Harmonization and qualification of an IFN-γ Enzyme-Linked ImmunoSpot assay (ELISPOT) to measure influenza-specific cell-mediated immunity within the FLUCOP consortium

Regulatory workshop on standardisation of clinical procedures, endpoints and data robustness of human challenge studies - A stakeholder meeting report

Inno4Vac, a public-private partnership funded by the IMI2/EU/EFPIA Joint Undertaking (IMI2 JU), brings together academic institutions, SMEs, and pharmaceutical companies to accelerate and de-risk vaccine development. The project has made significant strides in the selection and production of challenge agents for influenza, respiratory syncytial virus (RSV), and toxigenic Clostridioides difficile for controlled human infection model studies (CHIMs). A regulatory workshop held on March 20, 2024, addressed the standardisation of clinical procedures, ethical considerations, endpoints, and data integrity, highlighting the ongoing initiatives related to these CHIMs. Key discussions focused on refining trial protocols to balance statistical power with participant burden, overseen by a data safety monitoring board. The meeting emphasised the importance of harmonizing CHIM protocols to ensure robust, reproducible, and transparent research. Mandatory trial registration and adherence to the Findable, Accessible, Interoperable, and Reusable (FAIR) data principles were recommended to enhance data reuse and scientific value. This report consolidates efforts to standardise CHIM protocols, essential for accelerating therapeutic innovations and advancing global health research.

IFN-γ/IL-2 Double-Color FluoroSpot Assay for Monitoring Human Primary T Cell Activation: Validation, Inter-Laboratory Comparison, and Recommendations for Clinical Studies

The enzyme-linked immunosorbent spot (EliSpot) assay and its fluorescence-based version, FluoroSpot, are sensitive immunoassays commonly used to quantify antigen-specific T and B lymphocytes and other immune cells in peripheral blood or homogenized tissues. Due to their high sensitivity, these assays are popular in clinical trials to evaluate the efficacy of immunotherapy and vaccines, which involve a high level of scrutiny to ensure valid study results. Besides industry consensus white papers and other research publications, there is no formal guidance for the industry on how to validate EliSpot and FluoroSpot assays to ensure their accurate performance for immune monitoring in clinical trials. Herein, we describe a comprehensive in vitro study using healthy human donor peripheral blood mononuclear cells (PBMCs) and model antigens to validate a double-color FluoroSpot assay for monitoring antigen-specific lymphocytes by detecting and quantifying IFN-γ and IL-2-producing lymphocytes. Validation parameters, acceptance criteria set-up, and assay limits-limit of detection (LOD), minimum positive control response, lower and upper limits of quantification (LLOQ and ULOQ)-were determined, and assay performance was demonstrated by assessing precision, specificity, linearity, and robustness. In addition, an inter-laboratory comparison demonstrated concordance between assay results from two laboratories. In summary, this study outlines a robust approach to EliSpot and FluoroSpot validation and demonstrates that the IFN-γ/IL-2 FluoroSpot assay is suitable for the reliable detection of antigen-specific immune responses from PBMC samples across laboratories and meets the current regulatory requirements for bioanalytical method validation.

RBD-displaying OMV nanovaccine boosts immunity against SARS-CoV-2

BACKGROUND

Since the emergence of SARS-CoV-2, the causative agent of COVID-19, the global health landscape has confronted an unprecedented and formidable challenge. The SARS-CoV-2 receptor-binding domain (RBD) is a key antigen in vaccine design. However, its low immunogenicity has been a hurdle, resulting in the production of minimal anti-RBD antibodies even when combined with alum adjuvant. Outer membrane vesicles (OMVs), secreted by Gram-negative bacteria, are nanospherical structures that can display or deliver antigens while also providing adjuvant activity through pathogen-associated molecular patterns (PAMPs).

RESULTS

In this study, we utilized the SpyTag (ST)/SpyCatcher (SC) bioconjugation system to couple OMV and SARS-CoV-2 RBD in vitro. We successfully prepared a 'plug-and-display' nanovaccine OMV-RBD, which demonstrated good safety profiles and promoted the uptake of antigens by DCs and the maturation of BMDCs by activating TLR3 and NOD2 signaling pathways. Both intranasal and intramuscular immunization with OMV-RBD vaccine elicited robust antigen-specific humoral and cellular immune responses. Importantly, the induced antibodies effectively inhibited the binding of RBD to human angiotensin-converting enzyme 2 (hACE2) and neutralized SARS-CoV-2 pseudoviruses.

CONCLUSIONS

This vaccine platform offers an alternative strategy for developing recombinant subunit vaccines against SARS-CoV-2, potentially enhancing immune responses and improving vaccine efficacy.

