An improved europium release assay for complement-mediated cytolysis

In vitro beta-cell killing models using immune cells and human pluripotent stem cell-derived islets: Challenges and opportunities

Type 1 diabetes (T1D) is a disease of both autoimmunity and β-cells. The β-cells play an active role in their own demise by mounting defense mechanisms that are insufficient at best, and that can become even deleterious in the long term. This complex crosstalk is important to understanding the physiological defense mechanisms at play in healthy conditions, their alterations in the T1D setting, and therapeutic agents that may boost such mechanisms. Robust protocols to develop stem-cell-derived islets (SC-islets) from human pluripotent stem cells (hPSCs), and islet-reactive cytotoxic CD8+ T-cells from peripheral blood mononuclear cells offer unprecedented opportunities to study this crosstalk. Challenges to develop in vitro β-cell killing models include the cluster morphology of SC-islets, the relatively weak cytotoxicity of most autoimmune T-cells and the variable behavior of in vitro expanded CD8+ T-cells. These challenges may however be highly rewarding in light of the opportunities offered by such models. Herein, we discuss these opportunities including: the β-cell/immune crosstalk in an islet microenvironment; the features that make β-cells more sensitive to autoimmunity; therapeutic agents that may modulate β-cell vulnerability; and the possibility to perform analyses in an autologous setting, i.e., by generating T-cell effectors and SC-islets from the same donor.

Live Cell Labeling with Terpyridine Derivative Proligands to Measure Cytotoxicity Mediated by Immune Cells

Immunotherapy using immune checkpoint inhibitors and CAR-T cells has revolutionized treatment for patients with malignant tumors. However, measuring tumor cell cytotoxicity mediated by immune effector cells in clinical laboratories has been difficult due to the requirement for radioactive substances. In this study, a series of novel terpyridine derivative proligands were synthesized, and a non-radioactive cellular cytotoxicity assay using the newly synthesized compounds was developed for use in preclinical and clinical studies for cancer immunotherapy. Once internalized into target cells, the compounds are hydrolyzed by esterases, resulting in the intracellular accumulation of the negatively charged terpyridine derivatives. When the labeled target cells are recognized and killed by immune effector cells, the integrity of the cell membrane is disrupted, and the terpyridine derivatives are released. Upon combining the culture supernatant with europium (Eu³⁺ ), the cytotoxicity of immune effector cells for the target cells can be quantitatively determined by measuring the intensity of the Eu³⁺ /ligand-derived time-resolved fluorescence. Thus, the assay developed in this study would facilitate the development of novel cancer immunotherapies.

Pushing the frontiers of T-cell vaccines: accurate measurement of human T-cell responses

There is a need for novel approaches to tackle major vaccine challenges such as malaria, tuberculosis and HIV, among others. Success will require vaccines able to induce a cytotoxic T-cell response--a deficiency of most current vaccine approaches. The successful development of T-cell vaccines faces many hurdles, not least being the lack of consensus on a standardized T-cell assay format able to be used as a correlate of vaccine efficacy. Hence, there remains a need for reproducible measures of T-cell immunity proven in human clinical trials to correlate with vaccine protection. The T-cell equivalent of a neutralizing antibody assay would greatly accelerate the development and commercialization of T-cell vaccines. Recent advances have seen a plethora of new T-cell assays become available, including some like cytometry by time-of-flight with extreme multiparameter T-cell phenotyping capability. However, whether it is historic thymidine-based proliferation assays or sophisticated new cytometry assays, each assay has its relative advantages and disadvantages, and relatively few of these assays have yet to be validated in large-scale human vaccine trials. This review examines the current range of T-cell assays and assesses their suitability for use in human vaccine trials. Should one or more of these assays be accepted as an agreed surrogate of T-cell protection by a regulatory agency, this would significantly accelerate the development of T-cell vaccines.

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.

Improved protocol for colorimetric detection of complement-mediated cytotoxicity based on the measurement of cytoplasmic lactate dehydrogenase activity

The conventional cytotoxicity detection protocols based on the lactate dehydrogenase (LDH) activity assays rely upon the quantitation of the enzyme activity released into the assay medium upon cell lysis. In the case of complement-mediated cytotoxicity this results in the need to take into account the high and variable background LDH activity from the serum added to the cells. Using primate-derived COS-7 and pig kidney PK15 cell lines we show that, in the case of adherent cells, it is possible to overcome this drawback and to measure cell death caused by complement attack, by quantifying the amount of LDH activity retained by the undamaged cells. The modified assay is therefore quicker to carry out than the conventional procedure and cheaper because fewer control readings must be taken.

