Binding of modified alpha-1-antichymotrypsin to mitogen-stimulated human lymphocyte membrane: a model for immune suppression.

Alpha-1-antichymotrypsin (ACT), an acute phase reactant protein elevated during acute inflammation, and its derivatives (asialo ACT and acid-exposed asialo ACT) were investigated their effect on lymphocyte proliferative responses, and evidence for binding to lymphocyte membranes as well as the characteristics of this binding were investigated. Acid-exposed asialo ACT significantly reduced 3H-thymidine incorporation into human peripheral lymphocytes stimulated by phytohemagglutinin (PHA) though native ACT could not inhibit the mitogen-induced lymphoproliferation and asialo ACT moderately inhibited it. In order to determine the interaction of ACT and its derivatives to lymphocyte membranes, the binding of 125I-labelled ACT and its derivatives to membranes of intact lymphocyte and extracted lymphocyte membranes was examined. The binding of 125I-labelled native ACT and asialo ACT to resting and PHA-stimulated lymphocyte membrane was low. And the binding of 125I-labelled acid-exposed asialo ACT to resting lymphocyte membrane was also low. However, when lymphocytes were stimulated by mitogens the binding of 125I-labelled acid-exposed asialo ACT increased significantly. The binding of 125I-labelled acid-exposed asialo ACT to the membrane extracted from PHA-stimulated lymphocytes was time-dependent and saturation was reached at 120 min at 37 degrees C. One mg of membrane could bind a maximum of approximately 83 pmol of acid-exposed asialo ACT with dissociation constant of 0.73 microM. Other unlabelled serum glycoproteins such alpha-1-antitrypsin, alpha-1-acid glycoprotein, transferrin, and proteinase inhibitors including chymostatin, leupeptin and soy bean trypsin inhibitor did not compete with 125I-labelled acid-exposed asialo ACT for binding sites in simultaneous competition assays.