A Gremlin 1-expressing splenic niche cell population restrains chronic myeloid leukemia by antagonizing the BMP pathway.

The spleen plays a critical role in the pathogenesis of leukemia. However, our understanding of the splenic niche is very limited. Herein, we report that induced expression of the secreted protein Gremlin 1 in a mouse model restrains chronic myeloid leukemia (CML) progression and synergizes with tyrosine kinase inhibitor treatment, whereas blockade of Gremlin 1 promotes CML development. Intriguingly, the effect of Gremlin 1 is most evident in the spleen but not in the bone marrow. Gremlin 1 induces apoptosis of leukemic stem cells via antagonizing the BMP pathway. Single-cell RNA sequencing and experimental validation together show that Gremlin 1 marks a unique stromal cell population in the spleens of both mice and humans. Genetic ablation of Gremlin 1+ cells leads to accelerated CML progression. Collectively, Gremlin 1 and Gremlin 1+ cells are key defensive niche components in the spleen that limit CML progression, revealing an unprecedented mechanism for the body to fight off leukemia.