ANALYSIS OF MHC CLASS-II PRESENTATION OF PARTICULATE ANTIGENS BY B-LYMPHOCYTES

To generate Ab responses to most protein Ags, B cells must first degra de proteins in endocytic compartments and then display antigenic pepti des bound to MHC class II molecules. T helper lymphocytes recognize th ese complexes and stimulate the B cell to synthesize Ab. Although Ab p lay a key role in host defense against bacteria, it is believed that B cells are incapable of internalizing particulate Ags. However, we fin d that B lymphoblastoid cell lines and LPS-activated B lymphocytes can present particulate Ag up to 10(5)-fold more efficiently compared wit h soluble Ag. Moreover, particulate Ags are presented efficiently by u nstimulated B cells when they bind to surface Ig. In comparison to B c ells, macrophages in general presented particulate Ags 10-to 1000-fold more efficiently and could also present Ag from particles of a much w ider range of sizes. We document by ultrastructural and immunofluoresc ence analysis that B lymphoblastoid cell lines bind and internalize th ese particles. The internalization and presentation of the particulate Ag is inhibited by cytochalasin B. In contrast, a similar morphologic analysis of normal lymphocytes demonstrated that while Ag beads are b ound to the cell surface, they are internalized only rarely. These res ults suggest there may be both surface and intracellular pathways for the presentation of particulate Ags by B cells. Interestingly, for bot h macrophages and B cells, the epitopes generated from particulate and soluble Ags were not identical quantitatively or qualitatively, indic ating that there are differences in how these forms of Ag are processe d and presented.