Inducible CXCL12/CXCR4-dependent extramedullary hematopoietic niches in the adrenal gland

Adult hematopoietic Stem and Progenitor Cells (HSPCs) reside in the bone marrow hematopoietic niche, which regulates HSPC quiescence, self-renewal, and commitment in a demand-adapted manner. While the complex bone marrow niche is responsible for adult hematopoiesis, evidence exists for simpler, albeit functional and more accessible, extramedullary hematopoietic niches. Inspired by the anecdotal description of retroperitoneal hematopoietic masses occurring at higher frequency upon hormonal dysregulation within the adrenal gland, we hypothesized that the adult adrenal gland could be induced into a hematopoietic supportive environment in a systematic manner, thus revealing mechanisms underlying de novo niche formation in the adult. Here we show that upon splenectomy and hormonal stimulation, the adult adrenal gland of mice can be induced to recruit and host functional HSPCs, capable of serial transplantation, and that this phenomenon is associated with de novo formation of platelet-derived growth factor receptor α (PDGFRα) expressing stromal nodules. We further show in CXCL12-GFP reporter mice that adrenal glands contain a stromal population reminiscent of the CXCL12-Abundant Reticular (CAR) cells which compose the bone marrow HSPC niche. Mechanistically, HSPC homing to hormonally-induced adrenal glands was found dependent on the CXCR4/CXCL12 axis. Mirroring our findings in mice, we found reticular CXCL12+ cells co-expressing master niche-regulator FOXC1 in primary samples from human adrenal myelolipomas, a benign tumor composed of adipose and hematopoietic tissue. Our findings reignite long-standing questions regarding hormonal regulation of hematopoiesis and provide a novel model to facilitate the study of adult-specific inducible hematopoietic niches which may pave the way to therapeutic applications.