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Induction of maturation and activation of human dendritic cells: A mechanism underlying the beneficial effect of Viscum albumas complimentary therapy in cancer
© Elluru et al; licensee BioMed Central Ltd. 2008
Received: 17 July 2007
Accepted: 04 June 2008
Published: 04 June 2008
Viscum album (VA) preparations have been used as a complimentary therapy in cancer. In addition to their cytotoxic properties, they have also been shown to have immunostimulatory properties. In the present study, we examine the hypothesis that the VA preparations induce activation of human DC that facilitates effective tumor regression.
Four day old monocyte-derived immature DCs were treated with VA Qu Spez at 5, 10 and 15 μg/ml for 48 hrs. The expression of surface molecules was analyzed by flow cytometry. The ability of Qu Spez-educated DC to stimulate T cells was analyzed by allogeneic mixed lymphocyte reaction and activation of Melan-A/MART-1-specific M77-80 CD8+T cells. Cytokines in cell free culture supernatant was analyzed by cytokine bead array assay.
VA Qu Spez stimulated DCs presented with increased expression of antigen presenting molecule HLA-DR and of co-stimulatory molecules CD40, CD80 and CD86. The VA Qu Spez also induced the secretion of inflammatory cytokines IL-6 and IL-8. Further, Qu Spez-educated DC stimulated CD4+T cells in a allogeneic mixed lymphocyte reaction and activated melanoma antigen Melan-A/MART-1-specific M77-80 CD8+T cells as evidenced by increased secretion of TNF-α and IFNγ.
The VA preparations stimulate the maturation and activation of human DCs, which may facilitate anti-tumoral immune responses. These results should assist in understanding the immunostimulatory properties of VA preparations and improving the therapeutic strategies.
VA preparations are aqueous extracts from Viscum album (also known as European mistletoe) consisting of different types of lectins [1–3]. In addition to mistletoe lectins (ML), biologically active components of VA preparations include viscotoxins, several enzymes, peptides (such as viscumamide), amino acids, thiols, amines, polysaccharides, cyclitoles, lipids, phytosterols, triterpines, flavonoids, phenylpropanes and minerals [3, 4]. VA preparations have been used as a complimentary therapy in cancer. Several studies have reported the clinical benefits of VA preparations in cancer patients [5, 6]. Treatment with VA preparations or purified ML has also been shown to be associated with tumor regression in several experimental models [7, 8]. The mechanisms underlying the anti-tumoral activity of VA preparations are complex and not completely understood. The proposed mechanisms include induction of apoptosis of tumor cells and lymphocytes, inhibition of angiogenesis and stimulation of the cellular compartment of the immune system [9–14].
During the course of tumor development, the tumor evades the immune system through the secretion of various factors such as VEGF, IL-10 and PGE2 that have been shown to inactivate the immune system . The different pathways of immune evasion by tumors involve: induction of immune tolerance, resistance to killing by immune effector cells, and imparting functional paralysis of professional antigen presenting cells (APCs) such as dendritic cells (DCs) .
DCs are the professional APCs that are specialized in the uptake of antigens and their transport from peripheral tissues to the lymphoid organs [16, 17]. Because of their capacity to stimulate naive T cells, DCs have a central role in the initiation of primary immune responses . DCs reside in periphery as immature cells with a high ability to endocytose target antigens . Upon receiving appropriate stimuli and in the context of inflammation, DCs undergo maturation process characterized by increased surface expression of antigen presenting HLA molecules and co-stimulatory molecules such as CD80 and CD86 and secrete several pro-inflammatory cytokines .
Tumor cells suppress the maturation and activation process of DCs . Thus, several studies have demonstrated that DCs that reside in the tumor site or in the vicinity of tumor are immature with a decreased ability to stimulate T cells [22, 23]. In addition, tumor cells secrete several anti-inflammatory cytokines such as IL-10 and TGF, which can suppress the functions of DCs [24–26]. In view of the anti-tumoral and immunostimulatory properties of VA preparations, and the central role of DCs in anti-tumoral immune response, we examined the hypothesis that VA preparations stimulate the DCs, which in part explains the mechanisms underlying the beneficial effect of VA preparations in cancer therapy.
Antibodies and reagents
Recombinant human (rh) interleukin-4 (IL-4) was obtained from R&D Systems (Lille, France), and rh granulocyte macrophage-colony-stimulating factor (rh GM-CSF), rhIL-2 and rhTNFα were obtained from Immunotools (Friesoythe, Germany). FITC-conjugated monoclonal antibodies (mAb) to HLA-DR and CD80, PE-conjugated mAbs to CD86, CD40 and CD83 and APC-conjugated mAbs to CD11c were obtained from BD biosciences (France).
