- Email: [email protected]
The role of Fc-receptors in antigen cross-presentation by human dendritic cell subsets
Abstracts / Molecular Immunology 51 (2012) 5–41
lished as a key regulatory signal in endocytic trafficking. As de-ubiquitinating enzymes (DUBs), proteases specific for cleavage of ubiquitin from modified substrate proteins, impart reversibility to ubiquitination, they are well suited for spatiotemporal control of dynamic cellular processes. Using a flow cytometry-based approach, we screened a siRNA library targeting 90 human DUBs for effects on presentation of MHC-II on the surface of MelJuSo cells. Silencing of roughly 15% of the library was found to significantly alter expression or peptide loading properties of MHC-II. Of particular interest were the MIT domain containing DUBs USP8, USP54, AMSH and AMSH-LP from ubiquitin-specific protease and JAMM domain metallo-protease families, respectively. Loss of USP8 incurs enlarged MVBs characterized by accumulated ubiquitinated cargo, while depletion of USP54 results in clustering of subcellular organelles, including the MVBs. Current studies focusing on establishing the roles of their respective MIT domains in endosomal transport. By contrast to USP8 and USP54, knockdown of AMSH produces scattering of the late endosomal compartment, while co-expression of a putative novel ESCRT protein Tollip with catalytically inactive AMSH-D348A induces profound vacuolarization of the late endosomes. The latter phenotype is accompanied by clustering of Tollip-positive endosomes, akin to the effect of AMSHD348A on the ESCRT-0 complex organized around the adaptor protein Hrs. Ongoing work is addressing the molecular determinants underlying AMSH function in this context. doi:10.1016/j.molimm.2012.02.017 Protein topology and epitope position play a critical role in shaping CD8 T cell immunodominance to the Toxoplasma gondii parasite Harshita Grover a , Virginie Vasseur b , Virginie Féliu b , Jeremy Wang a , Jon Boyle c , Hamlet Chu a , Coraline Chéneau b , Ellen Robey a , Nilabh Shastri a , Nicolas Blanchard b,∗ a
University of California, Berkeley, USA Center of Pathophysiology of Toulouse-Purpan, France c University of Pittsburgh, PA, USA b
CD8 T cells protect from intracellular pathogens such as the Toxoplasma gondii (T. gondii) parasite. Despite T. gondii complexity, CD8 T cell responses target only a restricted panel of antigens, a phenomenon known as immunodominance. In T. gondii-infected mice, at least three natural antigens are presented by Ld MHC I molecules and recognized by CD8 T cells. The GRA6-derived HF10 epitope induces large and protective CD8 T cell populations but the contribution of subdominant epitopes to protection and the mechanisms underlying immunodominance hierarchy are unknown. Here, we investigated the molecular bases of CD8 immunodominance in T. gondii-infected mice. Using genetically modified parasites, we show that the two subdominant responses cannot compensate for protection afforded by the HF10-specific response. Rather than affinity for MHC I or T cell precursor frequency, we report that the critical parameters controlling HF10 immunodominance are the topology of GRA6 insertion in the membrane of the parasitophorous vacuole and the C-terminal position of HF10 within GRA6. Grafting another endogenous Toxo-derived epitope to GRA6 C-terminus (the SM9 epitope) indeed enhanced its processing and presentation. In a natural context of infection, our results emphasize the importance of the epitope position within the antigenic protein for its immunogenicity. They shed light on the poorly understood mechanisms that shape immunodominance in parasite infections. doi:10.1016/j.molimm.2012.02.018
9
Tracking endosomal membrane integrity in single antigen presenting cells Alexandre Bobard a,∗ , Marianne Burbage a , Roxanne Simeone a , Ignacio Cebrian b , Ariel Savina b , Roland Brosch a , Sebastian Amigorena b , Jost Enninga a a b
Pasteur Institute, Paris Curie Institute, Paris
