A highly efficient tumor-infiltrating MDSC differentiation system for discovery of anti-neoplastic targets, which circumvents the need for tumor establishment in mice.

Myeloid-derived suppressor cells (MDSCs) exhibit potent immunosuppressive activities in cancer. MDSCs infiltrate tumors and strongly inhibit cancer-specific cytotoxic T cells. Their mechanism of differentiation and identification of MDSC-specific therapeutic targets are major areas of interest. We have devised a highly efficient and rapid method to produce very large numbers of melanoma-infiltrating MDSCs ex vivo without inducing tumors in mice. These MDSCs were used to study their differentiation, immunosuppressive activities and were compared to non-neoplastic counterparts and conventional dendritic cells using unbiased systems biology approaches. Differentially activated/deactivated pathways caused by cell type differences and by the melanoma tumor environment were identified. MDSCs increased the expression of trafficking receptors to sites of inflammation, endocytosis, changed lipid metabolism, and up-regulated detoxification pathways such as the expression of P450 reductase. These studies uncovered more than 60 potential novel therapeutic targets. As a proof of principle, we demonstrate that P450 reductase is the target of pro-drugs such as Paclitaxel, which depletes MDSCs following chemotherapy in animal models of melanoma and in human patients. Conversely, P450 reductase protects MDSCs against the cytotoxic actions of other chemotherapy drugs such as Irinotecan, which is ineffective for the treatment of melanoma.

Assessing T-cell responses in anticancer immunotherapy: Dendritic cells or myeloid-derived suppressor cells?

Since dendritic cells operate as professional antigen-presenting cells (APCs) and hence are capable of jumpstarting the immune system, they have been exploited to develop a variety of immunotherapeutic regimens against cancer. In the few past years, myeloid-derived suppressor cells (MDSCs) have been shown to mediate robust immunosuppressive functions, thereby inhibiting tumor-targeting immune responses. Thus, we propose that the immunomodulatory activity of MDSCs should be carefully considered for the development of efficient anticancer immunotherapies.

Fc-dependent depletion of tumor-infiltrating regulatory T cells co-defines the efficacy of anti-CTLA-4 therapy against melanoma.

Treatment with monoclonal antibody specific for cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), an inhibitory receptor expressed by T lymphocytes, has emerged as an effective therapy for the treatment of metastatic melanoma. Although subject to debate, current models favor a mechanism of activity involving blockade of the inhibitory activity of CTLA-4 on both effector (T eff) and regulatory (T reg) T cells, resulting in enhanced antitumor effector T cell activity capable of inducing tumor regression. We demonstrate, however, that the activity of anti-CTLA-4 antibody on the T reg cell compartment is mediated via selective depletion of T reg cells within tumor lesions. Importantly, T reg cell depletion is dependent on the presence of Fcγ receptor-expressing macrophages within the tumor microenvironment, indicating that T reg cells are depleted in trans in a context-dependent manner. Our results reveal further mechanistic insight into the activity of anti-CTLA-4-based cancer immunotherapy, and illustrate the importance of specific features of the local tumor environment on the final outcome of antibody-based immunomodulatory therapies.

Immune modulation by genetic modification of dendritic cells with lentiviral vectors.

Our work over the past eight years has focused on the use of HIV-1 lentiviral vectors (lentivectors) for the genetic modification of dendritic cells (DCs) to control their functions in immune modulation. DCs are key professional antigen presenting cells which regulate the activity of most effector immune cells, including T, B and NK cells. Their genetic modification provides the means for the development of targeted therapies towards cancer and autoimmune disease. We have been modulating with lentivectors the activity of intracellular signalling pathways and co-stimulation during antigen presentation to T cells, to fine-tune the type and strength of the immune response. In the course of our research, we have found unexpected results such as the surprising immunosuppressive role of anti-viral signalling pathways, and the close link between negative co-stimulation in the immunological synapse and T cell receptor trafficking. Here we review our major findings and put them into context with other published work.

PD-L1 co-stimulation, ligand-induced TCR down-modulation and anti-tumor immunotherapy.

