The mononuclear phagocytic system (MPS) is the primary innate immune cell group in male reproductive tissues, maintaining the balance of pro-inflammatory and immune tolerance. This article aims to outline the role of mononuclear macrophages in the immune balance of the testes and epididymis, and to understand the inner immune regulation mechanism. A review of pertinent publications was performed using the PubMed and Google Scholar databases on all articles published prior to January 2021. Search terms were based on the following keywords: ‘MPS’, ‘mononuclear phagocytes’, ‘testes’, ‘epididymis’, ‘macrophage’, ‘Mφ’, ‘dendritic cell’, ‘DC’, ‘TLR’, ‘immune’, ‘inflammation’, and ‘polarization’. Additionally, reference lists of primary and review articles were reviewed for other publications of relevance. This review concluded that MPS exhibits a precise balance in the male reproductive system. In the testes, MPS cells are mainly suppressed subtypes (M2 and cDC2) under physiological conditions, which maintain the local immune tolerance. Under pathological conditions, MPS cells will transform into M1 and cDC1, producing various cytokines, and will activate T cell specific immunity as defense to foreign pathogens or self-antigens. In the epididymis, MPS cells vary in the different segments, which express immune tolerance in the caput and pro-inflammatory condition in the cauda. Collectively, MPS is the control point for maintaining the immune tolerance of the testes and epididymis as well as for eliminating pathogens.

C-type natriuretic peptide (CNP) is highly expressed in male reproductive tissues, such as the epididymis. The aim of this study is to explore the role of CNP in the maturation of rat epididymal spermatozoa. First, the expression levels of CNP and its specific natriuretic peptide receptor-B (NPR-B) were detected in various tissues of rats and epididymis at different stages after birth. Then a castrated rat model was established to analyze the relationship between testosterone and CNP/NPR-B expression in the epididymis. Finally, CNP and different inhibitors (NPR-B inhibitors, cGMP inhibitors) were used to incubate epididymal sperm in vitro to examine sperm mobility and expression of sperm maturation-related factors. The results showed CNP/NPR-B mRNAs were expressed in all tissues of rats, but were extremely highly expressed in male genital ducts (seminal vesicle, prostate and epididymis). The expression of CNP/NPR-B in epididymis was the highest at birth and the fifth week after birth. In the epididymis, CNP/NPR-B were highly expressed in the caput and located in the epididymal epithelial cells. After castration, the expression of CNP/NPR-B decreased sharply and was restored quickly after testosterone supplementation. In vitro, CNP could significantly promote the acquisition of epididymal sperm motility through the NPR-B/cGMP pathway and induce the expression of sperm maturation-related factors (such as Bin1b, Catsper 1, Dnah17, Fertilin). This study shows that CNP plays a role in epididymal sperm maturation. The mechanism of CNP is to promote the acquisition of epididymal sperm fluidity through the NPR-B/cGMP signaling pathway and also to regulate sperm maturation-related genes. Moreover, the expression of CNP/NPR-B was regulated by testosterone.

Objectives: This study aimed to investigate the mechanisms underlying impaired sperm motility in high-fat diet (HFD)-induced obesity. Methods: Six-week-old male C57BL/6J mice were used to establish an HFD-induced obesity model. Sperm motility and male reproductive function were then evaluated. Macrophage polarization was evaluated by flow cytometry, and key differentially expressed genes were identified by RNA-seq analysis of epididymal adipose tissue, the epididymis, and testes. Finally, pyruvate carboxylase (PC) and the PC inhibitor erianin (Eri) were used to examine the role of adipocyte-derived PC in regulating macrophage polarization. Results: HFD-induced obese mice exhibited abnormal testicular and epididymal morphology, with reduced sperm motility and viability. Flow cytometry revealed that macrophages were polarized toward the M1 phenotype in epididymal adipose tissue and the epididymis in the HFD group. RNA-seq analysis demonstrated that PC was the most significantly upregulated gene in epididymal adipose tissue. Furthermore, PC expression increased during adipocyte maturation in vitro. Mature adipocytes promoted macrophage polarization toward the M1 phenotype, while Eri attenuated this effect. In addition, supernatants from LPS-induced M1 macrophages contained elevated levels of inflammatory cytokines and significantly reduced sperm motility and viability. Conclusions: HFD-induced obesity impaired sperm quality and male reproductive function. The underlying mechanism may involve increased PC expression, which drives epididymal macrophages toward M1 polarization and triggers local chronic inflammation, ultimately compromising sperm motility and viability.