| The cell surface glycoprotein CD8 is involved in mediating the adhesion of CD8{dollar}sp+{dollar} T lymphocytes to their class I expressing targets. Central to this role is the ability of CD8 to act as a coreceptor by binding to the {dollar}alpha3{dollar} domain of MHC class I and stabilizing the interaction of the T cell receptor with antigenic peptide presented in the {dollar}alpha1/alpha2{dollar} cleft of the same class I molecule. Once bound, CD8 contributes to the transduction of intracellular signals that are important for activation.; To define the molecular nature of the interactions between the CD8{dollar}alpha/alpha{dollar} homodimer and HLA class I, we generated a panel of mutants of the Ig variable-like domain of human CD8{dollar}alpha{dollar} and tested the ability of mutant CD8{dollar}alpha/alpha{dollar} homodimers to bind to class I in a transient cell-cell adhesion assay. The results of this study clearly demonstrated that amino acids located on the top CDR-containing surface and on the A/B strand surface of the molecule were important for binding. The involvement of the top face and opposite sides of the CD8{dollar}alpha/alpha{dollar} homodimer is consistent with a bivalent model for coordinate binding of two class I molecules. The additional finding that the positive surface potential of CD8 plays a role in binding complements previous work demonstrating the importance of a negatively charged loop on class I for CD8{dollar}alpha/alpha{dollar}-class I interaction.; The ability of the CD8{dollar}alpha/alpha{dollar} homodimer to bind to the class Ib molecule, HLA-G, was also established using our cell-cell binding assay. Since HLA-G is expressed at the maternal-fetal interface on a subset of trophoblast cells, these results suggest a role for CD8{dollar}alpha/alpha{dollar}-HLA-G interaction during development.; Lastly, we have characterized a secreted form of human CD8{dollar}alpha{dollar} and shown by RNAse protection and sequence analysis that it arises from the alternative splicing of the same mRNA that encodes the membrane form of CD8. Message for secreted CD8{dollar}alpha{dollar} is present in CD8{dollar}sp+{dollar} T cell lines, resting and activated peripheral blood lymphocytes. The function of this secreted CD8 counterpart remains to be determined. |
