CD4 and viral infectious disease: CD4-deficient, MHC class II-deficient mice or CD4-deficient mice with a transgenic class II-specific T Cell Receptor (TCR) provide the most evidence for unconventional MHC class II-restricted CD8+ T cells elicited by either bacterial or viral infections in mice15–19, although there is some evidence for very low frequency unconventional CD8+ T cells in wild-type B6 mice18.