Title: The Role of GM-CSF/IL-3/IL-5 Receptor Common β Subunit (CBS) in Hematopoietic Stem and Progenitor Cell (HSPC) Expansion, Monocytosis and Atherosclerosis
Description: Atherosclerosis is caused by the recruitment of monocytes to the subendothelial space where they develop into macrophage foam cells and give rise to atherosclerotic plaques. Elevated numbers of blood monocytes are strongly associated with coronary heart disease likely because increased numbers of monocytes drive their infiltration into atherosclerotic plaques. Studies using animal models of atherosclerosis including Apolipo protein E knockout (Apoe-/-) mice and ATP binding cassette transporter A1(ABCA1) and ATP binding cassette transporter G1 (ABCG1) knockout (Abca1-/-Abcg1-/-) mice demonstrated that cellular cholesterol retention leads to Hematopoietic Stem and Progenitor Cell (HSPC) expansion that underlies monocytosis and neutrophilia, coinciding with increased cell surface expression of the common β subunit of the Granulocyte-macrophage colony-stimulating factor (GM-CSF) /Interleukine 3 (IL-3 receptor (CBS) in the bone marrow HSPCs and increased GM-CSF production in the spleen. These observations suggest that CBS signalling could contribute to atherogenesis by promoting myeloid expansion.

To investigate the role of CBS in the cholesterol driven HSPC expansion and monocytosis during atherogenesis, we generated Apoe-/-Cbs-/- mice through cross-breeding and transplanted Low-density lipoprotein (LDL) receptor knockout (Ldlr-/-) mice with bone marrow from Apoe-/-Cbs-/- or Apoe-/- mice followed by western type diet (WTD) feeding. To confirm that any potential effect of CBS deficiency is specific to the context of atherosclerosis associated monocytosis, we also transplanted Ldlr-/- mice with wild type (WT) or Cbs-/- bone marrow. Despite similar levels of plasma cholesterol among all groups, the Apoe-/-Cbs-/- bone marrow transplanted mice had fewer circulating monocytes and neutrophils, accompanied by reduced HSPC proliferation in both the bone marrow and the spleen. The spleen has been identified as an important site for extramedullary production of monocytes during atherosclerosis and acute inflammatory responses. To investigate the role of CBS in extramedullary monocytosis during atherogenesis, we compared WT and Apoe-/- mice fed on a WTD and found increased GM-CSF-producing Innate Response Activator (IRA) B cells in the spleen of Apoe-/- mice, reflecting increased CBS level on IRA B cells and peritoneal B1a cells that are precursors of IRA B cells.

CBS deficiency dramatically reduced the number and proliferative activity of IRA B cells in the spleen, suggesting that CBS mediates enhanced GM-CSF production in the spleen by promoting IRA B cell expansion. Despite the significant reduction of monocyte level, Apoe-/-Cbs-/- bone marrow transplanted mice showed similar levels of atherosclerotic lesion compared with Apoe-/- bone marrow transplanted ones. A closer examination of the lesions revealed decreased macrophage content but increased necrotic core formation in the Apoe-/-Cbs-/- bone marrow recipients. This could be explained by decreased dendritic cells (DCs) and Regulatory T cells (Tregs) in the spleen, which are known to exert atheroprotective effects through anti-inflammatory cytokines. Another contributing factor appeared to be the decreased expression of ABCG1, an ABC transporter that protects against macrophage apoptosis by maintaining cholesterol homeostasis. Taken together, our findings indicate that CBS plays a critical role in promoting monocytosis during atherogenesis by mediating HSPC proliferation in the bone marrow and GM-CSF production in the spleen through IRA B cell expansion, and these events likely contribute to macrophage accumulation in the atherosclerosis lesions.

The effect of CBS on atherogenesis is complex since it affects the development of multiple cell types that exert opposing roles during atherosclerosis. To further elucidate the relation between CBS, monocytosis and atherosclerosis, a cell- specific knock out approach might be informative.