Quantification of tumor-specific T lymphocytes with the ELISPOT assay

A Highly Sensitive Flow Cytometric Approach to Detect Rare Antigen-Specific T Cells: Development and Comparison to Standard Monitoring Tools

Personalized vaccines against patient-unique tumor-associated antigens represent a promising new approach for cancer immunotherapy. Vaccine efficacy is assessed by quantification of changes in the frequency and/or the activity of antigen-specific T cells. Enzyme-linked immunosorbent spot (ELISpot) and flow cytometry (FCM) are methodologies frequently used for assessing vaccine efficacy. We tested these methodologies and found that both ELISpot and standard FCM [monitoring CD3/CD4/CD8/IFNγ/Viability+CD14+CD19 (dump)] demonstrate background IFNγ secretion, which, in many cases, was higher than the antigen-specific signal measured by the respective methodology (frequently ranging around 0.05-0.2%). To detect such weak T-cell responses, we developed an FCM panel that included two early activation markers, 4-1BB (CD137) and CD40L (CD154), in addition to the above-cited markers. These two activation markers have a close to zero background expression and are rapidly upregulated following antigen-specific activation. They enabled the quantification of rare T cells responding to antigens within the assay well. Background IFNγ-positive CD4 T cell frequencies decreased to 0.019% ± 0.028% and CD8 T cells to 0.009% ± 0.013%, which are 19 and 13 times lower, respectively, than without the use of these markers. The presented methodology enables highly sensitive monitoring of T-cell responses to tumor-associated antigens in the very low, but clinically relevant, frequencies.

Radiotherapy and High-Dose Interleukin-2: Clinical and Immunological Results of a Proof of Principle Study in Metastatic Melanoma and Renal Cell Carcinoma

High-dose interleukin-2 (HD IL-2) has curative potential in metastatic melanoma (MM) and renal cell carcinoma (RCC). Radiotherapy (RT) kills cancer cells and induces immunomodulatory effects. Prospective trials exploring clinical and immunological properties of combined RT/HD IL-2 are still needed. We designed a phase II, single-arm clinical trial for patients with MM and RCC. The treatment schedule consisted of 3 daily doses of 6-12 Gy of RT to 1-5 non-index metastatic fields, before IL-2 at the first and third treatment cycle. HD IL-2 was administered by continuous infusion for 72 hours and repeated every 3 weeks for up to 4 cycles, thereafter every 4 weeks for a maximum of 2 cycles. The primary endpoint was the immunological efficacy of the combined RT/HD IL-2 treatment (assessed by IFN-γ ELISPOT). Nineteen out of 22 patients were evaluable for immunological and clinical response. Partial response occurred in 3 (15.7%) patients and stable disease was observed in 7 (36.8%). The disease control rate was 52.6% after a median follow up of 39.2 months. According to Common Terminology Criteria for Adverse Events 4.0 (CTCAE 4.0), the majority of toxicities were grade 1-2. Immunological responses were frequent and detected in 16 (84.2%) patients. Increased levels of IL-8 and IL-10 in melanoma, circulating effector memory CD4+ and intratumoral CD8+ T cells in both tumor types were detected after therapy. Overall the treatment was well tolerated and immunologically active. Immunomonitoring and correlative data on tumor and peripheral blood cell subsets suggest that this combination treatment could be a promising strategy for patients progressing after standard treatments.

A cancer vaccine-mediated postoperative immunotherapy for recurrent and metastatic tumors

Vaccines to induce effective and sustained antitumor immunity have great potential for postoperative cancer therapy. However, a robust cancer vaccine simultaneously eliciting tumor-specific immunity and abolishing immune resistance continues to be a challenge. Here we present a personalized cancer vaccine (PVAX) for postsurgical immunotherapy. PVAX is developed by encapsulating JQ1 (a BRD4 inhibitor) and indocyanine green (ICG) co-loaded tumor cells with a hydrogel matrix. Activation of PVAX by 808 nm NIR laser irradiation significantly inhibits the tumor relapse by promoting the maturation of dendritic cells and eliciting tumor infiltration of cytotoxic T lymphocytes. A mechanical study reveals that NIR light-triggered antigen release and JQ1-mediated PD-L1 checkpoint blockade cumulatively contribute to the satisfied therapeutic effect. Furthermore, PVAX prepared from the autologous tumor cells induces patient-specific memory immune response to prevent tumor recurrence and metastasis. The PVAX model might provide novel insights for postoperative immunotherapy.

Cancer vaccines represent a promising personalized therapeutic approach to treating cancer. Here, the authors report the efficacy in a metastatic model of a cancer vaccine-mediated postoperative immunotherapy, based on the coencapsulation of the JQ1 and a photosensitizer ICG together with inactivated tumor cells into a hydrogel matrix.

Membrane Microplates for One- and Two-Color ELISPOT and FLUOROSPOT Assays

Membranes are widely used as protein blotting matrices for a large variety of research applications including western blotting and enzyme-linked immunospot assay (ELISPOT). The largest advantage of using membranes versus solid plastic support is the porosity of membranes allowing for immobilization of high concentrations of proteins and antibodies which, in turn, increases the sensitivity of detection. Similar to plastic surfaces, polyvinylidene difluoride (PVDF) and nitrocellulose membranes create good microenvironment for live cells cultured in vitro and do not interfere with cellular physiology. It appears that PVDF-backed microplates are a golden standard for ELISPOT assays: such plates are inexpensive, easy to use and after assay development, membranes can be removed from the plates and archived. Given the convenience and reliability of membrane microplates, they are widely used in ELISPOT assays for basic research and clinical trials. The ELISPOT assay is an antibody "sandwich" technique aimed at trapping cell-secreted molecules between capture and detection antibodies, followed by either chromogenic enzymatic or fluorescence detection. This review covers the principles of the ELISPOT assay on membrane microplates including single-color and two-color detection techniques with the emphasis on assay design, choosing membrane microplates, and troubleshooting protocols.

Vaccine therapies for patients with glioblastoma

Glioblastoma (GBM) is a high-grade glial tumor with an extremely aggressive clinical course and a median overall survival of only 14.6 months following maximum surgical resection and adjuvant chemoradiotherapy. A central feature of this disease is local and systemic immunosuppression, and defects in patient immune systems are closely associated with tumor progression. Immunotherapy has emerged as an important adjuvant in the therapeutic armamentarium of clinicians caring for patients with GBM. The fundamental aim of immunotherapy is to augment the host antitumor immune response. Active immunotherapy utilizes vaccines to stimulate adaptive immunity against tumor-associated antigens. A vast array of vaccine strategies have advanced from preclinical study to active clinical trials in patients with recurrent or newly diagnosed GBM, including those that employ peptides, heat shock proteins, autologous tumor cells, and dendritic cells. In this review, the rationale for glioma immunotherapy is outlined, and the prevailing forms of vaccine therapy are described.

