Human mesenchymal stem cells constitutively express the serine protease granzyme B. (P1097)

Human mesenchymal stem cells (MSC) are known for their immunoregulatory potential and are allegedly involved in tolerance induction. Several immune cells have been shown to exhibit their regulatory effects via expression of the cytotoxic serine protease granzyme B including regulatory T cells and plasmacytoid dendritic cells. Since MSC were recently found to express the GrB inhibitor PI-9, which is primarily expressed by cytotoxic cells, we hypothesized that MSC may also express GrB. Using various methods including Western immunoblotting, ELISpot, FACS analysis we found that MSC freshly isolated from healthy donor bone marrow biopsies indeed express considerable amounts of GrB protein. ELISpot assays demonstrated that GrB was not only expressed but also secreted by MSC. Moreover, a colorimetric GrB activity assay confirmed that GrB was secreted in its enzymatically active form. Importantly, when testing the effect of various cytokines we found that G-CSF, IFNg and TPO were capable of further enhancing GrB expression in MSC. Future studies have to elucidate which aspects of the suppressive activity of MSC possibly depend on GrB and which culture conditions may support a suppressive phenotype of these cells including GrB expression. Our results may open novel approaches to the use of MSC as immunoregulatory cellular therapeutics in inflammatory conditions such as graft-versus-host disease.

HIV triggers interleukin 21-mediated induction of granzyme B-secreting B cells with T cell regulatory potential. (P1441)

Certain regulatory lymphocyte subpopulations like regulatory T cells can suppress T cell expansion by granzyme B (GrB) secretion. Recently, we found that B cells can also produce GrB and acquire regulatory potential in response to interleukin (IL-)21. Since HIV is associated with elevated serum IL-21 levels, we hypothesized that GrB-expressing B cells may be induced during HIV infection. Here, we show that infection of CD4+ T cells with HIV 1 (NL4-3) induces strong expression of IL-21 without upregulation of CD40 ligand. We demonstrate that such IL-21+CD40Llow T cells secrete IL-21, thereby inducing GrB in cocultured B cells rather than supporting plasma cell differentiation. In line with these results, serum levels of both IL-21 and GrB are significantly higher in HIV-infected patients before HAART as compared to healthy controls. Up to 60% of freshly isolated B cells from HIV-infected patients, but not healthy control B cells, express GrB. Of note, coculture of HIV-infected CD4+ T cells with GrB+ B cells results in GrB transfer, and strongly suppresses both T cell proliferation and viral replication as indicated by significantly reduced p24 levels. The observed effects are enhanced by IL-21, and reduced by GrB inhibition. In summary, GrB induction in HIV-specific B cells may play a role in decelerating expansion of HIV-infected CD4+ T cells, while interfering with their terminal differentiation into plasma cells.

Anti-viral vaccines license T cell responses by suppressing granzyme B levels in human plasmacytoid dendritic cells. (P4510)

Human plasmacytoid dendritic cells (pDC) are important modulators of adaptive T cell responses during viral infections. Recently, we found that human pDC produce the serine protease granzyme B (GrB), thereby regulating T cell proliferation in a GrB-dependent manner. Here, we demonstrate that intrinsic GrB production by pDC is significantly inhibited in vitro and in vivo by clinically used vaccines against viral infections such as tick-borne encephalitis (TBEV). We show that pDC GrB levels inversely correlate with the proliferative response of co-incubated T cells and that GrB suppression by a specific antibody or a GrB substrate inhibitor results in enhanced T cell proliferation, suggesting a predominant role of GrB in pDC-dependent T cell licensing. Functionally, we demonstrate that GrBhigh, but not GrBlow pDC transfer GrB to T cells, and may degrade the zeta-chain of the T cell receptor in a GrB-dependent fashion, thereby providing a possible explanation for the observed T cell suppression by GrB-expressing pDC. Modulation of pDC-derived GrB activity represents a previously unknown mechanism by which both anti-viral and vaccine-induced T cell responses may be regulated in vivo. Our results provide novel insights into pDC biology during vaccinations and may contribute to an improvement of prophylactic and therapeutic vaccines.

