Functional amyloids in insect immune response

The innate immune system of insects consists of humoural and cellular responses that provide protection against invading pathogens and parasites. Defence reactions against these latter include encapsulation by immune cells and targeted melanin deposition, which is usually restricted to the surface of the foreign invader, to prevent systemic damage. Here we show that a protein produced by haemocytes of Heliothis virescens (Lepidoptera, Noctuidae) larvae, belonging to XendoU family, generates amyloid fibrils, which accumulate in large cisternae of the rough endoplasmic reticulum and are released upon immune challenge, to form a layer coating non-self objects entering the haemocoel. This amyloid layer acts as a molecular scaffold that promotes localised melanin synthesis and the adhesion of immune cells around the non-self intruder during encapsulation response. Our results demonstrate a new functional role for these protein aggregates that are commonly associated with severe human diseases. We predict that insects will offer new powerful experimental systems for studying inducible amyloidogenesis, which will likely provide fresh perspectives for its prevention.

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► Haemocytes of Heliothis virescens larvae generate amyloid fibrils, which coat non-self objects entering the haemocoel.
► The amyloid layer promotes localised melanin synthesis and mediates the adhesion of immune cells to the non-self intruder.
► This mechanism prevents systemic melanogenesis, which would be fatal.