Selection of CD8(+) T cells with highly focused specificity during viral persistence in the central nervous system

The relationships between T cell populations during primary viral infection and persistence are poorly understood. Mice infected with the neurotropic JHMV strain of mouse hepatitis virus mount potent regional CTL responses th at effectively reduce infectious virus; nevertheless, viral RNA persists in the central nervous system (CNS), To evaluate whether persistence influenc es Ag-specific CD8(+) T cells, functional TCR diversity was studied in sple en and CNS-derived CTL populations based on differential recognition of var iant peptides for the dominant nucleocapsid epitope. Increased specificity of peripheral CTL from persistently infected mice for the index epitope com pared with immunized mice suggested T cell selection during persistence. Th is was confirmed with CD8(+) T cell clones derived from the CNS of either a cutely (CTLac) or persistently (CTLper)infected mice. Whereas CTLac clones recognized a broad diversity of amino acid substitutions, CTLper clones exh ibited exquisite specificity for the wild-type sequence. Highly focused spe cificity was CD8 independent but correlated with longer complementarity-det ermining regions 3 characteristic of CTLper clonotypes despite limited TCR alpha/beta-chain heterogeneity. Direct ex vivo analysis of CNS-derived mono nuclear cells by IFN-gamma, enzyme-linked immunospot assay confirmed the se lection of T cells with narrow Ag specificity during persistence at the pop ulation level. These data suggest that broadly reactive CTL during primary infection are capable of controlling potentially emerging mutations. By con trast, the predominance of CD8(+) T cells with dramatically focused specifi city during persistence at the site of infection and in the periphery suppo rts selective pressure driven by persisting Ag.