Dendritic cell-based tumor vaccinations in epithelial ovarian cancer: a systematic review

After decades of extensive research, epithelial ovarian cancer still remains a lethal disease. Multiple new studies have reported that the immune system plays a critical role in the growth and spread of ovarian carcinoma. This review summarizes the development of dendritic cell (DC) vaccinations specific for ovarian cancer. So far, DC-based vaccines have induced effective antitumor responses in animal models, but only limited results from human clinical trials are available. Although DC-based immunotherapy has proven to be clinically safe and efficient at inducing tumor-specific immune responses, its clear role in the therapy of ovarian cancer still needs to be clarified. The relatively disappointing low-response rates in early clinical trials point to the need for the development of more effective and personalized DC-based anticancer vaccines. This article reviews the basic mechanisms, limitations and future directions of DC-based anti-ovarian cancer vaccine development.

Defining ELISpot cut-offs from unreplicated test and control wells

In the absence of replication of wells, empirical criteria for enzyme-linked immunospot (ELISpot) positivity use fixed differences or ratios between spot forming units (SFU) counts between test and control. We propose an alternative approach which first identifies the optimally variance-stabilizing transformation of the SFU counts, based on the Bland-Altman plot of the test and control wells. The second step is to derive a positivity threshold from the difference in between-plate distribution functions of the transformed test and control SFU counts. This method is illustrated using 1309 assay results from a cohort study of influenza in Vietnam in which some, but not all, of the peptide pools have clear tendencies for SFU counts to be higher in test than control wells.

Resting of Cryopreserved PBMC Does Not Generally Benefit the Performance of Antigen-Specific T Cell ELISPOT Assays

T cell monitoring is increasingly performed using cryopreserved PBMC. It has been suggested that resting of PBMC after thawing, that is, culturing them overnight in test medium, produces higher antigen-induced spot counts in ELISPOT assays. To evaluate the importance of overnight resting, we systematically tested cryopreserved PBMC from 25 healthy donors. CEF peptides (comprising CMV, EBV and flu antigens) were used to stimulate CD8 cells and mumps antigen to stimulate CD4 cells. The data show that resting significantly increased antigen-elicited T cell responses only for CEF high responder PBMC. The maximal gain observed was doubling of spot counts. For CEF low responders, and for mumps responders of either low- or high reactivity levels, resting had no statistically significant effect on the observed spot counts. Therefore, resting is not a generally applicable approach to improve ELISPOT assay performance, but can be recommended only for clinical subject cohorts and antigens for which it has a proven benefit. Because resting invariably leads to losing about half of the PBMC available for testing, and because doubling the PBMC numbers plated into the assay reliably doubles the antigen-induced spot counts, we suggest the latter approach as a simple and reliable alternative to resting for enhancing the performance of ELISPOT assays. Our data imply that resting is not required if PBMC were cryopreserved and thawed under conditions that minimize apoptosis of the cells. Therefore, this study should draw attention to the need to optimize freezing and thawing conditions for successful T cell work.

ELISPOT Assay for Monitoring Cytotoxic T Lymphocytes (CTL) Activity in Cancer Vaccine Clinical Trials

The profiling and monitoring of immune responses are key elements in the evaluation of the efficacy and development of new biotherapies, and a number of assays have been introduced for analyzing various immune parameters before, during, and after immunotherapy. The choice of immune assays for a given clinical trial depends on the known or suggested immunomodulating mechanisms associated with the tested therapeutic modality. Cell-mediated cytotoxicity represents a key mechanism in the immune response to various pathogens and tumors. Therefore, the selection of monitoring methods for the appropriate assessment of cell-mediated cytotoxicity is thought to be crucial. Assays that can detect both cytotoxic T lymphocytes (CTL) frequency and function, such as the IFN-γ enzyme-linked immunospot assay (ELISPOT) have gained increasing popularity for monitoring clinical trials and in basic research. Results from various clinical trials, including peptide and whole tumor cell vaccination and cytokine treatment, have shown the suitability of the IFN-γ ELISPOT assay for monitoring T cell responses. However, the Granzyme B ELISPOT assay and Perforin ELISPOT assay may represent a more direct analysis of cell-mediated cytotoxicity as compared to the IFN-γ ELISPOT, since Granzyme B and perforin are the key mediators of target cell death via the granule-mediated pathway. In this review we analyze our own data and the data reported by others with regard to the application of various modifications of ELISPOT assays for monitoring CTL activity in clinical vaccine trials.

ELISPOT assay for neuroscience research: studying TNFα secretion from microglial cells

The major application of ELISPOT assays is to study secretion of cytokines and chemokines from immune system cells. We adapted this assay to study TNFα secretion from microglial BV2 cells, which are similar in physiology to microglia in the nervous system. Stimulation of BV2 cells with 1 μg/mL LPS resulted in a robust secretion of TNFα. Unlike uniform round spots formed by TNFα secreted by immune system cells, BV2 cells produced spots with short zigzag "tails" indicating that BV2 cells were actively moving during the incubation. In spite of irregular shapes, spots could be easily counted using an ELISPOT reader. Our study has shown the feasibility of employing an ELISPOT assay as a tool for neuroscience research to study the mechanisms underlying protein secretion from microglial cells. In addition, due to its convenient format, ELISPOT can be used for high-throughput screening of the potency of novel drugs to stimulate or inhibit cytokine secretion by microglial cells in the brain.

Immunological monitoring of the tumor immunoenvironment for clinical trials

Monitoring of immunotherapeutic clinical trials has undergone a considerable change in the last decade resulting in a general agreement that immune monitoring should guide the development of cancer vaccines. The emphasis on immune cell functions and quantitation of antigen-specific T cells have been playing a major role in the attempts to establish meaningful correlations between therapy-induced alterations in immune responses and clinical endpoints. However, one significant unresolved issue in modern immunotherapy is that when a tumor-specific cellular immune response is observed following the course of immunotherapy, it does not always lead to clinically proven cancer regression. This disappointing lack of a correlation between the tumor-specific cytotoxic immune responses and the clinical efficacy of immunotherapy may be explained, among other reasons, by the notion that the analysis of any single immunological parameter is not sufficient to provide clinically feasible information about the complex interactions between different cell subsets in the peripheral blood and immune, tumor, and stromal cells in the tumor milieu. By contrast, a systemic approach is required for improving the quality of a serial monitoring to ensure that it adequately and reliably measures potential changes induced in patients by administered vaccines or immunomodulators. Comprehensive evaluation of the balance between the immunostimulatory and immunosuppressive compartments of the immune system could be critical for a better understanding of how a given immunotherapy works or does not work in a particular clinical trial. New approaches to characterize tumor-infiltrating leukocytes, their phenotypic, biochemical, and genetic characteristics within the tumor microenvironment need to be developed and validated and should complement current monitoring techniques. These immune-monitoring assays for the local tumor immunoenvironment should be developed, validated, and standardized for reliability and consistency in order to establish the overall performance standards.

