Bioactivity of Androgens Within the Testes and Serum of Normal Men

Calcium and vitamin D homoeostasis in male fertility

Calcium and vitamin D have well-established roles in maintaining calcium balance and bone health. Decades of research in human subjects and animals have revealed that calcium and vitamin D also have effects on many other organs including male reproductive organs. The presence of calcium-sensing receptor, vitamin D receptor, vitamin D activating and inactivating enzymes and calcium channels in the testes, male reproductive tract and human spermatozoa suggests that vitamin D and calcium may modify male reproductive function. Functional animal models have shown that vitamin D deficiency in male rodents leads to a decrease in successful mating and fewer pregnancies, often caused by impaired sperm motility and poor sperm morphology. Human studies have to a lesser extent validated these findings; however, newer studies suggest a positive effect of vitamin D supplementation on semen quality in cases with vitamin D deficiency, which highlights the need for initiatives to prevent vitamin D deficiency. Calcium channels in male reproductive organs and spermatozoa contribute to the regulation of sperm motility and capacitation, both essential for successful fertilisation, which supports a need to avoid calcium deficiency. Studies have demonstrated that vitamin D, as a regulator of calcium homoeostasis, influences calcium influx in the testis and spermatozoa. Emerging evidence suggests a potential link between vitamin D deficiency and male infertility, although further investigation is needed to establish a definitive causal relationship. Understanding the interplay between vitamin D, calcium and male reproductive health may open new avenues for improving fertility outcomes in men.

Male contraception development: monitoring effective spermatogenesis suppression utilizing a user-controlled sperm concentration test compared with standard semen analysis

OBJECTIVE

To determine whether a user-controlled sperm concentration test compared with standard semen analysis can effectively monitor spermatogenesis suppression for male contraception.

DESIGN

Single center, prospective sub study of the ongoing clinical trial: "Study of daily application of Nestorone and testosterone combination gel for male contraception."

SETTING

Research institute at an academic medical center.

PARTICIPANT(S)

Couples participating in the male contraceptive clinical trial.

INTERVENTIONS

None.

MAIN OUTCOME MEASURE(S)

The ability by participants to monitor sperm suppression to a threshold compatible with contraceptive efficacy utilizing a user-controlled test verified by sperm concentration determined by standard laboratory methods.

RESULT(S)

Thirty-eight men participating in a hormonal male contraceptive clinical trial provided multiple samples during spermatogenesis suppression for this substudy. Participants, employing a user-controlled test, correctly identified the absence of sperm (a negative test) in 100% of their laboratory-confirmed azoospermic samples (n = 122). Participants also identified 100% of samples (n = 73) with sperm >0.2 million/mL as positive. Sperm counts between 0.01 and 0.2 million/mL were identified as negative in 96% of samples. Trial participants noted the overall ease of using the test with respect to sample preparation, test timing, and result interpretation, and that they could accurately use this test at home without difficulty.

CONCLUSION(S)

Participants undergoing spermatogenesis suppression in a hormonal male contraceptive trial performed user-controlled test to determine whether their semen sperm concentration was ≤ or >0.2 million/mL. Compared with standard semen analyses, participants correctly identified 100% of samples with sperm counts >0.2 million/mL as positive (Sensitivity 100%). A positive result when the couple is using a male contraceptive method triggers the need for semen analysis by a laboratory while the couple uses another method of contraception. Participants correctly diagnosed samples ≤0.2 million sperm/mL as negative in 99% of samples (specificity 99%). A negative result indicates a sperm concentration ≤0.2 million/mL, well below the threshold of ≤1 million/mL offering contraceptive efficacy demonstrated by prior studies. At-home sperm concentration test would minimize the need for users to return to the clinic to monitor suppression of spermatogenesis, decreasing cost and burden of male contraception trials and increasing practicality of the method.

CLINICAL TRIAL REGISTRATION NUMBER

NCT: 03452111.

