Latent Herpesvirus Infection Augments Experimental Pulmonary Fibrosis

Graduate Program in Immunology, Department of Internal Medicine, University of Michigan Medical School, Ann Arbor, MI, USA.
American Journal of Respiratory and Critical Care Medicine (Impact Factor: 13). 03/2010; 181(5):465-77. DOI: 10.1164/rccm.200905-0798OC
Source: PubMed


No effective treatment exists for idiopathic pulmonary fibrosis, and its pathogenesis remains unclear. Accumulating evidence implicates herpesviruses as cofactors (either initiating or exacerbating agents) of fibrotic lung disease, but a role for latent herpesvirus infection has not been studied.
To develop a murine model to determine whether latent herpesvirus infection can augment fibrotic responses and to gain insight into potential mechanisms of enhanced fibrogenesis.
Mice were infected with murine gammaherpesvirus 14 to 70 days before a fibrotic challenge with fluorescein isothiocyanate or bleomycin so that the virus was latent at the time of fibrotic challenge. Measurements were made after viral infection alone or after the establishment of fibrosis.
gammaHerpesvirus is latent by 14 days post infection, and infection 14 to 70 days before fibrotic challenge augmented fibrosis. Fibrotic augmentation was not dependent on reactivation of the latent virus to a lytic state. Total cell numbers and fibrocyte numbers were increased in the lungs of latently infected mice administered fibrotic challenge compared with mock-infected mice that received fibrotic challenge. Latent infection up-regulates expression of proinflammatory chemokines, transforming growth factor-beta1, and cysteinyl leukotrienes in alveolar epithelial cells.
Latent gammaherpesvirus infection augments subsequent fibrotic responses in mice. Enhanced fibrosis is associated with the induction of profibrotic factors and the recruitment of fibrocytes. Our data complement existing human and animal data supporting the hypothesis that gammaherpesviruses can serve as initiating cofactors in the pathogenesis of pulmonary fibrosis.

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    • "Proof of concept experiments of the involvement of herpesviruses in lung fibrosis come from experimental models of pulmonary fibrosis with the MHV-68 murine gamma-herpesvirus. Importantly, experimentally established pulmonary latent infection of mice with MHV-68 could confer higher susceptibility to bleomycin or FITC-induced fibrosis [79] in comparison to the uninfected control mice, thereby supporting a multiple/recurrent hit hypothesis where the herpesvirus presence alters the lung microenvironment and acts as a cofactor in experimentally induced models of pulmonary fibrosis. "
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    • "Nevertheless, some attempts have been made to identify mechanisms that could contribute to such exacerbation events. For example, viral infections have frequently been observed in cases of lung fibrosis [11], and viral accelerations of lung fibrosis have been successfully modeled in mice and are characterized by Th1 cytokine and fibrogenic growth factor expression that is consistent with activation of innate immunity [12], [13]. Viral particles are recognized by Pathogen Associated Molecular Pattern (PAMP) receptors in the Toll-Like Receptor (TLR) family and are able to activate an innate immune response in the lung. "
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    • "We have shown previously that alveolar epithelial cells and mesenchymal cells are target cells for viral infection within the lung [31,32]. Furthermore, lung epithelial cells [27] and fibroblasts [28) have been reported to express TLR-9. "
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    ABSTRACT: We have shown previously that murine gammaherpesvirus 68 (γHV68) infection exacerbates established pulmonary fibrosis. Because Toll-like receptor (TLR)-9 may be important in controlling the immune response to γHV68 infection, we examined how TLR-9 signaling effects exacerbation of fibrosis in response to viral infection, using models of bleomycin- and fluorescein isothiocyanate-induced pulmonary fibrosis in wild-type (Balb/c) and TLR-9-/- mice. We found that in the absence of TLR-9 signaling, there was a significant increase in collagen deposition following viral exacerbation of fibrosis. This was not associated with increased viral load in TLR-9-/- mice or with major alterations in T helper (Th)1 and Th2 cytokines. We examined alveolar epithelial-cell apoptosis in both strains, but this could not explain the altered fibrotic outcomes. As expected, TLR-9-/- mice had a defect in the production of interferon (IFN)-β after viral infection. Balb/c fibroblasts infected with γHV68 in vitro produced more IFN-β than did infected TLR-9-/- fibroblasts. Accordingly, in vitro infection of Balb/c fibroblasts resulted in reduced proliferation rates whereas infection of TLR-9-/- fibroblasts did not. Finally, therapeutic administration of CpG oligodeoxynucleotides ameliorated bleomycin-induced fibrosis in wild-type mice. These results show a protective role for TLR-9 signaling in murine models of lung fibrosis, and highlight differences in the biology of TLR-9 between mice and humans.
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