Monocytes express the serine protease granzyme B - potential involvement in antigen processing and presentation. (P5033)

In recent years, dendritic cells derived from monocytes (mo-DC) have been extensively studied as novel cell-therapeutic candidates for the treatment of various tumors. In order to elicit tumor-specific CTL responses, DC need to be capable of cross-presenting antigens. Recently, the serine protease granzyme B (GrB) was found to be crucially involved in cleavage and cross-presentation of antigens. We therefore hypothesized that monocytes may be able to express GrB under certain circumstances, comparable to other DC precursors including mDC and pDC. Using various methods including FACS analysis, ELISpot and Western immunoblot we found that monocytes can express GrB depending on various external stimuli. Importantly, we found that the TLR4 ligand LPS appears to play a central role in the induction of GrB in monocytes, and that the presence of antigen-stimulated B cells synergistically enhances GrB expression in monocytes. Moreover, when maturing monocytes into mo-DC, the IL-2 family cytokine IL-7 further supported the expression of GrB in the emerging mo-DC population. This is the first report describing the expression of GrB protein in monocytes and mo-DC in response to various cytokine and TLR stimuli. Our results may open novel approaches to the improvement of mo-DC-related cellular vaccines based on the expression of certain proteases such as GrB.

Interleukin-21-induced granzyme B-expressing B lymphocytes regulate T cells and infiltrate tumors (P1088)

The role of B cells in tumor infiltrations is controversially discussed. Different studies suggest that certain tumor-infiltrating B cell populations exhibit regulatory potential. Here, we demonstrate that the microenvironment of various solid tumors contains granzyme B (GrB)-expressing B cells adjacent to IL-21-providing T cells. Since GrB-mediated effector T cell modulation is known from regulatory T cells (Treg) and plasmacytoid dendritic cells, we hypothesized the existence of similar mechanisms in B cells. Here we show that IL-21 induces B cells expressing high levels of GrB and controlling T cell proliferation by GrB-dependent degradation of the T cell receptor ζ-chain. Detailed characterization of IL-21-induced GrB+ B cells reveals a CD19+CD38+CD1d+IgM+CD147+ phenotype and expression of additional regulatory molecules including IL-10, CD25 and IDO. Of note, IL-21-mediated GrB induction integrates both BCR- and TLR-mediated signals and is enhanced in the presence of B cell CD5 expression. This is the first report demonstrating that IL-21 induces GrB+ human regulatory B cells, which can be detected in tumor infiltrations, and which may contribute to the modulation of cellular adaptive immune responses by Treg-like mechanisms. Our findings may stimulate the development of novel diagnostic and cell therapeutic approaches to the management of malignant, autoimmune and graft-versus-host pathologies.

Tumor-infiltrating lymphocytes in breast cancer tissue comprise granzyme B-expressing B cells with cytotoxic potential (127.23)

Certain subtypes of breast cancer are infiltrated by lymphocytes including B cells, correlating them with a favorable prognosis. Recently, we found that B cells can differentiate into granzyme B (GrB)-secreting cells with cytotoxic potential towards certain tumors. After screening paraffin-embedded tissue sections of various tumors, we show here for the first time that breast carcinoma is infiltrated by CD19+ B cells which express significant amounts of GrB. Testing a panel of different cytokines described to be associated with anti-tumor immune responses revealed interleukin (IL-) 21 as a highly potent stimulus to induce GrB in B cells. Induction of GrB by IL-21 was strongly enhanced in the presence of B cell receptor engagement, and was associated with activation of JAK1/3 and STAT1/3, signaling molecules also activated in NK cells and CTL. Given these parallels with other cytotoxic cells, we tested whether or not GrB-secreting B cells are able to directly induce apoptosis in breast cancer cells. Indeed, we found that IL-21-activated, GrB-secreting B cells showed significant cytotoxic activity against breast cancer cells in-vitro, whereas unstimulated B cells had no such effects. The data presented here support the hypothesis that GrB-secreting B cells may be directly involved in anti-cancer immunity. Our findings suggest, that B cell plasticity is higher than previously thought and that B cells may play an immunological role beyond antibody and cytokine production.

