Release of Immune Complexes bound to Erythrocyte Complement Receptor (CR1), with Particular Reference to the Role of Factor I

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Abstract
The release of 125I‐bovine serum albumin (BSA)‐anti‐BSA immune complexes (IC) hound to human erythrocyte complement receptors (E‐CR1) was studied. IC were complement‐solubilized in normal human serum (NHS), and reacted with human erythrocytes at conditions optimal for binding of the IC to E‐CRI. E‐CRI‐bound IC could he released by the addition of NHS or purified factor I. Factor I‐deficient or I‐depleted serum mediated no release, and addition of purified factor 1 restored the release. Factor H was not required for the release of IC. The kinetics of IC release was influenced by the NHS concentration, the presence of EDTA, and the time of prior storage of the erythrocytes at 4°C. NHS (1:5 to 1:10) in the presence of EDTA caused nearly maximal release within 10–20 min at 37°C. In the absence of EDTA the NHS‐induced IC release was markedly slower. IC released within she first 30 min showed significant rebinding to new E. The release of IC was not associated with loss of the IC binding activity of E‐CRI. The NHS‐mediated release of IC could be inhibited by rabbit anti‐CRI and by a mixture of protease inhibitors. Release induced by purified factor 1 was also inhibited by protease inhibitors. The affinity of IC binding to E‐CRI was reduced alter cleavage of CRI‐bound C3b‐IC to iC3h‐IC by factor I.

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