Clearance of Herpes Simplex Virus Type 2 by CD8+ T Cells Requires Gamma Interferon and either Perforin- or Fas-Mediated Cytolytic Mechanisms

Herpes simplex virus type 2 (HSV-2) infects epithelial cells in the genital mucosa, spreads to the sensory ganglia via retrograde transport, and establishes a lifelong latent infection in sensory neurons (50). The virus periodically reactivates and descends sensory neurons via anterograde transport, resulting in development of recurrent lesions at or near the site of primary infection or in shedding of infectious virus in the absence of disease symptoms. The primary and recurrent lesions of immunocompetent individuals are generally self limiting and are resolved primarily by cell-mediated immune mechanisms. Recurrent disease is less well controlled in immunocompromised individuals, resulting in more frequent recurrences and sometimes severe mucocutaneous disease manifestations. Studies of HSV infection in human immunodeficiency virus (HIV)-infected individuals suggested that the severity of HSV disease could be inversely correlated with the number of HSV-specific CD8+ T cells (39). Studies of recurrent HSV lesions in immunocompetent humans have demonstrated the early infiltration of CD4+ T cells and macrophages, local production of IFN-γ, and late arrival of CD8+ T cells at the site of HSV infection. Both CD4+ and CD8+ T lymphocytes capable of IFN-γ secretion and HSV-specific cytolysis have been isolated from human herpetic lesions (10) and clearance of infectious virus, and resolution of lesions has been correlated with the detection of HSV-specific cytolytic T-lymphocyte activity (10, 22-23). However, the role for these cytolytic and noncytolytic immune mechanisms in resolution of HSV-2 genital infections is not well understood. Murine models of HSV-2 genital infection have also demonstrated the importance of cell-mediated immunity in clearance of HSV and have provided evidence for both cytolytic and noncytolytic mechanisms in resolution of vaginal HSV-2 infections. Mice depleted of T cells are unable to resolve a genital HSV infection, but mice depleted of either CD4+ or CD8+ subset can ultimately resolve the infection, although clearance is delayed (24, 28, 30, 34-35). HSV-specific T cells exhibiting ex vivo cytolytic function have been isolated from the vaginal lumen of HSV-2-inoculated mice at a time concomitant with virus clearance (32). T-cell-produced IFN-γ has also been detected in vaginal secretions within 24 h of HSV-2 challenge of HSV-immune mice, and neutralization of IFN-γ by treatment with specific antibody has been shown to severely impair resolution of a primary genital HSV-2 infection as well as rechallenge infections of HSV-immune mice (30, 35). In contrast, other studies utilizing an HSV-1 model failed to detect a dominant role for IFN-γ and instead suggested that a mechanism involving major histocompatibility complex (MHC) class I was more important for resolution of HSV infection (15). We examined the requirement for IFN-γ and cytolytic mechanisms in CD8+ T-cell-mediated virus clearance using an adoptive transfer strategy. Ovalbumin (OVA)-specific CD8+ T cells transferred to irradiated recipients cleared an engineered thymidine kinase-deficient, OVA-expressing virus (HSV-2 tk− OVA) from the genital mucosa. Virus clearance was abrogated by treatment of recipients with anti-IFN-γ antibody or by transfer of OT-I CD8+ T cells from mice genetically deficient in IFN-γ. To examine the requirement for cytolytic mechanisms to clear HSV-2 from the genital epithelium, mice deficient in perforin and Fas-Fas ligand interactions were challenged with HSV-2 tk− OVA. While a significant decrease in virus titer was observed soon after viral challenge, mice lacking both perforin- and Fas-mediated cytolytic mechanisms were unable to completely clear the infection. These data suggest that both IFN-γ and T-cell-mediated cytolytic mechanisms are required for complete clearance of HSV-2 from the genital epithelium.