Naive T cell activation requires signaling by the T cell receptor and by nonclonotypic cell surface receptors. The most important costimulatory protein is the monovalent homodimer CD28, which interacts with CD80 and CD86 expressed on antigen-presenting cells. Here we present the crystal structure of a soluble form of CD28 in complex with the Fab fragment of a mitogenic antibody. Structural comparisons redefine the evolutionary relationships of CD28-related proteins, antigen receptors and adhesion molecules and account for the distinct ligand-binding and stoichiometric properties of CD28 and the related, inhibitory homodimer CTLA-4. Cryo-electron microscopy-based comparisons of complexes of CD28 with mitogenic and nonmitogenic antibodies place new constraints on models of antibody-induced receptor triggering. This work completes the initial structural characterization of the CD28-CTLA-4-CD80-CD86 signaling system.

The intermolecular contact regions between monomers of the homodimeric DNA binding protein ParR and the interaction between the glycoproteins CD28 and CD80 were investigated using a strategy that combined chemical crosslinking with differential MALDI-MS analyses. ParR dimers were modified in vitro with the thiol-cleavable cross-linker 3,39-dithio-bis~succinimidylproprionate!~DTSSP!, proteolytically digested with trypsin and analyzed by MALDI-MS peptide mapping. Comparison of the peptide maps obtained from digested cross-linked ParR dimers in the presence and absence of a thiol reagent strongly supported a "head-to-tail" arrangement of the monomers in the dimeric complex. Glycoprotein fusion constructs CD28-IgG and CD80-F ab were cross-linked in vitro by DTSSP, characterized by nonreducing SDS-PAGE, digested in situ with trypsin and analyzed by MALDI-MS peptide mapping~6 thiol reagent!. The data revealed the presence of an intermolecular cross-link between the receptor regions of the glycoprotein constructs, as well as a number of unexpected but nonetheless specific interactions between the fusion domains of CD28-IgG and the receptor domain of CD80-F ab . The strategy of chemical cross-linking combined with differential MALDI-MS peptide mapping~6 thiol reagent! enabled localization of the interface region~s! of the complexes studied and clearly demonstrates the utility of such an approach to obtain structural information on interacting noncovalent complexes.

Background: Histone deacetylase inhibitors (HDACIs) induce hyperacetylation of core histones modulating chromatin structure and affecting gene expression. These compounds are also able to induce growth arrest, cell differentiation, and apoptotic cell death of tumor cells in vitro as well as in vivo. Even though several genes modulated by HDAC inhibition have been identified, those genes clearly responsible for the biological effects of these drugs have remained elusive. We investigated the pharmacological effect of the HDACI and potential anti-cancer agent Trichostatin A (TSA) on primary T cells.