I matinib mesylate (Gleevec) therapy remains the standard of care for patients with chronic myelogenous leukemia (CML).3 Designed as a selective competitive inhibitor of the Abelson (ABL) tyrosine kinases (BCR-ABL, v-ABL, c-ABL), this drug leads to growth arrest or apoptosis (1, 2). Imatinib also displays strong activity against the platelet-derived growth factor receptor, c-kit receptor, ABL-related gene, and their fusion proteins (1-3) and thus has also been used for the therapy of gastrointestinal stromal tumors with mutations in c-kit (2).However, the specificity of this molecule may be broader than originally expected, and it is becoming increasingly evident that imatinib also inhibits key tyrosine kinases in immune cells. The exact nature of imatinib effects (activation or suppression) on lymphocytes or dendritic cells remains controversial. Inhibition of CD4 ϩ or CD8 ϩ T cell proliferation and activation by imatinib has been documented (4 -8). Some reports have further highlighted the negative effect of imatinib on the TCR-induced ZAP70 signaling pathway identifying the leukocyte-specific protein tyrosine kinase (Lck) as a potential molecular target (4, 9). Similarly, a negative modulation of dendritic cell (DC) development by imatinib and a down-regulation of their Ag-presenting function have also been described (10 -12). In contrast to these findings, it has been reported that imatinib does not impede the immunogenicity of DC (13) and may enhance their Ag-presenting function (14). Additionally, some reports indicate that imatinib may foster DC-NK reciprocal activation, thereby promoting the antitumoral function of NK cells (15). CD4 ϩ CD25 ϩ regulatory T lymphocytes (Treg) critically contribute to the maintenance of self-tolerance and to the prevention of autoimmunity in animals and humans (16, 17). These suppressive cells have also been highlighted as major contributors in the establishment and persistence of cancer-induced immune tolerance (18,19). Treg expansion detected in the blood, lymph nodes, and spleens of tumor-bearing hosts (20 -23) may result from the conversion of CD4 ϩ CD25 Ϫ T cells into CD4 ϩ CD25 ϩ Treg (24) or from the proliferation of naturally occurring Treg (25). Tumorinduced Treg compromise the function of anti-tumor effector CD8 ϩ T cells, curtail CD4 ϩ T cell help, impede Ag-presenting cell activity (18,23,26) and therefore represent a major obstacle for successful cancer immunotherapy. In support of this concept, studies in humans and in animal models have demonstrated that attempts to disrupt Treg suppressive activity promote antitumoral immunity (20,27,28). Different strategies have thus been evaluated to deplete or inactivate Treg and include the use of anti-CD25 Abs, the IL-2/diphtheria toxin fusion protein, the immunotoxin The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.1 This work was supported in p...

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-β1-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.

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 antigen presenting cells has been the foundation for the development and utilization of these cells as vaccines in cancer immunotherapy. DCs are also endowed with the non-conventional 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 Treg 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 (Th-1 KDCs) was dependent on inducible nitric oxide synthase (iNOS) expression and nitric oxide (NO) production. Importantly, Th-1 KDCs were capable of presenting the acquired antigens from the killed tumor cells to T lymphocytes in vitro or in vivo. These observations open new possibilities for the application of KDCs in cancer immunotherapy.

IntroductionThe primary objective of cancer immunotherapy is to promote tumor elimination through the activation of innate and adaptive immune responses. Successful immunotherapy relies on vaccination strategies endowed with the dual capability of inducing tumor-specific lymphocytes while overcoming the mechanisms of immune tolerance. CD4 ϩ CD25 ϩ FoxP3 ϩ regulatory T lymphocytes (Tregs) critically contribute to the occurrence and persistence of tumor-induced tolerance. 1 An increase in the frequency of these immunosuppressive cells in cancer patients has been widely reported. Treg expansion observed during tumor progression may result from the proliferation of naturally occurring Tregs (nTregs) or from the conversion of CD4 ϩ CD25 Ϫ FoxP3 Ϫ T cells into CD4 ϩ CD25 ϩ FoxP3 ϩ Tregs (iTregs). 2,3 Tregs dampen immune responses by suppressing the function of the effectors CD4 ϩ , CD8 ϩ , and natural killer (NK) cells [4][5][6][7] and by inhibiting dendritic cell activation. [8][9][10] Because Tregs are one of the main barriers for the eradication of tumors by immune cells, their therapeutic depletion or their functional inactivation using drugs or antibodies improves responses to cancer immunotherapy, such as dendritic cell-based vaccines. [11][12][13][14][15][16] However, the selective elimination or inactivation of Tregs constitutes a major challenge because these cells share the same surface markers as activated conventional, nonsuppressive T cells. Indeed, antibody-based approaches indistinguishably target both Tregs and activated effector T lymphocytes. Likewise, chemotherapeutic agents such as cyclophosphamide, which are used to eliminate Tregs, do not target these cells selectively.Several reports have indicated that the adoptive transfer of allogeneic T cells may increase the efficacy of tumor immunotherapy by providing adjuvant/"danger" signals to the host immune cells. 17,18 A method has been optimized allowing for the efficient generation in vitro of a large number of allogeneic CD3/CD28 cross-linked T helper-1 (Th-1) memory T cells. 19 Adoptive transfer of these Th-1 lymphocytes stimulates anticancer immunity and significantly improves the survival of mice lethally injected with BCL1 leukemia cells. 19,20 This effect partly stems from cytokine production by activated T lymphocytes, which foster the establishment of protective type-1 immune responses. 18 However, the effects of type I cytokines, including interferon-␥ (IFN-␥), on Tregs have been discrepant in previous studies. As an essential effector cytokine for cell-mediated immunity, exogenous or autocrine IFN-␥ has been reported to negatively regulate Treg generation. 21,22 Other studies have found that IFN-␥ enhances activation-induced cell death and that it thereby may regulate the expansion and persistence of effector T cells by promoting apoptosis. 23,24 In the present study, we report that effector-memory CD4 ϩ Th-1 (emTh-1) cells are capable not only of fostering the establishment of type-1 immune responses, but also of critically impairing t...

We have previously reported on the anti-tumoral potential of a chaperone-rich cell lysate (CRCL) vaccine. Immunization with CRCL generated from tumors elicits specific T and NK cell-dependent immune responses leading to protective immunity in numerous mouse tumor models. CRCL provides both a source of tumor antigens and danger signals leading to dendritic cell activation. In humans, tumor-derived CRCL induces dendritic cell activation and CRCL-loaded dendritic cells promote the generation of cytotoxic T lymphocytes in vitro. The current study was designed to identify the signaling events and modifications triggered by CRCL in antigen presenting cells. Our results indicate that tumor-derived CRCL not only promotes the activation of dendritic cells, but also significantly fosters the function of macrophages that thus appear as major targets of this vaccine. Activation of both cell types is associated with the induction of the MAP kinase pathway, the phosphorylation of STAT1, STAT5 and AKT and with transcription factor NF-κB activation in vitro and in vivo. These results thus provide important insights into the mechanisms by which CRCL-based vaccines exert their adjuvant effects on antigen presenting cells.

Summary 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 non-conventional 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 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-STAT1 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.

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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.