Tick-borne pathogens induce differential expression of genes promoting cell survival and host resistance in Ixodes ricinus cells

Abstract

Background: There has been an emergence and expansion of tick-borne diseases in Europe, Asia and North America in recent years, including Lyme disease, tick-borne encephalitis and human anaplasmosis. The primary vectors implicated are hard ticks of the genus Ixodes. Although much is known about the host response to these bacterial and viral pathogens, there is limited knowledge of the cellular responses to infection within the tick vector. The bacterium Anaplasma phagocytophilum is able to bypass apoptotic processes in ticks, enabling infection to proceed. However, the tick cellular responses to infection with the flaviviruses tick-borne encephalitis virus (TBEV) and louping ill virus (LIV), which cause tick-borne encephalitis and louping ill respectively, are less clear.

Results: Infection and transcriptional analysis of the Ixodes ricinus tick cell line IRE/CTVM20 with the viruses LIV and TBEV, and the bacterium A. phagocytophilum, identified activation of common and distinct cellular pathways. In particular, commonly-upregulated genes included those that modulate apoptotic pathways, putative anti-pathogen genes, and genes that influence the tick innate immune response, including selective activation of toll genes.

Conclusion: These data provide an insight into potential key genes involved in the tick cellular response to viral or bacterial infection, which may promote cell survival and host resistance.

Keywords: Anaplasma phagocytophilum; Apoptosis; Flavivirus; Immunology; Ixodes ricinus; Tick cell; Toll; Transcriptomics.

Fig. 1

a Replication of LIV (grey) and TBEV (black) in IRE/CTVM20 cells, as demonstrated by log₁₀ mean virus copies/16S copies detected in cellular-derived RNA (dashed lines), and virus titre (PFU/ml) quantified in cell supernatant (solid bars). b Replication of Anaplasma phagocytophilum in IRE/CTVM20 cells, as demonstrated by mean Msp4 copies normalised against mean 16S copies in cellular-derived RNA. Error bars represent the standard deviation. *P < 0.05; **P < 0.01

Fig. 2

Distribution of differentially upregulated (a) and downregulated (b) genes, following infection of IRE/CTVM20 cells with A. phagocytophilum, LIV or TBEV

Fig. 3

Summary of differential gene expression for selected genes, highlighted green (upregulated), red (downregulated) or black (no significant change), following infection of IRE/CTVM20 cells with A. phagocytophilum (Ap), LIV or TBEV

Fig. 4

Pathogen-specific effect on biological processes in IRE/CTVM20 cells following infection with tick-borne pathogens at 72 and 120 hpi, as demonstrated by differential expression of (a) toll genes ISCW022740, ISCW007724, ISCW017724 and ISCW007727, and (b) MyD88 gene ISCW008802, where A. phagocytophilum, LIV and TBEV are represented by light grey bars, dark grey bars and black bars respectively. Statistical significance denoted by *P < 0.05; **P < 0.01