Effects of pharmacologically induced Leydig cell testosterone production on intratesticular testosterone and spermatogenesis†

The Leydig cells of the mammalian testis produce testosterone (T) in response to luteinizing hormone (LH). In rats and men with reduced serum T levels, T replacement therapy (TRT) will raise T levels, but typically with suppressive effects on sperm formation. The rate-determining step in T formation is the translocation of cholesterol to the inner mitochondrial membrane, mediated by protein-protein interactions of cytosolic and outer mitochondrial membrane proteins. Among the involved proteins is cholesterol-binding translocator protein (TSPO) (18 kDa TSPO). We hypothesized that in contrast to TRT, the administration of the TSPO agonist N,N-dihexyl-2-(4-fluorophenyl)indole-3-acetamide (FGIN-1-27), by stimulating the ability of the Leydig cells to produce T, would result in the elevation of serum T levels while maintaining intratesticular T concentration and therefore without suppression of spermatogenesis. Age-related reductions in both serum and intratesticular T levels were seen in old Brown Norway rats. Both exogenous T and FGIN-1-27 increased serum T levels. With exogenous T, serum LH and Leydig cell T formation were suppressed, and intratesticular T was reduced to below the concentration required to maintain spermatogenesis quantitatively. In contrast, FGIN-1-27 stimulated Leydig cell T formation, resulting in increased serum T without reductions in intratesticular T concentrations or in testicular sperm numbers. FGIN-1-27 also significantly increased serum and intratesticular T levels in rats made LH-deficient by treatment with the gonadotropin-releasing hormone antagonist cetrorelix. These results point to a possible approach to increasing serum T without negative effects on spermatogenesis, based upon stimulating T production by the Leydig cells themselves rather than administering T exogenously.

Determination of Intraprostatic and Intratesticular Androgens

Androgens represent the main hormones responsible for maintaining hormonal balance and function in the prostate and testis. As they are involved in prostate and testicular carcinogenesis, more detailed information of their active concentration at the site of action is required. Since the introduction of the term intracrinology as the local formation of active steroid hormones from inactive precursors of the adrenal gland, mainly dehydroepiandrosterone (DHEA) and DHEA-S, it is evident that blood circulating levels of sex steroid hormones need not reflect their actual concentrations in the tissue. Here, we review and critically evaluate available methods for the analysis of human intraprostatic and intratesticular steroid concentrations. Since analytical approaches have much in common in both tissues, we discuss them together. Preanalytical steps, including various techniques for separation of the analytes, are compared, followed by the end-point measurement. Advantages and disadvantages of chromatography-mass spectrometry (LC-MS, GC-MS), immunoanalytical methods (IA), and hybrid (LC-IA) are discussed. Finally, the clinical information value of the determined steroid hormones is evaluated concerning differentiating between patients with cancer or benign hyperplasia and between patients with different degrees of infertility. Adrenal-derived 11-oxygenated androgens are mentioned as perspective prognostic markers for these purposes.

Perfluoroalkyl substances cause Leydig cell dysfunction as endocrine disruptors

Perfluoroalkyl substances (PFASs) are a group of man-made organic substances. Some of PFASs have been classified as persistent organic pollutants and endocrine disruptors. They might interfere with the male sex endocrine system, causing the abnormal development of the male reproductive tract and failure of pubertal onset and infertility. The present review discusses the development and function of two generations of Leydig cells in rodents and the effects of PFASs on Leydig cell development after their exposure in gestational and postnatal periods. We also discuss human epidemiological data for the effects of PFASs on male sex hormone levels. The structure-activity relationship of PFASs on Leydig cell steroidogenesis and enzyme activities are also discussed.

