The ability to process microbial antigens and present them at the surface of cells is an important aspect of our innate ability to clear infections. It is generally accepted that antigens in the cytoplasm are loaded in the endoplasmic reticulum and presented at the cell surface on major histocompatibility complex (MHC) class I molecules, whereas peptides present in endo/phagocytic compartments are presented on MHC class II molecules. Despite the apparent segregation of the class I and class II pathways, antigens from intracellular pathogens including mycobacteria, Escherichia coli, Salmonella typhimurium, Brucella abortus and Leishmania, have been shown to elicit an MHC class-I-dependent CD8+ T-cell response, a process referred to as cross-presentation. The cellular mechanisms allowing the cross-presentation pathway are poorly understood. Here we show that phagosomes display the elements and properties needed to be self-sufficient for the cross-presentation of exogenous antigens, a newly ascribed function linked to phagocytosis mediated by the endoplasmic reticulum.

Summary Many immune system receptors signal through cytoplasmic tyrosine-based motifs (ITAMs), but how receptor ligation results in ITAM phosphorylation remains unknown. Live cell imaging studies showed a close interaction of the CD3ε cytoplasmic domain of the T cell receptor (TCR) with the plasma membrane through fluorescence resonance energy transfer (FRET) between a C-terminal fluorescent protein and a membrane fluorophore. Electrostatic interactions between basic CD3ε residues and acidic phospholipids enriched in the inner leaflet of the plasma membrane were required for binding. The nuclear magnetic resonance (NMR) structure of the lipid-bound state of this cytoplasmic domain revealed deep insertion of the two key tyrosines into the hydrophobic core of the lipid bilayer. Receptor ligation thus needs to result in unbinding of the CD3ε ITAM from the membrane to render these tyrosines accessible to Src kinases. Sequestration of key tyrosines into the lipid bilayer represents a previously unrecognized mechanism for control of receptor activation.

Phagocytosis is a key aspect of our innate ability to fight infectious diseases. In this study, we have found that fusion of the endoplasmic reticulum (ER) with the macrophage plasmalemma, underneath phagocytic cups, is a source of membrane for phagosome formation in macrophages. Successive waves of ER become associated with maturing phagosomes during phagolysosome biogenesis. Thus, the ER appears to possess unexpectedly pluripotent fusion properties. ER-mediated phagocytosis is regulated in part by phosphatidylinositol 3-kinase and used for the internalization of inert particles and intracellular pathogens, regardless of their final trafficking in the host. In neutrophils, where pathogens are rapidly killed, the ER is not used as a major source of membrane for phagocytosis. We propose that intracellular pathogens have evolved to adapt and exploit ER-mediated phagocytosis to avoid destruction in host cells.

Antigen presentation is essential for establishing immune tolerance and for immune responses against infectious disease and cancer. Although antigen presentation can be mediated by autophagy, here we demonstrate a pathway for mitochondrial antigen presentation (MitAP) that relies on the generation and trafficking of mitochondrial-derived vesicles (MDVs) rather than on autophagy/mitophagy. We find that PINK1 and Parkin, two mitochondrial proteins linked to Parkinson's disease (PD), actively inhibit MDV formation and MitAP. In absence of PINK1 or Parkin, inflammatory conditions trigger MitAP in immune cells, both in vitro and in vivo. MitAP and the formation of MDVs require Rab9 and Sorting nexin 9, whose recruitment to mitochondria is inhibited by Parkin. The identification of PINK1 and Parkin as suppressors of an immune-response-eliciting pathway provoked by inflammation suggests new insights into PD pathology.