Abstract While our PO1 aims at positioning anti-glycan immunotherapy as a new approach to treat severe bacterial diseases, we will contribute some essential basic studies to it by deconstructing the immune recognition of glycans by T and B cells. This work is made possible by the chemistry groups (Projects 1, 2, and 3) who will provide unique immunogens and reagents and will fit next to the bacteriology and structural biology components of the consortium to perform in vivo studies and structure determinations. The integration of the immunology results within this framework will inform the chemists on how to modify glycan epitopes and their linkages to optimize immune recognition and in vivo effector functions. Our effort will be divided between three specific aims: Aim 1: Establish the molecular basis of glycan recognition by T cells. We have shown that cognate recognition of the glycopeptide (GP) by T cells occurred. We will determine its importance in driving high affinity anti-glycan antibodies and study the repertoire diversity of these cells. The biophysical and structural basis of MHC-GP T cell recognition will be examined as well. We hypothesize that cognate glycan recognition by T cells is important for driving high affinity B cell maturation. Aim 2: Establish the molecular basis of glycan recognition by B cells. We will examine the kinetics of germinal center formation upon immunization and follow the maturation of anti- glycan B cells towards high affinity by isolating single B cells with antigen specific reagents and sequencing their receptors. Biophysical studies after recombinant expression of antibodies, paired with the structural effort of Core 3 should allow to decipher the basic rules of glycan antibody recognition. Here, recognition of capsular antigens will be compared to the recognition of teichoic acids, peptidoglycans and O-linked saccharides. Aim 3: Effectiveness of the vaccinal immune response to glycan in mice. As we evaluate in standard models the protection capacity of anti-glycan antibodies, we will compare vaccine to passive immunotherapy. The possibility to optimize this latter intervention will be explored with recombinant antibodies aimed at avoiding Staphylococcus aureus ability to foil the B cell response. While we have shown that we could produce monoclonal antibodies of high affinity against bacterial glycans protective in challenges models, here, we should provide fundamental mechanistic insights into the recognition of glycans by B and T cells. Our team will challenge this knowledge in the context of one of the most threatening bacterial infections of man: Staphylococcus aureus.