Platelets Enhance Dendritic Cell Responses against Staphylococcus aureus through CD40-CD40L.

Staphylococcus aureus is a major human pathogen that can cause mild to severe life-threatening infections in many tissues and organs. Platelets are known to participate in protection against S. aureus by direct killing and by enhancing the activities of neutrophils and macrophages in clearing S. aureus infection. Platelets have also been shown to induce monocyte differentiation into dendritic cells and to enhance activation of dendritic cells. Therefore, in the present study, we explored the role of platelets in enhancing bone marrow-derived dendritic cell (BMDC) function against S. aureus We observed a significant increase in dendritic cell phagocytosis and intracellular killing of a methicillin-resistant Staphylococcus aureus (MRSA) strain (USA300) by thrombin-activated platelets or their releasates. Enhancement of bacterial uptake and killing by DCs is mediated by platelet-derived CD40L. Coculture of USA300 and BMDCs in the presence of thrombin-activated platelet releasates invokes upregulation of the maturation marker CD80 on DCs and enhanced production of the proinflammatory cytokines tumor necrosis factor alpha (TNF-α), interleukin 12 (IL-12), and IL-6. Overall, these observations support our hypothesis that platelets play a critical role in the host defense against S. aureus infection. Platelets stimulate DCs, leading to direct killing of S. aureus and enhanced DC maturation, potentially leading to adaptive immune responses against S. aureus.