Targeting Myeloid Checkpoint Molecules in Combination With Antibody Therapy: A Novel Anti-Cancer Strategy With IgA Antibodies?

Abstract

Immunotherapy with therapeutic antibodies has shown a lack of durable responses in some patients due to resistance mechanisms. Checkpoint molecules expressed by tumor cells have a deleterious impact on clinical responses to therapeutic antibodies. Myeloid checkpoints, which negatively regulate macrophage and neutrophil anti-tumor responses, are a novel type of checkpoint molecule. Myeloid checkpoint inhibition is currently being studied in combination with IgG-based immunotherapy. In contrast, the combination with IgA-based treatment has received minimal attention. IgA antibodies have been demonstrated to more effectively attract and activate neutrophils than their IgG counterparts. Therefore, myeloid checkpoint inhibition could be an interesting addition to IgA treatment and has the potential to significantly enhance IgA therapy.

Keywords: CD47-SIRPalpha axis; IgA; antibodies; cancer immonotherapy; immune checkpoint; macrophages; myeloid checkpoints; neutrophils (PMNs).

Figure 1

An overview of putative immune checkpoints molecules regulating myeloid cell function in human tumor microenvironments (TME). Receptor-ligand interaction including the CD47-SIRPα axis, sialoglycans-Siglec axis and HLA1 (B2M)-LILRB 1/2 axis in the immune synapse between the myeloid cells, such as neutrophils and macrophages and the tumor cells. Src family kinases phosphorylate the ITIM upon binding of checkpoint molecules to their respective receptors. The following recruitment and activation of SHP-1 and SHP-2 suppresses the anti-tumor immune responses. Consequently, tumor cells are able to evade immune surveillance.

Figure 2

Regulation of CD47 expression in cancer cells. An overview of the mechanisms that cause CD47 overexpression in cancer. When the TNF receptor and the IL-1 receptor are activated by extracellular TNF-α and IL-1, NFκB is recruited and translocated to the nucleus, where it binds to a super-enhancer to promote CD47 expression. Extracellular IL-6 induces STAT3 signaling. Phosphorylated STAT3 complex and other transcription factors, including SNAI1, ZEB1, MYC, HIF-1, PKM2-β-catenin-BRG1-TCF4 complex enhance CD47 expression by directly binding to the CD47 promotor. Extracellular IFN-γ increased the CD47 expression, albeit the exact mechanism is unknown. Subsequently, at a post-transcription level, microRNAs could bind to the 3’ untranslated region of CD47 mRNA, causing translation to be disrupted.

Figure 3

IgG- and IgA-mediated activation of neutrophils. Neutrophils express various Fc receptors, the two most abundant of which, FcγRIIa and CD16b, are important in modulating activation upon IgG ligation. FcγRIIb is expressed nearly 9-fold greater than FcγRIIa. FcγRIIa is an activating receptor that binds to IgG in a 1:1 stoichiometry and signals via one ITAM motif. Downstream ITAM signaling activates effector functions such as ADCC. Moreover, neutrophils express FcγRIIb, which lacks an active intracellular signaling domain and functions as a scavenger receptor for IgG. IgA binds to FcαRI expressed on neutrophils, an activating Fc receptor in a 1:2 stoichiometry. A total of four ITAMs cause a strong activation of ADCC.

Figure 4

Regulation of neutrophil-mediated tumor cell death. The balance of pro-phagocytic and anti-phagocytic signals determines the fate of the tumor cell. Pro-phagocytic signals are elicited by Fc receptor engagement to IgA-opsonized tumor cells, resulting in Fc activation and phosphorylation of downstream ITAM tyrosines. Calreticulin is tethered to the cell surface by membrane glycans and interacts with lipoprotein receptor-related protein 1 (LRP1) receptor expressed on neutrophils. Likewise, SLAMF7 (CD319) binds to macrophage-1 antigen (MAC-1, α_Mβ₂), these interactions promote tumor cell killing by neutrophils. In contrast, overexpression of CD47 on tumor cells interact with SIRPα to inhibit neutrophil activation. Similarly, if expressed, the sialoglycans-Siglec axis, and HLA1 (B2M)-LILRB axis if expressed decrease immune responses, allowing tumor cells to evade immune surveillance.

Figure 5

Strategies for inhibiting myeloid checkpoints. 1) Genetic knock out of target genes involved in the inhibitory pathway. 2) Specific blocking of target checkpoint molecules with mAbs or soluble ligand-Fc fusion proteins to inhibit receptor binding and checkpoint axis activation. 3) Bispecific antibodies that target both TAA and checkpoint molecules simultaneously to avoid off-target side effects.4) Biologics that alter the structure of the target protein, preventing it from binding to the receptor, or that inhibit expression or block the target protein.