Critical Roles of Spätzle5 in Antimicrobial Peptide Production Against in Malpighian Tubules.

The dimeric cytokine ligand Spätzle (Spz) is responsible for Toll pathway activation and antimicrobial peptide (AMP) production upon pathogen challenge in . Here, we indicated that Spz5 has a functional role in response to bacterial infections. We showed that the highest expression of is induced by . However, knockdown reduced larval survival against and . To evaluate the molecular mechanism underlying the observed survival differences, the role of in AMP production was examined by RNA interference and microbial injection. AMPs that are active against Gram-negative and -positive bacteria, including tenecins, attacins, coleoptericins, taumatin-like-proteins, and cecropin-2, were significantly downregulated by RNAi in the Malpighian tubules (MTs) following a challenge with and . However, upon infection with the mRNA levels of most AMPs in the ds-injected group were similar to those in the control groups. Likewise, the expression of the transcription factors NF-κB, , and were noticeably suppressed in the MTs of -silenced larvae. Moreover, -infected knockdown larvae showed decreased antimicrobial activity in the MTs and hindgut compared with the control group. These results demonstrate that Spz5 has a defined role in innate immunity by regulating AMP expression in MTs in response to .