Validation of the performance and suitability of a new class of FBS optimized for use in single-cell functional assays

Theoretical and practical considerations for validating antigen-specific B cell ImmunoSpot assays

Owing to their ability to reliably detect even very rare antigen-specific B cells in cellular isolates such as peripheral blood mononuclear cells (PBMC), and doing so robustly in a high throughput-compatible manner, B cell ELISPOT/FluoroSpot (collectively "B cell ImmunoSpot") tests have become increasingly attractive for immune monitoring in regulated settings. Presently, there are no guidelines for the qualification and validation of B cell ImmunoSpot assay results. Here, we propose such guidelines, building on the experience acquired from T cell ImmunoSpot testing in an environment adhering to the requirements of regulatory bodies yet taking the unique features of B cell assays into account. A streamlined protocol is proposed that permits the performance of all tests needed for the formal validation of an antigen-specific B cell ImmunoSpot assay in only three experiments, utilizing 2.2 × 107 PBMC per donor. Subsequently, utilizing only 1-2 × 106 PBMC per sample (obtainable from 1 to 2 mL of blood), a validated multiplexed assay enables accurate quantification of the frequency of antigen-specific memory B cell-derived blasts secreting IgM, IgG, IgA or IgE antibodies. Collectively, such multiplexed B cell ImmunoSpot assays offer immense value for B cell immune monitoring programs due to their ease of implementation, scalability, applicability to essentially any antigenic system, economy of PBMC utilization, and last but not least, the high content information gained.

Decreased variability in the site-specific results during participation in the External Quality Assurance Program Oversight Laboratory (EQAPOL) proficiency program for IFN-gamma enzyme-linked immunospot (IFN-γ ELISpot) assay

The NIAID DAIDS-sponsored External Quality Assurance Program Oversight Laboratory (EQAPOL) manages an interferon-gamma (IFN-γ) enzyme-linked immunospot (ELISpot) external proficiency program. The ELISpot program evaluates the accuracy and variability of results across laboratories. The variability in the program is quantified via the dispersion, which is the ratio of the variance over the mean of the background-corrected spot-forming cells (SFC) replicates obtained under stimulation with different peptide pools (CMV, CEF). This report includes the longitudinal analysis of the ELISpot program cohort composed of 22 laboratories from 2011 to 2022 to assess whether the within-lab variability has improved over time. Random intercept models of the dispersion over time showed a significant decrease in overall dispersion from an average of approximately 1.8 in 2011 to approximately 1.25 in 2022. Out of the 21 sites, 16 sites (4 being statistically significant) had a negative trend for dispersion over time. Our finding of a reduction of overall within-lab variability demonstrates the need for and benefit of proficiency testing programs.

Performance and Stability of New Class of Fetal Bovine Sera (FBS) and Its Lyophilized Form in ELISpot and FluoroSpot Assays: Applications for Monitoring the Immune Response in Vaccine, and Cell and Gene Immunotherapy in Clinical Trials

Interferon-gamma (IFNγ) ELISpot and FluoroSpot are widely used assays to detect functional cell responses in immunotherapy clinical studies. Recognized for their importance in vaccine development studies to quantitate immune responses, these assays have more recently risen to the forefront in cell and gene therapy as well as cancer immunotherapy fields where responses against cancer neoantigens are not easily detectable above assay background. Here, we test a new class of fetal bovine serum (FBS), CultraPure FBS, in ex vivo ELISpot and FluoroSpot assays and cultured FluoroSpot assays following in vitro expansion. Several CultraPure FBS lots that have been specially formulated through the process of lyophilization (lyo-FBS) were compared to liquid CultraPure FBS. We stimulated human PBMCs with antigen-specific peptide pools diluted in media supplemented with liquid CultraPure FBS or lyo-FBS and found equivalent cytokine production with negligible to no assay background with both liquid and lyo-FBS formats. Moreover, the lyo-FBS showed lot-to-lot consistency and 90-day refrigerated (4 °C) stability in both ex vivo direct and in vitro cultured assays. In addition, we present here a method using lyo-FBS for the expansion of low-frequency antigen-specific T cells, mimicking the low frequency seen with cancer neoantigens by utilizing a cultured FluoroSpot assay. Our results demonstrate the presence of Granzyme B, interferon-gamma (IFNγ), and tumor necrosis factor (TNF) production by antigen-specific polyfunctional T cells following a 9-day culture using media supplemented with lyo-FBS.