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.