CD2-iCre mice reveal a role for ABCB7 in B cell development and bone marrow homeostasis

Abstract

Abstract ABCB7 is a mitochondrial transporter that exports iron-sulfur (Fe-S) cluster intermediates from the mitochondria to the cytoplasm. Using CD2-iCre ABCB7 cKO mice, we found ABCB7 is required for early B cell development. These mice have a severe block in pro-B cell development at the Fr. C to Fr. C′ transition, with reduced expression of intracellular μ and pre-BCR. CD19-cre and CD23-cre ABCB7 cKO mice have normal B cell development and peripheral B cell populations, indicating ABCB7 is required only during early B cell development. We observed reduced expression of Runx1, Ikaros, Foxo1, E47 (E2A), and Pax5 in CD2-iCre ABCB7 cKO pro-B cells. Interestingly, we also observed decreased BACH2 expression, which indicates elevated heme synthesis is occurring in the absence of ABCB7. We hypothesize that in the absence of ABCB7, developing B cells enhance heme synthesis to dispose of accumulating mitochondrial iron. Using a hen egg lysozyme BCR transgene, we were able to rescue B cell development in CD2-iCre ABCB7 cKO mice. Intriguingly, at the age of 5–8 weeks old, CD2-iCre ABCB7 cKO mice become lethargic and hunched. CBCs reveal an increase in peripheral blood neutrophils. Both the spleen and bone marrow become highly infiltrated by CD11b+Ly6Glow/+Ly6C− cells that are immature and mature neutrophils. As other knockout mice with pro-B cell blocks do not have neutrophilia, we considered if increased iron or heme is responsible for our observation. As heme can stimulate TLR4, we propose that excess heme from developing ABCB7-deficent B cells activates macrophages. In support of this, hemin induced GM-CSF production in BMDM cultures. We are currently exploring the mechanism by which excess heme production can stimulate the production of neutrophils.