A platelet-mediated system for shuttling blood-borne bacteria to [CD8α.sup.+] dendritic cells depends on glycoprotein GPIb and complement C3

The acquisition of pathogen-derived antigen by dendritic cells (DCs) is a key event in the generation of cytotoxic [CD8.sup.+] T cell responses. In mice, the intracellular bacterium Listeria monocytogenes is directed from the blood to splenic [CD8α.sup.+] DCs. We report that L. monocytogenes rapidly associated with platelets in the bloodstream in a manner dependent on GPIb and complement C3. Platelet association targeted a small but immunologically important portion of L. monocytogenes to splenic [CD8α.sup.+] DCs, diverting bacteria from swift clearance by other, less immunogenic phagocytes. Thus, an effective balance is established between maintaining sterility of the circulation and induction of antibacterial immunity by DCs. Other Gram-positive bacteria also were rapidly tagged by platelets, revealing a broadly active shuttling mechanism for systemic bacteria.