The peptidoglycan recognition protein PGRP-LC is a major activator of the imd/Relish pathway in the Drosophila immune response. Three transcripts are generated by alternative splicing of the complex PGRP-LC gene. The encoded transmembrane proteins share an identical intracellular part, but each has a separate extracellular PGRP-domain: x, y, or a. Here we show that two of these isoforms play unique roles in the response to different microorganisms. Using RNA interference in Drosophila mbn-2 cells, we found that PGRP-LCx is the only isoform required to mediate signals from Grampositive bacteria and purified bacterial peptidoglycan. By contrast, the recognition of Gram-negative bacteria and bacterial lipopolysaccharide requires both PGRPLCa and LCx. The third isoform, LCy, is expressed at lower levels and may be partially redundant. Two additional PGRP domains in the gene cluster, z and w, are both included in a single transcript of a separate gene, PGRP-LF. Suppression of this transcript does not block the response to any of the microorganisms tested.Insects and mammals use similar mechanisms and molecular pathways to recognize and eliminate invading microorganisms. The best studied example is the humoral immune response of the fruit fly Drosophila melanogaster, where two major immune pathways signal the presence of microbes and mediate the production of antimicrobial peptides, the imdRelish pathway and the Toll/Dif pathway (1-4). Members of the peptidoglycan recognition protein (PGRP) 1 family were recently found to play key roles in the activation of these pathways (5-9). PGRPs have been found in many organisms, including vertebrates, and the family includes peptidoglycanbinding pattern recognition molecules as well as peptidoglycandegrading amidases (10 -13). Several PGRP genes are found in the Drosophila genome (14). One of them, PGRP-LC, is required for the activation of the imd/Relish pathway and is likely to encode a pattern recognition molecule for the humoral immune response (6 -8). Surprisingly, we found that PGRP-LC mediates the induction by bacterial lipopolysaccharide (LPS) as well as peptidoglycan (6), although the effect of LPS was recently questioned (15). In the present study we show that two different isoforms of PGRP-LC display distinct functions in the response to LPS and peptidoglycan. EXPERIMENTAL PROCEDUREScDNA Library Screen-A ZAP Express cDNA expression library made from induced l(3)mbn-1 larvae (16) was screened using Hybond-NX membranes (Amersham Biosciences) and radioactive probes against the PGRP domains x, y, w, and z. We screened between 3 and 11 ϫ 10 5 plaques with the different probes, corresponding to 0.6 -2.2 times the estimated complexity of the library. The probes were made as described for Northern blot detection. Positive pBK-CMV phagemid clones derived from the ZAP Express vector (Stratagene) were obtained by in vivo excision using the ExAssist helper phage (Stratagene). EST clones of PGRP-LF were obtained from the Berkeley Drosophila Genome Project/Howard Hughes Medical In...

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A new genetic study has shown that the phagocytic ability of Drosophila blood cells, the hemocytes, may be important for the further induction of an antibacterial response in other tissues.