The impact of harmonization on ELISPOT assay performance

During more than 25 years of application in immunological sciences, ELISPOT has been established as a routine, robust, versatile, and reliable assay. From basic research to clinical immune monitoring, ELISPOT is being used to address the quantification and (to a lesser extent) functional characterization of immune cells secreting different molecules in the context of health and disease, immune intervention, and therapy in humans and other species [Kalyuzhny (Ed.) (2005) Handbook of Elispot: methods and protocols, Vol. 302, Humana Press Inc., Totowa, NJ]. Over the last decade, ELISPOT assays have been increasingly implemented as an immune-monitoring tool in clinical trials [Schmittel et al. J Immunother 23:289-295, 2000; Whiteside Immunol Invest 29:149-162, 2000; Nagata et al. Ann N Y Acad Sci 1037:10-15, 2004; Cox et al. (2005) Cellular immune assays for evaluation of vaccine efficacy in developing countries., In Manual of Clinical Immunology Laboratory (Rose, N. R., Hamilton, R. G., and Detrick, B., Eds.), p 301, ASM Press, Washington, DC; Cox et al. Methods 38:274-282, 2006]. While the principles of the original protocol have changed little since its first introduction [Czerkinsky J Immunol Methods 110:29-36, 1988], individual laboratories have adapted assay procedures based on experimental needs, availability of reagents and equipment, obtained recommendations, and gained experience, leading to a wide disparity of applied ELISPOT protocols with inevitable consequences. This chapter addresses the resulting challenges for ELISPOT use in clinical trial settings, and discusses the influence of harmonization strategies as a tool for overcoming these challenges. Furthermore, harmonization is discussed in the context of assay standardization and validation strategies.

ELISPOT assay as a tool to study oxidative stress in lymphocytes

Enzyme-linked immuno spot (ELISPOT) assay is widely used for vaccine development, cancer and AIDS research, and autoimmune disease studies. The output of ELISPOT assay is a formation of colored spots which appear at the sites of cells releasing cytokines, with each individual spot representing a single cytokine-releasing cell. We worked out a protocol to study oxidative stress in human peripheral blood lymphocytes by determining their potency to secrete IFN-gamma, IL-2, IL-4, IL-5, IL-8, and TNF-alpha in response to acute treatment with hydrogen peroxide. We show that hydrogen peroxide-induced oxidative stress can cause a ∼twofold decrease in the number of lymphocytes secreting the TH1 cytokines IFN-gamma and IL-2, as well as chemokines IL-8 and TNF-alpha. However, the number of cells secreting TH2 cytokines IL-4 and IL-5 in hydrogen -peroxide-treated group did not change. It appears that oxidative stress may affect TH1-TH2 cytokine secretion -balance which, in turn, may underlie developments of various pathological conditions. This protocol can be easily modified to study the effects of many other oxidative stress compounds.

Combining ELISPOT and ELISA to measure amounts of cytokines secreted by a single cell

Enzyme-linked immunospot (ELISPOT) assay allows for the determination of the frequency of -cytokine-secreting cells, but does not answer the question of how much cytokine is secreted per cell. In our study, we combined ELISPOT and ELISA assays and developed a protocol to calculate the amount of IFN gamma secreted by each cell. A suspension of human peripheral blood mononuclear cells was split into two pools and cells from one pool were cultured in a regular ELISPOT plate, whereas cells from the other pool were cultured in an uncoated, "blank," ELISPOT plate. After finishing the incubations, the amount of IFN gamma was measured by ELISA in culture media collected from both plates. The "blank" plate served to measure a total amount of secreted IFN gamma, whereas the ELISPOT plate served to measure the amount of unbound (UB) IFN gamma. Subtracting the amount of unbound IFN gamma from its total amount and dividing it by the number of spots in the ELISPOT plate allows for the calculation of the average amount of IFN gamma in a spot formed by a single cell.

Immunologic monitoring of cellular responses by dendritic/tumor cell fusion vaccines

Although dendritic cell (DC)- based cancer vaccines induce effective antitumor activities in murine models, only limited therapeutic results have been obtained in clinical trials. As cancer vaccines induce antitumor activities by eliciting or modifying immune responses in patients with cancer, the Response Evaluation Criteria in Solid Tumors (RECIST) and WHO criteria, designed to detect early effects of cytotoxic chemotherapy in solid tumors, may not provide a complete assessment of cancer vaccines. The problem may, in part, be resolved by carrying out immunologic cellular monitoring, which is one prerequisite for rational development of cancer vaccines. In this review, we will discuss immunologic monitoring of cellular responses for the evaluation of cancer vaccines including fusions of DC and whole tumor cell.

ELISpot assay as a tool to study oxidative stress in peripheral blood mononuclear cells

Enzyme-Linked Immuno Spot (ELISpot) assay is widely used for vaccine development, cancer and AIDS research, and autoimmune disease studies. The output of an ELISpot assay is a formation of colored spots which appear at the sites of cells releasing cytokines, with each individual spot representing a single cytokine-releasing cell.We have shown that hydrogen peroxide-induced oxidative stress was causing ∼twofold decrease in the number of lymphocytes secreting the TH1 cytokines IFN-gamma and IL-2, as well as chemokine IL-8 and cytokine TNF alpha. However, the number of cells secreting TH2 cytokines IL-4 and IL-5 in hydrogen peroxide-treated group did not change. Our ELISpot data indicate that oxidative stress may affect TH1-TH2 cytokine secretion balance which, in turn, may underlie developments of various pathological conditions. We adopted ELISpot assay for studying oxidative stress in human peripheral blood lymphocytes by analyzing the acute effect of hydrogen peroxide treatment on the frequency of cells secreting IFN-gamma, IL-2, IL-4, IL-5, IL-8, and TNF-alpha.

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.

Dual-color ELISPOT assay for the simultaneous detection of IL-2 and/or IFN-gamma secreting T cells

The enzyme-linked immunospot (ELISPOT) assay measures the secretion intensity of effector molecules released by immune cells in response to ex vivo antigenic stimulation, as well as the frequency of these responding cells. This assay is highly sensitive, quantitative, easy to use, and amenable to high-throughput screening. For these reasons, the ELISPOT assay is considered by many as a gold standard for monitoring cellular immune responses. Until recently, ELISPOT assays using chromophores to detect the T cell secretion of cytokines were limited to the characterization of a single effector molecule. Notably, studies evaluating the immune response to chronic viral infections often measured IFN-γ secretion by ELISPOT because of the known antiviral effects of this cytokine as well as its correlation to the cytotoxic capacity of T cells. However, maintenance of both IFN-γ and IL-2 secretion by pathogen-specific T cells has been linked to a more favorable clinical outcome in human immunodeficiency virus (HIV) and Leishmania infections. Therefore, an ELISPOT assay able to simultaneously characterize T cell responses in terms of IL-2 and IFN-γ secretion is potentially relevant for the monitoring of immune responses to certain infectious agents. In this protocol, we describe an ELISPOT assay for the simultaneous detection of IL-2 and IFN-γ upon stimulation with viral peptides.

