Kai Sontheimer

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

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 GrB-dependent, 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 inhibit, GrB secretion by pDCs. GrB-secreting pDCs may play a regulatory role for immune evasion of tumors, antiviral immune responses, and autoimmune processes. Our results provide novel information about the complex network of pDC-T-cell interactions and may contribute to an improvement of prophylactic and therapeutic vaccinations.

Interleukin 21-induced granzyme B-expressing B cells infiltrate tumors and regulate T cells

The pathogenic impact of tumor-infiltrating B cells is unresolved at present, however, some studies suggest that they may have immune regulatory potential. Here, we report that the microenvironment of various solid tumors includes B cells that express granzyme B (GrB, GZMB), where these B cells can be found adjacent to interleukin (IL)-21-secreting regulatory T cells (Treg) that contribute to immune tolerance of tumor antigens. Because Tregs and plasmacytoid dendritic cells are known to modulate T-effector cells by a GrB-dependent mechanism, we hypothesized that a similar process may operate to modulate regulatory B cells (Breg). IL-21 induced outgrowth of B cells expressing high levels of GrB, which thereby limited T-cell proliferation by a GrB-dependent degradation of the T-cell receptor ζ-chain. Mechanistic investigations into how IL-21 induced GrB expression in B cells to confer Breg function revealed a CD19(+)CD38(+)CD1d(+)IgM(+)CD147(+) expression signature, along with expression of additional key regulatory molecules including IL-10, CD25, and indoleamine-2,3-dioxygenase. Notably, induction of GrB by IL-21 integrated signals mediated by surface immunoglobulin M (B-cell receptor) and Toll-like receptors, each of which were enhanced with expression of the B-cell marker CD5. Our findings show for the first time that IL-21 induces GrB(+) human Bregs. They also establish the existence of human B cells with a regulatory phenotype in solid tumor infiltrates, where they may contribute to the suppression of antitumor immune responses. Together, these findings may stimulate novel diagnostic and cell therapeutic approaches to better manage human cancer as well as autoimmune and graft-versus-host pathologies.

Human B Cells Secrete Granzyme B When Recognizing Viral Antigens in the Context of the Acute Phase Cytokine IL-21

Human B cells are currently not known to produce the proapoptotic protease granzyme B (GrB) in physiological settings. We have discovered that BCR stimulation with either viral Ags or activating Abs in the context of the acute phase cytokine IL-21 can induce the secretion of substantial amounts of GrB by human B cells. Importantly, GrB response to viral Ags was significantly stronger in B cells from subjects recently vaccinated against the corresponding viruses as compared with unvaccinated subjects. GrB-secreting B cells featured a homogeneous CD19+CD20+CD27−CD38−IgD− phenotype, improved survival, and enhanced expression of costimulatory, Ag-presenting and cell-adhesion molecules. B cell-derived GrB was enzymatically active and its induction required the activation of similar signaling pathways as those in CTLs. Our findings suggest that GrB-secreting B cells support the early antiviral immune response against viruses with endosomal entry pathways, thereby counteracting overwhelming viral replication at the beginning of an infection until virus-specific T cells from draining lymph nodes arrive at the site of infection. Our data may also explain the elevated serum GrB levels found in the early phase of various viral diseases.

Human B cells differentiate into granzyme B-secreting cytotoxic B lymphocytes upon incomplete T-cell help

Recently, CD4+ T helper cells were shown to induce differentiation of human B cells into plasma cells by expressing interleukin (IL‐)21 and CD40 ligand (CD40L). In the present study we show, that in the absence of CD40L, CD4+ T cell‐derived IL‐21 induces differentiation of B cells into granzyme B (GzmB)‐secreting cytotoxic cells. Using fluorescence‐activated cell sorting (FACS) analysis, ELISpot and confocal microscopy, we demonstrate that CD4+ T cells, activated via their T‐cell receptor without co‐stimulation, can produce IL‐21, but do not express CD40L and rapidly induce GzmB in co‐cultured B cells in an IL‐21 receptor‐dependent manner. Of note, we confirmed these results with recombinant reagents, highlighting that CD40L suppresses IL‐21‐induced GzmB induction in B cells in a dose‐dependent manner. Surprisingly, although GzmB‐secreting B cells did not express perforin, they were able to transfer active GzmB to tumor cell lines, thereby effectively inducing apoptosis. In contrast, no cytotoxic effects were found when effector B cells were activated with IL‐2 instead of IL‐21 or when target cells were cultured with IL‐21 alone. Our findings suggest GzmB+ cytotoxic B cells may have a role in early cellular immune responses including tumor immunosurveillance, before fully activated, antigen‐specific cytotoxic T cells are on the spot. CD40 ligand determines whether IL‐21 induces differentiation of B cells into plasma cells or into granzyme B‐secreting cytotoxic cells.

CD5+ B cells from individuals with systemic lupus erythematosus express granzyme B

Recently, we reported that IL‐21 induces granzyme B (GzmB) and GzmB‐dependent apoptosis in malignant CD5+ B cells from patients with chronic lymphocytic leukemia. Several autoimmune diseases (AD) are associated with enhanced frequencies of CD5+ B cells. Since AD are also associated with elevated IL‐21 and GzmB levels, we postulated a link between CD5+ B cells, IL‐21 and GzmB. Here, we demonstrate that IL‐21 and GzmB serum levels are highly correlated in subjects with systemic lupus erythematosus (SLE) and that freshly isolated CD5+ SLE B cells constitutively express GzmB. IL‐21 directly induced GzmB expression and secretion by CD5+ B cells from several AD and from cord blood in vitro, and the simultaneous presence of BCR stimulation strongly enhanced this process. Furthermore, IL‐21 suppressed both viability and expansion of CD5+ B cells from SLE individuals. In summary, our study may explain the elevated levels of IL‐21 and GzmB in SLE and other AD. Moreover, our data suggest that IL‐21 may have disease‐modifying characteristics by inducing GzmB in CD5+ B cells and by suppressing their expansion. Our results provide the rationale for further evaluation of the therapeutic potential of IL‐21 in certain AD such as SLE.