Precision Killing of M2 Macrophages with Phage-Displayed Peptide-Photosensitizer Conjugates

SIMPLE SUMMARY: Tumor-associated M2 macrophages impair the anti-tumor immune response and negatively impact clinical outcomes. Hence, a strategy that eradicates these immunosuppressive cells could hold great therapeutic potential. Here, we demonstrated that phage-displayed peptides conjugated to the IR700 photosensitizer can kill M2 macrophages after near-infrared light irradiation, while leaving M1 macrophages unaffected. Additionally, combining IR700 with a phage displaying a cancer-specific peptide killed both cancer cells and M2 macrophages. The finding that the wild type M13 phage has some tropism to M2 macrophages would expand the scope for phage-based therapeutics. ABSTRACT: Among the immunosuppressive cells recruited to the tumor microenvironment, macrophages are particularly abundant and involved in angiogenesis, metastasis, and resistance to current cancer therapies. A strategy that simultaneously targets tumor cells and macrophages, particularly pro-tumoral M2 macrophages, would have significant clinical impact for various types of solid malignancies. By the use of phage display technology, we have recently developed a synthetic peptide, named NW, which binds to M1 and M2 macrophages with high affinity. Additional affinity selection on M2 macrophages identified only dominant peptides whose binding motifs are similar to that of the NW peptide. To reduce the frequency of selecting such dominating peptides, the peptide library was affinity selected on M2 macrophages blocked with NW peptide. This approach resulted in the selection of peptides that bind to M2, but not M1 macrophages. To explore the therapeutic potential of the selected peptides, the M13 phage-displayed peptides were conjugated to the photosensitizer IR700, which has been used for cancer photoimmunotherapy. The phage displaying a dominant peptide (SPILWLNAPPWA) killed both M1 and M2 macrophages, while those displaying the M2-specific peptides killed M2 macrophages only upon near-infrared light exposure. A significant fraction of the M2 macrophages were also killed with the untargeted M13 phage-IR700 conjugates. Hence, M2 macrophages can also be selectively targeted by the wild type M13 phage, which displayed a significant tropism to these cells. The benefits of this photoimmunotherapy include an automatic self-targeting ability of the wild type M13 phage, and the option of genetic manipulation of the phage genome to include tumor targeting peptides, allowing the killing of both M2 macrophages and cancer cells.