Molecular Characterization and Protective Efficacy of a Novel Protein (EnSSB) Containing a Single-Stranded DNA-Binding Domain from Eimeria necatrix

SSB proteins play essential roles in DNA replication, recombination, and repair in bacteria, archaea, and eukarya. This study investigates the transcript levels, identification, expression and purification, subcellular localization, and immune protective potential of the SSB-like proteins of Eimeria necatrix (EnSSB), exploring its role in the development of E. necatrix and its potential as a candidate antigen for a subunit vaccine against avian coccidiosis. The level of EnSSB gene transcription was highest in unsporulated oocysts (UO), followed by gametocytes (GAM) (p < 0.05). The gene consisted of an open reading frame of 1488 nucleotides encoding a protein of 495 amino acid residues with a predicted molecular weight of 53.31 kDa. EnSSB contained a SSB domain with a conserved OB (oligonucleotide/oligosaccharide binding) fold. The molecular mass of the native protein, as determined by Western blot analysis, was ~58 kDa in second-generation merozoites (MZ-2) and UO. In addition to the 58 kDa band, four other bands (~98 kDa, ~82 kDa, ~36 kDa and ~28 kDa) were detected in GAM. No bands were detected in MZ-3. Indirect immunofluorescence and immuno-electron microscopy localized EnSSB in the cytoplasm of macrogametocytes but not in wall-forming bodies and oocyst wall. Animal challenge experiments demonstrated that rEnSSB elicited robust IgY responses, increased splenic T lymphocytes and body weight gain, reduced intestinal lesion scores and oocyst shedding, and presented anticoccidial index (ACI) more than 160. These findings not only offer a foundation for understanding the role of EnSSB protein in regulating the development of E. necatrix, but also present a potential protective antigen of E. necatrix for the development of a subunit vaccine against avian coccidiosis.