Granzyme B produced by human plasmacytoid dendritic cells suppresses T-cell expansion

Abstract

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...

Trafficking properties of plasmacytoid dendritic cells in health and disease

“…A novel mechanism was proposed recently, where PDCs may exert tolerogenic, instead of immunogenic, functions. In fact, it was reported that IL-10 can induce human circulating PDCs to suppress T-cell proliferation through the secretion of GrB [30]. This finding is of interest given the high amounts of IL-10 produced in the tumour microenvironment and might explain the pro-tumorigenic effect of this cytokine observed in some experimental conditions [75].…”

“…More recent data have shown that PDCs, especially when unstimulated or alternatively stimulated, or in certain anatomical locations, act as tolerogenic instead of immunogenic cells. This may occur via different mechanisms, such as secretion of granzyme B (GrB) to impair T-cell proliferation [30] or by production of indoleamine-2,3-dioxygenase (IDO) to promote regulatory T-cell generation [31,32].…”

Antiviral Vaccines License T Cell Responses by Suppressing Granzyme B Levels in Human Plasmacytoid Dendritic Cells

“…This possibility is supported by recent findings that immune cell subsets other than CTL or NK cells are able to produce and secrete active GrB under certain circumstances. Subsets capable of GrB production include B cells (10,11), hematopoietic progenitor cells (12), basophils (13), mast cells (14), IFN-a-activated monocyte-derived DC (15), and pDC (6,16). Of note, pDC can produce GrB in amounts that considerably exceed GrB levels produced by classical cytotoxic lymphocytes (6).…”

“…P lasmacytoid dendritic cells (pDC) are a unique subset of DC linking innate and adaptive immunity, thereby modulating immune responses in the course of viral, autoimmune, and neoplastic diseases (1)(2)(3)(4). Apart from production of extraordinarily high IFN-a levels and their capacity to rapidly initiate Ag-specific antiviral CD8 + T cell responses by direct proteasome-independent cross-presentation of exogenous viral Ags (5), pDC are able to secrete large amounts of the serine protease granzyme B (GrB), particularly after treatment with IL-3 and IL-10, which can result in a suppression of T cell proliferation (6).…”

“…Subsets capable of GrB production include B cells (10,11), hematopoietic progenitor cells (12), basophils (13), mast cells (14), IFN-a-activated monocyte-derived DC (15), and pDC (6,16). Of note, pDC can produce GrB in amounts that considerably exceed GrB levels produced by classical cytotoxic lymphocytes (6). Apart from its cytotoxic functions, GrB can exert alternative effects such as degradation of viral proteins important for assembly and replication (17), cytokine-enhancing effects (18), receptor cleavage (19), matrix degradation and remodeling (20), and immunosuppression (9,21,22), including the recently published pDC-GrBmediated inhibition of T cell proliferation (6).…”