AUTOPHAGY 2016, VOL. 12, NO. 9, 1681–1682 http://dx.doi.org/10.1080/15548627.2016.1203488
AUTOPHAGIC PUNCTUM
ATGs help MHC class II, but inhibit MHC class I antigen presentation €nza Monica Loia, Monique Gannageb, and Christian Mu a Viral Immunobiology, Institute of Experimental Immunology, University of Zurich, Zurich, Switzerland; bDepartment of Pathology and Immunology, School of Medicine, University of Geneva, Geneva, Switzerland
ABSTRACT
ARTICLE HISTORY
We have recently shown that the LC3/Atg8 lipidation machinery of macroautophagy is involved in the internalization of MHC class I molecules. Decreased internalization in the absence of ATG5 or ATG7 leads to MHC class I surface stabilization on dendritic cells and macrophages, resulting in elevated CD8C T cell responses during viral infections and improved immune control. Here, we discuss how the autophagic machinery supports MHC class II restricted antigen presentation, while compromising MHC class I presentation via internalization and degradation.
Received 6 June 2016 Revised 8 June 2016 Accepted 13 June 2016 KEYWORDS
AAK1; Atg8/LC3; influenza virus; intracellular antigen processing; LC3-associated phagocytosis
The major histocompatibility complex (MHC) constitutes a set of molecules that are essential for the adaptive immune response. Their role is to present pathogen-derived antigen fragments on the cell surface for recognition by the appropriate T lymphocytes. Classically, endogenous antigens are processed by the proteasome in the cytosol or nucleus, loaded on MHC class I molecules in the endoplasmic reticulum and recognized by CD8C T lymphocytes. In contrast, exogenous antigens are taken up by endocytosis or phagocytosis and degraded by lysosomal proteolysis. These are then loaded on MHC class II molecules in late endosomes, so-called MHC class II-containing compartments (MIIC), and presented to CD4C T cells. However, there are exceptions to this paradigm. Extracellular, endocytosed antigen can also be presented on MHC class I molecules by cross-presentation, primarily by specialized antigen presenting cells, such as dendritic cells (DCs). Similarly, peptides of intracellular origin can be loaded onto MHC class II molecules and these ligands can be generated via several routes, including autophagy.
frequently with MIICs in human B cells, DCs and epithelial cell lines, as well as in mouse thymic epithelial cells (Fig. 1). This endogenous self-protein processing via macroautophagy contributes to both positive and negative CD4C T cell selection via presentation on MHC class II molecules in the mouse thymus as well as to herpesvirus-specific CD4C T cell responses in vivo. In contrast, exogenous antigen processing for MHC class II presentation benefits from the macroautophagy machinery via LC3-associated phagocytosis (LAP). During LAP, phagosomes get coated with LC3B and require reactive oxygen species production by the NADPH oxidase CYBB/NOX2 to acquire or maintain this coat (Fig. 1). Phagosome-associated LC3B seems to accelerate fusion with lysosomes in mouse macrophages, whereas in human macrophages, conventional and plasmacytoid DCs, LAP vesicles seem to be stabilized and to maintain antigen, resulting in prolonged MHC class II presentation. Thus the autophagic machinery seems to support both endogenous as well as exogenous antigen processing for MHC class II presentation via macroautophagy and LAP, respectively.
ATGs in MHC class II antigen presentation
ATGs in MHC class I antigen presentation
Twenty to 30 percent of natural MHC class II ligands originate from intracellular cytosolic and nuclear proteins, including the essential macroautophagy proteins LC3, GABARAP and GABARAPL2. Accordingly, targeting of antigens to phagophores (the autophagosome precursor) by fusing proteins to the N terminus of LC3B enhances MHC class II presentation of viral and tumor antigens up to 20-fold. Moreover, upon autophagy induced by starvation, MHC class II presentation of these cytosolic proteins increases by 50%, while membrane protein presentation remains unchanged. Indeed, autophagosomes fuse
In contrast to this supportive role for MHC class II-restricted antigen presentation, macroautophagy has been reported to restrict ubiquitinated antigen supply for proteasomal processing (Fig. 1). This results in decreased MHC class I presentation of the respective model antigens to CD8C T cells. Along similar lines of autophagy restricting antigen presentation by MHC class I molecules, we recently showed that murine antigen-presenting cells deficient for core components of macroautophagy, namely the LC3 lipidation machinery (such as ATG5 or ATG7), have elevated surface MHC class I expression
CONTACT Christian M€ unz [email protected] Viral Immunobiology, Institute of Experimental Immunology, University of Zurich, Winterhurerstrasse 190, CH-8057 Zurich, Switzerland Punctum to: Loi M, M€ uller A, Steinbach K, Niven J, Barreira da Silva R, Paul P, Ligeon L-A, Caruso A, Albrecht RA, Becker AC, Annaheim N, Nowag H, Dengjel J, Garcia-Sastre A, Merkler D, M€ unz C and Gannage M. Macroautophagy proteins control MHC class I levels on dendritic cells and shape anti-viral CD8C T cell responses. Cell Reports 2016; 15(5):1076-87. © 2016 Taylor & Francis
