Recognition and Response in Innate Immunity

Two systems of pathogen recognition and signal transduction mediate the humoral immune response in Drosophila melanogaster. The Imd pathway mediates responses to Gram (-) bacteria and Gram (+) bacilli; the Toll pathway, mediates responses to fungi and other Gram (+) bacteria. Both pathways activate transcription factors in the NF-kappaB/Rel family.

Using genetic, bioinformatic, and biochemical approaches, we have elucidated the sequence code that enables elaboration of specific effector responses. Combining expression data from wild-type and mutant backgrounds with an unbiased analysis of upstream regulatory regions, we have defined consensus kappaB sites responsive to each pathway (1).

(1) Bioinformatically Defined KappaB Code
Assaying a range of sequence elements in a cultured cell assay, we have confirmed the validity of the kappa B code in vivo. In addition, binding assays with recombinant forms of the Imd transcription factor Relish and the Toll transcription factor Dorsal demonstrate that the sequence code acts at the level of transcription factor recognition and binding (2). Variation in site sequence and number (3) thus underlies specificity in humoral responses.
(3) KappaB Sites Mediate Specific Immune Responses
(2) Binding of Relish (R) and Dorsal (D) to KappaB Sites

Current Research

We have expanded our analysis of transcriptional responses to innate immune signaling pathways across a range of Drosophila species, focusing initially on Toll signaling. These studies reveal strong conservation of the sequence code for Toll responsiveness. Global analysis of effector genes by RNAseq has defined a core set of Toll responsive genes shared across the genus, as well as species specific effectors. We are currently using a molecular genetic approach to delineate the function of these effectors, for many of which the innate immune function has not previously been characterized.