In May 2019, Anna Huttenlocher's team from the University of Wisconsin-Madison published a review titled "Neutrophil plasticity in the tumor microenvironment" in the international journal Blood1, discussing the polarization and plasticity of neutrophils, as well as the effects of neutrophils of different phenotypes on the tumor microenvironment, and promoting the role of neutrophils in tumorigenesis, development, Understanding of the role of the transfer process.
Neutrophil polarization and plasticity
Since neutrophils respond accordingly to various types of inflammatory stimuli, they have the ability to adapt to different environments, so there are neutrophil populations of different phenotypes in different inflammatory backgrounds. In 2009, Fridlender et al. first proposed a naming pattern for anti-tumor neutrophils and pro-tumor neutrophils, called N1 and N2, respectively. They found that an immunosuppressive cytokine transforming growth factor(TGF-β β), overexpressed by tumor cells, polarized neutrophils to a pro-tumor phenotype (N2), so a decrease in neutrophils led to a slight decline in tumor growth in mouse models. Using the TGF-β receptor inhibitor SM16 to block TGF-β leads to aggregation of neutrophils with an anti-tumor phenotype (N1). In this case, a decrease in neutrophils promotes tumor growth. In addition to increased N1 neutrophil polarization after TGF-β blockade, N1 neutrophil polarization is also thought to be induced by interferon 1 (IFNs). Overall, these results suggest that the polarization of neutrophils may be driven by the tumor microenvironment.
In addition, neutrophils themselves can directly or indirectly affect the tumor microenvironment. The authors first described their direct effects on the tumor microenvironment, and then went into further detail about how pro-inflammatory or anti-inflammatory neutrophils affect the tumor microenvironment. Figure 1 shows the different functions performed by neutrophils in the tumor microenvironment. Due to the difficulty of manipulating primitive neutrophils, much of the published literature on tumor infiltration neutrophils (TANs) has been conducted in mice. However, the authors focus primarily on research using human neutrophils and highlight the need for further progress in this area.
Figure 1: Neutrophils in the tumor microenvironment and their functions
Direct action of neutrophils on tumor cells:
Neutrophils can kill tumor cells in different situations and promote tumor growth. One of the most well-known examples of direct cytotoxicity of neutrophils killing tumor cells is the production of ROS (superoxides and hydrogen peroxide), but the mechanism and role of ROS in the tumor microenvironment is unknown. Studies have shown that ROS-mediated cell killing has been shown to be dependent on the expression of tumor cells TRPM2, an H2O2-dependent Ca2+ channel that, once activated, causes a large influx of calcium ions, which in turn leads to cell death (Figure 2). In addition to ROS, neutrophils also secrete RNS, such as nitric oxide, peroxynitrite, etc., to induce the elimination of cancer cells.
Figure 2: TRPM2-mediated neutrophil cell killing mechanism2
Neutrophils also directly promote tumor development, metastasis, and angiogenesis. The production of ROS and RNS not only directly kills tumor cells through cytotoxicity, but also induces oxidative damage to DNA and genetic instability, so ROS and RNS can also have a tumor-promoting effect. In addition, certain enzymes released by neutrophils can also promote tumor development, metastasis, and angiogenesis, such as myelooperoxidase (MPO), neutrophil elastinase (NE), and matrix metalloproteinase (MMPs). Moreover, neutrophils typically produce neutrophil extracellular trapping nets (NETs) during infection, which, when released in the tumor microenvironment, can stimulate the migration and invasion of cancer cells. NE and MPO can influence the development of tumors by regulating the production of NETs.
Pro-tumor effect of pro-inflammatory neutrophils:
Pro-inflammatory neutrophils recruit other immune cells that have different effects on the tumor microenvironment, including those that promote tumor growth. Pro-inflammatory neutrophils in the tumor microenvironment secrete chemoattractant protein-1, which secrete chemoattractant protein-1 and CCL17, which recruit monocytes and T-regulatory cells, respectively. T-regulatory cells are widely thought to inhibit other inflammatory T cell populations, thereby promoting tumor growth. But the role of the recruitment of monocytes in tumor progression is unclear, and it can induce both tumor growth and inhibit tumor growth, depending on the polarized state in the tumor microenvironment.
In addition, cytokines and growth factors secreted by TANs can also promote spillover and metastasis of tumor cells, such as IL-8.
