Cultured human monocytes, granulocytes and a monoblastoid cell line (THP-1) synthesize and secrete immunosuppressive acidic protein (a type of alpha 1-acid glycoprotein).

Immunosuppressive acidic protein (IAP, pI 3.0) is a type of alpha 1-acid glycoprotein (alpha 1-AG). The secretion of IAP into the culture fluids of different subpopulations of human peripheral blood leukocytes was examined by a newly devised passive hemagglutination (PHA)-inhibition test. Human peripheral monocytes, an established monoblastoid cell line (THP-1) and peripheral granulocytes produced IAP. However, neither T nor B lymphocytes, nor lymphoblastoid cell lines induced by TCGF or EB virus respectively, produced IAP. The IAP concentration reached a maximum (215 ng/ml) in the culture fluids of peripheral monocytes (1 X 10(6)/ml) when monocytes were stimulated by the addition of either immune complex, carrageenan or endotoxin. The synthesis de novo and shedding of IAP by THP-1 were demonstrated by the immunoprecipitation of radioactive IAP in the culture fluids of [3H]leucine-labeled cells. SDS-polyacrylamide gel electrophoresis of the immunoprecipitates showed two peaks of radioactivity, one comigrated with authentic IAP at 50,000 daltons, and the other at 38,000 daltons, suggesting that two different forms of IAP (and/or alpha 1-AG) are produced from human monocytes.