PD-1 engagement on the surface of effector T cells strongly suppresses their cytotoxic function, which constitutes a major obstacle for T cell-mediated anti-tumor activities. Surprisingly, PD-1 is strongly upregulated in T cells, engaging its ligand PD-L1 during antigen presentation. However, our recent published data may provide an explanation for this apparent contradiction.

Lentiviral vector followed by protein immunisation breaks tolerance against the self-antigen Her1 and results in lung cancer immunotherapy.

BACKGROUND: Lung cancer remains a leading cause of cancer mortality, and so the aim of the present study was to develop a therapeutic vaccine protocol. METHODS: We constructed a lentiviral vector (LV) expressing the extracellular domain (ECD) of murine Her1, an antigen associated with poor prognosis in lung cancer. RESULTS: A single LV injection, followed by two Her1 protein boosts, was effective in reducing the metastatic burden of Lewis lung carcinoma in mice. The Her1 LV immunisation generated CD8+ T cells that recognised Her1 ECD presented by dendritic cells, and that also homed to Her1-expressing tumours. Protein boosting further increased the CD8+ T cell response and generated anti-Her1 antibodies; in the antibody response, Her1 LV priming increased Th1-dependent immunoglobulin G2c production. CONCLUSIONS: The ability of this vaccine protocol to break both T cell and B cell tolerance to a self-antigen likely explains its effectiveness.

Selective activation of intracellular signalling pathways in dendritic cells for cancer immunotherapy.

One of the major challenges in achieving effective anti-cancer immunotherapy is to counteract immunological tolerance. Most tumor-associated antigens (TAAs) are sensed as self. Hence, naturally occurring tolerance towards them has to be overcome. Fortunately, there is increasing evidence that anti-tumor immune responses occur and play a crucial role in the success of well-established anti-neoplastic therapies such as radiotherapy and chemotherapy. In fact, their effectiveness relies on signalling by pattern recognition receptors such as Toll-like receptors (TLRs). TLR signal transduction involves activation of a few well-known pathways, of which nuclear factor κB (NF-κB) and mitogen activated protein kinases (MAPKs) are possibly the best characterized. Therefore, constitutive activation of these pathways in immune cells can potentially enhance anti-tumor immunity, especially when targeted to professional antigen presenting cells (APCs) such as dendritic cells (DCs). Several strategies have been devised to test this hypothesis, including constitutive activation of TLRs, NF-κB and MAPKs (extracellular-signal regulated kinase (ERK), p38 and c-Jun kinase 1 (JNK1)). Activation of these pathways in mouse and human DCs has differential effects in immunogenicity and in many cases, enhanced antitumor immunity in pre-clinical models, establishing the basis for future clinical applications.

PD-L1 co-stimulation contributes to ligand-induced T cell receptor down-modulation on CD8+ T cells.

T cell receptor (TCR) down-modulation after antigen presentation is a fundamental process that regulates TCR signal transduction. Current understanding of this process is that intrinsic TCR/CD28 signal transduction leads to TCR down-modulation. Here, we show that the interaction between programmed cell death 1 ligand 1 (PD-L1) on dendritic cells (DCs) and programmed death 1 (PD-1) on CD8 T cells contributes to ligand-induced TCR down-modulation. We provide evidence that this occurs via Casitas B-lymphoma (Cbl)-b E3 ubiquitin ligase up-regulation in CD8 T cells. Interference with PD-L1/PD-1 signalling markedly inhibits TCR down-modulation leading to hyper-activated, proliferative CD8 T cells as assessed in vitro and in vivo in an arthritis model. PD-L1 silencing accelerates anti-tumour immune responses and strongly potentiates DC anti-tumour capacities, when combined with mitogen-activated kinase (MAPK) modulators that promote DC activation.

Selective ERK activation differentiates mouse and human tolerogenic dendritic cells, expands antigen-specific regulatory T cells, and suppresses experimental inflammatory arthritis.

