Primed immune responses to Gram-negative peptidoglycans confer infection resistance in silkworms [Microbiology]

April 4th, 2014 by Miyashita, A., Kizaki, H., Kawasaki, K., Sekimizu, K., Kaito, C.

A heightened immune response, in which immune responses are primed by repeated exposure to a pathogen, is an important characteristic of vertebrate adaptive immunity. In the present study, we examined whether invertebrate animals also exhibit a primed immune response. The LD50 of Gram-negative enterohemorrhagic Escherichia coli (EHEC) O157:H7 Sakai in silkworms was increased 100-fold by pre-injection of heat-killed Sakai cells. Silkworms pre-injected with heat-killed cells of a Gram-positive bacterium, Staphylococcus aureus, did not have resistance to Sakai. Silkworms pre-injected with EHEC peptidoglycans, cell surface components of bacteria, were resistant to Sakai infection. Silkworms pre-injected with S. aureus peptidoglycans, however, were not resistant to Sakai. Silkworms pre-injected with heat-killed Sakai cells showed persistent resistance to Sakai infection even after pupation. Repeated injection of heat-killed Sakai cells into the silkworms induced earlier and greater production of antimicrobial peptides than a single injection of heat-killed Sakai cells. These findings suggest that silkworm recognition of Gram-negative peptidoglycans leads to a primed immune reaction and increased resistance to a second round of bacterial infection.