Dorothee Schmid

Innate and Adaptive Immunity through Autophagy

The two main proteolytic machineries of eukaryotic cells, lysosomes and proteasomes, receive substrates by different routes. Polyubiquitination targets proteins for proteasomal degradation, whereas autophagy delivers intracellular material for lysosomal hydrolysis. The importance of autophagy for cell survival has long been appreciated, but more recently, its essential role in both innate and adaptive immunity has been characterized. Autophagy is now recognized to restrict viral infections and replication of intracellular bacteria and parasites. Additionally, this pathway delivers cytoplasmic antigens for MHC class II presentation to the adaptive immune system, which then in turn is able to regulate autophagy. At the same time, autophagy plays a role in the survival and the cell death of T cells. Thus, the immune system utilizes autophagic degradation of cytoplasmic material, to both restrict intracellular pathogens and regulate adaptive immunity.

β-amyloid is a substrate of autophagy in sporadic inclusion body myositis

Sporadic Inclusion Body Myositis (sIBM) is the most common acquired muscle disease in patients above 50 years of age. Apart from inflammation in the skeletal muscle, overexpression of amyloid precursor protein (APP) and intracellular accumulation of its proteolytic fragment β‐amyloid play a central role in the pathogenesis of sIBM. In neurodegenerative disorders, similar aggregations of aberrant proteins have recently been shown to be susceptible to autophagic degradation. Therefore, we analyzed macroautophagy of APP in human muscle cell lines and sIBM muscle biopsies.

Autophagy in innate and adaptive immunity against intracellular pathogens

Autophagy delivers cytoplasmic constituents for lysosomal degradation. Recent studies have demonstrated that this pathway mediates resistance to pathogens and is targeted for immune evasion by viruses and bacteria. Lysosomal degradation products, including pathogenic determinants, are then surveyed by the adaptive immune system to elicit antigen-specific T cell responses. CD4+ T helper cells have been shown to recognize nuclear and cytosolic antigens via presentation by major histocompatibility complex (MHC) class II molecules after autophagy. Furthermore, some sources of natural MHC class II ligands display characteristics of autophagy substrates, and autophagosomes fuse with late endosomes, in which MHC class II loading is thought to occur. Although MHC class II antigen processing via autophagy has so far mainly been described for professional antigen-presenting cells like B cells, macrophages, and dendritic cells, it might be even more important for cells with less endocytic potential, like epithelial cells, when these express MHC class II at sites of inflammation. Therefore, autophagy might contribute to immune surveillance of intracellular pathogens via MHC class II presentation of intracellular pathogen-derived peptides.

Immune Surveillance via Self Digestion

The adaptive immune system is orchestrated by CD4+ T cells. These cells detect peptides presented on Major Histocompatiblity Complex (MHC) class II molecules, which are loaded in late endosomes with products of lysosomal proteolysis. One pathway by which proteins gain access to degradation in lysosomes is macroautophagy. We recently showed that constitutive macroautophagy can be detected in cells relevant for the immune system, including dendritic cells. In these antigen presenting cells, autophagosomes frequently fused with MHC class II antigen loading compartments and targeting of Influenza matrix protein 1 (MP1) for macroautophagy enhanced MHC class II presentation to MP1-specific CD4+ T cell clones up to 20 fold. Our findings indicate that macroautophagy is a constitutive and efficient pathway of antigen delivery for MHC class II presentation. We suggest that this pathway samples intracellular proteins for immune surveillance and induction of tolerance in CD4+ T cells, and could be targeted for improved MHC class II presentation of vaccine antigens. Addendum to: MHC Class II Antigen Loading Compartments Continuously Receive Input from Autophagosomes Dorothee Schmid, Marc Pypaert and Christian Münz Immunity 2006; In press

Localization and MHC Class II Presentation of Antigens Targeted for Macroautophagy

Intracellular antigens can be presented on major histocompatibility complex (MHC) class II molecules after degradation via macroautophagy. To enhance MHC class II presentation of potential vaccine antigens, we have developed a method to target antigens for autophagic degradation via fusion to the Atg8/LC3 protein: Atg8/LC3 is specifically incorporated into autophagosomes via coupling to phosphatidylethanolamine, and subsequently degraded in MHC class II loading compartments (MIICs). Antigens fused to the N-terminus of Atg8/LC3 follow the same pathway and get preferentially presented on MHC class II molecules. The localization of Atg8/LC3 fusion antigens in MIICs can be visualized by confocal microscopy, and MHC class II presentation can be quantified in a presentation assay with antigen-specific CD4(+) T-cell clones. These assays are good measures of autophagosome formation and lysosomal degradation of macroautophagy cargo and therefore are useful for studying regulation of the autophagic pathway under various experimental conditions and physiological perturbations.