The CD169+ macrophage actin cytoskeleton regulates membrane topography, immune complex mobility and B cell activation

B cells circulate through the lymph nodes, spleen, and bone marrow in search of cognate antigen. In lymph nodes B cells bind immune complexes on the surface of antigen presenting cells as macrophages, follicular dendritic cells (FDCs) and dendritic cells (DCs). It has been shown in vivo that B cells can acquire immune complexes from CD169+ macrophages through their B cell antigen receptor (BCR). Here we isolate primary CD169+ macrophages from lymph nodes of unimmunised mice and show that immune complex organisation is controlled by F-actin in vitro. Using single particle tracking (SPT) we show that immune complex diffusion, spatial organisation and clustering are influenced by F-actin. Further, we show that CD169+ macrophages are mechanosensitive. They adapt to substrate stiffness by remodelling actin, which alters membrane topography and further restricts immune complex diffusion. Finally, our results suggest that antigen presenting cells, like CD169+ macrophages, are not passive antigen depositories but have an active role in facilitating immune complex organisation through their actin cytoskeleton and can modulate B cell signalling responses.