Bone Marrow Stromal Antigen 2 Facilitates Activation of Hematopoietic Stem Cell through the Induction of ERK Signaling
Authors: Florez Marcus A , Le Duy , Jeong Youngjae , Kain Bailee N. , Cabrea-Perez Javier , Park Dongsu , King Katherine Y. 
Published: November 2021

Abstract
Chronic infections are prevalent worldwide and contribute to bone marrow failure syndromes. Long-term homeostatic production of all blood cells is dependent on the ability of hematopoietic stem cells (HSCs) to remain mostly quiescent and, upon activation, must carefully balance between their ability to self-renew and differentiate. However, infections trigger inflammatory cytokines, such as interferon-gamma (IFNy), that facilitate HSC proliferative exhaustion by disrupting both quiescence and self-renewal, thereby contributing to cytopenias and bone marrow failure syndromes. Our previously published results indicate that the IFN-induced surface protein, Bone Marrow Stromal Antigen 2 (BST2), facilitates HSC activation, and the loss of BST2 protected against the depletion of HSCs during chronic infection. However, the mechanism by which BST2 impacts HSCs activation is unknown. BST2 is known to promote activation of intracellular signaling in both dendritic cells and cancer cell types. In particular BST2 is a known activator of the NFkB signaling pathway. We hypothesized that BST2 could promote activation of HSCs through the induction of intracellular signaling pathways.
Intracellular signaling pathways have been shown to be important for the activation of not only HSCs but also downstream immune cells. Indeed, the loss of BST2 has been shown to impair the function of T cells, NK cells, and antigen presenting cells in the presence of viral infections via impaired induction of NFkB signaling. However, there have been no studies on the role of BST2 during bacterial infections. To evaluate the role of BST2 in hematopoiesis during a bacterial infection, we obtained BST2KO mice and characterized hematopoietic populations in the presence or absence of Mycobacterium avium infection, a model bacterial infection that generates high levels of IFNy. We observed no changes in activation or production of cytokines from macrophage, NK, CD8, or neutrophil populations. Surprisingly we observed enhanced dendritic cell activation at day 15, 30, and 42 as measured by co-stimulatory molecules and MHC II expression. Additionally, we observed increased tetramer-specific T cell numbers early on during the infection. Interestingly, these results pair with decreased bacterial load at day 30 and day 42 despite having the similar spleen sizes. These results suggest that BST2 regulates the activation of dendritic cells, which may trigger more robust earlier T cell responses early in the course of infection.
Our previous work showed that the loss of BST2 protects against HSC activation by displacement of HSCs from CXCL12-abundant reticular (CAR) cells in the niche. To determine whether HSC activation is related to distancing from the vascular niche or increased proximity to an activated osteoblastic niche, we performed intravital imaging using osteocalcin-GFP reporter mice. We observed no change in proximity to the osteoblastic niche after administration of recombinant IFNy using either WT or BST2KO HSCs. These results suggest that the effects of BST2 may be more related to distancing from CAR cells than migration towards other niche locations.
To evaluate the mechanisms underlying BST2-dependent HSC activation, we performed RNA sequencing on WT versus BST2KO HSCs in the presence of IFNy. GSEA-pathway analysis of differentially expressed genes revealed that BST2KO HSCs have a decreased response to IFNy and diminished activation of the P13K-AKT-MTOR signaling pathways. Western blot analysis on hematopoietic stem cell progenitor cells (HSPCs) indicate a decrease in phosphorylation of the ERK signaling pathway in BST2KO HSPCs. These results were analogous to work performed in 32D cells in vitro, in which ERK-phosphorylation has been shown to be important for both HSC maintenance and proliferation. Future studies are aimed at assessing the impact of ERK inhibition on HSC activation during various inflammatory stresses.
Collectively our results show that the loss of BST2 in downstream immune cells surprisingly enhances dendritic cell and T cell activation in response to bacterial infection. BST2 enhances HSC activation through activation of downstream ERK signaling. Future targeting of BST2 and/or ERK signaling could have important consequences for patients that suffer from cytopenias, bone marrow failure, and engraftment failure due to high levels of inflammatory stimuli.

Disclosures
No relevant conflicts of interest to declare.