Comprehensive overview of murine epididymal mononuclear phagocytes and lymphocytes: Unexpected populations arise

Regional modulation of toll-like receptor signaling pathway genes in acute epididymitis in mice

BACKGROUND

Region-specific immune environments in the epididymis influence the immune responses to uropathogenic Escherichia coli (UPEC) infection, a relevant cause of epididymitis in men. Toll-like receptors (TLRs) are essential to orchestrate immune responses against bacterial infections. The epididymis displays region-specific inflammatory responses to bacterial-derived TLR agonists, such as lipopolysaccharide (LPS; TLR4 agonist) and lipoteichoic acid (LTA; TLR2/TLR6 agonist), suggesting that TLR-associated signaling pathways could influence the magnitude of inflammatory responses in epididymitis.

OBJECTIVES

To investigate the expression and regulation of key genes associated with TLR4 and TLR2/TLR6 signaling pathways during epididymitis induced by UPEC, LPS, and LTA in mice.

MATERIAL AND METHODS

Epididymitis was induced in mice using UPEC, ultrapure LPS, or LTA, injected into the interstitial space of the initial segment or the lumen of the vas deferens close to the cauda epididymidis. Samples were harvested after 1, 5, and 10 days for UPEC-treated animals and 6 and 24 h for LPS-/LTA-treated animals. Ex vivo epididymitis was induced by incubating epididymal regions from naive mice with LPS or LTA. RT-qPCR and Western blot assays were conducted.

RESULTS

UPEC infection up-regulated Tlr2, Tlr4, and Tlr6 transcripts and their associated signaling molecules Cd14, Ticam1, and Traf6 in the cauda epididymidis but not in the initial segment. In these epididymal regions, LPS and LTA differentially modulated Tlr2, Tlr4, Tlr6, Cd14, Myd88, Ticam1, Traf3, and Traf6 expression levels. NFKB and AP1 activation was required for LPS- and LTA-induced up-regulation of TLR-associated signaling transcripts in the cauda epididymidis and initial segment, respectively.

CONCLUSION

The dynamic modulation of TLR4 and TLR2/TLR6 signaling pathways gene expression during epididymitis indicates bacterial-derived antigens elicit an increased tissue sensitivity to combat microbial infection in a spatial manner in the epididymis. Differential activation of TLR-associated signaling pathways may contribute to fine-tuning inflammatory responses along the epididymis.

Immunoregulatory mechanisms between epithelial clear cells and mononuclear phagocytes in the epididymis

INTRODUCTION

One of the most intriguing aspects of male reproductive physiology is the ability of the epididymis to prevent the mounting of immune responses against the onslaught of foreign antigens carried by spermatozoa while initiating very efficient immune responses versus stressors. Epithelial clear cells are strategically positioned to work in a concerted manner with region-specific heterogeneous subsets of mononuclear phagocytes to survey the epididymal barrier and regulate the balance between inflammation and immune tolerance in the post-testicular environment.

OBJECTIVE

This review aims to describe how clear cells communicate with mononuclear phagocytes to contribute to the unique immune environment in which sperm mature and are stored in the epididymis.

MATERIALS/METHODS

A comprehensive systematic review was performed. PubMed was searched for articles specific to clear cells, mononuclear phagocytes, and epididymis. Articles that did not specifically address the target material were excluded.

RESULTS

In this review, we discuss the unexpected roles of clear cells, including the transfer of new proteins to spermatozoa via extracellular vesicles and nanotubes as they transit along the epididymal tubule; and we summarize the immune phenotype, morphology, and antigen capturing, processing, and presenting abilities of mononuclear phagocytes. Moreover, we present the current knowledge of immunoregulatory mechanisms by which clear cells and mononuclear phagocytes may contribute to the immune-privileged environment optimal for sperm maturation and storage.

DISCUSSION AND CONCLUSION

Notably, we provide an in-depth characterization of clear cell-mononuclear phagocyte communication networks in the steady-state epididymis and in the presence of injury. This review highlights crucial concepts of mucosal immunology and cellcell interactions, all of which are critical but understudied facets of human male reproductive health.

New aspects of activin biology in epididymal function and immunopathology

The activins (A and B) and their binding protein, follistatin, play crucial roles in development, immunoregulation and inflammation throughout the body. In the male reproductive tract of the mouse, activin A and B production is largely confined to the initial segment and proximal caput of the epididymis and the efferent ducts, under normal conditions, with very low expression in the corpus, cauda and vas deferens. However, activin A protein is present throughout the epididymis and vas deferens and is largely associated with the epithelium and interstitial macrophages. Conversely, the activin-binding protein follistatin is produced in the distal epididymis, with very high expression in the vas deferens. Activin activity in the distal tract is inhibited by follistatin, and the activin-follistatin balance is important for regulating coiling of the duct during epididymal development. In further experiments, as described in this report, in situ hybridisation was used to localise activin A mRNA principally to cells in the periductal zone and interstitium in the efferent ducts and proximal caput. Activin B mRNA, on the other hand, was localised to periductal cells in the efferent ducts and proximal epididymis and, most notably, to epithelial cells in the initial segment. Activin A is implicated in the regulation of mononuclear phagocyte function and immune responses in the caput and stimulates the expression of the key immunoregulatory protein, indoleamine 2,3-dioxygenase in this region. Activin A production in the corpus and cauda increases dramatically during bacterial epididymitis in mice, promoting inflammation and fibrosis and causing damage to the epithelium and obstruction of the epididymal duct. Consequently, it appears that the activin-follistatin axis is crucial for maintaining normal epididymal structure and function, but disruption of this balance during inflammation has deleterious effects on male fertility. Follistatin has therapeutic potential in ameliorating the proinflammatory and profibrotic effects of activins.

