Antibody Avidity Maturation Following Booster Vaccination with an Intranasal Adenovirus Salnavac Vaccine

Abstract

Background: The COVID-19 pandemic has led to the rapid development of new vaccines and methods of testing vaccine-induced immunity. Despite the extensive research that has been conducted on the level of specific antibodies, less attention has been paid to studying the avidity of these antibodies. The avidity of serum antibodies is associated with a vaccine showing high effectiveness and reflects the process of affinity maturation. In the context of vaccines against SARS-CoV-2, only a limited number of studies have investigated the avidity of antibodies, often solely focusing on the wild-type virus following vaccination. This study provides new insights into the avidity of serum antibodies following adenovirus-based boosters. We focused on the effects of an intranasal Salnavac booster, which is compared, using a single analytical platform, to an intramuscular Sputnik V.

Methods: The avidity of RBD-specific IgGs and IgAs was investigated through ELISA using urea and biolayer interferometry.

Results: The results demonstrated the similar avidities of serum antibodies, which were induced by both vaccines for six months post-booster. However, an increase in antibody avidity was observed for the wild-type and Delta variants, but not for the BA.4/5 variant.

Conclusions: Collectively, our data provide the insights into antibody avidity maturation after the adenovirus-based vaccines against SARS-CoV-2.

Keywords: COVID-19; SARS-CoV-2; Salnavac vaccine; Sputnik V vaccine; antibody maturation; avidity; biolayer interferometry.

Figure 1

Affinity of anti-RBD monoclonal IgG antibodies. (A) Representative sensorgrams for mAbs with high (left panel), medium (middle panel), and low (right panel) affinity. The dotted line indicates the time between the association and dissociation steps. (B) The RBD (WT) binding capacity of mAbs with (middle) or without (left) 8 M urea treatment is shown. Dependence affinity indexes for the mAb concentration (right) are also shown. The means of the triplicate measurements and SD are presented. The frame indicates the chosen concentration of mAbs to determine AIs. (C) Spearman’s correlation between the affinity indexes of mAbs and k_on, k_off, and K_D. (D) Summary table of the mAb affinity parameters.

Figure 2

The RBD binding and avidity of serum antibodies in the boosted individuals. (A) Vaccination regimen, sampling timepoints, and serological tests. (B) Anti-RBD (WT, Delta, and BA.4/5) IgG responses in sera from Sputnik V- or Salnavac-boosted individuals. (C) Comparative analysis of anti-RBD IgG levels in individuals boosted with Sputnik V and Salnavac. The number of “plus” symbols or “ns” corresponds to the p-value as described below. (D) Anti-RBD (WT, Delta, and BA.4/5) IgA responses in sera from individuals boosted with Sputnik V or Salnavac. (E) Comparative analysis of anti-RBD IgA levels in individuals boosted with Sputnik V and Salnavac. (F) Representative sensorgrams of sera from one donor at the T1, T2, and T3 timepoints. The red dotted line indicates the time between the association and dissociation steps. (G) k_offs of serum samples over time obtained from Sputnik V- (top) and Salnavac-boosted (bottom) individuals. (H) Temporal changes in the AIs of RBD-specific IgG and IgA serum antibodies from Sputnik V- (top) and Salnavac-boosted (bottom) individuals. (I) Spearman’s correlation between the avidity indexes of sera and k_off values at different timepoints. (J,K) Avidity indexes of IgG (J) and IgA (K) RBD (WT, Delta, and BA.4/5)-specific serum antibodies from boosted individuals. (L,M) Spearman’s correlation between levels of IgG (L) and IgA (M) RBD-specific serum antibodies and avidity indexes of sera and k_off values at different timepoints. Statistics were calculated using 2-way ANOVA with Sidak’s multiple-comparison test (B–E,J,K) and the Kruskal–Wallis test with Dunn’s multiple-comparison test (G,H). Asterisks and hashes indicate the differences between different timepoints in Sputnik V- and Salnavac-boosted individuals, respectively. Pluses (C) indicate differences between Sputnik V- and Salnavac-boosted donors. * (#) p < 0.05; ** (##, ++) p < 0.01; *** (###, +++) p < 0.001; **** (####) p < 0.0001; ns—non-significant. In (B,D,J,K), half-transparent dots indicate individual values, solid dots indicate the mean values, and bars are presented as SD. In (G,H), the median ± IQRs are presented.

Figure 3

Comparative analysis of the avidity and neutralizing capacity of the sera from boosted individuals. (A) Neutralization antibody titers (ID₅₀) against SARS-CoV-2 (WT) pseudoviral particles before and after Sputnik V (left) and Salnavac (right) booster vaccination as measured by pVNA. (B) Comparisons of neutralizing antibody titers between Sputnik V- and Salnavac-boosted individuals. (C) Spearman’s correlations between the k_off values and neutralization titers (pVNA) of boosted donors’ sera at different timepoints. Statistics were calculated using the Kruskal–Wallis test with Dunn’s multiple-comparison test (A,B). * p < 0.05; **** (####) p < 0.0001; ns—non-significant. In (A,B), the median ± IQR are presented. In (B,C), dots indicate individual values.

Figure 4

Surrogate chip-based virus neutralization test on sera from Sputnik V- and Salnavac-boosted individuals. (A) The chip’s print layout for the virus neutralization test. The chip was printed with RBD (WT, Delta, and BA.4/5), irrelevant protein (BSA), or buffers (print, block, or PBS) as the negative control, and AlexaFluor-488 labeled IgG as the positive control. (B) Representative photographs of the chips after RBD-ACE2 interaction with (low panel) and without (upper panel) the blocking of the serum. (C) Comparisons of neutralization measured by sVNT between Sputnik V- and Salnavac-boosted individuals. Statistics were calculated using 2-way ANOVA with Sidak’s multiple-comparison test (C). Asterisks and hashes indicate differences between different timepoints in the Sputnik V- and Salnavac-boosted groups of donors, respectively. * (#) p < 0.05; ## p < 0.01. In (C), half-transparent dots indicate individual values, solid dots indicate the mean values, and bars are presented as SD.