Monocyte-derived leukemia-associated macrophages facilitate extramedullary distribution of T-cell acute lymphoblastic leukemia cells.

Macrophages play important roles in both physiological and pathological processes and arise from successive waves of embryonic and adult hematopoiesis. Monocyte-derived macrophages (MOMF) exert distinct functions under pathological conditions, and leukemia-associated macrophages (LAM) show considerable diversities in activation and functional phenotype. However, their origin and pathological roles have not been well elucidated. Here we used wild-type (WT) and CCR2-/- mice to study the pathological roles of monocyte-derived LAM in extramedullary tissues in models of Notch1-induced T-cell acute lymphoblastic leukemia (T-ALL). MOMF existed in the resting liver and spleen (SP). In SP, Ly6C+ monocytes gave rise to the Ly6C+ macrophage subset. Furthermore, an increase of monocyte-derived LAM, including the Ly6C+ subset, was detected in the extramedullary tissues in leukemic mice. More monocyte-derived LAM, including Ly6C+ LAM, was detected in the spleens of leukemic mice transplanted with exogeneous mononuclear cells. Moreover, Ly6C+ LAM exhibited increased M1-related characteristics and contributed to sterile inflammation. In CCR2-/- leukemic mice, reduced Ly6C+ LAM, relived sterile inflammation, and reduced distribution of leukemia cells were detected in extramedullary tissues. Additionally, monocyte-derived Ly6C+ LAM expressed high levels of CCL8 and CCL9/10. Blocking CCR1 and CCR2 relieved hepatosplenomegaly and inhibited the extramedullary distribution of leukemia cells in T-ALL mice. Collectively, our findings reveal the multifaceted pathological roles of monocyte-derived LAM in T-ALL progression.