Many inflammatory processes directly impact the function of the central nervous system (CNS) and give rise to human diseases. For example, acute infection of the CNS can induce a variety of disease states such as meningitis and encephalitis. Meningitis occurs when microbes infect the lining of the brain, whereas encephalitis is usually caused by infection of the brain itself. Epidemiological studies estimate that viral meningitis is induced with a peak monthly incidence of 1 in 100,000 persons, particularly in temperate climates. The disease is associated with symptoms that include fever, headache, stiffness of the neck, and seizures. Enteroviruses are the most common cause of viral meningitis, accounting for approximately 75-90% of the cases. Other meningitis-inducing viruses in humans include herpesviruses, human immunodeficiency virus-1, arbovirus, mumps virus, and lymphocytic choroimeningitis virus (LCMV). While complications associated with enterovirus-induced meningitis (the most common viral meningitis) in adults are rare, and are often seen in the immunocompromised, studies have shown that infection of children less than one year of age can result in mild to moderate neurological disability by the age of 5. At the other end of the spectrum, herpesviruses induce an array of CNS disorders that include encephalitis, myelitis, and meningitis, and these disorders have a very high mortality rate if left untreated. Because so many microbes have the capacity to infect and injure the CNS, it is important to uncover potential routes to pathogenesis. One of our main interests is to mechanistically define the impact of acute infections on the CNS and establish treatments to ameliorate adverse symptoms associated with these infections. We study viral (LCMV, vesicular stomatitis virus), parasitic (plasmodium berghei), and fungal (cryptococcus neoformans) infections to identify the similarities and differences in how the immune system responds to these different challenges. We also study sterile inflammatory responses (i.e., traumatic brain injury) to provide insights into how CNS immune cells respond to damage in the absence of an infectious agent. To advance our understanding of neural-immune interactions during CNS inflammatory diseases, we utilize a contemporary approach referred to as intravital two-photon laser scanning microscopy (TPM), which allows us to watch immune cells operate in the living brain in real time. This is accomplished by using fluorescently tagged immune cells and pathogens. By using fluorescent tags, the position of the pathogen can be studied in relation to innate (e.g. microglia, monocytes, macrophages, neutrophils, dendritic cells) and adaptive (e.g. microbe-specific CD8 T cells, CD4 T cells, B cells) immune cells as a disease develops. We can also administer therapeutic compounds into the viewing window (transcranial delivery) and watch how this locally influences the inflammatory process in real time. This powerful approach allows us to evaluate the efficacy of potential therapeutics at the site of disease. Using the LCMV model of viral meningitis, we demonstrated by TPM that virus-specific cytotoxic lymphocytes (CTL) drive acute onset seizures during meningitis by massively recruiting myelomonocytic cells (monocytes and neutrophils), which damage meningeal blood vessels and compromise the blood-cerebral spinal fluid (CSF) barrier. Virus-specific CTL participate in myelomonocytic cell recruitment by directly producing chemokines (CCL3, 4, and 5) that attract them. These data revised our thinking about viral meningitis by demonstrating that CTL do not always cause pathogenesis by releasing of cytotoxic effector molecules;rather, they can also contribute to CNS disease by recruiting pathogenic innate immune cells. Breakdown of vasculature by innate immune cells appears to be a general inflammatory reaction induced by CNS infection, as we have observed similar pathology following encephalitic virus (e.g. vesicular stomatitis virus) and parasitic (e.g. plasmodium berghei) infection. We are in the process of determining whether viral and parasitic CNS infections share common molecular mediators of vascular breakdown and also attempting to therapeutically disconnect the pathogenic link between innate and adaptive immune cells that results in vascular pathology. We also recently discovered that type I interferon (IFN-I) is an overarching master regulator of CNS immunity and may be amenable to therapeutic manipulation following infection. The CNS is inhabited by an elaborate network of specialized antigen presenting cells that include microglia, macrophages, and dendritic cells. These cells participate in CNS homeostasis, and when prompted, can initiate vigorous inflammatory responses associated with outcomes varying from tissue repair to neurological disease. Deciphering the innate pathways that trigger these cells to respond dynamically in the living brain is critical to the development of CNS immunomodulatory therapies. We sought novel mechanistic insights into how the brain responds innately to the establishment of a viral infection by conducting the first comprehensive genomic and real-time imaging analyses of a pure innate immune response mounted against a noncytopathic arenavirus (i.e., LCMV). LCMV does not kill the cells it infects, thus eliminating release of damage associated molecular pattern molecules that drive sterile injury responses. To determine how the brain responded innately to a noncytopathic virus, we analyzed patterns of gene expression at different stages of viral persistence. This revealed that the brain initially mounts a robust innate immune response (585 differentially regulated genes) that is silenced over time. Many of these were type I interferon stimulated genes (ISGs) that clustered into an interactive network indicative of coordinated anti-viral programming. To determine how this program translated into innate immune cell dynamics, we used TPM to visualize myeloid sentinels in real-time as they mounted their response to infection. Myeloid sentinels responded vigorously to infection through enhanced vascular patrolling and morphological transformations that promoted viral sequestration. Interestingly, this entire innate program (at the genomic and dynamic levels) was silenced as virus established widespread persistence in the brain. These data suggested that LCMV was able to quench the anti-viral program over time and that the brain contained a bottleneck in its innate program. This bottleneck was determined to be IFN-I signaling. To our surprise, we revealed that all myeloid cell dynamics and innate gene expression were completely silenced in the absence of IFN-I signaling, despite elevated viral loads. Importantly, we have identified an Achilles heel in the brains innate defense to a noncytopathic viral infection. The program has no redundancy and is linked exclusively to IFN-I. If IFN-I signaling is deactivated, the innate response is completely silenced, which likely explains why so many neurotropic viruses have acquired strategies to dampen the IFN-I pathway. Our results also suggest that it should also be possible to therapeutically dampen IFN-I signaling and ameliorate some of the adverse effects associated with CNS infection.

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Nayak, Debasis; Roth, Theodore L; McGavern, Dorian B (2014) Microglia development and function. Annu Rev Immunol 32:367-402
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