Insights into tick-pathogen interactions - a single cell RNA sequencing approach of transcriptional changes during ehrlichial infection
Tick-borne diseases represent a significant threat to human and animal health worldwide. In the United States, the blacklegged tick, Ixodes scapularis (I. scapularis), serves as a competent vector for several bacterial pathogens, including Ehrlichia muris eauclairensis (EME). The I. scapularis embryonic cell line (ISE6) is a valuable tool for propagating tick-borne pathogens and studying tick-pathogen interactions. In this study, we examined the cellular complexity of ISE6 cells and their response to EME infection. Single-cell RNA sequencing revealed 15 distinct cell clusters present. Although ISE6 cells are heterogeneous, they do not display transcriptional similarity to any known tick tissues. Notably, this lack of similarity did not influence their susceptibility to EME infection. Our results demonstrated that EME infection induces time-dependent transcriptional changes in ISE6 cells: early infection is characterized by upregulation of genes associated with stress adaptation, mitochondrial function, and metabolic pathways, whereas late infection leads to broad downregulation of genes involved in the cell cycle, DNA replication, and cytoskeletal organization. These findings enhance our understanding of ehrlichial interactions with ISE6 cells and reinforce the utility of this cell line as a resource for isolating and propagating arthropod endosymbionts and tick-borne pathogens. IMPORTANCEThis study provides a single-cell resolution framework for interpreting tick cell line biology during infection with a medically relevant ehrlichial pathogen. Using scRNA-seq, we show that the I. scapularis embryonic-derived ISE6 cell line comprises multiple transcriptionally distinct cell states, yet these states do not map cleanly onto canonical tick tissue signatures, even when compared against a curated reference tissue atlas. Despite this heterogeneity, EME broadly infects ISE6 cell population, indicating that susceptibility is not restricted to a specific cell type. We further define a time-dependent arthropod vector response in which early infection is marked by activation of stress and metabolic adaptation response, followed by late-stage inhibition of key signaling, transcriptional, and proliferative pathways as bacterial burden increases. Together, these findings strengthen the biological interpretation of ISE6 as an in vitro model for tick-pathogen interactions and provide a resource for future mechanistic studies of ehrlichial persistence, replication, and vector competence.