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Summary: Periostin (POSTN) is a multifunctional extracellular component that regulates cell-matrix interactions and cell-cell crosstalk. POSTN deletion significantly decreases leukemia burden in mice; however, the underlying mechanisms by which POSTN promotes B cell acute lymphoblastic leukemia (B-ALL) progression remain largely unknown. Here, we demonstrate that bone marrow (BM)-derived mesenchymal stromal cells (BM-MSCs) express higher levels of POSTN when co-cultured with B-ALL cells in vitro and in vivo. POSTN deficiency in BM-MSCs significantly decreases CCL2 expression in co-cultured B-ALL cells in vitro and in vivo. Moreover, POSTN treatment increases expression of CCL2 in B-ALL cells by activating the integrin-ILK-NF-κB pathway. Conversely, CCL2 treatment upregulates expression of POSTN in BM-MSCs via STAT3 activation. Furthermore, there is a positive correlation between POSTN expression and CCL2 level in the BM of mice and patients with B-ALL. These findings suggest that B-ALL cell-derived CCL2 contributes to the increased leukemia burden promoted by BM-MSC-derived POSTN. : Ma et al. show that BM-MSC-derived periostin promotes leukemia progression by activating the integrin-ILK-NF-κB-CCL2 pathway in leukemia cells and that leukemia cell-derived CCL2 increases periostin expression in BM-MSCs by activating STAT3. This work identifies critical crosstalk between leukemia cells and stromal cells via periostin and CCL2 in B-ALL progression. Keywords: periostin, CCL2, MSC, B-ALL, leukemia, bone marrow, extracellular matrix, matricellular protein