RON2, a novel gene in Babesia bigemina, contains conserved, immunodominant B-cell epitopes that induce antibodies that block merozoite invasion

Juan Mosqueda,Massaro Wilson Ueti,Miguel Angel Mercado-Uriostegui,Angelina Rodriguez,Ikuo Igarashi,Ruben Hernandez-Ortiz,Mario Hidalgo-Ruiz,Juan Alberto Ramos-Aragon,Diego Josimar Hernandez-Silva,Diana Alexandra Calvo-Olvera,Shin-Ichiro Kawazu

doi:10.1017/s0031182019001161

Abstract

Bovine babesiosis is the most important protozoan disease transmitted by ticks. In Plasmodium falciparum, another Apicomplexa protozoan, the interaction of rhoptry neck protein 2 (RON2) with apical membrane antigen-1 (AMA-1) has been described to have a key role in the invasion process. To date, RON2 has not been described in Babesia bigemina, the causal agent of bovine babesiosis in the Americas. In this work, we found a ron2 gene in the B. bigemina genome. RON2 encodes a protein that is 1351 amino acids long, has an identity of 64% (98% coverage) with RON2 of B. bovis and contains the CLAG domain, a conserved domain in Apicomplexa. B. bigemina ron2 is a single copy gene and it is transcribed and expressed in blood stages as determined by RT-PCR, Western blot, and confocal microscopy. Serum samples from B. bigemina-infected bovines were screened for the presence of RON2-specific antibodies, showing the recognition of conserved B-cell epitopes. Importantly, in vitro neutralization assays showed an inhibitory effect of RON2-specific antibodies on the red blood cell invasion by B. bigemina. Therefore, RON2 is a novel antigen in B. bigemina and contains conserved B-cell epitopes, which induce antibodies that inhibit merozoite invasion.

Highlights

Bovine babesiosis is the most important protozoan disease transmitted by ticks
In Plasmodium falciparum, apical membrane antigen-1 (AMA-1) is translocated onto the merozoite surface where it can interact with the rhoptry neck protein 2 (RON2), forming a structure known as a ‘moving junction’ (MJ), an irreversible step that commits the parasite to invasion
This complex is discharged towards the red blood cells (RBCs), and RON2 is integrated into the RBC membrane where it acts as an AMA-1 ligand on the parasite surface (Silvie et al, 2004; Shen and Sibley, 2012)

Summary

Introduction

Bovine babesiosis is the most important protozoan disease transmitted by ticks. It is caused by intraerythrocytic parasites of the genus Babesia that belong to the phylum Apicomplexa. It is postulated that formation of the MJ is initiated when RON2 is secreted from the rhoptries in a complex formed of RON4, 5, and 8 (Alexander et al, 2005; Straub et al, 2009; Besteiro et al, 2011) This complex is discharged towards the RBC, and RON2 is integrated into the RBC membrane where it acts as an AMA-1 ligand on the parasite surface (Silvie et al, 2004; Shen and Sibley, 2012). Blocking this interaction halts merozoite invasion, suggesting that RON2 may be a target for vaccine development

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