PIAS1 and STAT-3 impair the tumoricidal potential of IFN-γ-stimulated mouse dendritic cells generated with IL-15.

Primarily defined by their antigen-presenting property, dendritic cells (DCs) are being implemented as cancer vaccines in immunotherapeutic interventions. DCs can also function as direct tumor cell killers. How DC cytotoxic activity can be efficiently harnessed and the mechanisms controlling this nonconventional property are not fully understood. We report here that the tumoricidal potential of mouse DCs generated from myeloid precursors with GM-CSF and IL-15 (IL-15 DCs) can be triggered with the Toll-like receptor (TLR) 4 ligand lipopolysaccharide to a similar extent compared with that of their counterparts, conventionally generated with IL-4 (IL-4 DCs). The mechanism of tumor cell killing depends on the induction of iNOS expression by DCs. In contrast, interferon (IFN)-γ induces the cytotoxic activity of IL-4 but not IL-15 DCs. Although the IFN-γ-STAT-1 signaling pathway is overall functional in IL-15 DCs, IFN-γ fails to induce iNOS expression in these cells. iNOS expression is negatively controlled in IFN-γ-stimulated IL-15 DCs by the cooperation between the E3 SUMO ligase PIAS1 and STAT-3, and can be partially restored with PIAS1 siRNA and STAT-3 inhibitors.

Allogeneic effector/memory Th-1 cells impair FoxP3+ regulatory T lymphocytes and synergize with chaperone-rich cell lysate vaccine to treat leukemia.

Therapeutic strategies combining the induction of effective antitumor immunity with the inhibition of the mechanisms of tumor-induced immunosuppression represent a key objective in cancer immunotherapy. Herein we demonstrate that effector/memory CD4(+) T helper-1 (Th-1) lymphocytes, in addition to polarizing type-1 antitumor immune responses, impair tumor-induced CD4(+)CD25(+)FoxP3(+) regulatory T lymphocyte (Treg) immunosuppressive function in vitro and in vivo. Th-1 cells also inhibit the generation of FoxP3(+) Tregs from naive CD4(+)CD25(-)FoxP3(-) T cells by an interferon-γ-dependent mechanism. In addition, in an aggressive mouse leukemia model (12B1), Th-1 lymphocytes act synergistically with a chaperone-rich cell lysate (CRCL) vaccine, leading to improved survival and long-lasting protection against leukemia. The combination of CRCL as a source of tumor-specific antigens and Th-1 lymphocytes as an adjuvant has the potential to stimulate efficient specific antitumor immunity while restraining Treg-induced suppression.

Th-1 lymphocytes induce dendritic cell tumor killing activity by an IFN-γ-dependent mechanism.

Dendritic cells (DCs) encompass a heterogeneous population of cells capable of orchestrating innate and adaptive immune responses. The ability of DCs to act as professional APCs has been the foundation for the development and use of these cells as vaccines in cancer immunotherapy. DCs are also endowed with the nonconventional property of directly killing tumor cells. The current study investigates the regulation of murine DC cytotoxic function by T lymphocytes. We provide evidence that CD4(+) Th-1, but not Th-2, Th-17 cells, or regulatory T cells, are capable of inducing DC cytotoxic function. IFN-γ was identified as the major factor responsible for Th-1-induced DC tumoricidal activity. Tumor cell killing mediated by Th-1-activated killer DCs was dependent on inducible NO synthase expression and NO production. Importantly, Th-1-activated killer DCs were capable of presenting the acquired Ags from the killed tumor cells to T lymphocytes in vitro or in vivo. These observations offer new possibilities for the application of killer DCs in cancer immunotherapy.

Differential mediator production by dendritic cells upon toll-like receptor stimulation does not impact T cell cytokine expression.

Dendritic cells are key components of successful immunological responses bridging innate and adaptive defenses. In this study we wanted to know whether ligation of toll-like receptors (TLR) expressed by dendritic cells would induce differential proinflammatory mediator expression and whether these dendritic cells would differentially impact T cell function. For this purpose bone marrow-derived dendritic cells from OTII mice were used. The dendritic cells showed detectable levels of TLR1, 2, 4, 6, 7, 8 and 9, with TLR2 and TLR4 expressed at the highest levels. To determine whether TLR ligation differentially influenced proinflammatory mediator expression the dendritic cells were stimulated with peptidoglycan (PGN) or lipopolysaccharide (LPS) for TLR2 or TLR4, respectively. Comparisons were made to dendritic cells exposed to TNF-alpha or saline as controls. Whereas, both LPS and PGN were equally effective at inducing CXCL1 and TNF-alpha expression from the dendritic cells, LPS was unique at inducing CCL2 expression, and PGN was unique at inducing IL-1beta expression. Despite these differences, LPS and PGN treated dendritic cells were equally effective at eliciting IFN-gamma expression from T cells in an antigen-specific manner. These data indicate that ligation of TLR by components of Gram+ and Gram- bacteria differentially influence dendritic cell proinflammatory mediator expression, and that differential mediator production by dendritic cells upon TLR stimulation does not impact T cell cytokine production.

T Lymphocyte Inhibition by Tumor-Infiltrating Dendritic Cells Involves Ectonucleotidase CD39 but Not Arginase-1.

T lymphocytes activated by dendritic cells (DC) which present tumor antigens play a key role in the antitumor immune response. However, in patients suffering from active cancer, DC are not efficient at initiating and supporting immune responses as they participate to T lymphocyte inhibition. DC in the tumor environment are functionally defective and exhibit a characteristic of immature phenotype, different to that of DC present in nonpathological conditions. The mechanistic bases underlying DC dysfunction in cancer responsible for the modulation of T-cell responses and tumor immune escape are still being investigated. Using two different mouse tumor models, we showed that tumor-infiltrating DC (TIDC) are constitutively immunosuppressive, exhibit a semimature phenotype, and impair responder T lymphocyte proliferation and activation by a mechanism involving CD39 ectoenzyme.

Myeloid-derived suppressor cells from tumor-bearing mice impair TGF-ß-induced differentiation of CD4+CD25+FoxP3+ Tregs from CD4+CD25-FoxP3- T cells.

MDSCs and Tregs play an essential role in the immunosuppressive networks that contribute to tumor-immune evasion. The mechanisms by which tumors promote the expansion and/or function of these suppressive cells and the cross-talk between MDSC and Treg remain incompletely defined. Previous reports have suggested that MDSC may contribute to Treg induction in cancer. Herein, we provide evidence that tumor-induced gr-MDSCs, endowed with the potential of suppressing conventional T Lc, surprisingly impair TGF-ß1-mediated generation of CD4(+)CD25(+)FoxP3(+) iTregs. Furthermore, gr-MDSCs impede the proliferation of nTregs without, however, affecting FoxP3 expression. Suppression of iTreg differentiation from naïve CD4(+) cells by gr-MDSC occurs early in the polarization process, requires inhibition of early T cell activation, and depends on ROS and IDO but does not require arginase 1, iNOS, NO, cystine/cysteine depletion, PD-1 and PD-L1 signaling, or COX-2. These findings thus indicate that gr-MDSCs from TB hosts have the unanticipated ability to restrict immunosuppressive Tregs.