Chemokine regulation of neutrophil function in tumors

Highlights

• Neutrophils are endowed with both pro- and antitumor functions.

• Circulating neutrophils are a heterogeneous population.

• Anti-tumoral (N1) neutrophils are present in early stages of tumors and metastasis while in later stages pro-tumoral (N2) neutrophils predominate.

• CXC chemokines binding CXCR1 and CXCR2 are abundantly produced by tumors to recruit neutrophils.

• After extravasation neutrophils acquire a new pattern of chemokine receptors including CCR2 that promote anti-tumoral activity.

Abstract

The role of neutrophils in cancer and metastasis is still debated and controversial since they have been shown to be endowed with both pro- and antitumor functions. These contradictory results seem to be now explained by recent discoveries of tumor-associated neutrophils plasticity and multiple neutrophil subsets.

Chemokines and chemokine receptors are known to tightly regulate the release of neutrophils from the bone marrow, their passage into circulation and transmigration into the tissues as well as tumor infiltration. It is emerging that chemokine receptors are differentially expressed by neutrophil subsets and they affect not only their recruitment but also their effector functions.

Here we are resuming human and murine data suggesting that therapeutic modulation of neutrophil activity through the targeting of specific chemokines or chemokine receptors can improve their anti-tumoral properties.

Introduction

Neutrophils, also called polymorphonuclear leukocytes (PMNs), are the most abundant leukocytes population in circulation and they are essential effectors of the inflammatory response and defense against pathogens. Even if neutrophils are mostly considered for their anti-microbial function, more recently it has been described their involvement in several additional functions, both in physiological and pathological conditions, being able to regulate and activate the innate and adaptive immune responses [1]. Interestingly it is emerging an important role of neutrophils in cancer biology [2], [3].

Cancer cells activate mechanisms similar to the ones that regulate neutrophils recruitment to inflammatory sites, indeed they express various ligand (CXCL8, CXCL5 and CXCL6) for neutrophils receptors CXCR1 and CXCR2 that actively recruit neutrophils to tumor sites. Therefore neutrophils represent an important component of the tumor microenvironment and neutrophilia, recurrent in advanced cancer patients, has been associated with poor prognosis in many tumors.

A recent meta-analysis of published papers indicated that their presence in tumor tissue is associated with poor prognosis [4]. Furthermore an elevated ratio of peripheral neutrophils-to-lymphocytes (NLR) has been recognized as a poor prognostic indicator in various cancers [5].

In addition, neutrophils have been found to have a key role in the establishment of the pre-metastatic niche [6]. Nevertheless, neutrophil role in cancer is still not completely elucidated because both pro- and antitumor functions have been described [7] (Table 1).

Tumor associated neutrophils (TANs), recruited to tumor sites by signals produced by cancer cells and tumor microenvironment, are able to sustain cancer progression through several mechanisms. TAN_s are induced to release enzymes, contained in their granules, such as neutrophil elastase (ELA2), neutrophils collagenase (MMP8) and neutrophils gelatinase B (MMP9), that can promote tumor cells invasion by remodeling extracellular matrix (ECM) or directly acting on tumor cells [8]. Neutrophils were shown to support tumor growth and invasion via secretion of protumoral cytokines and growth factors (EGF, TGFβ, PDGF, HGF, VEGF). For example, hepatocyte growth factor (HGF) has been demonstrated to promote the invasion of human pulmonary adenocarcinoma cells [9]. PMNs can enhance VEGF production and tumor cell invasion even through the production of oncostatin M [10], [11], a member of IL-6 family, or releasing MMP9. In addition, granule enzymes are able to proteolytically activate the proangiogenic factors EGF, TGFβ, and PDGF from the extracellular matrix (ECM) [6].

Neutrophils can also enhance tumorigenesis through the release of reactive oxygen species (ROS, by myeloperoxidase and NADPH oxidase activity) and reactive nitrogen species (RNS, by nitric oxide synthase), which can contribute to further DNA damage and genetic instability [12].

Some molecules produced by neutrophils are also able to directly promote tumor cell proliferation. ELA2, prostaglandin E2 (PGE2) and leukotrienes activate intracellular signaling cascades which lead to tumor cell proliferation [13], [14]. In particular, leukotrienes support proliferation of metastasis initiating cells in different mouse model of breast cancer [15].

Another mechanism by which neutrophils can promote tumor metastasis is the facilitation of the adhesion of tumor cells to endothelial cells at the extravasation site. This mechanism can be mediated by neutrophil extracellular traps (NETs), chromatin fibers released by activated neutrophils and considered to be one of their antimicrobial mechanism. Tumor cells become trapped within the NETs and increase their adhesion to hepatic and pulmonary microvasculature. NETs can also promote cancer cell proliferation inhibiting apoptosis [16].

Furthermore PMN carry out protumoral activity inhibiting antitumoral immune responses. Indeed upon CXCL8 stimulation, neutrophils produce arginase 1, an inhibitor of T cell function [17]. It was also reported that neutrophils upregulate programmed death-ligand 1 (PD-L1) and suppress T-cell proliferation [18]. Finally, neutrophils were shown to promote tumor growth through the production of TGFβ, a cytokine with immunosuppressive effect on other immune cells [19]. Moreover, they produce chemokines and cytokines that actively recruit tumor-supporting cells to the tumor bed [20].

Despite the predominant outline of neutrophils protumoral functions, other studies revealed the abilities of these cells to have anti tumoral effects. In some cases, mechanisms described having a protumoral role were found to inhibit tumor growth. Indeed it has been shown that, even if ROS contribute to cell dead and tissue damage in the tumor microenvironment, they can be cytotoxic for tumor cells [21], [22]. Granot and his colleagues described how neutrophils can inhibit the metastatic seeding of breast carcinoma cells in the lungs by killing tumor cells via the generation of high levels of hydrogen peroxide [23].

Neutrophils are also able to kill tumor cells by antibody-dependent cell-mediated cytotoxicity [24] and they are important effectors of anticancer monoclonal antibodies based therapies [25]. Another killing mechanism is reported for interferon (IFN)-activated neutrophils that are able to release biologically active tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL/APO2 ligand), a molecule exerting selective apoptotic activities toward tumor cells [26], [27]. Neutrophil also mediate Fas-mediated apoptosis of cancer cells [28].

Finally, neutrophils are able to promote adaptive immune response to tumors. They can directly stimulate T cells through expression of OX-40L and 4-1BBL, costimulatory molecules that enhance proliferation of CD4⁺ and CD8⁺ T cells and increase their cytotoxic abilities in early stage of human lung cancer [29]. Indirect mechanisms by which neutrophils stimulate adaptive immune response against the tumor are release of NETs, [30] and production of cytokines and chemokines attracting monocytes and lymphocytes at tumor site, promoting immune cells crosstalk and therefore immune response [31].