Persistent viral infection elevates central nervous system MHC class I through chronic production of interferons

Persistence of even the stealthiest viruses can perturb immune function either to the benefit or detriment of the host. Lymphocytic choriomeningitis virus (LCMV) establishes lifelong, systemic persistence when introduced in utero or at birth. Despite a highly evolved host-pathogen relationship, LCMV cannot escape detection by the innate immune system, which results in chronic stimulation of the type 1 IFN pathway in adult carrier mice. In this study we demonstrate that IFN-beta is chronically up-regulated in peripheral lymphoid and nonlymphoid tissues (but not the CNS) of mice persistently infected from birth with LCMV and that dendritic cells (DCs) represent at least one source of IFN-beta. Interestingly, chronic stimulation of this innate pathway significantly elevated MHC class I expression in the CNS as well as the periphery. Elevated MHC I expression was dependent on IFN-alphabeta receptor but not MyD88-dependent signaling, as only genetic deletion of the former reduced MHC I to normal levels. An increase in circulating virus was also observed in the IFN-alphabeta receptor deficient carrier mice, signifying that type I IFN continually exerts anti-viral pressure during a LCMV carrier state. Finally, to determine whether heightened CNS MHC I could be therapeutically corrected, we purged LCMV carrier mice of their persistent infection using adoptive immunotherapy. This treatment significantly reduced CNS MHC I expression. Collectively, these data demonstrate that even a well adapted pathogen can chronically stimulate the innate immune system and consequently alter the expression of Ag presenting machinery in an immunologically specialized compartment like the CNS.