Abstract STUDY QUESTION Is the distribution of immune cells and the testicular vasculature altered in testicular biopsies from patients with Klinefelter syndrome (KS)? SUMMARY ANSWER Increased numbers of macrophages and mast cells, an increased expression of decorin and an increased blood vessel density were found in KS samples compared to controls. WHAT IS KNOWN ALREADY Most KS patients are infertile due to an early germ cell loss. From puberty onwards, testicular fibrosis can be detected. How this fibrotic process is initiated remains unknown. STUDY DESIGN, SIZE, DURATION In this study, the number of macrophages, mast cells and their secretory products were evaluated in KS, Sertoli cell only (SCO) and control patient samples. The association between immune cell numbers and level of fibrosis in KS tissue was examined. In addition, the vascularization within these testicular tissue biopsies was studied. For immunohistochemical evaluation, KS patients at different stages of testicular development were included: prepubertal (aged 4–7 years; n = 4), peripubertal (aged 11–17 years; n = 21) and adult (aged >18 years; n = 37) patients. In addition, testicular tissue biopsies of adult SCO (n = 33) and control samples for the three KS age groups (prepubertal n = 9; peripubertal n = 5; adult n = 25) were analysed. Gene expression analysis was performed on adult testicular tissue from KS (n = 5), SCO (n = 5) and control (n = 5) patients. PARTICIPANTS/MATERIALS, SETTING, METHODS Adult (>18 years) KS, SCO and control testicular tissue biopsies were obtained during a testicular sperm extraction procedure. KS peripubertal (11–18 years), prepubertal (<11 years) and age-matched control biopsies were obtained from the biobank of the university hospital. Immunohistochemistry was used to determine the tubular structure (H/PAS), the number of spermatogonia (MAGE-A4), macrophages (CD68) and mast cells (tryptase) and the blood vessel density (Von Willebrand factor). In addition, quantitative real-time polymerase chain reaction was used to determine the expression of secretory products of macrophages and mast cells (tryptase, tumour necrosis factor alpha and decorin). MAIN RESULTS AND THE ROLE OF CHANCE A significant increase in the number of macrophages (P < 0.0001) and mast cells (P = 0.0008) was found in the peritubular compartment of testes of adult KS patients compared to control samples. However, no association between the number of immune cells and the degree of fibrosis was observed. In adult SCO samples, a significant increase was seen for peritubular macrophage (P < 0.0001) and mast cell (P < 0.0001) numbers compared to control samples. In the interstitial compartment, a significant increase in mast cell number was found in adult SCO samples compared to KS (P < 0.0001) and control (P < 0.0001) tissue. A significant difference (P = 0.0431) in decorin expression could be detected in adult KS compared to control patients. Decorin expression was mostly seen in the walls of the seminiferous tubules. When comparing the vascularization between KS patients and age-matched controls, a significant increase (P = 0.0081) in blood vessel density could be observed only in prepubertal KS testicular tissue. LARGE SCALE DATA N/A. LIMITATIONS, REASONS FOR CAUTION As controls for this study, testicular tissue biopsies of men who underwent a vasectomy reversal or orchiectomy were used, but these men may not represent fertile controls. In addition, a high variability in immune cell numbers, secretory products expression and number of blood vessels could be observed amongst all patient samples. WIDER IMPLICATIONS OF THE FINDINGS Increased numbers of macrophages and mast cells have previously been described in non-KS infertile men. Our results show that these increased numbers can also be detected in KS testicular tissue. However, no association between the number of macrophages or mast cells and the degree of fibrosis in KS samples could be detected. Decorin has previously been described in relation to fibrosis, but it has not yet been associated with testicular fibrosis in KS. Our results suggest a role for this proteoglycan in the fibrotic process since an increased expression was observed in adult KS tissue compared to controls. Impaired vascularization in KS men was suggested to be responsible for the KS-related disturbed hormone levels. Our results show a significant difference in blood vessel density, especially for the smallest blood vessels, between prepubertal KS samples and age-matched controls. This is the first study to report differences between KS and control testicular tissue at prepubertal age. STUDY FUNDING/COMPETING INTEREST(S) The project was funded by grants from the Vrije Universiteit Brussel (E.G.) and the scientific Fund Willy Gepts from the UZ Brussel (D.V.S.). D.V.S. is a post-doctoral fellow of the Fonds voor Wetenschappelijk Onderzoek (FWO; 12M2819N). No conflict of interest is declared for this research project.

Klinefelter syndrome (KS) is a quite common disorder with an incidence of 1-2 in 1000 newborn males. Most patients are diagnosed in the light of a clinical check-up when consulting a fertility clinic with an unfulfilled child wish. Infertility in KS patients is caused by a massive germ cell loss, leading to azoospermia in more than 90% of the adult patients. Most seminiferous tubules in the adult KS testis are degenerated or hyalinized and testicular fibrosis can be observed, starting from puberty. However, focal spermatogenesis can be found in the testis of some patients. This offers the opportunity to extract spermatozoa from the testis by testicular sperm extraction. However, TESE is only successful in about half of the KS adults seeking to father children. The reason for the germ cell loss remains unclear. To date, it is still debated whether the testicular tissue changes and the germ cell loss seen in KS is directly caused by an altered X-linked gene expression, the altered somatic environment or a deficiency in the germ cells. In this review, we provide an overview of the current knowledge about the germ cell loss in KS patients.

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About half of testicular sperm extraction (TESE) procedures in men with non-obstructive azoospermia (NOA), including men with Klinefelter syndrome (KS), are unsuccessful. To avoid unnecessary invasive surgery, biomarkers for spermatozoa were studied. In addition, markers for spermatogonia in testis tissue were explored. This study aimed to find biomarkers in the semen and/or urine of NOA patients to predict the presence of spermatogonia in the testis. Differentially expressed miRNAs were identified (1) between samples from patients with and without a positive TESE procedure as well as (2) between TESE-negative patients with and without spermatogonia. A total of thirteen upregulated miRNAs (ten in seminal plasma and three in urine) were found in the TESE-negative/spermatogonia-positive group compared to the TESE-negative/spermatogonia-negative group. These miRNAs could be potential biomarkers for spermatogonia; however, more research is necessary to validate their predictive power.