Abstract 158: CD40L Regulates Hematopoietic Stem and Progenitor Cell Biology by Inducing Adhesion-Mediated Quiescence

Abstract

The self-renewal and differentiation capacity of hematopoietic stem and progenitor cells (HSPCs) is an important regulator of the way the mature immune system behaves and how it reacts upon challenges. The co-stimulatory protein CD40L is well-known for its role in immune activation and chronic inflammatory diseases, especially in atherosclerosis. We hypothesized that CD40L not only regulates mature immune cell activation, but also plays a role in the development of these cells by regulating HSPC biology. Here we show that CD40L deficiency increased the colony-forming potential of HSPCs by 62,1%. FACS analysis revealed that the bone marrow of CD40L -/- mice contained 34,4% more myeloid-biased (Lin-cKit+) progenitors, 147,2% more myeloid/lymphoid-biased (Lin-Sca-1+) progenitors and 46,6% more primitive (Lin-Sca-1+-cKit+) HSPCs. Competitive bone marrow transplantation demonstrated that the abscence of CD40L resulted in a 13.4% increased reconstitution of peripheral blood leukocytes. Lineage analysis showed an advantage towards the lymphoid lineage, as B cells were increased by 14.6%, T cells by 41.3%, and NKT cells by 50.9%. The adhesion of HSPCs on CD40L -/- bone marrow stroma was reduced by 58%. Cell cycle analysis revealed that CD40L -/- HSPCs were less quiescent compared to WT cells. CD40L -/- mice had delayed hematopoietic recovery and a 37.5% reduced survival rate after 5-FU treatment. CD40L -/- mice did not survive a bone marrow transplantation. All effects were independent from CD40, the classical receptor of CD40L. In conclusion, our data suggest that CD40L regulates adhesion-mediated quiescence of HSPCs. These results provide a novel CD40L-mediated mechanism of protecting the HSPC compartment against exhaustion during immune activation and chronic inflammatory disease like atherosclerosis, thereby securing long term hematopoiesis.