Abstract
TGF‐β1 was examined for the ability to regulate Ag‐presentation by B cells, using A20‐HL B lymphoma cells bearing TNP‐specific IgM receptors. Treatment of A20‐HL cells with TGF‐β1 at 1 ng/ml, a concentration that inhibited proliferation, enhanced presentation of Ag internalized via surface IgM (sIgM), but not via fluid‐phase pinocytosis. TGF‐β1‐treatment slightly enhanced surface expression of sIgM, but not of MHC class II molecules. The treatment accelerated recovery of sIgM expression after its removal by ligation with TNP‐OVA, and induced prolonged intracellular residence of TNP‐OVA internalized via sIgM, which co‐localized with intracellular MHC class II molecules. TGF‐β1‐treatment increased accumulation of newly synthesized intracellular MHC class II molecules that were localized in compartments positive for lysosome‐associated membrane protein 1, although cellular protein synthesis was decreased by the treatment. The accumulated intracellular MHC class II molecules were triggered to the cell surface by ligation of sIgM. Finally, TGF‐β1‐treatment induced Igα‐phosphorylation in response to lower concentrations of TNP‐OVA. On the basis of these findings, we conclude that TGF‐β1‐treatment of A20‐HL cells selectively enhances the ability to present Ag internalized via sIgM, not via fluid‐phase pinocytosis, through accelerating sIgM recovery, increasing accumulation of intracellular MHC class II molecules and enhancing the ability of sIgM ligation to induce Igα‐phosphorylation.