1682
M. LOI ET AL.
Figure 1. Macroautophagy supports antigen presentation on MHC class II, but compromises it on MHC class I molecules. (A) Macroautophagy involves the engulfment of cytoplasmic material and the formation of autophagosomes, which then can fuse with MHC class II-containing compartments. Antigenic peptides are then loaded onto MHC class II molecules, which can be delivered to the plasma membrane for CD4C T cell stimulation. (B) During LC3-associated phagocytosis (LAP) phagosomes are decorated with LC3B. After LC3B cleavage from their membrane, phagosomes fuse with MHC class II loading compartments. Their cargo is degraded and fragments are loaded onto MHC class II molecules. LAP facilitates lysosome-phagosome fusion or prolonged antigen processing for presentation by MHC class II in a cell type–specific manner. (C) Macroautophagy degrades intracellular proteins, which otherwise might serve as substrates for proteasomes. The resulting proteasome products give rise to MHC class I ligands after import into the ER by the canonical MHC class I antigen processing pathway. (D) In addition, macroautophagy attenuates MHC class I-restricted presentation by recruiting the internalization machinery to the MHC class I molecules at the cell surface. Here, AAK1 associates with, presumably membrane-bound, LC3B and triggers MHC class I internalization resulting in a diminished stimulation of CD8C T cells.
levels. These DC and macrophage populations stimulate CD8C T cells more efficiently in vitro and are associated with enhanced CD8C T cell responses during influenza A and lymphocytic choriomeningitis virus infection in vivo, resulting in improved immune control of influenza A virus. The increased MHC class I surface levels on these antigen-presenting cell populations seems to result from defective internalization of MHC class I molecules dependent on AAK1 (AP2 associated kinase 1), which is recruited to MHC class I molecules via binding to lipidated, presumably surface-coupled LC3B (Fig. 1). AAK1 phosphorylates components of the AP2 complex that is involved in clathrinmediated endocytosis. Accordingly, a significant pool of MHC class I molecules have been reported to reside in DCs within endosomes in addition to the endoplasmic reticulum, but the origin and intracellular trafficking of these vesicular MHC class I pools are not yet clearly defined. It was shown that a tyrosinebased sorting signal, associated with clathrin-dependent endocytosis, in the MHC I cytoplasmic tail is required for proper MHC class I internalization and trafficking to endosomal compartments. We found indeed that such vesicular MHC class I pools are compromised in the absence of ATG-dependent MHC class I internalization. Thus, membrane-associated LC3B seems to recruit the internalization machinery for MHC class I molecules, thereby dampening CD8C T cell responses during viral infections.
ATGs in receptor internalization This role for ATGs in MHC class I internalization is not the first description implicating the autophagic machinery in receptor endocytosis. The macroautophagy machinery has been previously described to play a role in trafficking of
plasma membrane molecules, such as EGFR, and for clathrin-mediated endocytosis in plasma membrane internalization for autophagosome formation. Indeed, lipidated LC3B interacts with components of the AP2 complex, which is directly phosphorylated by AAK1, for APP (amyloid b precursor protein) internalization and degradation. One AP2 subunit has been described to contain a LC3-interacting region (LIR), which is required for this internalization. Furthermore, also in clathrin itself, LIRs have been discovered and verified to interact with LC3 in cell-free biochemical assays. Thus, LC3B lipidation seems to assist different clathrin-dependent internalization and degradation steps for surface molecules, including MHC class I.
Conclusions The molecular machinery of macroautophagy not only serves for transporting cytoplasmic constituents for lysosomal degradation but it also influences vesicular transport in a broader sense and orchestrates T cell immunity. Indeed, autophagy proteins enable cells to expand the repertoire of antigens presented on MHC class II molecules while in their absence more stimulatory MHC class I molecules are present on the surface of DCs to elicit elevated CD8C T cell responses. Further studies will need to understand how one might selectively regulate these different macroautophagy functions on MHC-restricted antigen presentation for therapeutic interventions to boost adaptive immunity.
