Testicular macrophages: development and function in health and disease

Testicular aging: mechanism, management and future therapy

Testicular aging results in degeneration in testicular function, including decreased testosterone production and quality of sperm. Decreased testosterone level is associated with a range of systemic diseases and comorbidities, including cardiovascular disease, cognitive decline, depression, osteoporosis, frailty, increased body fat, and metabolic syndrome. In addition, with the rapid development of industrialization and increasing environmental pollution, the quality of male semen continues to decline globally. Currently, the average age of first marriage and childbirth for men is delayed, and the birth rate has been declining year by year. At present, the therapies for testosterone level decline in clinical practice are relatively limited. Therefore, studying the triggering and delaying mechanisms of testicular aging is significant for improving male health and protecting male fertility. This review summarizes the mechanisms and treatment strategies for male reproductive aging.

INSL3 promotes macrophage polarization to an immunosuppressive phenotype via the cAMP downstream signaling pathway and Akt/mTOR pathway

Insulin-like peptide 3 (INSL3) is a small peptide hormone produced almost exclusively by testicular Leydig cells in males and thus serves as an essential biomarker of the maturation and functionality of these cells. Accumulated evidence suggests that INSL3 is a crucial factor affecting testicular descent during fetal development by regulating the growth of the gubernaculum. However, the physiological roles of INSL3 in adults remain unclear. Here, we reported that relaxin family peptide 2 (RXFP2), the receptor of INSL3, is expressed on macrophages, and treatment with INSL3 can promote M2 macrophage polarization via the Akt/mTOR/S6K and PKA/CREB pathways. In addition, INSL3 can inhibit macrophage phagocytosis and promote their migration via the Epac and PKA signaling pathways, respectively. These findings reveal a new role for INSL3 in regulating macrophage function and shed new light on our understanding of the role of INSL3 in adulthood.

Unwrapping the immunological alterations in testicular germ cell tumors: From immune homeostasis to malignancy and emerging immunotherapies

BACKGROUND

Testicular germ cell tumors (TGCTs), derived from primordial germ cells, are rare malignancies with high curative potential. However, the emergence of new evidence indicating that 15% of patients experience tumor progression, leading to death, underscores the need for innovative therapeutics.

OBJECTIVES

This review aimed to explore the immune status in maintaining testicular health and the immune-related aspects of malignancy. Furthermore, it presents an overview of current data on the use of immunotherapy for TGCT patients.

RESULTS AND DISCUSSION

Recent advances in immunology have opened a promising avenue for studying diseases and highlighted its role in treating diseases. While the immunopathological facets of TGCTs are not fully understood, investigations suggest a complex interplay among testis-resident immune cells, testis-specific cells (i.e., Sertoli cells (SCs) and Leydig cells (LCs)), and immune-regulating mediators (e.g., sex hormones) in the normal testicle that foster the testicular immune privilege (TIP). Although TIP plays a crucial role in sperm production, it also makes testis vulnerable to tumor development. In the context of cancer-related inflammation, disruption of TIP leads to an imbalanced immune response, resulting in chronic inflammation that can contribute to testicular tissue dysfunction or loss, potentially aiding in cancer invasion and progression.

CONCLUSION

Comparing the immune profiles of normal and malignant testes is valuable and may provide insights into different aspects of testicular immunity and immune-based treatment approaches. For patients resistant to chemotherapy and with a poor prognosis, immunotherapy has shown promising results. However, its effectiveness in treating resistant TGCTs or preventing tumor recurrence is still uncertain.

