Complete activation of B cells relies on their capacity to extract surface-tethered 35 antigens at the immune synapse by either exerting mechanical forces or promoting 36 their proteolytic degradation through lysosome secretion. Whether antigen extraction 37 can be tuned by local cues originating from the lymphoid microenvironment has not 38 been investigated. We here show that Galectin-8 – an extracellular glycan-binding 39 protein that regulates interactions between cells and matrix proteins – facilitates 40 synapse formation upon BCR recognition of surface-tethered antigens. As a 41 consequence of this, B cell arrest phases are longer in the presence of Galectin-8, 42 which translates in increased antigen extraction and presentation to T lymphocytes in 43 vivo. We further show that Galectin-8 triggers a faster recruitment and secretion of 44 lysosomes at the B cell-antigen contact site, suggesting that enhanced antigen 45 extraction in the presence of Galectin-8 might result from increased antigen 46 proteolysis. Thus, extracellular cues can determine how B cells sense and extract 47 surface-tethered antigens and thereby tune B cell responses in vivo.