Plasmacytoid dendritic cells activated by influenza virus and CD40L drive a potent THI polarization

Plasmacytoid dendritic cells (PDCs) are a subset of dendritic cells present in human blood and inflamed lymph nodes. Here we show that blood PDCs, whe n stimulated with influenza virus and CD40L in vitro, undergo a maturation process characterized by up-regulation of major histocompatibility complex proteins and adhesion and costimulatory molecules. In addition, PDCs down-r egulate CXCR3 and L-selectin, which mediate migration and homing of these c ells into the lymph node. Mature PDCs efficiently stimulate T cells and dri ve a potent THI polarization in vitro, which is mediated by the synergistic effect of interleukin 12 and type I interferon. In vivo, mature PDCs are f ound in secondary lymphoid organs, where they represent the principal sourc e of type I interferon during inflammation. Thus, PDCs probably participate in antiviral and pro-inflammatory responses, rather than in T(H)2 polariza tion and tolerance induction.