Effects of Heparan sulfate acetyl-CoA: Alpha-glucosaminide N-acetyltransferase (HGSNAT) inactivation on the structure and function of epithelial and immune cells of the testis and epididymis and sperm parameters in adult mice

Heparan sulfate (HS), an abundant component of the apical cell surface and basement membrane, belongs to the glycosaminoglycan family of carbohydrates covalently linked to proteins called heparan sulfate proteoglycans. After endocytosis, HS is degraded in the lysosome by several enzymes, including heparan-alpha-glucosaminide N-acetyltransferase (HGSNAT), and in its absence causes Mucopolysaccharidosis III type C (Sanfilippo type C). Since endocytosis occurs in epithelial cells of the testis and epididymis, we examined the morphological effects of Hgsnat inactivation in these organs. In the testis, Hgsnat knockout (Hgsnat-Geo) mice revealed statistically significant decrease in tubule and epithelial profile area of seminiferous tubules. Electron microscopy (EM) analysis revealed cross-sectional tubule profiles with normal and moderately to severely altered appearances. Abnormalities in Sertoli cells and blood-testis barrier and the absence of germ cells in some tubules were noted along with altered morphology of sperm, sperm motility parameters and a reduction in fertilization rates in vitro. Along with quantitatively increased epithelial and tubular profile areas in the epididymis, EM demonstrated significant accumulations of electrolucent lysosomes in the caput-cauda regions that were reactive for cathepsin D and prosaposin antibodies. Lysosomes with similar storage materials were also found in basal, clear and myoid cells. In the mid/basal region of the epithelium of caput-cauda regions of KO mice, large vacuolated cells, unreactive for cytokeratin 5, a basal cell marker, were identified morphologically as epididymal mononuclear phagocytes (eMPs). The cytoplasm of the eMPs was occupied by a gigantic lysosome suggesting an active role of these cells in removing debris from the epithelium. Some eMPs were found in proximity to T-lymphocytes, a feature of dendritic cells. Taken together, our results reveal that upon Hgsnat inactivation, morphological alterations occur to the testis affecting sperm morphology and motility parameters and abnormal lysosomes in epididymal epithelial cells, indicative of a lysosomal storage disease.

Myd88 Signaling Is Involved in the Inflammatory Response in LPS-Induced Mouse Epididymitis and Bone-Marrow-Derived Dendritic Cells

Epididymitis is an epididymal inflammation that may lead to male infertility. Dendritic cells (DCs) and myeloid differentiation primary response gene 88 (Myd88) were associated with epididymitis in rodents. However, the functions of Myd88 on epididymal DCs remain unclear. This study investigated the role of Myd88 in DCs for epididymitis. The Myd88 signaling pathway, phenotypes of DC subsets, and cytokines were investigated in lipopolysaccharide (LPS)-induced epididymitis in mice. CRISPR-Cas9 was used to knockout Myd88 in bone-marrow-derived dendritic cells (BMDCs) and immortalized mouse epididymal (DC2) cell line. In the vivo experiments, levels of the proinflammatory cytokines IL-1α, IL-6, IL-17A, TNF-α, IL-1β, MCP-1, and GM-CSF, mRNA for MyD88 related genes, and the percentages of monocyte-derived DCs (Mo-DCs) were significantly elevated in mice with epididymitis. In the vitro experiments, LPS significantly promoted the apoptosis of BMDCs. In addition, the concentration of inflammatory cytokines in BMDCs and DC2s were increased in the LPS group, while decreasing after the knockout of Myd88. These findings indicate that Myd88 on DCs is involved in the inflammation of epididymitis in mice, which may be a potential target for better strategies regarding the treatment of immunological male infertility.

Individual variation in and lateral asymmetry of mouse epididymal draining lymph nodes

PROBLEM

Draining lymph nodes (LNs) are pivotal sites for maintaining tolerance to self-antigens as well as eliciting immune responses to exogenous antigens. The epididymis is a male reproductive organ with a unique local immune environment. Although mice are the most commonly used laboratory animals for immunology research, there are no detailed descriptions of the anatomical location and function of LNs that drain the epididymis.

METHOD OF STUDY

Evans blue labeling was utilized to explore lymphatic drainage of the epididymis in eight- to ten-week-old male C57BL/6 mice. We confirmed the lymphatic drainage of the epididymis in mice using the objective technique of carboxyfluorescein succinimidyl ester (CFSE)-labeled cells.

RESULTS

By combined Evans blue labeling and fluorescent labeling, we found that 1) the patterns of epididymal LN drainage are highly heterogeneous between individual mice; 2) the leftside LNs participate in drainage more frequently than the right-side LNs; and 3) epididymal lymphatic drainage bypasses both the paraaortic and renal LNs in some mice.

CONCLUSIONS

These data highlighted the need to consider the individual variation in and lateral asymmetry of draining LNs when characterizing the regional immunology of the mouse epididymis.

CX3CR1 deficiency leads to impairment of immune surveillance in the epididymis

Mononuclear phagocytes (MPs) play an active role in the immunological homeostasis of the urogenital tract. In the epididymis, a finely tuned balance between tolerance to antigenic sperm and immune activation is required to maintain epididymal function while protecting sperm against pathogens and stressors. We previously characterized a subset of resident MPs that express the CX3CR1 receptor, emphasizing their role in antigen sampling and processing during sperm maturation and storage in the murine epididymis. Bacteria-associated epididymitis is the most common cause of intrascrotal inflammation and frequently leads to reproductive complications. Here, we examined whether the lack of functional CX3CR1 in homozygous mice (CX3CR1EGFP/EGFP, KO) alters the ability of MPs to initiate immune responses during epididymitis induced by LPS intravasal-epididymal injection. Confocal microscopy revealed that CX3CR1-deficient MPs located in the initial segments of the epididymis displayed fewer luminal-reaching membrane projections and impaired antigen capture activity. Moreover, flow cytometry showed a reduction of epididymal KO MPs with a monocytic phenotype under physiological conditions. In contrast, flow cytometry revealed an increase in the abundance of MPs with a monocytic signature in the distal epididymal segments after an LPS challenge. This was accompanied by the accumulation of CD103+ cells in the interstitium, and the prevention or attenuation of epithelial damage in the KO epididymis during epididymitis. Additionally, CX3CR1 deletion induced downregulation of Gja1 (connexin 43) expression in KO MPs. Together, our study provides evidence that MPs are gatekeepers of the immunological blood-epididymis barrier and reveal the role of the CX3CR1 receptor in epididymal mucosal homeostasis by inducing MP luminal protrusions and by regulating the monocyte population in the epididymis at steady state as well as upon infection. We also uncover the interaction between MPs and CD103+ dendritic cells, presumably through connexin 43, that enhance immune responses during epididymitis. Our study may lead to new diagnostics and therapies for male infertility and epididymitis by identifying immune mechanisms in the epididymis.

