Investigators at Leeds University have discovered that the proinflammatory cytokine IL-36gamma acts as a global early sensor of pathogenic invasion and discriminator of pathogenic and harmless microbes. The researchers found that although the native intracellular form of IL-36gamma is upregulated by microbes, it is liberated from cells and converted to its proinflammatory form only in cases of pathogen-mediated cell damage by the microbial proteases. The team reported its results in the December 15, 2020, online issue of Cell Reports.
Senior author Martin Stacey, who is a lecturer of immunology at Leeds University, told BioWorld Science that the work demonstrates that IL-36gamma acts as a link between microbial proteolytic activity and initiation of an immune response in epithelial tissue. Importantly, "our paper provides evidence that microbial triggers increase the production of proinflammatory IL-36 cytokines and initiate/promote the inflammation of skin lesions. But the release of this molecule in the extracellular space and the subsequent adaptive immune response is controlled by specific proteolytic virulence factors," he said.
IL-36 cytokines are mainly expressed on the barrier sites of the body, e.g., bronchial, intestinal and dermal epithelium. One of their most important biological functions is the bridging of innate and adaptive immune responses. The most prominent example for an altered IL-36gamma expression is the spectrum of psoriasis. In addition to inflammatory dermatoses, IL-36gamma also seems to play a role in infectious dermatoses. Given the importance of proteolytic processing in the activation of IL-36gamma at epithelial barriers and the role played by microbial proteases in establishing pathogenic invasion, Stacey and his team decided to explore the effects of pathogen-mediated release and proteolytic activation of IL-36gamma.
The team first treated the oral buccal epithelial cell line TR146 with various proteases and with both pathogenic (Aspergillus fumigatus conidia, Streptococcus pyogenes) and harmless microbes (Staphylococcus epidermis). Though there was an increase in the expression of IL-36gamma in the cell lysate, only the pathogen treated group showed a concomitant increase in the supernatant, indicating the presence of extracellular IL-36gamma only in presence of pathogens. The team found that IL-36gamma was released extracellularly only in presence of cellular membrane damage as a result of pathogenic invasion.
IL-36gamma was activated by proteases derived from a range of human pathogens. As IL-36gamma is produced as an inactive precursor that requires precise N-terminal processing to become biologically active, Stacey and his team next endeavored to determine the mechanism of microbial activation of IL-36gamma. During infection, S. pyogenes and A. fumigatus abundantly secrete the cysteine protease SpeB and serine protease AspF13, respectively.
While S. pyogenes belongs to the streptococcal bacterial family causing upper respiratory tract infections, A. fumigata is a fungus and an opportunistic pathogen, causing invasive infections with mortality rates in excess of 50% in immunocompromised individuals These virulence factors were necessary for the site-specific cleavage of IL-36gamma to generate biologically active IL-36gamma. In earlier work, Stacey and his team had demonstrated that "in psoriasis, IL-36gamma is processed by cathepsin-S to become bioactive," he said. "However, the mechanisms by which IL-36gamma was released and activated during microbial infection were not known, and this study demonstrates that, IL-36gamma is activated as a direct result of secreted proteolytic virulence factors by invasive epithelial pathogens."
The work now published by Stacey and his team illustrates the potential for microbial infection to act as a trigger for IL-36-mediated pathological inflammation in susceptible individuals. Earlier work has already shown that IL-36gamma-Ser18, the main product of cathepsin S-dependent IL-36gamma cleavage, can induce psoriasiform changes in human skin-equivalent models.
Stacey said that his team's work "is not only significant from a mechanistic perspective but may also provide new therapeutic strategies for the treatment of infection mediated epithelial conditions, especially as IL-36gamma is strongly upregulated in skin lesions and can discriminate between harmless commensal microbes and potentially damaging pathogens."
Scientifically, the findings provide new insights into a critical mechanism whereby host organisms can discriminate invasive pathogens from harmless microbes. Stacey said that the study "helps build a more complete picture of how IL-36-mediated signaling is actually initiated in the context of epithelial infection, giving us a greater understanding of immune defenses at epithelial barriers. A better characterization of the pathways contributing to the chronic inflammation following pathogenic invasion will facilitate the development of new effective drugs for mitigating the severe inflammation mediated damage, that is often the cause of morbidities." The team also plans to look at other IL-36gamma mediated chronic disorders, especially those with an infectious component, such as cervical cancer.