BackgroundImmunotherapy has achieved remarkable advances via a variety of strategies against tumor cells that evade immune surveillance. As important innate immune cells, macrophages play important roles in maintaining homeostasis, preventing pathogen invasion, resisting tumor cells and promoting adaptive immune response. CD47 is found to be overexpressed on tumor cells and act as a don’t eat me’ signal, which contributes to immune evasion. Macrophages mediated phagocytosis via blockade CD47/SIRPα (signal regulatory protein alpha) interaction was proved to induce effective antitumor immune response.MethodsA novel peptide pep-20, specifically targeting CD47 and blocking CD47/SIRPα interaction, was identified via high-throughput phage display library bio-panning. The capability to enhance the macrophage-mediated phagocytosis activities and antitumor effects of pep-20 were investigated. The mechanism of pep-20 to induce T-cell response was explored by ex vivo analysis and confirmed via macrophage depleting strategy. The structure-activity relationship and D-amino acid substitution of pep-20 were also studied. The antitumor effects and mechanism of a proteolysis resistant D-amino acid derivate pep-20-D12 combined with irradiation (IR) were also investigated.ResultsPep-20 showed remarkable enhancement of macrophage-mediated phagocytosis to both solid and hematologic tumor cells in vitro, and inhibited tumor growth in immune-competent tumor-bearing mice. Furthermore, pep-20 promoted macrophages to mobilize the antitumor T-cell response with minimal toxicity. Furthermore, systemic administration of the derivate pep-20-D12 showed robust synergistic antitumor efficacy in combination with IR.ConclusionIn summary, these results demonstrated that CD47/SIRPα blocking peptides, pep-20 and its derivate, could serve as promising candidates to promote macrophages-mediated phagocytosis and immune response in cancer immunotherapy.
The low response rate and adaptive resistance of PD‐1/PD‐L1 blockade demands the studies on novel therapeutic targets for cancer immunotherapy. We discovered that a novel immune checkpoint TIGIT expressed higher than PD‐1 in many tumors especially anti‐PD‐1 resistant tumors. Here, mirror‐image phage display bio‐panning was performed using the d‐enantiomer of TIGIT synthesized by hydrazide‐based native chemical ligation. d‐peptide DTBP‐3 was identified, which could occupy the binding interface and effectively block the interaction of TIGIT with its ligand PVR. DTBP‐3 showed proteolytic resistance, tumor tissue penetrating ability, and significant tumor suppressing effects in a CD8+ T cell dependent manner. More importantly, DTBP‐3 could inhibit tumor growth and metastasis in anti‐PD‐1 resistant tumor model. This is the first d‐peptide targeting TIGIT, which could serve as a potential candidate for cancer immunotherapy.
TIGIT, an immune checkpoint molecule widely expressed on NK cells, activated T cells and Tregs, has been involved in delivering inhibitory signals through the interaction with PVR. The blockade of TIGIT/PVR interaction is a promising approach in cancer immunotherapy. Here, we unexpectedly discovered the expression of TIGIT in murine tumor cells. To elucidate the mechanism of such intrinsic expression, TIGIT knockout murine colorectal CT26 and MC38 cell lines were generated by using CRISPR/Cas9 system. Although TIGIT knockout showed no effects on proliferation and colony formation of tumor cells in vitro, the tumor growth in mice was considerably inhibited. TIGIT knockout led to the increase of IFN-γ secretion by NK and CD8+ T cells. Further, in BABL/c nude mice, CD8+ T cells depleting mice and NK cells depleting nude mice, the promotion of tumor growth was significantly diminished, suggesting that both NK cells and CD8+ T cells were involved in the tumor promoting process mediated by intrinsic TIGIT. In addition, blocking TIGIT/PVR interaction by the antibody or recombinant PVR protein could elicit anti-tumor effects by facilitating the tumor infiltration and restoring the function of CD8+ T cells, and the antibody-mediate TIGIT blockade could inhibit MC38 tumor growth through blocking TIGIT expressed on tumor cells. We therefore propose a novel TIGIT/PVR interaction mode that tumor intrinsic TIGIT delivers inhibitory signals to CD8+ T cells and NK cells by engaging with PVR.
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