Low-Dose Ionizing Radiation and Male Reproductive Immunity: Elucidating Subtle Modulations and Long-Term Health Implications

Low-dose ionizing radiation (LDIR) is a prevalent environmental factor with profound impacts on male reproductive health, particularly on the testicular immune microenvironment. This review examines the multifaceted effects of LDIR, emphasizing its ability to induce genotoxic stress, oxidative damage, and epigenetic modifications in reproductive cells. These alterations compromise DNA repair, disrupt chromatin structure, and induce immune dysregulation. Immune cells such as macrophages, T cells, natural killer cells, and dendritic cells exhibit significant functional changes under LDIR exposure, destabilizing the immune privilege critical for normal spermatogenesis. The long-term health implications of LDIR include impaired sperm quality, reduced fertility, and transgenerational risks through heritable genomic instability. This review underscores the importance of exploring the mechanisms underlying immune dysregulation and developing effective protective strategies. While LDIR's full impact on male reproductive health remains to be elucidated, addressing the gaps in our understanding of immune microenvironmental changes is crucial for mitigating its adverse effects and improving reproductive health outcomes.

Galectin-1 and galectin-3 in male reproduction - impact in health and disease

The formation and differentiation of mature, motile male germ cells, which can fertilize the egg and ensure successful implantation and development of a healthy embryo, are essential functions of the testis and epididymis. Spermatogenesis is a complex, multistep process that results in the formation of motile haploid gametes, requiring an immunoregulatory environment to maintain tolerance to developing neo-antigens. Different cell types (Sertoli cells, macrophages), immunoregulatory factors and tolerance mechanisms are involved. In this context, possible effects of galectins on the immunoregulatory functions and fertilization ability of male germ cells are postulated. Galectins are pleiotropic lectins involved in the homeostasis, modulation of immune responses and pathological processes. Despite the well-recognized role of galectins in female reproduction, the functions of galectins in the male reproductive organs, particularly the testis and epididymis, remain largely unexplored. Among the galectins, galectin-1 and galectin-3 are the best-studied in these organs. This review summarizes the current knowledge of the cellular expression and the roles of galectin-1 and galectin-3 in testis and epididymis and discusses their functions in spermatogenesis, steroidogenesis, epididymal maturation of spermatozoa and inflammatory response.

Mouse testicular macrophages can independently produce testosterone and are regulated by Cebpb

BACKGROUND

Testicular macrophages (TM) have long been recognized for their role in immune response within the testicular environment. However, their involvement in steroid hormone synthesis, particularly testosterone, has not been fully elucidated. This study aims to explore the capability of TM to synthesize and secrete testosterone de novo and to investigate the regulatory mechanisms involved.

RESULTS

Transcriptomic analysis revealed significant expression of Cyp11a1, Cyp17a1, Hsd3b1, and Hsd17b3 in TM, which are key enzymes in the testosterone synthesis pathway. qPCR analysis and immunofluorescence validation confirmed the autonomous capability of TM to synthesize testosterone. Ablation of TM in mice resulted in decreased physiological testosterone levels, underscoring the significance of TM in maintaining testicular testosterone levels. Additionally, the study also demonstrated that Cebpb regulates the expression of these crucial genes, thereby modulating testosterone synthesis.

CONCLUSIONS

This research establishes that TM possess the autonomous capacity to synthesize and secrete testosterone, contributing significantly to testicular testosterone levels. The transcription factor Cebpb plays a crucial role in this process by regulating the expression of key genes involved in testosterone synthesis.

Molecular and epigenetic ex vivo profiling of testis cancer-associated fibroblasts and their interaction with germ cell tumor cells and macrophages

