Activation of the alternative pathway of complement: efficient fluid‐phase amplification by blockade of the regulatory complement protein β1H through sulfated polyanions

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Abstract
Current concepts of activation of the alternative pathway of complement (APC) focus on the central role of an amplification mechanism triggered by C3b which is covalently bound to the surface of activating substances. Using sulfated polyanions as model substances, an efficient fluid-phase activation of complement is demonstrated in contrast to solid-phase activation. It is shown that particulate high-molecular weight sulfated polyanions are capable of reversibly binding the guinea pig and human regulatory protein β1H. This fixation leads to an extensive activation of C3 and factor B because the regulatory function of β1H is blocked in the fluid-phase C3b-dependent amplification system of the APC. Addition of β1H to β1H-depleted C4-deficient guinea pig serum reconstitutes the physiological control mechanisms of the APC. Guinea pig β1H, purified to homogeneity, is described as a 160000 dalton protein of a single-chain structure. In addition, highly specific and sensitive test systems for β1H are described.

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