Response of Leucine-Rich Repeat Domain-Containing Protein in Haemaphysalis longicornis to Babesia microti Infection and Its Ligand Identification.

Haemaphysalis longicornis is a blood-feeding hard tick known for transmitting a variety of pathogens, including Babesia How the parasites in the imbibed blood become anchored in the midgut of ticks is still unknown. Leucine-rich repeat domain (LRR)-containing protein, which is associated with the innate immune reaction and conserved in many species, has been detected in H. longicornis and has previously been indicated in inhibiting the growth of Babesia gibsoni However, the detailed mechanism is unknown. In this study, one of the ligands for LRR from H. longicornis (HlLRR) was identified in Babesia microti, designated BmActin, using glutathione transferase (GST) pulldown experiments and immunofluorescence assays. Moreover, RNA interference of HlLRR led to a decrease in the BmActin mRNA expression in the midgut of fully engorged ticks which fed on B. microti-infected mice. We also found that the expression level of the innate immune molecules in H. longicornis, defensin, antimicrobial peptides (AMPs), and lysozyme, were downregulated after the knockdown of HlLRR. However, subolesin expression was upregulated. These results indicate that HlLRR not only recognizes BmActin but may also modulate innate immunity in ticks to influence Babesia growth, which will further benefit the development of anti-Babesia vaccines or drugs.