Spatial profiling of the spleen in mouse and human myelofibrosis reveals complement-driven immune-stromal interactions as a therapeutic target.

Splenomegaly is a defining feature of myelofibrosis, yet the contribution of splenic mesenchymal stroma to disease progression remains unclear. We combined spatial and single-nucleus transcriptomics of patient spleens with spatial and single-cell transcriptomics, as well as imaging analyses, of murine spleens to map extramedullary hematopoiesis niches. Activated red pulp reticular cells localize near hematopoietic stem and progenitor cells, and early disease is characterized by marginal zone disruption with lymphoid depletion preceding stromal remodeling. Trajectory analyses reveal a shift in reticular cells from hematopoiesis-supportive to inflammatory and pro-fibrotic states, driven by macrophage- and megakaryocyte-derived signals that activate complement and induce tumor necrosis factor α (TNF-α), transforming growth factor β (TGF-β), extracellular matrix, and Thbs1 programs. Non-hematopoietic complement component C3 deficiency or pharmacological C3 inhibition suppresses these pathways, restores splenic architecture, and reduces splenomegaly and bone marrow fibrosis. These findings identify complement-dependent stromal reprogramming as a mechanism governing hematopoietic niches and as a targetable axis in myelofibrosis.