Research Interests

The main interests of this laboratory are in two areas; antigen processing and the mechanisms of action of antiviral proteins stimulated by interferons. Studies of MHC class I molecules center on two problems. The first is the mechanism(s) governing the association of antigen-derived peptides with class I molecules in the endoplasmic reticulum (ER). Peptides are transported into the ER by the Transporter associated with Antigen Processing (TAP) after their generation from cytosolic proteins by proteasomal degradation. Peptide binding occurs in the context of the peptide loading complex, which consists of TAP, an associated transmembrane glycoprotein called tapasin, and an associated 'empty' class I molecule which is a heterodimer of the class I heavy chain and ß2microglobulin. Also in the complex are two housekeeping molecules, the chaperone calreticulin which associates with the glycan on the class I heavy chain, and ERp57, a thiol oxidoreductase that is disulfide linked to tapasin. How the peptide loading complex promotes peptide loading of class I molecules at the molecular level is under intensive investigation.

The second area of interest in MHC class I lies in the phenomenon of cross-presentation, which is essential for priming the response of naïve CD8-positive T cells to viral antigens in vivo. Cross-presentation is a particular property of dendritic cells that allows them to generate MHC class I complexes with peptides derived from protein antigens that are internalized by phagocytosis or pinocytosis. These two processes ultimately allow entry of external proteins into the cytosol, either by crossing the phagosomal membrane or the ER membrane after they gain access to the ER. The mechanisms allowing protein access to the ER and the translocation mechanism responsible for entry into the cytosol are two problems being studied.

Work on MHC class II currently focuses on the role of Gamma Interferon-inducible Lysosomal Thioreductase (GILT) in MHC class II-restricted antigen processing. The latter is important for the recognition by CD4-positive T cells of protein antigens containing disulfide bonds, demonstrated in a GILT knockout mouse.

A third area of interest within the antigen processing field centers on CD1d molecules, which are structurally similar to MHC class I molecules but bind lipids rather than peptides. We are trying to understand the role of lipid binding and of ER chaperones in the assembly of CD1 molecules, as well as the precise mechanisms governing lipid binding in endosomes and lysosomes. We have shown that saposins, small molecules essential for the degradation of sphingolipids, are required for lipid binding to CD1d molecules in the endocytic pathway. Saposins are capable of mobilizing monomeric lipid molecules from the endosomal or lysosomal bilayer which then allows them to bind to the CD1d molecules. The importance of CD1d in antiviral responses is highlighted by the efforts made by a number of viruses to reduce CD1d expression upon infection. We recently found that Herpes simplex virus-1 (HSV-1) does this by inhibiting CD1d re-expression on the surface of cells during recycling through the endosomal/lysosomal system. Upon HSV-1 infection, CD1d molecules accumulate in lysosomes and disappear from the cell surface.

Finally, we are interested in understanding the functions of interferon-inducible proteins, particularly a protein we identified and called viperin. We originally showed that viperin has an inhibitory effect on the growth of human cytomegalovirus. We have recently found that it also inhibits the growth of influenza A and Herpes simplex virus. How this works at the molecular level is under intensive study. In collaboration with Paula Kavathas' laboratory we have recently been studying the role of family of cytosolic, interferon-inducible proteins on infection by Chlamydia trachomatis. We have determined that a member of this family, called guanylate-binding proteins or GBPs, is of major importance in mediating the established inhibitory effects of interferon-gamma on the intracellular proliferation of C. trachomatis.