Page 105 - Haematologica Vol. 109 - July 2024

ARTICLE - CD47 inhibition is augmented with TLR3 agonism H.E. Ramsey et al.
noted in the assay populations where macrophages had been pretreated with poly(I:C) and AML samples had been treated with ALX90 or magrolimab (Figure 8B; Online Sup- plementary Figure S5). These data show that the capacity of CD47i is significantly enhanced when the appropriate macrophage populations are available.
Discussion
The majority of clinically investigated immunotherapies in AML have been focused on the adaptive immune system, such as chimeric antigen receptor T-cell and T-cell immune checkpoint inhibitors. However, focusing on macrophage and innate immunity is proving a meaningful adjunct of immuno- therapy. CD47, which functions as an anti-phagocytic signal, allows for CD47 expressing cells to escape phagocytosis by allowing for its cognate binding to SIRP-α on macrophages. However, many cancers overexpress CD47 as an evolutionary mechanism thereby evading phagocytosis. CD47 inhibition has real clinical potential, with CD47 inhibitors showing benefit in solid tumors and hematological cancers in ear- ly clinical trials. While the solid tumor microenvironment typical contains a plethora of macrophages,38-40 limitations of clinical benefit in hematologic malignancies may be rooted in the availability of macrophages, or at least M0/ M1 macrophages in the bone marrow microenvironment. As shown in our studies, CD47 blockade greatly augments the clearance of peripheral AML blasts, but has less suc- cess in the BM despite sufficient CD47 present on the cell surface of BM blasts. Although peripheral blood in PDX models contains low numbers of macrophages, there are enough M0/M1 macrophages to clear the engrated AML
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from the blood with CD47i. We suspected that a loss of the majority of resident medullary macrophages may be responsible for this lack of phagocytic activity in the BM and while the resident macrophages were diminished in PDX models, they were also polarized as tumor-permissive M2 macrophages/LAM. With the induction of macrophage populations via poly(I:C), inhibition of CD47 was not only potentiated as a monotherapy for medullary reduction of tumor, but allowed for better response when combined with standard of care.
It has been previously shown that CD47 expression, while significantly higher on AML cells than in comparison to normal BM, was variable between AML patients.41 Such findings might lead to the belief that high and low CD47 expression might determine drug efficacy, however, we found that CD47 intensity was not a biomarker of efficacy. The inter-patient variability of CD47 expression on AML blasts made no difference in effectivity of the drug in in vitro phagocytosis assays. Additionally, while MFI in our study reflected the quantity of CD47 expressed, the capacity to clear leukemia was no different between PDX models of high and low intensity CD47.
The concept of M2 macrophages driving tumor progres- sion by sustaining an immunosuppressive environment is a phenomenon of growing interest. Additional studies are beginning to bring to light the role of tumor suppressive M2 macrophages in hindering necessary therapies such as immune checkpoint blockade, radiotherapy and che- motherapy.42,43
T cells interact with LAM to directly or indirectly effect response to tumor.44 While the NSGS mice used in our modeling have negligible levels of T cells due to mutation in interleukin 2 receptor γ, M2 macrophages are dominant
Figure 8. Polyinosinic:polycytidylic acid enhances CD47 inhibitor-induced phagocytosis. (A) Phagocytosis assays using macro- phages from vehicle- or polyinosinic:polycytidylic acid (poly(I:C))-treated mouse bone marrow (BM) incubated with pHrodo-labeled cells from patient acute myeloid leukemia (AML) 18-12-001 and 19-03-007 reveal increases in phagocytosis in poly(I:C)-treated BM. (B) Healthy donor macrophages, when pretreated with poly(I:C) reveal increases in phagocytosis of CD47-treated AML (18-12- 001) cells. Statistical comparisons in 3 individual donors shown here as ALX monotherapy, poly(I:C) monotherapy and combination against IgG1 control. Ig: immunoglobulin.
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