Immunohistochemical diagnosis of Toxoplasma gondii: potential for cross-reactivity with Neospora caninum

A monoclonal antibody (MAB 3F5) was compared with a commercial polyclonal antibody for specificity for Toxoplasma gondii and cross-reactivity with Neospora caninum in paraffin-embedded tissue sections. Both antibody preparations reliably recognized T. gondii tachyzoites (RH isolate) in animal tissues but did not always label tissue cysts (ME-49 isolate). The commercial antibody strongly cross-reacted with N. caninum tachyzoites, but MAB 3F5 did not when the immunoperoxidase, immunofluorescence, or immunogold procedures were employed, nor did it cross-react with other apicomplexans, e.g., Isospora suis, Eimeria bovis, Sarcocystis cruzi, Hammondia hammondi, or Caryospora bigenetica. Immunoelectron microscopy revealed that MAB 3F5 bound to dense granules in extracellular T. gondii tachyzoites. In western blots of T. gondii tachyzoites, a major band at 38 kDa and a minor band at 32 kDa were labeled by MAB 3F5, but no labeling of the proteins of N. caninum tachyzoites or uninfected host cells occurred at the ultrastructural level or in immunoblots