Characterization Of IgA Research Articles

Characterization of IgA and IgM binding and internalization by surface-expressed human Fcα/μ receptor

The Fcα/μ receptor (Fcα/μR) is an unusual Fc receptor in that it binds to two different antibody isotypes, IgA and IgM. This receptor is of interest because it is thought to be involved in the capture of IgA- and IgM-immune complexes and antigen presentation. To further characterize this receptor, we were able to stably express human Fcα/μR on the surface of the 293T cell line. Using this system, we determined the affinity of the interactions of the receptor with IgA and IgM, which led to novel insights including the important finding that IgM polymers can bind to human Fcα/μR in the absence of J chain. This is in contrast to the polymeric immunoglobulin receptor (pIgR), which requires the presence of J chain to bind to polymeric IgA and IgM. The dissociation constants (Kd) of all of the different human IgA isotypes and allotypes for human Fcα/μR were determined, and we show that the N-linked glycans on IgA1 are not required for binding to the receptor. In addition, we demonstrate that IgA can be rapidly internalized by human Fcα/μR in the presence of cross-linking antibody.

Identification and characterization of IgA and Vicia villosa-binding T cell subsets in rheumatoid arthritis

Autoimmunity may be due to augmentation of immune responses by human CD8 cells which bind the lectin Vicia villosa (VV). We have investigated T cells in rheumatoid arthritis (RA) by double immunofluorescence flow cytometry, in order to assess VV-binding CD8 and CD4 cells from the peripheral blood and synovial fluid. A significant increase in CD8+VV adherent (P less than 0.0001) and CD4+VV adherent cells (P less than 0.001) was found in synovial fluid, as compared with peripheral blood from patients with RA. A significant increase in VV-binding CD8+ or CD4+ cells was, however, not found in the blood of patients with RA, as compared with controls. We suggest that the lack of VV-binding T cells separated from blood, in contrast to those from synovial fluid, may be due to an inhibiting agent expressing N-acetyl D-galactosamine. Indeed, IgA1 is rich in N-acetyl D-galactosamine, it inhibits VV binding to T cells and is significantly bound to CD8 cells (P less than 0.001). The IgA1 was then characterized and in about half the patients J chains and secretory component was found, suggesting that the IgA1 is of the polymeric and secretory variety. IgA bound to the T cells engaged the Fc alpha receptors and a significant decrease in the Fc alpha receptors was found in CD8 cells (P less than 0.0001) and CD4 cells (P less than 0.01). Desorption studies were then carried out on CD8 and CD4 cells which showed that a loss of cell-bound IgA1 was associated with an increase in VV binding. Conversely, adsorption of IgA to T cells was associated with a loss in VV binding. The results suggest that the failure of VV binding to CD8+ and CD4+ cells from peripheral blood of patients with RA can be ascribed to cell-bound IgA1. Cytophilic IgA1 may inhibit the function of CD8+VV binding cells, thereby preventing augmentation of the systemic immune response, consistent with the lack of extra-articular disease in these patients with RA.