Immunohistochemical definition of antigenic determinants of pregnancy-associated α2-glycoprotein (α2-PAG) using monoclonal antibodies

Human pregnancy‐associated α2‐glycoprotein (α2‐PAG) is a high molecular weight glycoprotein present in normal sera. The protein is present in high concentration in the sera of pregnant females and in abnormally low concentration in association with conditions connected with abnormalities of mucosal immunity. Indirect immunoperoxidase techniques using poly‐ and monoclonal antibodies were employed to identify pregnancy‐associated α2‐PAG in different tissues. Four monoclonal antibodies were selected from a battery of antibodies with defined specificities in order to ascertain reactivity with various epitopes of the antigen. The antibodies were applied to paraffin sections of breast, colon, salivary gland, and tonsil, and different fixation regimes were used in the preparation of the tissues. The polyclonal antibodies were found to stain plasma cells and epithelial lumina evenly in all the tissues included whereas the monoclonal antibodies were shown to stain certain components selectively. In breast and salivary glands, all four monoclonal antibodies could identify α2‐PAG, but in tonsil and colon, only two were reactive. This difference in epitope expression might reflect the internal processing of α2‐PAG, and lack of availability of certain epitopes may be indicative of functional blocking of certain domains.