Microenvironments that tumor cells encounter are different during the stages of cancer progression—primary tumor, metastasis, and at the metastatic site. This suggests potential differences in immune surveillance of primary tumor and metastasis. Epithelial–mesenchymal transition (EMT) is a key reversible process in which cancer cells transition into highly motile and invasive cells for dissemination. Only a tiny proportion successfully metastasize, supporting the notion of metastasis-specific immune surveillance. EMT involves extensive molecular reprogramming of cells conferring many clinically relevant features to cancer cells and affects tumor cell interactions within the tumor microenvironment. We review the impact of tumor immune infiltrates on tumor cell EMT and the consequences of EMT in shaping the immune microenvironment of tumors. The usefulness of EMT as a model to investigate metastasis-specific immune surveillance mechanisms are also explored. Finally, we discuss potential implications of EMT for tumor immunogenicity, as well as current immunotherapies and future strategies.

Metastasis is the primary cause of cancer-related mortality and can occur early through parallel progression along with the primary tumor or late after linear tumor progression (1). Epithelial–mesenchymal transition (EMT) is a reversible cellular process by which stationary epithelial cancer cells trans-differentiate into highly motile and invasive mesenchymal-like cells, giving rise to disseminating or circulating tumor cells that initiate tumor metastasis (2, 3). After reaching the target organ, the disseminated tumor cells undergo the reverse phenotypic conversion from mesenchymal back to epithelial through a process known as mesenchymal-epithelial transition (MET) (4).

During EMT, cells downregulate the expression of multiple epithelial junctional proteins, including E-cadherin, an adherens junction protein, leading to the dissolution of cell-to-cell contacts, the loss of apico-basal polarity, and the acquisition of front–rear polarity (3, 4). Inhibition of E-cadherin expression is a hallmark of EMT, and it is followed by the induction of proteins, including N-cadherin, extracellular matrix (ECM) components, and the enzymes that can degrade them. Cells also undergo a robust reorganization of actin-cytoskeletal architecture and a dramatic change in the cell shape. Together, these key events result in a migratory and invasive capacity that defines the mesenchymal phenotype (3, 4). The change in the epithelial and mesenchymal gene expression that occurs during EMT is regulated by multiple transcription factor families that include Snail, Twist, Zeb, and bHLH (5). The transcription factor involved and its role depend on the cell and tissue type, as well as on the signaling pathway that initiates EMT (5). The multifunctional cytokine TGF-β, which is rich in tumor microenvironments (TMEs) and correlates with poor patient prognosis, has emerged as a potent inducer of EMT (6, 7). Initiation and progression of TGF-β–induced EMT involves coordinated regulation of multiple signaling pathways by altering the expression or activation of their signaling components (6, 7). Several growth factors (EGF, HGF, FGF, IGF, and PDGF) and developmental cytokines (Wnt, Notch, and Hedgehog) are known to induce EMT (5). Similarly, inflammatory cytokines, including TNF-α, IL-6, IL-1, and IL-8, in the TME were also implicated in the induction of EMT (5, 8). All of the non–TGF-β cytokines can induce EMT through cross-talk with the TGF-β–dependent pathways or induce the expression of EMT transcription factors (8). In addition to imparting a migratory and invasive capacity, EMT was shown to endow resistance to chemotherapy and radiation and confer stem cell–like properties, and it is known to promote immunosuppressive mechanisms in the TME (5, 9). Together, these abilities may allow cancer cells to successfully navigate the highly inefficient process of metastasis and link EMT to major clinical aspects that are responsible for cancer-related mortality. In contrast, the process of MET is less-well characterized. Although inhibition of TGF-β signaling or BMP-induced miR-200 expression was shown to promote MET (10), the precise molecular mechanisms involved are not clear. For a comprehensive understanding of all changes that occur during EMT, the mechanisms that direct those changes, and the pathways that mediate those mechanisms, please refer to extensive and outstanding reviews (3, 5).

Mechanisms of tumor immune surveillance have been thoroughly investigated in the primary tumor setting where the competition between pro- and antitumor mechanisms dictates the outcome of tumor initiation and growth (11, 12). Accumulating evidence suggests the existence of metastasis-specific immune surveillance in epithelial malignancies (13, 14). Given the critical role of EMT in initiating and promoting tumor metastasis, we review the immunological consequences of EMT in tumor progression. We outline the impact of the immune microenvironment on tumor cell EMT and then describe how cells undergoing EMT may interact with immune cells in the primary tumor and during metastasis. Finally, we discuss the implications of molecular reprogramming that occurs during EMT on metastasis-specific immune surveillance, tumor immunogenicity, and immunotherapies.

Inflammation plays a critical role at every stage of tumor development. Cancer cells produce various cytokines and chemokines that can recruit a diverse array of immune cells into the tumor, including macrophages, neutrophils, dendritic cells (DCs), NK cells, mast cells, myeloid-derived suppressor cells, and T and B lymphocytes, that constitute the tumor-associated inflammation (11, 12). Interactions of cancer cells with the immune microenvironment are critical for tumor progression, starting with tumor initiation and proceeding to immune surveillance, promotion of metastasis, and response to therapy (15). All tumor-infiltrating immune cells are capable of producing multiple inflammatory mediators to modulate tumor progression. Cancer cells, by engaging in a dynamic cross-talk with immune cells, exhibit EMT/MET plasticity to adapt to the changing microenvironment that they encounter in the primary tumor, during metastasis and at the distant site (Fig. 1, Table I) (16). These diverse interactions and the inflammatory mediators that they produce, collectively and individually, can determine the course of tumor progression. In the early stages, they can trigger neoplastic transformation by inducing genomic instability through production of DNA-damaging agents. In the later stages, they promote metastasis through multiple mechanisms, including induction of EMT (15).

FIGURE 1.

Cancer and immune cell interactions during tumor progression. (1) Differential recruitment of various immune cell types is modulated by the TME. (2) TAMs, neutrophils, and monocytes are converted to an anti-inflammatory phenotype. (3) M2 TAMs, in conjunction with MDSCs, help to drive EMT in the TME. (4) These recruited cells also help to suppress tumor-infiltrating lymphocytes. Upon successful EMT, cells can maintain this phenotype via autocrine complement protein C3a production (5) and block complement-mediated cytotoxicity by upregulating CD59 (6). MDSCs can help metastasis formation by secreting versican to help the reverse process of MET and seed the premetastatic niche at the distant site from the primary tumor. (7) Increased PD-L1 and tumor autophagy aid in blocking CTL synapse formation and cytotoxicity (8). This metastatic process can be limited by NK cell–mediated cytotoxicity by targeting upregulated NKG2D ligands.

FIGURE 1.

Cancer and immune cell interactions during tumor progression. (1) Differential recruitment of various immune cell types is modulated by the TME. (2) TAMs, neutrophils, and monocytes are converted to an anti-inflammatory phenotype. (3) M2 TAMs, in conjunction with MDSCs, help to drive EMT in the TME. (4) These recruited cells also help to suppress tumor-infiltrating lymphocytes. Upon successful EMT, cells can maintain this phenotype via autocrine complement protein C3a production (5) and block complement-mediated cytotoxicity by upregulating CD59 (6). MDSCs can help metastasis formation by secreting versican to help the reverse process of MET and seed the premetastatic niche at the distant site from the primary tumor. (7) Increased PD-L1 and tumor autophagy aid in blocking CTL synapse formation and cytotoxicity (8). This metastatic process can be limited by NK cell–mediated cytotoxicity by targeting upregulated NKG2D ligands.

