Over 50% of patients who survive neuroinvasive infection with West Nile virus (WNV) exhibit chronic cognitive sequelae1,2. Although thousands of cases of WNV-mediated memory dysfunction accrue annually3, the mechanisms responsible for these impairments are unknown. The classical complement cascade, a key component of innate immune pathogen defence, mediates synaptic pruning by microglia during early postnatal development4,5. Here we show that viral infection of adult hippocampal neurons induces complement-mediated elimination of presynaptic terminals in a murine WNV neuroinvasive disease model. Inoculation of WNV-NS5-E218A, a WNV with a mutant NS5(E218A) protein6,7 leads to survival rates and cognitive dysfunction that mirror human WNV neuroinvasive disease. WNV-NS5-E218A-recovered mice (recovery defined as survival after acute infection) display impaired spatial learning and persistence of phagocytic microglia without loss of hippocampal neurons or volume. Hippocampi from WNV-NS5-E218A-recovered mice with poor spatial learning show increased expression of genes that drive synaptic remodelling by microglia via complement. C1QA was upregulated and localized to microglia, infected neurons and presynaptic terminals during WNV neuroinvasive disease. Murine and human WNV neuroinvasive disease post-mortem samples exhibit loss of hippocampal CA3 presynaptic terminals, and murine studies revealed microglial engulfment of presynaptic terminals during acute infection and after recovery. Mice with fewer microglia (Il34−/− mice with a deficiency in IL-34 production) or deficiency in complement C3 or C3a receptor were protected from WNV-induced synaptic terminal loss. Our study provides a new murine model of WNV-induced spatial memory impairment, and identifies a potential mechanism underlying neurocognitive impairment in patients recovering from WNV neuroinvasive disease.

Pattern recognition receptor (PRR) detection of pathogen-associated molecular patterns (PAMPs), such as viral RNA, drives innate immune responses against West Nile virus (WNV), an emerging neurotropic pathogen. Here we demonstrate that WNV PAMPs orchestrate endothelial responses to WNV via competing innate immune cytokine signals at the blood-brain barrier (BBB), a multicellular interface with highly specialized brain endothelial cells that normally prevents pathogen entry. While Th1 cytokines increase the permeability of endothelial barriers, type I interferon (IFN) promoted and stabilized BBB function. Induction of innate cytokines by pattern recognition pathways directly regulated BBB permeability and tight junction formation via balanced activation of the small GTPases Rac1 and RhoA, which in turn regulated the transendothelial trafficking of WNV. In vivo, mice with attenuated type I IFN signaling or IFN induction (Ifnar−/− Irf7−/−) exhibited enhanced BBB permeability and tight junction dysregulation after WNV infection. Together, these data provide new insight into host-pathogen interactions at the BBB during neurotropic viral infection.

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Francisella tularensis is a gram-negative intracellular bacterium that can induce lethal respiratory infection in humans and rodents. However, little is known about the role of innate or adaptive immunity in protection from respiratory tularemia. In the present study, the role of interleukin-12 (IL-12) in inducing protective immunity in the lungs against intranasal infection of mice with the live vaccine strain (LVS) of F. tularensis was investigated. It was found that gamma interferon (IFN-␥) and IL-12 were strictly required for protection, since mice deficient in IFN-␥, IL-12 p35, or IL-12 p40 all succumbed to LVS doses that were sublethal for wild-type mice. Furthermore, exogenous IL-12 treatment 24 h before intranasal infection with a lethal dose of LVS (10,000 CFU) significantly decreased bacterial loads in the lungs, livers, and spleens of wild-type BALB/c and C57BL/6 mice and allowed the animals to survive infection; such protection was not observed in IFN-␥-deficient mice. The resistance induced by IL-12 to LVS infection was still observed in NK cell-deficient beige mice but not in CD8 ؊/؊ mice. These results demonstrate that exogenous IL-12 delivered intranasally can prevent respiratory tularemia through a mechanism that is at least partially dependent upon the expression of IFN-␥ and CD8 T cells.