The Role of Mononuclear Phagocytes in the Testes and Epididymis

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.

The regional distribution of resident immune cells shapes distinct immunological environments along the murine epididymis

The epididymis functions as transition zone for post-testicular sperm maturation and storage and faces contrasting immunological challenges, i.e. tolerance towards spermatozoa vs. reactivity against pathogens. Thus, normal organ function and integrity relies heavily on a tightly controlled immune balance. Previous studies described inflammation-associated tissue damage solely in the distal regions (corpus, cauda), but not in the proximal regions (initial segment, caput). To understand the observed region-specific immunity along the epididymal duct, we have used an acute bacterial epididymitis mouse model and analyzed the disease progression. Whole transcriptome analysis using RNAseq 10 days post infection showed a pro-inflammatory environment within the cauda, while the caput exhibited only minor transcriptional changes. High-dimensional flow cytometry analyses revealed drastic changes in the immune cell composition upon infection with uropathogenic Escherichia coli. A massive influx of neutrophils and monocytes was observed exclusively in distal regions and was associated with bacterial appearance and tissue alterations. In order to clarify the reasons for the region-specific differences in the intensity of immune responses, we investigated the heterogeneity of resident immune cell populations under physiological conditions by scRNASeq analysis of extravascular CD45+ cells. Twelve distinct immune cell subsets were identified, displaying substantial differences in distribution along the epididymis as further assessed by flow cytometry and immunofluorescence staining. Macrophages constituted the majority of resident immune cells and were further separated in distinct subgroups based on their transcriptional profile, tissue location and monocyte-dependence. Crucially, the proximal and distal regions showed striking differences in their immunological landscapes. These findings indicate that resident immune cells are strategically positioned along the epididymal duct, potentially providing different immunological environments required for addressing the contrasting immunological challenges and thus, preserving tissue integrity and organ function.

Purinergic signaling in the male reproductive tract

Purinergic receptors are ubiquitously expressed throughout the body and they participate in the autocrine and paracrine regulation of cell function during normal physiological and pathophysiological conditions. Extracellular nucleotides activate several types of plasma membrane purinergic receptors that form three distinct families: P1 receptors are activated by adenosine, P2X receptors are activated by ATP, and P2Y receptors are activated by nucleotides including ATP, ADP, UTP, UDP, and UDP-glucose. These specific pharmacological fingerprints and the distinct intracellular signaling pathways they trigger govern a large variety of cellular responses in an organ-specific manner. As such, purinergic signaling regulates several physiological cell functions, including cell proliferation, differentiation and death, smooth muscle contraction, vasodilatation, and transepithelial transport of water, solute, and protons, as well as pathological pathways such as inflammation. While purinergic signaling was first discovered more than 90 years ago, we are just starting to understand how deleterious signals mediated through purinergic receptors may be involved in male infertility. A large fraction of male infertility remains unexplained illustrating our poor understanding of male reproductive health. Purinergic signaling plays a variety of physiological and pathophysiological roles in the male reproductive system, but our knowledge in this context remains limited. This review focuses on the distribution of purinergic receptors in the testis, epididymis, and vas deferens, and their role in the establishment and maintenance of male fertility.

Contribution of epididymal epithelial cell functions to sperm epigenetic changes and the health of progeny

BACKGROUND

Spermatozoa acquire their motility and fertilizing abilities during their maturation through the epididymis. This process is controlled by epididymal epithelial cells that possess features adapted to sense and respond to their surrounding environment and to communicate with spermatozoa. During the past decade, new intercellular communication processes have been discovered, including the secretion and transport of molecules from the epithelium to spermatozoa via extracellular vesicles (EVs), as well as sensing of the intraluminal milieu by cellular extensions.

OBJECTIVE AND RATIONALE

This review addresses recent findings regarding epididymal epithelial cell features and interactions between spermatozoa and the epididymal epithelium as well as epigenetic modifications undergone by spermatozoa during transit through the epididymal microenvironment.

SEARCH METHODS

A systematic search was conducted in Pubmed with the keyword 'epididymis'. Results were filtered on original research articles published from 2009 to 2021 and written in the English language. One hundred fifteen original articles presenting recent advancements on the epididymis contribution to sperm maturation were selected. Some additional papers cited in the primary reference were also included. A special focus was given to higher mammalian species, particularly rodents, bovines and humans, that are the most studied in this field.

OUTCOMES

This review provides novel insights into the contribution of epididymal epithelium and EVs to post-testicular sperm maturation. First, new immune cell populations have been described in the epididymis, where they are proposed to play a role in protecting the environment surrounding sperm against infections or autoimmune responses. Second, novel epididymal cell extensions, including dendrites, axopodia and primary cilia, have been identified as sensors of the environment surrounding sperm. Third, new functions have been outlined for epididymal EVs, which modify the sperm epigenetic profile and participate in transgenerational epigenetic inheritance of paternal traits.

WIDER IMPLICATIONS

Although the majority of these findings result from studies in rodents, this fundamental research will ultimately improve our knowledge of human reproductive physiopathologies. Recent discoveries linking sperm epigenetic modifications with paternal environmental exposure and progeny outcome further stress the importance of advancing fundamental research on the epididymis. From this, new therapeutic options for infertile couples and better counseling strategies may arise to increase positive health outcomes in children conceived either naturally or with ART.

