Insect Immunity: Expression of the Two Major Inducible Antibacterial Peptides, Defensin and Diptericin, in Phormia Terranovae

Injections of low doses of bacteria into larvae of Phormia terranovae induce the appearance of potent bactericidal peptides in the blood, among which predominate the anti-Gram positive insect defensins and the anti-Gram negative diptericins. Insect defensins show significant homologies to mammalian (including human) microbicidal peptides present in polymorphonuclear leukocytes and macrophages. We report the molecular cloning of cDNAs and primer extension studies which indicate that insect defensin is produced as a prepro-peptide yielding mature defensin A (40 residues) after cleavage of a putative signal peptide (23 residues) and a prosequence (34 residues). Previous studies have established that diptericin (82 residues) is matured from a pre-peptide by cleavage of a putative signal peptide (19 residues) and C-terminal amidation. Using oligonucleotide probes complementary to the sequences of the mRNAs for defensin and diptericin, we show by in situ hybridization that both antibacterial peptides are concomitantly synthesized by the same cells: thrombocytoids, a specialized blood cell type, and adipocytes. Transcriptional studies based on hybridization of RNAs to cDNAs of defensin and diptericin indicate that the transcription of both genes is induced regardless of the nature of the stimulus (injection of Gram positive or Gram negative bacteria, lipopolysaccharides). Even a sterile injury applied to axenically raised larvae is efficient in inducing the transcription of both genes suggesting that the local disruption of the integument aspecifically initiates a signalling mechanism which the thrombocytoids and the adipocytes are able to interpret. The transcription of immune genes is relatively short lived and a second challenge yields a response similar to that of the first stimulus, indicating that the experimental insects do not keep a 'memory' of their first injection.