Possible influence of vitamin D on male reproduction

Vitamin D is a versatile signaling molecule with an established role in the regulation of calcium homeostasis and bone health. In recent years the spectrum of vitamin D target organs has expanded and a reproductive role is supported by the presence of the vitamin D receptor (VDR) and the vitamin D metabolizing enzymes in the gonads, reproductive tract, and human spermatozoa. Interestingly, expression levels of VDR and the vitamin D inactivating enzyme CYP24A1 in human spermatozoa serve as positive predictive markers of semen quality and are higher expressed in spermatozoa from normal than infertile men. VDR mediates a non-genomic increase in intracellular calcium concentration, sperm motility, and induces the acrosome reaction. Furthermore, functional animal model studies have shown that vitamin D is important for sex steroid production, estrogen signaling, and semen quality. Cross-sectional clinical studies have supported the notion of a positive association between serum 25-hydroxyvitamin D (25-OHD) level and semen quality in both fertile and infertile men. However, it remains to be determined whether this association reflects a causal effect. The VDR is ubiquitously expressed and activated vitamin D is a regulator of insulin, aromatase, and osteocalcin. Hence, it is plausible that the influence of vitamin D on gonadal function may be mediated indirectly through other vitamin D regulated endocrine factors. Recent studies have indicated that vitamin D supplementation may be beneficial for couples in need of assisted reproductive techniques as high serum vitamin D levels were found to be associated with a higher chance of achieving pregnancy. Randomized clinical trials are needed to determine whether systemic changes in vitamin D metabolites can influence semen quality, fertility, and sex steroid production in infertile men. In this review known and possible future implications of vitamin D in human male reproduction function will be discussed.

Concepts in diagnosis and therapy for male reproductive impairment

An accurate medical history and directed physical examination are essential in diagnosis of male infertility. We review the hormonal assessments and specific genetic analyses that are useful additional tests, and detail other evidence-based examinations that are available to help guide therapeutic strategies. By contrast with female infertility treatments-especially hormonal manipulations to stimulate or enhance oocyte production-spermatogenesis and sperm quality abnormalities are much more difficult to affect positively. In general, a healthy lifestyle can improve sperm quality. A few men have conditions in which evidence-based therapies can increase their chances for natural conception. In this second of two papers in The Lancet Diabetes and Endocrinology Series on male reproductive impairment, we examine the agreements and controversies that surround several of these conditions. When we are not able to cure, correct, or mitigate the cause of conditions such as severe oligozoospermia, non-remedial ductal obstruction, and absence of sperm fertilising ability, assisted reproductive technologies, such as in-vitro fertilisation (IVF) with intracytoplasmic sperm injection (ICSI), can be used as an adjunctive measure to allow for biological paternity. Not considered possible just two decades ago, azoospermia due to testicular failure, including 47,XXY (Klinefelter syndrome), is now treatable in approximately 50% of cases when combining surgical harvesting of testicular sperm and ICSI. Although genetic fatherhood is now possible for many men previously considered sterile, it is crucial to discover and abrogate causes as best possible, provide reliable and evidenced-based therapy, consider seriously the health and wellness of any offspring conceived, and always view infertility as a possible symptom of a more general or constitutional disease.

Spermatogenesis in humans and its affecting factors

Spermatogenesis is an extraordinary complex process. The differentiation of spermatogonia into spermatozoa requires the participation of several cell types, hormones, paracrine factors, genes and epigenetic regulators. Recent researches in animals and humans have furthered our understanding of the male gamete differentiation, and led to clinical tools for the better management of male infertility. There is still much to be learned about this intricate process. In this review, the critical steps of human spermatogenesis are discussed together with its main affecting factors.

The regulation of spermatogenesis by androgens

Testosterone is essential for maintaining spermatogenesis and male fertility. However, the molecular mechanisms by which testosterone acts have not begun to be revealed until recently. With the advances obtained from the use of transgenic mice lacking or overexpressing the androgen receptor, the cell specific targets of testosterone action as well as the genes and signaling pathways that are regulated by testosterone are being identified. In this review, the critical steps of spermatogenesis that are regulated by testosterone are discussed as well as the intracellular signaling pathways by which testosterone acts. We also review the functional information that has been obtained from the knock out of the androgen receptor from specific cell types in the testis and the genes found to be regulated after altering testosterone levels or androgen receptor expression.

The immune privilege of testis and gravid uterus: same difference?

The fetus in the gravid uterus and the developing spermatogenic cells in the adult testis both comprise special challenges for the host immune system. Protection of the neoantigens of the fetus and male germ cells from immune attack, defined as immune privilege, is fundamental for the propagation of species. Immune privilege is not simply the absence of leukocytes, but involves immune and non-immune cells acting synergistically together at multiple levels to create a unique tolerogenic environment. A number of the pathways are shared by the testis and gravid uterus. Amongst them steroid hormones, namely testosterone in the male and progesterone in the female, seem to function as key molecules that govern the local production of immunoregulatory factors which finally control the overall immune environment.

