Overview of the cellular immunity against JC virus in progressive multifocal leukoencephalopathy

The human polyomavirus JC (JCV) infects most healthy adults without causing any disease. In the setting of severe deficit of cell-mediated immunity, such as in acquired immunodeficiency syndrome (AIDS), malignancies or in organ transplant recipients, JCV can reactivate and cause progressive multifocal leukoencephalopathy (PML), a deadly demyelinating disease of the central nervous system. The humoral immune response, measured by the presence of virus-specific immunoglobulin G (IgG) in the blood or by intrathecal synthesis of IgG in the cerebrospinal fluid (CSF), is unable to contain the progression of PML. CD4+ T lymphocytes recognize extracellular viral proteins that have been degraded into peptides through the exogenous pathway and presented on major histocompatibility complex (MHC) class II molecules at the surface of antigen-presenting cells. Consistent with their underlying immunosuppression, the proliferative response of CD4+ T lymphocytes to mitogens or JCV antigens is reduced in PML patients. CD8+ cytotoxic T lymphocytes recognize intracellularly synthesized viral proteins that have been degraded into peptides through the endogenous pathway, and presented on MHC class I molecules at the surface of virus-infected cells. One of such JCV peptide, the VP1_p100 ILMWEAVTL, has been characterized as a cytotoxic T lymphocyte (CTL) epitope in HLA-A* 0201+ PML survivors. Staining with the corresponding A*0201/JCV VP1_p100 tetrameric complex showed that vp1_P100-stimulated peripheral blood mononuclear cells (PBMCs) of 5/7 (71%) PML survivors had JCV-specific CTL, versus none of 6 PML progressors (P =.02). This cellular immune response may therefore be crucial in the prevention of PML disease progression and the tetramer staining assay may be used as a prognostic marker in the clinical management of these patients.