Imd pathway

[2][3] Together the Toll and Imd pathways have formed a paradigm of insect immune signalling; as of September 2, 2019, these two landmark discovery papers have been cited collectively over 5000 times since publication on Google Scholar.

[6] Insects lacking Imd signalling either naturally or by genetic manipulation are extremely susceptible to infection by a wide variety of pathogens and especially bacteria.

[7] Activated Relish then forms dimers that move into the nucleus and bind to DNA leading to the transcription of antimicrobial peptides and other effectors.

A number of secreted PGRPs have amidase activity that downregulate the Imd pathway by digesting peptidoglycan into short, non-immunogenic fragments.

[10] Imd signalling regulates a number of effector peptides and proteins that are produced en masse following immune challenge.

Flies lacking multiple antimicrobial peptide genes succumb to infections by a broad suite of Gram-negative bacteria.

[13] The fly Diptericin A gene is essential for defence against the bacterium Providencia rettgeri (also suggested by an earlier evolutionary study[16]).

[18] These works accompany others on antimicrobial peptides and effectors regulated by the Drosophila Toll pathway, which also display a specific importance in defence against certain fungi or bacteria.

The modern interpretation is now that specific molecules might provide a somewhat redundant layer of defence, but also single peptides can have critical importance, individually, against relevant microbes.

It is thought that plant-feeding aphids have lost Imd signalling as they bear a number of bacterial endosymbionts, including both nutritional symbionts that would be disrupted by aberrant expression of antimicrobial peptides, and defensive symbionts that cover for some of the immune deficiency caused by loss of Imd signalling.

[26] It has also been suggested that antimicrobial peptides, the downstream components of Imd signalling, may be detrimental to fitness and lost by insects with exclusively plant-feeding ecologies.

Simplified diagram of Imd signalling as found in the fruit fly .
Fruit flies infected by GFP-producing bacteria. Red-eyed flies lacking antimicrobial peptide genes are susceptible to infection, while white-eyed flies have a wild-type immune response.
Pea aphids have lost Imd signalling