Preeclampsia (PE) is a specific syndrome of human pregnancy, and the leading cause of morbidity, mortality and premature delivery between 2 and 7% of pregnancies [1]. The clinical diagnosis is based on the development of hypertension (BP ≥ 140 x 90 mmHg) and proteinuria (≥300mg/24 h) [2] that occurs from the 20^th week of pregnancy or in the first days after birth [3].

It has been reported that PE is characterized by severe systemic inflammatory response, endothelial cell dysfunction, platelet aggregation, coagulation system activation and increased generalized vascular resistance [4, 5], which appear to contribute significantly to the pathophysiology of the disease [6]. Therefore, in PE it is observed activation of inflammatory cells such as monocytes and granulocytes, and endothelial cells [5,7], excessive production of proinflammatory cytokines [8–11] as well as lower production of regulatory cytokines such as Interleukin-10 (IL-10), and transforming growth factor (TGF-β) [12,13].

The excessive activation of intravascular monocytes/macrophages and granulocytes in PE suggests that innate immune system activation can negatively influences the pregnancy progress. Monocytes from women with PE are endogenously activated and release significantly higher concentrations of tumor necrosis factor alpha (TNF-α), superoxide anion and hydrogen peroxide compared to monocytes from normotensive pregnant women. These results confirm that PE is characterized by oxidative stress and that maternal circulating monocytes may represent an important source of free radicals and cytokines during the inflammatory disease [11,14]. Peripheral blood leukocyte activation state in pregnant women with PE is associated with genes related to inflammation [15]. There is an association between increased activation of nuclear transcription factor kappa B (NF-KB) and higher production of TNF-α and IL-1β by mononuclear cells of pre-eclamptic pregnant women as compared with normotensive pregnant women [16]. Recently, we reported that monocytes from women with PE are classically activated and produce higher levels of the inflammatory cytokines TNF-α and IL-12 associated with higher expression of TLR4 and CD64 surface markers. These results provide evidence that the systemic inflammatory environment in PE may differentiate and polarize the monocytes to the M1 phenotype [17].

Hyperuricemia has been observed in women with PE associated with the disease severity [18]. Positive correlation between increased production of TNF-α, superoxide anion and elevated serum uric acid level was observed in PE [14]. Also, high association between plasma levels of uric acid, heat shock protein 70, IL-1β, IL-12 and TNF-α has been reported in severe cases of PE [19]. Thus, high levels of uric acid in the plasma of patients with PE may represent a direct contribution to the pathogenesis of PE by its potential to promote inflammation [20] due to its association with proinflammatory cytokines.

Uric acid (monosodium urate-MSU) crystals may activate an intracellular complex called inflammasome, a multi-protein structure which is important for processing and release of IL-1β and IL-18 [21,22]. Inflammasome formation depends on receptors known as pattern recognition receptors (PRRs) expressed by cells of the innate immune system, such as monocytes, macrophages and dendritic cells [23,24]. Among the various PRRs, nod-like intracellular receptors with a pyrin domain (NLRP), are responsible for inflammasome formation in response to pathogens known as pathogen-associated molecular patterns (PAMPs) contributing to the host defense against infections [25] or also bind endogenous products of host cells, named danger-associated molecular patterns (DAMPs) [23,26,27]. Three NLR proteins have been identified to form inflammasome: NLRP1, NLRP3 and NLRC4 [22]. The NLRP3 inflammasome is the most studied inflammasome, and capable of sensing a wide variety of alarm signals [28]. In addition, some evidence indicates that NLRP3 plays an important role in inflammatory diseases such as atherosclerosis, gout, type I and type 2 diabetes [22,29,30]. After interaction with the ligand, several NLRP3 identical proteins form an oligomer and each NLRP3 of the oligomer binds to an adapter protein called ASC (apoptosis-associated speck-like protein). This binds to the inactive precursor form of caspase-1 enzyme, which becomes active and cleaves pro-IL-1β, resulting in biologically active IL-1β generation which subsequently is secreted into the extracellular medium [31,32].

Several endogenous molecules, considered “danger signals” such as MSU crystals, ATP, β-amyloid or hyaluronan) when in high concentrations, induce NLRP3 inflammasome activation, playing an important role in inflammatory diseases [27]. Conforti-Andreoni and collaborators [33] reported the importance of uric acid, together with the signaling pathway of the nuclear transcription factor-kB (NF-kB), in the inflammasome activation of dendritic cells as well as IL-1β and IL-18 production. According to Rock and collaborators [34], uric acid plays a pathophysiological role as a local alarm signal that alerts the immune system against cellular injury and aids in the development of innate and adaptive immune responses.

The NLRP3 receptor is described as uric acid sensor that induces IL-1β production in trophoblast cell lines. Thus, hyperuricemia associated to the PE could be related to activation of the NLRP3 inflammasome in trophoblast, thereby increasing the IL-1β levels in the placenta and contributing to the pathogenesis of PE [35]. Recently, a study of polimorphisms in inflammasome genes was analyzed in preeclamptic women, and showed association of NLRP1 variant rs12150220 (L155H) with development of preeclampsia, suggesting a role of this inflammasome receptor in the pathogenesis of this disorder [36].

