The 0 Domain of the Qa - 2 Molecule Is Defective for CD 8 Binding and Cytotoxic T Lymphocyte Activation

Qa-2 is a nonclassical class I molecule encoded by the Q7 gene within the mouse major histocompatibility complex (MHC). Results from previous experiments on Qa-2, and on a chimeric Ld molecule (LQ3) in which the 0 domain is encoded by Q7b , suggested that the a3 domain of Qa-2 does not carry out the functions typical ofthe0 domains in other classical and nonclassical class I antigens. Class I molecules that contain the Qa-2 tx3 domain are poorly recognized by primary cytotoxic T lymphocytes (CTLs), and do not function normally in either positive or negative selection in vivo. By employing a cell-cell adhesion assay we demonstrate directly that the Qa-2 0 domain in the context of the LQ3 hybrid molecule cannot bind to human CD8, although other mouse class 10 domains bind efficiently. In addition, CD8-dependent CTLmediated lysis of target cells, in a system which requires mouse CD8-class I a3 domain interactions, is deficient in cells that express the Qa-2 0 domain . When combined with our earlier work on LQ3 transgenic mice, these results provide additional molecular support for the hypothesis that interaction with CD8 is required for both positive and negative selection ofclass I restricted T cells in the thymus. As the Qa-2 0 domain sequence does not differ from the previously defined minimal CD8 binding sequence of other class I molecules, these results also suggest that additional amino acids in the0 domain must be critical for CD8 binding and CTL activation.