Detection of Antibody Responses Against SARS-CoV-2 in Plasma and Saliva From Vaccinated and Infected Individuals

Abstract

Antibodies (Abs) are essential for the host immune response against SARS-CoV-2, and all the vaccines developed so far have been designed to induce Abs targeting the SARS-CoV-2 spike. Many studies have examined Ab responses in the blood from vaccinated and infected individuals. However, since SARS-CoV-2 is a respiratory virus, it is also critical to understand the mucosal Ab responses at the sites of initial virus exposure. Here, we examined plasma versus saliva Ab responses in vaccinated and convalescent patients. Although saliva levels were significantly lower, a strong correlation was observed between plasma and saliva total Ig levels against all SARS-CoV-2 antigens tested. Virus-specific IgG1 responses predominated in both saliva and plasma, while a lower prevalence of IgM and IgA1 Abs was observed in saliva. Antiviral activities of plasma Abs were also studied. Neutralization titers against the initial WA1 (D614G), B.1.1.7 (alpha) and B.1.617.2 (delta) strains were similar but lower against the B.1.351 (beta) strain. Spike-specific antibody-dependent cellular phagocytosis (ADCP) activities were also detected and the levels correlated with spike-binding Ig titers. Interestingly, while neutralization and ADCP potencies of vaccinated and convalescent groups were comparable, enhanced complement deposition to spike-specific Abs was noted in vaccinated versus convalescent groups and corresponded with higher levels of IgG1 plus IgG3 among the vaccinated individuals. Altogether, this study demonstrates the detection of Ab responses after vaccination or infection in plasma and saliva that correlate significantly, although Ig isotypic differences were noted. The induced plasma Abs displayed Fab-mediated and Fc-dependent functions with comparable neutralization and ADCP potencies, but a greater capacity to activate complement was elicited upon vaccination.

Natural SARS-CoV-2 Infection Affects Neutralizing Activity in Saliva of Vaccinees

“…There are some limitations to our study: this was a non-randomized observational study, and it was not planned to investigate neither the production of the different antibody subtypes (i.e., IgA, IgG, and IgM) nor their maintenance over time post infection and/or vaccination. Also, while the main antiviral function of Ab is to neutralize virions, they may also display non-neutralizing effector functions mediated via their Fc fragments (i.e., Ab-dependent cellular phagocytosis, Ab-dependent cellular cytotoxicity, and Ab-dependent activation of classical complement cascade), whose activity should be monitored for completeness, as already performed in previous studies ( 48 ). Moreover, the study was not prospective, and therefore, it was limited by sample availability and suffers from some variability in specimen collection time points.…”

SARS-CoV-2 Spike- and Nucleoprotein-Specific Antibodies Induced After Vaccination or Infection Promote Classical Complement Activation

“…In saliva, which is often used as a proxy for mucosal responses, both IgA and IgG to SARS-CoV-2 are readily detectable after infection ( 52 , 53 ), but this is less so after vaccination alone ( 54 ). In contrast, IgM is typically not a major component of the antibody repertoire detected in saliva ( 55 ). Although there are significant levels of complement in the upper and lower respiratory tract ( 56 , 57 ), the relative predominance of IgA may mean there is less activation of the complement cascade even between the upper and lower respiratory tracts.…”

Guardians of the oral and nasopharyngeal galaxy: IgA and protection against SARS‐CoV‐2 infection*

“…Measuring sIgA in saliva can be challenging, and Luminex‐style bead‐based assays seem particularly prone to high background binding of non‐specific salivary IgA 81,82 . Other types of assays report low sensitivity detecting sIgA, where vaccine‐induced IgG in saliva is detectable while IgA is not 83 .…”