Antigen presenting cells ingest and process foreign particles and microorganisms from the environment before processing them and presenting them to other immune cells. In the case of bacteria, such as Shigella or Listeria, the endocytic membranes are breached and the pathogen escapes into the host cytoplasm. We have developed fluorescence imaging approaches and automated image analysis algorithms to monitor specifically whether the bacterial pathogens internalized by host cells remain membrane bound or whether they reach the host cytoplasm. Exemplary, this has revealed the unexpected rapid entry of Shigella flexneri into the host cytoplasm. Using our approach, we performed comparative work on the intracellular localization of numerous bacterial pathogens, including different species of Mycobacterium. Imaging revealed that the RD-1 region encoding a type-VII-secretion system is required and sufficient for the rupture of mycobacteria-containing vacuoles resulting in necrotic host cell death. Interestingly, our approach can also be adapted for the study of the intracellular processing of other type of antigens, such as inert particles. With this, we have been able to characterize the cellular route following particle engulfment that leads to the presentation of antigens on the cellular surface. Concerning this, the processed antigenic peptides on the so-called MHC classes I and II elicits efficient pathogen destruction by cytotoxic processes and/or antibody production. Several pathways for antigen cell surface presentation have been characterized. Classical MHC-I and II presentation pathways are well characterized and respectively allow the elimination of intracellular and extracellular pathogens. Another type of immune response, the so-called cross presentation, is a central process providing immunity against cancer, infection and grafts is not fully characterized. By loading phagocytes with particles carrying our model antigens, we assessed if the antigen went through a vacuolar versus cytosolic pathway upon phagocytosis. Our data show that a number of phagocytic cells were able to translocate antigens from the phagosome to the cytosol that has been described for the MHC class I cross presentation pathway. Furthermore, we identified cellular factors that are implicated in integrity of the endocytic membranes linking the endocytic machinery with the endoplasmic reticulum. Together, these findings shed light on the capacity of different phagocytes to export antigens from endosomal compartments to the cytosol. Given the robustness of our assay, this also opens the opportunity to screen for cellular factors involved in cytosolic translocation of antigens following phagocytosis. Our studies improve our understanding of antigen presentation at the cellular level in order to boost immunity and will bring new insights into the cellular mechanisms against infection and cancer. doi:10.1016/j.molimm.2012.02.019 The role of Fc-receptors in antigen cross-presentation by human dendritic cell subsets Thijs Flinsenberg, Ewoud Compeer, Femke Amelung, Jaap Boelens, Marianne Boes ∗ University Medical Centre Utrecht Introduction: The cross-presentation of endocytosed antigen in the form of peptide/class I MHC complexes plays a central role in
10
Abstracts / Molecular Immunology 51 (2012) 5–41
the elicitation of CD8+ cytotoxic T lymphocytes (CTLs) that mediate anti-viral and anti-tumour immune responses. Knowledge of the mechanisms behind cross-presentation may create opportunities for creating better anti-viral and anti-tumour cellular therapies. In mice, antigen uptake via Fc receptors (FcR) enhances crosspresentation. We aim to increase cross presentation in several human dendritic cell (DC) subsets by targeting FcR, thereby improving the elicitation of virus specific CTLs. Methods: CD1c+ or CD141+ primary or cultured immature monocyte derived human DCs (iDCs) were pulsed with IgGopsonized and non-opsonized freeze-thaw lysates of CMV-positive cells, synthetic CMV protein or peptide. Uptake via FcR was blocked by using human anti-CD16, -CD32 and -CD64 F(ab )2-fragments. As functional read-out, we measured activation of antigen specific HLA-A2-restricted CTLs by flow-cytometry (6 h of co-culture; production of IFN-y, TNF and surface-expression of lysosome associated membrane protein LAMP-1). Results: We show uptake of CMV protein and lysates by more than 80% of iDCs. The amount of internalized antigen was not affected by prior opsonization with polyclonal IgG or anti-CMV pp65 IgG. All analyzed human DC subtypes cross-presented pp65 protein resulting in CTL activation. CD141+ DC but not CD1c+ or monocyte-derived DC effectively processed and presented pp65-containing lysates as peptide/HLA A2 complexes. Moreover, IgG opsonization of protein improved antigen cross-presentation, which can be completely abrogated by prior incubating the DC with human Fc fragments. Conclusion: Targeting FcR improves antigen cross-presentation in human DC subtypes. We are currently clarifying mechanisms that mediate FcR mediated antigen cross-presentation by cell biological and confocal microscopy-based methods.
