The mechanisms of IgE uptake by human alveolar macrophages and a human B-lymphoblastoid cell line (Wil-2wt)

Human alveolar macrophages (HAM) internalized more IgE (81%) than human Wil-2wt B-lymphoblastoid cells (28%) suggesting a difference in the metabolic processing of the specific IgE receptor (CD23) or, alternatively, the presence of another functionally distinct receptor. The mannose receptor (MR), demonstrated to be present on the AM, may fulfil this role as IgE is heavily mannosylated and binds to a greater extent to concanavalin A (Con A) (which has specificity for oligomannose oligosaccharide chains) than other antibody isotypes. The hypothesis of a second IgE receptor was tested using mannan which is a competitive inhibitor of ligand binding to the MR and mannosylated bovine serum albumin (MBSA) which binds with avidity to the MR. Mannan (0.1 mg/ml) decreased internalized MBSA uptake in the HAM at 37-degrees suggesting the presence of the specific MR. In contrast, Wil-2wt cells did not bind MBSA. Mannan also reduced IgE uptake in the HAM at 37-degrees but had no effect on IgE uptake by Wil-2wt cells. Anti-CD23 monoclonal antibody (mAb) 135 also partially reduced membrane IgE uptake in HAM while completely inhibiting it by Wil-2wt cells. However, there did not appear to be competition for binding sites between IgE and MBSA in HAM. If only CD23 is involved in IgE uptake by HAM its function appears to be different to that in Wil-2wt cells. Definite involvement of the MR in IgE uptake will require further investigation as it may have an important role in allergic states.