The α;3 domain of the Qa-2 molecule is defective for CD8 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 L^d molecule (L^Q3) in which the α3 domain is encoded by Q7^b, suggested that the a3 domain of Qa-2 does not carry out the functions typical of the α3 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 α3 domain in the context of the L^Q3 hybrid molecule cannot bind to human CD8, although other mouse class I α3 domains bind efficiently. In addition, CD8-dependent CTLmediated lysis of target cells, in a system which requires mouse CD8-class I α3 domain interactions, is deficient in cells that express the Qa-2 α3 domain. When combined with our earlier work on L^Q3 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 α3 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 the α3 domain must be critical for CD8 binding and CTL activation.