Expression of genes for heme metabolism in B and plasma cells. (A and B) The relative expression levels of HO-1 mRNA were determined at indicated developmental stages (A) or in B220^low pro-B and pre-B cells of indicated genotypes (B) by qPCR. (C-E) Expression of indicated mRNAs (C, CD91 and CD163; D, HRG-1; E, ferroportin) in splenic B cells was determined. Cells were treated with LPS for indicated periods with or without heme. Two-fold dilutions of cDNA were compared in panel C. ND, not detectable. (F) A model for the function of heme in B cells. Heme regulates plasma cell differentiation and HO-1 expression by binding to Bach2. Because HO-1 degrades heme, the regulatory loop maintains intracellular heme levels within a certain range.