Hidden from the host: Spiroplasma bacteria infecting Drosophila do not cause an immune response, but are suppressed by ectopic immune activation

Abstract

Insects and other arthropods have an effective innate immune system that can clear infections with bacteria and other microorganisms. Despite this ability, one group of bacteria, the spiroplasmas, survive unharmed within the haemolymph of a wide range of arthropod hosts. We investigated the interaction between one member of this clade, a relative of Spiroplasma poulsonii, and the immune system of its Drosophila host. Expression of antimicrobial genes in spiroplasma-infected flies did not differ from wild-type controls either in the naturally infected state, nor after septic shock. We therefore concluded that spiroplasma infection did not induce an immune response in its host, but that this absence of response was unlikely to be because the bacterium inhibited response. Further experiments revealed immune reactions induced ectopically did reduce parasite titre. We therefore conclude that this bacterium has a novel form of interaction with its host, being hidden from the host immune system, but potentially suppressible by it.

Rickettsia peacockii, an endosymbiont of Dermacentor andersoni, does not elicit or inhibit humoral immune responses from immunocompetent D. andersoni or Ixodes scapularis cell lines

“…It did not appear that R. peacockii constrained lysozyme or defensin expression by infected tick cells, as infection did not influence expression by infected but otherwise unstimulated cells or decrease the responsiveness of IDE12 or DAE15 cells to bacterial stimulation. Similarly, Spiroplasma infecting D. melanogaster did not upregulate expression of antimicrobial peptides, or prevent upregulation of antimicrobial peptides after stimulation by inactivated E. coli and spores of the entomopathogenic fungus Beauveria bassiana (29). The tickendosymbiont relationship may be synonymous to the Drosophila response to Spiroplasma.…”

“…Bourtzis et al (28) found that Wolbachia, an endosymbiotic bacterium that infects a wide range of arthropods, did not appear to induce or suppress defensin expression in Drosophila simulans or Aedes albopictus. It is not known how Wolbachia avoids eliciting immune responses, but living inside cells may be an important adaptation that hides Wolbachia from arthropod immune systems (29). In nature, R. peacockii inhabits the ovaries of female D. andersoni and is rarely found in hemolymph (16).…”

Bacterial strategies to overcome insect defences

“…Although the bacterium is widely distributed in different tissues, it accumulates at high concentrations in haemolymph. The presence of Spiroplasma does not induce expression of any of seven antimicrobial genes in Drosophila 55 . The absence of an immune response does not result from immunosuppression, as the response can be induced by septic injury in Drosophila melanogaster that is already infected by Spiroplasma 55 .…”

“…The presence of Spiroplasma does not induce expression of any of seven antimicrobial genes in Drosophila 55 . The absence of an immune response does not result from immunosuppression, as the response can be induced by septic injury in Drosophila melanogaster that is already infected by Spiroplasma 55 . The absence of a response upon Spiroplasma infection results from an absence of elicitors; the lack of wall structure, including peptidoglycan, might explain the success of Spiroplasma in colonizing many species of insects.…”

“…The first strategy is used by a pathogen from the Spiroplasma genus that can infect flies without being detected and also without interfering with the immune response 55 …”