Murine alpha-2-macroglobulin: localization on a subpopulation of macrophages

We have previously shown that mouse peritoneal macrophages synthesize and secrete alpha-2-macroglobulin (alpha 2M) in culture. We have now examined whether alpha 2M delineates a subpopulation of murine macrophages. Mouse alpha 2M was purified from plasma by gel filtration and immunoabsorption to remove IgM. Purified alpha 2M was an active proteinase inhibitor, had a pI of 4.8, and a relative molecular mass (Mr) of 765 kD which dropped to 194 kD on reduction. Antisera raised against mouse alpha 2M were not cross-reactive with guinea pig or human alpha 2M, or with antigens in guinea pig, human, bovine or equine serum. One to 18 per cent of freshly isolated bone marrow, resident peritoneal or induced peritoneal macrophages displayed immunoreactive cytoplasmic and surface alpha 2M as detected by single cell immunoperoxidase assay or flow cytometry. alpha 2M was not detected in or on thymocytes or bone marrow lymphocytes. In culture, bone marrow-derived macrophages displayed peak levels of cytoplasmic alpha 2M on day 7 and two peaks of surface alpha 2M on days 2-3 and 5-7. The fact that alpha 2M can function as an anti-proteinase, can modulate lymphokine production and is present on inflammatory macrophages suggests a regulatory role for macrophages bearing and secreting this molecule at tissue sites of inflammation.