Dynamic regulation of B cell recruitment in T-dependent humoral immune response. (cognate B-Th cells interactions, 2-photon imaging, viral escape mutants) Project summary In this project we will study the regulation of T-dependent humoral (antibody) immune response (THIR) to learn how it can be manipulated to promote efficient humoral protection of healthy, as well as chronically sick or immunocompromised, individuals against infections. For generation of long-term, high-affinity humoral response, rare antigen-specific B cells have to acquire foreign antigen and then receive help from cognate Th cells. While the signaling and transcription factors promoting B - Th cell interactions and long-term humoral immunity have been extensively investigated, the molecular mechanisms important for recruitment of B cells into THIR in vivo are still poorly understood. Discovering novel ways to optimize the speed and efficiency of initial cognate interactions between B and Th cells is critical for induction of THIR when antigen or T cell help availability is limited. In addtion, it is important for vaccine prophylaxis in the case of rapidly spreading pandemic infections. Therefore, the objective of this application is to determine which factors regulate recruitment of B cells into THIR, and their fate when Ag or T cell help availability is limited. Our central hypothesis is that recruitment of B cells into THIR depends on (i) accessibility of Ag to B cells, and on molecular factors that (ii) regulate B cell responsiveness to T cell help, (iii) promote B-T cell encounters, and (iv) determine B cell fate if T cell help is not acquired. Our preliminary in vivo data suggests that single transient Ag acquisition by B cells may be sufficient to prime B cells for T cell help and participate in the germinal center and B cell memory responses. However, which molecular factors regulate the time that B cells are capable of acquiring T cell help and B cell differentiation fate in vivo is not known; and whether interactions between rare activated B and Th cells are promoted by molecular cues that attract them to each other or stabilize cognate interactions have not been addressed. In addition, whether limiting amounts of viral escape mutants could be recognized by the humoral immune system during ongoing viral infection is unclear. We will address these questions using transgenic lymphocytes and model antigens, as well as the natural mouse viral infection model - Murine Norovirus, two- photon intravital imaging and quantitative analysis of the data, and will characterize novel mechanisms that control THIR. Such results are expected to have an important positive impact because in addition to advancing the field of adaptive immunity in general, they could lead to improvements in existing vaccination strategies and suggest new ways to boost the immune system to rapidly fight ongoing infections.