Suppression of antigen presentation by the adenovirus E3-19 K protein; a crystallographic analysis of E3-19 K/MHC I interaction
doi:10.1016/j.molimm.2012.02.020
doi:10.1016/j.molimm.2012.02.022
Remodeling of the endosomal pathway during antigen crosspresentation by human dendritic cells
The tapasin-related protein TAPBPR modulates the ER export rate of MHC class I
Ewoud Compeer, Thijs Flinsenberg, Marianne Boes ∗
Louise Boyle ∗ , Keith Porter, Jessica Boname, Marian Burr, Peysh Patel, David Rhodes, Paul Lehner, John Trowsdale
University Medical Centre Utrecht The cross-presentation of endocytosed antigen in the form of peptide/class I MHC complexes plays a central role in the elicitation of CD8+ T cell clones for virus and tumor-directed immune responses. To clarify mechanisms in antigen cross-presentation, we studied transport mechanisms of peptide/MHC I complexes in human dendritic cells (DC). Earlier studies using DC from MHC class II-EGFP knock-in mice showed that Toll-like receptor (TLR) triggering induces the remodeling of MHCII-eGFP+ compartment into tubular structures in a microtubule dependent manner. By use of live cell confocal microscopy techniques, we confirm in human monocyte-derived DC the rapid transformation of vesicular late endosomes into tubular structures upon detection of TLR signals. Recent mouse-based work demonstrates that obstruction of the recycling pathway abrogates antigen cross-presentation, while leaving classical class I and class II MHC presentation intact. We investigated remodeling of recycling endosomal compartments using a cross-presentation model of uptake of viral antigen by human DC, fluorophore-conjugated transferrin and antigenspecific CD8+ T cell clones. We demonstrate a requirement for both TLR signaling and T cell contact for tubular remodeling of recycling compartments to occur. We propose that the rapid endosomal remodeling of endosomal compartments and polarization towards the CD8+ T cell enables efficient T cell activation. doi:10.1016/j.molimm.2012.02.021
Marlene Bouvier a,∗ , Lenong Li a , Yasameen Muzahim b a
University of Illinois at Chicago/Department of Microbiology and Immunology b University of Illinois at Chicago/Honors College
The cell-surface presentation of viral antigens by MHC class I molecules is critical for eliminating infected cells. In turn, viruses have evolved strategies to interfere with the process of antigen presentation. The adenovirus (Ad) E3-19 K protein binds to and retains MHC class I molecules in the endoplasmic reticulum (ER), blocking their egress to the cell surface. We characterized for the first time interaction between Ad type 2 (Ad2) E3-19 K and HLA-A2 using X-ray crystallography. Our (unpublished) structure to 2.1 A˚ resolution reveals that Ad2 E3-19 K interacts with the N-terminus of the ␣1-helix and C-terminus of the ␣2-helix of HLA-A2, i.e., the C-terminal end of the groove. E3-19 K binds away from the opening of the groove. This mode of binding is entirely consistent with our biochemical studies of E3-19 K/MHC I interaction, including the ability of E3-19 K to associate with mature and immature MHC class I molecules. Importantly, the Ad2 E3-19 K/HLA-A2 structure reveals how E3-19 K uses it variable and conserved regions to bind to MHC I, and how it accommodates polymorphism in MHC I. Our structure also allows to understand the ability of E3-19 K of different Ad serotypes and subgroups to associate with MHC I in an allele- and locus-specific manner. Overall, the Ad2 E3-19 K/HLA-A2 structure reveals determinants in E3-19 K that are critical for targeting MHC class I molecules. It also permits to understand better how E3-19 K has evolved to subvert antigen presentation.