Monitoring immune responses after glioma vaccine immunotherapy

Immunotherapy provides the ideal candidate of therapeutic attack against malignant gliomas because it allows for targeting of cancer cells without the potential for nonspecific toxicity. This is important when glial tumor cells spread far through normal brain tissue. Current vaccine therapies are in clinical trials and are showing beneficial responses. Given that the inflammatory response may make serial radiographic imaging more difficult to interpret, newer methodologies of immunomonitoring must be developed to assess the biologic efficacy of these immunotherapies. This article reviews methods of monitoring the immune system after vaccination against malignant gliomas. Improvements in immunomonitoring should lead to an increase in the efficiency of identifying viable avenues of therapeutic research, and assess the efficacy of those currently employed.

Intracellular delivery of major histocompatibility complex class I-binding epitopes: dendritic cells loaded and matured with cationic peptide/poly(I:C) complexes efficiently activate T cells

Based on their role for the induction of T-cell responses, dendritic cells (DCs) are popular candidates in cancer vaccine development. We established a novel single-step intracellular delivery of peptide/poly(I:C) complexes for antigen loading and Toll-like receptor-3 (TLR3)-mediated maturation of human DCs using a cell-penetrating peptide (tat(49-57): RKKRRQRRR) as delivery vector. Towards this end, a cationic tat-sequence was fused with an antigenic, major histocompatibility complex (MHC) class I-binding melanoma epitope (Melan-A/Mart-1 sequence: ELAGIGILTV) and then mixed with negatively charged poly(I:C) dsRNA to form peptide/nucleic acid complexes. Flow cytometry and confocal laser scanning microscopy confirmed intracellular localization of TLR3 in monocyte-derived immature DCs (iDCs). Peptide/poly(I:C) complexes were readily internalized by iDCs without negatively affecting cell viability. They induced DC maturation and secretion of bioactive interleukin (IL)-12p70. When peptide/poly(I:C) complex-loaded DCs were used for autologous T cell stimulation, epitope-specific interferon-gamma secretion was quantitatively superior in comparison to peptide-loaded DCs matured by a cytokine cocktail, as detected by enzyme-linked immunospot assays. Thus, complexes of cationic antigenic peptides and poly(I:C) might be of great utility for a TLR3-mediated DC maturation and intracellular peptide targeting in a single step. Resulting DCs induce a strong expansion/activation of antigen-specific T cells in the context of an IL-12p70 secretion.

ELISPOT assay on membrane microplates

Membranes used for western blotting can be also used for ELISPOT, an enzyme-linked immunospot assay, which allows determining frequencies of cytokine-secreting immune system cells. In addition to their high antibody-retaining capacity PVDF and NC membranes provide good support to immune system cells cultured in vitro and do not affect their physiology. ELISPOT assays utilizing membrane-backed microplates are used in many areas of research including vaccine development, HIV research, cancer and infection disease research, autoimmune disease, and allergy research.ELISPOT utilizes the same antibody "sandwich" technique as enzyme-linked immunosorbent assay, but unlike the latter ELISPOT belongs to state-of-the-art techniques when outcome of the assay depends on skills and accuracy of the operator, a thorough selection of matched pairs of capture and detection antibodies, and using appropriate staining reagents. This review covers basics of ELISPOT assay including its immunochemical design, selection of reagents and membrane microplates, and some troubleshooting recommendations.

Mannan-MUC1-pulsed dendritic cell immunotherapy: a phase I trial in patients with adenocarcinoma

PURPOSE

Tumor antigen-loaded dendritic cells show promise for cancer immunotherapy. This phase I study evaluated immunization with autologous dendritic cells pulsed with mannan-MUC1 fusion protein (MFP) to treat patients with advanced malignancy.

EXPERIMENTAL DESIGN

Eligible patients had adenocarcinoma expressing MUC1, were of performance status 0 to 1, with no autoimmune disease. Patients underwent leukapheresis to generate dendritic cells by culture ex vivo with granulocyte macrophage colony-stimulating factor and interleukin 4 for 5 days. Dendritic cells were then pulsed overnight with MFP and harvested for reinjection. Patients underwent three cycles of leukapheresis and reinjection at monthly intervals. Patients with clinical benefit were able to continue with dendritic cell-MFP immunotherapy.

RESULTS

Ten patients with a range of tumor types were enrolled, with median age of 60 years (range, 33-70 years); eight patients were of performance status 0 and two of performance status 1. Dendritic cell-MFP therapy led to strong T-cell IFNgamma Elispot responses to the vaccine and delayed-type hypersensitivity responses at injection sites in nine patients who completed treatments. Immune responses were sustained at 1 year in monitored patients. Antibody responses were seen in three patients only and were of low titer. Side effects were grade 1 only. Two patients with clearly progressive disease (ovarian and renal carcinoma) at entry were stable after initial therapy and went on to further leukapheresis and dendritic cell-MFP immunotherapy. These two patients have now each completed over 3 years of treatment.

CONCLUSIONS

Immunization produced T-cell responses in all patients with evidence of tumor stabilization in 2 of the 10 advanced cancer patients treated. These data support further clinical evaluation of this dendritic cell-MFP immunotherapy.

Immunological monitoring of cancer vaccine therapy

Immunological treatment of malignant diseases in humans aiming at the induction and proliferation of antigen-specific T cells has made rapid progress in recent years. A growing number of tumour-associated antigens, potentially synergistic combinations with adjuvants, and various routes of application provide new opportunities for cancer vaccination. Therefore, a highly accurate assessment of vaccine-induced T cell responses is required. Three T cell assays (tetramers, intracellular cytokine flow cytometry and ELISPOT assay) have emerged as first-line methods for monitoring T cell induction during vaccination. These assays are relatively easy to perform, reliable, sensitive and allow an ex vivo T cell analysis at the single cell level. Although at this stage assays are not a defined surrogate marker for clinical efficacy, they already provide information concerning the immunological potency of a given vaccine. In particular, comparing immune responses under various treatment conditions will help to develop more clinically efficient tumour vaccination. Novel assays, such as CD107 staining, human leukocyte antigen/green fluorescent protein-antigen-presenting cells or microarrays, and assays determining functions, such as proliferation assays, are beginning to complement first-line monitoring assays.

