New secondary metabolites with immunosuppressive activity from the phytopathogenic fungus Bipolaris maydis

Three previously undescribed compounds, including a meroterpenoid, guignardone T (1), and two ophiobolin-type sesterterpenoids, maydispenoids A and B (2 and 3), along with four known compounds (4-7), were isolated from the phytopathogenic fungus Bipolaris maydis collected from Anoectochilus roxburghii (Wall.) Lindl leaves. The structures of all undescribed compounds were elucidated by spectroscopic analysis, electronic circular dichroism (ECD) calculations and single-crystal X-ray diffraction. Structurally, maydispenoids A was characterized by a fascinating decahydro-3-oxacycloocta[cd]pentalene fragment. It is notable that the compounds 2 and 3 exhibited potential inhibitory activity in anti-CD3/anti-CD28 monoclonal antibodies (mAbs) stimulated murine splenocytes proliferation, with IC₅₀ values of 5.28 and 9.38 μM, respectively, and also suppress the murine splenocytes proliferation activated by lipopolysaccharide (LPS), with IC₅₀ values of 7.25 and 16.82 μM, respectively. This is the first report of ophiobolin-type sesterterpenoids as immunosuppressor, and may provide new chemical templates for the development of new immunosuppressive drugs for autoimmune disease treatment.