Concentrations of Mistletoe Lectins and Viscotoxins in VA Qu Spez
Concentration used (μg/ml)
VA Qu Spez
VA Qu Spez
VA Qu Spez
Generation and culture of monocyte-derived human dendritic cells
Peripheral blood mononuclear cells (PBMC) were isolated from buffy coats of healthy donors purchased from Hopital Hotel Dieu, Etablissement Français du Sang (06/EFS/029, dated 29.05.2006), upon ethical approval for the use of such materials. The percentage of monocytes in the PBMC preparations was in the range of 9 to 14%. Monocytes were positively isolated using CD14 beads (Miltenyi Biotec, France). The purity of the monocytes after purification is > 98%. Immature DCs were generated by culturing monocytes for 4 days in RPMI 1640 containing 10% FCS, 50 U/ml penicillin, 50 μg/ml streptomycin, rhIL-4 (500 IU/106 cells), and rhGM-CSF (1000 IU/106 cells). Half of the medium, including all supplements, was replaced on second day.
Analysis of the expression of surface molecules by flow cytometry
To investigate the effect of VA Qu Spez on DCs, 0.5 × 106 immature four-day old DCs were either untreated or treated with TNFα (15 ng) or VA preparations (5, 10 and 15 μg) for 48 h. On day 6, cell surface staining was performed with specifically labeled mAbs and proceeded for flow-cytometry (LSR II, BD Biosciences, France). Ten thousand events were recorded and analyzed for each sample. Data were analyzed by BD FACSDIVA software (BD Biosciences, France).
Mixed lymphocyte reaction (MLR) with allogenic CD4+ T cells
Responder CD4+ T cells used for allogeneic MLR assays were isolated from PBMC of healthy donors using a negative isolation kit (Dynal biotech-Invitrogen, France). DCs following 48 hr treatment with VA Qu Spez were washed extensively and were seeded with 1 × 105 responder allogeneic T cells at DC:T cell ratios of 1:10, 1:20 and 1:40. After 4 days, the cells were pulsed for 16 h with 0.5 μCi (0.037 MBq) of (3H)thymidine. Radioactive incorporation was measured by standard liquid scintillation counting. The proliferation of cells was measured as counts per minute (mean ± SEM of triplicate values) after subtracting values of responder T cell cultures alone.
Anergy assay to determine the activation status of the CD4 T cells in the co-culture with VA-treated DCs
The anergy assay was performed according to a modified protocol originally described by Steinbrink et al . Briefly, four-day old DCs were treated for 48 hrs with VA Qu Spez (15 μg/ml) or untreated or TNFα (15 ng/ml). Responder CD4+ T cells were then co-cultured during the first incubation at a density of 105 cells with 104 DC for 72 hrs. Then, T cells from the co-cultures were isolated by using CD4+ beads (Miltenyl Biotech) and rested for 24 hrs in the culture medium containing 2 U/ml IL-2. Subsequently, CD4+ T cells were re-stimulated with DCs generated from the same donor as that used for the first stimulation and have undergone similar VA Qu Spez treatment. After 48 hrs, the cells were pulsed for 16 h with 0.5 μCi (0.037 MBq) of (3H)thymidine. Radioactive incorporation was measured by standard liquid scintillation counting. The proliferation of cells was measured as counts per minute (mean ± SEM of triplicate values). Tests were conducted in triplicates. Additionally, the levels of cytokines TNF-α and IFNγ in the co-culture were analysed.
Activation of melanoma specific cytotoxic T cell (CTL) clones by VA Qu Spez-treated DCs
The melan-A-specific CTL clone M77-80 that was derived from tumor infiltrating lymphocytes of melanoma patient M77 is a kind gift from Dr. Nathalie Labarriere and Dr. Francine Jotereau [28, 29]. The VA Qu Spez-treated DCs from HLA matched donor (HLA-A2, 104/well/200 μl RPMI 1640 medium supplemented with 10% AB serum) were cultured overnight with M77 CTLs (105) in 96 well round-bottomed plates along with the MART-1 peptide (1 μM) and 25 IU/mL rh IL-2. The activation of M77-80 was analyzed by measuring IFNγ and TNFα in the cell free-supernatants.
Analysis of cytokines
Cytokines in the cell-free culture supernatant were quantified using BD CBA Human Inflammation kit and Human Th1/Th2 kits (BD Biosciences, France).
Statistical significance was determined using the Mann-Whitney U test.