Evaluating ELISPOT summary measures with criteria for obtaining reliable estimates

The ELISPOT assay is a commonly used technique for quantifying the occurrence of T lymphocyte cells secreting a cytokine after stimulation with an antigen or peptide. The assay endpoint, the number of spot-forming cells (SFC) at a specific concentration of effector cells, is typically estimated using either a simple arithmetic mean or the predicted value from a linear regression model. We compare statistical modeling approaches for summarizing these assays using data from the Pediatric AIDS Clinical Trial Group (PACTG) study 299. A simulation study was conducted to compare methods under controlled conditions. Assuming the optimal effector cell concentration is known, we demonstrate that the simple mean is appropriate if assays are conducted at the same concentration for all samples. Normalizing simple means to a summary concentration using results from a range of concentrations is not valid. A random effects or mixed model is superior to the simple mean when a large within-assay (subject) variance relative to between-subject variance exists. When assays are conducted over a range of effector cell concentrations for each individual, the theoretical linearity assumption of the regression model is often violated and can result in biased estimates. In this case, nonlinear models provide more accurate estimation. Collecting data over a range of concentrations allows reassessment of the optimal cell concentration after the data are generated.

Identification of a new HLA-A*0201-restricted CD8+ T cell epitope from hepatocellular carcinoma-associated antigen HCA587

For the development of peptide-based cancer immunotherapies, we aimed to identify specific HLA-A*0201-restricted CTL epitopes in hepatocellular carcinoma (HCC) associated antigen HCA587, which has been identified as a member of the cancer/testis (CT) antigens highly expressed in HCC. We first combined the use of an HLA-A*0201/peptide binding algorithm and T2 binding assays with the induction of specific CD8(+) T cell lines from normal donors by in vitro priming with high-affinity peptides, then IFN-gamma release and cytotoxicity assays were employed to identify the specific HLA-A*0201 CD8(+) T cell epitope using peptide-loaded T2 cells or the HCA587 protein(+) HCC cell line HepG2. In the six candidate synthesized peptides, two peptides showed higher binding ability in T2 binding assays. No. 2 peptide, encompassing amino acid residues FLAKLNNTV (HCA587(317-325)), was able to activate a HCA587-specific CD8(+) T-cell response in human lymphocyte cultures from two normal donors and two HCC patients, and these HCA587-specific CD8(+) T cells recognized peptide-pulsed T2 cells as well as the HCA587 protein(+) HCC cell line HepG2 in IFN-gamma release and cytotoxicity assays. The results indicate that no. 2 peptide is a new HLA-A*0201-restricted CTL epitope capable of inducing HCA587-specific CTLs. Our data suggest that identification of this new HCA587/HLA-A*0201 peptide FLAKLNNTV may facilitate the design of peptide-based immunotherapies for the treatment of HCA587-bearing HCC patients.

Cellular immunotherapy: antigen recognition is just the beginning

Advances in molecular and cellular biology have illustrated both the flexibility and complexity involved in host immune responses. Understanding this response is vital to the further development of therapeutic strategies that involve manipulation of the cellular immune response to target tumors. Mobilized, tumor antigen-specific T cells, the core for most immunotherapeutic strategies, are highly regulated, and capable of a wide spectrum of functional responses. Due to differences in murine and human immunity, broad-scale immune monitoring, particularly high-throughput ex vivo analysis of human immune responses, promises to determine what comprises an effective immunotherapy. Such understanding will lead to more sophisticated clinical trials, earlier determination of efficacy and individualized protocols.

Escape from immune surveillance does not result in tolerance to tumor-associated antigens

Despite expression of tumor-associated or tumor-specific antigens by most tumors, evasion of protective T-cell immunity is the rule rather than the exception. Understanding whether tumor immune escape primarily represents T-cell neglect, anergy/tolerance, or quantitative limits of an existent immune response is central to developing new strategies to enhance antitumor immunity. The authors studied the immune response to MB49, a tumor that naturally expresses HY. Immune surveillance was effective following low-dose tumor inocula, since normal female mice showed a diminished incidence and slower growth rate of MB49 compared with T-cell-depleted female mice and male mice. Following high-dose tumor inoculation, females developed large, progressive tumors but continued to demonstrate immune responses to class I and class II restricted HY epitopes. The HY reactive T cells remained capable of executing HY immune responses since T cells adoptively transferred from MB49-bearing animals mediated accelerated HY skin graft rejection compared with those taken from naive mice. Thus, MB49 does not induce immune tolerance to HY but rather escapes immune surveillance largely due to quantitative limits of the immune response. Treatment of tumor-bearing animals with rhIL7 significantly increased the number of T cells responding to HY but did not alter tumor growth rate. These results demonstrate that escape from immune surveillance does not necessarily imply immune tolerance to tumor antigens and that immunotherapy need not overcome tumor-induced tolerance per se, and suggest that substantial opportunities remain in tumor-bearing hosts to amplify weak but persistent antitumor immune responses.

Modified ELISPOT

The use of the IFN-γ ELISPOT assay to evaluate cellular immune responses has gained increasing popularity, especially as a surrogate measure for CTL responses. We developed and validated some modifications of the IFN-γ ELISPOT assay to optimize immunological monitoring of various cancer vaccine trials. Taking into consideration that the main mechanism of cell-mediated cytotoxicity is the release of cytolytic granules that contain, among others, cytolytic protein Granzyme B (GrB), we developed the GrB ELISPOT assay. Extensive studies demonstrated that the GrB ELISPOT assay is specific, accurately measures the rapid release of GrB, is more sensitive than the 51Cr-release assay, and that it may be successfully applied to measuring CTL precursory frequency in PBMC from cancer patients. Assuming that immunological assays that demonstrate recognition of native tumor cells (tumor-specific) may be more clinically relevant than assays that demonstrate recognition of tumor protein or peptide (antigen-specific), we developed and validated the Autologous Tumor IFN-γ ELISPOT assay using PBMC from idiotype vaccinated lymphoma patients as effectors and autologous B cell lymphoma tumor cells as targets. The precursor frequency of tumor-reactive T cells was significantly higher in the postvaccine PBMC, compared with prevaccine samples in all patients tested. Furthermore, the specificity of these T cells was established by the lack of reactivity against autologous normal B cells. These results demonstrate the feasibility of evaluating tumor-specific T cell responses when autologous, primary tumor cells are available as targets. Modifications of ELISPOT assay described in this chapter allow more comprehensive assessment of low frequency tumor-specific CTL and their specific effector functions and can provide valuable insight with regards to immune responses in cancer vaccine trials.