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Table I.
Summary of cancer–immune cell interactions and their functional consequences
Cell TypeIn Primary TumorDuring TransitionAt Metastatic Site
Effects of tumor-infiltrating immune cells on cancer cell EMT 
 TAMs Recruited by CCL2 (17, 18Prevent anoikis (24, 25Promote metastasis (17
  Secrete TGF-β, PDGF, EGF, TNF-α, IL-1, and IL-6 (11, 17, 18Tumor cell survival (21, 2325 
  Induce EMT (11, 17Degrade ECM (21, 23 
  Promote angiogenesis and tumor growth (11, 16Aid intra- and extravasation (22, 23 
 Monocytes Secrete TGF-β, PDGF, EGF, TNF-α, IL-1, and IL-6 (11, 17, 21Aid intra- and extravasation (27pMOs inhibit metastasis (28
   Induce EMT?   
 Neutrophils Recruited by CXCL15 and HMGB1 (32NETs trap cancer cells (30, 31Promote metastasis (30, 31
  Promote tumor growth?   
 MDSCs Secrete MMPs, CXCL5, CXCL12, VEGF, bFGF, HGF, TGF-β (45 Establish premetastatic niche by versican secretion (46
  Induce EMT (44  
  Promote tumor growth (4043  
 Platelets  Shield from NK cells (38Promote metastasis (34
   Sustain EMT via TGF-β (36 
   Aid in extravasation (37 
 Complement Induces EMT via C3a → TWIST (48C3a sustains EMT (48 
  Promotes tumor growth (48  
 T cells Promote tumor growth (51  
 Induce EMT via Tregs?   
Impact of EMT on immune cell functions 
 Macrophages Promote M1→M2 via IL-4, GM-CSF, TGF-β (27  
  M2 via TGF-β–induced IRAK-M (49  
  Promote tumor growth (49  
 Complement  Induce CD59 on tumor cells (59Promote metastasis (59
   Resistance to CDC (59 
 T cells Induce Tregs (51)Evasion of CTLs (52Tumor autophagy blocks CTL synapse formation (52Promote metastasis (51
 NK cells Secrete soluble ligands (69)Promote tumor growth? Induce activating ligands [NKG2D ligand (69), nectins (71, 72)] Inhibit metastasis? (68
  Suppress inhibitory ligands (epithelial cadherin) (69)Secreted soluble NKG2D ligand leads to suppression?  
Cell TypeIn Primary TumorDuring TransitionAt Metastatic Site
Effects of tumor-infiltrating immune cells on cancer cell EMT 
 TAMs Recruited by CCL2 (17, 18Prevent anoikis (24, 25Promote metastasis (17
  Secrete TGF-β, PDGF, EGF, TNF-α, IL-1, and IL-6 (11, 17, 18Tumor cell survival (21, 2325 
  Induce EMT (11, 17Degrade ECM (21, 23 
  Promote angiogenesis and tumor growth (11, 16Aid intra- and extravasation (22, 23 
 Monocytes Secrete TGF-β, PDGF, EGF, TNF-α, IL-1, and IL-6 (11, 17, 21Aid intra- and extravasation (27pMOs inhibit metastasis (28
   Induce EMT?   
 Neutrophils Recruited by CXCL15 and HMGB1 (32NETs trap cancer cells (30, 31Promote metastasis (30, 31
  Promote tumor growth?   
 MDSCs Secrete MMPs, CXCL5, CXCL12, VEGF, bFGF, HGF, TGF-β (45 Establish premetastatic niche by versican secretion (46
  Induce EMT (44  
  Promote tumor growth (4043  
 Platelets  Shield from NK cells (38Promote metastasis (34
   Sustain EMT via TGF-β (36 
   Aid in extravasation (37 
 Complement Induces EMT via C3a → TWIST (48C3a sustains EMT (48 
  Promotes tumor growth (48  
 T cells Promote tumor growth (51  
 Induce EMT via Tregs?   
Impact of EMT on immune cell functions 
 Macrophages Promote M1→M2 via IL-4, GM-CSF, TGF-β (27  
  M2 via TGF-β–induced IRAK-M (49  
  Promote tumor growth (49  
 Complement  Induce CD59 on tumor cells (59Promote metastasis (59
   Resistance to CDC (59 
 T cells Induce Tregs (51)Evasion of CTLs (52Tumor autophagy blocks CTL synapse formation (52Promote metastasis (51
 NK cells Secrete soluble ligands (69)Promote tumor growth? Induce activating ligands [NKG2D ligand (69), nectins (71, 72)] Inhibit metastasis? (68
  Suppress inhibitory ligands (epithelial cadherin) (69)Secreted soluble NKG2D ligand leads to suppression?  

Tumor-associated macrophages.

Tumor-associated macrophages (TAMs) are one of the major components of the immune cell infiltrates observed in the TME. They are derived from inflammatory monocytes that are recruited largely by MCP-1/CCL2 chemokines (17, 18). TAMs are implicated in a multitude of tumor-promoting functions, including angiogenesis, immune suppression, and EMT (11, 16). Genetic ablation of CSF1, a major lineage regulator of macrophages, or deleting its direct effector, Ets2, results in macrophage depletion and a marked reduction in metastasis in the PyMT model of breast cancer (19, 20). In the same model, recruitment of TAMs results in a TME rich in TGF-β, a potent inducer of EMT, along with mitogenic growth factors like PDGF and EGF. This leads to induction of EMT by TAMs, thus promoting metastasis (11, 17). TAMS are also a source of TNF-α, IL-6, IL-1, and matrix metalloproteinases (MMPs), which are known to enhance TGF-β–induced EMT and subsequent invasion (21). Consistently, analysis of primary tumors from patients with non-small lung cancer revealed a positive correlation among intratumoral macrophage densities, EMT markers, TGF-β levels, and tumor grade (16). Although the precise mechanisms by which TAMs mediate tumor progression in vivo are still unknown, they are implicated in every step of the metastatic cascade. Following EMT, intravasation of cancer cells into vasculature is facilitated by perivascular TAMs (22). During this process, cancer cells secrete CSF1 for the recruitment of TAMs; in turn, TAMs produce EGF and activate EGFR signaling in cancer cells. Together with other cytokines in the TME, TAMs induce an EMT-like phenotype with enhanced motility, invasion, and ECM degradation to promote intravasation of cancer cells (23). Once in circulation, cancer cells have to survive anoikis. Many inflammatory mediators derived from immune cells and cancer cells, including TNF-α, IL-6, and epiregulin, can promote cancer cell survival by activating pathways such as NF-κB and Stat3 (24, 25). In a mouse model of breast cancer, TAMs were shown to promote cancer cell survival by physically interacting with them (23).

Monocytes.

Monocytes are a diverse set of cells with several subtypes that have distinct function in the TME. A monocyte subpopulation, metastasis-associated monocytes (MAMs), preferentially migrates to metastatic sites rather than to primary tumors in breast and colorectal cancers (26). Similar to TAMS, MAMs are also derived from inflammatory monocytes, recruited by CCL2, and acquire prometastatic phenotype. MAMs were shown to promote cancer cell extravasation and survival at metastatic sites. Importantly, neutralizing CCL2 blocked recruitment of MAMs and inhibited cancer cell extravasation (27). In addition to the classical inflammatory monocytes that differentiate into TAMs or MAMs, a recent study demonstrated a critical role for nonclassical patrolling monocytes (pMO) in tumor metastasis (28). This study showed that pMOs accumulate in the microvasculature of lung and inhibit lung metastasis in multiple mouse models. pMOs reduce metastasis by interacting with cancer cells in the vessels and later recruit and activate NK cells (28). It would be interesting to study the effects of MAMs and pMOs on cancer cells, which, unlike TAMs, promote EMT and metastasis. For instance, it would be fascinating to determine whether MAMs promote MET at metastatic sites to facilitate successful colonization by cancer cells.