C-Type Natriuretic Peptide Plays an Anti-Inflammatory Role in Rat Epididymitis Induced by UPEC

Human epididymitis is mainly caused by retrograde urinary tract infection with uropathogenic Escherichia coli (UPEC). This disease is an important factor (accounting for 20-30%) causing male infertility. C-type natriuretic peptide (CNP), a protein composed of 22 amino acids, is proved to play an immunoregulatory role in respiratory and cardiovascular systems. CNP is expressed extremely high in the epididymis, but whether CNP plays the same role in acute epididymitis is unclear. At first, we established an acute caput epididymitis model in rats with UPEC and treated them with CNP to measure inflammatory damage. Then RNA-seq transcriptome technology was used to reveal potential signal pathways. Secondly, the turbidity and activity of UPEC were assessed using a microplate reader and the amount of UPEC by agar plates after incubation with CNP. Thirdly, macrophages in caput epididymis were tested by immunohistochemistry (IHC). Meanwhile, lipopolysaccharide (LPS) with or without CNP was used to stimulate the macrophage (RAW264.7) in vitro and to detect the expression level of pro-inflammatory factors. Finally, the macrophage (RAW264.7) was treated with CNP, 8-Br-cGMP [cyclic guanosinc monophosphate (cGMP) analog] and KT5823 [protein kinase G (PKG) inhibitor], and the expression level of nuclear factor-k-gene binding (NF-kB) signal pathway was examined. The results showed that the damage of epididymis induced by UPEC as well as the pro-inflammatory factors could be alleviated significantly with CNP treatment. CNP could inhibit the activity and numbers of bacteria in both in vivo and in vitro experiments. Moreover, CNP repressed the invasion, and the expression of pro-inflammatory factors (such as NF-kB, IL-1β, IL-6, TNF-α) in macrophages and its effect could be inhibited by KT5823. Therefore, we drew a conclusion from the above experiments that CNP alleviates the acute epididymitis injury induced by UPEC. On one hand, CNP could inhibit the growth of UPEC. On the other hand, CNP could decrease invasion and inflammatory reaction of macrophages; the mechanism was involved in inhibiting NF-kB signal pathway through the cGMP/PKG in macrophages. This research would open up the possibility of using CNP as a potential treatment for epididymitis.

Examination of testicular lumicrine regulation of activins and immunoregulatory genes in the epididymal caput

BACKGROUND

Immunoregulatory genes encoding activin A (Inhba) and B (Inhbb), and indolamine 2,3-dioxygenase-1 (Ido1) are highly expressed in the murine caput epididymidis, which also has a network of intraepithelial mononuclear phagocytes. This environment is postulated to promote immunological tolerance to epididymal sperm. The factors regulating the immunoregulatory agents in the epididymal caput are poorly understood.

OBJECTIVES

This study aimed to investigate the potential role of testicular lumicrine factors in regulating activin and other immune-related genes in the caput epididymidis.

MATERIALS AND METHODS

The efferent ducts in adult C57/Bl6 mice were exposed and ligated bilaterally. Serum and tissues were collected seven days later. Animals with bilateral sham ligation and animals with no ligations (collectively referred to as the "intact" group) were used as controls.

RESULTS

Pressure-induced seminiferous epithelial damage due to intratubular fluid accumulation was observed in all ligated testes. Testicular inhibin was significantly increased and testosterone was elevated in some animals following bilateral ligation, but serum testosterone, serum LH, and serum inhibin were normal. Ligation caused epithelial regression in the initial segment, with similar but less severe effects in other caput segments. Activin A staining by immunohistochemistry in the epithelium was reduced in bilateral ligation, particularly in the initial segment, with moderately reduced staining intensity in the rest of the caput. Inhba expression within the caput was not significantly affected by bilateral ligation, but Inhbb was reduced by more than 60%. Transcripts encoding the macrophage-specific receptor Cx3cr1 were significantly reduced following bilateral ligation, but other immune cell markers, Ido1, and inflammatory genes were unaffected.

CONCLUSION

These data indicate that testicular lumicrine secretion regulates several genes that are preferentially expressed in the initial segment, but has marginal effects on genes such as those encoding activin A and IDO1, which are expressed more widely in the caput.

Differential tissue-specific damage caused by bacterial epididymo-orchitis in the mouse

Ascending bacterial urinary tract infections can cause epididymo-orchitis. In the cauda epididymidis, this frequently leads to persistent tissue damage. Less coherent data is available concerning the functional consequences of epididymo-orchitis on testis and caput epididymidis. This in vivo study addresses the functional and spatial differences in responsiveness of murine epididymis and testis to infection with uropathogenic Escherichia coli (UPEC). Whole transcriptome analysis (WTA) was performed on testis, caput, corpus and cauda epididymidis of adult C57BL/6 J wildtype mice. Following UPEC-induced epididymo-orchitis in these mice, epididymal and testicular tissue damage was evaluated histologically and semi-quantitatively at 10 days and 31 days post-inoculation. Expression of inflammatory markers and candidate antimicrobial genes were analysed by RT-qPCR. WTA revealed distinct differences in gene signatures between caput and cauda epididymidis, particularly amonst immunity-related genes. Cellular and molecular signs of testicular inflammation and disruption of spermatogenesis were noticed at day 10, but recovery was observed by day 31. In contrast to the cauda, the caput epididymidis did not reveal any signs of gross morphological damage or presence of pro-inflammatory processes despite confirmed infection. In contrast to beta-defensins, known UPEC-associated antimicrobial peptides (AMP), like Lcn2, Camp and Lypd8, were inherently highly expressed or upregulated in the caput following infection, potentially allowing an early luminal protection from UPEC. At the time points investigated, the caput epididymidis was protected from any obvious infection/inflammation-derived tissue damage. Studies addressing earlier time-points will conclude whether in the caput epididymidis a pro-inflammatory response is indeed not essential for effective protection from UPEC.

