Studies on the mechanism of solubilization of immune precipitates by serum.

Abstract

Antigen (Ag)-antibody (Ab) aggregates prepared with several different antigens are solubilized by fresh serum at 37 degrees C (complex-release activity of serum or CRA). The rate of solubilization varies in different systems and is strongly influenced by the affinity of Ab for the Ag in the immune precipitate. With a given Ag-Ab precipitate, the maximum amount of complex that can be solubilized by individual sera is independent of the initial concentration of complexes and cannot be increased by prolonged incubation. CRA occurs in the absence of C2 and C4, but not in the absence of C3 and factor B of the properdin pathway. Addition of C2 to C2-deficient serum or C4 to C4-deficient serum enhances CRA. Solubilization does not involve extensive degradation of the complexed antibody, as might be detected by acrylamide gel electrophoresis of released antibody after reduction and alkylation to separate H and L chains. Immune precipitates can also be solubilized by incubation with monovalent fragments (Fab or Fab') of antibodies against determinants of the Ab molecules in the immune precipitate. In contrast, F(ab')2 fragments decrease the solubility of the immune precipitates. In view of these findings, we propose that CRA is mediated by the binding of functionally monovalent C fragments (C3 and C4) onto Ab molecules in the precipitates.

Decreased capacity to solubilize immune complexes in sera from patients with systemic lupus erythematosus

“…These results are consistent with the observation that both complement pathways are involved in systemic lupus erythematosus. The observation that, in some cases, the levels of C4 were decreased while the solubilization capacity of these sera remained within normal limits is in agreement with the nonessential role of the classical pathway in the solubilization phenomenon (14,21).…”

“…The solubilization appears to be the result of a particular dissociation and subsequent decrease in the size of complexes after fixation of C3. The integrity of the alternative pathway is required for the production of this phenomenon (14,15). The in vivo significance of this phenomenon is not well known, but there is evidence that immune complexes play an important role in the development of tissue lesions in animal and human diseases (16-1 8).…”

The internal thioester and the covalent binding properties of the complement proteins C3 and C4

“…This was interpreted as indicating that the interaction between activated C3 fragments and the target surface components was a very strong hydrophobic one. Another phenomenon, which was not addressed in a coherent manner, was the means by which C3b could bind to so wide a range of unrelated surfaces, including cell membranes Dalmasso & MullerEberhard, 1967), bacterial cell wall components (Johnston et al, 1969), immune aggregates (Theofilopoulos et al, 1974;Czop & Nussenzweig, 1976), zymosan (a large mannan complex from yeast cell walls) (Nicholson et al, 1974), and the artificial particle Sepharose (Goldstein et al, 1976). The key experiment, which provided the first evidence for the covalent binding of C3, was the analysis, by SDS-PAGE, of the membrane polypeptides of sheep erythrocytes after they had been treated sequentially with antibody, CI, C4, C2, and '251-labeled C3.…”

Modulation of Renal Disease in MRL/lpr Mice Genetically Deficient in the Alternative Complement Pathway Factor B

“…cis elements responsible for the constitutive and regulated expression of Bf are located in the C2-Bf intergenic region (4). Factor B not only is a component of the alternative complement pathway but also serves as a B cell growth factor (12,13), stimulates mononuclear cell cytotoxicity (14,15), induces macrophage spreading (16), and solubilizes immune complexes (ICs) (17)(18)(19). Bf is found in glomeruli in immune complex glomerulonephritis (20), is produced by glomeruli during inflammation (20), and forms split products (5).…”