Dept. of Molecular Microbiology and Immunology
Oregon Health & Science University
Murine cytomegalovirus (MCMV) has been used for many years as a model pathogen for studying persistent virus infections. The virus has devised several mechanisms for immune evasion, one of the best characterized, is the encoding of genes that interfere with MHC class I. Three immune evasion genes encoded by MCMV (m04, m06 and ml52) all function by interfering with MHC class I presentation of peptides to CDS T cells. While there is no evidence that these immune evasion genes are responsible for viral persistence, they have been conserved within the MCMV genome for millions of years. This dissertation will describe work done to characterize the effectiveness of the individual immune evasion genes at blocking functional antigen-specific CDS T cell responses. The studies performed in this dissertation will demonstrate that the coordinated function of MCMV's three immune evasion genes results in a powerful inhibition of lysis of infected cells by CDS T cells. Removal of any one immune evasion gene enables lysis by at least some CDS T cells. We have shown that there is no preferential downregulation of epitopes bound to H2-Kb or H2-Db isoforms by m06 and ml52. m04 's effects on inhibiting CTL killing are epitope specific and m04 is not involved in the global downregulation of MHC class I caused by m06 and ml52. The NKG2D ligand RAE-1 can function as a costimulatory on antigen presenting cells and is downregulated by ml52. We have shown that the impact ofNKG2D ligand downregulation on CDS T cell effector functions is modest, and does not account for the dramatic effect of ml52 on inhibition of CDS T cell killing. Finally the immune evasion genes are able to inhibit CDS T cell killing even when cognate peptide can be detected on the surface of the MCMV infected cell. CDS T cells can kill cells infected with a mutant MCMV, missing all three immune evasion genes (TKO), pulsed with cognate peptide. However, TAP-/- cells, which have reduced the total MHC class I on the surface, infected with TKO are not killed by antigen-specific CDS T cells. Restoration of total cell surface MHC class I, with the addition of non-cognate peptide to stabilize the MHC class I on the TAP-/- cells, results in the killing ofTAP-/- TKO-infected cells. We have also shown that CDS T cells take longer to kill virally infected cells as compared to uninfected cells expressing equal amounts of cognate peptide MHC. We believe the reason CDS T cells cannot kill MCMV infected cells is due a reduction in total MHC class I levels, which leads to a weaker interaction between the CDS T cell and the infect cells. This weaker interaction prevents the T cell from killing the virally infected cell either by reducing the conjugation time between the T cell and the infected cells or by altering the synapse structure so the lytic granules are not effectively targeted to the infected cell.
School of Medicine
Pinto, Amelia Kahler, "Inhibition of CD8 T cell effector functions by murine cytomegalovirus immune evasion genes" (2007). Scholar Archive. 825.