Immunoelectron-microscopic localization of a proliferation-associated antigen Ki-67 in MCF-7 cells

Immunocytochemistry using the monoclonal antibody Ki-67 is a commonly used method to assess proliferative activity of malignant tumours, Ki-67 reacts with proliferating cells with an antigen, whose structure, function and exact locations are unknown. We studied the subcellular location of Ki-67 in MCF-7 cells using immunoelectron microscopy. In the interphase cells, Ki-67 immunoreactivity was localized in the nucleolus, mainly in the nucleolar cortex. In particular, areas of the granular component of the nucleolus were strongly stained. Weak spot-like nucleoplasmic immunostaining was also seen outside the nucleolus. During prophase Ki-67 antigen was localized on the surfaces of the condensed chromatin and during metaphase on the surface of the chromosomes. After cell division and prior to formation of new nucleoli, Ki-67 immunoreactivity was located in the nucleoplasm. Quantification of Ki-67 immunofluorescence signal by flow cytometry revealed highest Ki-67 levels in mitotic cells. The location of Ki-67 is very similar to certain recently described proteins of nucleolar preribosomes suggesting that Ki-67 may also be a component of the preribosomes.