Differential sensitivity of IgA proteins of different subclasses and allotypes to reduction of disulfide bonds and digestion by streptococcal protease

Heterogeneity of binding of monoclonal IgA to staphylococcal protein A is related to the IgA polymerization state

The human IgA2-lambda myeloma protein Fel consists of covalent dimers and monomers which are partially self-associated. Affinity chromatography of this protein on staphylococcal protein A-Sepharose revealed that approximately 8% of the protein was retained and eluted by acid buffer. Although retained protein Fel was highly aggregated, in the presence of dissociating agents mostly monomeric form was found. Affinity rechromatography and electrophoresis of both fractions from affinity chromatography revealed that the retained fraction possessed substantially higher affinity for SpA than did the nonretained one. This could be due to the multivalency of protein Fel aggregates.

Fc receptor for IgA on erythrocytes

Fc receptor for human IgA was detected on rabbit, sheep and human ORh- red blood cells. This Fc alpha receptor was able to bind human secretory or myeloma IgA, IgA1 or IgA2 and alpha chains. The ligand binding was detected by complement mediated lysis and agglutination with specific anti-IgA serum. Blocking experiments showed the inability of Fab(IgA) fragment or of secretory component to inhibit the IgA binding.

Binding of human IgA myeloma proteins to protein A. Evidence for different binding mechanisms

The binding of fifteen IgA myeloma proteins to protein A was studied using affinity chromatography on protein A-Sepharose CL-4B. The percentage to which the proteins bound was variable from 1% to 11% with the exception of IgA(GED) with a binding capacity of 22%, and IgA(KLO) with a binding capacity of 84%. The binding of the proteins IgA(GED) and IgA(KLO) was studied further. IgA(GED) was a monomer, Iga(KLO) a polymer. The characteristics of the binding of these proteins were different. The protein A-reactive fraction of IgA(KLO), but not of IgA(GED) remained fully reactive on repeated protein A chromatography. Furthermore, the binding of IgA(GED) could be reduced to about 3% by either a decrease in pH to 4.5 or an increase in NaCl concentration to 2.0M, whereas the binding of IgA(KLO) was similarly reduced by a decrease in pH but its binding only reduced to half of the original value on a similar increase of NaCl concentration. In addition, F(ab')2 fragments of IgA(KLO), but not of IgA(GED), bind to protein A-Sepharose CL-4B, whereas IgA1 protease-produced Fab fragments of neither of the proteins did so, nor did pepsin-produced Fab' fragments. This suggests that the binding of F(ab')2 fragments of IgA(KLO) needs an intact hinge region.

A redox function for the Calpha2 domain of IgA immunoglobulin

IgA1 populations were reduced over a range of dithiothreitol concentrations and the products were examined by sodium dodecyl sulfate polyacrylamide gel electrophoresis and peptide mapping. The study showed that IgA was more resistant to reduction than other immunoglobulin classes (IgG, IgM). SDS-polyacrylamide gel electrophoresis showed that intersubunit bond and H-L bond were the most labile disulfide in polymeric and monomeric IgA, respectively. Peptide mapping revealed that the itrachain disulfide of the C alpha 2 domain was more or equally sensitive to the reduction than H-L and intersubunit bonds. Electrochemical experiments demonstrated that this bond had redox properties and the possibility involving disulfide exchange is discussed.

Studies of serum immunoglobulin binding to synovial fibroblast cell cultures from patients with rheumatoid arthritis

Using a sensitive 125I-protein A (PrA) binding assay to detect cell surface IgG, we have studied seven different synovial fibroblast cell cultures from patients with rheumatoid arthritis (RA). When these cultures were incubated in the presence of serum from 18 autologous and allogeneic RA patients (all seropositive), we were unable to detect significant IgG binding. Since IgM rheumatoid factor (RF) can block PrA binding, sera were absorbed with aggregated IgG to remove RF without affecting the results. Similar studies on three cell lines with seven rheumatoid sera were performed by antibody-dependent cell-mediated cytotoxicity. No significant cytotoxicity was observed. Since antibodies to collagen are present in rheumatoid sera, several cultures were incubated with ascorbic acid (12.5 microgram/ml) to optimize synthesis of cell surface collagen. These culture conditions did not affect serum immunoglobulin binding by the 125I-PrA assay. Thus, we can find no evidence for a direct humoral immune mediation of synovial proliferation in rheumatoid arthritis. These data do not support the hypothesis that the inflammatory process within the synovium of RA patients is an immunologic response to a fibroblast-associated antigen in the synovial membrane.