BLT esterase activity as an alternative to chromium release in cytotoxic T cell assays

Long chikungunya? An overview to immunopathology of persistent arthralgia

Chikungunya fever (CF) is caused by an arbovirus whose manifestations are extremely diverse, and it has evolved with significant severity in recent years. The clinical signs triggered by the Chikungunya virus are similar to those of other arboviruses. Generally, fever starts abruptly and reaches high levels, followed by severe polyarthralgia and myalgia, as well as an erythematous or petechial maculopapular rash, varying in severity and extent. Around 40% to 60% of affected individuals report persistent arthralgia, which can last from months to years. The symptoms of CF mainly represent the tissue tropism of the virus rather than the immunopathogenesis triggered by the host's immune system. The main mechanisms associated with arthralgia have been linked to an increase in T helper type 17 cells and a consequent increase in receptor activator of nuclear factor kappa-Β ligand and bone resorption. This review suggests that persistent arthralgia results from the presence of viral antigens post-infection and the constant activation of signaling lymphocytic activation molecule family member 7 in synovial macrophages, leading to local infiltration of CD4+ T cells, which sustains the inflammatory process in the joints through the secretion of pro-inflammatory cytokines. The term "long chikungunya" was used in this review to refer to persistent arthralgia since, due to its manifestation over long periods after the end of the viral infection, this clinical condition seems to be characterized more as a sequel than as a symptom, given that there is no active infection involved.

Role of the granzyme family in rheumatoid arthritis: Current Insights and future perspectives

Rheumatoid arthritis (RA) is a complex autoimmune disease characterized by chronic inflammation that affects synovial tissues of multiple joints. Granzymes (Gzms) are serine proteases that are released into the immune synapse between cytotoxic lymphocytes and target cells. They enter target cells with the help of perforin to induce programmed cell death in inflammatory and tumor cells. Gzms may have a connection with RA. First, increased levels of Gzms have been found in the serum (GzmB), plasma (GzmA, GzmB), synovial fluid (GzmB, GzmM), and synovial tissue (GzmK) of patients with RA. Moreover, Gzms may contribute to inflammation by degrading the extracellular matrix and promoting cytokine release. They are thought to be involved in RA pathogenesis and have the potential to be used as biomarkers for RA diagnosis, although their exact role is yet to be fully elucidated. The purpose of this review was to summarize the current knowledge regarding the possible role of the granzyme family in RA, with the aim of providing a reference for future research on the mechanisms of RA and the development of new therapies.

Widespread discrepancy in Nnt genotypes and genetic backgrounds complicates granzyme A and other knockout mouse studies

Granzyme A (GZMA) is a serine protease secreted by cytotoxic lymphocytes, with Gzma-/- mouse studies having informed our understanding of GZMA's physiological function. We show herein that Gzma-/- mice have a mixed C57BL/6J and C57BL/6N genetic background and retain the full-length nicotinamide nucleotide transhydrogenase (Nnt) gene, whereas Nnt is truncated in C57BL/6J mice. Chikungunya viral arthritis was substantially ameliorated in Gzma-/- mice; however, the presence of Nnt and the C57BL/6N background, rather than loss of GZMA expression, was responsible for this phenotype. A new CRISPR active site mutant C57BL/6J GzmaS211A mouse provided the first insights into GZMA's bioactivity free of background issues, with circulating proteolytically active GZMA promoting immune-stimulating and pro-inflammatory signatures. Remarkably, k-mer mining of the Sequence Read Archive illustrated that ≈27% of Run Accessions and ≈38% of BioProjects listing C57BL/6J as the mouse strain had Nnt sequencing reads inconsistent with a C57BL/6J genetic background. Nnt and C57BL/6N background issues have clearly complicated our understanding of GZMA and may similarly have influenced studies across a broad range of fields.

Development of a reporter gene method to measure the bioactivity of anti-CD38 × CD3 bispecific antibody

Background

A T cell-redirecting bispecific antibody (bsAb) consisting of a tumor-binding unit and a T cell-binding unit is a large group of antibody-based biologics against death-causing cancer diseases. The anti-CD38 × anti-CD3 bsAb (Y150) is potential for treating multiple myeloma (MM). When developing a cell-based reporter gene bioassay to assess the activities of Y150, it was found that the expression of CD38 on the human T lymphocyte cells (Jurkat) caused the nonspecific activation, which interfered with the specific T cells activation of mediated by the Y150 and CD38(+) tumor cells.

