Tuning antiviral CD8 T-cell response via proline-altered peptide ligand vaccination.

Viral escape from CD8+ cytotoxic T lymphocyte responses correlates with disease progression and represents a significant challenge for vaccination. Here, we demonstrate that CD8+ T cell recognition of the naturally occurring MHC-I-restricted LCMV-associated immune escape variant Y4F is restored following vaccination with a proline-altered peptide ligand (APL). The APL increases MHC/peptide (pMHC) complex stability, rigidifies the peptide and facilitates T cell receptor (TCR) recognition through reduced entropy costs. Structural analyses of pMHC complexes before and after TCR binding, combined with biophysical analyses, revealed that although the TCR binds similarly to all complexes, the p3P modification alters the conformations of a very limited amount of specific MHC and peptide residues, facilitating efficient TCR recognition. This approach can be easily introduced in peptides restricted to other MHC alleles, and can be combined with currently available and future vaccination protocols in order to prevent viral immune escape.

Reversed T Cell Receptor Docking on a Major Histocompatibility Class I Complex Limits Involvement in the Immune Response.

The anti-viral T cell response is drawn from the naive T cell repertoire. During influenza infection, the CD8+ T cell response to an H-2Db-restricted nucleoprotein epitope (NP366) is characterized by preferential expansion of T cells bearing TRBV13+ T cell receptors (TCRs) and avoidance of TRBV17+ T cells, despite the latter dominating the naive precursor repertoire. We found two TRBV17+ TCRs that bound H-2Db-NP366 with a 180° reversed polarity compared to the canonical TCR-pMHC-I docking. The TRBV17 ß-chain dominated the interaction and, whereas the complementarity determining region-3 (CDR3) loops exclusively mediated contacts with the MHC-I, peptide specificity was attributable to germline-encoded recognition. Nevertheless, the TRBV17+ TCR exhibited moderate affinity toward H-2Db-NP366 and was capable of signal transduction. Thus, the naive CD8+ T cell pool can comprise TCRs adopting reversed pMHC-I docking modes that limit their involvement in the immune response.

The influenza virus-specific CTL immunodominance hierarchy in mice is determined by the relative frequency of high-avidity T cells.

Virus-specific CTL responses typically fall into reproducible hierarchies with particular epitopes eliciting either immunodominant or subdominant responses after viral challenge. The recently acquired capacity to directly enumerate naive CTL precursors (CTLps) in both mice and humans has implicated CTLp frequency as a key predictor of immune response magnitude after Ag challenge. However, recent studies have indicated that naive CTLp frequencies do not necessarily predict the size of the Ag-driven response, indicating an important role for differential CTLp recruitment and/or expansion. This study characterizes the early emergence of various influenza epitope-specific CTL responses at multiple sites in C57BL/6 mice, and probes the role of Ag dose and TCR avidity in dictating immune response hierarchies. Despite large naive CTLp numbers, subdominance was found to arise largely as a consequence of the abrupt and premature cessation of CTL proliferation, at least for one epitope specificity. Investigation into the possible drivers of the poor proliferation observed for subdominant specificities showed that the immunodominance hierarchy endured irrespective of epitope abundance, and correlated with the prevalence of high-avidity T cells in both the naive and immune compartments. Our study strongly indicates that the quality, and not simply the quantity, of antiviral CTLs dictate response magnitude.

Reproducible selection of high avidity CD8+ T-cell clones following secondary acute virus infection.

The recall of memory CD8(+) cytotoxic T lymphocytes (CTLs), elicited by prior virus infection or vaccination, is critical for immune protection. The extent to which this arises as a consequence of stochastic clonal expansion vs. active selection of particular clones remains unclear. Using a parallel adoptive transfer protocol in combination with single cell analysis to define the complementarity determining region (CDR) 3α and CDR3ß regions of individual T-cell receptor (TCR) heterodimers, we characterized the antigen-driven recall of the same memory CTL population in three individual recipients. This high-resolution analysis showed reproducible enrichment (or diminution) of particular TCR clonotypes across all challenged animals. These changes in clonal composition were TCRα- and ß chain-dependent and were directly related to the avidity of the TCR for the virus-derived peptide (p) + major histocompatibility complex class I molecule. Despite this shift in clonotype representation indicative of differential selection, there was no evidence of overall repertoire narrowing, suggesting a strategy to optimize CTL responses while safeguarding TCR diversity.