Characterisation of the immune response following natural herpes zoster reactivation and during post-herpetic neuralgia

Abstract

Varicella Zoster Virus (VZV) is responsible for two clinically distinct diseases; varicella (or chickenpox) during primary infection, and herpes zoster (or shingles) following reactivation. Herpes zoster involves active viral replication within the dorsal root ganglia (DRG), as well as in the innervated skin. Post-herpetic neuralgia (PHN) is the most common complication of herpes zoster. During PHN patients experience intense persisting pain generally in the same distribution as the original herpes zoster rash. There is very little known about the underlying pathology and pathogenesis of PHN. One of the main reasons underlying this lack of understanding is the high species-specificity of VZV; animal and in vitro models are quite limited and study requires suitable human tissue. Previous studies involving limited histological examinations have revealed the presence of an immune infiltration and general cell loss within the DRG innervating the affected area. Our laboratory gained access to rare DRG samples that were surgically-excised from a PHN-affected patient in an attempt to control their pain. Additionally we have obtained DRG samples obtained at post-mortem examination from herpes zoster-affected patients and normal control patients. This has provided a unique opportunity to perform a detailed characterisation of the immune response to the virus within human sensory ganglia, as well as investigate viral gene expression and viral genome levels within the samples. An immune infiltration was detected in the PHN-affected patient material, and was composed of both CD4+ and CD8+ T cells with cytolytic potential, as well as B cells. Immunofluorescent investigation did not demonstrate any viral gene expression within the tissue, however molecular techniques showed that the VZV genome was present in the PHN-affected ganglia material at a level approximately seven-fold greater than that seen in a ganglion obtained from a patient with an active herpes zoster rash at the time of death. In both primary infection and reactivation, VZV infection involves a similar exanthematous stage, and skin lesions are histopathologically indistinguishable from each other. However our understanding of the immune response in the skin during PHN is very limited. We obtained skin biopsy material from PHN-affected and control patients at least 6 months post-resolution of their herpes zoster-affected rash. This allowed the detailed examination of dendritic cell and T cell subsets present within the skin biopsy material of both patient types to identify any factors that may predispose an individual to ongoing pain or resolution. The skin material from the PHN-affected patients contained a greater number of the immune cell subsets examined. Yet, there was no VZV antigen expression detected within these tissue samples. We also gained access to an extensive tissue bank comprised of skin biopsies obtained from patients following diagnosis of herpes zoster. Each patient was followed for at least 6-12 months, and at each visit a biopsy was taken from the most painful area of skin and clinical details recorded. This tissue material was utilised in blinded immunofluorescent assays to identify different subsets of dendritic cells and T cells. There was no VZV antigen expression identified within this tissue material, however when biopsies were taken lesions were avoided. Different T cell and dendritic cell subsets were examined over time, and matched with the patients’ clinical histories in order to identify any factor that may correlate with reported pain quality and levels. Despite investigating multiple factors and cell types there was no marker identified that predisposed to ongoing pain or a prompt recovery. Overall this thesis provides the first evidence of an ongoing chronic inflammatory process consisting predominantly of both CD4+ and CD8+ T cells within DRG from a PHN-affected patient many years post resolution of the herpes zoster rash. Within this chronically inflamed tissue, there is a large amount of VZV-specific DNA. In stark contrast to this, there does not seem to be any ongoing immunohistological process within the skin of patients post-herpes zoster. These findings have increased our knowledge on the immune reaction within human ganglia and skin following herpes zoster and during PHN and suggest that the best immune correlates of VZV diseases arising from VZV reactivation in ganglia are those that occur within the affected DRG rather than those that occur at cutaneous sites.