Because elevated serum concentrations of IL-1β and IL-18 are often correlated to malignancies, it was suggested that all members of the NLR family are associated with pro-tumoral activities. Nevertheless, numerous studies, especially the comprehensive review by Terlizzi et al., reported that members of this family can exhibit both pro-and anti-tumoral activities, depending on the type of cancer and whether they function in inflammasome dependent or independent pathways
[31].
While inflammasome activation in cancer is supposed to control its expansion, some stimuli of inflammasomes can behave as tumor promoters through the induction of chronic inflammation that rather facilitates tumor development. This sight is contrasted in animal models of colon cancer in which the activation of some inflammasome complexes is associated with tumor protection. For instance, NLRC4- and caspase-1-deficient mice have been reported to develop increased colonic inflammation, responsible for higher colon adenocarcinoma burden, in an azoxymethane/dextran sulfate sodium (AOM/DSS) mouse model. NLRC4 and caspase-1 were inferred to exert a protective function in that model via a direct effect on epithelial cell proliferation
[31]. In addition, knockdown of NLRP6 in mice increased their risk of developing colorectal cancer, suggesting its significant role in the onco-suppressive activity
[32]. Conversely, NLRP3, the most studied NLR has been reported to be associated with pro- and anti-carcinogenic roles. In the DSS/AOM cancer model, NLRP3 has been reported to play a protective role
[33]. However, this member has been associated with poor survival rate of colorectal cancer
[34] and to higher susceptibility to melanoma
[35] and myeloma
[36]. In addition, NLRP3 has been reported to suppress NK (natural killer) and T cell-mediated anti-tumor actions and immune-editing in a mouse model of carcinogen-induced sarcoma and metastatic melanoma
[37]. This phenomenon was mediated by IL-1β-dependent recruitment of immune suppressive cells, such as myeloid-derived suppressor cells (MDSCs) and Treg cells
[37]. Taken together, these findings strongly suggest that the roles of NLRs in human cancers are yet to be elucidated.
4.1. NLRP7 and Gestational Trophoblastic Diseases
Distinct from normal placental development, GTDs are a rare subgroup of placental pathologies, encompassing PHM or CHM and their non-molar counterpart such as CC, which constitutes the most aggressive form of placental cancer
[38]. CC is a highly proliferative and invasive tumor as trophoblast cells forming the tumor metastasize into multiple organs, including the vagina, lungs and brain
[39][38]. CC has an estimated incidence of 2 to 7 in 100,000 pregnancies in Europe and North America. This incidence is higher in Asia and Africa, with 5 to 202 in 100,000 pregnancies
[38]. CHM is a morbid pathology that is associated with a high risk (20%) for patients to develop post-molar CC
[38]. More often, CC may also develop after normal delivery. The incidence of this type of CC is 1 per 67,000 live births
[40][41]. Recent studies have shown that 50% of patients with recurrent HM have mutations in the gene
nlrp7 [38]. While the association of biallelic mutations in
nlrp7 with recurrent HM is well established, its role in the development of GTDs, especially CC, is poorly understood and often controversial
[5].
4.2. NLRP7 and Choriocarcinoma
Since the identification of
nlrp7 as a highly mutated gene in recurrent HMs, no study has been conducted to determine whether deregulations in the expression of this gene may contribute to the change in the behavior of the tumor trophoblast cells and their metastasis. Recently, the role of NLRP7 in these processes were investigated
[42]. Three approaches were used to define the role of NLRP7; (i) a clinical study in which human sera and placentae were used that were collected from normal pregnant women and from patients with CHM or CC; (ii) an in vitro study in which the influence of NLRP7 were investigated, knockdown on the tumorigenesis of the choriocarcinoma cell line, JEG3, which used both 2D and 3D culture systems; and (iii) an in vivo study in which an orthotopic model were used of CC and a metastatic model of this cancer
[43]. It was demonstrated that NLRP7 was upregulated in tumor cells, and in CHM and CC placentae. In JEG3 cells, NLRP7 increased proliferation and 3D organization of malignant cells.
NLRP7 increased expression in JEG3 cells and in CHM and CC tissues strongly suggested that its inflammasome is highly activated. Nevertheless, no production or secretion of mature IL-1β have been observed in JEG3 cells. It was strongly suggests that NLRP7 may function in an inflammasome-independent pathway in malignant trophoblast cells. This statement is in line with previous studies reporting that overexpression of NLRP7 exerts negative feedback on the production and maturation of IL-1β.