Antitumor effect of pro-inflammatory neutrophils:
Pro-inflammatory neutrophils in the tumor microenvironment can also inhibit tumor growth by coordinating the recruitment and function of other immune cells. In particular, neutrophils are able to present antigens to T cells, stimulating adaptive immune responses and the production of IFN-γ to inhibit tumor growth. Neutrophils can also interact with T cells through NETs. As mentioned earlier, NETs can promote cancer metastasis, but they can also help clear tumor cells by directly activating T cells by lowering the activation threshold of T cells. In addition, studies have shown that neutrophils stimulated by IFN-γ secrete IL-18 to recruit and further activate NK cells to play an antitumor effect. In addition to inhibiting tumor growth by recruiting other immune cells, neutrophils can also mediate antibody-dependent cytotoxicity to kill cancer cells.
Inflammatory molecules secreted by neutrophils can also reduce the burden on tumors. In 2009, Fridlender et al. found an increased expression of TNFα in mouse N1 cells, which was shown to promote tumor cell death by excitating neutrophils to release superoxides. However, the role of this cytokine in the tumor microenvironment is unclear, and its role in different neutrophil polarization states needs further study.
Antitumor effect of anti-inflammatory neutrophils:
Examples of anti-tumor behavior of anti-inflammatory neutrophils with immunosuppressive effects are rarely described in the literature, possibly due to the fact that there is little research in this area, rather than the lack of these functions by anti-inflammatory neutrophils. There is evidence that neutrophils can regulate the secretion of IL-17, reducing tumor growth. Neutrophils inhibit γδ T cells that produce IL-17 by ROS-induced oxidative stress, which promote tumor growth and metastasis by secreting IL-17. However, another recent study found that IL-17-γδ T cells polarize neutrophils into an immunosuppressive phenotype, which in turn promotes tumor growth. Therefore, the interaction between neutrophils and these T cells in the tumor microenvironment is very complex and requires further study.
Pro-tumor effect of anti-inflammatory neutrophils:
Anti-inflammatory neutrophils have an immunosuppressive effect in cancer, mainly by inhibiting the recruitment of other immune cells in the tumor microenvironment to promote tumor growth. In particular, neutrophils can release ROS, as well as enzymes such as arginase 1, which can inhibit T cells' responses to the tumor microenvironment. Interestingly, the IL-8 secreted by cancer cells can induce neutrophils to release arginase into the tumor microenvironment, degrading extracellular arginine, and this amino acid is necessary for T cell activation and proliferation, so such neutrophils suppress the T cell response. Neutrophils can also inhibit NK cell activity through MPO, release of hydrogen peroxide, and expression of CXCR4 on the cell membrane. In addition, neutrophils through the contact-dependent mechanism of TNFα and nitric oxide can induce apoptosis of CD-8+ T cells in the tumor environment to inhibit the immune response, ultimately promoting tumor growth.
All in all, recent studies have highly highlighted the complexity and importance of understanding neutrophil biology in the tumor microenvironment. By modulating tumor microenvironmental components or other immune cells, neutrophils have both pro-tumor and tumor-suppressing functions. However, little is known about understanding the correlation between these different neutrophil populations, and whether they behave as subtypes, or whether the neutrophil phenotype is dynamically malleable over time. Identifying surface markers of neutrophils in order to track neutrophils in the tumor microenvironment in time and space can facilitate understanding of different neutrophil populations in humans. And based on the discovery of cell surface markers, certain neutrophil phenotypes can be targeted for therapy, thereby improving the success rate and therapeutic effect of immunotherapy applications in cancer patients.
In December 2017, Etienne Meylan's team from the Swiss Federal Institute of Technology in Lausanne published an article in the journal Cell Reports titled "Neutrophils and Snail Orchestrate the Establishment of a Pro-tumor Microenvironment in Lung Cancer", using a mouse lung cancer model. They found that the reduction in neutrophils caused a huge reconstruction of the lung tumor microenvironment, and T cells began to "pour" into the tumor, making the tumor sensitive to anti-PD-L1 immunotherapy. And neutrophils will promote tumor cells to secrete Snail protein, to resist drugs, and promote tumor recurrence and metastasis, while Snail can also promote the infiltration of neutrophils, forming a vicious circle, maintaining a microenvironment that supports tumor growth.