Imaging the Vascular Bone Marrow Niche During Inflammatory Stress [New Methods]

Rationale: Inflammatory stress induced by exposure to bacterial lipopolysaccharide (LPS) causes hematopoietic stem cell (HSC) expansion in the bone marrow niche, generating a cellular immune response. As an integral component of the hematopoietic stem cell niche, the bone marrow vasculature regulates the production and release of blood leukocytes, which protect the host against infection but also fuel inflammatory diseases. Objective: We aimed to develop imaging tools to explore vascular changes in the bone marrow niche during acute inflammation. Methods and Results: Using the Toll-like receptor ligand LPS as a prototypical danger signal, we applied multi-parametric, -modality and -scale imaging to characterize how the bone marrow vasculature adapts when hematopoiesis boosts leukocyte supply. In response to LPS, ex vivo flow cytometry and histology showed vascular changes to the bone marrow niche. Specifically, proliferating endothelial cells gave rise to new vasculature in the bone marrow during hypoxic conditions. We studied these vascular changes with complementary intravital microscopy and PET/MR imaging. Fluorescence and PET integrin αVβ3 imaging signal increased during LPS-induced vascular remodeling. Vascular leakiness, quantified by albumin-based in vivo microcopy and MRI, rose when neutrophils departed and hematopoietic stem and progenitor cells (HSPC) proliferated more vigorously. Conclusions: Introducing a tool set to image bone marrow either with cellular resolution or non-invasively within the entire skeleton, this work sheds light on angiogenic responses that accompany emergency hematopoiesis. Understanding and monitoring bone marrow vasculature may provide a key to unlock therapeutic targets regulating systemic inflammation.