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Neospora caninum, the causative agent of neosporosis, is a recently identified apicomplexan parasite which is structurally and biologically closely related to, but antigenically distinct from, Toxoplasma gondii. Molecules associated with the surfaces of N. caninum tachyzoites are likely to participate in the host cell entry process, could be involved in the interaction of the parasite with the immune system, and they could influence the pathogenesis of neosporosis. Isolated N. caninum tachyzoites were extracted with the non-ionic detergent Triton X-114 and were further analysed using a polyclonal anti-N. caninum antiserum. Immunoblots revealed several reactive bands, 1 of which represented a glycoprotein of approximately 36 kDa (Nc-p36). This molecule was present in 2 isolates of Neospora (NC-1 and Liverpool), but was absent in Toxoplasma (RH-strain) tachyzoites. Immunofluorescence and pre-embedding immunogold transmission electron microscopy employing affinity-purified anti-Nc-p36 antibodies showed that the Nc-p36 is a cell surface-associated protein. Immunogold on-section labelling of LR-White-embedded parasites, fixed prior and at defined time-points after host cell entry, demonstrated the presence of this molecule on the surface as well as within the dense granules of N. caninum tachyzoites.

Morphologically, the tachyzoites and the tissue cysts of Neospora caninum are difficult to distinguish from those of other cyst-forming apicomplexan parasites such as Toxoplasma gondii. Several stage-specific antigens have been identified in T. gondii tachyzoites and bradyzoites, and respective antibodies are useful tools for discriminating between the 2 stages during tachyzoite-bradyzoite interconversion in T. gondii infections. Whereas several cell surface- and dense granule-associated proteins have been identified and characterized in N. caninum tachyzoites, not much is known about antigenic components expressed in N. caninum bradyzoites. In this study, the differential expression of the 2 N. caninum surface proteins Nc-p43 and Nc-p36 and the dense granule protein Nc-p33 (NCDG1) within tachyzoites and bradyzoites of N. caninum has been investigated.

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