Immunology. Accessory to murder

Cross-presentation: underlying mechanisms and role in immune surveillance

It was originally thought that a cell's major histocompatibility complex (MHC) class I molecules presented peptides derived exclusively from proteins synthesized by the cell itself. However, in some circumstances, antigens from the extracellular environment can be presented on MHC class I molecules and stimulate CD8(+) T-cell immunity, a process termed cross-presentation. Cross-presentation was originally discovered as an obscure phenomenon in transplantation immunity. However, it is now clear that it is a major mechanism by which the immune system monitors tissues and phagocytes for the presence of foreign antigen. Cross-presentation is the only pathway by which the immune system can detect and respond to viral infections or mutations that exclusively occur in parenchymal cells rather than in bone marrow-derived antigen-presenting cells (APCs). Professional APCs, such as dendritic cells, are the principal cells endowed with the capacity to cross-present antigens. In this process, the APCs acquire proteins from other tissue cells through endocytic mechanisms, especially phagocytosis or macropinocytosis. The internalized antigen can then be processed through at least two different mechanisms. In one pathway, the antigen is transferred from the phagosome into the cytosol, where it is hydrolyzed by proteasomes into oligopeptides that are then transported by the transporter associated with antigen processing to MHC class I molecules in the endoplasmic reticulum or phagosomes. In a second pathway, the antigen is cleaved into peptides by endosomal proteases, particularly cathepsin S, and bound by class I molecules probably in the endocytic compartment itself. Depending on the nature of the antigen, one or both of these pathways can contribute to cross-presentation in vivo. The outcome of cross-presentation can be either tolerance or immunity. Which of these outcomes occurs is thought to depend on whether antigens are acquired by themselves alone, leading to tolerance, or with immunostimulatory signals, leading to immunity. One source of such signals is from dying cells that release immunostimulatory 'danger' signals that promote the generation of immunity to their cellular antigens. In addition to the critical role of cross-presentation in normal immune physiology, this pathway has considerable potential for being exploited for developing subunit vaccines that elicit both CD4(+) and CD8(+) T-cell immunity.

Macrophages and dendritic cells use the cytosolic pathway to rapidly cross-present antigen from live, vaccinia-infected cells

Professional APCs (pAPC) can process and present on their own MHC class I molecules Ags acquired from Ag donor cells (ADC). This phenomenon of cross-presentation is essential in the induction of CD8(+) T cell responses to viruses that do not infect pAPC and possibly contributes to the induction of CD8(+) responses to many other viruses. However, little is known about the mechanisms underlying this process. In this study, we show that dendritic cells and macrophages cross-present a model Ag supplied by vaccinia virus-infected ADC via the cytosolic route. Strikingly, we also found that cross-presentation of Ags provided by vaccinia-infected cells occurs within a couple of hours of pAPC/ADC interaction, that the duration of cross-presentation lasts for only 16 h, and that cross-presentation can occur at early times of infection when the ADC are still alive.

Immunity to polyoma virus infection and tumorigenesis

Because oncogenic DNA viruses establish persistent infections in humans, continuous immunosurveillance for neoplastic cells is required to prevent virus-induced tumors. Antigen-specific CD8+ T lymphocytes are critical in vivo effectors for eliminating virus-infected and virus-transformed cells. Investigation into the induction, regulation, and maintenance of CD8+ T cells specific for these viruses is hindered by the lack of tractable animal models that mimic natural infection. Resistance to tumors induced by polyoma virus, a persistent natural mouse DNA virus, is mediated by polyoma-specific CD8+ T cells. Mice susceptible to polyoma virus tumorigenesis mount a smaller, albeit still considerable, expansion of anti-polyoma CD8+ T cells; importantly, these antiviral CD8+ T cells lack cytotoxic activity while retaining the phenotype of cytotoxic T lymphocyte (CTL) effectors. In this review, we will discuss potential in vivo mechanisms that regulate the functional competence of anti-polyoma CD8+ T cells, particularly in the context of chronic antigenic stimulation provided by persistent viral infections and tumors.

Bacterial toxins as tools for mucosal vaccination

Several studies have demonstrated that the biological properties of secreted bacterial toxins could be harnessed for the induction of mucosal and systemic immunity following application at epithelial surfaces. Although the properties and potential application of several of these toxins will be discussed in this review, special focus will be placed on Pseudomonas aeruginosa exotoxin A (PE). A non-toxic form of PE (ntPE) into which antigenic epitopes can be integrated appears to be a particularly promising vaccination tool, which is able to cross the polarized epithelia of the gastrointestinal, respiratory and reproductive tracts and selectively target macrophages and dendritic cells.

Antigen processing for MHC class I restricted presentation of exogenous influenza A virus nucleoprotein by B-lymphoblastoid cells

In general, exogenous proteins are processed by antigen-presenting cells in the endosomes for major histocompatibility complex (MHC) class II presentation to CD4+ T cells, while proteins synthesized endogenously are processed in the cytoplasm for MHC class I presentation to CD8+ T cells. However, it is recognized that exogenous proteins can be processed for MHC class I presentation also, and evidence in favour of alternatives to the conventional MHC class I processing and presentation pathway is accumulating. Here, we show that exogenous recombinant influenza A virus nucleoprotein (rNP) is processed for MHC class I presentation to CD8+ cytotoxic T lymphocytes (CTL) by EBV-transformed, B-lymphoblastoid cell lines (B-LCL). Processing of rNP for HLA-B27-associated presentation seemed to follow the conventional MHC class I pathway predominantly, as presentation was diminished in the presence of lactacystin and brefeldin A, but was less sensitive to chloroquine and NH4Cl. HLA-B27-associated presentation was also observed using cells lacking a functional transporter associated with antigen processing, suggesting that alternative pathways may be exploited for processing of rNP.