Immuno-oncology is an emerging field that is revolutionizing cancer treatment. Although most immunomodulatory strategies have focused on increasing the response of T cells, there has been a recent increase in interest in taking advantage of the natural killer cell (NK) compartment. The clinical responses to adoptive immunotherapy based on NK cells, however, were thwarted by the profound immunosuppression induced by the tumor microenvironment, particularly severe in the context of solid tumors. Tumorassociated macrophages (TAMs) have a prominent role in suppressing modulation of the tumor microenvironment, and these cells have been shown to be involved in inactivation of NK cells stimulated in vitro and transferred adoptively to cancer patients. The interaction between TAMs and NK cells can therefore be a prospective therapeutic target to improve cell therapies against cancer, and molecules that target these two cells have the potential to be immunoadjuvant agents.
Keywords: Immune Response; NK Cells; TAMs; Cancer
Abbreviations:NK: Natural Killer; TAMs: Tumor-Associated Macrophages; CTLs: Cytotoxic T lymphocytes; ADCC: Antibody-Dependent Cell Cytotoxicity; IL: Interleukin; TNF-α: Tumor Necrosis Factor-α; TGF: Tumor Growth Factor; VTGF: Vascular Tumor Growth Factor; FGF: Fibroblast Growth Factor
The development of cancer involves six categorical
characteristics (cancer hallmarks):
1) Unregulated cell proliferation (due to self-sufficiency in growth signaling or insensitivity to inhibitory growth signals).
2) Evasion of programmed cell death.
3) Sustained angiogenesis.
4) Tissue invasion and metastasis.
5) Inflammation associated with the tumor and 6) Evasion of the immune response [1,2].
Pathologists have recognized that virtually all neoplastic lesions are infiltrated by cells from both the innate and adaptive immune responses, whose density varies from subtle infiltration to severe inflammation [3,4]. The immune system plays a role in combating the formation and progression of insipient neoplasms, late tumors and micro metastases, and clinical epidemiology gives evidence of this role of the immune response in some forms of human cancer [5,6]. For example, patients with ovarian and colon tumors that are heavily infiltrated with CD8+ cytotoxic T lymphocytes (CTLs) and natural killer cells (NK) have a better prognosis than those who lack these cells . In mice, deficiencies in the function of CTLs, CD4+ T helper lymphocytes, or NK cells led to an increase in the incidence of neoplasms [7,8].
According to this logic, in immunocompetent individuals, solid tumors have somehow managed to evade detection by the various aspects of the immune system . Suppressor cells are found (predominantly) in the tumor microenvironment, such as regulatory T cells (Treg), a subtype of CD4+ T lymphocyte capable of suppressing immune responses , immature myeloid cells with suppressive capacity (MDSCs)  and tumor-associated macrophages (TAMs; which often have a suppressor profile - M2) . In addition, in immunological interactions between the tumor and the host, highly immunogenic cancer cells can prevent immune destruction by disabling components of the immune system. For example, neoplastic cells can paralyze CTLs and infiltrating NK cells through the secretion of TGF-β and other immunosuppressive factors (Yang, et al.) . NK cells are innate effector cells that play a crucial role in inhibiting the development of cancer, due to their ability to recognize and lyse transformed cells without the need for prior sensitization . This ability drew a lot of attention to NK cells as promising immunotherapeutic agents . Its antitumor activity is regulated through a sophisticated network of inhibitory receptors and activators .
The first include Killer-cell immunoglobulin-like receptors
(KIR), CD94 / NKG2A and ILT2 / LIR-1 / CD85j, all capable of making
NK cells tolerant after binding to their own type I molecules of the
histocompatibility complex (MHC I) [15,16] The TGF-β receptor
also plays a role in suppressing cytokine-induced NK cell activation
; in addition, a relevant inhibitory receptor expressed by NK
cells in cancer is programmed cell death protein 1 (PD-1) .
Activating receptors include natural NCRs cytotoxicity receptors
(NKp30, NKp44, NKp46), type C lectin receptors [NKG2D (NK
group 2, member D) and CD94 / NKG2C], the DNAM-1 adhesion /
coactivation receptor (accessory DNAX-1 molecule) and the 2B4,
CS1 (CD2 cell surface glycoprotein subset 1) and NTB-A (NK, T,
B antigen) [15,16]. These receptors interact with ligands highly
expressed in the target cells after tumor transformation, viral
infection and cell stress and trigger natural cytotoxicity of NK cells.
In addition, NK cells express a CD16 receptor, which binds to the
IgG Fc fragment and mediates antibody-dependent cell cytotoxicity
However, to the detriment of their important antitumor role, the number and / or function of these lymphocytes is largely reduced during the progression of cancer. The correlation between the deficient function of NK cells and the development of metastases has been established in head and neck cancer [20,21], pharyngeal  and other solid tumors [23,24]. NK cells in the peripheral blood of patients with cervical cancer have lower levels of expression of the activation receptors (NKp46, NKp30 and NKG2D) and the magnitude of this negative regulation is correlated with tumor progression . Such observations stimulated the development of therapeutic approaches that aim to restore cytotoxicity mediated by intratumoral NK cells, blocking their inhibitory receptors or reducing the immunosuppressive factors present in the tumor microenvironment. NK cells do not attack healthy own tissues, nor do they induce a storm of inflammatory cytokines, enabling their use in allogeneic adoptive cell therapy [14,26,27].
