Inhibition of phagocytic activity of plasmatocytes isolated from Galleria mellonella by entomogenous fungi and their secondary metabolites

The phagocytic activity of isolated plasmatocytes from Metarhizium anisopliae- or Beauveria bassiana-infected Galleria mellonella larvae was examined and compared to that observed from untreated larvae, Mycosis reduced plasmatocyte phagocytic activity against either yeast cells or blastospores of both entomopathogenic fungi, In contrast, the phagocytic activity of isolated plasmatocytes from diseased larvae against hydrophilic silica beads was only moderately inhibited compared to controls, In addition, the influence of solubilized or surface bound fungal secondary metabolites from M. anisopliae (destruxin A and E, cytochalasin B and D) on plasmatocytes in monolayers was investigated. Fungal cyclic peptides dissolved in sublethal concentrations were found to inhibit phagocytic activity of plasmatocytes in a dose-dependent manner. Fungal blastospores coated with secondary metabolites were not ingested, The phagocytic activity of plasmatocytes obtained from larvae which were preinjected with sublethal concentrations of destruxin A was not significantly reduced, The corresponding inhibition of phagocytic activity found in toxin-treated plasmatocytes in vitvo and in plasmatocytes isolated from infected larvae support the assumption that entomopathogenic fungi release secondary metabolites to impair the cellular recognition and defense reactions of their host. (C) 1997 Elsevier Science Ltd.