Steroid hormones (SHs) are lipophilic molecules derived from cholesterol and synthesized in the adrenal cortex (glucocorticoids, mineralocorticoids, and adrenal androgens), the testes (testicular androgens, oestrogen), and the ovary and placenta (oestrogens and progestagens or progestins). SHs reach their target cells via the blood, where they are bound to carrier proteins, and because of their lipophilic nature pass the cell membrane by simple diffusion. Within the target cells SHs bind to steroid hormone receptors (SHRs), the key mediators of SH action, which are complexed to chaperones, e.g. heat shock protein 90 (Hsp90), that help other proteins to fold and prevent aggregation. SHRs are intracellular transcription factors that can be activated, among other possibilities, by the specific and high affinity binding of ligand to exert positive or negative effects on the expression of target genes. Binding of agonistic or antagonistic ligands leads to different allosteric changes of SHRs making them competent to exert positive or negative effects on the expression of target genes by different mechanisms. (i) After dissociation of chaperones the liganded SHR-complexes can bind to chromatin organized DNA sequences in the vicinity of target genes, termed hormone response elements (HREs). The HRE-recruited hormone-receptor-complexes are then able to initiate chromatin remodelling and to relay activating or repressing signals to the target genes transcription machinery; (ii) through protein-protein interactions with other sequence-specific transcription factors, SHRs can also regulate the activity of many genes that are switched on, for instance, during stress or an inflammatory response; (iii) the SH response can also be integrated in the intracellular signalling network via cross-talk of SHRs with signal transduction pathways that transmit extracellular signals via membrane receptors and activation of protein kinase cascades to nuclear transcription factors that activate various target genes. By all these different mechanisms SHRs modulate numerous and specific responses in a large variety of cells, whereby their particular effect depends on the physiological, cellular and genetic context.

Despite the immune-privileged status of the male genital tract, infection and inflammation of the male genital tract are important etiological factors in male infertility. A common observation in clinical and experimental orchitis as well as in systemic infection and inflammation are decreased levels of testosterone. Emerging data point to an immunosuppressive role of testosterone. In our study, we substituted testosterone levels in experimental autoimmune orchitis (EAO) in rat by s.c. testosterone implants. EAO development was reduced to 17% when animals were treated with low-dose testosterone implants (3 cm long, EAO+T3) and to 33% when rats were supplied with high-dose testosterone implants (24 cm, EAO+T24) compared with 80% of animals developing disease in the EAO control group. In the testis, testosterone replacement in EAO animals prevented the accumulation of macrophages and significantly reduced the number of CD4+ T cells with a strong concomitant increase in the number of regulatory T cells (CD4+CD25+Foxp3+) compared with EAO control. In vitro testosterone treatment of naive T cells led to an expansion of the regulatory T cell subset with suppressive activity and ameliorated MCP-1–stimulated chemotaxis of T lymphocytes in a Transwell assay. Moreover, expression of proinflammatory mediators such as MCP-1, TNF-α, and IL-6 in the testis and secretion of Th1 cytokines such as IFN-γ and IL-2 by mononuclear cells isolated from testicular draining lymph nodes were decreased in the EAO+T3 and EAO+T24 groups. Thus, our study shows an immunomodulatory and protective effect of testosterone substitution in the pathogenesis of EAO and suggests androgens as a new factor in the differentiation of regulatory T cells.

Uropathogenic Escherichia coli (UPEC) is the most common etiological cause of urogenital tract infections and represents a considerable cause of immunological male infertility. We examined TLR 1–11 expression profiles in testicular cells and the functional response to infection with UPEC. All testicular cell types expressed mRNAs for at least two TLRs and, in particular, synthesis of TLR4 was induced in testicular macrophages (TM), Sertoli cells (SC), peritubular cells (PTC), and peritoneal macrophages (PM) after UPEC exposure. Even though MyD88-dependent pathways were activated as exemplified by phosphorylation of mitogen-activated protein kinases in TM, SC, PTC, and PM and by the degradation of IκBα and the nuclear translocation of NF-κB in PTC and PM, treatment with UPEC did not result in secretion of the proinflammatory cytokines IL-1α, IL-6, and TNF-α in any of the investigated cells. Moreover, stimulated production of these cytokines by nonpathogenic commensal E. coli or LPS in PM was completely abolished after coincubation with UPEC. Instead, in SC, PTC, TM, and PM, UPEC exposure resulted in activation of MyD88-independent signaling as documented by nuclear transfer of IFN-related factor-3 and elevated expression of type I IFNs α and β, IFN-γ-inducible protein 10, MCP-1, and RANTES. We conclude that in this in vitro model UPEC can actively suppress MyD88-dependent signaling at different levels to prevent proinflammatory cytokine secretion by testicular cells. Thus, testicular innate immune defense is shifted to an antiviral-like MyD88-independent response.

CD4+CD25+Foxp3+ Treg cells are crucial for the maintenance of immunological homeostasis. Androgens significantly induce Foxp3 expression in humans and regulate the differentiation of Treg cells. A functional androgen receptor–binding site is identified within the Foxp3 locus leading to epigenetic changes of histone H4.

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