A capsular myofibroblastic niche maintains hematopoietic stem cells in the spleen

The spleen is a key site for extramedullary hematopoiesis that hosts a rare population of functional hematopoietic stem cells (HSCs). While the microenvironment that supports extramedullary hematopoiesis response has gained interest, a niche for splenic HSCs at steady-state remains undescribed. Here, we have uncovered a red-pulp-specific, myofibroblastic niche that supports murine splenic HSCs within a ≈ 200-μm-wide capsular zone. Detailed spatial-distribution and perturbation analysis showed the importance of myofibroblasts in maintaining HSCs in a quiescent state. Unlike reported for the adult bone marrow, the HSCs in splenic niche were not spatially associated with vascular components. G-CSF-mediated chemokine alteration and 5-FU-induced proliferation resulted in HSCs shifts away from the splenic capsule. Interestingly, upon regaining quiescence, the HSCs re-occupied niches close to capsular myofibroblasts. Proteomic interactome profiles confirmed the relevance of capsular myofibroblasts for splenic HSCs and identified potential niche regulators of HSC maintenance. Together, this study demonstrates a dynamic HSC localization in the spleen and its niche context at homeostasis and under stress. It offers a model to uncover novel regulators crucial for HSC function.