Glycosaminoglycans enhance complement hemolytic efficiency: Theoretical considerations for GAG-complement-saliva interactions

Abstract

When human serum is diluted and pre-incubated at 37†C in low ionic strength buffer (LIS, u = 0.07; made iso-osmotic with dextrose), a spontaneous activation of complement (C) is observed as determined by C4 and C3 electrophoretic conversion. In this paper it is postulated that most species of glycosaminoglycans (GAG) restricted non-specific fluid phase complement consumption induced by LIS, an effect which conserved complement and thereby enhanced the subsequent residual serum C mediated hemolytic activity. The capacity of glycosaminoglycans, to have modulated the hemolytic activity at low ionic strength, depended on the charge of the GAG species tested. In general, the GAG regulatory effects may have been due to GAG mediated restriction of spontaneous non-specific fluid phase C1 auto-activation, and/or restriction of activated C1 activity. Such effects would result in the subsequent reduction of the spontaneous fluid phase C4 and C3 consumption. Although the precise mechanisms responsible for the effects were not identified, it is speculated that the potentiation of Cl inhibitor function and direct effects on C1 might be involved. Overall, the relative specific activities of the glycosaminoglycans, on a weight basis, in mediating the fluid phase C regulatory effect were heparin > dermatan sulfate > chondroitin-6-sulfate > chondroitin-4-sulfate > hyaluronic acid and keratan sulfate. When much higher concns of heparin (≥ 0.2/ μg/ml) were used, complement mediated lysis of EA was inhibited, probably due to the direct inhibition of C1, even C1 which may have bound to the sensitized erythrocytes (EA). Results similar to that of heparin were obtained using > 1 mg/ml of dermatan sulfate or dextran sulfate. In contrast, pre-incubation of human serum in LIS with high concns (up to 10 mg/ml) of hyaluronic acid or chondroitin-4-sulfate, which are much less charged, continued to result only in the restriction of hemolytically non-specific (fluid phase) C consumption, resulting in a higher residual complement hemolytic activity. A theory is developed that the binding of polyionic GAG to C1 and to C1 INH may provide a charged local environment which simulates a relatively higher ionic strength. Chemical degradation of hyaluronic acid or chondroitin-4 or -6 sulfate resulted in lowering of this C modulating effect, indicative of the importance of the structural integrity of these charged glyco-saminoglycans. Interestingly, thermal treatment of hyaluronic acid which alters the exposure of surface charges, reversed its C conserving effect, however, similar heat treatments did not change the regulatory activity of the other glycosaminoglycans.