Robust induction of functional humoral response by a plant-derived Coronavirus-like particle vaccine candidate for COVID-19

The COVID-19 pandemic caused by SARS-CoV-2 highlights the urgent need for globally deployable, durable vaccines. While neutralizing antibodies are a key correlate of immunity, binding antibodies and their effector functions might play a crucial role in long-term protection, particularly against variants. The plant-made CoVLP vaccine, licensed in Canada, shows efficacy in preventing COVID-19. This vaccine uses viral-like particles (VLPs) presenting SARS-CoV-2 spike protein in a native-like conformation, potentially enhancing immune response. This study uses systems serology to analyze the humoral response induced by two doses of this CoVLP vaccine, with and without AS03 adjuvant, to assess the durability and functionality of the antibody response and its effectiveness against variants of concern.

The literature review emphasizes the importance of both neutralizing and non-neutralizing antibodies in COVID-19 immunity and the limitations of currently available vaccines in terms of global access and waning immunity. Studies showing the correlation between binding antibodies and vaccine efficacy across platforms are referenced. Existing research on the durability of binding antibodies compared to neutralizing antibodies and the role of Fc-effector functions in protection are discussed. Previous findings on the immunogenicity of the plant-made CoVLP vaccine and the role of adjuvants are also reviewed.

The study used samples from a Phase 1 clinical trial where participants received two doses of CoVLP at three dose levels (3.75 µg, 7.5 µg, or 15 µg), with or without AS03 adjuvant. Systems serology was employed to assess SARS-CoV-2 specific antibody isotype/subclass titers, FcγR binding profiles, and Fc-functional activity at peak immunogenicity (day 42) and six months later (day 201). Multiplexed Luminex assays were used for antibody isotyping and FcγR binding. Effector functions including antibody-dependent cellular phagocytosis (ADCP), antibody-dependent neutrophil phagocytosis (ADNP), antibody-dependent complement deposition (ADCD), and antibody-dependent NK cell activation were also assessed. Glycan analysis was conducted to investigate the role of Fc glycosylation in antibody function. Statistical analyses included Mann-Whitney U-test, Benjamini-Hochberg correction, principal component analysis (PCA), least absolute shrinkage and selection operator (LASSO), and partial least square discriminant analysis (PLS-DA).

AS03-adjuvanted CoVLP induced significantly stronger cross-isotype responses to SARS-CoV-2 wild-type antigens (Spike, RBD, S1, S2) compared to the non-adjuvanted vaccine at day 42. While antibody titers declined over six months, IgG1 responses remained detectable. PCA showed distinct separation between adjuvanted and non-adjuvanted groups. LASSO and PLS-DA analysis identified key features (RBD and S1-specific FcγR binding antibodies, Spike isotype/subclass levels, complement deposition antibodies, phagocytosis-inducing antibodies) enriched in the adjuvanted group at both time points. Network analysis showed strong correlations between antibody, FcγR-binding, and effector functions in the adjuvanted group, particularly for phagocytic antibodies binding to FcγR2a and FcγR2b at day 201. FcγR2A binding levels remained robust across the Spike, S1, and S2 domains at day 201. Functional assays showed significantly higher levels of ADCD, ADCP, ADNP, and ADNK in the adjuvanted group at day 42, with ADCP and ADNK remaining high at day 201. Glycan analysis showed lower fucosylation in the AS03 group at day 201. The adjuvanted vaccine induced robust IgG1 and IgG3 titers against Alpha, Beta, and Delta variants at day 42, with FcγR2A binding persisting at day 201 for all variants. IgG3 response was triggered against Omicron but did not bind to FcγR.

The findings demonstrate that the plant-derived CoVLP vaccine, particularly when adjuvanted with AS03, elicits robust, functional, and durable antibody responses. The preserved Fc-effector functions, especially opsonophagocytosis (ADCP), might contribute significantly to long-term protection against SARS-CoV-2 and its variants, even in the context of waning neutralizing antibody titers. AS03's role in enhancing both magnitude and quality of humoral responses is highlighted, potentially via increased antigen uptake and presentation. The broader response against S1 and S2 domains suggests protection against variants. The persistence of functional antibodies provides an early line of defense, complementing the T-cell response. The unique presentation of the spike protein in the VLP format likely contributes to its ability to generate these highly functional B cell responses.

The study shows that the AS03-adjuvanted CoVLP vaccine induces a robust and durable humoral immune response, with preserved Fc-effector functions against multiple SARS-CoV-2 variants of concern. This suggests that this vaccine platform offers a promising approach to combatting COVID-19, especially in contexts with limitations in vaccine deployment. Further research should focus on the long-term durability of the response and its efficacy in larger populations, including older adults and individuals with co-morbidities.

The study had a relatively small sample size from a Phase 1 clinical trial, limiting generalizability. The participants were healthy adults aged 18-55, excluding other demographic groups. The limited sample volume prevented investigation of antibody function toward specific regions of Spike protein and variants. Therefore, larger-scale studies are needed to confirm the results and assess long-term efficacy in diverse populations.