Germ cell tumors (GCT) are the most common solid tumors in young men of age 15 - 40. In previous studies, we profiled the interaction of GCT cells with cells of the tumor microenvironment (TM), which showed that especially the 3D interaction of fibroblasts (FB) or macrophages with GCT cells influenced the growth behavior and cisplatin response as well as the transcriptome and secretome of the tumor cells, suggesting that the crosstalk of these cells with GCT cells is crucial for tumor progression and therapy outcome. In this study, we shed light on the mechanisms of activation of cancer-associated fibroblasts (CAF) in the GCT setting and their effects on GCT cells lines and the monocyte cell line THP-1. Ex vivo cultures of GCT-derived CAF were established and characterized molecularly and epigenetically by performing DNA methylation arrays, RNA sequencing, and mass spectrometry-based secretome analysis. We demonstrated that the activation state of CAF is influenced by their former prevailing tumor environment in which they have resided. Hereby, we postulate that seminoma (SE) and embryonal carcinoma (EC) activate CAF, while teratoma (TER) play only a minor role in CAF formation. In turn, CAF influence proliferation and the expression of cisplatin sensitivity-related factors in GCT cells lines as well as polarization of in vitro-induced macrophages by the identified effector molecules IGFBP1, LGALS3BP, LYVE1, and PTX3. Our data suggests that the vital interaction of CAF with GCT cells and with macrophages has a huge influence on shaping the extracellular matrix as well as on recruitment of immune cells to the TM. In conclusion, therapeutically interfering with CAF and / or macrophages in addition to the standard therapy might slow-down progression of GCT and re-shaping of the TM to a tumor-promoting environment. Significance: The interaction of CAF with GCT and macrophages considerably influences the microenvironment. Thus, therapeutically interfering with CAF might slow-down progression of GCT and re-shaping of the microenvironment to a tumor-promoting environment.

Single-cell insights into mouse testicular toxicity under peripubertal exposure to di(2-ethylhexyl) phthalate

Male fertility depends on normal pubertal development. Di-(2-ethylhexyl) phthalate (DEHP) is a potent antiandrogen chemical, and exposure to DEHP during peripuberty can damage the developing male reproductive system, especially the testis. However, the specific cellular targets and differentiation processes affected by DEHP, which lead to testicular toxicity, remain poorly defined. Herein, we presented the first single-cell transcriptomic profile of the pubertal mouse testis following DEHP exposure. To carry out the experiment, 2 groups (n = 8 each) of 3-week-old male mice were orally administered 0.5% carboxymethylcellulose sodium salt or 100 mg/kg body weight DEHP daily from postnatal day 21-48, respectively. Using single-cell RNA sequencing, a total of 31 distinct cell populations were identified, notably, Sertoli and Leydig cells emerged as important targets of DEHP. DEHP exposure significantly decreased the proportions of Sertoli cell clusters expressing mature Sertoli markers (Sox9 and Ar), and selectively reduced the expression of testosterone synthesis genes in fetal Leydig cells. Through cell-cell interaction analyses, we observed changed numbers of interactions in Sertoli cells 1 (SCs1), Leydig cells 1 (LCs1), and interstitial macrophages, and we also identified cell-specific ligand gene expressions in these clusters, such as Inha, Fyn, Vcam1, and Apoe. Complementary in vitro assays confirmed that DEHP directly reduced the expression of genes related to Sertoli cell adhesion and intercellular communication. In conclusion, peripubertal DEHP exposure reduced the number of mature Sertoli cells and may disrupt testicular steroidogenesis by affecting the testosterone synthesis genes in fetal Leydig cells rather than adult Leydig cells.

Size-dependent deleterious effects of nano- and microplastics on sperm motility

INTRODUCTION

Growing concerns regarding the reproductive toxicity associated with daily life exposure to micro-/nano-plastics (abbreviated as MNPs) have become increasingly prevalent. In reality, MNPs exposure involves a heterogeneous mixture of MNPs of different sizes rather than a single size.

METHODS

In this study, an oral exposure mouse model was used to evaluate the effects of MNPs of four size ranges: 25-30 nm, 1-5 µm, 20-27 µm, and 125-150 µm. Adult male C57BL/6 J mice were administered environmentally relevant concentrations of 0.1 mg MNPs/day for 21 days. After that, open field test and computer assisted sperm assessment (CASA) were conducted. Immunohistochemical analyses of organ and cell type localization of MNPs were evaluated. Testicular transcriptome analysis was carried out to understand the molecular mechanisms.