Application of non-radioactive europium (Eu3+) release assay to a measurement of human natural killer activity of healthy and patient populations

Europium (Eu3+) release assay is a non-radioactive method for a measurement of cytotoxicity of lymphocytes and has several advantages compared with a conventional 51Cr release assay. However, the Eu3+ release assay has not been applied to a natural killer (NK) activity measurement of a large number of the human population mainly due to a lack of comparability with the 51Cr release assay. With some modifications of the procedures and careful manipulation of cells, constant and reproducible results were obtained by the Eu3+ release assay. NK activity of several individuals was measured by the Eu3+ release assay and was compared with data obtained by 51Cr release assay performed simultaneously. The obtained values by the two methods were almost identical. We applied the Eu3+ method to measure NK activity of a large number of individuals, including 68 apparently healthy donors and 36 autoimmune and 21 cancer patients. Some of these diseases are known to show abnormal NK activity. The obtained cytotoxicities were mostly consistent with the previously reported data obtained by the 51Cr release assay. These results indicated that the Eu3+ release assay could be used as an alternative method for a measurement of human NK activity of mass population including patients.

Time-resolved fluorometric assay for natural killer activity using target cells labelled with a fluorescence enhancing ligand

A time-resolved fluorometric assay for the measurement of natural killer cell activity against target cells labelled with the acetoxymethyl ester of the fluorescence enhancing ligand 2,2':6',2"-terpyridine-6,6"-dicarboxylic acid (TDA) is described. The hydrophobic esterified form of TDA (bis(acetoxymethyl) 2,2':6',2"-terpyridine-6,6"-dicarboxylate, BATDA) diffuses readily through the cell membrane of viable cells. BATDA is hydrolysed by intracellular esterases resulting in accumulation of membrane impermeable TDA inside the target cells. After incubation of labelled K-562 cells with effector cells the TDA released from lysed cells into the supernatant is chelated with Eu3+. The natural killer cell activity is then quantified by measuring the intense fluorescence of the EuTDA chelates formed. Target cells are rapidly labelled when incubated with BATDA, TDA is released from target cells faster than 51Cr, the spontaneous release permits a short-term release assay to be set up and the detection of EuTDA is fast (5 min/96 well plate). Furthermore, this non-radioactive method permits the use of complex culture media since, in contrast to methods based on prompt fluorometry, the problem with autofluorescence can be avoided by the use of time-resolved fluorometry.

Increased tumor cell reactivity and complement-dependent cytotoxicity with mixtures of monoclonal antibodies against different gangliosides

Melanomas and other cancers of neuroectodermal origin express multiple cell-surface gangliosides in patterns that vary significantly even within the same tumor type. Monoclonal antibodies (mAb) against four of these gangliosides (GM2, GD2, 9-O-acetyl-GD3 and GD3) were tested alone and in combination on 14 tumor cell lines (7 melanomas, 3 neuroblastomas, 3 sarcomas and 1 astrocytoma) using flow cytometry and complement-dependent cytotoxicity (CDC) assays. Increased tumor cell recognition and CDC resulting from the combination of three or four mAb were found in 14/14 tested cell lines, and this was most striking when each mAb was used at suboptimal concentration. At these concentrations, the average mean fluorescence intensity of the 14 cell lines with individual mAb was between 3.0 and 6.8 and increased to 10.8 and 18.8 with the three- and four-mAb mixtures. The average percentage CDC-specific release with individual mAb was 2.0%-8.3%, and 12.3% and 16.6% with the three- and four-mAb combinations. The number of cell lines showing significant mean fluorescence intensity and CDC increased from 2-8/14 with single mAb to 13-14/14 with the mixtures of three or four mAb. Our experimental results support the rationale for active immunization with a polyvalent ganglioside vaccine or passive therapy with a combination of mAb to different gangliosides in patients with tumors of neuroectodermal origin. In addition, our studies have demonstrated that 9-O-acetyl-GD3 is a surprisingly effective target for immune attack, although it is a minor constituent of these cells.

Differentiation of cytotoxicity using target cells labelled with europium and samarium by electroporation

We report the simultaneous use of europium-DTPA (Eu-DTPA) and samarium-DTPA (Sm-DTPA) in cytotoxicity experiments to analyze simultaneously LAK and NK cell lysis and to differentiate between specific target lysis and bystander killing. The target cells were either labelled with Eu-DTPA or Sm-DTPA chelates by electroporation, which permits the use of target cell lines or primary leukemic B cells (B-CLL) that cannot be labelled by the conventional dextran-sulphate method. The release of europium and samarium reaches a maximum at comparable time intervals (2-3 h). Due to the shorter counting interval within the samarium window the labelling efficiency is about ten times less efficient compared to europium. Using europium as label for the LAK target Daudi and samarium as label for the NK sensitive cell line K562 the differentiation of LAK versus NK activity can be performed in a single culture assay. Also, the killing of B cells and bystander cells by cytotoxic T cells was analyzed in a system where T cells were redirected to B cells through CD3 x CD19 bispecific antibodies. In fact, no bystander killing was noted when bispecific antibodies were used to bridge cytotoxic T cells to the B cells. This approach provides a simple non-radioactive method for evaluating cytotoxicity against two different cells in a single culture well.