University Of Cambridge The current model of MHC class I antigen processing involves a number of proteins, namely calnexin, calreticulin, ERp57, tapasin, TAP1 and TAP2, and ERAP1/2 which together assist in the folding and peptide loading of the MHC class I heterodimer in the ER. A tapasin-related transcript of unknown function, named TAPBPR, is encoded on chromosome 12 in an MHC paralogue region. We have investigated the function of TAPBPR. We have identified that MHC class I heterodimers are a ligand for TAPBPR. Like tapasin, TAPBPR is an IFN- inducible protein which binds to class I in the ER. However the function of TAPBPR is clearly distinct from tapasin. When TAPBPR is over-expressed in cells, ER export of MHC class I is significantly slower and surface expression of MHC class I heterotrimers is considerably decreased. In agreement with this depletion of endogenous TAPBPR conversely increases the ER export rate of MHC class I. Our results also indicate a function for TAPBPR beyond the ER since TAPBPR can remain bound to class I while it is transported through the Golgi. The discovery of a new MHC class I-dedicated binding protein raises the question as to how it fits into the known MHC class I antigen processing and presentation pathway. doi:10.1016/j.molimm.2012.02.023
lished as a key regulatory signal in endocytic trafficking. As de-ubiquitinating enzymes (DUBs), proteases specific for cleavage of ubiquitin from modified substrate proteins, impart reversibility to ubiquitination, they are well suited for spatiotemporal control of dynamic cellular processes. Using a flow cytometry-based approach, we screened a siRNA library targeting 90 human DUBs for effects on presentation of MHC-II on the surface of MelJuSo cells. Silencing of roughly 15% of the library was found to significantly alter expression or peptide loading properties of MHC-II. Of particular interest were the MIT domain containing DUBs USP8, USP54, AMSH and AMSH-LP from ubiquitin-specific protease and JAMM domain metallo-protease families, respectively. Loss of USP8 incurs enlarged MVBs characterized by accumulated ubiquitinated cargo, while depletion of USP54 results in clustering of subcellular organelles, including the MVBs. Current studies focusing on establishing the roles of their respective MIT domains in endosomal transport. By contrast to USP8 and USP54, knockdown of AMSH produces scattering of the late endosomal compartment, while co-expression of a putative novel ESCRT protein Tollip with catalytically inactive AMSH-D348A induces profound vacuolarization of the late endosomes. The latter phenotype is accompanied by clustering of Tollip-positive endosomes, akin to the effect of AMSHD348A on the ESCRT-0 complex organized around the adaptor protein Hrs. Ongoing work is addressing the molecular determinants underlying AMSH function in this context. doi:10.1016/j.molimm.2012.02.017 Protein topology and epitope position play a critical role in shaping CD8 T cell immunodominance to the Toxoplasma gondii parasite Harshita Grover a , Virginie Vasseur b , Virginie Féliu b , Jeremy Wang a , Jon Boyle c , Hamlet Chu a , Coraline Chéneau b , Ellen Robey a , Nilabh Shastri a , Nicolas Blanchard b,∗ a
University of California, Berkeley, USA Center of Pathophysiology of Toulouse-Purpan, France c University of Pittsburgh, PA, USA b
CD8 T cells protect from intracellular pathogens such as the Toxoplasma gondii (T. gondii) parasite. Despite T. gondii complexity, CD8 T cell responses target only a restricted panel of antigens, a phenomenon known as immunodominance. In T. gondii-infected mice, at least three natural antigens are presented by Ld MHC I molecules and recognized by CD8 T cells. The GRA6-derived HF10 epitope induces large and protective CD8 T cell populations but the contribution of subdominant epitopes to protection and the mechanisms underlying immunodominance hierarchy are unknown. Here, we investigated the molecular bases of CD8 immunodominance in T. gondii-infected mice. Using genetically modified parasites, we show that the two subdominant responses cannot compensate for protection afforded by the HF10-specific response. Rather than affinity for MHC I or T cell precursor frequency, we report that the critical parameters controlling HF10 immunodominance are the topology of GRA6 insertion in the membrane of the parasitophorous vacuole and the C-terminal position of HF10 within GRA6. Grafting another endogenous Toxo-derived epitope to GRA6 C-terminus (the SM9 epitope) indeed enhanced its processing and presentation. In a natural context of infection, our results emphasize the importance of the epitope position within the antigenic protein for its immunogenicity. They shed light on the poorly understood mechanisms that shape immunodominance in parasite infections. doi:10.1016/j.molimm.2012.02.018
9
Tracking endosomal membrane integrity in single antigen presenting cells Alexandre Bobard a,∗ , Marianne Burbage a , Roxanne Simeone a , Ignacio Cebrian b , Ariel Savina b , Roland Brosch a , Sebastian Amigorena b , Jost Enninga a a b
Pasteur Institute, Paris Curie Institute, Paris