RESULTS

Our result showed that MNPs of different size ranges all impaired sperm motility, with a decrease in progressive sperm motility, linearity and straight-line velocity of sperm movement. Alterations did not manifest in animal locomotion, body weight, or sperm count. Noteworthy effects were most pronounced in the smaller MNPs size ranges (25-30 nm and 1-5 µm). Linear regression analysis substantiated a negative correlation between the size of MNPs and sperm curvilinear activity. Immunohistochemical analysis unveiled the intrusions of 1-5 µm MNPs, but not 20-27 µm and 125-150 µm MNPs, into Leydig cells and testicular macrophages. Further testicular transcriptomic analysis revealed perturbations in pathways related to spermatogenesis, oxidative stress, and inflammation. Particularly within the 1-5 µm MNPs group, a heightened perturbation in pathways linked to spermatogenesis and oxidative stress was observed.

CONCLUSIONS

Our data support the size-dependent impairment of MNPs on sperm functionality, underscoring the pressing need for apprehensions about and interventions against the escalation of environmental micro-/nano-plastics contamination. This urgency is especially pertinent to small-sized MNPs.

T cells in testicular germ cell tumors: new evidence of fundamental contributions by rare subsets

BACKGROUND

Immune cell infiltration is heterogeneous but common in testicular germ cell tumors (TGCT) and pre-invasive germ cell neoplasia in situ (GCNIS). Tumor-infiltrating T cells including regulatory T (Treg) and follicular helper T (Tfh) cells are found in other cancer entities, but their contributions to TGCT are unknown.

METHODS

Human testis specimens from independent patient cohorts were analyzed using immunohistochemistry, flow cytometry and single-cell RNA sequencing (scRNA-seq) with special emphasis on delineating T cell subtypes.

RESULTS

Profound changes in immune cell composition within TGCT, shifting from macrophages in normal testes to T cells plus B and dendritic cells in TGCT, were documented. In most samples (96%), the CD4+ T cell frequency exceeded that of CD8+ cells, with decreasing numbers from central to peripheral tumor areas, and to tumor-free, contralateral testes. T cells including Treg and Tfh were most abundant in seminoma compared to mixed tumors and embryonal carcinoma.

CONCLUSION

Despite considerable heterogeneity between patients, T cell subtypes form a key part of the TGCT microenvironment. The novel finding of rare Treg and Tfh cells in human testis suggests their involvement in TGCT pathobiology, with implications for understanding tumor progression, to assess patients' prognosis, and as putative targets for personalized immunotherapy.

Microplastics from face mask impairs sperm motility

The COVID-19 pandemic has resulted in unprecedented plastic pollution from single-used personal protective equipment (PPE), especially face masks, in coastal and marine environments. The secondary pollutants, microplastics from face masks (mask MP), rise concern about their detrimental effects on marine organisms, terrestrial organisms and even human. Using a mouse model, oral exposure to mask MP at two doses, 0.1 and 1 mg MP/day for 21 days, caused no change in animal locomotion, total weight, or sperm counts, but caused damage to sperm motility with increased curvilinear velocity (VCL). The high-dose mask MP exposure caused a significant decrease in linearity (LIN) of sperm motility. Further testicular transcriptomic analysis revealed perturbed pathways related to spermatogenesis, oxidative stress, inflammation, metabolism and energy production. Collectively, our findings substantiate that microplastics from face masks yield adverse effects on mammalian reproductive capacity, highlighting the need for improved plastic waste management and development of environmentally friendly materials.

