Evasion, suppression and tolerance of Drosophila innate immunity by a male-killing Spiroplasma endosymbiont.

How endosymbiotic bacteria cope with host insect immunity is poorly understood. Here we report previously unknown aspects of immunity-mediated interactions between male-killing/non-male-killing spiroplasmas and Drosophila host. The male-killing spiroplasma tended to reduce constitutive expression levels of some antimicrobial peptide genes, while the non-male-killing spiroplasma did not. In mutant flies whose innate immunity is constitutively up-regulated, infection densities of the male-killing spiroplasma were significantly suppressed but managed to increase during the aging of adult flies, indicating that the male-killing spiroplasma is resistant to mounted immune attacks. These findings suggest that not only immune evasion but also immune suppression and tolerance are involved in the establishment and maintenance of the insect-microbe symbiotic association.