Soluble peptidoglycan fragments stimulate antibacterial protein synthesis by fat body from larvae of Manduca sexta.

Both hemocytes and fat body from larvae of Manduca sexta, which have been injected with inducers of antibacterial protein synthesis, contain immunoreactive lysozyme. However, fat body is a richer source and has been demonstrated to synthesize and release lysozyme and cecropin-like peptides (bactericidins) in vitro. Fat body secretion of lysozyme and bactericidins is stimulated by addition of soluble peptidoglycan fragments to culture medium. The rate of lysozyme secretion by fat body varies as a function of peptidoglycan inducer concentration. These data are consistent with the hypothesis that, in vivo, bacteria must be phagocytized and partially degraded (processed) by hemocytes to generate a signal (peptidoglycan) that subsequently induces antibacterial protein synthesis by fat body.