Two populations of self-maintaining monocyte-independent macrophages exist in adult epididymis and testis

Macrophages are the principal immune cells of the epididymis and testis, but their origins, heterogeneity, development, and maintenance are not well understood. Here, we describe distinct populations of epididymal and testicular macrophages that display an organ-specific cellular identity. Combining in vivo fate-mapping, chimeric and parabiotic mouse models with in-depth cellular analyses, we found that CD64^hiMHCII^lo and CD64^loMHCII^hi macrophage populations of epididymis and testis arise sequentially from yolk sac erythro-myeloid progenitors, embryonic hematopoiesis, and nascent neonatal monocytes. While monocytes were the major developmental source of both epididymal and testicular macrophages, both populations self-maintain in the steady-state independent of bone marrow hematopoietic precursors. However, after radiation-induced macrophage ablation or during infection, bone marrow-derived circulating monocytes are recruited to the epididymis and testis, giving rise to inflammatory macrophages that promote tissue damage. These results define the layered ontogeny, maintenance and inflammatory response of macrophage populations in the male reproductive organs.

Region-specific transcriptomic and functional signatures of mononuclear phagocytes in the epididymis

In the epididymis, prevention of autoimmune responses against spermatozoa and simultaneous protection against pathogens is important for male fertility. We have previously shown that mononuclear phagocytes (MPs) are located either in the epididymal interstitium or in close proximity to the epithelium. In the initial segments (IS), these 'intraepithelial' MPs extend slender luminal-reaching projections between epithelial cells. In this study, we performed an in-depth characterisation of MPs isolated from IS, caput-corpus and cauda epididymis of CX3CR1EGFP+/- mice that express EGFP in these cells. Flow cytometry analysis revealed region-specific subsets of MPs that express combinations of markers traditionally described in 'dendritic cells' or 'macrophages'. RNA sequencing identified distinct transcriptomic signatures in MPs from each region and revealed specific genes involved in inflammatory and anti-inflammatory responses, phagosomal activity and antigen processing and presentation. Functional fluorescent in vivo labelling assays showed that higher percentages of CX3CR1+ MPs that captured and processed antigens were detected in the IS compared to other regions. Confocal microscopy showed that in the IS, caput and corpus, circulatory antigens were internalised and processed by interstitial and intraepithelial MPs. However, in the cauda only interstitial MPs internalised and processed antigens, while intraepithelial MPs did not take up antigens, indicating that all antigens have been captured before they reached the epithelial lining. Cauda MPs may thus confer a stronger protection against blood-borne pathogens compared to proximal regions. By identifying immunoregulatory mechanisms in the epididymis, our study may lead to new therapies for male infertility and epididymitis and identify potential targets for immunocontraception.

Differential Immune Response to Infection and Acute Inflammation Along the Epididymis

The epididymis is a tubular structure connecting the vas deferens to the testis. This organ consists of three main regions—caput, corpus, and cauda—that face opposing immunological tasks. A means of combating invading pathogens is required in the distally located cauda, where there is a risk of ascending bacterial infections originating from the urethra. Meanwhile, immune tolerance is necessary at the caput, where spermatozoa with immunogenic neo-antigens originate from the testis. Consistently, when challenged with live bacteria or inflammatory stimuli, the cauda elicits a much stronger immune response and inflammatory-inflicted damage than the caput. At the cellular level, a role for diverse and strategically positioned mononuclear phagocytes is emerging. At the mechanistic level, differential expression of immunoprotective and immunomodulatory mediators has been detected between the three main regions of the epididymis. In this review, we summarize the current state of knowledge about region-specific immunological characteristics and unveil possible underlying mechanisms on cellular and molecular levels. Improved understanding of the different immunological microenvironments is the basis for an improved therapy and counseling of patients with epididymal infections.

Lipopolysaccharide-induced epididymitis modifies the transcriptional profile of Wfdc genes in mice†

Whey-acidic protein four-disulfide core domain (WFDC) genes display putative roles in innate immunity and fertility. In mice, a locus on chromosome 2 contains 5 and 11 Wfdc genes in its centromeric and telomeric subloci, respectively. Although Wfdc genes are highly expressed in the epididymis, their contributions to epididymal function remain elusive. Here, we investigated whether Wfdc genes are regulated in response to lipopolysaccharide (LPS)-induced epididymitis, an inflammatory condition that impairs male fertility. We induced epididymitis in mice via (i) interstitial LPS injection into epididymal initial segment and (ii) intravasal LPS injection into the vas deferens towards cauda epididymis. Interstitial and intravasal LPS induced a differential upregulation of inflammatory mediators (interleukin 1 beta, interleukin 6, tumor necrosis factor, interferon gamma, and interleukin 10) in the initial segment and cauda epididymis within 72 h post-treatment. These changes were accompanied by a time-dependent endotoxin clearance from the epididymis. In the initial segment, interstitial LPS upregulated all centromeric (Slpi, Wfdc5, Wfdc12, Wfdc15a, and Wfdc15b) and five telomeric (Wfdc2, Wfdc3, Wfdc6b, Wfdc10, and Wfdc13) Wfdc transcripts at 24 and 72 h. In the cauda epididymis, intravasal LPS upregulated Wfdc5 and Wfdc2 transcripts at 24 h, followed by a downregulation of Wfdc15b and three telomeric (Wfdc6a, Wfdc11, and Wfdc16) gene transcripts at 72 h. Pharmacological inhibition of nuclear factor kappa B activation prevented LPS-induced upregulation of centromeric and telomeric Wfdc genes depending on the epididymal region. We show that LPS-induced inflammation differentially regulated the Wfdc locus in the proximal and distal epididymis, indicating region-specific roles for the Wfdc family in innate immune responses during epididymitis.

