A light and electron microscope investigation of the host-parasite relationship in the brains of mice with congenital toxoplasmosis.

Impregnation of the wall of intact tissue cysts in the brains of mice with congenital toxoplasmosis, with reduced silver salts and protargol silver suggested that it was composed, at least in part, of components derived from the neuronal cytoskeleton. Electron microscopy extended these observations and revealed that intact tissue cysts were separated from the extra-cellular compartment by a layer of neurofibrillae enclosed within the host cell membranes. Immunohistochemical staining confirmed that this layer contained neurofilament protein. Interior to this layer was a much convoluted parasitophorous vacuole membrane; exterior was the host cell membrane. In most cases, synaptic plates were noted on the outer plasma membrane. In no instance were tissue cysts observed either within neuroglial cells or in the absence of a host cell. Electron immunocytochemistry, using a rabbit polyclonal antiToxoplasma IgG as the primary layer in immunogold staining, revealed that Toxoplasma antigenwas widely distributed within the matrix of the cyst, being most concentrated in the proximity of the inner surface of the parasitophorous vacuole membrane. Relatively little Toxoplasma antigen was detected directly associated with cystozoites. Small amounts of antigen were detected directly associated with cystozoites, within host cell components exterior to the parasitophorous vacuole membrane and in the host neuropil, immediately adjacent to the tissue cyst. The inter-relationship between inflammatory lesions associated with small vessels, in close proximity to intact tissue cysts, suggest that there may be 'migration' of these cells, in an ordered sequence, towards the Toxoplasma antigen secreted from the tissue cysts