Elicitation of Both CD4+ and CD8+ T-Cell-Mediated Specific Immune Responses to HCA587 Protein by Autologous Dendritic Cells

We recently cloned a new member of cancer/testis antigen named HCA587, which was highly expressed in human hepatocellular carcinoma (HCC) tissues. To investigate it as a potential tumour-specific target for immunotherapy, the immunogenicity of this protein, especially the ability to induce specific cellular immune responses, was evaluated in the present study. As dendritic cells (DC) are the most potent antigen-presenting cells, DC-based vaccination has recently shown marked promise for the treatment of human malignancies by immunological intervention. Here, we demonstrate that autologous DC loaded with HCA587 protein could induce specific T-cell responses in healthy individuals by in vitro stimulations. Enzyme-linked immunospot analysis for interferon-gamma (IFN-gamma) secretion demonstrated HCA587-specific CD8(+) T cells in the antigen-stimulated peripheral blood lymphocytes, and the analysis of CD4(+) T cells by proliferation assay also showed antigen-specific reactivities in normal donors. Two-colour flow cytometric analysis of surface markers and intracellular cytokine expression demonstrated that HCA587-specific cytotoxic T lymphocytes exhibited a heterogeneous CD8(+)/CD56(+) expression, and a striking T-helper 1 cytokine bias (IFN-gamma(high)/IL-4(low)) was observed for both CD4(+) and CD8(+) HCA587-specific lymphocyte populations. We conclude that HCA587 is a potent immunogen that can induce CD4(+) and CD8(+) T-cell-mediated specific immune responses, and these findings propose HCA587 as a good candidate for the development of a therapeutic protein-based DC tumour vaccine for the treatment of HCC patients.

A novel ELISpot method for adherent cells

The purpose of this study was to develop an enzyme-linked immunospot assay (ELISpot assay) that can be used with human adherent cells. While standard enzyme-linked immunosorbent assays (ELISAs) are available and widely used and ELISpot assays are used for nonadherent lymphocytes, no ELISpot assay has been developed for adherent cells. We used primary human fibroblasts from four different tissues (myometrium, lung, gingiva, and orbit), either unstimulated or interleukin (IL)-1beta-activated, to evaluate an ELISpot assay. Antibody pairs for IL-6 and IL-8 were used and results were compared to a standard ELISA. We found that we could reliably detect IL-6 and IL-8 spots with as few as 10 fibroblasts. Optimal cell numbers were 50 cells per well incubated for 8 h, although spots appeared as early as 2 h after incubation. Spots were absent when cells, primary, or secondary anti-cytokine antibodies were omitted from the protocol. Spot number and size can be ascertained using current automated ELISpot reader technology. The frequency of IL-6 and IL-8-producing human fibroblasts could also be determined. For example, 60% of the lung fibroblasts express IL-6, but IL-8 can be detected from only 40% of the cells. Approximately 80% of the human orbital fibroblasts make IL-6, whereas approximately 50% generate IL-8 following IL-1beta stimulation. These new findings show that fibroblasts from different human tissues display different frequencies of cytokine production and this further supports the concept of fibroblast diversity. The sensitivity of this new ELISpot assay is adequate for cytokine detection in just a few cells, unlike the standard ELISA. It should permit ascertaining the frequency of fibroblasts and other adherent cells that produce cytokines and, if desired, can be used in tandem with a standard ELISA to determine total cytokine produced. Moreover, the assay is suitable for normal human adherent cells that are often short-lived and difficult to cultivate.

T-cell clonotypes in cancer

Cells of the immune system spontaneously recognize autologous tumor cells and T cells are believed to be the main effector cells for the immune surveillance of cancer. Recent advances in our understanding of basic and tumor immunology together with methodological developments implies that tumor specific T cells can now be studied functionally, phenotypically as well as molecularly. T cells recognize peptide antigens in the context of MHC molecules through the clonally distributed T-cell receptor (TCR), thus, the clonal distribution of the TCR offers the means to detect and track specific T cells based upon detection of the unique TCR. In this review, we present and discuss available data on TCR utilization of tumor specific T cells in murine models as well as spontaneous and treatment induced anti-tumor T-cell responses in humans.

A modified human ELISPOT assay to detect specific responses to primary tumor cell targets

Background

The desired outcome of cancer vaccination is to induce a potent T cell response which can specifically recognize and eliminate autologous tumor cells in vivo. Accordingly, immunological assays that demonstrate recognition of native tumor cells (tumor-specific) may be more clinically relevant than assays that demonstrate recognition of tumor protein or peptide (antigen-specific).

Methods

Towards this goal, we adapted the IFN-γ ELISPOT assay to measure immune responses against autologous primary tumor cells in vaccinated cancer patients. As a model system to develop the assay, we utilized peripheral blood mononuclear cells (PBMC) directly isolated from follicular lymphoma patients vaccinated with tumor-derived idiotype protein.

Results

After optimizing several variables, we demonstrated that the modified IFN-γ ELISPOT assay could be used to reliably and reproducibly determine the tumor-reactive T cell frequency in the PBMC of these patients. The precursor frequency of tumor-reactive T cells was significantly higher in the postvaccine PBMC, compared with prevaccine samples in all patients tested. Furthermore, the specificity of these T cells was established by the lack of reactivity against autologous normal B cells.

Conclusions

These results demonstrate the feasibility of quantitating tumor-specific T cell responses when autologous, primary tumor cells are available as targets.

Quantification and characterization of specific T-cells by antigen-specific cytokine production using ELISPOT assay or intracellular cytokine staining

The novel generation of sensitive T-cell assays facilitates the direct quantitation and characterization of specific T-cell responses. Functional T-cell assays such as the ELISPOT assay and the intracellular cytokine cytometry (ICC) employ the antigen-specific induction of cytokines to detect specific T-cells on a single cell level. ICC has the advantage that the simultaneous phenotypic characterization of the antigen-specific T-cells is possible. There is evidence now from clinical cancer vaccination trials, that there is a relationship between the detection of vaccine-induced T-cells by cytokine-based assays and clinical responses. As these assays become increasingly relevant in clinical practice to suggest issues of assay validation and quality control become of major importance.

Spontaneous regression of Hodgkin's disease: two case reports and a review of the literature

We present two cases of patients with Hodgkin's lymphoma who experienced spontaneous regressions of their disease. The first case was a 31-year-old man diagnosed with stage IIIA lymphocyte predominant Hodgkin's disease in 1994, who elected to be followed without any treatment. Over the subsequent 3 years, he experienced significant regression in his lymphadenopathy, and still remains asymptomatic of his disease 70 months after diagnosis. The second case was a 47-year-old man with a bulky anterior mediastinal mass found on a thoracic CT scan, ultimately diagnosed with stage IIB Nodular Sclerosing Hodgkin's Lymphoma. Repeat imaging of the chest performed two months later, just prior to initiating treatment, revealed that the mass had spontaneously decreased by >75% of its original size. Spontaneous regressions of Hodgkin's lymphoma are exceedingly rare. A review of the literature regarding spontaneous regressions of lymphoma and cancer in general is discussed.

