A pervasive (but possibly unwritten, and indeed potentially misleading) view in the field was that inherited symbionts do not interact with the insect immune system because of their intracellu-lar location. Humoral immunity involves secreted peptides, which are unlikely to be active intracel-lularly. The cascade leading to their production likewise is induced by free bacteria, not bacteria inside cells. Cellular immunity likewise involves recognition of bacteria that are extracellular, followed by their subsequent ingestion and lysis. Infected host cells are not targeted in this process. This makes the intracellular milieu a potential safe haven for bacteria, and it is tempting to suggest that the intracellular environment being a place of safety has in fact the driven the evolution of maternal inheritance for many bacteria. Under this thesis, entry into cells and adopting the intracellular habitat evolved as a way of escaping immune system activity. This entry into cells led to maternal inheritance, as bacteria were then found inside egg cells, and this to the evolution of both reproductive parasitism phenotypes and also evolution towards active contribution to host function.
This caricature is plausible, but is it reflected in evidence? With regard to innate immunity, it has been noted that Wolbachia-infected Drosophila do not upregulate genes encoding AMPs. However, Wolbachia infection does not prevent induction in response to septic shock, implying that the bacteria are likely to be avoiding activation of the system, rather than actively downregulating it (Bourtzis et al., 2000). In contrast to this, a study by Xi et al. (2008) in Drosophila S2 cell lines did establish genes associated with innate immunity as being upregu-lated in infected cells compared with uninfected ones. However, it was noted that the upregulated elements were not AMP genes (concordant with the whole-organism study), but those involved in the nuclear factor kB (NF-kB) signalling cascade leading to AMP induction. The inference from this data is not obvious, however, as these genes also have roles in non-immune contexts.
The humoral response is only one of the ways in which hosts disable or kill invading pathogens. Cellular responses including digestion of microbes in a phagolysosome, and also exposure to high levels of reactive oxygen species. Although humoral immunity may be ineffective, both of these mechanisms are potentially available to combat intracellular infections. With respect to the former, studies on Legionella and Coxiella have recently demonstrated the importance of secreted proteins carrying ankyrin-repeat motifs in manipulating the cellular environment, preventing fusion of endosomes with the vacuole in which the bacteria reside, and inhibiting endocytic maturation and the formation of a phagolysosome that would destroy the bacterium (Pan et al., 2008). It is notable that Wolbachia, the most common intracel-lular symbiont of insects, has an expanded range of genes containing ankyrin-repeat domains (Wu et al., 2004), and these represent an excellent area of focus with respect to how Wolbachia maintains itself inside cells.
The potential involvement of reactive oxygen species in defence against intracellular bacteria has been examined in Wolbachia infections in Drosophila S2 lines. Elevated levels of reactive oxygen species were observed, although it is unclear whether this was a host response to Wolbachia, or a result of Wolbachia respiration (Brennan et al., 2008). Proteomic analysis demonstrated increased levels of the superoxide dismutase and peroxid-ase enzymes of host origin, and also dismutase enzymes of Wolbachia origin, that may represent 'coping mechanisms' on the parts of host and bacterium. The proteins of host origin were broadly reflected in upregulated genes in the transcrip-tomic study of Xi et al. (2008).
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