The Immune Characteristics of the Epididymis and the Immune Pathway of the Epididymitis Caused by Different Pathogens

The epididymis is an important male accessory sex organ where sperm motility and fertilization ability develop. When spermatozoa carrying foreign antigens enter the epididymis, the epididymis shows "immune privilege" to tolerate them. It is well-known that a tolerogenic environment exists in the caput epididymis, while pro-inflammatory circumstances prefer the cauda epididymis. This meticulously regulated immune environment not only protects spermatozoa from autoimmunity but also defends spermatozoa against pathogenic damage. Epididymitis is one of the common causes of male infertility. Up to 40% of patients suffer from permanent oligospermia or azoospermia. This is related to the immune characteristics of the epididymis itself. Moreover, epididymitis induced by different pathogenic microbial infections has different characteristics. This article elaborates on the distribution and immune response characteristics of epididymis immune cells, the role of epididymis epithelial cells (EECs), and the epididymis defense against different pathogenic infections (such as uropathogenic Escherichia coli, Chlamydia trachomatis, and viruses to provide therapeutic approaches for epididymitis and its subsequent fertility problems.

From initial segment to cauda: a regional characterization of mouse epididymal CD11c+ mononuclear phagocytes based on immune phenotype and function

Successful sperm maturation and storage rely on a unique immunological balance that protects the male reproductive organs from invading pathogens and spermatozoa from a destructive autoimmune response. We previously characterized one subset of mononuclear phagocytes (MPs) in the murine epididymis, CX3CR1⁺ cells, emphasizing their different functional properties. This population partially overlaps with another subset of understudied heterogeneous MPs, the CD11c⁺ cells. In the present study, we analyzed the CD11c⁺ MPs for their immune phenotype, morphology, and antigen capturing and presenting abilities. Epididymides from CD11c-EYFP mice, which express enhanced yellow fluorescent protein (EYFP) in CD11c⁺ MPs, were divided into initial segment (IS), caput/corpus, and cauda regions. Flow cytometry analysis showed that CD11c⁺ MPs with a macrophage phenotype (CD64⁺ and F4/80⁺) were the most abundant in the IS, whereas those with a dendritic cell signature [CD64^- major histocompatibility complex class II (MHCII)⁺] were more frequent in the cauda. Immunofluorescence revealed morphological and phenotypic differences between CD11c⁺ MPs in the regions examined. To assess the ability of CD11c⁺ cells to take up antigens, CD11c-EYFP mice were injected intravenously with ovalbumin. In the IS, MPs expressing macrophage markers were most active in taking up the antigens. A functional antigen-presenting coculture study was performed, whereby CD4⁺ T cells were activated after ovalbumin presentation by CD11c⁺ epididymal MPs. The results demonstrated that CD11c⁺ MPs in all regions were capable of capturing and presenting antigens. Together, this study defines a marked regional variation in epididymal antigen-presenting cells that could help us understand fertility and contraception but also has larger implications in inflammation and disease pathology.

The epididymal immune balance: a key to preserving male fertility

Up to 15% of male infertility has an immunological origin, either due to repetitive infections or to autoimmune responses mainly affecting the epididymis, prostate, and testis. Clinical observations and epidemiological data clearly contradict the idea that the testis confers immune protection to the whole male genital tract. As a consequence, the epididymis, in which posttesticular spermatozoa mature and are stored, has raised some interest in recent years when it comes to its immune mechanisms. Indeed, sperm cells are produced at puberty, long after the establishment of self-tolerance, and they possess unique surface proteins that cannot be recognized as self. These are potential targets of the immune system, with the risk of inducing autoantibodies and consequently male infertility. Epididymal immunity is based on a finely tuned equilibrium between efficient immune responses to pathogens and strong tolerance to sperm cells. These processes rely on incompletely described molecules and cell types. This review compiles recent studies focusing on the immune cell types populating the epididymis, and proposes hypothetical models of the organization of epididymal immunity with a special emphasis on the immune response, while also discussing important aspects of the epididymal immune regulation such as tolerance and tumour control.

Granulocyte-macrophage colony stimulating factor (GM-CSF) is fully expressed in the genital tract, seminal plasma and spermatozoa of male pigs

Granulocyte-macrophage colony stimulating factor (GM-CSF) is a pro-inflammatory cytokine identified in boar seminal plasma (SP) but until now unexplored in terms of place of production and its association to spermatozoa. This study aimed to explore these aspects by evaluating the presence of GM-CSF in porcine reproductive organs (testes, epididymis and accessory sex glands), SP and mature spermatozoa (from cauda epididymis and ejaculated) using Western blot (WB), immunohistochemistry and immunocytochemistry. Positive labelling was obtained in tissues, SP and spermatozoa. In reproductive organs, WB revealed three forms of GM-CSF with different glycosylation degrees (15, 31 and 40 kDa). In SP and epididymal fluid, the GM-CSF appeared only in its active form while in spermatozoa the GM-CSF form present varied among sperm sources. Non-viable spermatozoa showed more GM-CSF than viable spermatozoa (14.87 ± 1.98 RU vs. 7.25 ± 0.52 RU) of fluorescence intensity. In conclusion, GM-CSF is widely present in the reproductive tract of male pigs, attached to the spermatozoa already in the epididymis as well as verted to SP. Consequently, the GM-CSF ought to regulate male genital tract and sperm function as well as mediating initial inflammatory responses and further mediating later immune actions by the female to semen deposition.

An atlas of cell types in the mouse epididymis and vas deferens

Following testicular spermatogenesis, mammalian sperm continue to mature in a long epithelial tube known as the epididymis, which plays key roles in remodeling sperm protein, lipid, and RNA composition. To understand the roles for the epididymis in reproductive biology, we generated a single-cell atlas of the murine epididymis and vas deferens. We recovered key epithelial cell types including principal cells, clear cells, and basal cells, along with associated support cells that include fibroblasts, smooth muscle, macrophages and other immune cells. Moreover, our data illuminate extensive regional specialization of principal cell populations across the length of the epididymis. In addition to region-specific specialization of principal cells, we find evidence for functionally specialized subpopulations of stromal cells, and, most notably, two distinct populations of clear cells. Our dataset extends on existing knowledge of epididymal biology, and provides a wealth of information on potential regulatory and signaling factors that bear future investigation.

