Page 97 - Haematologica Vol. 109 - July 2024

ARTICLE - Acute Myeloid Leukemia
TLR3 agonism augments CD47 inhibition in acute myeloid
leukemia
Haley E. Ramsey,1,2 Agnieszka E. Gorska,1 Brianna N. Smith,1 Andrew J. Monteith,1 Londa Fuller,1 Maria P. Arrate1 and Michael R. Savona1,3,4
1Department of Medicine, 2Program in Cancer Biology, 3Center for Immunobiology and 4Vanderbilt-Ingram Cancer Center, Vanderbilt University School of Medicine, Nashville, TN, USA
Abstract
CD47-SIRPa is a myeloid check point pathway that promotes phagocytosis of cells lacking markers for self-recognition. Tumor cells can overexpress CD47 and bind to SIRPa on macrophages, preventing phagocytosis. CD47 expression is en- hanced and correlated with a negative prognosis in acute myeloid leukemia (AML), with its blockade leading to cell clear- ance. ALX90 is an engineered fusion protein with high affinity for CD47. Composed of the N-terminal D1 domain of SIRPα genetically linked to an inactive Fc domain from human immunoglobulin (Ig) G, ALX90 is designed to avoid potential toxicity of CD47-expressing red blood cells. Venetoclax (VEN) is a specific B-cell lymphoma-2 (BCL-2) inhibitor that can restore apoptosis in malignant cells. In AML, VEN is combined with azanucleosides to induce superior remission rates, however treatment for refractory/relapse is an unmet need. We questioned whether the anti-tumor activity of a VEN- based regimen can be augmented through CD47 inhibition (CD47i) in AML and how this triplet may be enhanced. Human AML cell lines were sensitive to ALX90 and its addition increased efficacy of a VEN plus azacitidin (VEN+AZA) regimen in vivo. However, CD47i failed to clear bone marrow tumor burden in PDX models. We hypothesized that the loss of resident macrophages in the bone marrow in AML reduced efficiency of CD47i. Therefore, we attempted to enhance this medullary macrophage population with agonism of TLR3 via polyinosinic:polycytidylic acid (poly(I:C)), which led to expansion and activation of medullary macrophages in in vivo AML PDX models and potentiated CD47i. In summary, the addition of poly(I:C) can enhance medullary macrophage populations to potentiate the phagocytosis merited by therapeutic inhibition of CD47.
Introduction
CD47 is a ubiquitous cell surface protein that serves as myeloid checkpoint and is the ligand for the macrophage receptor, SIRPα. The immune system relies on the binding of CD47-SIRPα to recognize cells as ‘self’ leading to the moniker of the ‘don’t eat me’ signal.1,2 The CD47-SIRPα is a myeloid check point and is highly conserved in mammalian species. Cancer cells aberrantly overexpress CD47 to evade immune detection and destruction. As CD47 expression is prevalent in hematologic malignancies, trials targeting CD47 with monoclonal antibodies have been a recently explored strategy in the clinic.3,4
Targeting CD47 has led to immune response against tumor tissues, but also led to opsonization of RBC, as older RBC with less CD47 are more susceptible to premature clear- ance by CD47 antagonists, leading to anemia. Previous
groups have tried to address CD47 antagonist-induced hematotoxicity using dual targeting bispecific antibodies to CD47 and anti-SIRPα antibodies.5-7 The novel CD47 inhibitor evorpacept (ALX148) was engineered with an inactive Fc binding subunit and a high affinity CD47 binding domain. The inactive Fc domain prevents a pro-phagocytic signal and consequent clearance of RBC to improve the overall tolerability of CD47-targeted therapy.8 ALX90 is a small molecule inhibitor of CD47, with nanomolar level selectivity that serves as a tool compound for ALX148.8-10 ALX90 did not exhibit hemagglutination activity when compared to other CD47 antibody clones in human, monkey and murine blood samples. Similarly, the clinical analog, ALX148, led to no changes to the hematological parameters in treated mice.9 Acute myeloid leukemia (AML) is hallmarked by the acqui- sition of somatic mutations and chromosomal structural abnormalities, the arrest of normal cellular differentiation,
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Correspondence: M.R. Savona michael.savona@vanderbilt.edu
Received: Accepted: Early view:
June 29, 2023. December 15, 2023. December 28, 2023.
https://doi.org/10.3324/haematol.2023.283850
©2024 Ferrata Storti Foundation Published under a CC BY-NC license