Murine gammaherpesvirus-68 infects microglia and induces high levels of pro-inflammatory cytokine production

Viral infection of the pregnant cervix predisposes to ascending bacterial infection

Preterm birth is the major cause of neonatal mortality and morbidity, and bacterial infections that ascend from the lower female reproductive tract are the most common route of uterine infection leading to preterm birth. The uterus and growing fetus are protected from ascending infection by the cervix, which controls and limits microbial access by the production of mucus, cytokines, and antimicrobial peptides. If this barrier is compromised, bacteria may enter the uterine cavity, leading to preterm birth. Using a mouse model, we demonstrate, to our knowledge for the first time, that viral infection of the cervix during pregnancy reduces the capacity of the female reproductive tract to prevent bacterial infection of the uterus. This is due to differences in susceptibility of the cervix to infection by virus during pregnancy and the associated changes in TLR and antimicrobial peptide expression and function. We suggest that preterm labor is a polymicrobial disease, which requires a multifactorial approach for its prevention and treatment.

Vesicular stomatitis virus infects resident cells of the central nervous system and induces replication-dependent inflammatory responses

Vesicular stomatitis virus (VSV) infection of mice via intranasal administration results in a severe encephalitis with rapid activation and proliferation of microglia and astrocytes. We have recently shown that these glial cells express RIG-I and MDA5, cytosolic pattern recognition receptors for viral RNA. However, it is unclear whether VSV can replicate in glial cells or if such replication is required for their inflammatory responses. Here we demonstrate that primary microglia and astrocytes are permissive for VSV infection and limited productive replication. Importantly, we show that viral replication is required for robust inflammatory mediator production by these cells. Finally, we have confirmed that in vivo VSV administration can result in viral infection of glial cells in situ. These results suggest that viral replication within resident glial cells might play an important role in CNS inflammation following infection with VSV and possibly other neurotropic nonsegmented negative-strand RNA viruses.

Age-dependent pathogenesis of murine gammaherpesvirus 68 infection of the central nervous system

Gammaherpesvirus infection of the central nervous system (CNS) has been linked to various neurological diseases, including meningitis, encephalitis, and multiple sclerosis. However, little is known about the interactions between the virus and the CNS in vitro or in vivo. Murine gammaherpesvirus 68 (MHV-68 or (gamma)HV-68) is genetically related and biologically similar to human gammaherpesviruses, thereby providing a tractable animal model system in which to study both viral pathogenesis and replication. In the present study, we show the successful infection of cultured neuronal cells, microglia, and astrocytes with MHV-68 to various extents. Upon intracerebroventricular injection of a recombinant virus (MHV-68/LacZ) into 4-5-week-old and 9-10-week-old mice, the 4-5-week-old mice displayed high mortality within 5-7 days, while the majority of the 9-10-week-old mice survived until the end of the experimental period. Until a peak at 3-4 days post-infection, viral DNA replication and gene expression were similar in the brains of both mouse groups, but only the 9-10-week-old mice were able to subdue viral DNA replication and gene expression after 5 days post-infection. Pro-inflammatory cytokine mRNAs of tumor necrosis factor-alpha, interleukin 1beta, and interleukin 6 were highly induced in the brains of the 4-5-week-old mice, suggesting their possible contributions as neurotoxic factors in the agedependent control of MHV-68 replication of the CNS.

Neurogenic exacerbation of microglial and astrocyte responses to Neisseria meningitidis and Borrelia burgdorferi