Stem Leydig cells support macrophage immunological homeostasis through mitochondrial transfer in mice

As testicular mesenchymal stromal cells, stem Leydig cells (SLCs) show great promise in the treatment of male hypogonadism. The therapeutic functions of mesenchymal stromal cells are largely determined by their reciprocal regulation by immune responses. However, the immunoregulatory properties of SLCs remain unclear. Here, we observe that SLCs transplantation restore male fertility and testosterone production in an ischemia‒reperfusion injury mouse model. SLCs prevent inflammatory cascades through mitochondrial transfer to macrophages. Reactive oxygen species (ROS) released from activated macrophages inducing mitochondrial transfer from SLCs to macrophages in a transient receptor potential cation channel subfamily member 7 (TRPM7)-mediated manner. Notably, knockdown of TRPM7 in transplanted SLCs compromised therapeutic outcomes in both testicular ischemia‒reperfusion and testicular aging mouse models. These findings reveal a new mechanism of SLCs transplantation that may contribute to preserve testis function in male patients with hypogonadism related to immune disorders.

Exploration of Lactiplantibacillus plantarum P101 ameliorated the alcohol-induced testicular dysfunction based on metabolome analysis

The decline in male sperm quality caused by multiple factors has become a widespread concern. Alcohol excessive consumption is one of the factors that induce testicular dysfunction. Testicular dysfunction caused by alcohol abuse is related to oxidative stress and inflammation. Probiotics can ameliorate alcohol-induced testicular dysfunction. However, the specific mechanism is not explicit. This study aimed to elucidate the underlying mechanism by which Lactiplantibacillus plantarum P101 ameliorates the alcohol-induced testicular dysfunction. The model of alcohol-induced testicular dysfunction in C57B/6 male mice was established according to the National Institute on Alcohol Abuse and Alcoholism, and Lactiplantibacillus plantarum P101 supplementation was orally administered to mice during the experiment. The results showed that Lactiplantibacillus plantarum P101 promoted androgen production, reduced testis inflammation, and improved testis antioxidant capacity, thereby improving sperm quality and sperm motility and ultimately ameliorating alcohol-induced testicular disorder. Three key metabolite pathways and six key metabolites were identified by metabolome analysis.

High-fructose consumption suppresses insulin signaling pathway accompanied by activation of macrophage and apoptotic markers in rat testis

Dietary high-fructose may cause metabolic disturbances; however, its effect on the reproductive system is little understood. The insulin signaling pathway is critical in testicular development, maintenance of microcirculation and spermatogenesis. Therefore, in this study, we aimed to investigate the impact of dietary high-fructose on insulin signaling pathway as well as macrophage and apoptotic markers in testicular tissue of rats. Fructose was administered to male Wistar rats as a 20% solution in drinking water for fifteen-week. Gene expression of ir-β, irs-1, irs-2, pi3k, akt, mtor, and enos in the testicular samples was determined by real-time PCR. Protein expression of IR, IRS-1, IRS-2, PI3K, Akt, phospho-Akt (p-Akt), mTOR, eNOS, phospho-eNOS (p-eNOS), and GLUT5 was established by analysis of Western Blot. Testicular expression of occludin, CD163, CD68, caspase-8, and caspase-3 was analyzed by using immunohistochemical assay. Testicular level of fructose was measured by colorimetric method. Dietary high-fructose decreased mRNA expressions of irs-1, irs-2, pi3k, and mtor in the testicular tissue of rats. Also, this dietary intervention impaired protein expressions of IR, IRS-1, IRS-2, PI3K, p-Akt, mTOR, eNOS, and p-eNOS as well as p-Akt/Akt and p-eNOS/eNOS ratios in the testis of rats. However, a high-fructose diet increased the expression of CD163, CD68, caspase-8 and caspase-3, but decreased that of occludin, in the testicular tissue of rats. The high-fructose consumption in rats suppresses testicular insulin signaling but activates macrophages-related factors and apoptotic markers. These changes induced by dietary fructose could be related to male reproductive dysfunction.

Novel treatment and insight for irradiation-induced injuries: Dibucaine ameliorates irradiation-induced testicular injury by inhibiting fatty acid oxidation in primary Leydig cells

BACKGROUND

Male infertility is a worldwide problem but few treatments, especially irradiation-induced testicular injury. The aim of this research was to investigate novel drugs for the treatment of irradiation-induced testicular injury.

