CARBOXYPEPTIDASE-M IS IDENTICAL TO THE MAX.1 ANTIGEN AND ITS EXPRESSION IS ASSOCIATED WITH MONOCYTE TO MACROPHAGE DIFFERENTIATION

The two monoclonal antibodies MAX.1 and MAX.11 recognize cell surface antigens that are almost undetectable on monocytes but highly expresse d on differentiated macrophages. Biochemical characterization revealed that both antibodies detect the same 58-64-kDa glycoprotein anchored to the plasma membrane by glycosyl-phosphatidylinositol linkage. We pu rified the MAX.1/11 antigen by immunoaffinity chromatography using mon oclonal antibody MAX.11. The NH2-terminal amino acid sequence was dete rmined and turned out to be identical to the NH2-terminal sequence of the membrane-bound carboxypeptidase M. By precipitation with antibodie s MAX.1 and MAX.11, membrane preparations of macrophages and placental microvilli were almost completely depleted of enzyme activity, indica ting that the two antibodies indeed recognize carboxypeptidase M. Immu noreactivity of both antibodies correlates with the reported tissue di stribution of enzyme activity. Expression of carboxypeptidase M on mRN A level and enzymatic activity markedly increase during in vitro diffe rentiation of monocytes, according to the described increase in MAX.1 and MAX.11 antigen expression. Moreover, in vitro differentiated macro phages show the highest specific activity yet described in any tissue. In addition, carboxypeptidase M expression could be detected in HL-60 , U937, and THP-1 myeloid cell lines. Vitamin D-3-induced monocytic di fferentiation resulted in an increased carboxypeptidase M expression i n all three cell lines. Further studies are needed to elucidate the fu nctional role of carboxypeptidase M during monocytic differentiation a nd activation.