ABSTRACT – Project 3 The focus of Project 3 is to study antigen-specific B cell and plasma cell responses in the context of two timely and fundamental topics in vaccinology: (i) Immunology of COVID-19 vaccines, and (ii) the impact of the microbiota on immune responses to vaccination. The COVID-19 pandemic caused by the novel coronavirus SARS-CoV-2 (CoV-2), and the vaccines developed to combat this pathogen, have underscored a need for greater understanding of primary antibody responses in humans. We will use a systematic panel of cutting- edge humoral immunity analyses to thoroughly characterize antibodies elicited by two CoV-2 vaccines, and the B cell and plasma cell clonal populations required for B cell memory and sustained antibody titers. Our focus will be on the serological, B cell and plasma cell responses elicited by a lipid nanoparticle mRNA vaccine (Pfizer-BioNTech), and a Matrix M-adjuvanted recombinant protein vaccine (Novavax). Combining these analyses with studies of innate immunity (Project 1) and T cell (Project 2) responses to these vaccines should highlight cellular mechanisms correlated with the strength and durability of antibody responses. Rare serious anaphylactoid adverse reactions have been reported for mRNA vaccines, particularly in individuals with a history of food allergy, and those with IgG antibodies specific for polyethylene glycol (PEG). We will examine potential B cell contributions to these anaphylactoid reactions, using specimens from affected individuals who received SARS-COV-2 mRNA vaccines. Finally, we will address the role of the microbiome on humoral immunity to vaccination, with a similar strategy of serological, memory B cell and plasma cell analyses in participants with or without temporarily ablated microbiota following antibiotic treatment. Of particular importance in the aforementioned studies, we will not only analyze peripheral blood B cells and plasmablasts, but also monitor lymph node germinal center reactions by fine-needle aspiration, and sample bone marrow plasma cells in the same participants, to comprehensively study humoral immunity to vaccination in humans. The combined impact of these investigations will likely be clinically significant in guiding the development of future vaccination strategies by uncovering the B cell and plasma cell specificities, differentiation pathways, and longevity stimulated by new SARS-CoV-2 vaccine platforms, and in clarifying the role of the microbiome in vaccine responses to novel antigens.