Considering that high levels of uric acid are often observed in pregnancies complicated by PE, and are associated with oxidative stress and elevated levels of IL-1β and TNF-α produced by monocytes, the present study aimed to evaluate the role of NLRP1 and NLRP3 inflammasomes in endogenous or monosodium urate-induced activation of monocytes from pregnant women with PE. Gene expression of NLRP1 and NLRP3 receptors as well as their association with IL-1β, TNF-α and IL-18 production by these cells were determined.

In pregnant women with PE elevated serum levels of uric acid associated with increased production of pro-inflammatory cytokines such as IL-1β and TNF-α by mononuclear cells from peripheral blood [16], suggest participation of hyperuricemiain the pathogenesis of PE. It is known that uric acid crystals (MSU) act as DAMPs activating the complex NLRP3 inflammasome [40]. The results of the present study show higher gene expression of NLRP1 and NLRP3 receptors, caspase-1 as well as IL-1β, IL-18 and TNF-α in peripheral blood monocytes of pregnant women with PE. These results suggest the endogenous activation of these cells, associated with increased production of inflammatory cytokines compared with monocytes from normotensive pregnant women.

It was observed that the gene expression of NLRP1, NLRP3 and caspase-1 have similar profiles to each other and higher expression in monocytes from pre-eclamptic women compared with normotensive pregnant women and non-pregnant women groups. A recent study [35], showed that uric acid activates the inflammasome in trophoblast cells, leading to IL-1β secretion and, suggesting that it is a novel mechanism for induction of inflammation in the maternal-fetal interface and causing adverse pregnancy outcomes, including preeclampsia. Caspases are important cysteinyl aspartate proteases in apoptosis and inflammation mechanisms [41]. Caspase-1 is an important inflammatory protein synthesized as pro-caspase-1, and becomes biologically activated when inflammasome starts its generation process [42,43]. It is recognized in the literature that NLRP1, NLRP3, AIM2 and IPAF inflammasomes are responsible for caspase-1 activation process, leading to the processing and release of IL-1β and IL-18 [44].

According to Martinon et al. [21], monosodium urate triggers the increase in gene expression of NLRP3 and caspase-1 in lineage of THP1 monocytes. Our results corroborate with these authors, showing that the stimulus with MSU induces increased NLRP3 and caspase-1 expression in monocytes from women with PE and from non-pregnant women, while the NLRP1 gene showed only a tendency to increased expression by monocytes of pregnant women with PE without statistical significance. Our results about reduction of gene expression of caspase-1 and IL-1β by monocytes stimulated with MSU in the presence of Glybenclamide, an inflammasome inhibitor [45] suggests that caspase-1 and IL-1β expression are dependent on NLRP3 activation by MSU. However, the precise molecular details of NLRP3 inflammasome in response to uric acid in preeclampsia remain to be elucidated. In the present study, the demonstrated capacity of NLRP3 inflammasome in inducing IL-1β production suggests a role for this mechanism in the systemic inflammatory response observed in preeclampsia.

The gene expression of NLRP1 and NLRP3 receptors, caspase-1, IL-1β, IL-18 and TNF-α, as well as the production of these cytokines was higher in monocytes from pregnant women with PE stimulated with MSU compared with normotensive pregnant women.

However, when compared to monocytes from healthy women, this increase is significant only in transcriptional, but not in protein level. This difference between gene expression and production of cytokines could be related to the fact that multiple ribosomes can translate a single mRNA molecule at the same time. Ribosomes groups called polysome, allow the simultaneous production of multiple strings of amino acids (polypeptides). These polypeptides may be complete or require further processing to become mature and to acquire a three-dimensional conformation [46].

The production of IL-1β, IL-18 and TNF-α by monocytes from non-pregnant women stimulated with MSU was increased, reaching a maximum level of production, similar to that observed in pregnant women with PE. These results confirm the ability of uric acid in exerting stimulatory effect on inflammatory activity of these cells by inflammasome activation represented by high expression of the genes encoding the NLRP3 receptor and caspase-1 protein, as well as increase in the inflammatory cytokines IL-1β and IL-18 production after cell stimulation in NP group.

In preeclamptic women there was no significant increase in gene expression and production of IL-1β after monocyte stimulation with MSU as showed in Fig 2. This phenomenon may be occurred probably because the cells were already significantly activated in PE, and producing higher endogenous levels of this cytokine. Siljee et al. [47] suggest that increased serum levels of IL-1β during the first trimester of pregnancy, in the absence of other inflammatory proteins, can be considered an early biomarker of PE onset, and an important pathogenic factor for the development of this disease.

