Selection of CTL escape mutants in mice infected with a neurotropic coronavirus: Quantitative estimate of TCR diversity in the infected central nervous system

Variant viruses mutated in the immunodominant cytotoxic T cell epitope surf ace (S) glycoprotein S-510-518 are selected in mice chronically infected wi th mouse hepatitis virus, strain JHM. We determined whether this selection occurred in the presence of an oligoclonal or polyclonal T cell response us ing soluble MHC/peptide tetramers in direct ex vivo analyses of CNS-derived lymphocytes, A total of 42% (range, 29-60%) of CD8 T cells in the CNS of m ice with acute encephalitis recognized epitope S510-518, A total of 34% (ra nge, 18-62%) of cells from mice with hind limb paralysis (and chronic demye lination) were also epitope specific, even though only virus expressing mut ated epitope is detected in these animals. Sequence analysis of the beta-ch ain CDR3 of 487 tetramer S-510-518-positive cDNA clones from nine mice show ed that a majority of clonotypes were identified in more than one mouse. Fr om these analyses, we estimated that 300-500 different CD8 T cell clonotype s responsive to epitope S-510-518 were present in each acutely infected bra in, while 100-900 were present in the CNS of each mouse with chronic diseas e, In conclusion, a polyclonal CD8 T cell response to an epitope does not p l preclude the selection of T cell escape mutants, and epitope-specific T c ells are still present at high levels even after RNA-encoding wild-type seq uence is no longer detectable.