Combination of monoclonal antibodies improves immunohistochemical diagnosis of Neospora caninum

Histological analysis is commonly used for a conclusive diagnosis of neosporosis. Immunohistochemistry (IHC) using monoclonal (mAb) and polyclonal (pAb) antibodies can improve diagnosis; however, the use of pAb may induce cross-reactivity with other related parasites. The aims of this study were to compare the performance of mAbs and their combinations with that of pAb in IHC and evaluate the usefulness of mAb to identify Neospora caninum infection in aborted bovine fetal tissues. For this purpose, mAbs targeting NcSRS2 (4.15.15) or NcGRA7 (4.11.5 and 1/24-12) and one pAb collected from a rabbit inoculated with N. caninum tachyzoites were tested by IHC. Artificial standardized tissue sections were prepared as positive controls using homogenized bovine brain spiked with cultured tachyzoites of N. caninum. The numbers of labeled parasites were counted in each positive control section. In addition, four equal proportional combinations of the mAbs were also analyzed in the IHC. Finally, the pAb and the best combination of mAbs obtained in the positive control experiments were tested with tissue sections of naturally-infected cattle. To confirm analytical specificity, mAbs and a pAb were tested with Toxoplasma gondii and Besnoitia besnoiti positive control slides and tissues sections from naturally infected cattle containing Sarcocystis spp. and B. besnoiti antigens. The mAb 4.15.15 detected 57% of the total parasites in sections while 4.11.5 and 1/24-12 were able to detect 49% and 41%, respectively. For the mAb combinations (I: 1/24-12 + 4.11.5, II: 1/24-12 + 4.15.15, III: 4.15.15 + 4.11.5, IV: 1/24-12 + 4.11.5 + 4.15.15), the detection capacity was 32.4%, 79.4%, 66.6% and 60.7% for each combination, respectively. The best mAb combination (1/24-12 and 4.15.15) and the pAb serum detected 100% (18/18) of naturally-infected animals. Sarcocystis spp. or B. besnoiti were not detected by mAb combinations in IHC, however the pAb cross-reacted with Sarcocystis spp. cysts. These results confirm the usefulness of mAb application in IHC to N. caninum.