Differential activation of inflammatory pathways in testicular macrophages provides a rationale for their subdued inflammatory capacity

Cytokines in Male Fertility and Reproductive Pathologies: Immunoregulation and Beyond

Germline development in vivo is dependent on the environment formed by somatic cells and the differentiation cues they provide; hence, the impact of local factors is highly relevant to the production of sperm. Knowledge of how somatic and germline cells interact is central to achieving biomedical goals relating to restoring, preserving or restricting fertility in humans. This review discusses the growing understanding of how cytokines contribute to testicular function and maintenance of male reproductive health, and to the pathologies associated with their abnormal activity in this organ. Here we consider both cytokines that signal through JAKs and are regulated by SOCS, and those utilizing other pathways, such as the MAP kinases and SMADs. The importance of cytokines in the establishment and maintenance of the testis as an immune-privilege site are described. Current research relating to the involvement of immune cells in testis development and disease is highlighted. This includes new data relating to testicular cancer which reinforce the understanding that tumorigenic cells shape their microenvironment through cytokine actions. Clinical implications in pathologies relating to local inflammation and to immunotherapies are discussed.

Specific immune cell and cytokine characteristics of human testicular germ cell neoplasia

STUDY QUESTION

Which immune cells and cytokine profiles are characteristic for testicular germ cell neoplasia and what consequences does this have for the understanding of the related testicular immunopathology?

SUMMARY ANSWER

The unique immune environment of testicular germ cell neoplasia comprises B cells and dendritic cells as well as high transcript levels of IL-6 and other B cell supporting or T helper cell type 1 (Th1)-driven cytokines and thus differs profoundly from normal testis or inflammatory lesions associated with hypospermatogenesis.

WHAT IS KNOWN ALREADY

T cells are known to be the major component of inflammatory infiltrates associated with either hypospermatogenesis or testicular cancer. It has previously been reported that B cells are only involved within infiltrates of seminoma samples, but this has not been investigated further.

STUDY DESIGN, SIZE, DURATION

Immunohistochemical characterisation (IHC) of infiltrating immune cells and RT-qPCR-based analysis of corresponding cytokine microenvironments was performed on different testicular pathologies. Testicular biopsies, obtained from men undergoing andrological work-up of infertility or taken during surgery for testicular cancer, were used in this study. Samples were grouped as follows: (i) normal spermatogenesis (n = 18), (ii) hypospermatogenesis associated with lymphocytic infiltrates (n = 10), (iii) samples showing neoplasia [germ cell neoplasia in situ (GCNIS, n = 26) and seminoma, n = 18].

PARTICIPANTS/MATERIALS, SETTING, METHODS

IHC was performed using antibodies against T cells (CD3+), B cells (CD20cy+), dendritic cells (CD11c+), macrophages (CD68+) and mast cells (mast cell tryptase+). Degree and compartmental localisation of immune cells throughout all groups analysed was evaluated semi-quantitatively. RT-qPCR on RNA extracted from cryo-preserved tissue samples was performed to analyse mRNA cytokine expression, specifically levels of IL-1β, IL-6, IL-17a, tumour necrosis factor (TNF)-α (pro-inflammatory), IL-10, transforming growth factor (TGF)-β1 (anti-inflammatory), IL-2, IL-12a, IL-12b, interferon (IFN)-γ (Th1-driven), IL-4, IL-5, IL-13, IL-23a (Th2-driven), CXCL-13, CXCL-10 and CCL-5 (chemokines).

MAIN RESULTS AND THE ROLE OF CHANCE

This is the first study showing a direct linkage between the distribution pattern of immune cells in hypospermatogenesis versus testicular cancer and analysis of a wide range of 17 related cyto- and chemokines. A fundamental difference between testicular inflammation patterns associated with different testicular inflammatory conditions either containing or lacking neoplastic cells was demonstrated. In hypospermatogenesis, T cells were detected, whereas B cells and dendritic cells were almost absent. Within GCNIS and seminoma, in addition to T cells, high numbers of dendritic cells and B cells were found, the latter additionally organised in cell clusters, whereas mast cells were absent. Transcripts encoding pro-inflammatory cytokines (IL-1β, IL-6 and TNF-α), anti-inflammatory cytokines (TGF-β1), Th1-driven cytokines (IL-2 and IFN-γ) as well as chemokines (CXCL-13, CXCL-10 and CCL-5) were all significantly increased in testicular germ cell neoplasia (P ≤ 0.01), suggesting the presence of a pro-tumorigenic environment. In contrast, Th2-related cytokines (IL-5, IL-13 and IL-23a) characterised the environment within samples showing normal spermatogenesis as well as hypospermatogenesis. One of the most important outcomes is the pivotal role of IL-6 in testicular cancer that opens potential novel diagnostic and/or immune-therapeutic perspective for testis cancer.

