Two key steps contribute to tumor metastasis. First, tumor cells proliferate and angiogenesis, then tumor cells break through the barrier of the extracellular matrix and invade outside. Subsequently, tumor cells shape solid tumors at the secondary site.

    The tumor microenvironment at the primary tumor site is consists of TAMs, TAFs (Tumor-associated fibrocytes), MDSC (myeloid inhibitory cells), mastocytes, and so on. These cells could promote tumor progression by secreting multiple cytokines and chemokines to affect tumor cell proliferation, the expression level of metastasis-related genes, inducing angiogenesis, and degradation of chemokine extracellular matrix.

    Success proliferation of metastatic cells at the metastatic target organ is the basis to form a metastatic tumor. Different target organs can release different cytokines by various kinds of cells to recruit tumor cells, promote tumor proliferation, induce angiogenesis, and eventually form metastasis. During this process, local or metastatic tumor cells can also release cytokines to cultivate a beneficial microenvironment [5].

4. How to acquire TAMs?

    TAMs are a kind of highly heterogeneous collection with diverse activation modes and markers among different tumor tissues. Moreover, the exact inducer of TAM is still not clear. Therefore, common methods to acquire certain TAMs are to isolate them from target tumor tissues in vivo or from the cancer cell-macrophage co-culture system in vitro ^[6][7][6, 7].

5.The molecular sources involved in forming TAMs in cervical cancer

    Multiple sources-derived molecules could promote the formation of TAMs in cervical cancer.

    Molecules from cervical cancer cells: it is well known that continuous high risky human papillomavirus (HPV) infection leads to cervical cancer. The HPV genome encodes E6 and E7 proteins. Accumulated data has shown that E6 and E7 could promote the transcription of COX-2 and IL-10 ^[8][9][10][8, 9,10]. Meanwhile, studies show that IL-10 induces monocytes to the M2 subtype [11]. Moreover, IL-10 can also inhibit the classic activation of macrophages by the JAK1/Tyk2/STAT3 pathway [12]. COX-2 mediates PEG2 production, while PGE2 has a predominant impact on the phenotype of the M2-like macrophage [13].

    Molecules from seminal plasma: seminal plasma (SP) could activate COX-2 and increase prostaglandin E2 receptor expression in cervical adenocarcinoma cells ^[14][15][14, 15]. In vitro stimulation of cervical cells with normal seminal plasma resulted in significantly elevated concentrations of secreted IL-6 [16]. Moreover, prostaglandin E2 and IL-6 acted in M2-like macrophage polarization [13]. These results indicate that SP may affect the differentiation of monocytes via prostaglandin or IL-6 in cervical cancer. However, more experimental verification of the effect of the molecules in SP on macrophages is expected to be performed.

    Molecules from anaerobic microenvironment: Overexpressed Nrp-1 in hypoxia-primed cervical cancer cells was necessary for hypoxic cervical TME to recruit and polarize macrophages towards the M2-like phenotype [17].

6.What are the roles of TAMs in cervical cancer?

    First, TAMs affect the proliferation of cervical cancer cells. For example, Bacillus Calmette - Guerin (BCG) promotes macrophage polarization towards the M1 phenotype and enhanced the transition of M2 to M1 macrophages. The results also revealed that polarized M1 macrophages induced by BCG decreased the protein expression of phosphorylated (p‑)retinoblastoma (Rb)/E2F transcription factor 1 (E2F1), inhibits the proliferation, and promotes the apoptosis of cervical cancer cells [18].

    Second, TAMs may affect the invasion of cervical cancer cells. According to the quantitative analysis results, the median value of the TAM density was higher in the CC group (5,540.14) than in the CIN I-III group (2,502.17) and the chronic cervicitis group (1,403.31), with statistical significance in all three groups (pP<0.001, for between-group comparisons) [19]. Though there are many studies on the effect of TAMs on the invasion of cancer cells ^[20][21][22][20,21,22], less research has been reported in cervical cancer at the cell level.

