Figure 2.

XBP-1 activation is a differentiation-dependent event in B cells, and the lack of XBP-1 leads to IRE-1 upregulation. (A) XBP-1 deficiency does not lead to accumulation of misfolded proteins. Four-day LPS-stimulated XBP-1^WT/MD4 and XBP-1^KO/MD4 plasmablasts were treated with or without 30 μM thapsigargin for 3 h before lysis. Lysates were treated with the indicated concentrations of the cross-linker bis[sulfosuccinimidyl]suberate (BS³) and immunoblotted for calreticulin. (B) B cells purified from spleens of either XBP-1^WT/MD4 and XBP-1^KO/MD4 mice (upper three panels) or XBP-1^WT/μS−/− and XBP-1^KO/μS−/− mice (lower three panels) were cultured in LPS (20 μg/ml) to induce differentiation. Cell lysates were immunoblotted for XBP-1, p97 (AAA-ATPase) and actin. (C) XBP-1^WT/μS−/− and XBP-1^KO/μS−/− B cells were stimulated by LPS to induce differentiation. Lysates were immunoblotted for IRE-1, p97 and actin.