METHODS

We administered dibucaine (0.8 mg/kg) intraperitoneally to male mice (6 mice per group) after five consecutive daily 0.5 Gy whole-body irradiation, and evaluated its ameliorating efficacy by testicular HE staining and morphological measurements. Drug affinity responsive target stability assay (Darts) were used to find target protein and pathway; mouse primary Leydig cells were isolated and to explore the mechanism (Flow cytometry, Western blot, and Seahorse palmitate oxidative stress assays); finally rescue experiments were completed by combining dibucaine with fatty acid oxidative pathway inhibitors and activators.

RESULTS

The testicular HE staining and morphological measurements in dibucaine treatment group was significantly better than that in irradiation group (P < 0.05); sperm motility and mRNA levels of spermatogenic cell markers were also higher than those in the latter (P < 0.05). Darts and Western blot results showed that dibucaine targets CPT1A and downregulate fatty acid oxidation. Flow cytometry, Western blot, and Palmitate oxidative stress assays of primary Leydig cells demonstrated that dibucaine inhibits fatty acid oxidation in Leydig cells. Dibucaine combined with etomoxir/baicalin confirmed that its inhibition of fatty acid oxidation was beneficial in ameliorating irradiation-induced testicular injury.

CONCLUSIONS

In conclusion, our data suggest that dibucaine ameliorates irradiation-induced testicular injury in mice by inhibiting fatty acid oxidation in Leydig cells. This will provide novel ideas for the treatment of irradiation-induced testicular injury.

Activin A and CCR2 regulate macrophage function in testicular fibrosis caused by experimental autoimmune orchitis

Experimental autoimmune-orchitis (EAO), a rodent model of chronic testicular inflammation and fibrosis, replicates pathogenic changes seen in some cases of human spermatogenic disturbances. During EAO, increased levels of pro-inflammatory and pro-fibrotic mediators such as TNF, CCL2, and activin A are accompanied by infiltration of leukocytes into the testicular parenchyma. Activin A levels correlate with EAO severity, while elevated CCL2 acting through its receptor CCR2 mediates leukocyte trafficking and recruits macrophages. CCR2 + CXCR4 + macrophages producing extracellular matrix proteins contribute widely to fibrogenesis. Furthermore, testicular macrophages (TMs) play a critical role in organ homeostasis. Therefore, we aimed to investigate the role of the activin A/CCL2-CCR2/macrophage axis in the development of testicular fibrosis. Following EAO induction, we observed lower levels of organ damage, collagen deposition, and leukocyte infiltration (including fibronectin+, collagen I+ and CXCR4+ TMs) in Ccr2-/- mice than in WT mice. Furthermore, levels of Il-10, Ccl2, and the activin A subunit Inhba mRNAs were lower in Ccr2-/- EAO testes. Notably, fibronectin+ TMs were also present in biopsies from patients with impaired spermatogenesis and fibrotic alterations. Overexpression of the activin A antagonist follistatin reduced tissue damage and collagen I+ TM accumulation in WT EAO testes, while treating macrophages with activin A in vitro increased the expression of Ccr2, Fn1, Cxcr4, and Mmp2 and enhanced migration along a CCL2 gradient; these effects were abolished by follistatin. Taken together, our data indicate that CCR2 and activin A promote fibrosis during testicular inflammation by regulating macrophage function. Inhibition of CCR2 or activin A protects against damage progression, offering a promising avenue for therapeutic intervention.

