Diverse Neutrophil Functions in Cancer and Promising Neutrophil-Based Cancer Therapies

Abstract

Neutrophils represent the most abundant cell type of leukocytes in the human blood and have been considered a vital player in the innate immune system and the first line of defense against invading pathogens. Recently, several studies showed that neutrophils play an active role in the immune response during cancer development. They exhibited both pro-oncogenic and anti-tumor activities under the influence of various mediators in the tumor microenvironment. Neutrophils can be divided into several subpopulations, thus contradicting the traditional concept of neutrophils as a homogeneous population with a specific function in the innate immunity and opening new horizons for cancer therapy. Despite the promising achievements in this field, a full understanding of tumor-neutrophil interplay is currently lacking. In this review, we try to summarize the current view on neutrophil heterogeneity in cancer, discuss the different communication pathways between tumors and neutrophils, and focus on the implementation of these new findings to develop promising neutrophil-based cancer therapies.

Keywords: cancer therapy; neutrophil heterogeneity; tumor microenvironment; tumor-associated neutrophils.

Figure 1

Neutrophil heterogeneity during tumor development. In the peripheral blood of cancer patients, three distinct populations of circulating neutrophils can be found: NDNs, LDNs, and g-MDSCs. Tumors recruit neutrophils via various mediators. These mediators include G-CSF [19], CXCL1 [20], CXCL2 [21], CXCL5 [22], CXCL8 [23], CXCL12 [24], IL-10 [19], IL-17 [25], and TGF-β [26]. After infiltration into the tumor microenvironment, neutrophils gain an N1 or N2 phenotype under the action of IFN-β [27] or TGF-β [14], respectively. Neutrophils in their turn reshape the tumor microenvironment: N1 TANs secrete pro-inflammatory anti-tumor mediators [14,28], while N2 TANs support tumor progression and angiogenesis and enhance the immunosuppressive tumor microenvironment [24,28]. NDNs—normal-density neutrophils, LDNs—low-density neutrophils, g-MDSCs—granulocytic-myeloid-derived suppressor cells, G-CSF—granulocyte colony-stimulating factor, CXCL—C-X-C motif chemokine ligand, CCL—C-C motif chemokine ligand, IL—interleukin, TGF-β—transforming growth factor beta, IFN-β—interferon beta, TNF-α—tumor necrosis factor alpha, ROS—reactive oxygen species, VEGF—vascular endothelial growth factor, MMP9—matrix metallopeptidase 9, TME—tumor microenvironment.

Figure 2

Mechanisms of anti-tumor (A) and pro-tumor (B) activities of neutrophils. (A) Mechanisms of neutrophil anti-tumor activities. (A1) Neutrophils exhibit direct anti-tumor activity via the production of ROS and RNS. Neutrophil-derived H₂O₂ activates TRPM2 and kills tumor cells in a CA⁺²-dependent manner [92]. Tumor-derived HGF interacts with MET on neutrophils and stimulates NO production, which mediates the destruction of tumor cells [93]. Moreover, NETs can display anti-tumor effects [95,96]. (A2) Neutrophils kill antibody-coated tumor cells via ADCC in a mechanism called trogoptosis [101]. (A3) Neutrophils alter the immune responses in the tumor microenvironment. Neutrophils stimulate macrophages to produce IL-12, which leads to the polarization of CD4⁻CD8⁻ unconventional αβ T cells, which exhibit IFN-γ-dependent anti-tumor activity [104]. Moreover, neutrophils enhance CD8+ T cell reactivity, reflected in CD69 expression and IFN-γ secretion and inhibit γδ17 T cells [105,106]. (B) Mechanisms of neutrophil pro-tumor activities. (B1) Neutrophils produce ROS and RNS, which can cause genotoxicity and contribute to tumorigenesis [110,111]. (B2) Neutrophils participate in creating an immunosuppressive tumor microenvironment by expressing PD-L1 on their surface, producing high levels of iNOS and ARG and secreting immunosuppressive mediators such as CCL17 and IL-10 [28,119,120,121]. (B3) Neutrophils support tumor angiogenesis via the secretion of several factors: VEGF [24], MMP9 [24], IL-8 [126], and Bv8 [127]. (B4) Neutrophils promote tumor growth and metastasis by producing NE [116], PGE2 [115], TGF-β [137], TNF-α [137], TRF [138], Gas6 [140], and NETs [143]. ROS—reactive oxygen species; RNS—reactive nitrogen species; TRPM2—transient receptor potential cation channel, subfamily M, member 2; HGF—hepatocyte growth factor; MET—mesenchymal–epithelial transition tyrosine kinase receptor; NETs—neutrophil extracellular traps; ADCC—antibody-dependent cellular cytotoxicity; IL—interleukin; IFN—interferon; PD-L1—programmed death-ligand 1; iNOS—inducible nitric oxide synthase; ARG—arginase; CCL17—C-C motif chemokine ligand 17; VEGF—vascular endothelial growth factor; MMP9—matrix metallopeptidase 9; Bv8—prokineticin 2; NE—neutrophil elastase; PGE2—prostaglandin E2; TGF-β—transforming growth factor beta; TNF-α—tumor necrosis factor alpha; TRF—transferrin; Gas6—growth arrest specific 6.

