Therapeutic Blockade of Macrophage Colony Stimulating Factor (CSF-1) Delays AML Progression in Mice In Vivo

Macrophage colony-stimulating factor (M-CSF or CSF-1) plays a role in regulating innate immune responses promoting macrophage growth and differentiation. We hypothesized CSF-1 may also play a role in growth and progression of Acute Myeloid Leukaemia (AML). The aim of this study is to investigate the role of CSF-1 in survival and chemo-resistance of leukaemia stem and progenitor cells (LSPC). We further hypothesized that blocking CSF-1R signalling in LSPC may dampen leukaemia survival in vitro and delay leukaemia progression in vivo.

AML was induced in mice by injecting murine Haematopoietic Stem and Progenitor Cells (HSPC) transduced with either MLL-AF9 or AML1- ETO fusion oncogenes for the development of either monomyelocytic or granulocytic leukaemia respectively. We found CSF-1R, the main receptor for CSF-1, was expressed on these acute myeloid leukaemia cells, thus it is possible that CSF-1 provide supportive microenvironment for leukemic growth. To identify whether CSF-1 in the bone marrow (BM) niche is essential for growth of malignant LSPC, we harvested normal or- leukaemic blasts from BM for in vitro studies using Long-Term Culture-Initiating-Cell (LT-CIC) assays. In these assays AML LSPC or normal HSPC cells were co-cultured with mesenchymal stromal cells (MSC) from either wildtype mice (MSC that produce CSF-1) or MSC from OP/OP mice (unable to produce functional CSF-1). We found normal (wild-type) HSPC were able to proliferate, survive and produce LT-CIC in the absence of niche-provided CSF-1, however AML blasts could not, unless rescued by addition of recombinant CSF-1 (100 ng/mL) in vitro. Together these data suggest CSF-1 signalling may be critical for AML LSPC but not normal HSPC.

Next we investigated in mice whether therapeutic CSF-1 blockade could similarly dampen AML survival or progression in vivo. Cohorts of mice were injected with luciferase-expressing monomyelocytic (MLL-AF9) BM leukaemic blasts, then 7 days later administered the small molecule CSF-1 antagonist (GW2580, 160mg/kg daily for 10 days) or vehicle control. Leukaemia progression was tracked by biweekly bioluminescence and testbleeds for appearance of GFP+ leukaemia blasts in blood. We found therapeutic blockade of CSF-1 significantly reduced tumour burden in these mice by both bioluminescence and testbleed analysis. Mice were also monitored for duration of survival. As anticipated by the observed reduction in leukaemia burden, therapeutic CSF-1 blockade also significantly extended the duration of overall mouse survival (P<0.005, n= 8 mice/ group).

Together these studies suggest therapeutic CSF-1 blockade may show promise as an adjunct therapy to help reduce tumour burden and improve success of AML leukaemia therapies.