Natural killer and NKT cells in the male reproductive tract

Natural killer (NK) cells are important effector lymphocytes that play a pivotal role in the innate and adaptive immune responses to tumors and viral infection. NKT cells are a heterogeneous group of T cells that share properties with both T cells and NK cells. They display immunoregulatory properties as they facilitate the cell-mediated immune response to tumors and infectious diseases, and inhibit cell-mediated immunity associated with autoimmune diseases and allograft rejection. However, the roles of NK and NKT cells in the male reproductive tract remain largely unexplored, in particular, NKT cells, tissue distribution, and state of health or disease. Infection and inflammation of the male genital tract are thought to be the primary etiological factors of male infertility. In this review, we considered this complex and rapidly growing field. We summarize the recent findings and the characterization and roles of NK and NKT cells in the male reproductive tract, including the testis, epididymis, prostate, seminal vesicle, and semen, to enhance our understanding of the immunological mechanisms of male infertility and for the design effective vaccines for male reproductive health in the future.

Region-specific immune responses to autoimmune epididymitis in the murine reproductive tract

Epididymitis is a common pathology of the male reproductive tract, potentially leading to infertility. Studies on bacterial epididymitis indicate that the cauda epididymis is more susceptible to inflammatory damage than the caput. These regional differences in immunoregulation are further investigated using an experimental autoimmune epididymo-orchitis model. Adult mice were immunized against testicular antigens and tissues were collected at 30 and 50 days following the first immunization. Epididymitis developed progressively; 70% of the mice developed disease at 30 days after the initial immunization and 93% at 50 days. Epididymitis was characterized by epithelial damage, immune cell infiltrates and fibrosis in the cauda, with minimal changes in the corpus, while the caput was unaffected. The incidence of epididymitis was greater than that of orchitis but similar to vasitis. The severity of epididymitis was positively correlated with the orchitis severity. Expression of key genes implicated in epididymal immunoregulation, inflammation and fibrosis, such as Ido1, Tnf, Tgfb1, Ccl2, Il1b, Il10, Cx3cl1 and Col1a1, was unchanged in the caput but increased in proportion to damage severity in the cauda at 50 days. Activin receptor mRNA expression in the cauda was negatively correlated with disease severity. These data suggest that the cauda is highly susceptible to inflammatory damage following an autoimmune challenge but the caput is minimally affected. This may be because the cauda is required to combat ascending infections through a robust inflammatory response, while the caput provides a more tolerogenic environment in order to protect the auto-antigenic sperm released from the testis.

A framework for high-resolution phenotyping of candidate male infertility mutants: from human to mouse

Male infertility is a heterogeneous condition of largely unknown etiology that affects at least 7% of men worldwide. Classical genetic approaches and emerging next-generation sequencing studies support genetic variants as a frequent cause of male infertility. Meanwhile, the barriers to transmission of this disease mean that most individual genetic cases will be rare, but because of the large percentage of the genome required for spermatogenesis, the number of distinct causal mutations is potentially large. Identifying bona fide causes of male infertility thus requires advanced filtering techniques to select for high-probability candidates, including the ability to test causality in animal models. The mouse remains the gold standard for defining the genotype-phenotype connection in male fertility. Here, we present a best practice guide consisting of (a) major points to consider when interpreting next-generation sequencing data performed on infertile men, and, (b) a systematic strategy to categorize infertility types and how they relate to human male infertility. Phenotyping infertility in mice can involve investigating the function of multiple cell types across the testis and epididymis, as well as sperm function. These findings will feed into the diagnosis and treatment of male infertility as well as male health broadly.

Differential expression and localisation of TGF-β isoforms and receptors in the murine epididymis

Testes produce spermatozoa that transit through and are stored in the epididymis where they acquire their fertilising capacities. Spermatozoa appear in the genital tract at puberty, long after the immune system was trained to self-antigens. As a consequence, this organ has to set strategies to tolerate sperm antigens to avoid autoimmune responses that would specifically target and destroy them. A recent study pointed the Transforming Growth Factor-beta (TGF-β) signalling in the dendritic cells as a crucial mechanism for epididymal tolerance to spermatozoa. In the mouse, TGF-β exists under three isoforms, and three distinct receptors have been described. Using RT-qPCR, immunohistochemistry and ELISA techniques, we investigated the expression and spatial distribution of the epididymal TGF-β isoforms and of their receptors in young and adult mice. We showed that both ligands and receptors were produced by immune and non-immune cells in the epididymis, whatever the age mice have. These data bring new clues as to the mechanisms of peripheral tolerance to sperm cells in the murine epididymis and raise potential other implications of the cytokine isoforms.

Novel role of proton-secreting epithelial cells in sperm maturation and mucosal immunity

Epithelial cells are immune sensors and mediators that constitute the first line of defense against infections. Using the epididymis, a model for studying tubular organs, we uncovered a novel and unexpected role for professional proton-secreting 'clear cells' in sperm maturation and immune defense. The epididymal epithelium participates in the maturation of spermatozoa via the establishment of an acidic milieu and transfer of proteins to sperm cells, a poorly characterized process. We show that proton-secreting clear cells express mRNA transcripts and proteins that are acquired by maturing sperm, and that they establish close interactions with luminal spermatozoa via newly described 'nanotubes'. Mechanistic studies show that injection of bacterial antigens in vivo induces chemokine expression in clear cells, followed by macrophage recruitment into the organ. Injection of an inflammatory intermediate mediator (IFN-γ) increased Cxcl10 expression in clear cells, revealing their participation as sensors and mediators of inflammation. The functional diversity adopted by clear cells might represent a generalized phenomenon by which similar epithelial cells decode signals, communicate with neighbors and mediate mucosal immunity, depending on their precise location within an organ.