OBJECTIVE: Most therapeutic treatments for autoimmune arthritis rely on immunosuppressive drugs, which have side effects. Although a previous study by our group showed that specific ERK activation suppressed immune responses, its application in a therapeutic setting has never been tested. The aim of the present study was to define the ERK-dependent immunosuppressive mechanisms and to apply selective ERK activation for the treatment of experimental inflammatory arthritis. METHODS: A constitutively active ERK activator was coexpressed with a model antigen using lentivectors. Immunosuppressive mechanisms were characterized at the level of dendritic cell (DC) function, differentiation of antigen-specific Treg cells, and inhibition of inflammatory T cells. Administration of the ERK activator with antigen as a strategy to suppress inflammatory arthritis was tested in an experimental mouse model. RESULTS: Selective ERK activation induced mouse and human DCs to secrete bioactive transforming growth factor ß, a process required for suppression of T cell responses and differentiation of antigen-specific Treg cells. Treg cells strongly proliferated after antigen reencounter in inflammatory conditions, and these cells exhibited antigen-dependent suppressive activities. Inflammatory arthritis was effectively inhibited through antigen-specific mechanisms. Importantly, this strategy did not rely on identification of the initiating arthritogenic antigen. Equivalent mechanisms were demonstrated in human monocyte-derived DCs, setting the scene for a possible rapid translation of this approach to patients with rheumatoid arthritis. CONCLUSION: This strategy of selective ERK activation resulted in an effective therapeutic protocol, with substantial advantages over DC or T cell vaccination.

On the Mechanism of T cell receptor down-modulation and its physiological significance.

Effective, long-lasting immune responses largely depend upon T cell reponses. Antigen-specific T lymphocytes are activated and differentiate into effector T cells after antigen presentation by professional antigen presenting cells (APCs). However, T cell responses are tightly regulated to prevent T cell hyperactivation which may end up in autoimmune pathology. One of these regulatory mechanisms is ligand-induced TCR down-modulation, a process by which TCRs are removed from the T cell surface shortly after engagement with their cognate antigenic peptide associated to MHC molecules on the APC. TCR down-modulation is a complicated process. Here we briefly describe the three main models that attempt to clarify this mechanism in the context of T cell activation and function.

Targeting lentiviral vectors for cancer immunotherapy.

Delivery of tumour-associated antigens (TAA) in a way that induces effective, specific immunity is a challenge in anti-cancer vaccine design. Circumventing tumour-induced tolerogenic mechanisms in vivo is also critical for effective immunotherapy. Effective immune responses are induced by professional antigen presenting cells, in particular dendritic cells (DC). This requires presentation of the antigen to both CD4(+) and CD8(+) T cells in the context of strong co-stimulatory signals. Lentiviral vectors have been tested as vehicles, for both ex vivo and in vivo delivery of TAA and/or activation signals to DC, and have been demonstrated to induce potent T cell mediated immune responses that can control tumour growth. This review will focus on the use of lentiviral vectors for in vivo gene delivery to DC, introducing strategies to target DC, either targeting cell entry or gene expression to improve safety of the lentiviral vaccine or targeting dendritic cell activation pathways to enhance performance of the lentiviral vaccine. In conclusion, this review highlights the potential of lentiviral vectors as a generally applicable 'off-the-shelf' anti-cancer immunotherapeutic.

HIV-1 lentiviral vector immunogenicity is mediated by Toll-like receptor 3 (TLR3) and TLR7.

Lentiviral vectors are promising vaccine vector candidates that have been tested extensively in preclinical models of infectious disease and cancer immunotherapy. They are also used in gene therapy clinical trials both for the ex vivo modification of cells and for direct in vivo injection. It is therefore critical to understand the mechanism(s) by which such vectors might stimulate the immune system. We evaluated the effect of lentiviral vectors on myeloid dendritic cells (DC), the main target of lentiviral transduction following subcutaneous immunization. The activation of DC cultures was independent of the lentiviral pseudotype but dependent on cell entry and reverse transcription. In vivo-transduced DC also displayed a mature phenotype, produced tumor necrosis factor alpha (TNF-alpha), and stimulated naive CD8(+) T cells. The lentiviral activation of DC was Toll-like receptor (TLR) dependent, as it was inhibited in TRIF/MyD88 knockout (TRIF/MyD88(-/-)) DC. TLR3(-/-) or TLR7(-/-) DC were less activated, and reverse transcription was important for the activation of TLR7(-/-) DC. Moreover, lentivirally transduced DC lacking TLR3 or TLR7 had an impaired capacity to induce antigen-specific CD8(+) T-cell responses. In conclusion, we demonstrated TLR-dependent DC activation by lentiviral vectors, explaining their immunogenicity. These data allow the rational development of strategies to manipulate the host's immune response to the transgene.

