Alloreactivity from human viral specific memory T-cells.

The mechanisms by which alloreactive memory T-cells are generated in non-sensitized individuals have begun to be elucidated. It is generally accepted that a very high level of crossreactivity is an essential feature of the T-cell receptor. Indeed it has recently been shown that alloreactivity from viral specific memory T-cells is far more common than predicted, 45% of viral specific T-cell clones were found to be allo-HLA crossreactive. In this overview the evidence for crossreactive alloresponses from human viral specific memory T-cells is discussed with special emphasis on the unexpected high frequency of these crossreactive responses, the peptide and tissue specificity of the responses, and the mechanistic insights gleaned from the elucidation of the crystal structure of an allo-HLA crossreactive viral specific TCR. The possible implications for clinical solid organ and bone marrow transplantation and tolerance induction will be discussed.