Inno4Vac Workshop Report Part 1: Controlled Human Influenza Virus Infection Model (CHIVIM) Strain Selection and Immune Assays for CHIVIM Studies, November 2021, MHRA, UK

Controlled human infection models (CHIMs) are a critical tool for the understanding of infectious disease progression, characterising immune responses to infection and rapid assessment of vaccines or drug treatments. There is increasing interest in using CHIMs for vaccine development and an obvious need for widely available and fit-for-purpose challenge agents. Inno4Vac is a large European consortium working towards accelerating and de-risking the development of new vaccines, including the development of CHIMs for influenza, respiratory syncytial virus and Clostridioides difficile. This report (in two parts) summarises a workshop held at the MHRA in 2021, focused on how to select CHIM candidate strains of influenza and respiratory syncytial virus (RSV) based on desirable virus characteristics and which immune assays would provide relevant information for assessing pre-existing and post-infection immune responses and defining correlates of protection. This manuscript (Part 1) summarises presentations and discussions centred around influenza CHIMs and immune assays (a second manuscript summarises RSV CHIM and immune assays: Inno4Vac workshop report Part 2: RSV CHIM strain selection and immune assays for RSV CHIM studies, November 2021, MHRA, UK).

Validation of the Enzyme-Linked ImmunoSpot Analytic Method for the Detection of Human IFN-γ from Peripheral Blood Mononuclear Cells in Response to the SARS-CoV-2 Spike Protein

COVID-19 vaccine evaluations are mainly focused on antibody analyses, but there is growing interest in measuring the cellular immune responses from the researchers evaluating these vaccines. The cellular responses to several COVID-19 vaccines have been studied using the enzyme-linked immunospot (ELISPOT) assay for IFN-γ. However, the ELISPOT assay is no longer used only for research purpose and so the performance of this assay must be validated. Since the bioanalytical validation of ELISPOT-IFN-γ is essential for evaluating the method's effectiveness and establishing confidence in a vaccine's immunogenicity, the present work validates the ELISPOT-IFN-γ assay's performance in determining the frequency of IFN-γ-producing cells after stimulation with the SARS-CoV-2 spike protein. The validation was performed in peripheral blood mononuclear cells from volunteers immunized with anti-COVID-19 vaccines. According to the findings, the LOD was 17 SFU and the LLOQ was 22 SFU, which makes the method highly sensitive and suitable for evaluating low levels of cellular responses. The procedure's accuracy is confirmed by the correlation coefficients for the spike protein and anti-CD3+, being 0.98 and 0.95, respectively. The repeatability and intermediate precision tests were confirmed to be reliable by obtaining a coefficient of variation of ≤25%. The results obtained in this validation enable the assay to be employed for studying antigen-specific cells and evaluating cellular responses to vaccines.

Meeting Report From "Correlates of Protection for Next Generation Influenza Vaccines: Lessons Learned From the COVID-19 Pandemic"

BACKGROUND

This report summarizes the discussions and conclusions from the "Correlates of Protection for Next Generation Influenza Vaccines: Lessons Learned from the COVID-19 Pandemic" meeting, which took place in Seattle, USA, from March 1, 2023, to March 3, 2023.

CONCLUSIONS

Discussions around influenza virus correlates of protection and their use continued from where the discussion had been left off in 2019. While there was not much progress in the influenza field itself, many lessons learned during the coronavirus disease 2019 (COVID-19) pandemic, especially the importance of mucosal immunity, were discussed and can directly be applied to influenza correlates of protection.

Differential composition and yield of leukocytes isolated from various blood component leukoreduction filters

In Flanders, an estimated 300,000 leukoreduction filters are discarded as biological waste in the blood establishment each year. These filters are a possible source of fresh donor leukocytes for downstream purposes including research. We investigated leukocyte isolation from two types of filters either used for the preparation of platelet concentrates (PC-LRF) or erythrocyte concentrates (EC-LRF). Outcome parameters were leukocyte yield, differential count, turnaround time and effect of storage conditions. Leukocytes were harvested by reverse flow of a buffer solution. Control was the gold standard density gradient centrifugation of buffy coats. Total leukocyte number isolated from PC-LRF (1049 (± 40) x 106) was almost double that of control (632 (± 66) x 106) but the differential count was comparable. Total leukocyte number isolated from EC-LRF (78 (± 9) x 106) was significantly lower than control, but the sample was specifically enriched in granulocytes (81 ± 4%) compared to control (30 ± 1%). Isolation of leukocytes from either PC- or EC-LRF takes 20 min compared to 240 min for control density gradient centrifugation. Leukocyte viability is optimal when harvested on day 1 post donation (95 ± 0.9%) compared to day 3 (76.4 ± 2.4%). In conclusion, our study demonstrates that leukoreduction filters from specific blood component processing are easy to use and present a valuable source for viable leukocytes of all types.