Methods

Here, we first knocked-out the CD38 expression on Jurkat T cell line by CRISPR-Cas9 technology, then developed a stable monoclonal CD38(−) Jurkat T cell line with an NFAT-RE driving luciferase expressing system. Further based on the CD38(−) Jurkat cell, we developed a reporter gene method to assess the bioactivity of the anti-CD38 × anti-CD3 bsAb.

Results

Knocking out CD38 expression abolished the nonspecific self-activation of the Jurkat cells. The selected stable monoclonal CD38(−) Jurkat T cell line assured the robustness of the report genes assay for the anti-CD38 × anti-CD3 bsAb. The relative potencies of the Y150 measured by the developed reporter gene assay were correlated with those by the flow-cytometry-based cell cytotoxicity assay and by the ELISA-based binding assay.

Conclusions

The developed reporter gene assay was mechanism of action-reflective for the bioactivity of anti-CD38 × anti-CD3 antibody, and suitable for the quality control for the bsAb product.

CD56 regulates human NK cell cytotoxicity through Pyk2

Human natural killer (NK) cells are defined as CD56+CD3-. Despite its ubiquitous expression on human NK cells the role of CD56 (NCAM) in human NK cell cytotoxic function has not been defined. In non-immune cells, NCAM can induce signaling, mediate adhesion, and promote exocytosis through interactions with focal adhesion kinase (FAK). Here we demonstrate that deletion of CD56 on the NK92 cell line leads to impaired cytotoxic function. CD56-knockout (KO) cells fail to polarize during immunological synapse (IS) formation and have severely impaired exocytosis of lytic granules. Phosphorylation of the FAK family member Pyk2 at tyrosine 402 is decreased in NK92 CD56-KO cells, demonstrating a functional link between CD56 and signaling in human NK cells. Cytotoxicity, lytic granule exocytosis, and the phosphorylation of Pyk2 are rescued by the reintroduction of CD56. These data highlight a novel functional role for CD56 in stimulating exocytosis and promoting cytotoxicity in human NK cells.

Granzyme A in Chikungunya and Other Arboviral Infections

Granzyme A (GzmA) is secreted by cytotoxic lymphocytes and has traditionally been viewed as a mediator of cell death. However, a growing body of data suggests the physiological role of GzmA is promotion of inflammation. Here, we show that GzmA is significantly elevated in the sera of chikungunya virus (CHIKV) patients and that GzmA levels correlated with viral loads and disease scores in these patients. Serum GzmA levels were also elevated in CHIKV mouse models, with NK cells the likely source. Infection of mice deficient in type I interferon responses with CHIKV, Zika virus, or dengue virus resulted in high levels of circulating GzmA. We also show that subcutaneous injection of enzymically active recombinant mouse GzmA was able to mediate inflammation, both locally at the injection site as well as at a distant site. Protease activated receptors (PARs) may represent targets for GzmA, and we show that treatment with PAR antagonist ameliorated GzmA- and CHIKV-mediated inflammation.

Characterization of a single reporter-gene potency assay for T-cell-dependent bispecific molecules

T-cell-dependent bispecific antibodies (TDBs) are promising cancer immunotherapies that recruit patients' T cells to kill cancer cells. There are many TDBs in clinical trials, demonstrating their widely recognized therapeutic potential. However, their complex, multi-step mechanism of action (MoA), which includes bispecific antigen binding, T-cell activation, and target-cell killing, presents unique challenges for biological characterization and potency assay selection. Here, we describe the development of a single reporter-gene potency assay for a TDB (TDB1) that is MoA reflective and sensitive to binding of both antigens. Our reporter-gene assay measures T-cell activation using Jurkat cells engineered to express luciferase under the control of an NFkB response element. The potencies of select samples were measured both by this assay and by a flow-cytometry-based cell-killing assay using human lymphocytes as effector cells. Correlating the two sets of potency results clearly establishes our reporter-gene assay as MoA reflective. Furthermore, correlating potencies for the same panel of samples against binding data measured by binding assays for each individual arm demonstrates that the reporter-gene potency assay reflects dual-antigen binding and can detect changes in affinity for either arm. This work demonstrates that one reporter-gene assay can be used to measure the potency of TDB1 while capturing key aspects of its MoA, thus serving as a useful case study of selection and justification of reporter-gene potency assays for TDBs. Furthermore, our strategy of correlating reporter-gene potency, target-cell killing, and antigen binding for each individual arm serves as a useful example of a thorough, holistic approach to biological characterization for TDBs that can be applied to other bispecific molecules.