Figure 3

Tumor cells and CAFs modulate neutrophil function. Tumor cells and CAFs communicate with neutrophils through the production of EVs and several soluble factors. The main soluble factors are: TGF-β [14], SAA1 [119], CXCL1 [144], CXCL5 [163], IL-8 [165], Aβ [166], CTSC [174], and G-CSF [200]. The main effects of this communication are the polarization of neutrophils into the N2 phenotype and triggering NETosis. CAFs—cancer-associated fibroblasts, TGF-β—transforming growth factor beta, SAA1—serum amyloid A 1, CXCL—C-X-C motif chemokine ligand, IL-8—interleukin 8, Aβ—amyloid β, CTSC—cathepsin C, G-CSF—granulocyte colony-stimulating factor, TEVs—tumor-derived extracellular vesicles, CAF-EVs—CAF-derived extracellular vesicles.

Figure 4

Neutrophil in cancer therapy: potential approaches. Several neutrophil-based anticancer therapies have recently been investigated: (1) A suggested strategy is to inhibit neutrophil recruitment to the tumor microenvironment. TGF-β and CXCR2 inhibition is the first strategy that comes to mind, since they are widely involved in neutrophil recruitment to the tumor microenvironment [26,236]. More promising strategies are to block neutrophil immunosuppressive function or to restore neutrophil anti-tumor properties. (2) MET inhibitors [83], PLAG [237], COX inhibitors [238], and CCL20 inhibitors [123] could inhibit neutrophil immunosuppressive functions and restore the efficiency of ICIs. (3) To restore neutrophil anti-tumor activities, TGF-β inhibitors [14], lorlatinib [233], and some selected bioactive compounds (berberine [239], salidroside [240], and emodin [241]) are considered reliable choices. (4) NET inhibition is also a potential therapeutic approach that could be applied by the inhibition of NET production (PAD4 inhibitors [235]), the digestion of NETs (DNase I [235]), or the inhibition of different NET compounds (NE inhibitors [169]). (5) Recently, CAR-neutrophils were developed as a novel approach to use neutrophils in cancer therapy [17]. TGF-β—transforming growth factor beta, CXCR—CXC chemokine receptor, PD-L1—programmed death-ligand 1, PD-1—programmed cell death protein 1, MET—mesenchymal–epithelial transition tyrosine kinase receptor, PLAG—1-palmitoyl-2-linoleoyl-3-acetyl-rac-glycerol, COX—cyclooxygenase, CCL—C-C motif chemokine ligand, ICIs—immune checkpoint inhibitors, NET—neutrophil extracellular trap, PAD4—protein-arginine deiminase type-4, NE—neutrophil elastase, CAR—chimeric antigen receptor.