Similarly, when the D b NP 366 + Vb8.3 + CD8 + T cells induced by wt HK infection were sequenced, the majority of the TCRb clonotypes were detected with both the D b NP 366 and D b NPN3A tetramers (Table 3) . However, a switch in frequency was seen for some CDR3b-defined clonotypes, indicating selective binding of particular TCRs by the D b NPN3A tetramer. For example in M9 and M10 (Table 3) the 'public' SGGANTGQL CDR3b-sequence dominated the D b NP 366 + Vb8.3 + CD8 + T cell population, whereas this sequence was less commonly detected in the same mice by the D b NPN3A tetramer. The difference could, of course, reflect diversity in TCRa usage. Following clonotype selection with the D b NP 366 and D b NPN3A tetramers, sequencing of the secondary TCRb repertoire (M11-M13) induced by challenge with the homologous (PR8, then HK) viruses showed less divergence within the wt D b NP 366 + CD8 + T cell specific population. Interestingly, clonotypes like KGGS-NTGQL were enriched by the D b NPN3A tetramer in some but not other mice (Table 3 ; present in M9 but not M12) suggesting, again, the likely importance of Vb-Va chain pairing for recognition of the native D b NP 366 + CD8 + T cells with the mutant D b NPN3A tetramer. Thus, the analysis suggests that only a subset of the repertoire generated by wt infection is able to recognize the D b NPN3A epitope, though this population is more diverse than that generated in response to the mutant NPN3A virus. All the statistical differences ( Table 2, Table 3 ) were confirmed when the data were standardized to a number of sequences (data not shown).