LIMITATIONS, REASONS FOR CAUTION

Testicular tissue composed of immune cells as well as other somatic cells and germ cells does not allow identification of specific cytokine sources or single cell types, being responsible for establishing the overall cytokine environment. In this study, laser-assisted microdissection did not reach the required efficiency for RT-qPCR analyses. Therefore, in vitro models would be suggested for addressing the above-mentioned issue. Conclusions about cytokine levels in testes with GCNIS are based on a small number of samples.

WIDER IMPLICATIONS OF THE FINDINGS

The unique B cell presence and the significantly increased expression level of IL-6 in testicular germ cell neoplasia (P < 0.001) strengthen its special role in this disease. In line with current knowledge on other types of cancer, these results underline the relevance of further investigating the potential of IL-6 as early biomarker and target for therapeutic intervention in testicular germ cell neoplasia.

STUDY FUNDING/COMPETING INTERESTS

This study (and B.K. in person) was supported by the Deutsche Forschungsgemeinschaft (DFG) as part of the International Research Training Group between Justus Liebig University of Giessen and Monash University, Melbourne (GRK 1871/1) on 'Molecular pathogenesis on male reproductive disorders'. T.H., H.-C.S. and M.B. were supported by the LOEWE focus group 'MIBIE' (male infertility during infection & inflammation)-an excellence initiative of the German state government of Hessen. From the Australian side, K.L. was supported by NHMRC grants (Fellowship, ID1079646 and Project, ID1081987); K.L., S.I. and M.H. received scholarship (S.I.) and research funding (K.L., M.H.) from Monash University. The project also drew support from the Victorian Government's Operational Infrastructure Support Program. The authors have no competing interests to declare.

The macrophages in testis function

Testicular macrophages are the largest leukocyte population in the testis. Their environment is characterized by the seemingly contradictory needs for tolerance against the autoantigenic germ cells and the capacity to mount pro-inflammatory innate immune responses against invading pathogens. During the past years considerable progress has been made in our understanding how intracellular signaling events enable testicular macrophages to adequately respond to inflammatory stimuli with the capacity to clear pathogens, but avoid excessive tissue damage to maintain fertility. Moreover, new data add to our understanding that testicular macrophages play essential roles in normal testis homeostasis and fetal testicular development.

Developmental origins of male subfertility: role of infection, inflammation, and environmental factors

Male gamete development begins with the specification of primordial cells in the epiblast of the early embryo and is not complete until spermatozoa mature in the epididymis of adult males. This protracted developmental process involves extensive alteration of the paternal germline epigenome. Initially, epigenetic reprogramming in fetal germ cells results in removal of most DNA methylation, including parent-specific epigenetic information. The germ cells then establish sex-specific epigenetic information through de novo methylation and undergo spermatogenesis. Chromatin in haploid germ cells is repackaged into protamines during spermiogenesis, providing further widespread epigenetic reorganization. Finally, after fertilization, epigenetic reprogramming in the preimplantation embryo is necessary for regaining totipotency. These events provide substantial windows during which epigenetic errors either may be corrected or may occur in the germline. There is now increasing evidence that environmental factors such as exposure to toxicants, the parents' and individual's diet, and even infectious and inflammatory events in the male reproductive tract may influence epigenetic reprogramming. This, together with other damage inflicted on the germline chromatin, may result in negative consequences for fertility and health. Large epidemiological birth cohort studies have yielded insight into possible causative environmental factors. Together with experimental animal studies, a clearer view of environmental impacts on fetal development and their intergenerational and even transgenerational effects on reproductive health has emerged and is reviewed in this article.