The prophenoloxidase system in Drosophila participates in the anti-nematode immune response

Drosophila melanogaster relies on an evolutionarily conserved innate immune system to protect itself from potentially deadly pathogens. One of the earliest pathways activated after injury or infection is the melanization pathway, which is responsible for synthesizing and depositing melanin at the site of injury, or onto invading microbes. Three genes, PPO1-3, encoding prophenoloxidase (PPO), an inactive precursor of phenoloxidase (PO), are responsible for the production of melanin after their activation via immune challenge. One pathogen capable of infecting D. melanogaster are entomopathogenic nematodes. Steinernema carpocapsae nematodes exist in a mutualistic relationship with Xenorhabdus nematophila bacteria and are an important biological control agent for controlling insect pests. The nematode-bacteria complex (symbiotic nematodes) can be separated, creating "axenic" nematodes, devoid of their associated bacteria, which are still capable of infecting and killing D. melanogaster. In order to investigate how the D. melanogaster melanization pathway contributes to the anti-nematode immune response, symbiotic and axenic S. carpocapsae were used to study D. melanogaster survival, PPO gene expression, and activation of PPO to PO. Our research suggests that the expression of all three D. melanogaster PPO genes contributes to survival, however only PPO1 or PPO3 appear to be up-regulated during axenic or symbiotic nematode infection. Additionally, we present data suggesting that a complex regulatory system exists between PPOs, potentially allowing for the compensation of PPOs by one another. Further, we found that axenic nematode infection leads to higher levels of PO, suggesting that X. nematophila suppresses this activation. We also report for the first time the differentiation of lamellocytes, a specialized type of hemocytes in D. melanogaster, in response to symbiotic S. carpocapsae nematode infection. Our results suggest an important role played by the melanization pathway in response to nematode infection, and demonstrate how this response can be manipulated by S. carpocapsae nematodes and their mutualistic X. nematophila bacteria.