Research Areas

Class II MHC proteins

Class II MHC proteins bind peptides derived from cell surface, intravesicular, or endocytosed proteins, and present them to CD4+ helper T cells, as part of the mechanism by which the immune system responds to foreign material in the body. We have developed procedures for refolding and assembling the MHC-peptide complex, determined the X-ray crystal structure for a class II MHC protein in complex with a tightly bound peptide, and characterized a conformational change that occurs concurrent with peptide binding. In continuing work we are:
  • developing methods for prediction of which peptides will bind to an MHC protein
  • designing and synthesizing peptidomimetic compounds to occupy the binding site
  • determining the triggering mechanism for the conformational change.


Peptide Loading Mechanisms

Inside a cell, peptides are generated and brought in contact with nascent MHC proteins by specialized proteolytic machinery and intracellular trafficking mechanisms. One of these components is an enzyme that promotes peptide exchange on MHC proteins, in an unusual example of non-covalent macromolecular catalysis. We have determined that the binding reaction proceeds via a surprisingly elaborate kinetic mechanism, with conformational changes in both empty and peptide-loaded forms. In continuing work we are
  • determining structures for MHC proteins in complex with the chaperones and peptide exchange factors
  • investigating the catalytic mechanism for the exchange factor

Antigen loading in vivo

In most cells, protein degradation and MHC peptide loading occur in endosomal and lysosomal digestion compartments. We have recently discovered that dendritic cells carry empty MHC proteins on their surface, and can directly load peptide from the extracellular medium as well as generate peptides in the vicinity of the cell surface using secreted proteases. Dendritic cells are the "sentinel" cells of the immune system, with unique capabilities in activation and control of T-cells, and are under intensive investigation as the basis for anti-tumor vaccination strategies. In continuing work we are:
  • designing inhibitors of secreted dendritic cell proteases
  • investigating antigen presentation in the nervous system, which appears to involve similar mechanisms as in dendritic cells.

T-cell activation

The interaction of MHC-peptide complexes on the surface of a cell with antibody-like receptors on a T cell causes cellular activation and stimulation of an immune response. The molecular mechanism by which T cells are activated is unclear, but involves oligomerization of T-cell receptor subunits in the membrane plane. To approach the signaling mechanism, we have developed a novel model system using chemically-defined oligomers of MHC-peptide complexes, and used it to determine that receptor dimerization is the minimal requirement for T-cell activation. In continuing work we are:
  • determining the orientation dependence for T-cell activation
  • investigating the molecular mechanism responsible for triggering cytoplasmic signaling cascades
  • developing a quantative model of receptor-ligand interactions in this system
  • characterizing a lipid-induced conformational change in the TCR zeta subunit
  • developing fluorescent oligomers of MHC proteins for detection of specific T-cell populations in clinical samples
  • characterizing the T-cell populations responding to infection by influenza virus and HIV.