Disclosure of potential conflicts of interest No potential conflicts of interest were disclosed.
AUTOPHAGIC PUNCTUM
ATGs help MHC class II, but inhibit MHC class I antigen presentation €nza Monica Loia, Monique Gannageb, and Christian Mu a Viral Immunobiology, Institute of Experimental Immunology, University of Zurich, Zurich, Switzerland; bDepartment of Pathology and Immunology, School of Medicine, University of Geneva, Geneva, Switzerland
ABSTRACT
ARTICLE HISTORY
We have recently shown that the LC3/Atg8 lipidation machinery of macroautophagy is involved in the internalization of MHC class I molecules. Decreased internalization in the absence of ATG5 or ATG7 leads to MHC class I surface stabilization on dendritic cells and macrophages, resulting in elevated CD8C T cell responses during viral infections and improved immune control. Here, we discuss how the autophagic machinery supports MHC class II restricted antigen presentation, while compromising MHC class I presentation via internalization and degradation.
Received 6 June 2016 Revised 8 June 2016 Accepted 13 June 2016 KEYWORDS
AAK1; Atg8/LC3; influenza virus; intracellular antigen processing; LC3-associated phagocytosis
The major histocompatibility complex (MHC) constitutes a set of molecules that are essential for the adaptive immune response. Their role is to present pathogen-derived antigen fragments on the cell surface for recognition by the appropriate T lymphocytes. Classically, endogenous antigens are processed by the proteasome in the cytosol or nucleus, loaded on MHC class I molecules in the endoplasmic reticulum and recognized by CD8C T lymphocytes. In contrast, exogenous antigens are taken up by endocytosis or phagocytosis and degraded by lysosomal proteolysis. These are then loaded on MHC class II molecules in late endosomes, so-called MHC class II-containing compartments (MIIC), and presented to CD4C T cells. However, there are exceptions to this paradigm. Extracellular, endocytosed antigen can also be presented on MHC class I molecules by cross-presentation, primarily by specialized antigen presenting cells, such as dendritic cells (DCs). Similarly, peptides of intracellular origin can be loaded onto MHC class II molecules and these ligands can be generated via several routes, including autophagy.
frequently with MIICs in human B cells, DCs and epithelial cell lines, as well as in mouse thymic epithelial cells (Fig. 1). This endogenous self-protein processing via macroautophagy contributes to both positive and negative CD4C T cell selection via presentation on MHC class II molecules in the mouse thymus as well as to herpesvirus-specific CD4C T cell responses in vivo. In contrast, exogenous antigen processing for MHC class II presentation benefits from the macroautophagy machinery via LC3-associated phagocytosis (LAP). During LAP, phagosomes get coated with LC3B and require reactive oxygen species production by the NADPH oxidase CYBB/NOX2 to acquire or maintain this coat (Fig. 1). Phagosome-associated LC3B seems to accelerate fusion with lysosomes in mouse macrophages, whereas in human macrophages, conventional and plasmacytoid DCs, LAP vesicles seem to be stabilized and to maintain antigen, resulting in prolonged MHC class II presentation. Thus the autophagic machinery seems to support both endogenous as well as exogenous antigen processing for MHC class II presentation via macroautophagy and LAP, respectively.
ATGs in MHC class II antigen presentation
ATGs in MHC class I antigen presentation
Twenty to 30 percent of natural MHC class II ligands originate from intracellular cytosolic and nuclear proteins, including the essential macroautophagy proteins LC3, GABARAP and GABARAPL2. Accordingly, targeting of antigens to phagophores (the autophagosome precursor) by fusing proteins to the N terminus of LC3B enhances MHC class II presentation of viral and tumor antigens up to 20-fold. Moreover, upon autophagy induced by starvation, MHC class II presentation of these cytosolic proteins increases by 50%, while membrane protein presentation remains unchanged. Indeed, autophagosomes fuse
In contrast to this supportive role for MHC class II-restricted antigen presentation, macroautophagy has been reported to restrict ubiquitinated antigen supply for proteasomal processing (Fig. 1). This results in decreased MHC class I presentation of the respective model antigens to CD8C T cells. Along similar lines of autophagy restricting antigen presentation by MHC class I molecules, we recently showed that murine antigen-presenting cells deficient for core components of macroautophagy, namely the LC3 lipidation machinery (such as ATG5 or ATG7), have elevated surface MHC class I expression
CONTACT Christian M€ unz [email protected] Viral Immunobiology, Institute of Experimental Immunology, University of Zurich, Winterhurerstrasse 190, CH-8057 Zurich, Switzerland Punctum to: Loi M, M€ uller A, Steinbach K, Niven J, Barreira da Silva R, Paul P, Ligeon L-A, Caruso A, Albrecht RA, Becker AC, Annaheim N, Nowag H, Dengjel J, Garcia-Sastre A, Merkler D, M€ unz C and Gannage M. Macroautophagy proteins control MHC class I levels on dendritic cells and shape anti-viral CD8C T cell responses. Cell Reports 2016; 15(5):1076-87. © 2016 Taylor & Francis