Meeting Report: Controlled Human Influenza Virus Infection Model Studies: Current Status and Future Directions for Innovation

On November 13-14, 2023, the National Institute of Allergy and Infectious Diseases (NIAID) in partnership with the Task Force for Global Health, Flu Lab, the Canadian Institutes of Health Research, and the Centers for Disease Control and Prevention convened a meeting on controlled human influenza virus infection model (CHIVIM) studies to review the current research landscape of CHIVIM studies and to generate actionable next steps. Presentations and panel discussions highlighted CHIVIM use cases, regulatory and ethical considerations, innovations, networks and standardization, and the utility of using CHIVIM in vaccine development. This report summarizes the presentations, discussions, key takeaways, and future directions for innovations in CHIVIMs. Experts agreed that CHIVIM studies can be valuable for the study of influenza infection, immune response, and transmission. Furthermore, they may have utility in the development of vaccines and other medical countermeasures; however, the use of CHIVIMs to de-risk clinical development of investigational vaccines should employ a cautious approach. Endpoints in CHIVIM studies should be tailored to the specific use case. CHIVIM studies can provide useful supporting data for vaccine licensure but are not required and do not obviate the need for the conduct of field efficacy trials. Future directions in this field include the continued expansion of capacity to conduct CHIVIM studies, development of a broad panel of challenge viruses and assay reagents and standards that can be shared, streamlining of manufacturing processes, the exploration of targeted delivery of virus to the lower respiratory tract, efforts to more closely replicate natural influenza disease in CHIVIM, alignment on a definition of breadth to facilitate development of more broadly protective/universal vaccine approaches, and continued collaboration between stakeholders.

Unravelling influenza correlates of protection: lessons from human A/H1N1 Challenge

Mucosal immunity is important in protecting from upper respiratory tract influenza infection. Human challenge provides a unique model to define correlates of protection with baseline immune responses being correlated to the quantity and length of viral shedding and clinical outcomes. Here, we discuss recent work on mucosal and systemic correlates of protection (R. Bean, L. T. Giurgea, A. Han, L. Czajkowski, et al., mBio 15:e02372-23, 2024, https://doi.org/10.1128/mbio.02372-23) and place it in the context of previous work on mucosal immunity. We also discuss the importance of standardized assays to allow global comparison of relevant immune responses in defining correlates of protection. Correlates of protection are important for designing next-generation broadly protective influenza vaccines.

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.

Detection of SARS-CoV-2-Specific Cells Utilizing Whole Proteins and/or Peptides in Human PBMCs Using IFN-ƴ ELISPOT Assay

SARS-CoV-2 continues to threaten global public health, making COVID-19 immunity studies of utmost importance. Waning of antibody responses postinfection and/or vaccination and the emergence of immune escape variants have been ongoing challenges in mitigating SARS-CoV-2 morbidity and mortality. While a tremendous amount of work has been done to characterize humoral immune responses to SARS-CoV-2 virus and vaccines, cellular immunity, mediated by T cells, is critical for efficient viral control and protection and demonstrates high durability and cross-reactivity to coronavirus variants. Thus, ELISPOT, a standard assay for antigen-specific cellular immune response assessment, allows us to evaluate SARS-CoV-2-specific T-cell response by quantifying the frequency of SARS-CoV-2-specific cytokine-secreting cells in vitro. We have outlined a detailed procedure to study T-cell recall responses to SARS-CoV-2 in human peripheral blood mononuclear cells (PBMCs) following infection and/or vaccination using an optimized IFN-γ ELISPOT assay. Our methodologies can be adapted to assess other cytokines and are a useful tool for studying other viral pathogen and/or peptide-specific T-cell responses.

