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J. Biol. Chem., Vol. 279, Issue 20, 21121-21127, May 14, 2004

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Differential Activation of the NF-B-like Factors Relish and Dif in Drosophila melanogaster by Fungi and Gram-positive Bacteria^*

Marika Hedengren-Olcott¶, Michael C. Olcott||, Duane T. Mooney**, Sophia Ekengren, Bruce L. Geller**, and Barbara J. Taylor

From the Departments of Zoology, ^||Biochemistry and Biophysics, and ^**Microbiology, Oregon State University, Corvallis, Oregon 97331, the Wenner-Green Institute, Stockholm University, S-106 91 Stockholm, Sweden, and the Umeå Centre for Molecular Pathogenesis, Umeå University, S-901 87 Umeå, Sweden

The current model of immune activation in Drosophila melanogaster suggests that fungi and Gram-positive (G⁺) bacteria activate the Toll/Dif pathway and that Gram-negative (G^-) bacteria activate the Imd/Relish pathway. To test this model, we examined the response of Relish and Dif (Dorsal-related immunity factor) mutants to challenge by various fungi and G⁺ and G^- bacteria. In Relish mutants, the Cecropin A gene was induced by the G⁺ bacteria Micrococcus luteus and Staphylococcus aureus, but not by other G⁺ or G^- bacteria. This Relish-independent Cecropin A induction was blocked in Dif/Relish double mutant flies. Induction of the Cecropin A1 gene by M. luteus required Relish, whereas induction of the Cecropin A2 gene required Dif. Intact peptidoglycan (PG) was necessary for this differential induction of Cecropin A. PG extracted from M. luteus induced Cecropin A in Relish mutants, whereas PGs from the G⁺ bacteria Bacillus megaterium and Bacillus subtilis did not, suggesting that the Drosophila immune system can distinguish PGs from various G⁺ bacteria. Various fungi stimulated antimicrobial peptides through at least two different pathways requiring Relish and/or Dif. Induction of Attacin A by Geotrichum candidum required Relish, whereas activation by Beauvaria bassiana required Dif, suggesting that the Drosophila immune system can distinguish between at least these two fungi. We conclude that the Drosophila immune system is more complex than the current model. We propose a new model to account for this immune system complexity, incorporating distinct pattern recognition receptors of the Drosophila immune system, which can distinguish between various fungi and G⁺ bacteria, thereby leading to selective induction of antimicrobial peptides via differential activation of Relish and Dif.

Received for publication, December 18, 2003 , and in revised form, February 19, 2004.

^* This work was supported by Everett Boaz Comer. A preliminary account of this work was presented at the 43rd Annual Drosophila Research Conference, April 10–14, 2002, San Diego, CA (Suppl. 1038C, Flybase ID FBrf0146714). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Present address: Boyce Thompson Institute, Tower Rd., Ithaca, NY 14853.

^¶ To whom correspondence should be addressed: Dept. of Microbiology, 220 Nash Hall, Oregon State University, Corvallis, OR 97331. Tel.: 541-737-2260; Fax: 541-737-0496; E-mail: hedengrm{at}science.oregonstate.edu.

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