How the insect immune system interacts with an obligate symbiotic bacterium

Abstract

The animal immune system provides defence against microbial infection, and the evolution of certain animal-microbial symbioses is predicted to involve adaptive changes in the host immune system to accommodate the microbial partner. For example, the reduced humoral immune system in the pea aphid Acyrthosiphon pisum, including an apparently non-functional immune deficiency (IMD) signalling pathway and absence of peptidoglycan recognition proteins (PGRPs), has been suggested to be an adaptation for the symbiosis with the bacterium Buchnera aphidicola. To investigate this hypothesis, the interaction between Buchnera and non-host cells, specifically cultured Drosophila S2 cells, was investigated. Microarray analysis of the gene expression pattern in S2 cells indicated that Buchnera triggered an immune response, including upregulated expression of genes for antimicrobial peptides via the IMD pathway with the PGRP-LC as receptor. Buchnera cells were readily taken up by S2 cells, but were subsequently eliminated over 1-2 days. These data suggest that Buchnera induces in non-host cells a defensive immune response that is deficient in its host. They support the proposed contribution of the Buchnera symbiosis to the evolution of the apparently reduced immune function in the aphid host.

Figure 1.

Buchnera aphidicola in cell culture. (a) Incorporation into Drosophila S2 cells (at 1 h after challenge): the cells were probed with FITC-ApisP (green: specific to Buchnera 16S rRNA) with DAPI (blue: DNA stain) counterstain. (b). Integrity of isolated Buchnera cells assessed by BacLight viability assay, displayed as merged dual colour images; inset, cells killed by incubation at 70°C for 30 min before addition of stain. (c,d) Metabolic vitality of isolated Buchnera cells assessed by CTC assay. Cells were incubated for 30 min at room temperature (c) and at 70°C (d) before incubation with CTC stain for 1 h. Split images are displayed. For (b) green is viable and red is dead; for (c,d), all cells stain green and metabolically active cells additionally stain red. Scale bars, (a) 5 µm, (b,c,d) 10 µm.

Figure 2.

Time course of Buchnera-infection of S2 cells. (a) determined by FISH (604–842 cells scored) and (b) determined by qRT-PCR of the Buchnera gene dnaK (n = 3).