Sipa1 loss in host niche boosts anti-AML immune response

Abstract Background: Signal-induced proliferation-associated gene 1 (Sipa1), a RAP1 GTPase-activating protein, is expressed in both hematopoietic cells and stromal cells. It negatively regulates G-protein signaling pathways involving in cell adhesion and proliferation. We have demonstrated that Sipa1loss induces bone marrow (BM) mesenchymal stem and progenitor cells (MSPCs, CD45-TER119-CD31-CD44-CD51+SCA1+ and SCA1- subsets) alterations that drive the development of myeloproliferative neoplasm. Meanwhile, we have observed reduced Sipa1 expression in the BM MSPCs in mice with acute myeloid leukemia (AML) (Blood Adv. 2018, 2(5):534-548). However, the impact of the Sipa1 reduction on AML progression remains unclear. Methods: We have here explored the impact of Sipa1 expression in the host BM mesenchymal and immune cell niche by, 1) analyzing AML progression in Sipa1-/- and Sipa1+/+ recipient mice transplanted with MLL-AF9 AML cells with or without prior irradiation; 2) in vivo depletion of specific lymphoid subsets using neutralizing antibodies to determine their contributions; 3) immune and mesenchymal niche characterization using RNA sequencing and flow cytometry; 4) determining clinical correlation between Sipa1 expression in hematopoietic cells and treatment outcomes in patients with AML. Results: Remarkably, Sipa1 deficiency in host microenvironment led to delayed or no AML development, reflected in that 30% of the Sipa1-/- recipient mice did not develop AML while all Sipa1+/+ mice did after AML cell injection without prior irradiation. Notably, such a difference in AML development was not observed in lethally irradiated Sipa1-/- and Sipa1+/+ recipient mice where their immune cells were eliminated, indicating the critical involvement of Sipa1-/- immune cells in the AML inhibition. Further, in vivo depletion of NK cells by antibody neutralization could completely reverse the difference in AML development kinetics in the Sipa1-/- mice, suggesting that Sipa1 deficiency may boost NK cells anti-AML immune response. The enhanced anti-AML activity of Sipa1-/- NK cells was consistent with the increased maturation and activation of Sipa1-/- NK cells derived from spleen and blood. Furthermore, RNA sequencing suggested significant enrichment of the inflammatory genes associated with NK cell cytoxicity in Sipa1-/- BM MSCs (CD45-TER119-CD31-CD44-CD51+SCA1+) relative to that in Sipa1+/+ mice, providing another potential mechanism contributing to the increased anti-AML activity of Sipa1-/- NK cells, driven by the inflamed Sipa1-/- BM MSCs. Most importantly and notably, in line with these findings in mice, low SIPA1 expression in bone marrow mononuclear cells is associated with better survival and therapy responses to BET and mTOR inhibitors in AML patients (TCGA LAML dataset, and unpublished data from Prof. Caroline Heckman's lab in Helsinki University). Conclusions: Altogether, our study suggests that Sipa1 loss boosts anti-AML response of NK cells, possibly triggered by enhanced inflammatory host microenvironment. These findings along with the clinical correlation between low SIPA1 expression and better survival in AML patients point to the translational potential of these discoveries, meriting further investigation of therapeutic potential of Sipa1 deficient NK cells for AML and the underlying mechanisms.