Immunohistochemical localization of murine alpha 1-pregnancy-associated protein (alpha 1-PAP) in non-pregnant females: a comparative study with human pregnancy-associated alpha 2-glycoprotein (alpha 2-PAG).

A murine pregnancy-associated protein (alpha 1-PAP) was demonstrated by immunoperoxidase (PAP) staining within Ia positive cells in gut-associated lymphoid tissue and intestinal mucosae of normal female MF1 but not C57BL/10 mice. These immunohistochemical findings reflect the differences in serum alpha 1-PAP concentrations between the two strains, being 50-fold higher in the MF1 females. Staining for alpha 1-PAP was also detected within perivascular and periportal hepatocytes of MF1 mice. Using a combined indirect immunoperoxidase (PAP)/direct immunofluorescence procedure, cytoplasmic alpha 1-PAP was demonstrated in a proportion of plasma cells secreting the IgA isotype. In contrast with human pregnancy-associated alpha 2-glycoprotein (alpha 2-PAG) which was observed in the majority of IgA plasma cells within the lamina propria of human gastrointestinal mucosae, on average only 15% of IgA producers in corresponding mouse tissue were positive for alpha 1-PAP. As with the localization of alpha 2-PAG, no other classes of Ig-producing cells stained for alpha 1-PAP. These observations strengthen recent proposals that alpha 1-PAP is a murine analogue of human alpha 2-PAG, a glycoprotein with immunosuppressive properties.