Identification and Characterization of M2 Macrophage-targeting Peptide M2pep for the Delivery of Pro-apoptotic Peptides to Tumor-associated Macrophages

Abstract

Activated macrophages are made up of two broad subsets, M1 “classically activated” macrophages and the M2 “alternatively activated” macrophages. These subsets present diverse functional phenotypes, with M1 macrophages exhibiting a pro-inflammatory and microbicidal functions while M2 macrophages possess functional roles in resolution of inflammation and tissue remodeling. An imbalance in M1 versus M2 macrophages can contribute to disease progression. In cancer, populations of tumor-associated macrophages (TAMs) mediate immunosuppression and possess M2-like qualities such as poor antigen presentation, promotion of angiogenesis and tissue remodeling and repair. In addition, TAMs may promote the exhausted phenotype of CD8+ T cells, resulting in their inability to eliminate tumor cells. Not surprisingly, the extent of TAM accumulation within tumors generally correlates with poor disease prognosis. Despite the growing interest in tumor immunotherapy, there continues to be a dearth of TAM-specific targeting agents. In this work, an M2-selective peptide was isolated via a subtractive phage biopanning technique against whole cells. The phage biopanning resulted in identification of M2pep, a peptide that selectively binds and is internalized by murine M2 macrophages. Next, M2pep was characterized for binding to TAMs extracted from CT-26 syngeneic tumors as well as accumulation in TAMs in vivo. Administration of a fusion peptide of M2pep with KLA, a pro-apoptotic peptide, resulted in delayed tumor growth, increased mouse survival, and selective elimination of TAMs. Unfortunately, M2pep did not bind human M2 macrophages derived from peripheral blood mononuclear cells (PBMC), and consequently, whole-cell phage display biopanning against human macrophages was performed to obtain a similar human M2pep peptide. Despite six separate whole-cell panning attempts against human PBMC-derived M2 macrophages, no peptide with selective binding to human M2 macrophages was isolated. In addition, biopanning against Legumain and PD-L1 immobilized proteins, targets selected for their high expression on tumor-associated macrophages, failed to distinguish peptides with selective binding to their target protein. Work to discover peptides with selective human M2 macrophage binding properties is ongoing in the lab.