CYTOTOXIC T-LYMPHOCYTE RESPONSE TO A WILD-TYPE HEPATITIS-B VIRUS EPITOPE IN PATIENTS CHRONICALLY INFECTED BY VARIANT VIRUSES CARRYING SUBSTITUTIONS WITHIN THE EPITOPE

Mutations that abrogate recognition of a viral epitope by class I-rest ricted cytotoxic T lymphocyte (CTL) can lead to viral escape if the CT L response against that epitope is crucial for viral clearance. The li kelihood of this type of event is low when the CTL response is simulta neously directed against multiple viral epitopes, as has been recently reported for patients with acute self-limited hepatitis B virus (HBV) infection. The CTL response to HBV is usually quite weak, however, du ring chronic HBV infection, and it is generally acknowledged that this is a major determinant of viral persistence in this disease. If such individuals were to produce a mono- or oligospecific CTL. response, ho wever, negative selection of the corresponding mutant viruses might oc cur. We have recently studied two HLA-A2-positive patients with chroni c hepatitis B who, atypically, developed a strong HLA-A2-restricted CT L response against an epitope (FLPSDFFPSV) that contains an HLA-A2-bin ding motif located between residues 18-27 of the viral nucleocapsid pr otein, hepatitis B core antigen (HBcAg). These patients failed, howeve r, to respond to any of other HLA-A2-restricted HBV-derived peptides t hat are generally immunogenic in acutely infected patients who success fully clear the virus. Interestingly, DNA sequence analysis of HBV iso lates from these two patients demonstrated alternative residues at pos ition 27 (V --> A and V --> I) and position 21 (S --> N, S --> A, and S --> V) that reduced the HLA and T cell receptor-binding capacities o f the variant sequences, respectively. Synthetic peptides containing t hese alternative sequences were poorly immunogenic compared to the pro totype HBc18-27 sequence, and they could not be recognized by CTL clon es specific for the prototype peptide. While we do not know if the two patients were originally infected by these variant viruses or if the variants emerged subsequent to infection because of immune selection, the results are most consistent with the latter hypothesis. If this is correct, the data suggest that negative selection of mutant viral gen omes might contribute to viral persistence in a subset of patients wit h chronic HBV infection who express a narrow repertoire of anti-HBV CT L responses.