Antigen presenting cells ingest and process foreign particles and microorganisms from the environment before processing them and presenting them to other immune cells. In the case of bacteria, such as Shigella or Listeria, the endocytic membranes are breached and the pathogen escapes into the host cytoplasm. We have developed fluorescence imaging approaches and automated image analysis algorithms to monitor specifically whether the bacterial pathogens internalized by host cells remain membrane bound or whether they reach the host cytoplasm. Exemplary, this has revealed the unexpected rapid entry of Shigella flexneri into the host cytoplasm. Using our approach, we performed comparative work on the intracellular localization of numerous bacterial pathogens, including different species of Mycobacterium. Imaging revealed that the RD-1 region encoding a type-VII-secretion system is required and sufficient for the rupture of mycobacteria-containing vacuoles resulting in necrotic host cell death. Interestingly, our approach can also be adapted for the study of the intracellular processing of other type of antigens, such as inert particles. With this, we have been able to characterize the cellular route following particle engulfment that leads to the presentation of antigens on the cellular surface. Concerning this, the processed antigenic peptides on the so-called MHC classes I and II elicits efficient pathogen destruction by cytotoxic processes and/or antibody production. Several pathways for antigen cell surface presentation have been characterized. Classical MHC-I and II presentation pathways are well characterized and respectively allow the elimination of intracellular and extracellular pathogens. Another type of immune response, the so-called cross presentation, is a central process providing immunity against cancer, infection and grafts is not fully characterized. By loading phagocytes with particles carrying our model antigens, we assessed if the antigen went through a vacuolar versus cytosolic pathway upon phagocytosis. Our data show that a number of phagocytic cells were able to translocate antigens from the phagosome to the cytosol that has been described for the MHC class I cross presentation pathway. Furthermore, we identified cellular factors that are implicated in integrity of the endocytic membranes linking the endocytic machinery with the endoplasmic reticulum. Together, these findings shed light on the capacity of different phagocytes to export antigens from endosomal compartments to the cytosol. Given the robustness of our assay, this also opens the opportunity to screen for cellular factors involved in cytosolic translocation of antigens following phagocytosis. Our studies improve our understanding of antigen presentation at the cellular level in order to boost immunity and will bring new insights into the cellular mechanisms against infection and cancer. doi:10.1016/j.molimm.2012.02.019 The role of Fc-receptors in antigen cross-presentation by human dendritic cell subsets Thijs Flinsenberg, Ewoud Compeer, Femke Amelung, Jaap Boelens, Marianne Boes ∗ University Medical Centre Utrecht Introduction: The cross-presentation of endocytosed antigen in the form of peptide/class I MHC complexes plays a central role in
10
Abstracts / Molecular Immunology 51 (2012) 5–41
the elicitation of CD8+ cytotoxic T lymphocytes (CTLs) that mediate anti-viral and anti-tumour immune responses. Knowledge of the mechanisms behind cross-presentation may create opportunities for creating better anti-viral and anti-tumour cellular therapies. In mice, antigen uptake via Fc receptors (FcR) enhances crosspresentation. We aim to increase cross presentation in several human dendritic cell (DC) subsets by targeting FcR, thereby improving the elicitation of virus specific CTLs. Methods: CD1c+ or CD141+ primary or cultured immature monocyte derived human DCs (iDCs) were pulsed with IgGopsonized and non-opsonized freeze-thaw lysates of CMV-positive cells, synthetic CMV protein or peptide. Uptake via FcR was blocked by using human anti-CD16, -CD32 and -CD64 F(ab )2-fragments. As functional read-out, we measured activation of antigen specific HLA-A2-restricted CTLs by flow-cytometry (6 h of co-culture; production of IFN-y, TNF and surface-expression of lysosome associated membrane protein LAMP-1). Results: We show uptake of CMV protein and lysates by more than 80% of iDCs. The amount of internalized antigen was not affected by prior opsonization with polyclonal IgG or anti-CMV pp65 IgG. All analyzed human DC subtypes cross-presented pp65 protein resulting in CTL activation. CD141+ DC but not CD1c+ or monocyte-derived DC effectively processed and presented pp65-containing lysates as peptide/HLA A2 complexes. Moreover, IgG opsonization of protein improved antigen cross-presentation, which can be completely abrogated by prior incubating the DC with human Fc fragments. Conclusion: Targeting FcR improves antigen cross-presentation in human DC subtypes. We are currently clarifying mechanisms that mediate FcR mediated antigen cross-presentation by cell biological and confocal microscopy-based methods.