Hypochlorous acid: a natural adjuvant that facilitates antigen processing, cross-priming, and the induction of adaptive immunity.

The production of hypochlorous acid (HOCl) is a characteristic of granulocyte activation, a hallmark of the early phase of innate immune responses. In this study, we show that, in addition to its well-established role as a microbicide, HOCl can act as a natural adjuvant of adaptive immunity. HOCl enhances the T cell responses to the model Ag OVA, facilitating the processing and presentation of this protein via the class II MHC pathway. HOCl modification also enhances cross-presentation of the tumor Ag tyrosinase-related protein 2 via class I MHC. The adjuvant effects of HOCl are independent of TLR signaling. The enhanced presentation of HOCl-modified OVA is mediated via modification of the N-linked carbohydrate side chain rather than formation of protein aldehydes or chloramines. HOCl-modified OVA is taken up more efficiently by APCs and is degraded more efficiently by proteinases. Atomic force microscopy demonstrated that enhanced uptake is mediated via specific receptor binding, one candidate for which is the scavenger receptor lectin-like oxidized low-density lipoprotein receptor, which shows enhanced binding to chlorinated OVA. A function of HOCl is therefore to target glycoprotein Ags to scavenger receptors on the APC surface. This additional mechanism linking innate and adaptive immunity suggests novel strategies to enhance immunity to vaccines.

Lentiviral vectors transduce proliferating dendritic cell precursors leading to persistent antigen presentation and immunization.

Lentiviral vectors (LVs) are tools for in vivo gene delivery, to correct genetic defects or to deliver antigens for vaccination. It was reported that systemic injection of LVs in mice transduced cells in liver and spleen. Here we describe the reasons for, and consequences of, persistent gene expression in spleen. After 5 days of intravenous injection, a green fluorescence protein (GFP)-expressing LV was detected in lymphocytes, macrophages and all subsets of dendritic cells (DCs) in spleen. In the case of macrophages and DCs, the percentage of transduced cells increased between 5 and 30 days after injection. We used bromodeoxyuridine (BrdU) incorporation to show that the macrophages were largely nondividing, whereas the transduced DCs arose from dividing precursor cells and could be detected in spleen 2 months after injection. Expression of ovalbumin (OVA) in the LV reduced the number of transduced DCs in spleen after 30 days. However, the remaining transduced cells stimulated proliferation and activation of OVA-specific CD8(+) T cells transferred 2 months after LV injection. The mice also maintained cytolytic activity against OVA-pulsed targets. These results show that LVs transduce DC precursors, which maintain transduced DCs in spleen for at least 2 months, leading to prolonged antigen presentation and effective T-cell memory.

Apoptotic events induced by naturally occurring retinoids ATRA and 13-cis retinoic acid on human hepatoma cell lines Hep3B and HepG2.

Two hepatoma cell lines were incubated for 72 h with ATRA and its analog 13cisRA and according to MTT assay, Hep3B cells were highly susceptible whereas HepG2 cells were more resistant to the treatment. At the high concentration of 166 microM, retinoids were able to induce apoptosis in both cell lines and the highest effect was observed in HepG2 cells treated with ATRA. TUNEL-based photometric ELISA showed that at the same retinoid concentration tested by flow cytometry, both cell lines showed apoptosis whereas plasma membranes were not significantly disrupted. Inhibitors of apoptosis Bcl-xL and survivin were downregulated in Hep3B cells by treatment with both retinoids. Bax, a pro-apoptotic protein, was not significantly upregulated in Hep3B cells, but was slightly increased in HepG2 cells treated with 13cisRA. Both procaspase-3 and procaspase-8 were cleaved in Hep3B cells, suggesting apoptosis could be triggered through the extrinsic pathway. In the case of HepG2 cells, lack of caspase activation suggests a mechanism dependent on other kind of proteases.