Tissue-specific macrophages are important for the activation of innate immune responses and general organ homeostasis. Testicular macrophages (TM) reside in the testicular interstitial space and comprise the largest leukocyte population in the testis and are assumed to play a role in maintaining testicular immune privilege. Numerous studies have indicated that the interstitial fluid (IF) surrounding the TM has immunosuppressive properties, which may influence the TM phenotype. However, the identity of the immunosuppressive molecules present in the IF is poorly characterized. In this thesis it is shown that in the rat, IF shifts the M1 phenotype of granulocyte macrophage-colony stimulating factor induced bone marrow derived macrophages towards the M2 phenotype. M2 macrophages polarized by IF mimic the properties of TM such as increased expression of CD163, high secretion of IL-10 and low secretion of TNF-alpha. In addition, IF-polarized macrophages display immunoregulatory functions by inducing the expansion of immunosuppressive regulatory T cells. This thesis provides evidence that PGE2, PGI2, testosterone and corticosterone are important immunoregulatory molecules in the IF, playing a relevant role in determining the phenotype of TM. Except corticosterone, all of these factors are able to inhibit the NF-kB signaling pathway to suppress the production of pro-inflammatory cytokines and thus maintain an immunosuppressive microenvironment of the testis. Corticosterone was found to be the principal immunosuppressive molecule in the IF. Its receptor, the glucocorticoid receptor, was found to be present in TM immunohistochemically. In addition, TM locally produce small amounts of corticosterone, which suppress the expression of inflammatory genes and render TM refractory to inflammatory stimuli. Taken together, these results suggest that testicular corticosterone shapes the immunosuppressive function and phenotype of TM. This steroid hormone may therefore play also an important role in the establishment and maintenance of the immune privilege of the testis.
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