Neutrophils.

Studies demonstrated the pro- and antimetastatic effects of neutrophils during tumor progression. Depletion of neutrophils promoted lung metastasis in a mouse model of breast cancer (29). Consistent with this, neutrophils isolated from a tumor bearing mice showed cytotoxicity against cancer cells in vitro and adoptive transfer of these neutrophils blocked experimental lung metastasis (29). Neutrophils also produce unique structures called neutrophil extracellular traps (NETs), which are composed of extruded DNA and antimicrobial proteins. After surgical stress or infection, cancer cells were shown to become trapped in NETs that formed in liver and lung capillaries, promoting the development of micrometastasis (30, 31). In contrast, cancer cells were also shown to recruit neutrophils through CXCL15 or HMGB1 secretion (32). Recruited neutrophils were implicated in enhancing angiogenesis, intravasation of cancer cells, and suppression of cytotoxic CD8 T lymphocytes, resulting in the promotion of metastasis (11, 33). Interestingly, TGF-β was shown to induce a switch from an antimetastatic to a prometastatic phenotype in neutrophils in a mouse model of mesothelioma (32). Therefore, it is possible that the prometastatic functions of neutrophils are regulated by specific environmental factors in a similar manner to TAMs. Like TAMs, neutrophils may also modulate EMT, at least in the context in which they are known to promote metastasis.

Platelets.

Also known as thrombocytes, platelets are small, enucleated cellular structures and are second most abundant in the circulation after erythrocytes (34). The primary role of these cells is to stop bleeding (hemostasis) after tissue or vascular injury (34). Increased platelet numbers were associated with decreased patient survival in a number of tumor types, including breast, lung, pancreatic, and brain, suggesting a role for platelets in tumor progression (35). In circulation, platelets form platelet–cancer cell aggregates to aid and shield migrating cancer cells by multiple mechanisms and promote metastasis. In colon and breast cancer, platelets promote extravasation of cancer cells by inducing EMT, through direct contact and release of TGF-β (36). Platelet-specific ablation of TGF-β production or cancer cell–specific inhibition of NF-κB activation protected mice from tumor metastasis (36). In melanoma, platelet-derived ATP was shown to activate a purinergic receptor, P2Y, on endothelial cells to increase vascular permeability and promote cancer cell extravasation (37). In this case, genetic ablation of P2Y suppressed metastasis (37). Formation of platelet–cancer cell aggregates may also protect circulating cancer cells from NK cell– and T cell–mediated immune surveillance (38). In addition, platelet-derived cytokines, including PDGF, VEGF, and TGF-β, can promote cancer cell survival, angiogenesis, and EMT in the primary TME and promote metastasis (34). Given the dynamic molecular changes that occur during EMT, it is reasonable to expect potential differences between epithelial and mesenchymal phenotypes in their ability to interact with platelets. Investigating these differences may help in the targeting of platelet–cancer cell interactions for metastatic control.

Myeloid-derived suppressor cells.

Abnormal differentiation of the myeloid compartment in tumor-bearing mice and cancer patients results in the accumulation of immature immunosuppressive myeloid cells called myeloid-derived suppressor cells (MDSCs), reflecting their origin and function (39). MDSCs contribute to tumor progression by involving a variety of immune suppression–dependent and -independent mechanisms. MDSCs are known to produce a plethora of soluble factors, including MMPs, CXCL12, CXCL5, VEGF, bFGF, HGF, and TGF-β to promote angiogenesis, cancer cell invasion, and metastasis. Clinical relevance of MDSCs was demonstrated in multiple cancers in which the number of circulating MDSCs in patients correlated with advanced disease stage and metastasis (4043). In a spontaneous mouse model of melanoma, MDSCs recruited to the tumor site produced HGF and TGF-β to induce EMT in melanoma cells. Depletion of MDSCs suppressed melanoma metastasis by inhibiting cancer cell EMT (44). MDSCs are also known to promote metastasis by inducing cancer cell stemness in ovarian cancer (45). Intriguingly, MDSCs are implicated in the formation of premetastatic niches; MDSCs reach the niche before the cancer cells and condition it to promote cancer cell seeding by secretion of immunosuppressive factors, including S100A8/A9, bFGF, IL-10, and IL-4. Once cancer cells reach this metastatic niche, MDSCs are implicated in promoting MET in cancer cells by secreting versican (46). The lack of a clear understanding of the precise mechanism by which MDSCs are recruited to the premetastatic niche makes this a mysterious process.

Indirect mechanisms.

In addition to the above-described direct effects on cancer cells resulting from cell-to-cell interactions with immune cells, the inflammatory cytokines produced by all immune cell types can modulate EMT through indirect mechanisms. The transcription factor Snail, an important regulator of E-cadherin expression during EMT, is protected from degradation in response to TNF-α signaling. Thus, stabilized Snail aids in completing EMT and promotes cancer cell migration and metastasis (25). Similarly, other EMT transcription factors like Twist and Kiss are regulated by proinflammatory cytokines (47), including complement component C3a (48). Activation of Stat3 was implicated in Twist induction, and NF-κB–mediated induction was shown for Twist and Kiss expression (25, 47). EMT-induced cancer cell invasion requires extensive proteolysis of the ECM. In addition to cancer cells, inflammatory immune cells are an important source of ECM-degrading proteases, including MMP2 and MMP9. Again, cytokines like TNF-α, IL-6, and IL-1 are implicated in the induction of these proteases. After EMT, when metastatic cells enter the circulation, the same cytokines also promote the survival of tumor cells in circulation through activation of NF-κB– and Stat3-mediated survival pathways (12, 25, 47).

The other most important aspect of TME cross-talk is the ability of cancer cells to modulate immune responses within the tumor. The most common theme in these interactions is that cancer cells interfere with the antitumor responses by secreting soluble mediators that block the effector functions of the involved immune cells and reprogram them into cells of a regulatory phenotype. Robust morphological and molecular changes that occur during EMT support the idea that cells undergoing EMT have the potential to modulate the function and phenotypes of innate and adaptive immune cells in the TME (Fig. 1, Table I). However, only a few studies looked at the impact of EMT on the interactions between cancer and immune cells.

Impact on TAMs.

After the recruitment of macrophages into the TME, the reciprocal interaction between macrophages and cancer cells involves modulation of macrophage phenotype by cells undergoing EMT. Studies showed that cancer cells can skew macrophages more toward an anti-inflammatory macrophage (M2) phenotype that is associated with TAMs through the production of various factors, including IL-4, GM-CSF, and TGF-β (27). Although the precise mechanisms by which macrophages acquire the tumor-promoting TAM phenotype are not clear, recent studies, including ours, identified a role for TLR signaling. We demonstrated that tumor cell–derived TGF-β induces the expression of IRAK-M, a negative regulator of TLR signaling, in macrophages, promoting an M2 phenotype (49). Genetic ablation of IRAK-M in mice inhibited tumor growth by promoting an inflammatory macrophage (M1)-like phenotype in TAMs (49). In another example, screening for cancer cell–derived factors that promote macrophage activation identified ECM component versican in mouse lung cancer cells (46). Versican is also upregulated in many human tumors. This study demonstrated that versican activated macrophages through TLR2, and induced IL-6 and TNF-α, to generate a microenvironment that facilitates metastatic outgrowth of Lewis lung carcinoma cells (46). A similar skewing of neutrophils to a more tolerogenic phenotype by cancer cells was also reported in the TME (32); however, the differential impact of epithelial versus mesenchymal phenotypes was not assessed in any of the above studies. Using a Snail1-overexpression model, EMT cells were shown to induce differentiation of immature DCs into regulatory DCs, with low MHC class II expression (50).

