Role of newly synthesized MHC class II molecules in antigen-specific antigen presentation by B cells.

Using a B lymphoma, A20-HL, bearing IgM receptors for TNP, we have shown that presentation of an Ag taken up through the receptors is highly sensitive, whereas that of an Ag taken up nonspecifically is resistant to inhibition of protein synthesis or protein transport from the endoplasmic reticulum. To analyze the difference, we have examined the effect of protein synthesis inhibition on A20-HL cells in terms of internalization and fragmentation of a specific Ag, TNP-OVA, and distribution of MHC class II molecules. Inhibition of protein synthesis in A20-HL cells with emetine, an irreversible protein synthesis inhibitor, did not decrease the surface expression of anti-TNP receptors, or the kinetics of internalization of 125I-TNP-OVA. To detect fragmentation of TNP-OVA, A20-HL cells were incubated at 37 degrees C in the presence of 125I-TNP-OVA, and the cell lysate was analyzed in SDS-PAGE. The number of detectable fragments increased with the incubation period, and inhibition of protein synthesis did not change the electrophoretic pattern. Expression of MHC class II molecules on the surface of A20-HL cells was not affected by inhibition of protein synthesis. However, intracellular MHC class II molecules markedly decreased in amount in the emetine-treated cells. Thus, presentation of an Ag taken up through Ag receptors seems to be dependent on intracellular MHC class II molecules, whereas that of an Ag taken up nonspecifically does not, suggesting that the Ag-processing pathway in B cells for a specific Ag is different from that for a nonspecific one, at least partly.