Cell-mediated immunotherapy: a new approach to the treatment of malignant glioma

BACKGROUND

The dismal prognosis for patients harboring intracranial gliomas has prompted an intensive search for effective treatment alternatives such as immunotherapy. Our increased knowledge in basic immunology, glioma immunobiology, and molecular biology may lead to the development of effective, rational immunotherapy approaches.

METHODS

The authors reviewed the literature on glioma immunology, the status of tumor vaccine therapy and on novel techniques to monitor the tumor-specific immune response.

RESULTS

Experimental conditions currently exist whereby potent antitumor cell-mediated immune responses can be generated. However, clinically, no therapeutic regimen has proven effective. Obstacles to establishing an effective immunotherapy regimen are the lack of a well-defined glioma-specific antigen, the heterogeneity of tumor cells in gliomas, and the modulating effect of the glioma itself on the immune system. Unique strategies to overcome these barriers are being developed.

CONCLUSIONS

Novel strategies to generate an anti-glioma immune response through use of dendritic cell vaccination, directed cytokine delivery, gene-based immunotherapy, and reversal of tumor-induced immunosuppression are promising. These strategies carry the potential of overcoming the resistance of gliomas to immunotherapeutic manipulation and, undoubtedly, will become a part of our future therapeutic armamentarium.

Rational approaches to human cancer immunotherapy

Over most of the 20th century, immunotherapy for cancer was based on empiricism. Interesting phenomena were observed in the areas of cancer, infectious diseases, or transplantation. Inferences were made and extrapolated into new approaches for the treatment of cancer. If tumors regressed, the treatment approaches could be refined further. However, until the appropriate tools and reagents were available, investigators were unable to understand the biology underlying these observations. In the early 1990s, the first human tumor T cell antigens were defined and dendritic cells were discovered to play a pivotal role in antigen presentation. The current era of cancer immunotherapy is one of translational research based on known biology and rationally designed interventions and has led to a rapid expansion of the field. The beginning of the 21st century brings the possibility of a new era of effective cancer immunotherapy, combining rational, immunological treatments with conventional therapies to improve the outcome for patients with cancer.

A multidonor ELISPOT study of IL-1 beta, IL-2, IL-4, IL-6, IL-13, IFN-gamma and TNF-alpha release by cryopreserved human peripheral blood mononuclear cells

Utilization of cryopreserved peripheral blood mononuclear cells (PBMCs), rather than fresh ones collected from the same donor on different dates, overcomes the variability in sensitivity of these cells to activation agents. To understand the effect of cryopreservation, frozen PBMCs from eight healthy donors were studied to release T(H)1 or T(H)2 cytokines including IL- 1 beta, IL-2, IL-4, IL-6, IL-13, TNF-alpha and IFN-gamma using ELISPOT assay. The number of spot-forming cells (SFC) was determined using three concentrations of PBMCs (5 x 10(6), 5 x 10(5) and 5 x 10(4) cells/ml). PBMCs from all eight donors were found to retain their functional capacity to release T(H)1 or T(H)2 cytokines after freezing and thawing. When PBMCs were taken in concentrations 5 x 10(6) or 5 x 10(5) cells/ml, the density of IL-1 beta-, IL-2-, IL-6- and TNF-alpha-related spots in a well for most of the donors appeared to be overly high, making SFC quantification either difficult or impossible. To the contrary, PBMCs in concentration 5 x 10(4) produced distinct and quantifiable spots. The density of spots related to IL-4 and IL-13 release appeared to be optimal for SFC quantification when PBMCs were taken in concentration 5 x 10(6) whereas in 5 x 10(5) cells/ml the spot density was very low and absent in 5 x 10(4) cells/ml concentration group. No relationship between release levels of different cytokines was found, except IFN-gamma and IL-2 cytokine indicating that cryopreserved PBMCs with a high IFN-gamma response will likely have a high IL-2 response as well. Our results indicate that a release level of one cytokine may not be reliably predicted by knowing the level of the other. This implies that it is necessary to test cryopreserved PBMCs in a broad range of concentrations to determine one, which will be optimal for producing distinct and quantifiable spots.

The ELISPOT assay: an easily transferable method for measuring cellular responses and identifying T cell epitopes

Characterization of human leukocyte antigen (HLA) class I restricted epitopes derived from viral pathogens is imperative for formulating therapeutic interventions, as well as for vaccine design and monitoring. Sensitive, easy and cost-effective assays that measure the frequency of antigen-specific T lymphocytes are crucial for evaluating and improving vaccines and therapies. This paper reviews the ELISPOT technique that allows for quantifying HIV-specific T lymphocytes at the single cell level from peripheral blood by detection of antigen-induced cytokine secretion. The assay can be used successfully to quantify T cell immune responses in humans infected with different pathogens and to assess T cell immunogenicity of vaccines in phase I/II and III clinical trials. This review focuses on the ELISPOT methodology and discusses how it can be standardized and potentially used by multiple international laboratories attached to clinical trial sites.

Carcinoembryonic antigen as a target for therapeutic anticancer vaccines: a review

PURPOSE

To describe the features of carcinoembryonic antigen (CEA) that are important for its use in vaccination approaches and review the clinical experience with therapeutic vaccines targeting CEA.

METHODS

A PubMed search was performed on CEA, along with various qualifiers such as cancer vaccines, epitopes, and function. Relevant articles were reviewed.

RESULTS

CEA is a member of the immunoglobulin supergene family and may play a role in tumorigenesis. CEA protein is processed and presented on major histocompatibility complex (MHC) proteins for multiple alleles, including HLA A2, A3, and A24. T lymphocytes from healthy volunteers and cancer patients can recognize the processed epitopes of CEA and can become activated to lyse CEA-expressing tumors. Therapeutic vaccination approaches that have targeted CEA include vaccination with recombinant CEA protein, CEA anti-idiotype antibodies, and dendritic cells pulsed with agonist epitopes of CEA. Humoral responses have predominantly been induced with the first two approaches, whereas CD4 and CD8 responses, disease stabilization, and even objective clinical responses have been seen with the dendritic cell approach. Recently, CEA-poxvirus vectors encoding CEA and costimulatory molecules such as B7.1 have been shown to be safe and to induce increases in the frequency of T-cell precursors that recognize processed epitopes of CEA presented on MHC class 1 molecules. Disease stabilization has been seen in up to 37% of patients treated with these vaccines.

CONCLUSION

Tolerance to CEA in patients with cancer can be overcome with several different vaccination approaches, and such vaccinations are safe and immunologically active. Poxvirus-based vaccines can reproducibly generate T-cell responses to CEA and to tumors expressing CEA. Clinical activity has been seen with poxvirus or dendritic cell approaches. Other approaches are also being explored.

