NLRs, a family of cytoplasmic sensors that bind to inflammatory complexes, are gradually coming into the limelight; they are a representative class of receptors responsible for innate immune pattern recognition [19]. The NLR family possesses several distinctive structural features, including a central nucleotide-binding and oligomerization (NACHT) structural domain, which is usually surrounded by a C-terminal leucine-rich repeat (LRR) and an N-terminal caspase recruitment domain (CARD) or pyrin domain (PYD) [20]. The NLRP3 inflammasome is composed of the NLRP3 scaffold, apoptosis-associated speck-like protein containing CARD (ASC) junctions (PYCARD), and cystatinase-1 (caspase-1) together (Figure 1) [21]. Ratsimandresy RA et al. [22] found that NLRP3 is able to encode the PYD, which contributes significantly to the innate immune response pathways and is required for host defense against pathogens and the initiation of wound healing. Haneklaus M et al. [23] have shown that NLRP3 can provide an essential molecular mechanism in the induction of central IL-1β by activating caspase-1. IL-1β is a critical pro-inflammatory cytokine that drives the inflammatory process and restores homeostasis [23].

As a result of gene amplification from the last common ancestor, 22 human genes and many more mouse genes come together to form the NLR family. Among the structural domains found in NLR family members, the NACHT structural domain is the only structural domain common to all NLR family members and can activate the signaling complex through ATP-dependent oligomerization. LRR structural domains are thought to yield considerable influence on ligand perception and autoregulation, while the CARD and PYD structural domains may be instrumental in mediating homotypic protein-protein interactions for downstream signaling [19].

In general, the initiation of NLRP3 inflammasomes requires the realization of two signals. The first initiating stimulus, which can include any receptor signal (such as phosphorylated Toll-like receptor) leading to activation of the transcription factor NF-κB, the signal provided by NF-κB activators is essential for NLRP3 transcription [24]. However, under the premise of steady-state and sufficient concentrations, ASC and procaspase-1 are not required for transcriptional regulation to achieve activation of the inflammasome, nor is pro-IL-18, the substrate of caspase-1. They can promote caspase-1 activation through an acute mechanism that is independent of induction by inflammasome sensors [25]. Secondly, observations relying on pattern recognition receptor (PRR) signals have shown that free cytoplasmic NLRP3 remains in an inactive ubiquitinated state until the initiation signal stimulates its LRR structural domain ubiquitination, at which point NLRP3 is activated through a two-step deubiquitination mechanism initiated by Toll-like receptor signaling and mitochondrial reactive oxygen species (ROS) activation [26]. This discovery not only emphasizes the critical steps of the initiation program but also distinguishes it from the transcriptional step. Between the transcriptional up-regulation and post-translational modification of inflammasome components, PRR signal-dependent initiation establishes a high threshold; only by reaching this threshold can NLRP3 inflammasome activation be achieved. Another group of molecules that control the initiation of NLRP3 inflammasomes are the cellular inhibitor of apoptosis proteins (cIAPs), commonly considered programmed cell death. cIAP1 and cIAP2 have also been shown to be vital in the inflammasome pathway by acting on caspase-1 [27].

Research has shown that ROS, K⁺, Ca²⁺, and mitochondrial dysfunction are closely related to the activation of NLRP3 inflammasomes (Figure 2). NLRP3 inflammasomes can also be activated by many other molecules. In 2006, a study by Mariathasan S et al. [28] found that extracellular ATP released by damaged cells can activate caspase-1, thereby stimulating the activation of NLRP3 inflammasomes. In 2008, Halle A et al. [29] found that fibrillar peptide amyloid-β (aβ) also activated inflammasomes formed by the cytoplasmic receptor NLRP3, leading to the release of IL-1β; and in 2009, research by Yamasaki K et al. [30] found that hyaluronic acid could activate NLRP3 inflammasomes by stimulating IL-1β. Activation of the NLRP3 inflammasomes also monitors autometabolic stress. In 2006, Martinon F et al. [31] found that monosodium urate (MSU) and calcium pyrophosphate deposition (CPPD) were involved in the activation of NLRP3 inflammasomes, resulting in the production of active IL-1β. In 2009, Gasse P et al. [32] found that uric acid is released during cellular injury, and thus, uric acid-dependent pathways can also stimulate NLRP3 inflammasomes. In the same year, Griffith JW et al. [33] also demonstrated that the release of uric acid activates NLRP3 inflammasomes, which enhanced the host’s initial response to hemolysin. In 2010, Zhou R et al. [34] found that the activation of NLRP3 can improve glucose tolerance and insulin sensitivity. In addition, NLRP3 inflammasomes are activated by many factors. For example, trinitrophenyl chloride [35] and trinitro-chlorobenzene [36] are associated with pathologies related to contact hypersensitivity reactions; ultraviolet medium wave (UVB) [37] irradiation is linked to sunburn; SiO₂ [38–40] is closely connected to silicosis; and asbestos [39] is related to the pathologies of asbestosis.