Clinical responses to adoptive immunotherapy based on NK cells haven’t been promising, due to the profound immunosuppression induced by the tumor microenvironment, being particularly severe in the context of solid tumors . The tumor microenvironment makes NK cells dysfunctional, hindering cell proliferation, cytokine secretion, activator-receptor expressions and cytolytic activity [28-32]. As a result, adoptively transferred NK cells failed to demonstrate clinical benefits in solid tumors. For example, in a clinical trial, patients with metastatic melanoma or renal cell carcinoma showed no clinical response after the adoptive transfer of autologous NK cells activated in vitro, despite the fact that these cells efficiently lyse melanoma cells in culture . This study demonstrated that, after the adoptive transfer, it was discovered that NK cells were quiescent and could no longer lyse tumor cells in vitro and expressed significantly lower levels of the main activating receptor (NKG2D).
These findings suggested that cells in the tumor microenvironment, other than tumor cells, have a role in inhibiting NK cell function, and evidence has shown that TAMs can play a crucial role in this suppression . TAMs are found within the neoplastic tissue, as well as in the surrounding tissues  and can be pro- or anti-tumorigenic [35,36]. For, a suppressive profile predominates among TAMs . Macrophages are classified by literature into two main groups, M1 and M2 (although this polarization is a simplistic concept and there is, in fact, a complex phenotypic spectrum). M1 macrophages are involved in the inflammatory response, pathogen clearance and antitumor immunity, through the expression of proinflammatory cytokines, such as interleukin (IL) -1β, IL-6, IL-12, IL-23, tumor necrosis factor-α (TNF-α) and inducible nitric oxide synthase (iNOS or NOS2) [34,37,38]. In contrast, M2 macrophages are known to promote tissue repair and remodeling, angiogenesis and promote tumor progression .
M2 macrophages release anti-inflammatory cytokines, such as IL-10 and TGF-β, in addition to tumor growth factor (TGF), vascular growth factor (VTGF), other pro-angiogenic factors, such as the basic fibroblast growth factor (FGF2), and enzymes that degrade the extracellular matrix, including matrix-9 metalloproteinase (MMP-9) and other MMPs. Suppressive TAMs inhibit the Th1 antitumor response, while promoting the Th2 phenotype and Treg cell responses . In addition, IL-10 secreted by TAMs inhibits the local production of IL-12, a crucial cytokine to trigger the cytotoxicity of NK cells [41,42]. A recent study demonstrated that both M2 macrophages (of peritoneal origin and generated from bone marrow), and TAMs, in coculture with NK cells, substantially inhibit the activation of the latter and their cytotoxicity against tumor cells. Both macrophages, M2 and TAMs, are producers of the immunosuppressive cytokine TGF-β and the inhibition of this cytokine restored the cytotoxicity of NK cells in contact with the macrophages, implicating TGF-β in this inhibition mechanism . Therefore, reprogramming suppressive TAMs for a proinflammatory phenotype can change the course of the anti-tumor immune response.
Although it is known that M1 macrophages activate NK
cells during the antineoplastic response  and that there
is substantial evidence that TAMs have a prominent role in modulating the tumormicroenvironment, it is surprising that few
studies have investigated the role of TAMs in NK cell dysfunction
associated with neoplasms  and also, that the crosstalk
between these two cells as an immunotherapeutic strategy has not
been explored. The interaction between NK cells and TAMs can be
a prospective therapeutic target to improve the efficacy of NK cells
in cell therapies against cancer. For that reason, the manipulation
of cells of the immune system, including macrophages and NK cells,
have been tested as immunotherapy or even as adjuvants to other
techniques in the treatment of cancer . However, the difficulty
of sustaining the activation of these cells in the tumor suppressor
microenvironment is the main cause of clinical frustration; cellular
immunotherapy combining macrophages and activated NK cells
may be a way to overcome this obstacle.
Finally, it is relevant to mention that current treatment options for cancer, which often involve a combination of chemotherapy, radiation therapy and surgery, have been shown to be inefficient; in the case of glioblastomas, for example, the clinical course from the moment of diagnosis remains catastrophic and few patients achieve a 2.5-year post-diagnosis survival; the relative survival in the first year is 35%, 13.7% in the second year and less than 5% survive for 5 years after diagnosis [44,45]. Considering these limitations, efforts have been directed towards the development of new treatment strategies. In addition, another relevant point is that, while the usual chemotherapy against cancer causes early adverse events, compromising the defense mechanisms, the new classes of immune therapy through monoclonal antibodies and pro-inflammatory mediators, such as interleukins, can induce inflammatory responses. overwhelming and autoimmunity. Modulating the immune response through an agent that is not an immune mediator can generate a more specific response and with a lower risk of these side effects.
Competing of Interests
I declare that the authors have no competing of interests.
This work was supported by the following Brazilian foundations: Fundação de Amparo à Pesquisa do Estado de São Paulo (The São Paulo Research Foundation - FAPESP - grant number 2015/04194‐0) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (The Brazilian National Council for Scientific and Technological Development - CNPq - grant number 431465/2016‐9). Amanda Pires Bonfanti is a fellow of FAPESP (#2018/23559-7).
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