In vitro data have indicated that nitric oxide (NO) inhibits Leydig cell testosterone production, suggesting that NO may play a role in the suppression of steroidogenesis and spermatogenic function during inflammation. Consequently, we investigated expression of the inflammation-inducible isoform of NO synthase (iNOS) in the inflamed adult rat testis and the ability of a broad-spectrum inhibitor of NO production, L-nitro-L-arginine methyl ester, to prevent Leydig cell dysfunction during inflammation. Unexpectedly, immunohistochemical and mRNA data established that iNOS is expressed constitutively in Leydig cells and in a stage-specific manner in Sertoli, peritubular, and spermatogenic cells in the normal testis. Expression was increased in a dose-dependent manner in all these cell types during lipopolysaccharide (LPS)-induced inflammation. In noninflamed testes, treatment with the NO synthase inhibitor reduced testicular interstitial fluid formation and testosterone production without any effect on serum LH levels. Administration of the inhibitor did not prevent the suppression of testicular interstitial fluid and testosterone production that occurs within 6 h after LPS treatment. Collectively, these data indicate a novel role for iNOS in autocrine or paracrine regulation of the testicular vasculature, Leydig cell steroidogenesis, and spermatogenesis in the normal testis. The data suggest that increased NO is not the major cause of acute Leydig cell dysfunction in the LPS-treated inflammation model, although a role for NO in this process cannot be excluded, particularly at other time points. Moreover, up-regulation of iNOS may contribute to the seminiferous epithelium damage caused by LPS-induced inflammation.

Cytokine profiles in the testes of rats treated with lipopolysaccharide reveal localized suppression of inflammatory responses.

The majority of macrophages in the rat testis can be identified by the tissue-resident macrophage marker ED2. A smaller population of intratesticular macrophages do not express the ED2 antigen but are positive for the monocyte/macrophage marker ED1. Treatment of adult rats with the inflammatory stimulus lipopolysaccharide (LPS) had no effect on the number of testicular resident (ED2(+)) macrophages but caused a transient increase in ED1(+)ED2(-) monocyte-like macrophages (an average three-fold increase 12 h later). In both control and LPS-treated rat testes, a majority of macrophages that expressed ED1 and all Leydig cells were immuno-positive for the inducible isoform of nitric oxide synthase (iNOS). However, less than 6% of ED2(+) macrophages showed any iNOS expression, even after LPS treatment. This deficiency was confirmed by the finding that isolated ED2(+) testicular macrophages (>98% pure) stimulated with LPS did not produce NO in vitro. In contrast, resident macrophages from the peritoneum showed the expected NO response, and purified Leydig cells produced significant NO regardless of the presence or absence of LPS. Collectively, these data indicate the presence of at least two macrophage subsets in the adult rat testis: (1) the ED2(+) resident macrophages, which do not alter following LPS-treatment and mostly do not express iNOS or produce NO in response to an inflammatory stimulus, and (2) the ED1(+)ED2(-) monocyte-like macrophages, which increase in number after LPS-treatment and express iNOS even in the absence of exogenous inflammatory stimulation. It is highly probable that these different subsets have different functional roles within the testis.

Macrophages are numerous in the testicular interstitial tissue under normal conditions and increase during inflammation. The mechanisms involved are poorly characterized. Expression of the macrophage-regulating cytokines monocyte chemoattractant protein (MCP)-1 and macrophage colony-stimulating factor (M-CSF) was examined in the adult rat testis before and after an i.p. injection of an inflammatory stimulus, lipopolysaccharide (LPS). In the normal testis, M-CSF was readily observed using Northern blot and Western blot analysis. In contrast, MCP-1 was not detectable by Northern blot in the normal testis, but was detected using RT-PCR amplification and a sensitive ELISA. After LPS treatment, testicular MCP-1 mRNA and protein expression increased dramatically (up to 400-fold). In-situ hybridization for MCP-1 revealed that production was confined to the interstitium of the inflamed testis, in Leydig cells, peritubular cells, perivascular cells and monocyte-like macrophages, but not in tissue-resident macrophages. Unlike MCP-1, M-CSF mRNA and protein expression in the testis increased only marginally, if at all, after LPS treatment. These results suggest that MCP-1 stimulates the increase in intratesticular macrophages that accompanies LPS-induced inflammation in vivo. Together with M-CSF, MCP-1 may also play a role in maintaining the resident macrophage population of the normal testis.