Vaccine development is an expensive and time-consuming process that heavily relies on animal models. Yet, vaccine candidates that have previously succeeded in animal experiments often fail in clinical trials questioning the predictive value of animal models. Alternative assay systems that can add to the screening and evaluation of functional characteristics of vaccines in a human context before embarking on costly clinical trials are therefore urgently needed. In this study, we have established an in vitro system consisting of long-term cultures of unfractionated peripheral blood mononuclear cells (PBMCs) from healthy volunteers to assess (recall) T cell responses to vaccine candidates. We observed that different types of influenza vaccines (whole inactivated virus (WIV), split, and peptide vaccines) were all able to stimulate CD4 and CD8 T cell responses but to different extents in line with their reported in vivo properties. In-depth analyses of different T cell subsets revealed that the tested vaccines evoked mainly recall responses as indicated by the fact that the vast majority of the responding T cells had a memory phenotype. Furthermore, we observed vaccine-induced activation of T follicular helper cells, which are associated with the induction of humoral immune responses. Our results demonstrate the suitability of the established PBMC-based system for the in vitro evaluation of memory T cell responses to vaccines and the comparison of vaccine candidates in a human immune cell context. As such, it can help to bridge the gap between animal experiments and clinical trials and assist in the selection of promising vaccine candidates, at least for recall antigens.

Vaccine development relies on testing vaccine candidates in animal models. However, results from animals cannot always be translated to humans. Alternative ways to screen vaccine candidates before clinical trials are therefore desirable. Dendritic cells (DCs) are the main orchestrators of the immune system and the link between innate and adaptive responses. Their activation by vaccines is an essential step in vaccine-induced immune responses. We have systematically evaluated the suitability of two different human DC-based systems, namely the DC-cell line MUTZ-3 and primary monocyte-derived DCs (Mo-DCs) to screen immunopotentiating properties of vaccine candidates. Two different influenza vaccine formulations, whole inactivated virus (WIV) and subunit (SU), were used as model antigens as they represent a high immunogenic and low immunogenic vaccine, respectively. MUTZ-3 cells were restricted in their ability to respond to different stimuli. In contrast, Mo-DCs readily responded to WIV and SU in a vaccine-specific way. WIV stimulation elicited a more vigorous induction of activation markers, immune response-related genes and secretion of cytokines involved in antiviral responses than the SU vaccine. Furthermore, Mo-DCs differentiated from freshly isolated and freeze/thawed peripheral blood mononuclear cells (PBMCs) showed a similar capacity to respond to different vaccines. Taken together, we identified human PBMC-derived Mo-DCs as a suitable platform to evaluate vaccine-induced immune responses. Importantly, we show that fresh and frozen PBMCs can be used indistinctly, which strongly facilitates the routine use of this system. In vitro vaccine pre-screening using human Mo-DCs is thus a promising approach for evaluating the immunopotentiating capacities of new vaccine formulations that have not yet been tested in humans.

Vaccine co-administration can increase vaccination coverage. We assessed the safety, reactogenicity, and immunogenicity of concomitant administration of Ad26.COV2.S COVID-19 vaccine with seasonal influenza vaccines. This non-inferiority, Phase 3, randomised, double-blind study enrolled 859 healthy adults and was conducted between 02 November 2021 and 28 November 2022. Participants aged ≥18–64 years were randomised to receive a seasonal quadrivalent standard dose (SD) influenza vaccine (Afluria Quadrivalent, Seqirus) concomitantly with Ad26.COV2.S (Coad_SD) or placebo (0.9% NaCl; Control_SD) on Day 1 and placebo or Ad26.COV2.S on Day 29. Participants aged ≥65-years were randomised to the Coad_SD or Control_SD groups, or to Coad_HD or Control_HD groups that received a seasonal quadrivalent HD (high-dose) influenza vaccine (Fluzone High-Dose Quadrivalent, Sanofi Pasteur Inc) in the same schedules. The primary outcomes were haemagglutinin inhibition titres against the four influenza vaccine strains at Day 29, and SARS-CoV-2 Spike-specific antibodies at Day 29 in the Coad_SD group and Day 57 in the Control-SD group, with a non-inferiority margin (Control-SD group/Coad_SD group) of 1.5. Reactogenicity and safety were assessed in all participants (NCT05091307). Non-inferiority criteria for concomitant administration in the SD groups were met for SARS-CoV-2 Spike-specific antibodies (ratio 1.11, 95% CI 0.97–1.26) and haemagglutinin inhibition titres for all influenza strains (A/H3N2 1.23, 95% CI 1.05–1.45; B/Victoria 0.99, 95% CI 0.84–1.19; B/Yamagata, 1.03, 95% CI 0.88–1.21) except A/H1N1 (1.28, 95% CI 1.09–1.53) for which the upper limit of the 95% CI was >1.5. Concomitant administration of Ad26.COV2.S and SD influenza vaccine induced robust immune responses in terms of SARS-CoV-2 Spike-specific antibodies and haemagglutinin inhibition to all four influenza strains. Seroconversion and seroprotection rates against all influenza vaccine strains were comparable in the Coad and Control groups. Anti-Spike antibodies 28 days after receiving Ad26.COV2.S were similar whether administered with influenza vaccine or alone. Antibody responses persisted at least 6 months post-vaccination in all groups. The reactogenicity and safety profile following co-administration was consistent with the known safety profiles of the study vaccines. No safety concerns were identified. Coadministration was immunogenic and well tolerated in adults aged ≥65 years who received HD influenza vaccine. Co-administration of seasonal influenza vaccine with Ad26.COV2.S was immunogenic with an acceptable safety profile, supporting co-administration of these vaccines. Janssen Vaccines & Prevention BV and 10.13039/100012399 Biomedical Advanced Research and Development Authority .

Viral subunit vaccines are a safer and more tolerable alternative to whole inactivated virus vaccines. However, they often come with limited efficacy, necessitating the use of adjuvants. Using free and particle-bound viral antigens, we assessed whether size affects the uptake of those antigens by human monocyte-derived dendritic cells (Mo-DCs) and whether differences in uptake affect their capacity to stimulate cytokine production by T cells. To this end, influenza antigens and hepatitis B surface antigen (HBsAg) were covalently conjugated to polystyrene particles of 500 nm and 3 μm. Cellular uptake of the antigens, either unconjugated or conjugated, and their capacity to stimulate T cells within a population of human peripheral blood mononuclear cells (PBMCs) were measured by flow cytometry. Conjugation of both antigens to particles significantly increased their uptake by Mo-DCs. Moreover, both the 500 nm and 3 μm influenza conjugates induced significantly higher numbers of cytokine-producing CD4⁺ T cells and induced increased production of the pro-inflammatory cytokines IFNγ and TNFα. In contrast, conjugation of HBsAg to particles did not notably affect the T cell response. In conclusion, conjugation of antigen to 500 nm and 3 μm particles leads to increased antigen uptake by human Mo-DCs, although the capacity of such conjugates to induce T cell stimulation likely depends on the immunological status of the PBMC donor.