1682
M. LOI ET AL.
Figure 1. Macroautophagy supports antigen presentation on MHC class II, but compromises it on MHC class I molecules. (A) Macroautophagy involves the engulfment of cytoplasmic material and the formation of autophagosomes, which then can fuse with MHC class II-containing compartments. Antigenic peptides are then loaded onto MHC class II molecules, which can be delivered to the plasma membrane for CD4C T cell stimulation. (B) During LC3-associated phagocytosis (LAP) phagosomes are decorated with LC3B. After LC3B cleavage from their membrane, phagosomes fuse with MHC class II loading compartments. Their cargo is degraded and fragments are loaded onto MHC class II molecules. LAP facilitates lysosome-phagosome fusion or prolonged antigen processing for presentation by MHC class II in a cell type–specific manner. (C) Macroautophagy degrades intracellular proteins, which otherwise might serve as substrates for proteasomes. The resulting proteasome products give rise to MHC class I ligands after import into the ER by the canonical MHC class I antigen processing pathway. (D) In addition, macroautophagy attenuates MHC class I-restricted presentation by recruiting the internalization machinery to the MHC class I molecules at the cell surface. Here, AAK1 associates with, presumably membrane-bound, LC3B and triggers MHC class I internalization resulting in a diminished stimulation of CD8C T cells.
levels. These DC and macrophage populations stimulate CD8C T cells more efficiently in vitro and are associated with enhanced CD8C T cell responses during influenza A and lymphocytic choriomeningitis virus infection in vivo, resulting in improved immune control of influenza A virus. The increased MHC class I surface levels on these antigen-presenting cell populations seems to result from defective internalization of MHC class I molecules dependent on AAK1 (AP2 associated kinase 1), which is recruited to MHC class I molecules via binding to lipidated, presumably surface-coupled LC3B (Fig. 1). AAK1 phosphorylates components of the AP2 complex that is involved in clathrinmediated endocytosis. Accordingly, a significant pool of MHC class I molecules have been reported to reside in DCs within endosomes in addition to the endoplasmic reticulum, but the origin and intracellular trafficking of these vesicular MHC class I pools are not yet clearly defined. It was shown that a tyrosinebased sorting signal, associated with clathrin-dependent endocytosis, in the MHC I cytoplasmic tail is required for proper MHC class I internalization and trafficking to endosomal compartments. We found indeed that such vesicular MHC class I pools are compromised in the absence of ATG-dependent MHC class I internalization. Thus, membrane-associated LC3B seems to recruit the internalization machinery for MHC class I molecules, thereby dampening CD8C T cell responses during viral infections.
ATGs in receptor internalization This role for ATGs in MHC class I internalization is not the first description implicating the autophagic machinery in receptor endocytosis. The macroautophagy machinery has been previously described to play a role in trafficking of
plasma membrane molecules, such as EGFR, and for clathrin-mediated endocytosis in plasma membrane internalization for autophagosome formation. Indeed, lipidated LC3B interacts with components of the AP2 complex, which is directly phosphorylated by AAK1, for APP (amyloid b precursor protein) internalization and degradation. One AP2 subunit has been described to contain a LC3-interacting region (LIR), which is required for this internalization. Furthermore, also in clathrin itself, LIRs have been discovered and verified to interact with LC3 in cell-free biochemical assays. Thus, LC3B lipidation seems to assist different clathrin-dependent internalization and degradation steps for surface molecules, including MHC class I.
Conclusions The molecular machinery of macroautophagy not only serves for transporting cytoplasmic constituents for lysosomal degradation but it also influences vesicular transport in a broader sense and orchestrates T cell immunity. Indeed, autophagy proteins enable cells to expand the repertoire of antigens presented on MHC class II molecules while in their absence more stimulatory MHC class I molecules are present on the surface of DCs to elicit elevated CD8C T cell responses. Further studies will need to understand how one might selectively regulate these different macroautophagy functions on MHC-restricted antigen presentation for therapeutic interventions to boost adaptive immunity.
Disclosure of potential conflicts of interest No potential conflicts of interest were disclosed.
