Hematopoietic stem cells (HSCs) can self-renew and differentiate into different blood cell lineages. In adult mammals, this process primarily occurs in the bone marrow (BM), where specialised microenvironments, known as ‘niches’, tightly regulate HSC activity. Composed of stromal, endothelial, and mesenchymal cells, these niches control stem-cell quiescence, proliferation, and differentiation, ensuring balanced blood cell production under steady-state conditions.

Beyond the BM, the spleen also harbors a notable population of HSCs that can differentiate into different blood cell-types during conditions of stress, such as infection, inflammation, or injury. However, little is known about how HSCs are maintained within the spleen. A new study by researchers from the Indian Institute of Science Education and Research (IISER) Thiruvananthapuram, now identifies a previously unrecognized stem-cell niche within the spleen that helps maintain HSCs outside the BM. This niche is composed of capsular myofibroblasts, the cells that line the capsular region of the spleen, maintaining its structural integrity.

During stress, HSCs originating from the BM can travel through the bloodstream and seed other peripheral organs like the spleen, initiating extramedullary hematopoiesis (EMH), the production of blood cells outside the BM. Previous studies have largely focused on signals and cues that drive EMH during conditions like pregnancy, anemia, injury, and infections. However, the role of resident splenic HSCs and their local microenvironment during EMH has remained unclear.

To address this gap, the researchers mapped the spatial distribution of HSCs in the spleen using immunostaining approaches in mouse-derived tissue. They observed that during EMH, HSCs tend to accumulate within the red pulp of the spleen, a region enriched with myofibroblasts. Specifically, primitive HSCs—an early and highly quiescent subpopulation of HSCs—were located closer to these capsular myofibroblasts, whereas broader populations of HSCs and progenitor cells were distributed farther away. These findings suggest that capsular myofibroblasts may form a specialized niche that helps preserve the stemness of HSCs within the spleen.

In BM, hematopoietic niches are surrounded by blood vessels and stromal cells, forming a dynamic microenvironment where HSCs integrate diverse cellular and molecular cues. On the contrary, analysis of spleen tissue revealed little involvement of these structures in the maintenance and distribution of splenic HSCs, further pointing to the role of capsular myofibroblasts in regulating the hematopoietic niche.

To understand how the niche functions at the molecular level, the team next examined the expression of niche-related markers known to regulate HSC activity. They focused on the chemokine CXCL12 (SDF-1α), a key extrinsic regulator of HSC maintenance in the BM. In the spleen, SDF-1α was strongly expressed in the capsular and vascular tissues. Importantly, its expression was largely restricted to capsular myofibroblasts, supporting the presence of a niche-like environment in this region.

Next, the researchers examined the role of granulocyte colony-stimulating factor (G-CSF), a cytokine known to influence HSC mobilization and EMH by suppressing SDF-1α and other key regulators. In mice treated with G-CSF, the researchers noted a decrease in the levels of SDF-1α and other key hematopoietic regulators in splenic pulp and fibrous myofibroblasts. This decrease was accompanied by a marked increase in hematopoietic progenitor cells, suggesting a shift away from the quiescent state of primitive HSCs. G-CSF treatment also caused primitive HSCs to relocate away from the capsular region. Remarkably, as HSCs returned to a quiescent state, this effect was fully reversed, and primitive HSCs once again clustered closely with capsular myofibroblasts. Together, these findings highlight the central role of capsular myofibroblasts in establishing a splenic HSC niche.

The study also explored how the splenic capsular niche responds to stress. When the researchers induced myeloablation, a condition that depletes BM HSCs, they observed a rapid expansion of HSCs within the splenic red pulp. This expansion was accompanied by a transient shift away from the capsular niche, suggesting that the spleen activates its resident HSCs to compensate for compromised BM function.

To better understand the molecular basis of the niche, the team compared the proteomic profiles of splenic capsular cells and stromal cells. Capsular myofibroblasts were highly enriched in secretory proteins with hematopoietic regulatory functions. These results reinforce the idea that capsular myofibroblasts create a specialized molecular environment capable of directly supporting splenic HSCs.

Although the spleen has long been recognized as a site of EMH, the mechanisms governing HSC maintenance in this organ have remained largely unexplored. This study provides the first model of a hematopoietic niche in the capsular region of the spleen, revealing a previously unrecognised mechanism of EMH regulation. Beyond advancing our understanding of EMH, these findings could inform strategies for the therapeutic manipulation of HSC niches, for example, to enhance HSC maintenance and optimize EMH activation in the context of BM impairment or disease.