EMT and CTL functions.

Unlike innate immune cells, there is no evidence that T and B cells can directly modulate tumor cell phenotype, including induction of EMT, despite their contribution to the overall tumor progression. On the contrary, cells undergoing EMT were shown to induce the activation of immunosuppressive regulatory T cells (Tregs). Using TGF-β–induced and Snail1-overexpression models of EMT in melanoma cells, thrombospondin-1 produced during EMT was implicated in the induction of FOXP3 expression in CD4+ T cells (50). Inhibition of Snail1 or neutralization of thrombospondin-1 was sufficient to restore T cell infiltration and induction of antitumor immune responses in the B16-F10 melanoma tumors (50). In MCF-7 human breast cancer cells, acquisition of EMT phenotype was associated with the inhibition of CTL-mediated lysis (51). This inhibition was attributed to the dysfunctional immunologic synapse between CTLs and cancer cells, along with the induction of autophagy in cancer cells. Interestingly, inhibition of autophagy in cancer cells restored susceptibility to CTL-mediated cytotoxicity (51). This is consistent with the fact that the extensive actin cytoskeletal remodeling that occurs during EMT is also critical for the formation of an immunological synapse (52). Furthermore, this suggests that, in addition to the recognition of a tumor cell, the formation of a successful immunological synapse is critical for host immune surveillance. The observed differences in the ability of epithelial and mesenchymal-like cells to form immunological synapses may also contribute to the potential metastasis-specific immune surveillance. In a unique mouse model of melanoma in which tumor cells disseminate early, even before the primary tumor is detectable, disseminated tumor cells were kept dormant at distant sites, in part, by cytostatic CD8+ T cells. Depletion of these cells restored metastatic outgrowth, demonstrating immune control of metastasis (14). A more recent study demonstrated an important molecular link between EMT and CTL dysfunction. This study provided evidence that miR-200, a suppressor of EMT, targets PD-L1, which is a ligand for the CTL checkpoint receptor PD-1 (53). Transcription factor ZEB1, an EMT activator, induces PD-L1 expression on tumor cells by relieving the miR-200–mediated suppression of PD-L1, resulting in the suppression of CTL function and promotion of metastasis. These findings suggest that the EMT phenotype may serve as a biomarker to identify subgroups of patients who may respond to checkpoint inhibitors, such as PD-L1 and CTLA4 antagonists.

Regulation of complement-mediated cytotoxicity.

The complement pathway is recognized as a first line of defense in host immune surveillance against non-self microbial and tumor cells (54). The deposition and activation of complement component proteins in tumor tissues, coupled with increased expression of inhibitory complement regulatory proteins on tumor cells, illustrate the importance of the complement pathway in host immune surveillance against cancer (5558). In a recent study, we observed resistance to complement-dependent cytotoxicity (CDC) and induction of CD59 expression after TGF-β–induced EMT in lung cancer cells. CD59 is a potent inhibitor of membrane attack complex that mediates CDC. Inhibition of CD59 expression restored the susceptibility to CDC of cells that underwent EMT in vitro and blocked experimental metastasis by these cells (59). In contrast, complement-activating components are also implicated in promoting tumor progression. For example, complement component C3a was shown to trigger EMT in ovarian cancer cells through the induction of Twist, an EMT transcription factor (48). One possible explanation for such paradoxical effects is that EMT renders cancer cells resistant to complement-mediated cytotoxicity, after which the activating components of the complement might promote tumor progression by sustaining EMT.

EMT confers susceptibility to NK cell–mediated cytotoxicity?

NK cells were initially identified by their ability to kill tumor cells without prior sensitization (60, 61). An epidemiological study showed that low NK cell activity in blood correlates with high incidence of malignancies, suggesting a critical role for NK cells in the host’s immunosurveillance against cancer (62). As is the case for T cells, despite the presence of NK cells in the tumors, there is little evidence that they actively contribute to tumor progression, including induction of EMT. Consistently, tumor infiltration of NK cells was primarily associated with better patient prognosis or had no influence at all. In contrast, an immunosuppressive TME, which may also include cells undergoing EMT, renders tumor-infiltrating NK cells hyporesponsive, with low cytotoxic activity (63). The other major obstacle for NK cell–mediated immunosurveillance is their limited access to cancer cells in the tumor bed. Multiple studies showed that NK cells, when present, are preferentially localized to tumor stroma, with little or no direct contact with cancer cells (6366). Emerging data suggest that circulating NK cells are potent killers of cancer cells compared with organ-specific (67) or tumor-infiltrating (63) NK cells. In agreement with this hypothesis, circulating NK cells were shown to be crucial for prevention of metastasis (68), but the mechanisms involved are not clear.

Recently, acquisition of a mesenchymal-like phenotype was shown to increase the expression of NKG2D ligands, a major class of NK cell activators, rendering cells undergoing EMT more susceptible to NK cell–mediated cytotoxicity (69). Consistent with this, we observed a similar increase in the susceptibility of lung cancer cells to NK cell–mediated killing after TGF-β–induced EMT; however, the mechanism was independent of the NKG2D receptor (P.J. Chockley and V.K. Keshamouni, unpublished observations). This suggests that EMT cells may become susceptible through multiple mechanisms. Together, the above observations indicate that cells undergoing EMT, although contributing to the immunosuppressive microenvironment that inhibits NK cell–mediated immunosurveillance, become more susceptible to NK cell–mediated cytotoxicity when in circulation. This is consistent with the notion of metastasis-specific immunosurveillance and may contribute, in part, to the inefficiency of the metastatic process.

Additionally, several epithelial cell adhesion molecules whose expression is extensively modulated during EMT are identified as potential activating/inhibitory ligands for NK cells. For example, E-cadherin is a known inhibitory ligand for NK cells (70), and downregulation of its expression is a hallmark of EMT. Therefore, it is tempting to suggest that modulation of E-cadherin expression could be another potential mechanism by which cells undergoing EMT become more susceptible to NK cell–mediated cytotoxicity. Similarly, among other cell adhesion molecules, Cadm1, which is identified as an activating NK cell ligand (71), is frequently downregulated in different malignancies, and the nectin protein receptor CD96 is implicated in promoting spontaneous metastasis (72). Like NKG2D ligand, it would be important to assess whether these nonclassical ligands and their receptors are also modulated during EMT.

Each of the studies described above was carried out in isolation and in different models. However, the effects of different immune cell types in modulating various aspects of tumor progression are similar. Because many of these immune cells work together in other contexts, it is likely that they also work together during primary tumor growth and metastatic seeding. Initiation and progression of EMT involves a robust reprogramming of gene expression, changes in signaling and metabolic pathways, and reorganization of the cytoskeleton. Given the wide spectrum of changes that occur during EMT, it is reasonable to speculate how EMT can have a broad range of consequences for cancer cells, host immunosurveillance, and the efficacy of immune therapies (discussed below).

