There are no direct means to study class I MHC presentation in human normal or diseased cells. Using CMV-infected human cells and applying novel mAb that mimic T-cell receptor specificity directed toward the immunogenic epitope of the viral pp65 protein presented on HLA-A2 molecules, we directly imaged the dynamics of Ag presentation in infected cells. We demonstrate that following infection large intracellular pools of HLA-A2/pp65 complexes are localized to the Golgi. These HLA-A2/pp65 pools account for the majority of total HLA-A2 molecules in infected cells. Interestingly, these large pools are sequestered inside infected cells and only a small portion of them are exported to the cell surface. Virus-induced class I MHC down-regulation did not affect the intracellular pool of HLA-A2/ pp65 complexes. Our data also suggest that proteasome function influences the release of class I complexes to the membrane. We present herein a new and direct molecular tool to study the dynamics of viral Ag presentation that may further elucidate the balance between immune response versus viral escape.Key words: Ab . Ag presentation/processing . MHC . Virology Supporting Information available online IntroductionImmunity to CMV is complex and involves humoral and cellmediated responses [1][2][3][4][5]. Studies showed that both NK cells and CTL are of primary importance in the prevention of recurrence [6][7][8]. Many gene products participate in generating the CTL response, but the high frequencies of the viral protein pp65 found in subsequent studies showed that this protein is the chief target of the CTL-mediated immune response [9,10]. Cytosolic proteins, usually synthesized in the cells, such as these viral proteins, enter the class I MHC pathway of Ag presentation. The proteasome is a cytoplasmic multiprotein complex that proteolytically degrades ubiquitinated cytoplasmic proteins and probably generates a large portion of the peptides destined for display by class I MHC molecules. Peptides are delivered from the cytoplasm to the ER by the TAP molecules. Newly formed class I MHC dimmers in the ER associate with and bind peptides delivered by the TAP. Peptide binding stabilizes class I MHC molecules and permits their movement out of the ER, through the Golgi, to the cell surface. This pathway ensures that any cell synthesizing viral proteins can 1552be marked for recognition and killing by CD8 1 CTL [11,12]. Among all the pp65 peptides, CMV-specific CTL activity in HLA-A2-positive individuals was found to be mainly directed to the peptide pp65 495-503 NLVPMVATV [13].Characterization of class I MHC/peptide presentation is essential for studying the relationships between immunity and viral escape. However, there are no direct means available to study class I MHC/peptide presentation; for example, study of viralderived peptide/MHC complexes on the surface and inside cells during the course of infection. mAb with peptide-specific, MHCrestricted recognition patterns, termed TCR-like Ab, are currently the only tool for such studies. ...

CTLs act as the effector arm of the cell-mediated immune system to kill undesirable cells. Two processes regulate these effector cells to prevent self reactivity: a thymic selection process that eliminates autoreactive clones and a multistage activation or priming process that endows them with a license to kill cognate target cells. Hitherto no subsequent regulatory restrictions have been ascribed for properly primed and activated CTLs that are licensed to kill. In this study we show that CTLs possess a novel postpriming regulatory mechanism(s) that influences the outcome of their encounter with cognate target cells. This mechanism gauges the degree of Ag density, whereupon reaching a certain threshold significant changes occur that induce anergy in the effector T cells. The biological consequences of this Ag-induced postpriming control includes alterations in the expression of cell surface molecules that control immunological synapse activity and cytokine profiles and induce retarded cell proliferation. Most profound is genome-wide microarray analysis that demonstrates changes in the expression of genes related to membrane potential, TCR signal transduction, energy metabolism, and cell cycle control. Thus, a discernible and unique gene expression signature for anergy as a response to high Ag density has been observed. Consequently, activated T cells possess properties of a self-referential sensory organ. These studies identify a new postpriming control mechanism of CTL with anergenic-like properties. This mechanism extends our understanding of the control of immune function and regulation such as peripheral tolerance, viral infections, antitumor immune responses, hypersensitivity, and autoimmunity.

The cover image of this issue consists of confocal images of cytomegalovirus‐infected human fibroblast cells stained for HLA‐A2/pp65 complexes, taken from Makler et al. (pp. 1552–1565). In this article, the authors use antibodies that mimic TCR specificity to directly visualize the dynamics of antigen presentation in these cells. The authors show that, following infection, large intracellular pools of HLA‐A2/pp65 complexes are localized to the Golgi. Interestingly, these large pools are sequestered inside infected cells and only a small portion of them are exported to the cell surface.