Low Frequency of Cytotoxic T Lymphocytes against the Novel HLA-A*0201-Restricted JC Virus Epitope VP1 p36 in Patients with Proven or Possible Progressive Multifocal Leukoencephalopathy

Abstract

ABSTRACT JC virus (JCV)-specific cytotoxic T lymphocytes (CTL) in peripheral blood are associated with a favorable outcome in patients with progressive multifocal leukoencephalopathy (PML). However, the frequency of these cells in the peripheral blood mononuclear cells (PBMC) of PML patients is unknown. To develop a highly sensitive assay for detecting the cellular immune response against this virus, we performed a CTL epitope mapping study of JCV VP1 major capsid protein by using overlapping peptides. A novel HLA-A*0201-restricted epitope, the VP1 p36 peptide SITEVECFL, was characterized. The cellular immune response against JCV was assessed in 32 study subjects. By combining the results of the 51 Cr release assay on pooled peptides and staining with the HLA-A*0201/JCV VP1 p36 tetramer, VP1-specific CTL were detected in 10 of 11 PML survivors (91%) versus only 1 of 11 PML progressors (9%, P = 0.0003). VP1-specific CTL were also detected in two of two patients recently diagnosed with PML and in four of four human immunodeficiency virus-positive patients with possible PML. The frequency of CTL specific for the novel VP1 p36 and the previously described VP1 p100 epitopes was determined. In two patients, the frequency of CTL specific for the VP1 p36 or VP1 p100 epitopes, as determined by fresh blood tetramer staining (FBTS), ranged from 1/6,000 to 1/24,000 PBMC. A CTL sorting technique combining tetramer staining and selection with immunomagnetic beads allowed the detection of epitope-specific CTL in two cases that were determined to be negative by FBTS. The phenotype of these CTL in vivo was consistent with activated memory cells. These data suggest that, although present in low numbers, JCV-specific CTL may be of central importance in the containment of JCV spread in immunosuppressed individuals.