[23][44]. Recent studies from the literature demonstrated that IL-1β might negatively control the proliferation of trophoblast cells through the deregulation of the cell cycle
[45][46]. Importantly, Chow et al. demonstrated that another member of the NLR family, the NLRP3, promotes metastasis in an inflammasome independent manner and that knock-out mice for NLRP3 exhibit lower numbers of lung metastases upon intravenous inoculation of prostate or melanoma malignant cells
[37]. It has also been reported that overexpression of NLRP12 is associated with the aggravation of prostate cancer without any increase in the levels of mature IL-18 or IL-1β by these cells
[32]. Overall, these results roughly suggest that NLRP7, similar to NLRP12 and NLRP3, functions in an inflammasome independent manner in malignant cells
[32][47].
Importantly, the
in vivo study that used the orthotopic model of CC, which was injected within its placenta with NLRP7 invalidated-CC cells, showed higher maternal immune response and that the mice developed smaller tumors and displayed less metastases. Furthermore, a strong increase in the levels of IL-1β were observed, both locally in mouse placenta and in the maternal serum. This finding strongly suggests that the expression of NLRP7 by the trophoblast cells contributes to its camouflage by the maternal environment (
Figure 2). In line with this assumption, it was observed that malignant cells that were inactivated for NLRP7 exhibited significant decrease in the expression of proteins that contribute to maternal immune tolerance. These include PD-L1, HLA-G and hCG. The latter hormone has recently been reported to increase the activity of regulatory T cells (Treg) and to retain the tolerogenic activity of dendritic cells
[48][49]. Importantly, these findings strongly support a local immune tolerance that is mediated by malignant cells-secreted hCG. This hormone is known to act as a strong chemoattractant for T-suppressors that are apoptotic actors for T-lymphocytes.
Figure 2. Proposed model for the role of NLRP7 protein in the development of gestational choriocarcinoma.
4.4. HLA Family in Normal and Tumor Placenta
HLA (human leukocyte antigen) proteins are mainly expressed by the EVT. This family of proteins has been shown to be involved in the attenuation of the pool of immune cells present at the fetomaternal interface from implantation to delivery
[52][53]. The HLA family is composed of numerous members that are differentially involved in the immune tolerance during pregnancy. Among all HLA members, HLA-G is exclusively expressed on EVT
[54]. During pregnancy, HLA-G plays an immunosuppressive role rather than an antigen-presenting role
[15][50]. In non-pathological conditions, it is expressed only at the surface of the EVT, thymic epithelial cells, the cornea and in the cells facing the amniotic fluid
[52]. The main role of HLA-G is to inhibit cytotoxic T lymphocytes and Natural killer cells through an interaction with their ILT-2 and KIR receptors
[55]. Recent studies also showed that HLA-G regulates trophoblast invasion, a key parameter of placental development in normal and tumor conditions
[56]. In addition, soluble HLAG (sHLA-G) has been shown to impair the expression and function of different chemokines receptors in T, B, and NK cells through the ILT2 receptor
[55]. HLA-G has also been shown to be upregulated by βhCG in JEG3, suggesting that this hormone also contributes to the mechanism by which choriocarcinoma cells develop immune tolerance
[48][49].
4.5. PDL-1 in Normal and Tumor Trophoblast Cells
The survival of the trophoblast cells depend on their ability to evade the immune system through the inhibition of their anti-tumoral activity
[15][50]. A common ligand found in several aggressive cells is the protein PD-L1 (programmed death ligand -1), which mediates immunosuppression upon binding to its receptor PD-1, commonly expressed by immune cells
[57]. During normal pregnancy, the immunosuppressive role of PD-L1 is major, as it is expressed on the ST. PDL-1 interaction with these cells promotes an immune tolerance to the fetal tissues
[58].
In CC, PD-L1 is expressed by the ST and CT. PD-L1/PD-1 interaction provides an immune tolerance through the activation of the paternal antigen-specific naïve helper T cells Tregs
[59]. Importantly, it was demonstrated that NLRP7 knockdown caused a decrease in PD-L1 expression, suggesting that this protein is directly involved in the NLRP7-mediated immunosuppression
[42]. Altogether, these findings suggest that a tight relationship exists between the maternal immune system and the NLRP7 inflammasome.
5. Conclusion
It clearly appears that appropriate NLRP7 expression and NLRP7 inflammasome activity are essential during early pregnancy. However, further investigation is required to establish how HM-associated NLRP7 overexpression and variants might affect NLRP7 function and lead to reproductive wastage.
Overall, it appears that the NLRP7 mode of function will tightly depend on the cellular status. Under physiological conditions, NLRP7 will function in an inflammasome- dependent pathway to contribute to the maintenance of the required fine balance between pro-inflammatory and anti-inflammatory settings. This will be ensured through the processing of pro-IL-1β to IL-1β. This is what clearly has been reported in normal trophoblast cells and in placental explant model systems. The NLRP7 inflammasome activity can be exacerbated in the context of pregnancy pathologies such as FGR pregnancy, to overcome the stressful conditions of the trophoblast cells.