Importance of SLC26 Transmembrane Anion Exchangers in Sperm Post-testicular Maturation and Fertilization Potential

In mammals, sperm cells produced within the testis are structurally differentiated but remain immotile and are unable to fertilize the oocyte unless they undergo a series of maturation events during their transit in the male and female genital tracts. This post-testicular functional maturation is known to rely on the micro-environment of both male and female genital tracts, and is tightly controlled by the pH of their luminal milieus. In particular, within the epididymis, the establishment of a low bicarbonate (HCO₃^–) concentration contributes to luminal acidification, which is necessary for sperm maturation and subsequent storage in a quiescent state. Following ejaculation, sperm is exposed to the basic pH of the female genital tract and bicarbonate (HCO₃^–), calcium (Ca²⁺), and chloride (Cl^–) influxes induce biochemical and electrophysiological changes to the sperm cells (cytoplasmic alkalinization, increased cAMP concentration, and protein phosphorylation cascades), which are indispensable for the acquisition of fertilization potential, a process called capacitation. Solute carrier 26 (SLC26) members are conserved membranous proteins that mediate the transport of various anions across the plasma membrane of epithelial cells and constitute important regulators of pH and HCO₃^– concentration. Most SLC26 members were shown to physically interact and cooperate with the cystic fibrosis transmembrane conductance regulator channel (CFTR) in various epithelia, mainly by stimulating its Cl^– channel activity. Among SLC26 members, the function of SLC26A3, A6, and A8 were particularly investigated in the male genital tract and the sperm cells. In this review, we will focus on SLC26s contributions to ionic- and pH-dependent processes during sperm post-testicular maturation. We will specify the current knowledge regarding their functions, based on data from the literature generated by means of in vitro and in vivo studies in knock-out mouse models together with genetic studies of infertile patients. We will also discuss the limits of those studies, the current research gaps and identify some key points for potential developments in this field.

Epithelial dynamics in the epididymis: role in the maturation, protection, and storage of spermatozoa

Epithelial cells line the lumen of tubular organs and are key players in their respective functions. They establish a unique luminal environment by providing a protective barrier and by performing vectorial transport of ions, nutrients, solutes, proteins, and water. Complex intercellular communication networks, specific for each organ, ensure their interaction with adjacent epithelial and non-epithelial cells, allowing them to respond to and modulate their immediate environment. In the epididymis, several epithelial cell types work in a concerted manner to establish a luminal acidic milieu that is essential for the post-testicular maturation and storage of spermatozoa. The epididymis also prevents autoimmune responses against auto-antigenic spermatozoa, while ensuring protection against ascending and blood pathogens. This is achieved by a network of immune cells that are in close contact and interact with epithelial cells. This review highlights the coordinated interactions between spermatozoa, basal cells, principal cells, narrow cells, clear cells, and immune cells that contribute to the maturation, protection, selection, and storage of spermatozoa in the lumen of the epididymis.

In search of new paradigms for epididymal health and disease: innate immunity, inflammatory mediators, and steroid hormones

The primary job of the epididymis is to mature and protect the luminally transiting spermatozoa. Mounting evidence is showing that innate immune components [including Toll-like receptors (TLRs) and antimicrobial proteins, among which are β-defensins] and inflammatory mediators, under the primary influence of androgens, participate in the cellular and molecular processes that define this tissue. Here, we present an overview of the contributions of these signaling pathway components during epididymal homeostasis and discuss the hypotheses as to their involvement in epididymitis, the most common urological inflammatory condition in men, frequently impairing their fertility. Drawing primarily from rodent models, we also focus on how the distribution and functional expression of innate immune components are differentially regulated in the prenatal developing epididymis, providing new insights into the disruption of these signaling pathways throughout the lifespan. Male infertility is caused by a variety of conditions, such as congenital malformations, genetic and endocrine disorders, exposure to environmental toxicants, and inflammatory/infectious conditions. More than one-third of infertile men with an idiopathic condition cannot currently be adequately diagnosed. Thinking about the innate immunity and inflammation context of the epididymis may provide new insights and directions as to how these systems contribute to male fertility, as well as also uncover urological and andrological outcomes that may aid clinicians in diagnosing and preventing epididymal pathologies.

Of vessels and cells: the spatial organization of the epididymal immune system

BACKGROUND

One third of infertility cases in couples worldwide has an exclusive male origin and immune disorders, essentially due to repetitive infections, are emerging an cause of male infertility. As the place of sperm maturation, epididymis must be preserved from excessive immune responses that may arise following infections of the male genital tract. At the same time, epididymis must set and maintain a tolerogenic environment in order not to destroy sperm cells that enter the tissue at puberty, long after the immune system has been taught to recognize self pathogens. The immune cells that populate the epididymis have raised growing interest over the last thirty years but they may be not sufficient to understand the immune balance existing in this organ, between immune response to pathogens and tolerance to spermatozoa. Indeed, immune cells are the most motile cells in the organism and need blood and lymphatic vessels to traffic between lymphoid organs and sites of infection to induce efficient responses.

OBJECTIVES

To review the literature on the blood and lymphatic vessels, and on the immune cells present at steady state in the rodent epididymis (rat and mouse).

MATERIALS AND METHODS

PubMed database was searched for studies reporting on the spatial organization of the rodent epididymal vasculature and immune cell types at steady state. This search was combined with recent findings from our team.

RESULTS

At steady state, the rodent epididymis presents with dense blood and lymphatic networks, and a large panel of immune cells distributed across the interstitum and epithelium along the organ.

CONCLUSIONS

The immune system of the rodent epididymis is highly organized. Exploring its functions, especially in an infectious context, is the essential coming step before any transposition to human.