Structural, cellular and molecular aspects of immune privilege in the testis

The testis presents a special immunological environment, considering its property of immune privilege that tolerates allo- and auto-antigens. Testicular immune privilege was once believed to be mainly based on the sequestration of antigens from the immune system by the blood–testis barrier in the seminiferous epithelium. Substantial evidence supports the view that the combination of physical structure, testicular cells, and cytokines controls immune responses in the testis to preserve the structural and functional integrity of testicular immune privilege. Both systemic immune tolerance and local immunosuppression help maintain the immune privilege status. Constitutive expression of anti-inflammatory factors in testicular cells is critical for local immunosuppression. However, the testis locally generates an efficient innate immune system against pathogens. Disruption of these mechanisms may lead to orchitis and impair fertility. This review article highlights the current understanding of structural, cellular, and molecular mechanisms underlying the unique immune environment of the testis, particularly its immune privilege status.

Circulating antimüllerian hormone levels in boys decline during early puberty and correlate with inhibin B

OBJECTIVE

To investigate peripheral levels of inhibin B and antimüllerian hormone (AMH) in boys during peripuberty and in patients with congenital hypogonadotropic hypogonadism (HH).

DESIGN

Randomized, placebo-controlled trial (peripubertal boys); and cross-sectional clinical study (males with HH).

SETTING

University central hospital.

PATIENT(S)

Twenty-eight peripubertal boys with idiopathic short stature (ISS), 19 males with Kallmann syndrome.

INTERVENTION(S)

Letrozole (2.5 mg/day) or placebo in boys with ISS for 2 years.

MAIN OUTCOME MEASURE(S)

Longitudinal follow-up observation of serum AMH and its relationship with inhibin B during early puberty and the influence of high (letrozole-treated boys) and low (males with HH) gonadotropin exposure on circulating AMH.

RESULT(S)

In boys with ISS receiving placebo, the decrease in AMH levels and the increase in inhibin B levels were correlated. The serum AMH level had already declined before a clinically significant increase in testis volume or serum testosterone occurred. Letrozole did not appear to modulate the decline in AMH. The AMH levels were lower in boys and young adults with Kallmann syndrome and prepubertal testes (mean: 20.9 ± 4.7 ng/mL, n = 6) as compared with prepubertal ISS boys (102.3 ± 11.9 ng/mL).

CONCLUSION(S)

The gonadotropin-mediated early pubertal increase in inhibin B is tightly coupled to decrease in AMH levels and may reflect androgen-mediated differentiation of Sertoli cells. Profound gonadotropin deficiency is associated with low AMH levels, suggesting impaired development of the Sertoli cell population.

Non-classical actions of testosterone and spermatogenesis

Testosterone is essential to maintain spermatogenesis and male fertility. In the absence of testosterone stimulation, spermatogenesis does not proceed beyond the meiosis stage. After withdrawal of testosterone, germ cells that have progressed beyond meiosis detach from supporting Sertoli cells and die, whereas mature sperm cannot be released from Sertoli cells resulting in infertility. The classical mechanism of testosterone action in which testosterone activates gene transcription by causing the androgen receptor to translocate to and bind specific DNA regulatory elements does not appear to fully explain testosterone regulation of spermatogenesis. This review discusses two non-classical testosterone signalling pathways in Sertoli cells and their potential effects on spermatogenesis. Specifically, testosterone-mediated activation of phospholipase C and calcium influx into Sertoli cells is described. Also, testosterone activation of Src, EGF receptor and ERK kinases as well as the activation of the CREB transcription factor and CREB-mediated transcription is reviewed. Regulation of germ cell adhesion to Sertoli cells and release of mature sperm from Sertoli cells by kinases regulated by the non-classical testosterone pathway is discussed. The evidence accumulated suggests that classical and non-classical testosterone signalling contribute to the maintenance of spermatogenesis and male fertility.

