The origin and characteristics of MAb 5.4 specific for CEACAM1 and MAb 188.A2 specific for rat transferrin receptor have been described previously [33, 34] . Monoclonal antibody 9.2 (MAb 9.2) was provided by Drs. Werner Reutter and Oliver Baum at the Free University, Berlin, Germany [35] . Mouse anti-human HLA antibody was purchased from Sigma-Aldrich (Sigma-Aldrich Co., St. Louis, MO). The preparation of polyclonal rabbit antipeptide antibodies specific for the CEACAM1-4L and CEA-CAM1-4S has been previously described [36] . The secondary antibodies used for indirect immunofluorescence labeling were Alexa-488 conjugated goat anti-mouse and goat anti-mouse-HRP conjugated secondary antibody (Invitrogen, Carlsbad, CA, USA).