IMMUNOHISTOCHEMICAL DIAGNOSIS OF TOXOPLASMA-GONDII - POTENTIAL FOR CROSS-REACTIVITY WITH NEOSPORA-CANINUM

A monoclonal antibody (MAB 3F5) was compared with a commercial polyclo nal antibody for specificity for Toxoplasma gondii and cross-reactivit y with Neospora caninum in paraffin-embedded tissue sections. Both ant ibody preparations reliably recognized T. gondii tachyzoites (RW isola te) in animal tissues but did not always label tissue cysts (ME-49 iso late). The commercial antibody strongly cross-reacted with N. caninum tachyzoites, but MAB 3F5 did not when the immunoperoxidase, immunofluo rescence, or immunogold procedures were employed, nor did it cross-rea ct with other apicomplexans, e.g., Isospora suis, Eimeria bovis, Sarco cystis cruzi, Hammondia hammondi, or Caryospora bigenetica. Immunoelec tron microscopy revealed that MAB 3F5 bound to dense granules in extra cellular T. gondii tachyzoites. In western blots of T. gondii tachyzoi tes, 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 imm unoblots.