RNA-Seq analysis of chikungunya virus infection and identification of granzyme A as a major promoter of arthritic inflammation

Chikungunya virus (CHIKV) is an arthritogenic alphavirus causing epidemics of acute and chronic arthritic disease. Herein we describe a comprehensive RNA-Seq analysis of feet and lymph nodes at peak viraemia (day 2 post infection), acute arthritis (day 7) and chronic disease (day 30) in the CHIKV adult wild-type mouse model. Genes previously shown to be up-regulated in CHIKV patients were also up-regulated in the mouse model. CHIKV sequence information was also obtained with up to ≈8% of the reads mapping to the viral genome; however, no adaptive viral genome changes were apparent. Although day 2, 7 and 30 represent distinct stages of infection and disease, there was a pronounced overlap in up-regulated host genes and pathways. Type I interferon response genes (IRGs) represented up to ≈50% of up-regulated genes, even after loss of type I interferon induction on days 7 and 30. Bioinformatic analyses suggested a number of interferon response factors were primarily responsible for maintaining type I IRG induction. A group of genes prominent in the RNA-Seq analysis and hitherto unexplored in viral arthropathies were granzymes A, B and K. Granzyme A^-/- and to a lesser extent granzyme K^-/-, but not granzyme B^-/-, mice showed a pronounced reduction in foot swelling and arthritis, with analysis of granzyme A^-/- mice showing no reductions in viral loads but reduced NK and T cell infiltrates post CHIKV infection. Treatment with Serpinb6b, a granzyme A inhibitor, also reduced arthritic inflammation in wild-type mice. In non-human primates circulating granzyme A levels were elevated after CHIKV infection, with the increase correlating with viral load. Elevated granzyme A levels were also seen in a small cohort of human CHIKV patients. Taken together these results suggest granzyme A is an important driver of arthritic inflammation and a potential target for therapy.

Trial Registration: ClinicalTrials.gov NCT00281294

The largest chikungunya virus (CHIKV) epidemic ever recorded began in 2004 in Africa and spread across Asia reaching Europe and recently the Americas, with millions of cases reported. We undertook a detailed analysis of the mRNA expression profile during acute and chronic arthritis in an adult wild-type mouse model of CHIKV infection and disease. Gene induction profiles showed a high concordance with published human data, providing some validation of the mouse model. The host response was overwhelmingly dominated by type I interferon response genes, even after type I interferon induction was lost. The analysis also provided information on CHIKV RNA, with no adaptive viral genome changes identified. An important goal of the analysis was to identify new players in arthritic inflammation. Granzyme A was prominent in the RNA-Seq data and granzyme A deficient mice showed reduced arthritis, with no effects on viral loads. Arthritic disease could also be ameliorated in wild-type mice with a granzyme A inhibitor. Elevated circulating granzyme A levels were seen in non-human primates infected with CHIKV and in human CHIKV patients. Granzyme A thus emerges to be a major driver of CHIKV-mediated arthritic inflammation and a potential target for anti-inflammatory interventions.

