An analysis of the role of a MHC class II-restricted neo-self peptide in T cell repertoire formation

The specificity with which CD4+ T cells recognize PR8 S1, the major I-E$\rm\sp{d}$-restricted T cell determinant from hemagglutinin (HA), as a neo-self peptide in vivo was examined by analyzing HA transgenic (HA Tg) mice for their ability to respond to PR8 S1, or to a closely related analog (S1(K113)). T cell hybridomas specific for S1(K113) were generated from HA Tg and non-transgenic (non-Tg) mice, and examined for their cross-reactivity with the neo-self PR8 S1. Also, hybridoma TCR genes were sequenced in order to examine the genetic basis for T cell recognition of PR8 S1 as a neo-self peptide. The results indicated that negative selection eliminates PR8 S1-specific T cells from the repertoire of HA Tg mice, including those T cells which express a TCR clonotype dominant among S1(K113)-specific T cells from non-Tg mice. Nevertheless, HA Tg mice retained the ability to respond to the S1(K113) determinant, and some S1(K113)-specific TCRs recognized PR8 S1 as a partial agonist/antagonist peptide. Additionally, modified versions of the dominant TCR clonotype were identified from HA Tg mice that used altered junctional sequences and a novel V$\alpha$/V$\beta$ pairing to evade negative selection. The remaining S1(K113)-specific TCRs from HA Tg mice were highly diverse, indicating that tolerance to PR8 S1 as a self peptide does not limit the diversity of the T cell response to the closely related S1(K113) analog. The mechanism for negative selection of PR8 S1-specific T cells in HA Tg mice was examined by crossing these mice with transgenic mice expressing a PR8 S1-specific TCR (Ts1 Tg mice). Thymocytes expressing PR8 S1-specific TCRs were eliminated from HAxTs1 Tg mice through clonal deletion at the CD4/CD8 double positive stage of development. Overall, the findings described here provide insights into the ways in which TCRs react with class II-restricted self peptides during selection of T cell repertoire, and may also provide a model for the induction of autoimmunity by viruses that contain close homologs of self peptides