Characterization of one alpha 2-macroglobulin with anticomplementary activity from pregnant women

In 15 pregnant women during the third term of pregnancy, the immunomodulatory property of alpha 2-macroglobulin (alpha 2M) was initially detected by measuring the inhibitory effect on immune complement-dependent haemolysis of serum alpha 2M fractions obtained by gel filtration. By a two-step chromatography procedure consisting of gel filtration followed by anion-exchange chromatography, different sub-forms of alpha 2M in serum were separated. Amongst them, it was shown that the inhibition of complement activity was almost exclusively linked to one particular subform. Additional studies revealed that the observed effect was not due to proteases bound to alpha 2M during clotting since, by using protease-specific inhibitors, no change was observed in complement inhibition. This subform, though present at very low levels in control sera, appeared in strikingly increased amounts during the third trimester of pregnancy (35 mg/l) and comprised between 3 and 5% of the total alpha 2M. Results show that the increase of alpha 2M anticomplementary activity is linked to the increase in alpha 2M levels in serum.