Brief review on the roles of neutrophils in cancer development

Abstract Neutrophils, which are traditionally regarded as a hallmark of inflammation, are also a member of the intratumoral immune cells. The roles of neutrophils in cancer development are diverse and undefined. So far, they are known to be involved in tumor initiation and tumor cell proliferation and metastasis. They show heterogeneity in both phenotypes and functions during early versus late stage of cancer development. Because they are also associated with the clinical outcomes of various types of solid tumors, cancer treatments that target neutrophils might be highly effective. In this review, we briefly cover the latest findings on the multiple roles of neutrophils in cancer development and point out the future directions as well.


INTRODUCTION
Neutrophils are the most abundant type of WBCs in the human body.
The name "neutrophil" is derived from the neutral pink color that these cells develop after histologic staining with H&E. By contrast, basophils are stained a dark blue color and eosinophils are stained bright red.
NETs were found to be a protective component against pathogens, since they are important for neutrophils to isolate cancer cells with antimicrobial factors. 34,35 Neutrophils also promote tumor cell proliferation through the secretion of inducible NOS (iNOS) and arginase 1 (ARG1) to inhibit CD8 + T cells 36,37 (Fig. 1).
Furthermore, neutrophils promote cancer metastasis through many mechanisms. For example, in the initial stage of metastasis, neutrophils suppress the function of natural killer cells and enhance the extravasation of tumor cells. 38 At a later disease stage, they interact directly with circulating tumor cells (CTCs) to promote cell cycle progression of those cells and accelerate their seeding. 39 One paper selected CTCs and CTCs-associated WBCs in patients with breast cancer and mouse models, with single-cells RNA sequencing, in which they found that in most of cases, CTCs are associated with neutrophils, and CTCs interaction with neutrophils is mediated by VCAM1. 39 NETs also can induce cancer cells migration in vitro, 40 especially large amount of NETs production under infection. 40,41 Another piece of evidence to support this role of neutrophils is the finding that their depletion through the use of antigranulocyte receptor-1 (anti-Gr1) antibodies suppressed metastasis. 42,43 Surprisingly, studies have also found that the depletion of antimetastatic neutrophils regulated by chemokine (C-C motif) ligand 2 (CCL2) enhances metastasis. 44,45 Neutrophils can also promote metastasis through direct interaction with cancer cells. 42

HETEROGENEITY OF NEUTROPHILS IN CANCER
It is already broadly agreed that neutrophils play a critical role in cancer development, while antitumoral role of neutrophils is not well understood. The heterogeneity of neutrophils has been reviewed elsewhere. 46 which can migrate into tumor-draining lymph nodes and cross-present antigens to suppress T cells function. 68 Second, neutrophils may directly kill tumor cells by releasing ROS and RNS. 44,56,69 In addition, it is published many years ago that TGF-blockade is related to neutrophils antitumoral function. 36 Following TGF-blockade, tumorassociated CD11b + Ly6G + neutrophils increased rather than Ly6G − macrophages, and these intratumoral neutrophils play a significant role in tumor cytotoxicity of myeloid cells in anti-TGF-treatment in a CD8 + -dependent manner. 36  receptors by cancer cells enhance response to hypoxia, which disturb neutrophils retention in bone marrow. 78,79 KRAS signaling in cancer cells and PTEN or SMAD4 depletion will promote GM-CSF and CXCR2 ligand expression. [80][81][82] Cancer cells also regulate neutrophils metabolism. Cancer cell can induce NETs production from neutrophils to enhance metabolism, through extracellular RNAs from cancer cells. 83 Tumor microenvironment also induces metabolically adaptation for neutrophils in mitochondrial metabolism and oxidative phosphorylation to maintain immune suppression. 84

CLINICAL EVALUATION BY NEUTROPHILS
Many studies have found that the neutrophil levels in patients correspond to their clinical outcomes, and the neutrophil-to-lymphocyte ratio (NLR) has become the most significant prognostic marker for many types of cancer. An elevated NLR is correlated with worse outcomes in many types of cancer and even reduces the patient's response to some antitumor treatments, [93][94][95][96][97] indicating that neutrophils may promote, rather than suppress, tumor growth. For example, in patients with renal cell carcinoma, the NLR correlates positively with the level of C-reactive protein (CRP, 98 a marker of systemic inflammation that is usually also increased in these patients 99,100 ) and is much easier to determine compared with CRP testing. A 1% increase in the NLR corresponds to a 15% increase in the risk of cancer recurrence. 101 Moreover, an NLR of >1.98 is correlated to a larger tumor size, higher nuclear grade, histologic tumor necrosis, and sarcomatoid differentiation 102 and associated with pathologic renal sinus fat invasion. 103 Patients with an NLR of >4.0 have a worse prognosis. 102 Neutrophils as a predictive factor for cancer is not only that NLR can predict the outcome of cancer, but also that absolute number can be predictive to the effect of treatment. Bevacizumab (Bv), which targets on vascular endothelial growth factor (VEGF), a factor that recruits neutrophils expressing proangiogenic matrix metallopeptidase 9, is frequently used for glioblastoma (GBM) patients. The prognostic value for these patients can be basal neutrophils and Tregs. Counts below 3.9 giga per liter (G/L) for neutrophils and above 0.11G/L for Tregs showed prolonged survival. Neutrophils count is highly associated with response to Bv only in steroid-free patients, whereas Tregs count is irrelevant to steroid. 104 NLR is directly associated with the clinical outcome of patients with cancer, which is usually combined with lymphocyte-to-monocyte ratio for evaluation. 105 Sometimes monocyte-neutrophils ratio is also utilized, 106 whereas it is important to have an absolute NLR value for patients to evaluate cancer stage, adequate treatment, and estimate clinical outcome, which would be largely valuable information. Besides, it seems that absolute number of neutrophils might be an underline factor to select the best treatment, which might be a promising index in clinic.
It is also significant to explore the potential targets on neutrophils to hamper cancer cells proliferation and metastasis. Neutrophils or neutrophils-related targets CXCR2 inhibitors or anti-Ly6G together with checkpoint inhibitors have been suggested to use in clinic. 58

DISCLOSURES
The authors declare no conflicts of interest.