Identification of diagnostic antigens for South American Babesia caballi infections

Preparation of monoclonal antibodies against Bc48 and development of a rapid detection assay for infection with Babesia caballi in China

Babesia caballi (Nuttal, 1910) is one of the causative agents of equine piroplasmosis which causes economic losses to horse industry in China. There is an urgent need for rapid detection method for B. caballi infection in Xinjiang Province, China. To prepare monoclonal antibodies (mAbs) against Bc48 gene of B. caballi (Xinjiang local strains) and establish colloidal gold-immunochromatographic (ICT) assay for diagnosis of the disease, recombinant Bc48 was expressed and purified from Escherichia coli. With purified Bc48 as immunogen in mice, three hybridoma cells named 11F4, 1H2 and 7F4 secreting mAbs against Bc48 of B. caballi were obtained, which showed strong reaction with recombinant Bc48 and Bc48 gene transfected cells. Furthermore, colloidal gold labelled ICT assay based on purified Bc48 recombinant antigen and its mAb was developed. The ICT assay showed high sensitivity and specificity and no cross-reaction with Theileria equi (Laveran, 1901). Total of 56 horse serum samples collected from Xinjiang were tested by ICT and compared with the detection by commercial ELISA kit. The results showed that 32 out of 56 serum samples were positive by ICT and 33 were positive by ELISA. ICT assay had high coincidence (98%) to the reference ELISA kit. mAbs and ICT developed in this study could be provided as an efficient diagnosis tool for infection with B. caballi in horse in Xinjiang area.

Detection of Babesia caballi infection by enzyme-linked immunosorbent assay using recombinant 48-kDa merozoite rhoptry protein

The 48-kDa Babesia caballi merozoite rhoptry protein was expressed using a pGEX4T expression vector in Escherichia coli as glutathione S-transferase fusion protein (GST-BC48), and the expressed GST-BC48 was used in an ELISA to detect specific antibodies in serum samples. No cross-reaction was observed with sera from horses experimentally infected with Babesia equi. GST-BC48 ELISA was a highly sensitive and specific test when compared with the CFT. A total of 209 horse sera obtained from Central Mongolia were examined with the GST-BC48 ELISA and 46.4% (97/209) were found to be seropositive for B. caballi, suggesting that the GST-BC48 ELISA can be successfully used for both quarantine and epidemiological studies.

Protein analysis of Babesia caballi merozoites by two-dimensional polyacrylamide gel electrophoresis and western blotting

Babesia caballi merozoites were prepared by combining two improved methods of cultivation and purification of merozoites using Percoll-gradiation, and the protein compositions of merozoites were analyzed by two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting. The relative molecular masses of the major proteins and protein masses separated by electrophoresis were >94, 80-70, 50-45, 34-30, 30-28 and 18 kDa. By Western blotting, twelve proteins or protein groups were recognized by pooled sera from two horses experimentally infected with B. caballi. Among twelve proteins, five new proteins (54, 30-26, 24, and two 18 kDa) were identified, and the 48 kDa protein was revealed to consist of 2 components in the B. caballi merozoite. One protein (54 kDa) of B. caballi was also recognized by the pooled sera from two horses experimentally infected with B. equi.

Cloning and expression of a 48-kilodalton Babesia caballi merozoite rhoptry protein and potential use of the recombinant antigen in an enzyme-linked immunosorbent assay

A cDNA expression library prepared from Babesia caballi merozoite mRNA was screened with a monoclonal antibody BC11D against the rhoptry protein of B. caballi merozoite. A cDNA encoding a 48-kDa protein of B. caballi was cloned and designated BC48. The complete nucleotide sequence of the BC48 gene had 1,828 bp and was shown to contain no intron. Southern blotting analysis indicated that the BC48 gene contained more than two copies in the B. caballi genome. Computer analysis suggested that this sequence contained an open reading frame of 1,374 bp with a coding capacity of approximately 52 kDa. The recombinant protein expressed by the vaccinia virus vector in horse cells had an apparent molecular mass of 48 kDa, which was the same as that of the native B. caballi 48-kDa protein. Moreover, recombinant proteins expressed by the pGEX4T expression vector in Escherichia coli as glutathione S-transferase fusion proteins were used for antigen in an enzyme-linked immunosorbent assay (ELISA). The ELISA was able to differentiate very clearly between B. caballi-infected horse sera and B. equi-infected horse sera or noninfected normal horse sera. These results suggest that this simple and highly sensitive test might be applicable to the detection of B. caballi-infected horses in the field.

Inhibitory effect of monoclonal antibodies on the growth of Babesia caballi

Monoclonal antibodies (mAbs) were produced against Babesia caballi (USDA strain) to define a species-specific antigen for use in diagnosis and vaccine development. Eight positive clones of B. caballi mAbs determined by indirect immunofluorescent antibody test were selected for purification and further characterisation. Confocal laser microscopy showed that the antigens recognised by the mAbs were located on the surface/cytoplasm, central part, and/or anterior end of B. caballi parasites, with five different reactive patterns. These mAbs seemed to be species-specific, since they did not cross-react with Babesia equi-infected erythrocytes or uninfected erythrocytes. In Western blotting analysis, 18, 20, 34, 36, 48, and 155 kDa proteins of B. caballi merozoites were recognised by six different mAbs. When added to in vitro cultures, four of the mAbs significantly inhibited the in vitro growth of B. caballi parasites. These results provide a rationale for evaluating antigens for the development of diagnostic methods or vaccines.

Polyclonal antibody characterization of Babesia caballi antigens

In a previous study diagnostic B. caballi antigens with apparent molecular mass of 50 and 48 kDa were identified. Another antigen of 141 kDa was recognized by most but not all B. caballi sera tested. Here a further characterization of the three antigens is reported. Rabbits were vaccinated with gel-purified antigens and monospecific antibodies were obtained for the 141 and 48 kDa antigens. Antibodies raised against the 50 kDa antigen cross-reacted with the 48 kDa antigen, suggesting that these two antigens bear unique as well as common epitopes. After two-dimensional electrophoresis the 50 and 48 kDa antigens were present as horizontal bands over a pH range from approximately 5.0-7.0 with focused spots at a pH of 5.5 and 5.9, respectively. The 141 kDa antigen was not present after two-dimensional electrophoresis. None of the three antigens could be identified as a glycoprotein. Judging from the immunofluorescence antibody test staining pattern obtained with the rabbit sera the 141 kDa antigen is present on the surface of infected erythrocytes. The 50 and 48 kDa antigens are located in the parasite itself and probably not on the surface of infected erythrocytes. The punctate staining pattern observed with the 48 kDa antiserum suggests that this antigen might be located in or associated with the apical complex of the parasite.