Peptidoglycan recognition protein (PGRP)-LE and PGRP-LC act synergistically in Drosophila immunity.

In innate immunity, pattern recognition molecules recognize cell wall components of microorganisms and activate subsequent immune responses, such as the induction of antimicrobial peptides and melanization in Drosophila. The diaminopimelic acid (DAP)-type peptidoglycan potently activates imd-dependent induction of antibacterial peptides. Peptidoglycan recognition protein (PGRP) family members act as pattern recognition molecules. PGRP-LC loss-of-function mutations affect the imd-dependent induction of antibacterial peptides and resistance to Gram-negative bacteria, whereas PGRP-LE binds to the DAP-type peptidoglycan, and a gain-of-function mutation induces constitutive activation of both the imd pathway and melanization. Here, we generated PGRP-LE null mutants and report that PGRP-LE functions synergistically with PGRP-LC in producing resistance to Escherichia coli and Bacillus megaterium infections, which have the DAP-type peptidoglycan. Consistent with this, PGRP-LE acts both upstream and in parallel with PGRP-LC in the imd pathway, and is required for infection-dependent activation of melanization in Drosophila. A role for PGRP-LE in the epithelial induction of antimicrobial peptides is also suggested.

Overexpression of a pattern-recognition receptor, peptidoglycan-recognition protein-LE, activates imd/relish-mediated antibacterial defense and the prophenoloxidase cascade in Drosophila larvae.

In Drosophila, microbial infection activates an antimicrobial defense system involving the activation of proteolytic cascades in the hemolymph and intracellular signaling pathways, the immune deficiency (imd) and Toll pathways, in immune-responsive tissues. The mechanisms for microbial recognition are largely unknown. We report that, in larvae, the imd-mediated antibacterial defense is activated by peptidoglycan-recognition protein (PGRP)-LE, a PGRP-family member in Drosophila. Consistent with this, PGRP-LE binds to the diaminopimelic acid-type peptidoglycan, a cell-wall component of the bacteria capable of activating the imd pathway, but not to the lysine-type peptidoglycan. Moreover, PGRP-LE activates the prophenoloxidase cascade, a proteolytic cascade in the hemolymph. Therefore, PGRP-LE acts as a pattern-recognition receptor to the diaminopimelic acid-type peptidoglycan and activates both the proteolytic cascade and intracellular signaling in Drosophila immunity.