Approaches to the pharmacological modulation of plasmacytoid dendritic cells

Human plasmacytoid dendritic cells (pDC) are crucial for the modulation of adaptive immune responses in the course of neoplastic, viral and autoimmune diseases. In several of these disorders deregulated pDC-derived interferon-α (IFN-α), a key cytokine produced by pDC, plays a central role. Apart from IFN-α, pDC can produce a variety of other mediators, which are involved in immunological cross-talk. The most recently discovered are the cytotoxic serine protease granzyme B (GrB) and indoleamine 2,3-dioxygenase, which have been described to be involved in the suppression of effector T cell responses. Here we review the regulation of pDC function by a variety of immunomodulatory agents, which may be developed as future candidates for the therapy of a variety of diseases. Moreover, we introduce the novel concept of enhancing immune responses after vaccination in poor responders by increasing pDC-derived IFN-α and simultaneously inhibiting pDC-derived GrB secretion. Finally we discuss potential approaches to abrogate pDC-mediated tolerance induction against tumors and viral infections.