New flow cytometric assays for monitoring cell-mediated cytotoxicity

The exact immunologic responses after vaccination that result in effective antitumor immunity have not yet been fully elucidated and the data from ex vivo T-cell assays have not yet defined adequate surrogate markers for clinical efficacy. A more detailed knowledge of the specific immune responses that correlate with positive clinical outcomes should help to develop better or novel strategies to effectively activate the immune system against tumors. Furthermore, clinically relevant material is often limited and, thus, precludes the ability to perform multiple assays. The two main assays currently used to monitor lymphocyte-mediated cytoxicity in cancer patients are the (51)Cr-release assay and IFN-gamma ELISpot assay. The former has a number of disadvantages, including low sensitivity, poor labeling and high spontaneous release of isotope from some tumor target cells. Additional problems with the (51)Cr-release assay include difficulty in obtaining autologous tumor targets, and biohazard and disposal problems for the isotope. The ELISpot assays do not directly measure cytotoxic activity and are, therefore, a surrogate marker of cyotoxic capacity of effector T cells. Furthermore, they do not assess cytotoxicity mediated by the production of the TNF family of death ligands by the cytotoxic cells. Therefore, assays that allow for the simultaneous measurement of several parameters may be more advantageous for clinical monitoring. In this respect, multifactor flow cytometry-based assays are a valid addition to the currently available immunologic monitoring assays. Use of these assays will enable detection and enumeration of tumor-specific cytotoxic T lymphocytes and their specific effector functions and any correlations with clinical responses. Comprehensive, multifactor analysis of effector cell responses after vaccination may help to detect factors that determine the success or failure of a vaccine and its immunological potency.

A novel flow cytometric assay focusing on perforin release mechanisms of cytotoxic T lymphocytes

CD8(hi+) cytotoxic T lymphocytes (CTL) are major players in immune defense. In addition, they contribute to the maintenance of immune homeostasis. We now describe a hitherto unavailable, but simple assay to determine ex vivo lytic granule-based cytotoxic functions of human CD8(hi+) CTL subgroups in a clinical setting, under target cell free conditions. Ficoll-isolated peripheral blood lymphocytes from 17 healthy volunteers were stimulated either by phorbol 12-myristate 13-acetate (PMA) in combination with ionomycin or by antibody mediated crosslinking of the CD3 molecule on the T cell surface. Using perforin as a marker for lytic granules, the reduction of CTL granules over time intervals up to 120 min was quantified by FACScan flow cytometry. The kinetics of perforin reduction were compared to the kinetics of NA-CBZ-L-lysine-thiobenzyl ester hydrochloride (BLT)-esterase release and of CD63 upregulation. The reduction in the perforin(+) portion of CD8(hi+) CTLs was correlated inversely with BLT-esterase release and CD63 upregulation. At 30 and 120 min after PMA/ionomycin stimulation, 55 +/- 14% and 42 +/- 14%, respectively, of CD8(hi+) CTLs still stained perforin(+) (time point 0 min = 100%). Perforin-granule release induced by CD3-crosslinking occurred as fast within 30 min (55 +/- 17%), but over the 120 min time interval it was not as complete when compared to PMA/ionomycin-stimulated perforin-reduction. Thus, the combination of an established degranulation assay with the power of immuno flow cytometry allows one to investigate the cytotoxic capability of CTL-subtypes and the kinetics of perforin-granule release. In addition, the assay may prove useful in the elucidation of intracellular signaling cascades governing the perforin-granule release process.

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.

Dietary conjugated linoleic acid modulates phenotype and effector functions of porcine CD8(+) lymphocytes

In vivo vaccination and challenge studies have demonstrated that CD8(+) lymphocytes are essential for the development of cell-mediated protection against intracellular pathogens and neoplastic cells. Depletion of peripheral blood CD8(+) cells interferes with clearance of viruses and intracellular fungi, induction of delayed type hypersensitivity responses and antitumoral activity. In contrast to humans or mice, porcine peripheral CD8(+) lymphocytes are characterized by a heterogeneous expression pattern (i.e., CD8alphabeta and CD8alphaalpha) that facilitates the study of distinctive traits among minor CD8(+) cell subsets. A factorial (2 x 2) arrangement within a split-plot design, with 16 blocks of two littermate pigs as the experimental units for immunization treatment (i.e., unvaccinated or vaccinated with a proteinase-digested Brachyspira hyodysenteriae bacterin) and pig within block as the experimental unit for dietary treatment (soybean oil or conjugated linoleic acid) were used to investigate the phenotypic and functional regulation of CD8(+) cells by dietary conjugated linoleic acid (CLA). Dietary CLA supplementation induced in vivo expansion of porcine CD8(+) cells involving T-cell receptor (TCR)gammadeltaCD8alphaalpha T lymphocytes, CD3(-)CD16(+)CD8alphaalpha (a porcine natural killer cell subset), TCRalphabetaCD8alphabeta T lymphocytes and enhanced specific CD8(+)-mediated effector functions (e.g., granzyme activity). Expansion of peripheral blood TCRalphabetaCD8alphabeta cells was positively correlated (r = 0.89, P < 0.01) with increased percentages of CD8alphabeta(+) thymocytes. Functionally, CLA enhanced the cytotoxic potential of peripheral blood lymphocytes and proliferation of TCRgammadeltaCD8alphaalpha cells. Collectively, these results indicate that dietary CLA enhances cellular immunity by modulating phenotype and effector functions of CD8(+) cells involved in both adaptive and innate immunity.

