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Antigen presentation and T cell-mediated immunity: membrane trafficking in cross-presentation

Final Activity Report Summary - CROSS PRESENTATION (Antigen Presentation and T cell-mediated immunity: Membrane trafficking in Cross-Presentation)

Phagocytosis is a receptor-mediated process by which some specialised cells engulf large extracellular particles, such as dying cells, inert material or microorganisms. This process is a vital component of host defence against infection, since its object is to kill the microbe. Induction of adaptive immune responses against various microbes is also depends on phagocytosis. The effectiveness of the immune response requires a highly regulated mechanism, involving processing of the antigens and their presentation to specific T cells at the surface of Antigen-presenting cells (APCs), such as Dendritic cells (DC) and macrophages. Antigenic peptides are loaded on Major histocompatibility complex (MHC) molecules for presentation to T cells. Since the discovery of DCs more than 30 years ago, the progress made by different laboratories in the knowledge of DC biology has been extraordinary. Nevertheless, many of the properties that confer DCs their unique and high capacity for antigen processing remain to be elucidated. Macrophages (MO) and DCs are considered professional Antigen presenting cells (APC), although only DCs possess the unique ability to prime effective immune responses. Indeed, numerous studies have demonstrated that DCs have developed specific means to allow efficient antigen uptake, processing and presentation to T cells to initiate an immune response.

The trigger of cytotoxic immune responses via the activation of CD8+ T lymphocytes may occur through two pathways in DCs: direct priming and cross-priming. These two mechanisms differ by the source of the antigen and the type of cell that processes and presents the peptide antigen. In the case of the direct antigen presentation pathway, CD8+ T cells recognise their cognate peptide-MHC class I complexes on the surface of the cells that synthesised the Ag, such as malignant or virally infected cells. Alternatively, CD8+ T cells recognise peptide-MHC class I complexes on the surface of cells, which have captured exogenously synthesised Ag, and subsequently processed and presented the Ag bound to their own MHC class I molecules. This indirect pathway, called cross presentation, mediates the initiation of cytotoxic responses against bacteria, tumours and certain viruses that do not infect DCs. Contrary to MO which are no efficient for cross presenting Ags, DCs are the most competent APC for antigen cross presentation.

Cross-presentation implies that after internalisation, phagocytosed antigens somehow reach the MHC class I peptide loading pathway, which is normally restricted to endogenous antigens. In fact, it has been shown that cross presentation takes place in a specialised ER-phagosome mix compartment. However, the general process of phagosome maturation allowing exogenous antigens to be process for cross presentation is still unclear in DCs. Then, the main objective of my proposal was to analyse some aspects of the intracellular membrane trafficking steps involved in cross presentation. One of the most relevant points is to understand how Ag proteolysis is controlled along the internalisation pathway in DCs.

The results of my project have revealed a novel specialisation of the phagocytic pathway of DCs that contributes to efficient antigen cross presentation. The NADPH oxidase NOX2 is recruited to phagosomes generating low levels of superoxides and causing H+ consumption. Thus, the phagosomal pH is maintained above 7 for several hours after phagocytosis, which allows a fine control of the proteolytic activity. We also demonstrated that NOX2 membrane components are stored in 'secretory inhibitory lysosome-related organelles' and Rab27a plays a critical role in the recruitment of these vesicles to phagosomes. Altogether, the results of my project published in two first-author papers, suggest that phagocytic-endocytic proteolysis in DCs is aimed at degrading proteins 'partially' (processing) rather than 'totally', as occurs in macrophages and neutrophils.