ves1αgenes expression is the major determinant ofBabesia bovis-infected erythrocytes cytoadhesion to endothelial cells
Authors: Hakimi Hassan , Yamagishi Junya , Sakaguchi Miako , Verocai Guilherme G. , Kawazu Shin-ichiro , Asada Masahito 
Published: September 2024

Babesia boviscauses the most pathogenic form of babesiosis in cattle, resulting in high mortality in naive adults. This parasite invades red blood cells (RBCs) within the bovine hosts where they multiply and produce clinical disease.Babesia bovisexports numerous proteins into invaded RBCs changing its properties. Thus, the infected RBCs (iRBCs) are capable to cytoadhere in the microvasculature of internal organs and brain, leading to respiratory distress, neurologic signs, and mortality. Variant Erythrocyte Surface Antigen 1 (VESA1) is one of those exported proteins byB. boviswhich represents a major virulence factor due to its central role in immune evasion by antigenic variation and intravascular parasite sequestration. VESA1 is a heterodimer protein encoded byves1αandves1βmultigene family and localized on the ridges, the focal point for cytoadhesion. To gain further insights into the molecular mechanisms of cytoadhesion ofB. bovis, we panned the parasites with bovine brain microvasculature endothelial cells, which resulted in obtaining several clones with different cytoadherence abilities. The transcriptome analysis of 2 high and 2 low cytoadherent clones revealed thatves1αsequences were diversified, likely resulting from genomic recombination. On the other hand,ves1βsequences were almost identical among these 4 clones. Insertion and expression ofves1αof a clone with high binding intoef-1αlocus of a low binging clone increased cytoadherence confirming the role ofves1αsuggested by our transcriptome data. Whole genome sequencing of cytoadherent clones revealed active locus ofves1on chromosome 2. These results suggest that VESA1a proteins encoded byves1αgenes determine the cytoadherence specificity and/or cytoadherence strength ofB. bovisand they are in the active site for recombination.

Author summary

Babesia bovisis an apicomplexan intraerythrocytic protozoan parasite which causes the most pathogenic form of babesiosis in cattle. This pathogenicity is the result of parasite multiplication and cytoadherence of infected red blood cells (iRBCs) in the microvasculature of brain and internal organs and is mediated byB. bovissurface exposed ligand, Variant Erythrocyte Surface Antigen 1 (VESA1). Here using parasite panning assay, transcriptomics, and genetic tools, we showed that VESA1a is the main determinant ofB. boviscytoadhesion. VESA1 are large hypervariable proteins (>100kDa) consisting of VESA1a and VESA1b subunits encoded byves1αandves1βmultigene family. PanningB. boviswith bovine brain endothelial cells resulted in obtaining cytoadherent parasite clones with different binding abilities. Comparative transcriptome analysis revealed diversification ofves1αsequences. Insertion and expression ofves1αof a clone with high-binding ability in the genome of a low-binding clone increased cytoadherence confirming the role ofves1α.Mapping RNA-seq on the genome of cytoadherent clones revealed the locus of active transcription and this locus was suggested to be the active site for recombination which promoted the production of variants ofves1αwith different binding abilities. Altogether, our results provide new insights intoB. boviscytoadhesion and VESA1 biology.