Background:Cryptosporidium parvum is a parasitic protozoan, which develops inside the parasitophorous vacuole between the microvillous membrane of enterocytes in a wide variety of vertebrates, including human. Cryptosporidiosis is an important parasite that causes of severe diseases in the immunodeficient people especially AIDS patients. Cryptosporidiosis has been also reported as a common serious primary cause of outbreaks of diarrhea in newborn calves. The aim of this study was to establishcharacterization of a recombinant P23 as an immunogenic antigen in domestic isolates of C. parvum. Methods: Stool specimens of naturally infected calves were used for molecular detection of C. parvum by nested PCR. To obtain the recombinant P23 protein, totalmRNA was isolated from oocyst of C. parvum, and cDNA was synthesized. The cDNA was then amplified using specific primers for P23 gene. The RT-PCR product was cloned in pGEX-5X-2 vector and sequenced. The blast algorithm was used for homologous gene search and then P23 structure was predicted based on homology modeling and threading algorithm. The Immune Epitope Database (IEDB) was utilized for epitope identification and finally structural and functional annotations of recombinant P23 were compared with other homologous proteins. Results: The sequence analysis showed 98%-100% nucleotide sequence identityand 97%-100%arnino acid homologyto the known P23 sequences in GenBank. Epitope prediction of P23 reveled several peptide sites that can be a good candidate site for vaccine design. Conclusion: Since P23 is an immunodominant surface glycoprotein expressed in the early phase of infection and theimmunogenic epitopes are found in its residual chain of amino acid sequence, the recombinant P23 could be recommended as a favorable candidate for chemotherapy or prevention against C. parvuminfection.

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