Viral tropism for the testis and sexual transmission

The mammalian testis adopts an immune privileged environment to protect male germ cells from adverse autoimmune reaction. The testicular immune privileged status can be also hijacked by various microbial pathogens as a sanctuary to escape systemic immune surveillance. In particular, several viruses have a tropism for the testis. To overcome the immune privileged status and mount an effective local defense against invading viruses, testicular cells are well equipped with innate antiviral machinery. However, several viruses may persist an elongated duration in the testis and disrupt the local immune homeostasis, thereby impairing testicular functions and male fertility. Moreover, the viruses in the testis, as well as other organs of the male reproductive system, can shed to the semen, thus allowing sexual transmission to partners. Viral infection in the testis, which can impair male fertility and lead to sexual transmission, is a serious concern in research on known and on new emerging viruses. To provide references for our scientific peers, this article reviews research achievements and suggests future research focuses in the field.

Testicular Immunity and Its Connection with the Microbiota. Physiological and Clinical Implications in the Light of Personalized Medicine

Reproduction is a complex process, which is based on the cooperation between the endocrine-immune system and the microbiota. Testicular immunity is characterized by the so-called immune privilege, a mechanism that avoids autoimmune attacks against proteins expressed by spermatozoa. Testicular microbiota is connected with the gut microbiota, the most prevalent site of commensals inthe body. Both microbiotas take part inthe development of the immune system and protection againstpathogen invasion. Dysbiosis is caused by concurrent pathologies, such as obesity, diabetes, infections and trauma. The substitution of beneficial bacteria with pathogens may lead to destruction of spermatozoa directly or indirectly and, ultimately, to male infertility. Novel therapeutic interventions, i.e., nutritional interventions and supplementation of natural products, such as, probiotics, prebiotics, antioxidants and polyphenols, may lead to the restoration of the otherwise-impaired male reproductive potential, even if experimental and clinical results are not always concordant. In this review, the structure and immune function of the testis will be described with special reference to the blood-testisbarrier. The regulatory role of both the gut and testicular microbiota will be illustrated in health and disease, also emphasizing therapeutic attempts with natural products for the correction of male infertility, in the era of personalized medicine.

Reproductive immune microenvironment

About 10 %-12 % of couples in the world suffer from infertility, and immunological factors are being paid more and more attention. Attempts to induce peripheral immune tolerance in pregnant women by injecting husband cells have been widely promoted, but ultimately proved unsuccessful. Over the past two decades, our understanding of how the immune system is involved in gametogenesis and embryonic development, especially in early pregnancy, has undergone a major shift, going from the periphery to the local area of reproductive tissue. However, a holistic overview of immune responses in reproductive organs and tissues is currently lacking. Here, we further highlight the importance of regional immunity research for understanding reproductive health by reviewing the research mileage of the testis, ovary, and uterine immune microenvironment. We propose the concept of "reproductive immune microenvironment (RIM)" by summarizing the common features and basic functions of the tissue microenvironment in which immune cells reside, including the interstitial space of the testis, the ovarian stroma and the endometrium. The establishment of the concept of RIM not only focuses on the comprehensive description of the immune response in reproductive tissues, but also provides a macroscopic perspective for a deeper understanding of the immune etiology of reproductive system-related diseases.

Immune Cells as Critical Regulators of Steroidogenesis in the Testis and Beyond

Steroidogenesis is an essential biological process for embryonic development, reproduction, and adult health. While specific glandular cells, such as Leydig cells in the testis, are traditionally known to be the principal players in steroid hormone production, there are other cell types that contribute to the process of steroidogenesis. In particular, immune cells are often an important component of the cellular niche that is required for the production of steroid hormones. For several decades, studies have reported that testicular macrophages and Leydig cells are intimately associated and exhibit a dependency on the other cell type for their proper development; however, the mechanisms that underlie the functional relationship between macrophages and Leydig cells are unclear. Beyond the testis, in certain instances immune cells themselves, such as certain types of lymphocytes, are capable of steroid hormone production, thus highlighting the complexity and diversity that underlie steroidogenesis. In this review we will describe how immune cells are critical regulators of steroidogenesis in the testis and in extra-glandular locations, as well as discuss how this area of research offers opportunities to uncover new insights into steroid hormone production.