CCL19⁺ fibroblasts define a proliferative niche in chronic lymphocytic leukemia

Adaptive immune responses occur lymph nodes (LNs) in a microenvironment established by resident stromal cells. LNs are also a site of proliferation of chronic lymphocytic leukemia (CLL), a B cell cancer that alters LN structure in a stereotypic manner. To deeply characterize reactive and CLL LNs, we developed a single-cell RNA sequencing pipeline. We find that proliferation of CLL cells in proliferation centers (PCs), a CLL-specific niche, begins with transient upregulation of MYC, subsequent downregulation of which may limit CLL growth. PCs contain a distinct fibroblast population expressing CCL19 while CLL cells express the CCL19 receptor CCR7, providing a recruitment mechanism for CLL cells to PCs. Using informatic, spatial, and in situ analyses to identify ligand-receptor pairs involving PC CLL cells and nearby immune and stromal cells, we observe that PCs are enriched for macrophages expressing BAFF, the integrin αXβ2 heterodimer, and Galectin9, factors implicated in cell growth, adhesion, and immunosuppression. The most common predicted interactions in PCs involve CD74 and ligands such as MIF, and we find that CD74 blockade consistently inhibits CLL cell growth in culture. Our work highlights key features of the CLL proliferative niche and provides a roadmap for identifying vulnerabilities and new therapeutic strategies.