Suppression of antigen presentation by the adenovirus E3-19 K protein; a crystallographic analysis of E3-19 K/MHC I interaction
doi:10.1016/j.molimm.2012.02.020
doi:10.1016/j.molimm.2012.02.022
Remodeling of the endosomal pathway during antigen crosspresentation by human dendritic cells
The tapasin-related protein TAPBPR modulates the ER export rate of MHC class I
Ewoud Compeer, Thijs Flinsenberg, Marianne Boes ∗
Louise Boyle ∗ , Keith Porter, Jessica Boname, Marian Burr, Peysh Patel, David Rhodes, Paul Lehner, John Trowsdale
University Medical Centre Utrecht The cross-presentation of endocytosed antigen in the form of peptide/class I MHC complexes plays a central role in the elicitation of CD8+ T cell clones for virus and tumor-directed immune responses. To clarify mechanisms in antigen cross-presentation, we studied transport mechanisms of peptide/MHC I complexes in human dendritic cells (DC). Earlier studies using DC from MHC class II-EGFP knock-in mice showed that Toll-like receptor (TLR) triggering induces the remodeling of MHCII-eGFP+ compartment into tubular structures in a microtubule dependent manner. By use of live cell confocal microscopy techniques, we confirm in human monocyte-derived DC the rapid transformation of vesicular late endosomes into tubular structures upon detection of TLR signals. Recent mouse-based work demonstrates that obstruction of the recycling pathway abrogates antigen cross-presentation, while leaving classical class I and class II MHC presentation intact. We investigated remodeling of recycling endosomal compartments using a cross-presentation model of uptake of viral antigen by human DC, fluorophore-conjugated transferrin and antigenspecific CD8+ T cell clones. We demonstrate a requirement for both TLR signaling and T cell contact for tubular remodeling of recycling compartments to occur. We propose that the rapid endosomal remodeling of endosomal compartments and polarization towards the CD8+ T cell enables efficient T cell activation. doi:10.1016/j.molimm.2012.02.021
Marlene Bouvier a,∗ , Lenong Li a , Yasameen Muzahim b a
University of Illinois at Chicago/Department of Microbiology and Immunology b University of Illinois at Chicago/Honors College
The cell-surface presentation of viral antigens by MHC class I molecules is critical for eliminating infected cells. In turn, viruses have evolved strategies to interfere with the process of antigen presentation. The adenovirus (Ad) E3-19 K protein binds to and retains MHC class I molecules in the endoplasmic reticulum (ER), blocking their egress to the cell surface. We characterized for the first time interaction between Ad type 2 (Ad2) E3-19 K and HLA-A2 using X-ray crystallography. Our (unpublished) structure to 2.1 A˚ resolution reveals that Ad2 E3-19 K interacts with the N-terminus of the ␣1-helix and C-terminus of the ␣2-helix of HLA-A2, i.e., the C-terminal end of the groove. E3-19 K binds away from the opening of the groove. This mode of binding is entirely consistent with our biochemical studies of E3-19 K/MHC I interaction, including the ability of E3-19 K to associate with mature and immature MHC class I molecules. Importantly, the Ad2 E3-19 K/HLA-A2 structure reveals how E3-19 K uses it variable and conserved regions to bind to MHC I, and how it accommodates polymorphism in MHC I. Our structure also allows to understand the ability of E3-19 K of different Ad serotypes and subgroups to associate with MHC I in an allele- and locus-specific manner. Overall, the Ad2 E3-19 K/HLA-A2 structure reveals determinants in E3-19 K that are critical for targeting MHC class I molecules. It also permits to understand better how E3-19 K has evolved to subvert antigen presentation.
University Of Cambridge The current model of MHC class I antigen processing involves a number of proteins, namely calnexin, calreticulin, ERp57, tapasin, TAP1 and TAP2, and ERAP1/2 which together assist in the folding and peptide loading of the MHC class I heterodimer in the ER. A tapasin-related transcript of unknown function, named TAPBPR, is encoded on chromosome 12 in an MHC paralogue region. We have investigated the function of TAPBPR. We have identified that MHC class I heterodimers are a ligand for TAPBPR. Like tapasin, TAPBPR is an IFN- inducible protein which binds to class I in the ER. However the function of TAPBPR is clearly distinct from tapasin. When TAPBPR is over-expressed in cells, ER export of MHC class I is significantly slower and surface expression of MHC class I heterotrimers is considerably decreased. In agreement with this depletion of endogenous TAPBPR conversely increases the ER export rate of MHC class I. Our results also indicate a function for TAPBPR beyond the ER since TAPBPR can remain bound to class I while it is transported through the Golgi. The discovery of a new MHC class I-dedicated binding protein raises the question as to how it fits into the known MHC class I antigen processing and presentation pathway. doi:10.1016/j.molimm.2012.02.023