Although glial cells are recognized for their roles in maintaining neuronal function, there is growing appreciation of the ability of resident CNS cells to initiate and/or augment inflammation following trauma or infection. The tachykinin, substance P (SP), is well known to augment inflammatory responses at peripheral sites and its presence throughout the CNS raises the possibility that this neuropeptide might serve a similar function within the brain. In support of this hypothesis, we have recently demonstrated the expression of high affinity receptors for SP (Neurokinin-1 (NK-1) receptors) on microglia and shown that this tachykinin can significantly elevate bacterially induced inflammatory prostanoid production by isolated cultures of these cells. In the present study, we demonstrate that endogenous SP/NK-1R interactions are an essential component in the initiation and/or progression of CNS inflammation in vivo following exposure to two clinically relevant bacterial CNS pathogens, Neisseria meningitidis and Borrelia burgdorferi. We show that in vivo elevations in inflammatory cytokine production and decreases in the production of an immunosuppressive cytokine are markedly attenuated in mice genetically deficient in the expression of the NK-1R or in mice treated with a specific NK-1R antagonist. Furthermore, we have used isolated cultures of microglia and astrocytes to demonstrate that SP can augment inflammatory cytokine production by these resident CNS cell types following exposure to either of these bacterial pathogens. Taken together, these studies indicate a potentially important role for neurogenic exacerbation of resident glial immune responses in CNS inflammatory diseases, such as bacterial meningitis.

Characterization of nucleotide-binding oligomerization domain (NOD) protein expression in primary murine microglia

We demonstrate that primary microglia express nucleotide-binding oligomerization domain (NOD) proteins that are thought to serve as novel pattern recognition receptors for bacterial peptidoglycan motifs. NOD2 is constitutively present in microglia and is upregulated following exposure to Borrelia burgdorferi or Neisseria meningitidis. Its expression is also elevated following exposure to Toll-like receptor (TLR) ligands and muramyl dipeptide (MDP), a putative ligand for NOD2. Microglia express essential downstream effector molecules for NOD2-mediated cell responses, and MDP augments TLR ligand-induced inflammatory cytokine production. Together these data suggest that NOD2 may contribute to microglial immune responses to bacterial pathogens.

Marek's Disease Virus–Induced Transient Paralysis Is Associated with Cytokine Gene Expression in the Nervous System

Marek's disease (MD)-associated transient paralysis (TP) was experimentally induced in chickens by intraperitoneal inoculation of RB1B strain of Marek's disease virus (MDV). Between 7 and 11 days post-infection (d.p.i.), neck and limb paralysis was observed in 18% of infected chickens, which was associated with various degrees of edema, vacuolation, perivascular cuffing of mononuclear cells, and glial cell infiltration mainly in the cerebrum, cerebellum, and brain stem. The chickens that were infected but did not progress to develop TP until 12 d.p.i. also had similar lesions suggestive of encephalitis in the cerebrum, cerebellum, and brain stem. Chickens infected with MDV had more interleukin (IL)-6, IL-12, and interferon (IFN)-gamma in their brain tissues compared to uninfected chickens. Moreover, IL-18 was significantly increased in brain tissues of birds showing clinical signs of TP compared to uninfected birds. Importantly, the expression of IL-6, IL-18, and IFN- gamma in brain tissues of MDV-infected chickens with signs of TP was significantly increased compared to that in asymptomatic MDV-infected birds. MDV genome load in the brain of chickens showing clinical signs of TP was higher than that in asymptomatic MDV-infected chickens but was not statistically significant. The lesions in the cervical, thoracic, and lumbar spinal cord segments in MDVinfected chickens were characterized mainly by perivascular cuffing of mononuclear cells irrespective of the group. The expression of mRNA for IL-18 and IFN-gamma genes was not significantly different in spinal cord tissues of chickens with TP compared to clinically normal, MDV-infected and noninfected chickens. These results suggest possible underlying immunologic mechanisms for MDV-induced TP.

Functional expression of NOD2, a novel pattern recognition receptor for bacterial motifs, in primary murine astrocytes

