell contact between the effector CD4(+) T cells and the target cells. While the antiviral effector activity of CD8(+) T cells has been well documented, these results strongly suggest that HIV-1-specific CD4(+) T cells are capable of directly contributing to antiviral immunity.

Two human ULBP/RAET1 molecules with transmembrane regions are ligands for NKG2D

We characterized two novel members of the RAET1/ULBP gene cluster, RAET1E and RAET1G. The encoded proteins were similar to the ULBP in their class I-like alpha1 and alpha2 domains, but differed in that, instead of being GPI-anchored, their sequences were type 1 membrane-spanning molecules. Both proteins were capable of being expressed at the cell surface. Both proteins bound the activating receptor NKG2D, and RAET1G bound the human CMV protein UL16. The expression of diverse NKG2D-binding molecules in different tissues and with different properties is consistent with multiple modes of infection- or stress-induced activation.

Pervasive influence of hepatitis C virus on the phenotype of antiviral CD8+ T cells

Recent studies using MHC class I tetramers have shown that CD8(+) T cell responses against different persistent viruses vary considerably in magnitude and phenotype. At one extreme, hepatitis C virus (HCV)-specific CD8(+) T cell responses in blood are generally weak and have a phenotype that is perforin low and CCR7 high (early memory). At the other, specific responses to CMV are strong, perforin high, and CCR7 low (mature or effector memory). To examine the potential mechanisms behind this diversity, we compared CMV-specific responses in HCV-infected and healthy individuals. We find a striking difference in the phenotype of CMV-specific CD8(+) T cells between these groups. In the HCV-infected cohort, CMV-specific CD8(+) T cells lost markers associated with maturity; they had increased expression of CCR7 and reduced expression of Fas and perforin. They nevertheless responded to Ag in vitro in a manner similar to controls, with strong proliferation and appropriate acquisition of effector memory markers. The reduction in mature CD8 T cells in HCV-infected individuals may arise through either impairment or regulation of T cell stimulation, or through the early loss of mature T cells. Whatever the mechanism, HCV has a pervasive influence on the circulating CD8(+) T cell population, a novel feature that may be a hallmark of this infection.

The VITAL assay: a versatile fluorometric technique for assessing CTL- and NKT-mediated cytotoxicity against multiple targets in vitro and in vivo

Assessment of cell-mediated toxicity has traditionally been achieved by measuring the specific activity of enriched effector cell populations against antigen-loaded target cells labeled with radioactive isotopes in vitro. Fluorometric techniques are viewed as a promising alternative to the use of radioactive isotopes for these analyses. Direct assessment of cytotoxicity in vivo can be achieved by monitoring survival of injected fluorescent targets relative to a differentially labeled internal control population without specific antigen. We have developed this approach, incorporating the use of multiple target cell populations labeled with different dyes so that cytotoxicity can be assessed against titrated doses of a given antigen, or against a range of different antigens, simultaneously. We show that this assay, referred to as the VITAL assay, can be used to assess cytotoxic activity of CTL and iNKT cells in vivo and in vitro. CTL responses measured in vivo could be correlated with antigen doses used in immunization strategies, and also with the size of specific CTL populations enumerated in the blood with fluorescent MHC/peptide tetramers. The VITAL assay is, therefore, a sensitive technique allowing analysis of complex multi-epitope responses.

Recent advances in tumour antigen-specific therapy:In vivo veritas

Modern cancer therapies should strive not only to eliminate malignant tissues but also to preserve healthy tissues and the patient's quality of life. Antigen-specific immunotherapy approaches are promising for either aspect since they are designed to only act against tissues expressing 1 or more specified tumour antigens. In order to develop successful vaccine and adoptive transfer protocols, longitudinal monitoring of cancer patients taking part in clinical trials is mandatory. Here, in vivo expansion of antigen-specific cells, as well as their ex vivo functional status represent important parameters to be analysed. To obtain results that most closely reflect the cells' in vivo status, functional assays must be carried out with as little in vitro culturing as possible. The present minireview discusses recent advances in these domains.

