Assessment of Immunogenicity in SARS-CoV-2 Naïve and Convalescent Subjects Following COVID-19 Vaccination

WRIB 2022 -- Active vaccination leads to successful priming of adaptive immune responses, resulting in the development of antigen-specific T and B effector cells. Upon encountering the same antigen (by booster vaccine or exposure to infection), these effector lymphocytes proliferate and undergo clonal expansion and differentiation. As a result, B cells produce antigen-specific antibodies and T cells secrete pro-inflammatory cytokines (e.g., IFNg) and lytic granules (e.g., granzyme B) for promoting protective immunity. It's pivotal to monitor the quality and quantity of humoral as well as cellular immunogenicity following vaccination. Assessment of vaccine immunogenicity is relied on detection of effector molecules (e.g., cytokines for T cells; antibodies for B cells) secreted by these adaptive lymphocytes. Development of humoral immunity is directly evaluated and quantified by the levels and isotypes (IgM or IgG) of antibodies in the peripheral blood. The functionality of these antibodies can be further analyzed using neutralizing antibody assays. Detection of antigen-specific T-cell immunity requires additional efforts to prepare PBMCs from blood, and is often measured by cytokine production following ex vivo stimulation with the same antigens. To accommodate a large population during vaccine trials, a high-throughput assay such as Enzyme-linked immunospot (ELISpot) is often utilized for detection of antigen-specific T cells in PBMCs following ex vivo stimulation. Receptor binding domain (RBD) of the spike protein is the most essential target antigen on the SARS-CoV-2 viruses, given the blockade of viral entry to the host by the anti-RBD neutralizing antibody. Peptide pools scanning the RBD region (223 amino acids) are commonly used for ex vivo stimulation. However, it is not known whether this RBD peptide pool can elicit the most robust T-cell responses. The IFNg ELISpot assay was performed to evaluate the peptide pools.