Many conventional therapies, including chemotherapy, radiation, and targeted therapies, rely on the induction of antitumor immune responses for their optimum efficacy (73). However, the triggering of an antitumor immune response depends on the immunogenicity of a tumor, and the immunogenicity is dictated by the cancer cell antigenicity and a multitude of other factors produced in the TME (74). Several mechanisms, including genetic and epigenetic changes, are known to regulate Ag expression and Ag presentation, two major criteria that regulate tumor immunogenicity. What is not fully considered in this process is the plasticity of cancer cells to undergo EMT. It is now well established that induction of EMT involves robust modulation of cell surface proteins, isoform switching by alternative splicing, immune-modulatory cytokine secretion, and actin cytoskeletal remodeling (5, 75). Each of the above changes that occurs during EMT is capable of generating neoepitopes and modulates their presentation. As a result, EMT may alter tumor immunogenicity at a much faster rate than genetic effects, which are inherently slower. Cataloging molecular changes during EMT, particularly ones that have the potential to modulate immunogenicity, may identify novel Ags for use in designing primary tumor- and metastasis-specific immunotherapeutic strategies.

Studies have focused on understanding how a tiny proportion of disseminating cells escape host surveillance and metastasize. Unfortunately, very little attention has been paid to understanding the mechanisms that successfully clear >99% of tumor cells. Granted, it is the escape of <1% of cells that results in the lethal metastatic disease, but not exploiting the effective tumor-clearance mechanisms that are already used by the host may be a missed opportunity. Because EMT is critical for metastasis, exclusive focus on evasive or resistance mechanisms that cells acquire after EMT may have promoted an unintended bias: that cells undergoing EMT must be resistant to anything that these cells encounter. On the contrary, it is equally feasible that metastatic cells after EMT are also vulnerable to host immunosurveillance, as illustrated by the increased susceptibility to NK cell–mediated cytotoxicity (69). In other words, when cancer cells exit the immunosuppressive primary TME, it is possible that they pay a toll to metastasize by becoming more susceptible to host immunosurveillance and, thus, contribute to the inefficiency of metastasis; however, this concept needs further and more careful investigation. If proved, identifying the molecules and mechanisms that regulate these potential EMT-induced vulnerabilities may be critical for any metastasis-specific prevention strategies. Given the potential for the presence of metastasis-specific immune-surveillance mechanisms, in clinical trials, it may be important to somehow assess the efficacy of a given therapy on metastatic disease in tandem, even when there is no effect on the primary tumor.

EMT was implicated in conferring resistance to both conventional therapies (such as chemotherapy and radiation therapy) and targeted therapies (such as anti-EGFR small-molecule therapy). In addition, EMT-based gene signatures were shown to predict patient prognosis (76, 77). For example, a 20-gene signature that we derived from an EMT-associated secretory phenotype predicted patient survival in non-small cell lung cancer patients (78). More recently, primary tumors stratified based on an EMT score showed a strong enrichment for immune checkpoint molecules in mesenchymal-like tumors compared with epithelial-like tumors, including significant upregulation of PD-L1, PD-L2, PD-1, and CTLA4 (77). This suggests that EMT-based biomarkers can be valuable for identifying patients who will benefit from immune checkpoint blockade agents and other immunotherapies in cancer. Similarly, engineering chimeric AgR T cells or NK cells to specifically target mesenchymal-like cells may be a feasible metastasis-specific prevention strategy. Cell culture models of cytokine-induced EMT can be valuable tools in this endeavor, particularly for identifying specific targets expressed by mesenchymal-like cells.

This work was supported by National Institutes of Health/National Cancer Institute Grant CA132571-01, the Elizabeth A. Crary Fund (to V.G.K.), and National Institutes of Health T32 Immunology Training Grant AI 007413 for graduate education (to P.J.C.).

Abbreviations used in this article:

CDC

complement-dependent cytotoxicity

DC

dendritic cell

ECM

extracellular matrix

EMT

epithelial-mesenchymal transition

M1

inflammatory macrophage

M2

anti-inflammatory macrophage

MAM

metastasis-associated monocyte

MDSC

myeloid-derived suppressor cell

MET

mesenchymal-epithelial transition

MMP

matrix metalloproteinase

NET

neutrophil extracellular trap

pMO

patrolling monocyte

TAM

tumor-associated macrophage

TME

tumor microenvironment

Treg

regulatory T cell.