New CFSE-based assay to determine susceptibility to lysis by cytotoxic T cells of leukemic precursor cells within a heterogeneous target cell population

For the clinical evaluation of the efficacy of cellular immunotherapy it is necessary to analyze the effector functions of T cells against primary leukemic target cell populations which are usually considerably heterogeneous caused by differential maturation stages of the leukemic cells. An appropriate assay should not only allow the quantitative analysis of rapid cell death induction as measured by the conventional 51Cr release assay but also of the more slowly executing pathways of T-cell-induced apoptosis occurring within days instead of hours which cannot be measured using this method. Furthermore, it should dissect the differential susceptibility to T-cell-induced cell death of various target cell subpopulations and characterize the malignant precursor cells capable of producing malignant progeny. To fulfill these requirements we developed a new assay based on carboxyfluorescein diacetate succinimidyl ester (CFSE) labeling of the target cell population combined with antibody staining of specific cell populations and addition of fluorescent microbeads to quantitatively monitor target cell death occurring within a longer time frame up to at least 5 days. This new assay facilitates the analysis of differential recognition of distinct cell types within a heterogeneous target cell population and allows simultaneously evaluation of the proliferative status of surviving target cells in response to relevant cytokines.

Measuring T cell-mediated cytotoxicity using fluorogenic caspase substrates

Cytotoxic T lymphocytes (CTLs) play a major role in the immune response against viruses and other intracellular pathogens. In addition, CTLs are implicated in the control of tumor cells in certain settings. Accurate measures of CTL function are of critical importance to study the pathogenesis of infectious diseases and to evaluate the efficacy of new vaccines and immunotherapies. To this end, we have recently developed a flow cytometry-based CTL (FCC) assay that measures the CTL-induced caspase activation within target cells using cell permeable fluorogenic caspase substrates. This novel assay reliably detects, by flow cytometry or fluorescence/confocal microscopy, antigen-specific CTLs in a wide variety of human and murine systems, and is safer and more informative than the standard 51Cr-release assay. In addition, the flow cytometric CTL (FCC) assay provides an alternative method that is often more sensitive and physiologically informative when compared to previously described FCC assays, as it measures a biological indicator of apoptosis within the target cell. The FCC assay may thus represent a useful tool to further understand the molecular and cellular mechanisms that underlie CTL-mediated killing during tumorigenesis or following infection with viruses or other intracellular pathogens.

Sensitive and viable identification of antigen-specific CD8+ T cells by a flow cytometric assay for degranulation

Flow cytometric detection of antigen-specific CD8+ T cells has previously been limited to MHC-class I tetramer staining or intracellular cytokine production, neither of which measure the cytolytic potential of these cells. Here we present a novel technique to enumerate antigen-specific CD8+ T cells using a marker expressed on the cell surface following activation induced degranulation, a necessary precursor of cytolysis. This assay measures the exposure of CD107a and b, present in the membrane of cytotoxic granules, onto the cell surface as a result of degranulation. Acquisition of cell surface CD107a and b is associated with loss of intracellular perforin and is inhibited by colchicine, indicating that exposure of CD107a and b to the cell surface is dependent on degranulation. CD107a and b are expressed on the cell surface of CD8+ T cells following activation with cognate peptide, concordant with production of intracellular IFNgamma. Finally, CD107-expressing CD8+ T cells are shown to mediate cytolytic activity in an antigen-specific manner. Measurement of CD107a and b expression can also be combined with MHC-class I tetramer labeling and intracellular cytokine staining to provide a more complete assessment of the functionality of CD8+T cells expressing cognate T cell receptors (TCR).

The flow cytometric PKH-26 assay for the determination of T-cell mediated cytotoxic activity

We present a rapid flow cytometric and non-radioactive functional assay developed for the determination of the cytotoxic activity of T lymphocytes, natural killer cells, and lymphokine-activated killer cells. In contrast to indirect evaluation of cytotoxicity using radioactive assays, this assay is based on the quantitative and qualitative flow cytometric analysis of cell damage on a single cell level. Target cells are stained with PKH-26, a lipophilic dye that stably integrates into the cell membrane, without disturbing its surface marker expression. It, thus, permits the distinction between target and effector cells. After short term in vitro incubation (1.5-3h), AnnexinV-FITC (ann-FITC) staining allows to discriminate between apoptotic and non-apoptotic target cells. Data analysis is performed first by gating on PKH-26 positive target cells, followed by the analysis of the ann-FITC positive subpopulation. The percentage of cytotoxicity in the PKH-26 gated cell population is calculated by subtracting unspecific ann-FITC positive target cells, measured in appropriate controls without effector cells. Using in vitro generated antigen-specific cytotoxic T lymphocytes, we demonstrate that this flow cytometric assay is sensitive, correlates well with the standard 51Cr release assay, and is easy to handle.

