Pre- and post-embedding immunoelectron microscopy of Ki-M1P immunoreactive germinal center macrophages using ultra-small gold probes with silver enhancement.

The Ki-M1P protein is primarily detected in cells deriving from the monocyte/macrophage cell lineage. The aim of this study was to investigate the occurrence of Ki-M1P immunoreactivity in germinal center macrophages by immunohistochemical and immunocytochemical staining techniques. Ultra-small (0.8 nm) gold probes combined with silver enhancement were used as a detection system for pre- and post-embedding immunostaining both at the light and electron microscopic level. Ki-M1P-positive macrophages were observed at a constant frequency in the germinal centers of the follicles throughout the tonsillar lymphatic tissue. The specific immunostaining was localized in the cytoplasm of these cells. Electron microscopic examination demonstrated the presence of abundant lysosomes in the cytoplasm, and some of the germinal center macrophages contained phagocytosed cells (tingible bodies) showing signs of various degrees of digestion. Ki-M1P immunoreactivity, as revealed by depositions of silver-enhanced ultra-small gold probes, was confined to the periphery of the lysosomes and tingible bodies. The results obtained demonstrate that the use of silver-enhanced ultra-small gold probes is a highly sensitive and specific detection system for pre- and post-embedding immunostaining of the Ki-M1P protein, and provides, in general, a flexible system for combined light and electron microscopic examination of tissue antigens. Furthermore, in the cytoplasm of the germinal center macrophages a spatial association between the Ki-M1P protein and lysosomes and tingible bodies was observed. These findings may indicate that the Ki-M1P protein is connected with phagocytosis and/or processes related to intracellular digestion in these cells.