Mastering the Computational Challenges of Elispot Plate Evaluation

Protocol to measure the impact of Clostridioides difficile toxins on antibody responses using ELISA, ELISPOT, and toxin-neutralization assays

To determine the effects of bacterial toxins on the humoral immune response, accurate measurement of antibody quantity, specificity, and function is critical. Here, we present a murine injection protocol for bacterial toxins and vaccine antigens and collection of plasma via retro-orbital eye bleeds. We describe steps for measuring antigen-specific antibody titers, antibody-secreting plasma cell abundance, and in vitro function of those antibodies. This protocol may be adapted to measure the impact of several bacterial toxins on humoral immunity. For complete details on the use and execution of this protocol, please refer to Norman et al.1.

Validation of a double-color ELISpot assay of IFN-γ and IL-4 production in human peripheral blood mononuclear cells

The Enzyme-Linked ImmunoSpot (ELISpot) assay detects cytokines secreted during T cell-specific immune responses against pathogens. As this assay has acquired importance in the clinical setting, standard bioanalytical evaluation of this method is required. Here, we describe a formal bioanalytical validation of a double-color ELISpot assay for the evaluation of IFN-γ and IL-4 released by T helper 1 and T helper 2 cells, respectively. As recommended by international guidelines, the parameters assessed were: range and detection limits (limit of detection, LOD; upper and lower limit of quantification, ULOQ and LLOQ), Linearity, Relative Accuracy, Repeatability, Intermediate Precision, Specificity and Robustness. The results obtained in this validation study demonstrate that this assay meets the established acceptability criteria. ELISpot is therefore a reliable technique for measuring T cell-specific immune responses against various antigens of interest.

Important Considerations for ELISpot Validation

The ELISpot assay has a solid place in the immune monitoring field for over 40 years. It is an assay that can assess the function of single immune cells in a straightforward and easy-to-learn approach. Its use in basic research, translational, and clinical work has been documented in countless publications. Harmonization guidelines and invaluable tools for optimal assay performance and evaluation exist. However, the validation of an established ELISpot protocol has been left to diverse opinions about how to interpret and tackle typical validation parameters. This chapter addresses important considerations for ELISpot validation, including the interpretations of validation parameters for a meaningful description of assay performance.

Validation of an IFN-gamma ELISpot assay to measure cellular immune responses against viral antigens in non-human primates

Adeno-Associated Virus (AAV)-based gene therapy vectors are in development for many inherited human disorders. In nonclinical studies, cellular immune responses mediated by cytotoxic T cells may target vector-transduced cells, which could impact safety and efficacy. Here, we describe the bioanalytical validation of an interferon-gamma (IFN-γ)-based Enzyme-Linked Immunospot (ELISpot) assay for measuring T cell responses against viral antigens in cynomolgus monkeys. Since ELISpots performed with antigen-derived peptides offer a universal assay format, method performance characteristics were validated using widely available peripheral blood mononuclear cells (PBMCs) responsive to cytomegalovirus peptides. The limit of detection and confirmatory cut point were established using statistical methods; precision, specificity, and linearity were confirmed. Monkey PBMCs from an AAV5 gene therapy study were then analyzed, using peptide pools spanning the vector capsid and transgene product. AAV5-specific T cell responses were detected only in 2 of 18 monkeys at Day 28, but not at Day 13 and 56 after vector administration, with no correlation to liver enzyme elevations or transgene expression levels. No transgene product-specific T cell responses occurred. In conclusion, while viral peptide-specific IFN-γ ELISpots can be successfully validated for monkey PBMCs, monitoring peripheral T cell responses in non-clinical AAV5 gene therapy studies was of limited value to interpret safety or efficacy.