Safety assessment of Syzygium aromaticum flower bud (clove) extract with respect to testicular function in mice

The flower buds of Syzygium aromaticum (clove), a common food flavor, have been used as indigenous medicine for the treatment of male sexual disorders in Asian countries. However, the possible mechanism(s) by which it acts at testicular level remain obscure. Therefore, to investigate its effect on testicular function, chronic oral exposure of hexane extract of flower buds of Syzygium aromaticum in three doses (15 mg, 30 mg, and 60 mg/kg BW) were evaluated for a single spermatogenic cycle (35 days) in Parkes (P) strain mice. The treatment did not induce systemic toxicity at the doses tested. Lower dose (15 mg) of the extract increased the activities of Delta(5) 3 beta-HSD and 17 beta-HSD, and serum level of testosterone. The higher doses (30 and 60 mg) of extract inhibited these parameters and induced non-uniform degenerative changes in the seminiferous tubules associated with decrease in daily sperm production and depletion of 1C (round and elongated spermatids) population. Taken together these results suggest biphasic action of hexane extract of Syzygium aromaticum flower bud on testicular function, thereby advocating a cautious use of the flower bud as an aphrodisiac in indigenous systems of medicine in Asian countries.

Androgens transduce the G alphas-mediated activation of protein kinase A in prostate cells

Androgens regulate the development and function of male reproductive organs and play a crucial role in the onset and progression of prostate cancer. Androgen action is primarily mediated through the nuclear androgen receptor (AR) which acts as a ligand-dependent transcription factor. This mode of androgen action takes hours to manifest and is called the genomic pathway. The androgen-mediated genomic responses require activity of cyclic AMP (cAMP)-dependent protein kinase (PKA). Androgens also act through nongenomic pathways in certain cell types to evoke rapid responses (manifested in minutes) that are mediated through changes in ion currents and second messengers. Here, we show that androgen causes the rapid and cAMP-dependent activation of PKA in prostate cells. The androgen-induced PKA activation is not inhibited by nuclear AR antagonist bicalutamide and can be observed in cells that do not express nuclear AR gene. Reduction of G alphas expression with siRNA attenuates the androgen-mediated activation of PKA, which is required for the androgen-induced prostate cell proliferation. We conclude that androgen actively evokes a nongenomic signaling pathway to activate PKA that is needed for the genomic functioning of nuclear AR. The inhibition of PKA activation, together with standard AR-targeted therapies, may be more efficacious for treatment of patients with prostate cancer.

The androgen receptor can signal through Wnt/beta-Catenin in prostate cancer cells as an adaptation mechanism to castration levels of androgens

Background

A crucial event in Prostate Cancer progression is the conversion from a hormone-sensitive to a hormone-refractory disease state. Correlating with this transition, androgen receptor (AR) amplification and mutations are often observed in patients failing hormonal ablation therapies. β-Catenin, an essential component of the canonical Wnt signaling pathway, was shown to be a coactivator of the AR signaling in the presence of androgens. However, it is not yet clear what effect the increased levels of the AR could have on the Wnt signaling pathway in these hormone-refractory prostate cells.

Results

Transient transfections of several human prostate cancer cell lines with the AR and multiple components of the Wnt signaling pathway demonstrate that the AR overexpression can potentiate the transcriptional activities of Wnt/β-Catenin signaling. In addition, the simultaneous activation of the Wnt signaling pathway and overexpression of the AR promote prostate cancer cell growth and transformation at castration levels of androgens. Interestingly, the presence of physiological levels of androgen or other AR agonists inhibits these effects. These observations are consistent with the nuclear co-localization of the AR and β-Catenin shown by immunohistochemistry in human prostate cancer samples. Furthermore, chromatin immunoprecipitation assays showed that Wnt3A can recruit the AR to the promoter regions of Myc and Cyclin D1, which are well-characterized downstream targets of the Wnt signalling pathway. The same assays demonstrated that the AR and β-Catenin can be recruited to the promoter and enhancer regions of a known AR target gene PSA upon Wnt signaling. These results suggest that the AR is promoting Wnt signaling at the chromatin level.

