Ultrastructural cytochemical demonstration of peroxidase-positive monocyte granules: an additional method for studying the origin of mononuclear cells in encephalitic lesions.

Unlike lymphocytes, blood monocytes possess in their cytoplasm peroxidase-positive (azurophil) granules (ppg) which largely correspond to the homonymous organelles of neutrophil granulocytes. We tested whether ppg, demonstrated cytochemically at the submicroscopic level, could serve as markers of monocyte-derived reactive mononuclear cells in encephalitic lesions. Samples of cerebrocortical tissue from adult albino mice with experimental yellow fever virus encephalitis were incubated in a medium containing diaminobenzidine and H2O2 for localization of peroxidatic activity. Mononuclear cells exhibiting ppg were found (1) in the lumen of brain venules, (2) in different stages of migration through the walls of such vessels, (3) in perivascular areas, (4) in the glioneuropil, either loosely scattered or forming small clusters, (5) in a satellite position to neurons, and (6) in leptomeningitic inflitrates. Several mononuclear elements harboring ppg had assumed an elongated, rod cell-like outline. Amongst the peroxidase-negative mononuclears were fully developed brain macrophages and elements showing morphologic features characteristic of activated lymphocytes. Most mononuclear cells without ppg resembled the peroxidase-reactive ones. The results of this study provide direct evidence in favor of a monocytic origin of, at least, numerous reactive mononuclear elements in encephalitic lesions. The approach followed in the present study is not suitable for quantitative investigations of the histogenesis of mononuclear cells responding to brain injuries, since emigrated blood monocytes rapidly lose their ppg, particularly, when they display enhanced phagocytic activity.