The transcriptome of the lone star tick, Amblyomma americanum, reveals molecular changes in response to infection with the pathogen, Ehrlichia chaffeensis

The lone star tick, Amblyomma americanum, is an obligatory ectoparasite of many vertebrates and the primary vector of Ehrlichia chaffeensis, the causative agent of human monocytic ehrlichiosis. This study aimed to investigate the comparative transcriptomes of A. americanum underlying the processes of pathogen acquisition and of immunity towards the pathogen. Differential expression of the whole body transcripts in six different treatments were compared: females and males that were E. chaffeensis non-exposed, E. chaffeensis-exposed/uninfected, and E. chaffeensis-exposed/infected. The Trinity assembly pipeline produced 140,574 transcripts from trimmed and filtered total raw sequence reads (approximately 117M reads). The gold transcript set of the transcriptome data was established to minimize noise by retaining only transcripts homologous to official peptide sets of Ixodes scapularis and A. americanum ESTs and transcripts covered with high enough frequency from the raw data. Comparison of the gene ontology term enrichment analyses for the six groups tested here revealed an up-regulation of genes for defense responses against the pathogen and for the supply of intracellular Ca++ for pathogen proliferation in the pathogen-exposed ticks. Analyses of differential expression, focused on functional subcategories including immune, sialome, neuropeptides, and G protein-coupled receptor, revealed that E. chaffeensis-exposed ticks exhibited an upregulation of transcripts involved in the immune deficiency (IMD) pathway, antimicrobial peptides, Kunitz, an insulin-like peptide, and bursicon receptor over unexposed ones, while transcripts for metalloprotease were down-regulated in general. This study found that ticks exhibit enhanced expression of genes responsible for defense against E. chaffeensis.