Conclusion

Our findings suggest that the AR signaling through the Wnt/β-Catenin pathway should be added to the well established functional interactions between both pathways. Moreover, our data show that via this interaction the AR could promote prostate cell malignancy in a ligand-independent manner.

Transcripts of testicular gonadotropin-releasing hormone, steroidogenic enzymes, and intratesticular testosterone levels in infertile men

OBJECTIVE

To investigate the expressions of testicular GnRH and steroidogenic enzymes and their correlations with intratesticular T levels (ITT) and serum hormonal parameters in infertile men.

DESIGN

Prospective case study.

SETTING

University reproductive laboratory and clinics.

PATIENT(S)

Thirty-four azoospermic men.

INTERVENTION(S)

The mRNA transcript levels of GnRH-I, GnRH-II, GnRH-R, and five steroidogenic enzymes in the testes of azoospermic men were determined by quantitative real time polymerase chain reaction. The ITT level was determined by radioimmunoassay.

MAIN OUTCOME MEASURE(S)

Transcript levels of genes and ITT determination.

RESULT(S)

The mRNA transcript levels of GnRH-I, GnRH-II, GnRH-R, cytochrome P450 side-chain cleavage (CYP11A1), and 3beta-hydroxy-steroid dehydrogenase type 2 enzyme (HSD3B2) as well as the ITT levels were significantly increased in patients with spermatogenic failure, especially in men with Sertoli cell-only syndrome. GnRH-I and -II mRNA transcript levels positively correlated with HSD3B2 mRNA transcript levels, ITT levels, and serum FSH levels.

CONCLUSION(S)

Increased testicular GnRH transcripts, steroidogenic enzyme transcripts, and ITT levels are associated with spermatogenic failure in infertile men. Testicular GnRH is involved in the regulation of human spermatogenesis and steroidogenesis.

Junction restructuring and spermatogenesis: the biology, regulation, and implication in male contraceptive development

Spermatogenesis that occurs in the seminiferous epithelium of adult mammalian testes is associated with extensive junction restructuring at the Sertoli-Sertoli cell, Sertoli-germ cell, and Sertoli-basement membrane interface. While this morphological phenomenon is known and has been described in great details for decades, the biochemical and molecular changes as well as the mechanisms/signaling pathways that define changes at the cell-cell and cell-matrix interface remain largely unknown until recently. In this chapter, we summarize and discuss findings in the field regarding the coordinated efforts of the anchoring [e.g., adherens junction (AJ), such as basal ectoplasmic specialization (basal ES)] and tight junctions (TJs) that are present in the same microenvironment, such as at the blood-testis barrier (BTB), or at distinctly opposite ends of the Sertoli cell epithelium, such as between apical ectoplasmic specialization (apical ES) in the apical compartment, and the BTB adjacent to the basal compartment of the epithelium. These efforts, in turn, regulate and coordinate different cellular events that occur during the seminiferous epithelial cycle. For instance, the events of spermiation and of preleptotene spermatocyte migration across the BTB both take place concurrently at stage VIII of the epithelial cycle of spermatogenesis. Recent findings suggest that these events are coordinated by protein complexes found at the apical and basal ES and TJ, which are located at different ends of the Sertoli cell epithelium. Besides, we highlight important areas of research that can now be undertaken, and functional studies that can be designed to tackle different issues pertinent to junction restructuring during spermatogenesis.

Serum 17-hydroxyprogesterone strongly correlates with intratesticular testosterone in gonadotropin-suppressed normal men receiving various dosages of human chorionic gonadotropin

OBJECTIVE

To determine if serum concentrations of testosterone precursors would correlate with intratesticular testosterone (ITT) concentration measured directly by testicular aspiration and allow for a less invasive means of inferring ITT.

DESIGN

Controlled clinical study.

SETTING

Healthy volunteers in an academic research environment.

PATIENT(S)

Twenty-nine normal men.

INTERVENTION(S)

We determined ITT concentration by testicular aspiration before and after treatment in men receiving exogenous T to block endogenous gonadotropin production and randomly assigned to one of four doses of hCG (0, 125 IU, 250 IU, or 500 IU every other day) for 3 weeks.