Monocytes from normotensive pregnant women stimulated with MSU, in turn, have lower gene and protein expression of inflammatory cytokines IL-1β, IL-18 and TNF-α in relation to PE and NP groups. The lowest gene expression of these cytokines by monocytes from NT group could be due to the regulatory activity of IL-10 on these cells. The predominance of higher levels of this anti-inflammatory cytokine predominance in normal pregnancy acts to minimize the deleterious effects of excessive inflammatory response, being able to regulate the inflammatory response that occurs during pregnancy by controlling the IL-1β and TNF-α gene expression [48]. It is known that IL-10 exerts a potent inhibitory effect on the production of inflammatory cytokines such as IL-1β and TNF-α by activated monocytes [49]. However, in PE, this regulation does not occur adequately, and the balance is altered, with decreased IL-10 and increased TNF-α [50]. In previous work, it was shown that monocytes from preeclamptic pregnant women produce endogenous IL-10 significantly lower than in normotensive pregnant women, while TNF-α values are high [13], confirming the findings in the literature [12,51].

In the present study no significant differences were detected between preeclamptic and NP groups in relation to endogenous IL-18 gene expression and cytokine production. However, monocytes from these both groups produced higher levels of IL-18 after MSU stimulation (Figs 2B and 4B). Pro-IL-18 is constitutively present in healthy people monocytes [52]. Then, it was expected that the increase in gene and protein expression was not as obvious as that of other inflammatory cytokines. IL-18 is also secreted by inflammasome activation [53,54]. We also observed that there was more intense production of IL-1β than IL-18, demonstrated by high levels of IL-1β secreted by monocytes from the three groups. It is believed that this may be occurred because IL-18 also stimulates the secretion of Th2 cytokines with anti-inflammatory activity [55]. Thus, IL-18 induces Th1 and Th2 responses, acting as a cytokine with both inflammatory and anti-inflammatory profiles.

The production of proinflammatory cytokines is a critical step for an effective innate immune response and is also a mechanism by which this response influences the subsequent development of adaptive immune response [56]. IL-1β and IL-18 are examples of key cytokines which not only activates monocytes, macrophages and neutrophils but also specifically direct the development of the adaptive immune response of CD4+ T cells in humans and mice. The differentiation of CD4+ T cells in the presence of IL-1β and IL-18 results in Th17 and Th1 effector cells, respectively [57,58]. The predominant profile of Th17 cells on regulatory T cells in pregnant women with PE could act by modulating the Th1/Th2 balance [59], suggesting that both innate and adaptive immunity are involved in the excessive inflammatory response that occurs in this obstetric pathology [60].

The endogenous and MSU-induced gene expression of TNF-α, was higher in monocytes from pregnant women with PE compared to NT and NP groups, while the cytokine production was significantly higher in PE group than in the NT group. This increased TNF-α production by monocytes from pre-eclamptic pregnant women confirms previous results from the literature [11,13, 61], demonstrating that elevated serum uric acid levels correlate with increased production of TNF-α and free radicals released by monocytes from pregnant women with PE compared to normotensive pregnant women [14]. These elevated levels of TNF-α in peripheral blood of pre-eclamptic pregnant woman might activate NLRP1 and NLRP3 inflammasomes in monocytes by its stimulatory effect on the nuclear factor-κB transcription. The synthesis of TNF-α and IL-1β is controlled, in part, by NF-κB, which is more active in pregnant women with PE cells [16]. NF-κB regulates the transcription of genes related to inflammation [62,63] and TNF-α, in turn, acts stimulating NF-κB activation, and maintaining a cell cycle activation [64]. Thus, although not directly related to the inflammasome, TNF-α is an important inflammatory cytokine produced in the first activation signal of this multiprotein complex.

Activation of inflammasomes by MSU requires two signals. The first is the recognition of MSU by the extracellular toll-like (TLR) receptor, leading to the nuclear transcription factor (NF-κB) activation, and causing production of pro-IL-1β by monocytes, as previously described in gout [22]. The second signal depends on phagocytosis of MSU crystals and their recognition by NLRP3 inflammasome in monocytes, resulting in activation of caspase-1 and subsequent cleavage of pro-IL-1β to produce active IL-1β. According to Bauernfeind et al. [65], the need for this double stimulation to activate the inflammasome might be important for preventing uncontrolled activation of NLRP3, a responsible that could lead to devastating consequences in the host, as seen in autoinflammatory diseases.

In conclusion, the present results show, for the first time, that monocytes from preeclampticwomen have endogenous activation of the inflammasomes NLRP1 and NLRP3 and secrete higher levels of IL-1β, IL-18 and TNF-α. Stimulation of these cells with MSU, inducesincrease in gene expression of NLRP3 receptor and caspase-1, as weel as IL-18 production, that is more evident in pre-eclamptic pregnant and non-pregnant women groups. These results demonstrate the role of uric acid in NLRP3 inflammasome activation and release of inflammatory cytokines, suggesting the envolvement of this inflammatory complex in the pathogenesis of preeclampsia.