In this thesis, the immunomodulatory effects of selected natural products that are applied in complementary medicine were investigated.
The European mistletoe, Viscum album, is used as an additive anticancer therapy, and it can be observed that mistletoe induces the maturation of dendritic cells (DCs). In this work, it was investigated if the tumor-induced immunosuppression of DC maturation could be counteracted via mistletoe treatment. Moreover, the role of mistletoe lectins in this process was analyzed. Using monocyte-derived DCs, the effects of two different mistletoe preparations (Iscador® Qu Spez and abnobaVISCUM® Fraxini) on DC maturation were measured by quantifying CD83, CD86, and HLA-DR expression using flow cytometry and confocal microscopy. Additionally, changes in DC cytokine release were measured using bead array assays. A co-culture with T cells was carried out to assess the impact of mistletoe-treated DCs on T cell proliferation and function using CFSE staining and cytokine ELISA. The role of mistletoe lectins in this process was evaluated via the incubation of DCs with ML-depleted mistletoe preparations or anti-ML antibodies. The effects of mistletoe on DCs in an immunosuppressive tumor microenvironment were highlighted via the simultaneous incubation with a tumor supernatant. The results showed that DC maturation was induced by mistletoe treatment and that this process was dependent on MLs; however, the effects were not strong enough to produce a measurable activation of the T cells. The tumor supernatant effectively inhibited DC maturation and this process could be, in part, counteracted by mistletoe treatment in a ML-dependent manner. From these results it can be concluded that the recovery of DC maturation after tumor-induced immunosuppression is a possible mode-of-action of mistletoe in cancer treatment.
Equisetum arvense, the common horsetail, has also been traditionally used in the treatment of inflammatory diseases and is one of the plants with the highest silica amounts. First, preparation methods were tested to obtain high silica yields. To investigate if the immunomodulatory effects of Equisetum arvense are mediated by silica, the effects of Equisetum decoctions on certain immune parameters were interpreted in regard to their silica concentration. A silica standard solution was used as well. A flow cytometric analysis of the influences on T cell proliferation, apoptosis, and CD25/CD69 expression were measured, and influences on T cell function were assessed using IL-2 and IFN-γ ELISA. The data revealed that T cell activation and function was impaired in an IL-2 dependent mechanism by Equisetum decoctions and the silica standard solution. Using HPLC-UV-MS, isoquercitrin could be identified as the most abundant flavonoid and bioactivity testing revealed inhibitory effects on T cell proliferation as well. It could therefore be concluded that the immunosuppressive activity of Equisetum arvense preparations is mediated by both silica and isoquercitrin.
Cyanobacteria represent an interesting source for novel drug creations. Screening its effects on T cell proliferation and apoptosis revealed various interesting strains exhibiting immunosuppression but not cytotoxicity. Using bioactivity-guided fractionation, hapalindoles could be identified as active compounds in one strain, namely Hapalosiphon sp. Influences of different cyanobacteria strains on the IL-2 signaling pathways AP-1, NFAT, and NFκB have been measured using reporter cell lines and intracellular staining. Moreover, the effects of two cyanobacteria strains on the THP-1 cancer cell line have been measured, and both were able to inhibit proliferation via induction of apoptosis. The results revealed that cyanobacteria can be potent immune modulators and that further research should be conducted to describe their effects and active compounds as well as to evaluate possible applications in more detail.
Moreover, further elucidation of the immunomodulatory effects of Tricholoma populinum, green tea catechins, the active mutant of the plant cyclotide kalata B1 T20K, and Rosmarinus officinalis was carried out. Tricholoma populinum, a mushroom described to alleviate inflammatory diseases, led to a reduction in IL-8 secretion by HMC-1 mast cells while affecting cell viability only at high concentrations. The green tea catechins ECG and F- ECG reduced the viability of LNCaP and PC-3 prostate cancer cell lines, which was measured using the WST-1 assay. Additionally, the effects of ECG, F-ECG, and various other green tea catechins on T cell proliferation, the induction of apoptosis, and cell viability were measured. Some catechins were able to reduce T cell proliferation, and the effects of ECG on T cells were intensified by fluorination. These results support the use of green tea in the prevention of prostate cancer via its inhibition of tumorigenesis and reduction of inflammation. Moreover, the active mutant of plant cyclotide kalata B1 T20K and Rosmarinus officinalis, two natural products with known immunomodulatory effects, were screened for their potential to affect IL-2 signaling in T cells.
The investigations reported in this thesis describe the procedure for evaluating the immunomodulatory effects of natural products. The results revealed that natural products can be potent modulators of the human immune system, which provides a rationale for their use in complementary medicine.