Proficiency tests to evaluate the impact on assay outcomes of harmonized influenza-specific Intracellular Cytokine Staining (ICS) and IFN-ɣ Enzyme-Linked ImmunoSpot (ELISpot) protocols

The magnitude and quality of cell-mediated immune responses elicited by natural infection or vaccination are commonly measured by Interferon-ɣ (IFN-ɣ) Enzyme-Linked ImmunoSpot (ELISpot) and Intracellular Cytokine Staining (ICS). To date, laboratories apply a variety of in-house procedures which leads to diverging results, complicates interlaboratory comparisons and hampers vaccine evaluations. During the FLUCOP project, efforts have been made to develop harmonized Standard Operating Procedures (SOPs) for influenza-specific IFN-ɣ ELISpot and ICS assays. Exploratory pilot studies provided information about the interlaboratory variation before harmonization efforts were initiated. Here we report the results of two proficiency tests organized to evaluate the impact of the harmonization effort on assay results and the performance of participating FLUCOP partners. The introduction of the IFN-ɣ ELISpot SOP reduced variation of both background and stimulated responses. Post-harmonization background responses were all lower than an arbitrary threshold of 50 SFU/million cells. When stimulated with A/California and B/Phuket, a statistically significant reduction in variation (p < 0.0001) was observed and CV values were strongly reduced, from 148% to 77% for A/California and from 126% to 73% for B/Phuket. The harmonizing effect of applying an ICS SOP was also confirmed by an increased homogeneity of data obtained by the individual labs. The application of acceptance criteria on cell viability and background responses further enhanced the data homogeneity. Finally, as the same set of samples was analyzed by both the IFN-ɣ ELISpot and the ICS assays, a method comparison was performed. A clear correlation between the two methods was observed, but they cannot be considered interchangeable. In conclusion, proficiency tests show that a limited harmonization effort consisting of the introduction of SOPs and the use of the same in vitro stimulating antigens leads to a reduction of the interlaboratory variation of IFN-ɣ ELISpot data and demonstrate that substantial improvements for the ICS assay are achieved as comparable laboratory datasets could be generated. Additional steps to further reduce the interlaboratory variation of ICS data can consist of standardized gating templates and detailed data reporting instructions as well as further efforts to harmonize reagent and instrument use.

Immune responses in healthy adults elicited by a bivalent norovirus vaccine candidate composed of GI.4 and GII.4 VLPs without adjuvant

The development of an efficacious vaccine against norovirus is of paramount importance given its potential to reduce the global burden of norovirus-associated morbidity and mortality. Here, we report a detailed immunological analysis of a phase I, double-blind, placebo-controlled clinical trial performed on 60 healthy adults, ages 18 to 40. Total serum immunoglobulin and serum IgA against vaccine strains and cross-reactive serum IgG against non-vaccine strains were measured by enzyme immunoassays, whereas cell-mediated immune responses were quantified using intracellular cytokine staining by flow cytometry. A significant increase in humoral and cellular responses, e.g., IgA and CD4⁺ polypositive T cells, was triggered by the GI.4 Chiba 407 (1987) and GII.4 Aomori 2 (2006) VLP-based norovirus vaccine candidate rNV-2v, which is formulated without adjuvant. No booster effect was observed after the second administration in the pre-exposed adult study population. Furthermore, a cross-reactive immune response was elicited, as shown by IgG titers against GI.3 (2002), GII.2 OC08154 (2008), GII.4 (1999), GII.4 Sydney (2012), GII.4 Washington (2018), GII.6 Maryland (2018), and GII.17 Kawasaki 308 (2015). Due to viral infection via mucosal gut tissue and the high variety of potentially relevant norovirus strains, a focus should be on IgA and cross-protective humoral and cell-mediated responses in the development of a broadly protective, multi-valent norovirus vaccine.

Clinical trial registration

https://clinicaltrials.gov, identifier NCT05508178. EudraCT number: 2019-003226-25.

SARS-CoV-2 epitope-specific T cells: Immunity response feature, TCR repertoire characteristics and cross-reactivity

The devastating COVID-19 pandemic caused by SARS-CoV-2 and multiple variants or subvariants remains an ongoing global challenge. SARS-CoV-2-specific T cell responses play a critical role in early virus clearance, disease severity control, limiting the viral transmission and underpinning COVID-19 vaccine efficacy. Studies estimated broad and robust T cell responses in each individual recognized at least 30 to 40 SARS-CoV-2 antigen epitopes and associated with COVID-19 clinical outcome. Several key immunodominant viral proteome epitopes, including S protein- and non-S protein-derived epitopes, may primarily induce potent and long-lasting antiviral protective effects. In this review, we summarized the immune response features of immunodominant epitope-specific T cells targeting different SRAS-CoV-2 proteome structures after infection and vaccination, including abundance, magnitude, frequency, phenotypic features and response kinetics. Further, we analyzed the epitopes immunodominance hierarchy in combination with multiple epitope-specific T cell attributes and TCR repertoires characteristics, and discussed the significant implications of cross-reactive T cells toward HCoVs, SRAS-CoV-2 and variants of concern, especially Omicron. This review may be essential for mapping the landscape of T cell responses toward SARS-CoV-2 and optimizing the current vaccine strategy.