Silencing of the immune gene BmPGRP-L4 in the midgut affects the growth of silkworm (Bombyx mori) larvae.

Peptidoglycan recognition proteins (PGRPs) are one of the receptors in insects' immune pathways, essential for insects to recognize the exogenous pathogens in order to activate the Toll and IMD pathway. In the silkworm Bombyx mori, previous studies focused on the short PGRPs and less is known about the long PGRPs. In this study, a long PGRP in silkworm BmPGRP-L4 was cloned and its expression and function were analyzed. The results showed that BmPGRP-L4 contains a transmembrane region, a conserved PGRP domain, and an amidase-2 domain. The expression profile demonstrated that BmPGRP-L4 existed in diverse tissues including epidermis, fat body, midgut and silk glands, with remarkably high expression in the midgut in the 5 th instar. Oral infection of E. coli and S. aureus significantly induced BmPGRP-L4 in the midgut and epidermis, as well as in fat body and silk glands. Peptidoglycan also induced the expression of BmPGRP-L4 in midgut tissue ex vivo and BmN4 cells in vitro. RNAi of BmPGRP-L4 was effective in the midgut and epidermis, while the efficiency in the fat body was transient. RNAi-mediated knock-down of BmPGRP-L4 reduced the weight and growth of the silkworm, possibly due to its participation in the immune response and the regulation of the microbiota in the midgut lumen of the silkworm larvae. This article is protected by copyright. All rights reserved.