Surfactant protein (SP) D functions as a soluble pattern recognition molecule to mediate the clearance of pathogens by phagocytes in the innate immune response. We hypothesize that SP-D may also interact with dendritic cells, the most potent antigen presenting cell, to enhance uptake and presentation of bacterial antigens. Using mouse bone marrow-derived dendritic cells, we show that SP-D binds to immature dendritic cells in a dose-, carbohydrate-, and calcium-dependent manner, whereas SP-D binding to mature dendritic cells is reduced. SP-D also binds to Escherichia coli HB101 and enhances its association with dendritic cells. Additionally, SP-D enhances the antigen presentation of an ovalbumin fusion protein expressed in E. coli HB101 to ovalbumin-specific major histocompatibility complex class II T cell hybridomas. The enhancement of antigen presentation by SP-D is dose dependent and is not shared by other collectin-like proteins tested. These studies demonstrate that SP-D augments antigen presentation by dendritic cells and suggest that innate immune molecules such as SP-D may help initiate an adaptive immune response for the purpose of resolving an infection.
