MIDGUT-SPECIFIC IMMUNE MOLECULES ARE PRODUCED BY THE BLOODSUCKING INSECT STOMOXYS-CALCITRANS

We have cloned and sequenced two defensins, Smd1 and Smd2, from anteri or midgut tissue of the bloodsucking fly Stomoxys calcitrans. The DNA and N-terminal protein sequences suggest both are produced as preprope ptides. Smd1 differs from the classic defensin pattern in having an un usual six-amino acid-long N-terminal sequence. Both Smd1 and Smd2 have lower pI points and charge than insect defensins derived from fat bod y/hemocytes. Northern analysis shows both of these defensin molecules are tissue specific; both are produced by the anterior midgut tissue a nd, unlike the other insect defensins reported to date, neither appear s to be expressed in fat body or hemocytes, Northern analysis also sho ws that mRNAs for both defensins are constitutively produced in the an terior midgut tissues and that these transcripts are up-regulated in r esponse to sterile as well as a lipopolysaccharide-containing blood me al. However, anti-Gram-negative biological activity in the midgut is s ubstantially enhanced by lipopolysaccharide. These findings suggest th at the insect midgut has its own tissue-specific immune mechanisms and that this invertebrate epithelium is, like several vertebrate epithel ia, protected by specific antibacterial peptides.