Protective effects of Phaseolus vulgaris lectin against viral infection in Drosophila

Phytohemagglutinin (PHA) isolated from the family of Phaseolus vulgaris beans is a promising agent against viral infection; however, it has not yet been demonstrated in vivo. We herein investigated this issue using Drosophila as a host. Adult flies were fed lectin approximately 12 h before they were subjected to a systemic viral infection. After a fatal infection with Drosophila C virus, death was delayed and survival was longer in flies fed PHA-P, a mixture of L4, L3E1, and L2E2, than in control unfed flies. We then examined PHA-L4, anticipating subunit L as the active form, and confirmed the protective effects of this lectin at markedly lower concentrations than PHA-P. In both experiments, lectin feeding reduced the viral load prior to the onset of fly death. Furthermore, we found a dramatic increase in the levels of the mRNAs of phagocytosis receptors in flies after feeding with PHA-L4 while a change in the levels of the mRNAs of antimicrobial peptides was marginal. We concluded that P. vulgaris PHA protects Drosophila against viral infection by augmenting the level of host immunity.