On the role of HMGB1 and IL-1 in tissue barrier defense

Abstract

The specialized epithelial surfaces of the lymphoepithelial tissue in the airways and the seminiferous epithelium of the testis are sites of host-environmental interaction at the same time as they serve important physiological functions in gas exchange and reproduction. The host response of a broken epithelial barrier requires rapid mechanisms to control the microflora and to prevent invasion of pathogens. Cytokines are a heterogeneous group of proteins originally described as signaling molecules within the immune system. Today more than 100 cytokines have been reported including the proinflammatory cytokines, e.g., interleukin-1 (IL-1), interleukin-6 (IL-6), tumor necrosis factor- alpha (TNF-alpha), as an important subgroup. Antimicrobial peptides are key effector molecules in the innate immune response. To date about 500 antimicrobial gene derived factors have been described in a variety of species from microbes and plants to insects and mammals, including small peptides, larger proteins and protein fragments.

The present work aimed to explore the presence and function of factors contributing to barrier host defense in the upper airways and the testis. By in situ hybridization a constitutive and stage dependent expression of interleukin1 alpha (IL-1alpha) mRNA was localized to Sertoli cells of adult rat testis. Testicular IL-1alpha may serve as an immune stimulatory factor in the testis and the localization in Sertoli cells is compatible with a barrier site function. Adult and prepubertal rats were treated with endotoxin to induce a systemic inflammatory activation. The treatment caused induction of IL-1alpha, IL-1beta, tumor necrosis factor-alpha (TNF-alpha) and IL-6 expression in testicular macrophages in both adult and prepubertal testes. The constitutive Sertoli cell IL-1alpha expression was down-regulated. The level of bioactive IL-1 in adult testis remained unchanged despite the massive induction of IL-1beta. We therefore hypothesized that IL-1 antagonists, e.g., IL-1ra, were induced to balance the IL-1 activity. As high concentrations of IL-1 may have adverse effects, the observed reactions may contribute to the protection of developing germ cells from harmful influences of local inflammation.

High mobility group box chromosomal protein 1 (HMGB1) was purified from adenoids and human and rat testis. HMGB1 was found to possess pronounced and rapid antibacterial activity. The germinal centers of the adenoid and the Sertoli cells of the testis were identified as the main production sites of the HMGB 1. Surface secretions of the adenoid contained antimicrobial activity mainly consisting of alpha-defensins 1-3.

HMGB1 was found to interact with IL-1 in an IL-1 responsive thymocyte proliferation bioassay and also stimulated anti-CD3 activated T lymphocyte proliferation. The findings that HMGB 1 interacted with IL-1 to mediate biological responses prompted us to perform biosensor experiments to find out whether HMGB1 could bind to the IL-1 receptor(s) (IL-1R) HMGB 1 showed a dose-dependent binding to both IL-1 receptor type I and type II.

From the present results one can speculate that the presence of HMGB1 in Sertoli cells serves dual purposes, to directly kill invading microorganisms and to act as a co-activator of immune cells. The co-localization with IL-1alpha in adult testis amplifies this latter function. In both the testis and the adenoid HMGB 1 may serve as a first line of defense towards incoming bacteria.

Taken together the results suggest that HMGB 1 and IL-1 serve important functions in the epithelial barrier defense and that HMGB 1 exerts at least parts of its proinflammatory cytokinelike functions through the IL-1 RI.