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 huma n Wil-2wt B-lymphoblastoid cells (28%) suggesting a difference in the metabolic processing of the specific IgE receptor (CD23) or, alternati vely, the presence of another functionally distinct receptor. The mann ose receptor (MR), demonstrated to be present on the AM, may fulfil th is role as IgE is heavily mannosylated and binds to a greater extent t o concanavalin A (Con A) (which has specificity for oligomannose oligo saccharide chains) than other antibody isotypes. The hypothesis of a s econd IgE receptor was tested using mannan which is a competitive inhi bitor of ligand binding to the MR and mannosylated bovine serum albumi n (MBSA) which binds with avidity to the MR. Mannan (0.1 mg/ml) decrea sed internalized MBSA uptake in the HAM at 37-degrees suggesting the p resence of the specific MR. In contrast, Wil-2wt cells did not bind MB SA. 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 comp letely inhibiting it by Wil-2wt cells. However, there did not appear t o be competition for binding sites between IgE and MBSA in HAM. If onl y CD23 is involved in IgE uptake by HAM its function appears to be dif ferent 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.