1
Klein
C. A.
2009
.
Parallel progression of primary tumours and metastases.
Nat. Rev. Cancer
9
:
302
312
.
2
Bonnomet
A.
,
Brysse
A.
,
Tachsidis
A.
,
Waltham
M.
,
Thompson
E. W.
,
Polette
M.
,
Gilles
C.
.
2010
.
Epithelial-to-mesenchymal transitions and circulating tumor cells.
J. Mammary Gland Biol. Neoplasia
15
:
261
273
.
3
Kalluri
R.
,
Weinberg
R. A.
.
2009
.
The basics of epithelial-mesenchymal transition.
J. Clin. Invest.
119
:
1420
1428
.
4
Thiery
J. P.
,
Sleeman
J. P.
.
2006
.
Complex networks orchestrate epithelial-mesenchymal transitions.
Nat. Rev. Mol. Cell Biol.
7
:
131
142
.
5
Lamouille
S.
,
Xu
J.
,
Derynck
R.
.
2014
.
Molecular mechanisms of epithelial-mesenchymal transition.
Nat. Rev. Mol. Cell Biol.
15
:
178
196
.
6
Heldin
C. H.
,
Landström
M.
,
Moustakas
A.
.
2009
.
Mechanism of TGF-beta signaling to growth arrest, apoptosis, and epithelial-mesenchymal transition.
Curr. Opin. Cell Biol.
21
:
166
176
.
7
Massagué
J.
2008
.
TGFbeta in cancer.
Cell
134
:
215
230
.
8
Jing
Y.
,
Han
Z.
,
Zhang
S.
,
Liu
Y.
,
Wei
L.
.
2011
.
Epithelial-mesenchymal transition in tumor microenvironment.
Cell Biosci.
1
:
29
.
9
Zavadil
J.
,
Haley
J.
,
Kalluri
R.
,
Muthuswamy
S. K.
,
Thompson
E.
.
2008
.
Epithelial-mesenchymal transition.
Cancer Res.
68
:
9574
9577
.
10
Gregory
P. A.
,
Bert
A. G.
,
Paterson
E. L.
,
Barry
S. C.
,
Tsykin
A.
,
Farshid
G.
,
Vadas
M. A.
,
Khew-Goodall
Y.
,
Goodall
G. J.
.
2008
.
The miR-200 family and miR-205 regulate epithelial to mesenchymal transition by targeting ZEB1 and SIP1.
Nat. Cell Biol.
10
:
593
601
.
11
Kitamura
T.
,
Qian
B. Z.
,
Pollard
J. W.
.
2015
.
Immune cell promotion of metastasis.
Nat. Rev. Immunol.
15
:
73
86
.
12
Grivennikov
S. I.
,
Greten
F. R.
,
Karin
M.
.
2010
.
Immunity, inflammation, and cancer.
Cell
140
:
883
899
.
13
Slaney
C. Y.
,
Rautela
J.
,
Parker
B. S.
.
2013
.
The emerging role of immunosurveillance in dictating metastatic spread in breast cancer.
Cancer Res.
73
:
5852
5857
.
14
Eyles
J.
,
Puaux
A. L.
,
Wang
X.
,
Toh
B.
,
Prakash
C.
,
Hong
M.
,
Tan
T. G.
,
Zheng
L.
,
Ong
L. C.
,
Jin
Y.
, et al
.
2010
.
Tumor cells disseminate early, but immunosurveillance limits metastatic outgrowth, in a mouse model of melanoma.
J. Clin. Invest.
120
:
2030
2039
.
15
Elinav
E.
,
Nowarski
R.
,
Thaiss
C. A.
,
Hu
B.
,
Jin
C.
,
Flavell
R. A.
.
2013
.
Inflammation-induced cancer: crosstalk between tumours, immune cells and microorganisms.
Nat. Rev. Cancer
13
:
759
771
.
16
Gao
D.
,
Vahdat
L. T.
,
Wong
S.
,
Chang
J. C.
,
Mittal
V.
.
2012
.
Microenvironmental regulation of epithelial-mesenchymal transitions in cancer.
Cancer Res.
72
:
4883
4889
.
17
Pollard
J. W.
2009
.
Trophic macrophages in development and disease.
Nat. Rev. Immunol.
9
:
259
270
.
18
Lee
H. W.
,
Choi
H. J.
,
Ha
S. J.
,
Lee
K. T.
,
Kwon
Y. G.
.
2013
.
Recruitment of monocytes/macrophages in different tumor microenvironments.
Biochim. Biophys. Acta
1835
:
170
179
.
19
Lin
E. Y.
,
Nguyen
A. V.
,
Russell
R. G.
,
Pollard
J. W.
.
2001
.
Colony-stimulating factor 1 promotes progression of mammary tumors to malignancy.
J. Exp. Med.
193
:
727
740
.
20
Zabuawala
T.
,
Taffany
D. A.
,
Sharma
S. M.
,
Merchant
A.
,
Adair
B.
,
Srinivasan
R.
,
Rosol
T. J.
,
Fernandez
S.
,
Huang
K.
,
Leone
G.
,
Ostrowski
M. C.
.
2010
.
An ets2-driven transcriptional program in tumor-associated macrophages promotes tumor metastasis.
Cancer Res.
70
:
1323
1333
.
21
Bonde
A. K.
,
Tischler
V.
,
Kumar
S.
,
Soltermann
A.
,
Schwendener
R. A.
.
2012
.
Intratumoral macrophages contribute to epithelial-mesenchymal transition in solid tumors.
BMC Cancer
12
:
35
.
22
Wyckoff
J. B.
,
Wang
Y.
,
Lin
E. Y.
,
Li
J. F.
,
Goswami
S.
,
Stanley
E. R.
,
Segall
J. E.
,
Pollard
J. W.
,
Condeelis
J.
.
2007
.
Direct visualization of macrophage-assisted tumor cell intravasation in mammary tumors.
Cancer Res.
67
:
2649
2656
.
23
Condeelis
J.
,
Pollard
J. W.
.
2006
.
Macrophages: obligate partners for tumor cell migration, invasion, and metastasis.
Cell
124
:
263
266
.
24
Sullivan
N. J.
,
Sasser
A. K.
,
Axel
A. E.
,
Vesuna
F.
,
Raman
V.
,
Ramirez
N.
,
Oberyszyn
T. M.
,
Hall
B. M.
.
2009
.
Interleukin-6 induces an epithelial-mesenchymal transition phenotype in human breast cancer cells.
Oncogene
28
:
2940
2947
.
25
Wu
Y.
,
Deng
J.
,
Rychahou
P. G.
,
Qiu
S.
,
Evers
B. M.
,
Zhou
B. P.
.
2009
.
Stabilization of snail by NF-kappaB is required for inflammation-induced cell migration and invasion.
Cancer Cell
15
:
416
428
.
26
Qian
B.
,
Deng
Y.
,
Im
J. H.
,
Muschel
R. J.
,
Zou
Y.
,
Li
J.
,
Lang
R. A.
,
Pollard
J. W.
.
2009
.
A distinct macrophage population mediates metastatic breast cancer cell extravasation, establishment and growth.
PLoS One
4
:
e6562
.
27
Qian
B. Z.
,
Pollard
J. W.
.
2010
.
Macrophage diversity enhances tumor progression and metastasis.
Cell
141
:
39
51
.
28
Hanna
R. N.
,
Cekic
C.
,
Sag
D.
,
Tacke
R.
,
Thomas
G. D.
,
Nowyhed
H.
,
Herrley
E.
,
Rasquinha
N.
,
McArdle
S.
,
Wu
R.
, et al
.
2015
.
Patrolling monocytes control tumor metastasis to the lung.
Science
350
:
985
990
.
29
Granot
Z.
,
Henke
E.
,
Comen
E. A.
,
King
T. A.
,
Norton
L.
,
Benezra
R.
.
2011
.
Tumor entrained neutrophils inhibit seeding in the premetastatic lung.
Cancer Cell
20
:
300
314
.
30
Cools-Lartigue
J.
,
Spicer
J.
,
McDonald
B.
,
Gowing
S.
,
Chow
S.
,
Giannias
B.
,
Bourdeau
F.
,
Kubes
P.
,
Ferri
L.
.
2013
.
Neutrophil extracellular traps sequester circulating tumor cells and promote metastasis.
J. Clin. Invest.
123
:
3446
3458
.
31
Tohme
S.
,
Yazdani
H. O.
,
Al-Khafaji
A. B.
,
Chidi
A. P.
,
Loughran
P.
,
Mowen
K.
,
Wang
Y.
,
Simmons
R. L.
,
Huang
H.
,
Tsung
A.
.
2016
.
Neutrophil Extracellular Traps Promote the Development and Progression of Liver Metastases after Surgical Stress.
Cancer Res.
76
:
1367
1380
.
32
Fridlender
Z. G.
,
Sun
J.
,
Kim
S.
,
Kapoor
V.
