Posttranslational side chain modification of a viral epitope results in diminished recognition by specific T cells

A stretch of 16 amino acid residues within the nominal phosphoprotein of rabies virus was shown to carry an immunodominant epitope for class I- and class II-restricted T cells. The nominal phosphoprotein of rabies virus is thought to be heterogeneously phosphorylated at multiple serine and threonine residues. The synthetic peptide that expressed the T-cell epitope contained a single serine residue corresponding to position 196 of the protein. Phosphorylation of this serine within the synthetic peptide caused a significant decrease of the antigenic potency of the peptide. A similar effect was seen if the serine was replaced by an alanine or if the peptide was glycosylated at its acidic residues. These data suggest that T-cell-mediated recognition of antigen presented by major histocompatibility complex class I- or II-positive cells is impaired not only by point mutations but also by posttranslational side chain modifications of residues within viral epitopes.