Studies on the mechanisms of action of the immunomodulator Bestatin in various screening test systems

The effects of Bestatin, a low molecular weight metabolite of Streptomyces olivoreticuli on the human and mouse/rat immune system, have been studied in detail. To describe the activity of the immunomodulating dipeptide, it has been tested in vitro, ex vivo and in vivo in various experimental models. Bestatin simultaneously applied with selected antigens to mice was able to enhance the DTH response against a challenge injection of the respective antigen given into the footpad. Serum antibody levels against those antigens were uneffected. However, an increase of PFC could be found in those mice given high doses of Bestatin. On natural killer cell activity against Yac-1 tumor cells the dipeptide had no effect in low responder (DBA2/J) mice. In high responder mice (CBA/JCr), however, a significant increase of NK cell activity of spleen cells could be found, when the drug was given on day 0 or on days 0 to 3 and the test was performed on day 4. Bestatin had no effect on the generation of allogeneic cytotoxic T-lymphocytes in vivo or in vitro and even a suppressive effect on the induction of syngeneic antitumor CTL. Contrary to this suppressive effect, Bestatin increases in the popliteal lymph node assay the weights in a dose-dependent way. When mouse macrophages or human monocytes were either incubated in vitro with Bestatin or mice were treated with the dipeptide parenterally or orally and the macrophages from those mice were investigated, Bestatin induced in vitro and in vivo a dose-dependent increase in pinocytic uptake of radioactive colloidal gold. Also the oxidative metabolism was dose-dependently augmented as measured by chemiluminescence. Bestatin modulates the macrophage mediated cytotoxicity. In vitro or in vivo activated mononuclear phagocytes exhibited a dose-dependent increase in cytotoxic activity for several tumor target cells. A minimum ratio of 50:1 effector to target cells was necessary for this cytotoxic effect. A similar degree of activation was observed in macrophages from athymic nu/nu-mice or from endotoxin resistant C3H/HeJ-mice. Other parameters of macrophage activation were determined by measuring secretion of lysosomal enzymes and liberation of prostaglandins. Bestatin interacts with macrophages in vivo and in vitro by increasing their secretory activity of acid hydrolases (beta-glucuronidase, beta-galactosidase, and N-acetyl-beta-D-glucosaminidase). This release was dose- and time-dependent and not associated with any sign of cell death. Another class of mediators produced by macrophages after stimulation with Bestatin were the prostaglandins E2 and F2a.(ABSTRACT TRUNCATED AT 400 WORDS)