,
Cheng
G.
,
Ling
L.
,
Worthen
G. S.
,
Albelda
S. M.
.
2009
.
Polarization of tumor-associated neutrophil phenotype by TGF-beta: “N1” versus “N2” TAN.
Cancer Cell
16
:
183
194
.
33
Coffelt
S. B.
,
Kersten
K.
,
Doornebal
C. W.
,
Weiden
J.
,
Vrijland
K.
,
Hau
C. S.
,
Verstegen
N. J.
,
Ciampricotti
M.
,
Hawinkels
L. J.
,
Jonkers
J.
,
de Visser
K. E.
.
2015
.
IL-17-producing γδ T cells and neutrophils conspire to promote breast cancer metastasis.
Nature
522
:
345
348
.
34
Goubran
H. A.
,
Stakiw
J.
,
Radosevic
M.
,
Burnouf
T.
.
2014
.
Platelets effects on tumor growth.
Semin. Oncol.
41
:
359
369
.
35
van Es
N.
,
Sturk
A.
,
Middeldorp
S.
,
Nieuwland
R.
.
2014
.
Effects of cancer on platelets.
Semin. Oncol.
41
:
311
318
.
36
Labelle
M.
,
Begum
S.
,
Hynes
R. O.
.
2011
.
Direct signaling between platelets and cancer cells induces an epithelial-mesenchymal-like transition and promotes metastasis.
Cancer Cell
20
:
576
590
.
37
Schumacher
D.
,
Strilic
B.
,
Sivaraj
K. K.
,
Wettschureck
N.
,
Offermanns
S.
.
2013
.
Platelet-derived nucleotides promote tumor-cell transendothelial migration and metastasis via P2Y2 receptor.
Cancer Cell
24
:
130
137
.
38
Palumbo
J. S.
,
Talmage
K. E.
,
Massari
J. V.
,
La Jeunesse
C. M.
,
Flick
M. J.
,
Kombrinck
K. W.
,
Jirousková
M.
,
Degen
J. L.
.
2005
.
Platelets and fibrin(ogen) increase metastatic potential by impeding natural killer cell-mediated elimination of tumor cells.
Blood
105
:
178
185
.
39
Condamine
T.
,
Ramachandran
I.
,
Youn
J. I.
,
Gabrilovich
D. I.
.
2015
.
Regulation of tumor metastasis by myeloid-derived suppressor cells.
Annu. Rev. Med.
66
:
97
110
.
40
Diaz-Montero
C. M.
,
Salem
M. L.
,
Nishimura
M. I.
,
Garrett-Mayer
E.
,
Cole
D. J.
,
Montero
A. J.
.
2009
.
Increased circulating myeloid-derived suppressor cells correlate with clinical cancer stage, metastatic tumor burden, and doxorubicin-cyclophosphamide chemotherapy.
Cancer Immunol. Immunother.
58
:
49
59
.
41
Wang
L.
,
Chang
E. W.
,
Wong
S. C.
,
Ong
S. M.
,
Chong
D. Q.
,
Ling
K. L.
.
2013
.
Increased myeloid-derived suppressor cells in gastric cancer correlate with cancer stage and plasma S100A8/A9 proinflammatory proteins.
J. Immunol.
190
:
794
804
.
42
Zhang
B.
,
Wang
Z.
,
Wu
L.
,
Zhang
M.
,
Li
W.
,
Ding
J.
,
Zhu
J.
,
Wei
H.
,
Zhao
K.
.
2013
.
Circulating and tumor-infiltrating myeloid-derived suppressor cells in patients with colorectal carcinoma.
PLoS One
8
:
e57114
.
43
Sun
H. L.
,
Zhou
X.
,
Xue
Y. F.
,
Wang
K.
,
Shen
Y. F.
,
Mao
J. J.
,
Guo
H. F.
,
Miao
Z. N.
.
2012
.
Increased frequency and clinical significance of myeloid-derived suppressor cells in human colorectal carcinoma.
World J. Gastroenterol.
18
:
3303
3309
.
44
Cui
T. X.
,
Kryczek
I.
,
Zhao
L.
,
Zhao
E.
,
Kuick
R.
,
Roh
M. H.
,
Vatan
L.
,
Szeliga
W.
,
Mao
Y.
,
Thomas
D. G.
, et al
.
2013
.
Myeloid-derived suppressor cells enhance stemness of cancer cells by inducing microRNA101 and suppressing the corepressor CtBP2.
Immunity
39
:
611
621
.
45
Toh
B.
,
Wang
X.
,
Keeble
J.
,
Sim
W. J.
,
Khoo
K.
,
Wong
W.-C.
,
Kato
M.
,
Prevost-Blondel
A.
,
Thiery
J.-P.
,
Abastado
J.-P.
.
2011
.
Mesenchymal transition and dissemination of cancer cells is driven by myeloid-derived suppressor cells infiltrating the primary tumor.
PLoS Biol.
9
:
e1001162
.
46
Kim
S.
,
Takahashi
H.
,
Lin
W. W.
,
Descargues
P.
,
Grivennikov
S.
,
Kim
Y.
,
Luo
J. L.
,
Karin
M.
.
2009
.
Carcinoma-produced factors activate myeloid cells through TLR2 to stimulate metastasis.
Nature
457
:
102
106
.
47
Yu
H.
,
Pardoll
D.
,
Jove
R.
.
2009
.
STATs in cancer inflammation and immunity: a leading role for STAT3.
Nat. Rev. Cancer
9
:
798
809
.
48
Cho
M. S.
,
Rupaimoole
R.
,
Choi
H. J.
,
Noh
K.
,
Chen
J.
,
Hu
Q.
,
Sood
A. K.
,
Afshar-Kharghan
V.
.
2016
.
Complement Component 3 is regulated by TWIST1 and mediates epithelial-mesenchymal transition.
J. Immunol.
196
:
1412
1418
.
49
Standiford
T. J.
,
Kuick
R.
,
Bhan
U.
,
Chen
J.
,
Newstead
M.
,
Keshamouni
V. G.
.
2011
.
TGF-β-induced IRAK-M expression in tumor-associated macrophages regulates lung tumor growth.
Oncogene
30
:
2475
2484
.
50
Kudo-Saito
C.
,
Shirako
H.
,
Takeuchi
T.
,
Kawakami
Y.
.
2009
.
Cancer metastasis is accelerated through immunosuppression during Snail-induced EMT of cancer cells.
Cancer Cell
15
:
195
206
.
51
Akalay
I.
,
Janji
B.
,
Hasmim
M.
,
Noman
M. Z.
,
André
F.
,
De Cremoux
P.
,
Bertheau
P.
,
Badoual
C.
,
Vielh
P.
,
Larsen
A. K.
, et al
.
2013
.
Epithelial-to-mesenchymal transition and autophagy induction in breast carcinoma promote escape from T-cell-mediated lysis.
Cancer Res.
73
:
2418
2427
.
52
Abouzahr
S.
,
Bismuth
G.
,
Gaudin
C.
,
Caroll
O.
,
Van Endert
P.
,
Jalil
A.
,
Dausset
J.
,
Vergnon
I.
,
Richon
C.
,
Kauffmann
A.
, et al
.
2006
.
Identification of target actin content and polymerization status as a mechanism of tumor resistance after cytolytic T lymphocyte pressure.
Proc. Natl. Acad. Sci. USA
103
:
1428
1433
.
53
Chen
L.
,
Gibbons
D. L.
,
Goswami
S.
,
Cortez
M. A.
,
Ahn
Y.-H.
,
Byers
L. A.
,
Zhang
X.
,
Yi
X.
,
Dwyer
D.
,
Lin
W.
, et al
.
2014
.
Metastasis is regulated via microRNA-200/ZEB1 axis control of tumour cell PD-L1 expression and intratumoral immunosuppression.
Nat. Commun.
5
:
5241
.
54
Walport
M. J.
2001
.
Complement. First of two parts.
N. Engl. J. Med.
344
:
1058
1066
.
55
Fishelson
Z.
,
Donin
N.
,
Zell
S.
,
Schultz
S.
,
Kirschfink
M.
.
2003
.
Obstacles to cancer immunotherapy: expression of membrane complement regulatory proteins (mCRPs) in tumors.
Mol. Immunol.
40
:
109
123
.
56
Jurianz
K.
,
Ziegler
S.
,
Garcia-Schüler
H.
,
Kraus
S.
,
Bohana-Kashtan
O.
,
Fishelson
Z.
,
Kirschfink
M.
.
1999
.
Complement resistance of tumor cells: basal and induced mechanisms.
Mol. Immunol.
36
:
929
939
.
57
Varsano
S.
,
Rashkovsky
L.
,
Shapiro
H.
,
Ophir
D.
,
Mark-Bentankur
T.
.
1998
.