    Third, TAMs are involved in the blood and lymphatic angiogenesis of cervical cancer. The human lymphatic endothelial cells (HLEC), which were cultured in a conditioned medium of cervical cancer cell-macrophage coculture, formed significantly more tube-like structures in vitro when compared with those in conditioned mediums of LEC, normal cervical epithelium, single macrophage, and single cervical cancer cell [23]. Interleukin-10 (IL-10) derived from hypoxic TAMs adjacent to lymphatic vessels was a prerequisite for lymphangiogenesis through its induction of Sp1 upregulation in lymphatic endothelial cells ^[24][24]. Moreover, extracellular E7 oncoprotein in cervical cancer promotes macrophages to release IL-6, TNF alpha, and IL-1 which was reported to induce new blood vessels by activating cellular adhesion molecule expression [25].

    Last, TAMs could suppress the immune in cervical cancer. CD11b+ F4/80+ /ArginaseI+ IL10+ M2 macrophages are the main population of immune cells infiltrating HPV+ cervical tumor mouse model. Depletion of this population using clodronate-containing liposomes inhibits tumor growth and enhances tumor-specific CD8 T-cell responses. The results clearly indicated that TAMs facilitate tumor growth by inhibiting antitumor T-cell function [26]. M2 macrophages display a tolerogenic phenotype reflected by a lower expression of costimulatory molecules, an altered balance in IL-12p70 and IL-10 production, and a poor capacity to stimulate T cell proliferation and IFN-γ production in cervical cancer [27].

7. The therapeutic strategies to target TMAs in cervical cancer

    An abundance of molecules has been discovered to act in the formation of TAMs, so the translation of basic research to clinical application is a concerned subject.

    Polymethyl methacrylate (PMMA) is a synthetic polymer approved by the Food and Drug Administration for certain human clinical applications such as bone cement. PMMA 4 particles stimulate the highest level of TNF-alpha production by macrophages in vitro and yields the best result of antitumor protection in vivo [28]. A new synthetically prepared fluoroquinolone derivative 6-fluoro-8-nitro-4-oxo-1,4-dihydroquinoline-3-carboxylate (6FN) can induce RAW 264.7 macrophages cell-release of pro-and anti-inflammatory TH1, TH2 and TH17 cytokines with anti-cancer and/or anti-infection activities. Significant inhibition of growth in cancer cells HeLa is detected [29]. Indomethacin (Ind.) is a kind of prostaglandin inhibitor. The combination therapy of OK-432 and Ind. reinforce the macrophage-mediated immunopotentiation, resulting in a stronger antitumor effect [30]. Peritoneal macrophages obtained from patients with CC, when treated with sizofiran (SPG) and rIFN-gamma, show a potent tumoricidal quality [31]. Moreover, Sizofiran -immunotherapy combined with radiotherapy induces the cytotoxic activity of macrophages in cervical cancer patients [32]. The synthetic long peptide (SLP) vaccination could induce cytokine-producing T-cells to develop a strong capacity to skew M2-like macrophages to M1-like macrophages, which is necessary for cervical tumor tissue shrinkage [33]. Antibodies in serum from a recombinant vaccinia virus expressing the E2 gene of bovine papillomavirus are capable of activating cytotoxicity mediated by infiltrating macrophages for the efficient killing of papilloma tumor cells [34].

8. What we should explore next?

    Currently, the phenotypic identification of M1 and M2 macrophages is well established in mice. However, due to the substantial physiological and immunological disparities between mice and humans, murine models poorly mimic human inflammatory biology and murine markers are of limited use in human studies [35]. In the term of human macrophages study alone, the markers or secretions are variously caused by different cell types to generate macrophages (monocytic cell lines or primary cells from blood), different culture conditions, and different stimuli for polarization [36, 37, ^[36][37][38]38]. Moreover, increasing research suggests that there are not only M1 and M2 macrophages among TAMs, but at least 4 kinds of subtypes, including M1, M2a, M2b, and M2c macrophages [39]. Therefore, identifying signatures that could distinguish all the subtypes of TAMs cultured in the medium simulating the human environment is necessary. These studies will lay the foundation of further research about constructing prognostic TAM signatures for patient stratification and identifying specific therapeutic targets.