Role of indoleamine 2,3-dioxygenase in testicular immune-privilege

Gisela S Gualdoni,Vanesa A Guazzone,Cecilia V Pérez,Marcelo Hill,Patricia V Jacobo,Ignacio Anegon,Livia Lustig,Christian Höcht,Mónica B Frungieri,María E Matzkin,Cristian M Sobarzo

doi:10.1038/s41598-019-52192-8

Abstract

Male meiotic germ cell including the spermatozoa represent a great challenge to the immune system, as they appear long after the establishment of normal immune tolerance mechanisms. The capacity of the testes to tolerate autoantigenic germ cells as well as survival of allogeneic organ engrafted in the testicular interstitium have led to consider the testis an immunologically privileged site. Disruption of this immune privilege following trauma, tumor, or autoimmune orchitis often results in male infertility. Strong evidence indicates that indoleamine 2,3-dioxygenase (IDO) has been implicated in fetal and allograft tolerance, tumor immune resistance, and regulation of autoimmune diseases. IDO and tryptophan 2,3-dioxygenase (TDO) catalyze the same rate-limiting step of tryptophan metabolism along a common pathway, which leads to tryptophan starvation and generation of catabolites collectively known as kynurenines. However, the relevance of tryptophan metabolism in testis pathophysiology has not yet been explored. Here we assessed the in vivo role of IDO/TDO in experimental autoimmune orchitis (EAO), a model of autoimmune testicular inflammation and immunologically impaired spermatogenesis. EAO was induced in adult Wistar rats with testicular homogenate and adjuvants. Control (C) rats injected with saline and adjuvants and normal untreated rats (N) were also studied. mRNA expression of IDO decreased in whole testes and in isolated Sertoli cells during EAO. TDO and IDO localization and level of expression in the testis were analyzed by immunostaining and Western blot. TDO is expressed in granulomas from EAO rats, and similar protein levels were observed in N, C, and EAO groups. IDO was detected in mononuclear and endothelial cells and reduced IDO expression was detected in EAO group compared to N and C rats. This phenomenon was concomitant with a significant reduction of IDO activity in EAO testis measured by tryptophan and kynurenine concentrations (HPLC). Finally, in vivo inhibition of IDO with 1-methyl-tryptophan increased severity of the disease, demonstrating down regulation of IDO-based tolerance when testicular immune regulation was disrupted. We present evidence that an IDO-based mechanism is involved in testicular immune privilege.

Highlights

The testis is considered an immunoprivileged organ since it tolerates germ cell antigens appeared during the pubertal period, an event developing long after establishment of immunocompetence
Tryptophan catabolism into kynurenine by the IDO enzyme has been proposed as a putative mechanism in testicular immune tolerance, this possibility has never been evaluated in normal and pathological conditions in rats
We evaluated the expression and activity of IDO/ TDO in normal versus inflamed testis and assessed orchitis progression after treating rats immunized with sperm antigens and adjuvants with the IDO-specific inhibitor 1MT

Summary

Introduction

The testis is considered an immunoprivileged organ since it tolerates germ cell antigens appeared during the pubertal period, an event developing long after establishment of immunocompetence. Non-sequestered germ cell antigens egress from normal tubules and interact with circulating antibodies, forming immune complexes at the wall of ST. Non-sequestered germ cell antigens originated from fragments discarded by spermatids during spermiogenesis maintain physiological tolerance dependent on Treg. Fallarino et al.[9] demonstrated that isolated neonatal porcine SC prevent and revert diabetes in non obese diabetic mice, being cell or insulin therapy unnecessary. This result was explained by the restitution of systemic immune tolerance dependent on efficient tryptophan metabolism in the xenografts. IDO activity contributes to maternal tolerance in pregnancy[18], controls allograft rejection[19,20], and protects against autoimmunity such as, trinitrobenzene sulfonic acid colitis[21], rheumatoid arthritis[22], granulomatous diseases[14,23] and allergy[24,25]

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