PROJECT 2: SUMMARY The B cell and plasma cell populations that give rise to serum and mucosal antibodies will ultimately determine the effectiveness and duration of an individual’s humoral immune response to SARS-CoV-2 infection. We will use several mutually-supporting strategies to analyze these cells in the Boyd lab: single B cell phenotyping, B cell receptor (BCR) deep sequencing and determination of antigen specificity with DNA-barcoded antigen tetramers; bulk B cell immunoglobulin gene repertoire sequencing; and monoclonal antibody production from antigen-specific B cells. In complementary strategy, the Jardetzky lab will make use of yeast display libraries of patient-derived single-chain antibody variable fragments (ScFv) enriched for native heavy-light chain pairing to determine the antigen specificity of hundreds to thousands of antigen-specific clones per patient. In longitudinal peripheral blood samples and nasal biopsy samples we will thoroughly characterize antigen-specific B cell clones in patient responses to SARS-CoV-2. We hypothesize that with these data we will be able to determine which features of B cell clonal responses to SARS-CoV-2 are associated with differences in COVID-19 disease severity and differences between populations groups stratified by age, sex, ethnicity and pre-existing conditions, or dysregulated immunity in the context of checkpoint blockade treatment. We further hypothesize that analysis of memory B cell populations together with serological responses may predict which individuals will have longer-lasting humoral protection against reexposure to SARS-CoV-2. Finally, we will evaluate the B cell responses stimulated by natural infection compared to vaccination, beginning with the Covaxx peptide- based vaccine cohort, but with the expectation that additional vaccines will be approved for use during the period of this project funding. In addition to studying aspects of the B cell responses that differ among these clinical scenarios, we will search for features such as “convergent” virus-specific BCRs of highly similar sequences shared between different individuals that may have prognostic value, for example by revealing that an individual has a potent neutralizing antibody response to SARS-CoV-2. Our Aims are the following: Specific Aim 1: Analyze B cell responses in acute COVID-19 disease. Specific Aim 2: Evaluate the formation of B cell memory to SARS-CoV-2. Specific Aim 3: Analyze mucosal B cell and plasma cell responses to SARS-CoV-2 compared to responses of B cells in the blood.