Orchitis usually refers to unilateral or bilateral orchitis caused by viruses and bacteria [41], mainly categorized into bacterial orchitis, autoimmune orchitis, and other testicular immune disorders. As for bacterial orchitis caused by prostate bacterial infection and urinary tract infection, its common pathogens include UroPathogenic Escherichia coli (UPEC), Klebsiella pneumoniae, Pseudomonas aeruginosa, and Staphylococcus and Streptococcus genera [42]. Autoimmune orchitis is mainly divided into two categories: primary and secondary. NLRP3 inflammasomes and testicular immunity are also inextricably intertwined. The study by Zhou Y et al. [43] confirmed that NLRP3 inflammasomes are paramount in destroying testicular structures. Activation of the typical inflammasome pathway of NLRP3/caspase-1 and IL-1β induces the secretion of inflammatory cytokine secretion and immune response in Sertoli cells. It is proven that NLRP3 inflammasomes engage in DBP (dibutyl phthalates)-induced inflammation of testicular Sertoli cells and provide a new reference point for the treatment of testicular inflammation. Walenta L et al. [44] concluded that NLRP3 may be a new participant in testicular immune regulation. To verify this claim, they performed immunostaining on human testicular sections from idiopathic infertility samples and found increased NLRP3 levels in Sertoli cells and peritubular cells, suggesting that NLRP3 expression is associated with idiopathic male infertility.

Varicocele is an inflammatory syndrome caused by abnormal dilatation of the testicular veins, which is the most common abnormal condition in male reproductive diseases. It is characterized by a total tendon muscular injury and functional impairment of testicular interstitial cells. Varicocele is also defined as excessive dilation of the pampiniform plexus [58]. In the patient’s testicular tissues, elevated levels of ROS can lead to varicocele and low fertility [59]. Agarwal A et al. [60] have shown that ROS is negatively correlated with sperm concentration, viability, and sperm morphology. Desai N et al. [61] have also found that ROS causes sperm DNA damage and has adverse effects on fertilization potential and pregnancy rate. The above studies emphasize the outstanding contribution of ROS in activating NLRP3 inflammasomes, and the experimental model of varicocele also demonstrates the increase in NLRP3 inflammasome level expression [62, 63]. In this case, with the production of ROS and the activation of NLRP3 inflammasomes, an inflammatory state will occur in varicocele, damaging male reproductive tissues [59]. Whereas, the study of Sahin Z et al. [64] found that the levels of pro-inflammatory cytokines such as IL-1α and IL-1β also increased after varicocele induction in 11- and 13-week-old rats, and IL-1β is only expressed in the nuclei of spermatogonia and Sertoli cells. Increased IL-1α and IL-1β in varicocele alters homeostasis in favor of inflammation and immune response and adversely affects testicular tissue, leading to male infertility. At the current moment, more and more studies are linking immune disorders to the role of NLRP3 inflammasomes, providing new ideas and mechanisms for the disease [65]. Antonuccio P [63] and others have found that it is possible to combine seleno-L-methionine (Se) and polydeoxyribonucleotide (PDRN), an adenosine A2A receptor agonist, and that Se-PDRN improves all the renal tubular morphologic parameters and their ultrastructural features, increasing both testosterone levels and decreasing the expression of NLRP3, cysteinyl asparagin-1, and IL-1β, showing an overall positive effect on fertility. Given the many activators of NLRP3 inflammasomes, it remains to be further investigated whether we can turn our therapeutic attention to NLRP3 and provide a new approach to varicocele treatment while decreasing NLRP3 inflammasome expression.

Acute epididymitis is mostly an inflammation of the epididymis caused by an infection, the course of which is usually less than 6 weeks, and, in a few cases, accumulates to the testes, forming epididymo-orchitis [66]. In men under 35 years of age, the majority of cases are caused by sexually transmitted pathogens, such as Neisseria gonorrhoeae or Chlamydia trachomatis. Most cases in men older than 35 are infected by Escherichia coli or other gram-negative coliforms and usually occur in patients with urologic anomalies (e.g., benign prostatic hyperplasia), indwelling catheters, or recent urologic surgeries. And the main reason is a viral infection if it occurs in prepubertal boys [67]. Lin N et al. [68] found that sperm viability tended to decrease progressively in patients with chronic epididymitis, and the activation of NLRP3 inflammasomes promoted apoptosis and IL-1β production in the epithelial cells of the epididymis, thus leading to the progression of epididymal inflammation. Fan W et al. [69] found that a long-term high-fat diet leads to NLRP3 inflammasomes in reproductive tissues such as the epididymis. TNF-α and IL-6 expression levels are elevated, which are positively correlated with BMI and negatively correlated with sperm concentration and viability, causing a localized inflammatory response in the male reproductive tract and inducing epididymitis and other relevant diseases. Certainly, the effect of NLRP3 inflammasomes on infertility due to epididymitis deserves to be investigated in detail, and further efforts are needed.

In summary, the impression of NLRP3 inflammasomes in testicular, spermatic vein, and epididymal immune disorders is indispensable, and many testicular, spermatic vein, and epididymal immune disorders involve NLRP3 (Figure 3).