MHC I-dependent antigen presentation is inhibited by poliovirus protein 3A

The effects of poliovirus 3A protein expression and poliovirus infection on the presentation of hepatitis C virus antigens in cultured chimpanzee cells were examined. Expression of poliovirus 3A protein inhibits protein secretion when expressed in isolation and was sufficient to protect chimpanzee cells from lysis by hepatitis C virus-specific cytotoxic T cells in standard (51)Cr-release assays. Poliovirus infection also inhibited antigen presentation, as determined by decreased cytotoxic T cell activation. A mutation in 3A that abrogates the inhibition of protein secretion also abolished the effects of poliovirus on antigen presentation. These results demonstrate that the inhibition of secretion observed in poliovirus-infected cells substantially reduces the presentation of new antigens on the cell surface. These observations may reflect a general mechanism by which nonenveloped viruses such as poliovirus and other viruses that do not require a functional protein secretory apparatus can evade detection by the cellular immune response.

Cutting Edge: Differentiation of Antitumor CTL In Vivo Requires Host Expression of Stat1

Several lines of evidence suggest that an IFN-γ-producing, Th1/Tc1 phenotype may be optimal for tumor rejection. Recent work has indicated that IFN signaling on tumor cells is important for protection against carcinogenesis. However, the potential involvement of IFN signaling among host immune cells has not been carefully examined. To this end, Stat1-deficient mice were employed as tumor recipients. In contrast to wild-type mice, Stat1−/− mice failed to reject immunogenic tumors and did not support regression of poorly immunogenic tumors when treated with an IL-12-based vaccine. T cells from immunized Stat1−/− mice produced 50% of the levels of IFN-γ and lacked cytolytic activity compared with wild-type mice, and NK lytic activity also was not observed. Lack of cytolytic function correlated with a failure to up-regulate serine esterase activity. Thus, IFN-mediated signaling on host cells is required for the development of antitumor lytic effector cells.

A novel method for measuring CTL and NK cell-mediated cytotoxicity using annexin V and two-color flow cytometry

An assay based on two-color flow cytometry has been developed to measure CTL and NK cell-mediated cytotoxicity. After effector/target cells are incubated together, CTL or NK populations are stained with an effector cell specific PE-conjugated mAb. Subsequently, annexin V-FITC binds to cells expressing phosphatidylserine (an early marker of apoptosis) on the cell surface. Target cells are gated upon as PE-negative and quantified with respect to their annexin V positivity. The shift from annexin Vneg to annexin Vhi is a discrete event such that all target cells fall within discernible populations with respect to annexin V. There is a strong correlation between cytotoxicity measured with our assay and a standard 51Cr release assay (r2 = 0.989). The PE/annexin V assay shows increased sensitivity at early timepoints after target/effector cell mixing. In addition, this method allows for analysis of target cells at the single cell level. Therefore, we have described a promising new technique to measure in vitro cell-mediated cytotoxicity. It avoids the potential difficulties of working with radioactive isotopes, and offers increased sensitivity and versatility.

Extracellular Granzymes A and B in Humans: Detection of Native Species During CTL Responses In Vitro and In Vivo

Activated CTLs and NK cells induce apoptosis via multiple mechanisms, including that termed granule exocytosis. The latter pathway consists of vectorial secretion of perforin and a family of granule-associated serine proteases (granzymes) to the target cell. To establish whether granzymes are released extracellularly during cytolytic reactions in vivo, ELISAs that measure the native enzymes were developed and were found to specifically detect granzyme A (GrA) and granzyme B (GrB) at picogram concentrations. Low levels of GrA and GrB were present in plasma of healthy individuals (GrA, 33.5 pg/ml (median); GrB, 11.5 pg/ml (median)), whereas significantly higher levels were present in patients with ongoing CTL response, i.e., patients suffering from infections by EBV or HIV type 1. Markedly elevated levels were also noted in synovial fluid of patients with active rheumatoid arthritis. The measurement of soluble granzymes should be useful to assess clinical disorders associated with activated CTL and NK cells. Furthermore, these results suggest that granzymes mediate biologic effects beyond their described role in apoptotic cell death.

