Prolonged exposure to proinflammatory conditions, such as during chronic infection, can lead to the development of bone marrow failure syndromes including aplastic anemia. Indeed, we have previously shown that chronic infection activates hematopoietic stem cells (HSCs), ultimately leading to bone marrow suppression due to an IFNgamma (IFNγ)-dependent depletion of HSCs. Defining the mechanisms behind inflammation mediated HSC loss may provide important insight into how inflammation affects hematopoiesis in many clinical conditions including infection, ageing, and myelodysplastic syndrome.
HSC quiescence is maintained in large part through interactions with the surrounding cells of the bone marrow niche. In particular, HSCs closely colocalize with perivascular stromal cells that secrete high levels of the chemokine Cxcl12 and other hematopoietic maintenance factors such as angiopoietin, IL7, and stem cell factor. We hypothesized that IFNγ-induced activation may affect the positioning of HSCs relative to supporting cells within the bone marrow microenvironment. We used 3-D intravital imaging to assess the localization of HSCs relative to Cxcl12-abundant reticulocytes (CAR cells). When CMTMR-stained HSCs were transplanted into Cxcl12-GFP mice we found that IFNγ treatment led to a distancing of HSCs away from quiescence-enforcing CAR cells. In contrast, TNF-alpha treatment, which does not induce HSC proliferation at the concentration used, resulted in no such relocalization. Furthermore, no displacement occurred in response to IFNγ treatment when Ifngr1-deficient HSCs were transplanted, suggesting a cell-autonomous mechanism of relocalization.
RNA expression analysis and chemotaxis assays showed that IFNγ-treated HSCs are not impaired in their ability to respond to Cxcl12 signaling, indicating that a change in Cxcl12-Cxcr4 interactions does not account for the movement of HSCs away from CAR cells. To further assess HSC-specific changes that may account for IFNγ-dependent relocalization, we performed gene expression analysis of HSCs from control or IFNγ-treated mice. We saw no change in the expression of common HSCs receptors that are thought to play a role in maintaining HSC quiescence, such as cKit, Cdh2, Mpl, Itgb1, Itbg2, Itga4, and Itga1. However, the surface protein, Bst2, also known as tetherin, was significantly upregulated in HSCs upon IFNγ stimulation. This was confirmed at both the RNA and protein level. A prior report identified Bst2 as a noncanonical E-selectin ligand. This finding is particularly interesting given that HSCs found in close proximity to E-selectin-expressing endothelial cells exist in an activated, proliferative state. Furthermore, the interaction between HSCs and E-selectin+ endothelial cells was previously noted to be independent of canonical E-selectin ligands such as Psgl1, CD44, or the Lewis family antigens. Thus we hypothesized that IFNγ-induced upregulation of surface Bst2 may facilitate proliferation by promoting HSC binding to E-selectin. Indeed, in vitro E-selectin binding assays showed that IFNγ treatment increased binding of progenitor cells to E-selectin, but not to the closely related adhesin, P-selectin.
Inflammation plays an important role in the development of many cancers including acute myeloid leukemia (AML), leading us to investigate if Bst2 upregulation is an important factor in AML biology. Indeed, we found that increased levels of Bst2 are associated with poor survival in AML patients. Further, murine studies have shown that leukemic blasts have greatly increased E-selectin binding capacity following transformation. Using two independent AML cell lines, we found that IFNγ treatment upregulates Bst2 expression and increases E-selectin binding capacity. These findings suggest a mechanism by which inflammation could drive cell proliferation in AML.
In summary, we have uncovered a potential new mechanism for IFNγ-induced HSC activation, whereby HSCs are relocalized in a Bst2-dependent manner from the quiescence-enforcing CAR niche to an active E-selectin-positive niche. Furthermore, we demonstrate that IFNγ-mediated Bst2 expression also occurs in AML, suggesting a mechanistic link between inflammation and AML progression. This study may open up new potential therapeutic avenues for the treatment of patients with chronic infection, inflammatory diseases, and cancer.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.