Human lung cancer cell lines express cell membrane complement inhibitory proteins and are extremely resistant to complement-mediated lysis; a comparison with normal human respiratory epithelium in vitro, and an insight into mechanism(s) of resistance.
Clin. Exp. Immunol.
113
:
173
182
.
58
Donin
N.
,
Jurianz
K.
,
Ziporen
L.
,
Schultz
S.
,
Kirschfink
M.
,
Fishelson
Z.
.
2003
.
Complement resistance of human carcinoma cells depends on membrane regulatory proteins, protein kinases and sialic acid.
Clin. Exp. Immunol.
131
:
254
263
.
59
Goswami
M. T.
,
Reka
A. K.
,
Kurapati
H.
,
Kaza
V.
,
Chen
J.
,
Standiford
T. J.
,
Keshamouni
V. G.
.
2016
.
Regulation of complement-dependent cytotoxicity by TGF-β–induced epithelial-mesenchymal transition.
Oncogene
35
:
1888
1898
.
60
Jondal
M.
,
Pross
H.
.
1975
.
Surface markers on human b and t lymphocytes. VI. Cytotoxicity against cell lines as a functional marker for lymphocyte subpopulations.
Int. J. Cancer
15
:
596
605
.
61
Kiessling
R.
,
Klein
E.
,
Pross
H.
,
Wigzell
H.
.
1975
.
“Natural” killer cells in the mouse. II. Cytotoxic cells with specificity for mouse Moloney leukemia cells. Characteristics of the killer cell.
Eur. J. Immunol.
5
:
117
121
.
62
Imai
K.
,
Matsuyama
S.
,
Miyake
S.
,
Suga
K.
,
Nakachi
K.
.
2000
.
Natural cytotoxic activity of peripheral-blood lymphocytes and cancer incidence: an 11-year follow-up study of a general population.
Lancet
356
:
1795
1799
.
63
Platonova
S.
,
Cherfils-Vicini
J.
,
Damotte
D.
,
Crozet
L.
,
Vieillard
V.
,
Validire
P.
,
André
P.
,
Dieu-Nosjean
M. C.
,
Alifano
M.
,
Régnard
J. F.
, et al
.
2011
.
Profound coordinated alterations of intratumoral NK cell phenotype and function in lung carcinoma.
Cancer Res.
71
:
5412
5422
.
64
Halama
N.
,
Braun
M.
,
Kahlert
C.
,
Spille
A.
,
Quack
C.
,
Rahbari
N.
,
Koch
M.
,
Weitz
J.
,
Kloor
M.
,
Zoernig
I.
, et al
.
2011
.
Natural killer cells are scarce in colorectal carcinoma tissue despite high levels of chemokines and cytokines.
Clin. Cancer Res.
17
:
678
689
.
65
Carrega
P.
,
Morandi
B.
,
Costa
R.
,
Frumento
G.
,
Forte
G.
,
Altavilla
G.
,
Ratto
G. B.
,
Mingari
M. C.
,
Moretta
L.
,
Ferlazzo
G.
.
2008
.
Natural killer cells infiltrating human nonsmall-cell lung cancer are enriched in CD56 bright CD16(-) cells and display an impaired capability to kill tumor cells.
Cancer
112
:
863
875
.
66
Chow
M. T.
,
Sceneay
J.
,
Paget
C.
,
Wong
C. S.
,
Duret
H.
,
Tschopp
J.
,
Möller
A.
,
Smyth
M. J.
.
2012
.
NLRP3 suppresses NK cell-mediated responses to carcinogen-induced tumors and metastases.
Cancer Res.
72
:
5721
5732
.
67
Halfteck
G. G.
,
Elboim
M.
,
Gur
C.
,
Achdout
H.
,
Ghadially
H.
,
Mandelboim
O.
.
2009
.
Enhanced in vivo growth of lymphoma tumors in the absence of the NK-activating receptor NKp46/NCR1.
J. Immunol.
182
:
2221
2230
.
68
Sathe
P.
,
Delconte
R. B.
,
Souza-Fonseca-Guimaraes
F.
,
Seillet
C.
,
Chopin
M.
,
Vandenberg
C. J.
,
Rankin
L. C.
,
Mielke
L. A.
,
Vikstrom
I.
,
Kolesnik
T. B.
, et al
.
2014
.
Innate immunodeficiency following genetic ablation of Mcl1 in natural killer cells.
Nat. Commun.
5
:
4539
.
69
López-Soto
A.
,
Huergo-Zapico
L.
,
Galván
J. A.
,
Rodrigo
L.
,
de Herreros
A. G.
,
Astudillo
A.
,
Gonzalez
S.
.
2013
.
Epithelial-mesenchymal transition induces an antitumor immune response mediated by NKG2D receptor.
J. Immunol.
190
:
4408
4419
.
70
Schwartzkopff
S.
,
Gründemann
C.
,
Schweier
O.
,
Rosshart
S.
,
Karjalainen
K. E.
,
Becker
K. F.
,
Pircher
H.
.
2007
.
Tumor-associated E-cadherin mutations affect binding to the killer cell lectin-like receptor G1 in humans.
J. Immunol.
179
:
1022
1029
.
71
Faraji
F.
,
Pang
Y.
,
Walker
R. C.
,
Nieves Borges
R.
,
Yang
L.
,
Hunter
K. W.
.
2012
.
Cadm1 is a metastasis susceptibility gene that suppresses metastasis by modifying tumor interaction with the cell-mediated immunity.
PLoS Genet.
8
:
e1002926
.
72
Blake
S. J.
,
Stannard
K.
,
Liu
J.
,
Allen
S.
,
Yong
M. C.
,
Mittal
D.
,
Aguilera
A. R.
,
Miles
J. J.
,
Lutzky
V. P.
,
de Andrade
L. F.
, et al
.
2016
.
Suppression of metastases using a new lymphocyte checkpoint target for cancer immunotherapy.
Cancer Discov.
6
:
446
459
.
73
Zitvogel
L.
,
Kepp
O.
,
Kroemer
G.
.
2011
.
Immune parameters affecting the efficacy of chemotherapeutic regimens.
Nat. Rev. Clin. Oncol.
8
:
151
160
.
74
Blankenstein
T.
,
Coulie
P. G.
,
Gilboa
E.
,
Jaffee
E. M.
.
2012
.
The determinants of tumour immunogenicity.
Nat. Rev. Cancer
12
:
307
313
.
75
Philippar
U.
,
Roussos
E. T.
,
Oser
M.
,
Yamaguchi
H.
,
Kim
H. D.
,
Giampieri
S.
,
Wang
Y.
,
Goswami
S.
,
Wyckoff
J. B.
,
Lauffenburger
D. A.
, et al
.
2008
.
A Mena invasion isoform potentiates EGF-induced carcinoma cell invasion and metastasis.
Dev. Cell
15
:
813
828
.
76
Lou
Y.
,
Diao
L.
,
Parra Cuentas
E. R.
,
Denning
W. L.
,
Chen
L.
,
Fan
Y. H.
,
Byers
L. A.
,
Wang
J.
,
Papadimitrakopoulou
V. A.
,
Behrens
C.
, et al
.
2016
.
Epithelial-mesenchymal transition is associated with a distinct tumor microenvironment including elevation of inflammatory signals and multiple immune checkpoints in lung adenocarcinoma.
Clin. Cancer Res.
pii: clincanres.1434.2015.
77
Mak
M. P.
,
Tong
P.
,
Diao
L.
,
Cardnell
R. J.
,
Gibbons
D. L.
,
William
W. N.
,
Skoulidis
F.
,
Parra
E. R.
,
Rodriguez-Canales
J.
,
Wistuba
I. I.
, et al
.
2016
.
A patient-derived, Pan-cancer EMT signature identifies global molecular alterations and immune target enrichment following epithelial-to-mesenchymal transition.
Clin. Cancer Res.
22
:
609
620
.
78
Reka
A. K.
,
Chen
G.
,
Jones
R. C.
,
Amunugama
R.
,
Kim
S.
,
Karnovsky
A.
,
Standiford
T. J.
,
Beer
D. G.
,
Omenn
G. S.
,
Keshamouni
V. G.
.
2014
.
Epithelial-mesenchymal transition-associated secretory phenotype predicts survival in lung cancer patients.
Carcinogenesis
35
:
1292
1300
.

The authors have no financial conflicts of interest.