Authors: Nelson Curtis M, Herron Michael J, Wang Xin-Ru, Baldridge Gerald D, Oliver Jonathan D, Munderloh Ulrike G
Journal: Frontiers in veterinary science
Summary
# Editorial Summary: *Anaplasma phagocytophilum* Gene Expression in Equine and Human Infection *Anaplasma phagocytophilum*, the causative agent of equine granulocytic anaplasmosis, employs distinctly different molecular strategies depending on whether it infects mammalian immune cells or tick vectors—a finding with significant implications for understanding disease transmission and potential therapeutic targets. Curtis and colleagues used whole-genome microarray analysis to compare bacterial gene expression when the pathogen replicated within human neutrophil-like cells (HL-60), primary human granulocytes, and tick intestinal cells (ISE6), revealing that 122 genes were upregulated in mammalian cells versus tick cells, including numerous immunogenic surface proteins and secreted factors. Critically, approximately 29% of *Ap*'s genome comprises hypothetical proteins with unknown functions, yet these genes were disproportionately upregulated during infection initiation in both tick and host cells, with between 33–60% of differentially expressed genes belonging to this category and many predicted to be secreted or membrane-associated. The pathogen's ability to reprogram its transcriptome based on its cellular environment—mounting different molecular responses when binding versus replicating, and when transitioning between tick and mammalian hosts—underscores the sophisticated adaptation mechanisms that enable persistence and transmission. For equine practitioners, these findings suggest that targeting conserved hypothetical proteins involved in host cell invasion could offer novel avenues for intervention, whilst the distinct gene expression profiles between tick and mammalian infection highlight why tick control remains essential to breaking the transmission cycle of equine anaplasmosis.
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Practical Takeaways
- •Understanding Ap gene expression patterns in both tick vectors and mammalian hosts may inform future vaccine or therapeutic targets to prevent anaplasmosis in horses and other animals
- •Hypothetical proteins represent a significant knowledge gap in tick-borne pathogen biology and warrant investigation as potential virulence factors or host-interaction mediators
- •The differential gene expression between tick and mammalian infection environments suggests the pathogen employs distinct strategies for vector maintenance versus host infection
Key Findings
- •Ap upregulated 122 genes in human HL-60 cells versus ISE6 tick cells, including p44 paralogs and hypothetical proteins, with 47% predicted to be secreted or membrane-localized
- •Hypothetical genes constituting ~29% of the Ap genome played disproportionately central roles in infection establishment across both tick and mammalian cell models
- •Bacteria showed differential transcriptional responses depending on host cell origin, with ISE6-grown bacteria upregulating specific genes upon exposure to ISE6 cells versus HL-60 cells
- •Particular sets of hypothetical genes were consistently upregulated during bacterial binding and entry into both ISE6 and HL-60 cells, suggesting conserved infection mechanisms