Combination of MHC-peptide multimer-based T cell sorting with the Immunoscope permits sensitive ex vivo quantitation and follow-up of human CD8+ T cell immune responses

Identification of MHC-restricted antigens and progress in the induction and control of adaptive cytotoxic immune responses have led to renewed interest in immunotherapy as a treatment for severe pathologies such as cancer and autoimmune diseases. Reliable procedures for detecting and monitoring T cell responses induced by the treatment throughout a clinical trial are needed in order to design rational protocols with increased efficiency. We have attempted to develop such a procedure by combining T cell sorting using HLA-peptide complexes multimerized on magnetic beads together with the quantitative Immunoscope approach. Once a recruited patient has been typed for HLA and target antigens, relevant HLA--peptide multimers can be selected and used for sorting specific peripheral T cells prior to any treatment and at the peak of the expected response to treatment. Clonotypic primers specific for the TCR rearrangements of the specific T cell clones can then be designed and used for measuring the frequency of their TCR transcripts by quantitative PCR on blood samples or T cell subsets throughout the trial. In reconstruction experiments as well as in samples from one rheumatoid arthritis patient, we were readily able to detect and follow several T cell clones with a frequency as low as 10(-5) among CD8+ T cells. The main advantages of this procedure over other currently available assays are that it does not require any assumptions on the functional status of the specific T cells and it permits the monitoring of individual T cell clones whose phenotypic shift can thus be evaluated.

A panel of MHC class I restricted viral peptides for use as a quality control for vaccine trial ELISPOT assays

Vaccines in general and HIV vaccines in particular are focusing ever more on the induction of cellular immunity specifically the generation of cytotoxic T cells (CTL). As progress is made towards developing a safe and effective HIV vaccine, there is a need for a robust, sensitive and reproducible assay to evaluate vaccine-induced cellular immunogenicity in Phase II/III trials. The enzyme-linked immunospot (ELISPOT) assay fits these criteria and is a technology that is readily transferable and amenable to high-through-put screening. There is a need for reagents that can be used as positive controls and for optimizing and standardizing the assay. We selected a panel of 23 8-11 mer Influenza virus (Flu), Cytomegalovirus (CMV) and Epstein Barr virus (EBV) epitopes recognized by CD8 positive T cells and presented by 11 class I HLA-A and HLA-B alleles whose cumulative frequencies represent >100% of Caucasian individuals. We examined interferon-gamma (IFN-gamma) secretion in peripheral blood mononuclear cells (PBMC) incubated with the peptides using a modified ELISPOT assay. IFN-gamma secretion was detected in 15/17 (88%) HIV-1 seronegative and 14/20 (70%) HIV-1 seropositive individuals. Release of IFN-gamma in response to the pool of peptides was CD8+ T cell mediated and HLA restricted. In vitro stimulation of PBMC with individual peptides or the pool of peptides led to the expansion of T cells capable of killing target cells expressing the appropriate viral antigen in a CTL assay. The size, shape and appearance of the spots produced using this peptide panel provided a standard for the establishment of acceptance criteria of spots for the evaluation of ELISPOT plates using an automated reader system.

A simple method to reduce the background and improve well-to-well reproducibility of staining in ELISPOT assays

The enzyme-linked immunospot (ELISPOT) assay due to its high sensitivity is widely used for the detection of secreted cytokines at the single-cell level. Quality of staining is the key for accurate detection of spots and their subsequent quantification in ELISPOT assays. A simple technique employing regular aluminum foil has been developed to improve the quality of staining. In plates wrapped with aluminum foil non-specific background staining and artifacts were reduced or abolished. Application of aluminum foil allowed better contrast of specific spots as well as their uniform distribution across the filter membrane. This technique provides an inexpensive and reliable tool for large-volume immune system cells screening by improving well-to-well reproducibility in ELISPOT plates.

Flow cytometric determination of intracellular or secreted IFNgamma for the quantification of antigen reactive T cells

The detection of antigen-induced IFNgamma secretion at the single cell level can be used to identify and enumerate antigen-reactive T cells from peripheral blood. This study was performed to analyze the suitability of T cell enumeration by flow cytometry in comparison with the ELISPOT assay. Peripheral blood mononuclear cell (PBMC) samples from six HLA-A2+ healthy subjects were analysed for the frequency of influenza-reactive CD8+ T cells by flow cytometry detecting either intracellular IFNgamma (IC-FC) or secreted IFNgamma (S-FC). All samples were also analysed by IFNgamma ELISPOT assay. The frequency of influenza peptide-reactive T cells determined by IC-FC was 0.01 to 0.34% of CD8+ T cells and by ELISPOT assay 0.02 to 0.23% of CD8+ T cells (n=6 subjects) with a high inter-assay reproducibility and a close correlation between the assays (r=0.77, P<0.001). Little or no IFNgamma production was observed in unstimulated PBMC samples using either the IC-FC or the ELISPOT assay. In contrast, using S-FC large numbers of IFNgamma-secreting CD8+ T cells (0.37% to 5.55%, n=6 subjects) were detected in unstimulated PBMC. The frequency of influenza-reactive CD8+ T cells (0.57-5.19%, n=6 subjects) determined by S-FC did not correlate with the values from the IC-FC or ELISPOT assays. This comparative study shows the suitability of the determination of frequencies of antigen reactive T cells in PBMC by IC-FC. The advantage of IC-FC is the possibility to phenotype simultaneously antigen-reactive T cells.

Quantitation of antigen-reactive T cells in peripheral blood by IFNgamma-ELISPOT assay and chromium-release assay: a four-centre comparative trial

The ELISPOT assay is increasingly being used for the monitoring of the induction of antigen-reactive T cells in cancer vaccination trials. In order to evaluate the reliability of T cell frequency analysis with the ELISPOT assay, a comparative study was performed in four European laboratories. Six samples from healthy subjects were analyzed for the frequency of influenza-reactive CD8+ T cells in peripheral blood mononuclear cells (PBMC) by IFNgamma-ELISPOT assay. In addition, one laboratory determined cytotoxic T cell precursor (CTL) frequencies in these samples by limiting dilution chromium-release assay (LDA), and three laboratories performed a variant of the LDA, the multiple microculture assay (MMA). Consistent frequencies of influenza peptide-reactive T cells were obtained with the ELISPOT assay in all four laboratories. The numbers detected by ELISPOT assay correlated closely with those determined by LDA. In contrast, the frequencies obtained with the MMA differed considerably and showed little correlation with the other two assays. This study shows that it is possible to use the ELISPOT assay to determine with reliability antigen-reactive T cells in a multicenter setting. We suggest that this assay may be suitable for monitoring cancer vaccine trials.