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.
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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
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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.
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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
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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
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characterizing the T-cell populations
responding to infection by influenza virus and HIV.
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