Herpesvirus Infection Augments the Response to a Subsequent Fibrotic Challenge in the Lung Via the Recruitment of Fibrocytes and the Induction of Pro-fibrotic Factors.

Abstract

No effective treatment currently exists for pulmonary fibrosis. The cause of pulmonary fibrosis is unknown, and little is known about its pathobiology. While pulmonary fibrosis is likely multifactorial, evidence is accumulating to implicate gammaherpesviruses as cofactors in the pathogenesis of pulmonary fibrosis. We developed a murine model to test the hypothesis that latent gammaherpesvirus infection can augment the response to a subsequent pulmonary fibrotic challenge. Mice were infected intranasally with murine gammaherpesvirus (γHV-68) prior to an intratracheal fibrotic stimulus with either fluorescein isothiocyanate (FITC) or bleomycin. γHV-68 was latent in the lung by 14 days after infection. γHV-68 infection 14-70 days prior to the fibrotic stimulus augmented fibrosis measured 21 days later. Furthermore, latent γHV-68 infection induced fibrosis in response to a sub-threshold fibrotic challenge. Although FITC challenge initiated low-level lytic gene transcription by γHV-68, reactivation from latency was not necessary for viral-induced augmentation of fibrosis. Mechanisms that potentially contribute to latent γHV-68-induced augmentation of fibrosis were explored. Inflammation, CCL2, CCL12, transforming growth factor (TGF)-β1, and fibroctye number were increased in the lungs during γHV-68 latency. Inflammation, fibrocyte number, and TGF-β1 levels were further increased in the lungs of latently-infected mice that were administered FITC compared to mock-infected mice that received FITC. We found that γHV-68 infection can alter the phenotype of alveolar epithelial cells (AECs) to promote a pro-fibrotic environment. Specifically, AECs isolated from latently-infected mice produced more CCL2, CCL12, TGF-β1 and cysteinyl leukotrienes (cysLTs) than AECs from uninfected mice. While the actions of CCL2 and CCL12 to recruit fibrocytes and the actions of TGF-β1 to promote fibrocyte differentiation were known, the effects of cysLTs on fibrocytes were unknown. Our studies demonstrate that cysLTs are critical mediators of fibrosis with both autocrine and paracrine effects on fibrocyte proliferation via cysLT1 receptors. In sum, gammaherpesvirus infection augmented subsequent fibrosis in mice via the induction of pro-fibrotic factors and the recruitment of fibrocytes. Our data complement existing human and animal literature supporting a role for gammaherpesviruses as cofactors in the pathogenesis of pulmonary fibrosis and provide new mechanistic insight into the disease pathogenesis.