Four Color ImmunoSpot® Assays for Identification of Effector T-Cell Lineages

The Applications of ELISpot in the Identification and Treatment of Various Forms of Tuberculosis and in the Cancer Immunotherapies

ELISpot (enzyme-linked immunospot) is a powerful immunological tool for the detection of cytokine-secreting cells at a single-cell resolution. It is widely used for the diagnosis of various infectious diseases, e.g., tuberculosis and sarcoidosis, and it is also widely used in cancer immunotherapy research. Its ability to distinguish between active and latent forms of tuberculosis makes it an extremely powerful tool for epidemiological studies and contact tracing. In addition to that, it is a very useful tool for the research and development of cancer immunotherapies. ELISpot can be employed to assess the immune responses against various tumor-associated antigens, which could provide valuable insights for the development of effective therapies against cancers. Furthermore, it plays a crucial role to the evaluation of immune responses against specific antigens that not only could aid in vaccine development but also assist in treatment monitoring and development of therapeutic and diagnostic strategies. This chapter briefly describes some of the applications of ELISpot in tuberculosis and cancer research.

Unbiased, High-Throughput Identification of T Cell Epitopes by ELISPOT

Recent systematic immune monitoring efforts suggest that, in humans, epitope recognition by T cells is far more complex than has been assumed based on minimalistic murine models. The increased complexity is due to the higher number of HLA loci in humans, the typical heterozygosity for these loci in the outbred population, and the high number of peptides that each HLA restriction element can bind with an affinity that suffices for antigen presentation. The sizable array of potential epitopes on any given antigen is due to each individual's unique HLA allele makeup. Of this individualized potential epitope space, chance events occurring in the course of the T cell response determine which epitopes induce dominant T cell expansions. Establishing the actually-engaged T cell repertoire in each human subject, including the individualized peptides targeted, therefore requires the systematic testing of all peptides that constitute the potential epitope space in that person. The goal of comprehensive, high-throughput epitope mapping can be readily established by the methods described in this chapter.

ELISpot Assay for the Detection of ASFV-Specific Interferon-Gamma (IFN-γ)-Producing Cells

The enzyme-linked immunospot (ELISpot) assay is a technique of unparalleled sensitivity to determine the frequency of antigen-specific immune cells secreting an immunomodulatory mediator upon recall antigen stimulation, making it a valuable tool in vaccine research. Typically done in multi-well microplate format, it also allows a high-throughput analysis of numerous immune cell samples, e.g., from different donor subjects, especially with the help of automated plate readers and specialized software that currently exist in most laboratories. IFN-γ is a hallmark cytokine secreted especially by T-cell subsets in recall response to pathogens, and consequently the IFN-γ ELISpot assay is one of the most widely used. The cellular arm of the immune response is known to be fundamental in protection against virulent ASFV, and therefore this assay is frequently employed in ASFV vaccine research to evaluate the results from immunization experiments.The technique involves the use of plates with wells that have a membrane for base with a strong binding capacity for amino acids that thus can be densely coated with an antibody for IFN-γ. Upon adding cells and specific antigen or other control stimuli, responding cells will release IFN-γ that is captured by the antibody in close proximity and revealed using a second antibody (sandwich method) through either chromogenic or fluorescent methods, leading to the detection of a "spot" on the membrane for each positive cell. Here we detail our protocol to detect the frequency of ASFV antigen-specific IFN-γ-producing cells in immunized pig lymphocytes and give an example of a typical result using the technique.

Discordance Between the Predicted Versus the Actually Recognized CD8+ T Cell Epitopes of HCMV pp65 Antigen and Aleatory Epitope Dominance

CD8+ T cell immune monitoring aims at measuring the size and functions of antigen-specific CD8+ T cell populations, thereby providing insights into cell-mediated immunity operational in a test subject. The selection of peptides for ex vivo CD8+ T cell detection is critical because within a complex antigen exists a multitude of potential epitopes that can be presented by HLA class I molecules. Further complicating this task, there is HLA class I polygenism and polymorphism which predisposes CD8+ T cell responses towards individualized epitope recognition profiles. In this study, we compare the actual CD8+ T cell recognition of a well-characterized model antigen, human cytomegalovirus (HCMV) pp65 protein, with its anticipated epitope coverage. Due to the abundance of experimentally defined HLA-A*02:01-restricted pp65 epitopes, and because in silico epitope predictions are most advanced for HLA-A*02:01, we elected to focus on subjects expressing this allele. In each test subject, every possible CD8+ T cell epitope was systematically covered testing 553 individual peptides that walk the sequence of pp65 in steps of single amino acids. Highly individualized CD8+ T cell response profiles with aleatory epitope recognition patterns were observed. No correlation was found between epitopes' ranking on the prediction scale and their actual immune dominance. Collectively, these data suggest that accurate CD8+ T cell immune monitoring may necessitate reliance on agnostic mega peptide pools, or brute force mapping, rather than electing individual peptides as representative epitopes for tetramer and other multimer labeling of surface antigen receptors.

Natural T cell autoreactivity to melanoma antigens: clonally expanded melanoma-antigen specific CD8 + memory T cells can be detected in healthy humans

We used four-color ImmunoSpot® assays, in conjunction with peptide pools that cover the sequence of tyrosinase (Tyr), melanoma-associated antigen A3 (MAGE-A3), melanocyte antigen/melanoma antigen recognized by T cells 1 (Melan-A/MART-1), glycoprotein 100 (gp100), and New York esophageal squamous cell carcinoma-1 (NY-ESO-1) to characterize the melanoma antigen (MA)-specific CD8 + cell repertoire in PBMC of 40 healthy human donors (HD). Tyr triggered interferon gamma (IFN-γ)-secreting CD8 + T cells in 25% of HD within 24 h of antigen stimulation ex vivo. MAGE-A3, Melan-A/MART-1, and gp100 also induced recall responses in 10%, 7.5%, and 2.5% of HD, respectively. At this time point, these CD8 + T cells did not yet produce GzB (granzyme B). However, they engaged in GzB production after 72 h of antigen stimulation. By this 72-h time point, 57.5% of the HD responded to at least one, and typically several, of the MA. A closer characterization of the Tyr-specific CD8 + T cell repertoire indicated that it was low-affinity, and to primarily entail a stem cell-like subpopulation. Collectively, our data reveal pre-existing endogenous T cell immunity against melanoma antigens in healthy donors, and analogous to natural autoantibodies, we have termed this "natural T cell autoreactivity".

Reagent Tracker Dyes Permit Quality Control for Verifying Plating Accuracy in ELISPOT Tests

ELISPOT assays enable the detection of the frequency of antigen-specific T cells in the blood by measuring the secretion of cytokines, or combinations of cytokines, in response to antigenic challenges of a defined population of PBMC. As such, these assays are suited to establish the magnitude and quality of T cell immunity in infectious, allergic, autoimmune and transplant settings, as well as for measurements of anti-tumor immunity. The simplicity, robustness, cost-effectiveness and scalability of ELISPOT renders it suitable for regulated immune monitoring. In response to the regulatory requirements of clinical and pre-clinical immune monitoring trials, tamper-proof audit trails have been introduced to all steps of ELISPOT analysis: from capturing the raw images of assay wells and counting of spots, to all subsequent quality control steps involved in count verification. A major shortcoming of ELISPOT and other related cellular assays is presently the lack of audit trails for the wet laboratory part of the assay, in particular, the assurance that no pipetting errors have occurred during the plating of antigens and cells. Here, we introduce a dye-based reagent tracking platform that fills this gap, thereby increasing the transparency and documentation of ELISPOT test results.