Tissue-restricted T cell alloresponses across HLA barriers: selection and identification of leukemia-restricted CTL in HLA-mismatched stimulator-responder pairs

Exploiting the graft-versus-leukemia (GVL) effect in mismatched transplants requires its separation from graft-versus-host disease (GVHD). We generated leukemia-specific cytotoxic T lymphocytes (CTL) in three haplotype-mismatched, two class I-mismatched and two single HLA-A locus-matched stimulator-responder pairs. Six patients with chronic myelogenous leukemia and one patient with acute myeloid leukemia transformed from MDS were studied. CTL generated after 10 days stimulation with unselected leukemic peripheral blood mononuclear cells inhibited leukemic CFU-GM colony growth (>85% at 10:1 effector:target ratio) with no third-party colony inhibition. In five pairs, responders were cultured separately with leukemia cells, PHA-B or LCL from the stimulator. After 2-4 restimulations, the T cell repertoire was examined by flow analysis using Vbeta-specific antibodies. Test cultures (but not controls) showed preferential expansion of 1-4 Vbeta families either common to two or more stimulators or unique to a particular stimulator. Notably, we elicited leukemia-specific TCR Vbeta expansions on four out of five occasions. In two pairs, responder cells selected for the appropriate leukemia-specific Vbeta family were shown to have leukemia-specific cytotoxicity. These leukemia-restricted T-cells were CD8+ or CD4+ and CD25+ or CD57+. The results support the development of strategies to selectively deplete GVHD and conserve GVL reactivity in mismatched transplants.

Analysis of cell kinetics using a cell division marker: mathematical modeling of experimental data

We consider an age-maturity structured model arising from a blood cell proliferation problem. This model is "hybrid", i.e., continuous in time and age but the maturity variable is discrete. This is due to the fact that we include the cell division marker carboxyfluorescein diacetate succinimidyl ester. We use our mathematical analysis in conjunction with experimental data taken from the division analysis of primitive murine bone marrow cells to characterize the maturation/proliferation process. Cell cycle parameters such as proliferative rate beta, cell cycle duration tau, apoptosis rate gamma, and loss rate micro can be evaluated from CarboxyFluorescein diacetate Succinimidyl Ester + cell tracking experiments. Our results indicate that after three days in vitro, primitive murine bone marrow cells have parameters beta = 2.2 day(-1), tau = 0.3 day, gamma = 0.3 day(-1), and micro = 0.05 day(-1).

Real-time monitoring of immune responses

With the advent of cellular immunotherapy, the ability to monitor immune responses during treatment will be essential to evaluate the effectiveness of the new therapies. While the ultimate determinate of the success of immunotherapy trials will be clinical outcome, methods of monitoring immunity in real-time have become available that will assist in the development of immunotherapy strategies and in the prediction of individual patient prognosis during the course of treatment. The essentials of existing immune assays are described here with examples of how these techniques have been used previously. A perspective on which approaches will likely prove the most useful for monitoring immune responses in real-time during immunotherapy is also presented.

Targeting autoantigen-specific T cells and suppression of autoimmune encephalomyelitis with receptor-modified T lymphocytes

We demonstrate here the feasibility of antigen-specifically redirecting T cells against autoreactive T lymphocytes and thereby treating a model autoimmune disease. We created and transgenically expressed on T cells a heterodimeric chimeric receptor that genetically links an autoantigenic peptide, its restricting MHC, and the signal transduction domain of the T-cell receptor (TCR) zeta-chain. Engagement of the chimeric receptor by the TCR of autoreactive T cells activated the receptor-modified T cells in vitro and in vivo, inducing proliferation and cytolysis. Adoptively transferred receptor-modified T cells prevented and treated a model autoimmune disease, experimental allergic encephalomyelitis (EAE), even after epitope spreading had diversified the autoantigenic response. Treatment reduced disease severity and increased survival of affected animals, and was durable for >75 days. The receptor-modified cells acted both by strongly attenuating T-cell response to autoantigen as well as by shifting the residual response from an immunopathologic Th1 to a protective Th2 format.

Beyond 51Cr release: New methods for assessing HIV-1-specific CD8+ T cell responses in peripheral blood and mucosal tissues

Much scientific effort has been directed towards elucidating the complexities of cell-mediated immune responses to HIV-1(reviewed in [1,2]). These studies have attempted to explain the immune system's ultimate failure to contain viral replication, leading to development of AIDS disease, and to identify immune responses that will be useful in developing immunomodulatory therapies and novel vaccine strategies. Although many of the complex interactions involved in AIDS pathogenesis remain unsolved, great progress has been made in characterizing the kinetics, specificity and functional dynamics of HIV-1-specific T cell responses. These investigations have come at a time when advances in virology, cellular immunology and molecular biology have converged to provide a variety of methodological approaches not available at the onset of the AIDS pandemic. Application of these tools to other infectious diseases and immunopathological conditions will provide a fertile area of research for future years. This review focuses on recent developments in the assessment of HIV-1-specific T cell responses in peripheral blood and tissues, with a particular emphasis on flow cytometry-based approaches.