Simultaneous measurement of NK cell cytotoxicity against two target cell lines labelled with fluorescent lanthanide chelates

We describe a cytotoxicity assay which permits the simultaneous measurement of natural killer cell activity against two different cell lines. The target cell lines are labelled either with a fluorescent europium chelate or with a fluorescent terbium chelate and cell death is quantified by measuring the chelate release. K-562, Molt4 and Daudi cell lines have been used as targets. The release of the two chelates from the target cells can be detected with the help of time resolved fluorometry. As the measurements are made after background fluorescence has decayed no additional steps are needed to correct for the background from the medium. The assay procedure used for measurement of cytotoxicity against two target cell lines is very similar to the widely used 51Cr release assay.

Simultaneous measurement of natural killer cell cytotoxicity against each of three different target cell lines

A time-resolved fluorometric assay for the simultaneous measurement of natural killer cell activity against three different lanthanide diethylenetriaminopentaacetate (LaDTPA) labelled target cell lines is described. The target cell line K-562 was labelled with SmDTPA, the cell line Molt with TbDTPA and the cell line Raji with EuDTPA. After co-incubation of the three target cell lines with effector cells the fluorescence of the lanthanides released from the lysed target cells was measured in an enhancer solution in which they formed highly fluorescent complexes. It was possible to differentiate the specific release from the three target cell lines because the emission lines of the europium, samarium and terbium complexes formed in the enhancer solution are well separated from each other. The autofluorescence from culture media supplemented with serum was avoided by the use of time-resolved fluorometry. The results show that applying fluorometry based on the combination of spectral and temporal resolution to natural killer cell assays, makes possible the simultaneous determination of lysis in up to three target cell lines in complex culture medium.

Cytolytic antibodies to melanocytes in vitiligo

Patients with vitiligo have been found to have circulating antibodies to pigment cells. To evaluate the functional activity of these antibodies, a highly sensitive europium release assay was used to compare complement-mediated cytolysis of human melanocytes by sera of 56 patients with vitiligo (20 with active disease, 25 with inactive disease, 11 with unidentified disease activity) and 47 control individuals. Significant melanocyte lysis was mediated by 32 (57%) of the patients with vitiligo but by only three (6%) of the control sera (p < 0.001), and by 17 (85%) of 20 patients with active vitiligo versus 11 (44%) of 25 patients with inactive disease (p < 0.025). Mean melanocyte lysis by vitiligo sera was 24% versus 6% by control sera (p < 0.0001). A subset of 12 vitiligo sera with high titers of cytolytic antibodies to melanocytes (34% mean cytolysis) reacted minimally (< 2% mean cytolysis) to a panel of control cells that included human and murine melanomas, human fibroblasts, lung carcinoma, and rhabdomyosarcoma. These findings indicate that antibodies present in patients with vitiligo have the functional ability to selectively kill melanocytes and are more common in active disease. These observations support, but do not prove, the hypothesis that vitiligo is an autoimmune disease and that anti-pigment cell antibodies have a role in inducing the disease.

Modified procedure for labelling target cells in a europium release assay of natural killer cell activity

Lanthanide europium chelated to diethylenetriaminopentaacetate (EuDTPA) can be used to label target cells such as tumor cells and lymphocytes (Blomberg et al., 1986a,b; Granberg et al., 1988). This procedure has permitted the development of new non-radioactive methods for the detection of target cell cytolysis by natural killer (NK) cells (Blomberg et al., 1986a,b), cytotoxic T lymphocytes (CTL) (Granberg et al., 1988) or complement-mediated cytolysis (Cui et al., 1992). However, we had no success with this method because of a lack of comparability between human NK cell activity simultaneously measured by a classical 51Cr release assay (Seaman et al., 1981) and EuDTPA release assay (Blomberg et al., 1986a). Furthermore, cell division and cell viability were significantly impaired by the suggested concentrations of EuCl3. In this paper, we present a modified non-cytotoxic method for target cell labelling with EuDTPA while cells are growing in culture medium.

Fluorescent europium chelates as target cell markers in the assessment of natural killer cell cytotoxicity

A time-resolved fluorometric assay for the detection of natural killer cell activity against target cells labelled with the fluorescent chelate europium-6,6"-bis[N,N-bis(carboxymethyl)-aminomethyl]-4'-phenyl-2,2',6', 2"-terpyridine (EuCAPT) has been developed. In the assay released EuCAPT from lysed K-562 cells is measured in the supernatant after co-incubation of the target cells with effector cells. Thus, the performance of the assay is essentially similar to the previously described EuDTPA assay and the widely used 51Cr assay. EuCAPT is released from target cells lysed by effector cells faster than 51CrO4(2-) but somewhat slower than EuDTPA. In contrast to methods based on prompt fluorometry the autofluorescence from culture medium supplemented with serum can be avoided by the use of time-resolved fluorometry. The result shows that fluorescent europium chelates provide an alternative to radioactive markers currently used for the assessment of in vitro cellular cytotoxicity.