There is growing appreciation that resident brain cells can initiate and/or regulate inflammation after trauma or infection in the central nervous system (CNS). Recent studies from our laboratory have begun to shed light on the mechanisms by which astrocytes perceive bacterial challenges by demonstrating the functional expression of Toll-like receptors (TLR) in this cell type. In the present study, we demonstrate that astrocytes also express members of the novel nucleotide-binding oligomerization domain (NOD) family of proteins that can serve as cytosolic pattern recognition receptors. We show that isolated cultures of murine astrocytes constitutively express robust levels of NOD2, a molecule that can recognize a minimal peptidoglycan motif. Expression of NOD2 is significantly upregulated after exposure to two disparate and clinically relevant bacterial pathogens of the CNS, Borrelia burgdorferi and Neisseria meningitidis. Similarly, NOD2 protein expression is elevated after exposure to specific bacterial ligands for TLRs. Importantly, we show that astrocytes express Rip2 kinase, an essential downstream effector molecule for NOD-mediated cell responses, and demonstrate that this expression is upregulated after bacterial challenge. Furthermore, we confirm the functional nature of NOD2 in astrocytes by demonstrating that a specific ligand for this receptor induces significant inflammatory cytokine production and augments immune responses induced by TLR ligation. Taken together, the present demonstration that astrocytes express functional NOD2 proteins may represent a potentially important mechanism by which this glial cell type initiates either protective host responses within the brain or the progression of damaging CNS inflammation.

Murine gammaherpesvirus-68 elicits robust levels of interleukin-12 p40, but not interleukin-12 p70 production, by murine microglia and astrocytes

Murine gammaherpesvirus-68 (gamma HV-68) is a tractable model to investigate the pathophysiology of human gammaherpesvirus infections, including Epstein-Barr virus (EBV). Herpesvirus infections are thought to play a role in the pathology of damaging, inflammatory diseases states of the central nervous system (CNS), such as multiple sclerosis. The ability of the host to mount a strong cell-mediated immune response is critical in determining the outcome of viral infections. Interleukin (IL)-12 is an important inflammatory cytokine that plays a pivotal role in the development of protective cell-mediated immune responses to viral infections. Given recent reports of associations between gammaherpesvirus infections and inflammatory disorders of the CNS, the authors investigated the ability of gamma HV-68 to induce the production of bioactive IL-12 in resident CNS cell types. In the present study, the authors demonstrate that gamma HV-68 infection is a potent stimulus for IL-12p40 production by murine microglia and astrocytes. However, despite the elevated expression of mRNA encoding IL-12p40 subunit, concomitant with robust secretion of IL-12p40 protein, gamma HV-68 failed to elicit the production of the bioactive IL-12p70 heterodimer. This failure did not result from an absence of T lymphocyte-derived signals or interactions between CNS cell types as determined by coculture studies. Taken together, these data suggest that the resident CNS cell types, astrocytes and microglia, are not significant sources of proinflammatory IL-12p70 in response to gammaherpesvirus infection. Indeed, the production of IL-12p40 may point to an anti-inflammatory role for these cells during herpesvirus infections of the CNS.

Substance P Augments Borrelia burgdorferi-Induced Prostaglandin E2 Production by Murine Microglia

Substance P is a ubiquitous CNS neuropeptide and has recently been demonstrated to augment immune cell function during inflammatory events. Central to the ability of substance P to modulate immune cell function is the interaction of substance P with the substance P neurokinin-1 receptor expressed by a variety of immune cells, including microglia. CNS involvement during Lyme disease can occur when Borrelia burgdorferi, the causative agent of Lyme disease, gains access to the CNS. In the present study, we demonstrate that substance P augments B. burgdorferi-induced expression of mRNA encoding COX-2 and subsequent secretion of PGE(2) by cultured, murine microglia. Furthermore, this effect is associated with the ability of substance P to enhance B. burgdorferi-induced NF-kappa B activation, as demonstrated by increased nuclear localization of the p65 (RelA) subunit of NF-kappa B in these cells. Interestingly, we demonstrate that substance P augments B. burgdorferi-induced expression of mRNA encoding two PGE(2) receptors, E-prostanoid receptor subtypes 2 and 4, as well as each receptor protein. In addition, these effects are mediated via interactions between substance P and its high affinity receptor, as evidenced by the absence of augmented PGE(2) synthesis in the presence of a specific neurokinin-1 receptor antagonist or in cells genetically deficient in the expression of these receptors. Taken together, the present demonstration that substance P can exacerbate B. burgdorferi-induced inflammatory responses in microglia in vitro may indicate a role for this neuropeptide in the development of CNS inflammation observed during human neuroborreliosis.