Human plasmacytoid dendritic cells (pDCs) are crucially involved in the modulation of adaptive T-cell responses in the course of neoplastic, viral, and autoimmune disorders. In several of these diseases elevated extracellular levels of the serine protease granzyme B (GrB) are observed. Here we demonstrate that human pDCs can be an abundant source of GrB and that such GrB ؉ pDCs potently suppress T-cell proliferation in a GrBdependent, perforin-independent manner, a process reminiscent of regulatory T cells. Moreover, we show that GrB expression is strictly regulated on a transcriptional level involving Janus kinase 1 (JAK1), signal transducer and activator of transcription 3 (STAT3), and STAT5 and that interleukin-3 (IL-3), a cytokine secreted by activated T cells, plays a central role for GrB induction. Moreover, we find that the immunosuppressive cytokine IL-10 enhances, while Toll-like receptor agonists and CD40 ligand strongly
IntroductionPlasmacytoid dendritic cells (pDCs) represent a central link between innate and adaptive immunity and play a crucial role in viral, autoimmune, and neoplastic diseases. [1][2][3][4] One of their most prominent features is the ability of pDCs to produce and secrete large amounts of type I interferons (IFNs), thereby initiating and orchestrating antiviral immune responses. 1,5 pDCs can also function as antigen-presenting cells and stimulate effector T-cell responses. 6 However, pDC effects on T-cell subsets can include both T-cell activation (immunogenic function) 7 and induction of T-cell anergy (tolerogenic function). 7,8 pDCs therefore play an important role in fine-tuning cellular immune responses depending on the microenvironment. Several studies suggest that both the activated T cell-derived cytokine interleukin-3 (IL-3) 9 and the immunosuppressive cytokine IL-10 induce a rather tolerogenic pDC phenotype associated with suppression of T-cell responses. [10][11][12] In contrast, activation of pDCs in the presence of ligands for the pDC-characteristic Toll-like receptors (TLRs) TLR7 and TLR9 1 and for CD40 10 results in an immunogenic phenotype with such pDC triggering a proinflammatory immune response including T-cell activation and cytotoxicity. 6,13,14 Several inflammatory diseases including viral and autoimmune diseases have been found to be associated with elevated levels of extracellular granzyme B (GrB). Granzymes including GrB have been found to be locally elevated in bronchoalveolar lavage fluid from patients with chronic allergic asthma and in synovial fluid from patients with rheumatoid arthritis. 15 Infections with cytomegalovirus after renal transplantation, with the dengue fever virus or with HIV, have been associated with high serum levels of GrB. 15 Granzymes such as GrB represent a major constituent of the granules of cytotoxic cells, including cytotoxic T lymphocyte (CTL) and natural killer (NK) cells. The classical function of GrB is induction of apoptosis in target cells recognized by CTLs. 16,17 Evidence is growing that apart from its cytotoxic effec...