IL-1R1 signaling drives T cell activation in the CNS via effects on DC activation.

In 1935, the olfactory route was hypothesized to be a portal for virus entry into the central nervous system (CNS). This hypothesis was based on experiments in which nasophayngeal infection with poliovirus in monkeys was prevented from spreading to their CNS via transection of olfactory tracts between the olfactory neuroepithelium (ONE) of the nasal cavity and the olfactory bulb (OB). Since then, numerous neurotropic viruses have been observed to enter the CNS via retrograde transport along axons of olfactory sensory neurons whose cell bodies reside in the ONE. Importantly, this route of infection can occur even after subcutaneous inoculation of arboviruses that can cause encephalitis in humans. While the olfactory route is now accepted as an important pathway for viral entry into the CNS, it is unclear whether it provides a way for infection to spread to other brain regions. More recently, studies of antiviral innate and adaptive immune responses within the olfactory bulb suggest it provides early virologic control. Here we will review the data demonstrating that neurotropic viruses gain access to the CNS initially via the olfactory route with emphasis on findings that suggest the OB is a critical immunosensory effector organ that effectively clears virus.

The cardinal features of asthma include pulmonary inflammation and airway hyperresponsiveness (AHR). Classically, asthma, specifically allergic asthma, has been attributed to a hyperactive Th2 cell immune response. However, the Th2 cell-mediated inflammation model has failed to adequately explain many of the clinical and molecular aspects of asthma. In addition, the outcomes of Th2-targeted therapeutic trials have been disappointing. Thus, asthma is now believed to be a complex and heterogeneous disorder, with several molecular mechanisms underlying the airway inflammation and AHR that is associated with asthma. The original classification of Th1 and Th2 pathways has recently been expanded to include additional effector Th cell subsets. These include Th17, Th9 and Treg cells. Emerging data highlight the involvement of these new Th cell subsets in the initiation and augmentation of airway inflammation and asthmatic responses. We now review the roles of these recently classified effector Th cell subsets in asthmatic inflammation and the insights they may provide in addition to the traditional Th2 paradigm. The hope is that a clearer understanding of the inflammatory pathways involved and the mediators of inflammation will yield better targeted therapeutics.

Immune cell entry into the virally infected central nervous system (CNS) is vital for promoting viral clearance yet may contribute to neuropathology if not rigorously regulated. We previously showed that signaling through the interleukin 1 receptor (IL-1R1) is critical for effector T cell reactivation and virologic control within the CNS during murine West Nile virus (WNV) encephalitis. WNV-infected IL-1R1−/− mice also display increased parenchymal penetration of CD8+ T cells despite lack of CD4-mediated full activation, suggesting dysregulation of molecular components of CNS immune privilege. Here, we show that IL-1 signaling regulates the CNS entry of virus-specific lymphocytes, promoting protective immune responses to CNS viral infections that limit immunopathology. Analysis of blood-brain barrier (BBB) function in the WNV-infected IL-1R1−/− mice revealed no alterations in permeability. However, parenchymal proinflammatory chemokine expression, including CCL2, CCL5 and CXCL10, was significantly upregulated, whereas microvasculature CXCL12 expression was significantly decreased in the absence of IL-1 signaling. We show that during WNV infection, CD11b+CD45hi infiltrating cells (macrophages) are the primary producers of IL-1β within the CNS and, through the use of an in vitro BBB model, that IL-1β promotes CXCR4-mediated T cell adhesion to brain microvasculature endothelial cells (BMECs). Of interest, IFNγ+ and CD69+ WNV-primed T cells were able to overcome CXCL12-mediated adhesion via down-regulation of CXCR4. These data indicate that infiltrating IL-1β-producing leukocytes contribute to cellular interactions at endothelial barriers that impart protective CNS inflammation by regulating the parenchymal entry of CXCR4+ virus-specific T cells during WNV infection.