Question 1. For CD8+ T lymphocytes to recognize virally infected cells, the infected cells must — A. express functional HLA-DM molecules. B. express functional transporter associated with antigen presentation (TAP) 1 and 2 proteins. C. have trafficked through a germinal center. D. extinguish expression of its class I HLA molecules 1. Answer: B Explanation: In virus-infected cells, some viral proteins are degraded in the cellular proteasome, the major cellular protein degrading organelle. TAP-1 and TAP-2 are proteins that participate in the transport of peptide fragments from the proteasome into the endoplasmic reticulum where they are loaded into newly synthesized class I MHC molecules. Without TAP-1 or TAP-2, viral peptides are not loaded into class I molecules, and recognition by CD8+ T cells does not occur. HLA-DM is required for peptide loading into class II MHC molecules, permitting recognition by CD4+ T cells. B-lymphocytes must traffic through the germinal center in order to undergo somatic mutation and differentiation to high affinity antibody producing cells. Trafficking through the germinal center is not required for CD81 T cells devel- opment or differentiation. Some viruses down-regulate (extinguish) expression of the class I proteins of the cell they have infected. This is a strategy for avoiding the CD8 host immune response. Sustained class I molecule expression is necessary for recognition by CD8+ T cells. . Question 2. The HLA invariant chain — A. controls loading of viral peptide fragments into class I HLA molecules