CLEARANCE OF LIPOPOLYSACCHARIDE IN HEMOLYMPH OF THE SILKWORM BOMBYX-MORI - ITS ROLE IN THE TERMINATION OF CECROPIN MESSENGER-RNA INDUCTION

Clearance of lipopolysaccharide (LPS) in the hemolymph of silkworm, Bombyx mori, and its physiological role in the regulation of induction of an antibacterial protein, cecropin B, were investigated. In in vitro culture of fat body, cecropin B mRNA appeared 20 min after incubation with LPS and its accumulation persisted more than 72 h. No significant desensitization was observed for at least 24 h in vitro. On the contrary, cecropin B mRNA completely disappeared 24 h after injection of LPS in vivo. These results suggest that concentration and/or biological activity of LPS decreases in hemolymph with time. LPS concentration assay at various time intervals after injection revealed two phases of LPS clearance from hemolymph, an initial rapid (T-1/2 = 1.0 h) followed by a very slow phase (T-1/2 > 18 h). The threshold concentration of LPS to induce cecropin B mRNA in vivo was over 10-fold higher than that of in vitro. Although no cecropin mRNA accumulation was detected after 24 h of LPS injection, it appeared again by a reinjection of LPS at this time period. This result suggests that the reduction of LPS concentration directly terminates cecropin induction in vivo. We, therefore, conclude that B. mori has a LPS clearance system from hemolymph which is sufficient to terminate LPS mediated antibacterial protein induction.