A method for exposing hidden antigenic sites in paraformaldehyde-fixed cultured cells, applied to initially unreactive antibodies

We describe a simple protocol that allows the retrieval of masked or hidden intracellular antigens in cultured cells. The protocol is based on the exposure of paraformaldehyde-fixed and Triton X-100-permeabilized cells to guanidine hydrochloride (GdnHCl). We used it for localization of different antigens in BHK-21 cells by immunofluorescence microscopy. Our results showed that five out of six antibodies, initially unreactive, became excellent localization tools when used in conjunction with GdnHCl. Denaturation of fixed cells with GdnHCl did not affect the overall cell architecture, when monitored with different organelle-specific antibodies. In two cases out of 17 the antigenic sites were lost after denaturation. However, this problem could be partially overcome by including low amounts of glutaraldehyde in the fixative. We think that this method could be generally useful in immunofluorescence localization studies, particularly in cases where the antibodies are known to react only with denatured antigens.