VA Qu Spez enhances the expression of antigen presenting and co-stimulatory molecules on human DCs
VA Qu Spez induces the secretion of pro-inflammatory cytokines IL-6 and IL-8 by DCs
VA Qu Spez-treated DCs stimulate T cell proliferation
VA Qu Spez-treated DCs do not induce anergy of CD4+ T cells
To further confirm that VA Qu Spez-treated DCs do not impart CD4+T cell anergy, we analysed for the secretion of T cell cytokines TNF-α and IFNγ in the DC-CD4+T cell co-cultures. As shown in Figure 4b, CD4+ T cells that were re-stimulated/challenged during second cycle of stimulation with VA Qu Spez-treated DCs show increased secretion of above cytokines as compared to control DCs. These results suggest that maturation and activation of DCs induced by VA Qu Spez have functional repercussion on T cell activation and not T cell anergy.
VA Qu Spez-treated DCs stimulate melanoma specific M77-80 CTL clone
Although VA preparations are widely used in clinical practice and cancer therapy, their mechanisms of action are yet to be fully understood. In our previous studies, we have shown that in addition to cytotoxic properties, VA preparations have immunostimulatory effects that facilitate tumor regression in experimental models . However, to mount an effective anti-tumoral immune response, an increased expression of co-stimulatory molecules on the DCs, the sentinels of the immune system, accompanied by an enhanced secretion of pro-inflammatory cytokines that culminates in T cell proliferation is necessary.
DCs found within the tumor microenvironment are found to have a relatively immature phenotype characterized by low levels CD86, and surface HLA-DR expression and inability to produce pro-inflammatory cytokine [30, 31]. Clinical studies with mistletoe lectins have shown that VA preparations stimulate the cytokine secretion and function of monocytes, the precursors of DCs . The previous studies by Stein et al demonstrated that mistletoe extract and their isolated components influences the maturation of DC with an increased expression of co-stimulatory and antigen presenting molecules [33, 34]. Furthermore, we found that the up-regulation of these molecules is accompanied by the induction of inflammatory cytokines by the VA preparations and stimulation of tumor specific T cells. Together these results suggest that induction of maturation and activation of human DCs is one of the mechanisms underlying the beneficial effect of VA preparations as complimentary therapy in cancer.
Previously, it has been demonstrated that VA lectin induces the gene expression of IL-1 alpha, IL-1 beta, IL-6, TNF-α, IFN-γ and GM-CSF from PBMC . A recent clinical study has shown that the CD14+ monocytes from multiple myeloma patients could be induced to differentiate into functional DCs by culturing them with the cytokine cocktail consisting of GM-CSF, IL-4, IL-6, TNF-α and IL-1β for use in cancer immunotherapy . Our data demonstrates that VA Qu Spez-mediated maturation of DCs and secretion of pro-inflammatory cytokines (IL-6 and IL-8) has repercussion on the stimulation of CD4+ T cells and their cytokine secretion. It is interesting to note that VA Qu Spez-treated DCs do not induce anergy of T cells as shown by the induction of proliferation and the secretion of TNF-α and IFNγ by the CD4+ T cells. Thus, induction of DC-cytokines and T cell cytokines by VA Qu Spez represents a critical determinant in the development of effective innate immune responses against the tumor cells .
CD8+ cytotoxic T lymphocytes (CTLs) are critical for the elimination of tumor. Thus, therapies aimed at expansion of CTLs and their functions holds the key in mounting an effective anti-tumor immune response. The ability of the CTLs to recognize the processed peptides derived from the cellular genes, such as those encoding MART-1 or tyrosinase in melanoma, led to the recognition that protective immune responses are often directed towards tumor-associated, rather than tumor-specific, antigens [28, 29]. Using Melan-A/MART-1 specific M77-80 CTL clone, we have shown that DCs "educated" by VA Qu Spez can mount an anti-tumoral immune response as suggested by the increased levels of secretion of TNF-α and IFNγ by the CTLs in the co-culture. Further studies on the effect of the VA preparations on the DCs that have been subjected to inactivation by tumor factors, may provide strategies in dissecting the stimulatory effects of the VA preparations on the DCs.
VA preparations are known to have cytotoxic properties towards the tumor cells. They are also known to improve the quality of life in the cancer patients. However, the mechanisms by which VA preparations stimulate the immune system and exert beneficial effects in patients are not yet clear. We have demonstrated the role of the VA preparations in stimulating the DCs with implications in the induction of anti-tumor immunity. However, these in vitro results need to be validated further in the context of clinical studies. The elucidation of immunostimulatory mechanisms of VA preparations is critical in understanding their role as complimentary therapy in cancer.
The authors thank Prof. Francine Jotereau and Dr. Nathalie Labarriere (INSERM U601, Nantes, France) for the M77-80 CTL clone. The authors thank Rainier Dierdorf, Jean Chazarenc, Michael Werner and Marc Follmer for discussion. This work was supported by grants from Weleda AG, Switzerland, and by Institut National de la Santé et de la Recherche Médicale (INSERM) and Centre National de la Recherche Scientifique (CNRS), France. S.E is a recipient of fellowship from EGIDE, France.
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