Immunology & Inflammation

Induction of cytokines during the innate immune response to persistent viruses

Central hypothesis Theiler's VirusAll humans are infected with viruses throughout their lifetime. While most of these virus infections stimulate innate and adaptive immunity and are eliminated, it is estimated that each human is infected by 8-10 viruses that persistently stimulate the immune systems and are never eliminated. It is hypothesized that in some cases these persistent viral infections lead to diseases, such as Multiple Sclerosis (MS). Dr. Petro has had funding from the National Multiple Sclerosis Society, the Nebraska Center for Virology, and the Nebraska Center for Cell Signaling to investigate a mouse model of human MS in which a persistent viral infection in mice triggers symptoms similar to MS in humans. As a model for persistent viral infections, his laboratory is focusing upon the innate immune response to the Theiler's Murine Encephalomyelitis Virus (TMEV). In some strains of mice TMEV persistently infects macrophages of the immune system but is never eliminated within the lifetime of the individual mouse. In other strains of mice, TMEV is eliminated within a few weeks. As a result, susceptible mice with persistent TMEV infections end up with a neuroinflammatory disease similar to human MS. Dr. Petro has seen that failures within the innate immune system are causes of TMEV persistence. The innate immune response to viral infection depends on Interferon Response Factor-3 (IRF-3), Type I interferons (IFN-alpha/beta), IL-12, and IFN-gamma.

Other cytokines that are induced, such as IL-23, do not cause virus elimination but likely promote MS-like disease. In addition to innate cytokine production, apoptosis (programmed cell death) is triggered in virus-infected cells. Apoptosis prevents viral persistence and infection into surrounding cells. Induction of Type I interferons, repression of IL-12, and apoptosis are all dependent on a transcription factor called interferon regulatory factor 3 (IRF-3). Dr. Petro has found that IRF-3 of mice that cannot eliminate TMEV differs from IRF-3 of mice that can control this virus. Similar differences in IRF-3 are found in human populations. Therefore, interferon, IL-12, and apoptosis are altered in macrophages from susceptible mice infected with TMEV. Interestingly, TMEV and many viruses like it encode a protein, termed L-protein, that interacts with IRF-3 and modulates its activity and another protein L*-protein that interferes with apoptosis. The central hypothesis of this line of research in Dr. Petro’s lab is that differences in host IRF-3 or viral proteins that modulate IRF-3 underlie persistent infections with certain viruses.

Cytokine Expression

Persistent microbial infections lead to pathology in host tissue because of persistent inappropriate cytokines produced by the inflammatory and immune systems. Therefore, cytokines produced by cells of the bone microenvironment are being examined in in vitro and in vivo bone turnover systems to define more clearly their bone resorptive effects during chronic gingival infection. To gain an insight into the finer details of cytokine induction pathways, promoter constructs of inflammatory cytokine genes are being transfected into primary cells and cell lines from the bone microenvironment to study mechanisms of gene activation and cytokine production. In particular, the influence of bacterial virulence factors, estrogen levels, and new pharmaceutical candidates are being tested using these promoter constructs of inflammatory cytokine genes. Long-term goals include defining gene signaling factors which could be manipulated to direct cytokine production to yield net bone formation. Stem cell lines from individuals could be developed to reseed periodontal defects and stimulate bone regeneration.

Regulation of Cytokine Production

It is quite clear that CD4 T cell and APC cytokine production is induced by appropriate ligation of cell membrane proteins, transmembrane signaling cascades, and activation of cytokine-specific transcriptional factors interacting with promoters of cytokine genes. However, excessive cytokine induction occurs when inflammatory and immune responses are stimulated by persistent microbial infection. The Bioregulation Group is committed to elucidation of cytokine induction pathways and discovery of counter-measures that control cytokine expression. Dr. Petro is using a plant compound called resveratrol which has been shown to be anti-inflammatory and which controls immune responses. The central hypothesis here is that resveratrol mimicking of internal cell signaling and transcriptional mechanisms controls inflammation and immune responses. In the end, resveratrol induces counteracting cytokines or factors that dampen inflammatory and immune responses. New information about resveratrol could be used to promote protective immune responses so that infecting microbes are eliminated or controlled, chronic inflammation is prevented, and immune responses do not become exhausted. In order to achieve these goals, Dr. Petro is using this unique compound as a therapy in experimental model systems to manipulate signaling and transcriptional pathways.