HIV triggers interleukin 21-mediated induction of granzyme B-secreting B cells with antiviral properties (105.18)

Certain lymphocyte subsets including plasmacytoid dendritic cells and regulatory T cells can secrete granzyme B (GrB), thereby suppressing T cell expansion. Recently, we found that B cells can also produce GrB in response to interleukin (IL) 21. Since HIV has been shown to be associated with elevated serum IL-21 levels, we hypothesized that GrB-expressing B cells may be induced during HIV infection. Here, we demonstrate for the first time, that infection of CD4+ T cells with HIV 1 (NL4-3), but not mock infection, induces strong expression of IL-21. We further demonstrate that such T cells induce GrB in co-cultured B cells in an IL-21-dependent fashion. In support of these data, serum levels of both IL-21 and GrB are significantly higher in HIV-infected patients before HAART as compared to healthy controls. Up to 60% of B cells (36.2 ± 12.9%) from patients infected with HIV, but not normal B cells, express GrB. Importantly, co-culture of HIV-infected CD4+ T cells with GrB+ B cells resulted in GrB transfer, and strongly suppressed both, proliferation of T cells and virus replication as indicated by significantly reduced p24 levels. The observed effects were enhanced by IL-21, and reduced by GrB inhibition. In summary, we demonstrate that HIV induces IL-21 in CD4+ T cells, thereby indirectly triggering the development of GrB-secreting B cells with antiretroviral properties. GrB-secreting B cells may play a so far unappreciated role in decelerating the expansion of HIV.

Interleukin 21-activated B cells can kill tumor cells in a granzyme B-dependent manner (66.5)

Interleukin 21 (IL-21) is a promising cytokine for the treatment of tumors and infections. Recently, IL-21 was identified as an inducer of plasma cells when combined with CD40 ligand (CD40L). Using various immunological methods including FACS analysis, ELISA, ELISpot and fluorescence microscopy, we demonstrate here that in the absence of CD40L B cells (BC) rather differentiate into granzyme B (GzmB)-secreting cytotoxic cells instead of plasma cells. GzmB induction in BC requires IL-21 and BC receptor cross-linking, and is associated with phosphorylation of SYK, JAK1/3 and STAT1/3, signaling molecules which are also involved in the activation of CTL and NK cells. CD40L effectively suppresses GzmB in BC, suggesting that GzmB-secreting BC play a role early during inflammatory responses, before CD40L-expressing T cells are present. BC-derived GzmB is enzymatically active and induces apoptosis in tumor cell lines including HeLa cells and PC3 cells. Importantly, induction of apoptosis by GzmB-secreting B cells can be suppressed by neutralization of GzmB using the substrate inhibitor Ac-IEPD-CHO as well as a novel small molecule inhibitor of GzmB identified in our laboratory. Our data reveal a novel role of IL-21-activated BC, which involves GzmB secretion and cytotoxicity. Our findings may have implications for the understanding of tumor immunosurveillance and early anti-viral immune responses, and may open novel approaches for the immunotherapy of neoplastic and viral diseases.

Viral and bacterial antigens as well as agonists for TLR4 and TLR9 can trigger granzyme B secretion by human B cells in the presence of interleukin 21 (154.21)

B cells play a key role for adaptive immune responses by secreting antigen-specific immunoglobulins, for example in the course of infections. Recently, we found that human B cells are able to express granzyme B (GzmB) in response to IL-21. Here we demonstrate that viruses including tick-borne encephalitis, polio, hepatitis B, measles and human immunodeficiency virus (HIV) as well as Bacille Calmette-Guérin can enhance the GzmB response by B cells from healthy individuals. In contrast, B cells from patients with active HIV infection showed a lower capacity to produce GzmB in response to HIV, suggesting a novel pathogenetic mechanism in HIV patients involving B cells. Furthermore, we reveal that next to B cell receptor (BCR) stimulation ligation of toll-like receptors including TLR4 and TLR9 can be involved in B cell GzmB enhancement. After 2 days incubation, up to 77% of freshly isolated B cells directly respond with GzmB expression to stimulation with IL-21, BCR stimulation and TLR ligation. Of note, GzmB is secreted by B cells in an enzymatically active state, reaching levels comparable to those secreted by cytotoxic cells and its induction requires activation of similar signaling pathways as in CTL and NK cells. We suggest GzmB-secreting B cells may play a role in early immune responses, and may contribute to elevated serum GzmB levels found in viral diseases and sepsis. Further studies have to elucidate whether B cells exhibit cytotoxicity towards infected cells.