The assessment of antigen-specific CD8+ T cells through the combination of MHC class I tetramer and intracellular staining

Peptide-bound histocompatibility leukocyte antigen (HLA) class I tetramers enable a precise identification of antigen-specific CD8+ T cells using flow cytometry. The combination of this technology with intracellular staining techniques opens up significantly better ways of studying these cells than previously possible, allowing immunologists to look at their life cycle (activation and proliferation), manner of death (aging and apoptosis) and effector function (cytotoxic potential and cytokine production). In this review, we hope to provide an overview of these possibilities, as well as making specific suggestions about the use of intracellular staining techniques in the study of antigen-specific T cells. Understanding how antigen-specific cells develop and function in different circumstances and pathologies will be the key to unravelling the secrets of our cellular immune system.

The fluorolysis assay, a highly sensitive method for measuring the cytolytic activity of T cells at very low numbers

We have developed a highly sensitive cytolysis test, the fluorolysis assay, as a simple nonradioactive and inexpensive alternative to the standard 51Cr-release assay. P815 cells were stably transfected with a plasmid expressing the enhanced green fluorescent protein (EGFP) gene. These target cells were coated with or without cognate peptide or anti-CD3 Ab and then incubated with CD8(+) T cells to allow antigen-specific or nonspecific lysis. The degree of target cell lysis was measured using flow cytometry to count the percentage of viable propidium iodide(-) EGFP(+) cells, whose numbers were standardized to a reference number of fluorochrome-linked beads. By using small numbers of target cells (200-800 per reaction) and extended incubation times (up to 2 days), the antigen-specific cytolytic activity of one to two activated CD8(+) T cells of a CTL line could be detected. The redirected fluorolysis assay also measured the activity of very few (> or =6) primary CD8(+) T cells following polyclonal activation. Importantly, antigen-specific lysis by small numbers (> or =25) of primary CD8(+) T cells could be directly measured ex vivo. This exquisite sensitivity of the fluorolysis assay, which was at least 8-33-folds higher than an optimized 51Cr-release assay, allows in vitro and ex vivo studies of immune responses that would otherwise not be possible due to low CTL numbers or frequencies.

Visualization and quantification of T cell-mediated cytotoxicity using cell-permeable fluorogenic caspase substrates

We have developed a non-radioactive flow-cytometry assay to monitor and quantify the target-cell killing activities mediated by cytotoxic T lymphocytes (CTLs). This flow-cytometry CTL (FCC) assay is predicated on measurement of CTL-induced caspase activation in target cells through detection of the specific cleavage of fluorogenic caspase substrates. Here we show that this assay reliably detects antigen-specific CTL killing of target cells, and demonstrate that it provides a more sensitive, more informative and safer alternative to the standard 51Cr-release assay most often used to quantify CTL responses. The FCC assay can be used to study CTL-mediated killing of primary host target cells of different cell lineages, and enables the study of antigen-specific cellular immune responses in real time at the single-cell level. As such, the FCC assay can provide a valuable tool for studies of infectious disease pathogenesis and development of new vaccines and immunotherapies.

An improved flow cytometric assay for the determination of cytotoxic T lymphocyte activity

The cytotoxic activity of T lymphocytes, natural killer and lymphokine-activated killer cells is usually tested by radioactive assays, which detect the release of cytoplasmic contents after plasma membrane disintegration of dying cells. In contrast to this indirect evaluation of cytotoxicity, we describe here an improved fluorescence assay that is based on the direct quantitative and qualitative flow cytometric analysis of cell damage at a single cell level. Target cells are stained with PKH-26, a lipophilic dye that stably integrates into the cell membrane and permits distinction between target and effector cells. After 3 h of in vitro incubation, costaining with AnnexinV-FITC (ann-FITC) and propidium iodide (PI) permitted discrimination between vital, early apoptotic and necrotic cells. Data analysis is performed first by gating on PKH-26-positive target cells followed by the analysis of ann-FITC- and PI-positive subpopulations. The percentage of cytotoxicity in the PKH-26-gated cell population is calculated by subtracting non-specific ann-FITC- or PI-positive target cells, measured in appropriate controls without effector cells. Membrane staining of target cells such as primary melanoma cells or leukemic blasts revealed high and stable loading of PKH-26 without altering the viability or the immunogenicity of the cells. Using in vitro-generated antigen-specific cytotoxic T lymphocytes (CTL), we could demonstrate that this flow cytometric assay is sensitive and correlates well with the standard 51Cr release assay. In conclusion, the improved fluorescence assay is a simple and highly reproducible procedure for evaluating the specific cytotoxicity of T cells.