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Abstract
Adaptive B cell responses are initiated by B cell receptor-antigen engagement. Despite
its importance, the early stages of B cell interaction with antigen in vivo remain
elusive. Using fluorescent particulate antigen in combination with multiphoton microscopy
and immune staining, we identified the follicle-subcapsular sinus (SCS) boundary of
the lymph node as a site for B cell-antigen acquisition. In this macrophage-rich region,
antigen accumulated shortly after administration, where specific follicular B cells
were retained for long periods of time. These B cells acquired antigen cumulatively
and became the main antigen carriers inside the follicle before polarizing to the
B cell-T cell border. These observations not only provide evidence of a B cell role
as antigen transporters into the follicle, but also highlight the boundary between
the follicle and the SCS as a site for initiation of B cell responses.