Microenvironmental CXCL12 Deletion Enhances Flt3-ITD Acute Myeloid Leukemia Stem Cell Response to Therapy by Reducing p38 MAPK Signaling

Fms-like tyrosine kinase 3 (Flt3) tyrosine kinase inhibitors (Flt3-TKI) have improved outcomes for patients with Flt3-mutated acute myeloid leukemia (AML) but are limited by resistance and relapse, suggesting persistence of leukemia stem cells (LSC). Here we utilized a Flt3-internal tandem duplication (Flt3-ITD) and Tet2-deleted AML genetic mouse model to characterize Flt3-ITD AML LSC and their resistance to Flt3-TKI. We show that LSC were enriched within the primitive ST-HSC population. CXCl12-expressing osteoprogenitors were increased in the Flt3-ITD AML marrow and FLT3-ITD LSC showed increased expression of the CXCL12 receptor CXCR4. CXCL12 deletion from the microenvironment enhanced AML targeting by combined Flt3-TKI and chemotherapy treatment with enhanced LSC targeting. We found that both treatment and CXCL12 deletion partially reduced p38 mitogen-activated protein kinase (p38) signaling with increased, but not complete, reduction seen with combined treatment and CXCL12 deletion. Our studies suggest that p38 contributes to CXCL12-mediated maintenance of AML LSC following Flt3-TKI and chemotherapy treatment, but that p38 inhibition enhances AML LSC response to treatment to a greater extent than CXCL12 deletion alone. This study supports a role for p38 signaling in CXCL12-mediated protection of AML LSC from treatment and provide a rationale for targeting p38 signaling to enhance Flt3-ITD AML targeting. FACS-sorted ST-HSC were isolated from CXCL12 WT or CXCL12 KO mice transplanted with Flt3-ITD TET2 KO AML bone marrow then treated with either vehicle controls or combination cytarabine, doxorubicin, and quizartinib