THE ALPHA-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 previou s experiments on Qa-2, and on a chimeric L(d) molecule (L(Q3)) in whic h the alpha3 domain is encoded by Q7b, suggested that the alpha3 domai n of Qa-2 does not carry out the functions typical of the alpha3 domai ns in other classical and nonclassical class I antigens. Class I molec ules that contain the Qa-2 alpha3 domain are poorly recognized by prim ary cytotoxic T lymphocytes (CTLs), and do not function normally in ei ther positive or negative selection in vivo. By employing a cell-cell adhesion assay we demonstrate directly that the Qa-2 alpha3 domain in the context of the L(Q3) hybrid molecule cannot bind to human CD8, alt hough other mouse class I alpha3 domains bind efficiently. In addition , CD8-dependent CTL-mediated lysis of target cells, in a system which requires mouse CD8-class I alpha3 domain interactions, is deficient in cells that express the Qa-2 alpha3 domain. When combined with our ear lier work on L(Q3) transgenic mice, these results provide additional m olecular support for the hypothesis that interaction with CD8 is requi red for both positive and negative selection of class I restricted T c ells in the thymus. As the Qa-2 alpha3 domain sequence does not differ from the previously defined minimal CD8 binding sequence of other cla ss I molecules, these results also suggest that additional amino acids in the alpha3 domain must be critical for CD8 binding and CTL activat ion.