Immunoregulation by Chromatin-Modifying Modulators

The mammalian immune system effectively fights most viruses and other pathogenic invaders. At the same time a sophisticated multi-layered regulatory system is required to fine-tune and resolve the immune response to prevent immunopathology or autoimmunity. Based on a quantitative proteomics approach with lung tissue of influenza A virus-infected mice we previously had identified genes with conserved epigenetic domains but yet still undefined function in the immune response. These genes, the histone methyl-transferase Setdb2 and the PHD domain containing Phf11, are located in close genomic proximity right next to each other and can form fusion transcripts. Notably, human single nucleotide polymorphisms in both SETDB2 and PHF11 have been genetically mapped to increased susceptibility to asthma. Our preliminary data reveal that these genes are significantly up-regulated upon toll-like receptor stimulation and viral infection. Interferon receptor signaling was found to be indispensable for the transcriptional induction of Setdb2 and Phf11 and as such we consider them bona fide interferon-stimulated genes. Our unpublished data from gene-targeted mice suggest that Setdb2 is involved in the regulation of both innate and adaptive antiviral immune responses. In this project proposal we aim to mechanistically dissect the molecular contributions of the chromatin-modifying modulators Setdb2 and Phf11 to the immune system. A complementary approach of molecular cell biology, proteomics, biochemistry and immunology will be employed to delineate how these genes are regulated, which substrates are methylated by Setdb2 (histones vs. non-histone proteins) and whether Phf11 may modulate the immune-related function of Setdb2. We also aim to elucidate the individual roles of these genes in the regulation of the immune response using cell culture systems as well as available Setdb2 - targeted mice. We hypothesize that Setdb2 and Phf11 form a novel regulatory node that ensures appropriate control of the immune response. A better understanding of the underlying molecular processes in the context of immunoregulation may pave the way for improved therapies for infectious and sterile inflammatory diseases.

During seasonal flu epidemics, respiratory infections with influenza virus lead to a sudden onset of high fever, cough, headache, runny nose, sore throat and other signs of fatigue and general discomfort. High-risk groups such as very young, elderly or chronically ill people may develop severe disease, which results in approximately 500.000 deaths annually (WHO). These severe influenza cases are often associated with secondary bacterial infections, so-called superinfections.Despite the significant clinical and socioeconomic impact, the molecular mechanisms of how influenza virus causes superinfection remain poorly understood and preclude the development of novel therapies. Interestingly, past studies suggested that the virus is not the primary cause of disease. Instead, the trigger for disease seems to be the suppression of the immune system by the virus and, as a consequence, the increased susceptibility to bacterial superinfection. This is supported by the clinical observation that influenza patients often suffer from infections with bacteria such as streptococci. In this study, we used a superinfection model of influenza virus streptococci and uncovered a missing molecular link that may explain how viruses predispose to bacterial superinfection. We found that the molecule interferon, which plays a central role in the antiviral response of the patient, induces the enzyme Setdb2. Surprisingly, upon removal of Setdb2 by using tools of molecular biology, we observed a milder course of superinfection. To understand this phenomenon, we studied the molecular function of Setdb2. We found that this enzyme modifies the packaging of DNA and blocks the expression of antibacterial genes such as the chemoattractant Cxcl1. Cxcl1 was already known to be crucial for the recruitment of a specific type of cells of the immune system, called neutrophils, to eliminate the bacteria. Blocking Cxcl1 may provide a benefit to the host by avoiding unnecessary excessive inflammation during viral infection. At the same time, however, Setdb2 makes the organism more vulnerable to superinfection. Together, our findings discovered a novel crosstalk between the antiviral molecule interferon and the enzyme Setdb2, which weakens the antibacterial response as a consequence of the influenza virus infection and thereby leads to severe disease. Current and future research aims to elucidate the complex molecular interplay between the virus, Setdb2, the bacteria and the immune system. This may open new therapeutic avenues with the goal to potentially interfere with the function of Setdb2. Thereby, the immune system of influenza patients at risk may be reinvigorated leading to an increased antibacterial resistance. (This text was taken from the official press release of CeMM.)