Infection with a virus generates a polyclonal immune response with broad alloreactive potential.

Virus-specific T cells have been shown to cross-react with allogeneic HLA (allo-HLA) at a clonal level. However, the impact of a single virus on the allorepertoire has never been investigated at the polyclonal level. We made an inventory of the incidence and specificity of allo-HLA-cross-reactive-virus-specific CD8+ T cells in 24 healthy individuals. T cells were stained for 25 virus-specific tetramers, and mixed-lymphocyte reactions were performed against a panel of HLA-typed allostimulators. Allospecificity was confirmed by IFNγ-ELISA using T-cell clones against a panel of HLA-typed cell-lines. The polyclonal immune repertoire directed against CMV alone was associated with a memory response against six allo-HLA molecules. Besides, a single allostimulator activated memory T-cell responses with multiple viral specificities. Concluding, a single virus can substantially broaden the allo-HLA memory T-cell repertoire. This study only looked at CMV- and EBV-specific T cells, whereas the immune repertoire consists of T cells directed against many different viruses. Hence, transplant patients receiving an HLA-mismatched graft may already express a polyclonal repertoire of anti-donor-memory T cells before transplantation.