MAIN OUTCOME MEASURE(S)

The association between serum 17-hydroxyprogesterone (17OH-P), androstenedione, and DHEA and ITT.

RESULT(S)

With T administration alone, serum 17OH-P decreased significantly and increased significantly when 500 IU hCG was administered. End-of-treatment ITT strongly correlated with serum 17OH-P. Moreover, serum 17OH-P, but not androstenedione or DHEA, was independently associated with end-of-treatment ITT by multivariate linear regression.

CONCLUSION(S)

Serum 17OH-P is highly correlated with ITT in gonadotropin-suppressed normal men receiving T and stimulated with hCG. Serum 17OH-P is a surrogate biomarker of ITT and may be useful in research and in men receiving gonadotropin therapy for infertility.

The testis in immune privilege

The production, differentiation, and presence of male gametes represent inimitable challenges to the immune system, as they are unique to the body and appear long after the maturation of the immune system and formation of systemic self-tolerance. Known to protect germ cells and foreign tissue grafts from autoimmune attack, the 'immune privilege' of the testis was originally, and somewhat simplistically, attributed to the existence of the blood-testis barrier. Recent research has shown a previously unknown level of complexity with a multitude of factors, both physical and immunological, necessary for the establishment and maintenance of the immunotolerance in the testis. Besides the blood-testis barrier and a diminished capability of the large testicular resident macrophage population to mount an inflammatory response, it is the constitutive expression of anti-inflammatory cytokines in the testis by immune and particularly somatic cells, that represents an essential element for local immunosuppression. The role of androgens in testicular immune regulation has long been underestimated; yet, accumulating evidence now shows that they orchestrate the inhibition of proinflammatory cytokine expression and shift cytokine balance toward a tolerogenic environment. Furthermore, the role of the testicular dendritic cells in suppressing antigen-specific immunity and T-lymphocyte activation is discussed. Finally, the active role mast cells play in the induction and amplification of immune responses, both in infertile humans and in experimental models, highlights the importance of preventing mast cell activation to maintain the immune-privileged status of the testis.

The androgen microenvironment of the human testis and hormonal control of spermatogenesis

It is well established for both rat and man that the total testosterone concentration within the testis is far higher than that in serum. We know for the rat that intratesticular testosterone can be reduced by 50-60% without an adverse effect on spermatogenesis but that the required intratesticular testosterone concentration is still 10-fold greater than serum testosterone concentration. This kind of information, if available for the human, could prove invaluable for understanding and treating select men with infertility and in the development of male hormonal contraceptives. Unfortunately, we know little about the androgen content of intratesticular fluid within the human testis and nothing about the relationship between intratesticular androgens and human spermatogenesis. Using a newly developed minimally invasive technique for repetitive testicular sampling, our recent studies of the human have demonstrated that, as in the rat, there is a gradient between the concentration of testosterone in serum and within the testis; intratesticular testosterone levels were found to be 100-fold higher than serum testosterone levels in normal men. Using liquid chromatography tandem mass spectroscopy, we have shown that intratesticular 5alpha-dihydrotestosterone (DHT) levels are only 2% that of testosterone and, thus, despite greater affinity for the androgen receptor, intratesticular DHT is not significant in normal men. In order to assess how much of the testosterone within the human testis is bioactive, we adapted a highly sensitive recombinant protein mammalian cell-based bioassay to measure androgen bioactivity. The androgen bioactivity in the normal human testis is roughly two-thirds that of the total testosterone measurable by radioimmunoassay, despite the fact that the concentrations of the major androgen-binding proteins (sex hormone-binding globulin- and androgen-binding protein) are insufficient to account for this difference. This finding suggests that androgens may bind to other, as-yet-unknown molecules in the human testis. How, or if, this relates to spermatogenesis in the rat, or to man-to-man differences in the response to hormonal contraceptives, is not clear. We do not yet know how much testosterone is required within the human testis to either maintain or restore quantitatively normal spermatogenesis because, as yet, experimental studies comparable to those performed in the rat have not been feasible for the human.