The Role of Macrophages in Acute Myeloid Leukaemia Progression and Sensitivity to Chemotherapy

Acute Myeloid Leukaemia (AML) is a heterogeneous neoplasm that has notoriously low survival rates. Although treatment often leads to clinical remission, the relapse rate of AML is very high and the relapse is usually fatal. There are a multitude of recorded driving mutations found in AML patients, and individual patients often have several driving mutations within their disease. This has meant that treatment options are limited to cytotoxic drugs that target rapid cell cycling and come with significant side effects. The high relapse rate in AML is thought to be caused by a small population of Leukaemic Stem Cells (LSCs) that have decreased cell cycling compared to the leukaemia blasts that make up the majority of the disease burden. Hence when patients undergo treatment the quiescent LSCs survive in the bone marrow and are able to repopulate the leukaemia blasts once treatment has ceased. In the non-malignant bone marrow, the Haematopoietic Stem Cells (HSCs) reside within the HSC niche which contains different types of cells that support the HSCs. Macrophages are an important regulator of the HSC niche, depletion of the bone marrow macrophage population causes disruption of the function of HSC niches and mobilisation of the HSCs into the blood. LSCs can highjack bone marrow HSC niches to support them instead of the HSCs; this may be conveying protection to the LSCs leading to their increased survival during chemotherapy, and eventually relapses in AML patients. Tumour Associated Macrophages (TAMs) have been associated with several solid cancers and their presence indicates a poor prognosis for patients. Theses TAMs tend to be in the M2 polarization activation state; this gives them decreased immune functions and increase chemokine producing abilities.It has not been shown yet if there are changes in the macrophage populations within the bone marrow, and whether these macrophages are indeed acting as TAMs as they do in other cancers.The chemotherapy regime adapted from the literature was found to be lethal to our mice, I found that lowering the dose of doxorubicin and changing the route of administration. With the improved chemotherapy regime, I found that the F4/80+ CD169+ macrophages in the mouse BM that were less susceptible to induction chemotherapy.I found that the CD11b-DTR mouse model was not appropriate for long term survival experiments due to mouse mortality one week after DT injection. I was able to show however that the CD169DTR/WT mouse model was compatible with repeated cycles of DT injections and the sub-lethal irradiation of the mice prior to DT injection. Neither of these conditions reduced CD169+ macrophage depletion rates.In vitro experiments showed that the depletion of CD169+ macrophages in mice transplanted with MLL-AF9 leukaemia did not alter the disease progression or survival of mice. However, when combined with chemotherapy, the CD169+ macrophage depletion does increase the survival of mice with MLL-AF9 leukaemia. This increase in survival was not associated with a decrease in leukaemia burden in the BM of mice 2 or 7 days after the cessation of chemotherapy. The increase in survival of mice transplanted with MLL-AF9 leukaemia was not seen in mice transplanted with AML1-ETO9a leukaemia. Chemotherapy treatment of CD169 KO mice transplanted with MLL-AF9 leukaemia did not alter the survival of the mice however there may be changes in the BM leukaemia burden after chemotherapy in these mice.Examination of the macrophages in mice with AML revealed that the macrophages are altered significantly by the presence of the leukaemia cells. The expression profile of MHC class II, CD80, MGL1, PD-L1, CD62L, CD304 and TIM4 on BM macrophages indicates a shift in macrophage function and could represent an immunosuppressive role for BM macrophages during AML in mice. This was more evident in the mice transplanted with MLL-AF9 than in those transplanted with AML1-ETO9a, indicating that different oncogenes in AML lead to different interactions with the microenvironment.Overall, these findings indicate that the there is a population of macrophages in the BM of mice whose immunological function is significantly altered by the presence of AML cells and play a chemoprotective role in mice with MLL-AF9 driven leukaemia.