Herpes simplex virus type 1 renders infected cells resistant to cytotoxic T-lymphocyte-induced apoptosis

Many viruses interfere with apoptosis of infected cells, presumably preventing cellular apoptosis as a direct response to viral infection. Since cytotoxic T lymphocytes (CTL) induce apoptosis of infected cells as part of the "lethal hit," inhibition of apoptosis could represent an effective immune evasion strategy. We report here herpes simplex virus type 1 (HSV-1) interference with CTL-induced apoptosis of infected cells and show that HSV-1 inhibits the nuclear manifestations of apoptosis but not the membrane changes. The HL-60 cell line (human promyelocytic leukemia) undergoes apoptosis in response to many stimuli, including incubation with ethanol. After HSV-1 infection (strains E115 and 17+), ethanol-treated cells did not produce oligonucleosomal DNA fragments characteristic of apoptosis, as assayed by gel electrophoresis and enzyme-linked immunosorbent assay. Inhibition was detected 2 h after infection and increased over time. Importantly, HSV-1-infected cells were resistant to apoptosis induced by antigen-specific CD4+ CTL, despite the fact that CTL recognition and degranulation in response to infected targets remained intact. Unlike HSV-1, HSV-2 (strains 333 and HG52) did not inhibit DNA fragmentation. In contrast to the inhibition of DNA fragmentation by HSV-1, none of the HSV-1 or -2 strains interfered with the ethanol-induced exposure of surface phosphatidylserine characteristic of apoptosis, as determined by annexin V binding. These results demonstrate that genes of HSV-1 inhibit the nuclear manifestations of apoptosis but not the membrane manifestations, suggesting that these may be mediated via separate pathways. They also suggest that HSV-1 inhibition of CTL-induced apoptosis may be an important mechanism of immune evasion.

Analyzing cytotoxic T lymphocyte activity: a simple and reliable flow cytometry-based assay

A flow cytometry-based assay for analyzing cytotoxic T lymphocyte (CTL) activity is presented. This new approach is characterized by easy handling, the generation of highly reproducible data sets and is not dependent on the use of radioactivity. Before exposure to primed CTL effector cells the target cells were labeled with the green fluorescent dye DiO18(3) which is incorporated stably into the cell membrane. After a 4-h incubation period, samples were counterstained with the red fluorescent nuclear dye propidium iodide in order to permit discrimination between live and dead cells within both cell populations. The assay has been used to quantitate CTL effector activity against allogeneic lymphoblasts. Results derived from this novel flow cytometry assay show an excellent correlation (r = 0.988) with data obtained using the standard 51chromium release assay. An additional advantage of the assay is that freshly prepared splenocytes may be used as target cells because culturing and activation of target cells is no longer required. The results demonstrate that the fluorescent dyes DiO18(3) and propidium iodide in combination with flow cytometry permit accurate analysis of cytotoxic T cell activity.

A flow cytometric long-term cytotoxicity assay

A method to evaluate cytotoxic effects applicable over a wide range of incubation times has been developed. It is based on quantification by flow cytometry of dead and viable target cells stained by covalently binding the fluorescent dye fluorescein isothiocyanate (FITC). The staining with FITC did not affect cell viability and growth parameters and was stable enough to identify target cells for at least 2 days. The flow cytometric analysis of the cell mixture was performed in a test system with activated CD8+ lymphocytes as effector cells and melanoma M21 cells as targets in the presence of appropriate bispecific antibodies and revealed a rather complex pattern composed of several distinct cell subsets which were identified by use of antibodies to lymphocyte antigens. The assay compared favourably with results from a conventional 51Cr release assay obtained after 4 h and 8 h of incubation and from a target cell adherence assay obtained after 24 h of incubation. The application of the method described herein is especially advantageous for the evaluation of long-term cytotoxic effects. Furthermore, it provides valuable multi-parameter information which is useful for elucidating mechanisms of cytotoxicity.