Sec61 blockade by mycolactone inhibits antigen cross-presentation independently of endosome-to-cytosol export

Endoplasmic reticulum stress and quality control in relation to cisplatin resistance in tumor cells

The endoplasmic reticulum (ER) is a crucial organelle that orchestrates key cellular functions like protein folding and lipid biosynthesis. However, it is highly sensitive to disturbances that lead to ER stress. In response, the unfolded protein response (UPR) activates to restore ER homeostasis, primarily through three sensors: IRE1, ATF6, and PERK. ERAD and autophagy are crucial in mitigating ER stress, yet their dysregulation can lead to the accumulation of misfolded proteins. Cisplatin, a commonly used chemotherapy drug, induces ER stress in tumor cells, activating complex signaling pathways. Resistance to cisplatin stems from reduced drug accumulation, activation of DNA repair, and anti-apoptotic mechanisms. Notably, cisplatin-induced ER stress can dualistically affect tumor cells, promoting either survival or apoptosis, depending on the context. ERAD is crucial for degrading misfolded proteins, whereas autophagy can protect cells from apoptosis or enhance ER stress-induced apoptosis. The complex interaction between ER stress, cisplatin resistance, ERAD, and autophagy opens new avenues for cancer treatment. Understanding these processes could lead to innovative strategies that overcome chemoresistance, potentially improving outcomes of cisplatin-based cancer treatments. This comprehensive review provides a multifaceted perspective on the complex mechanisms of ER stress, cisplatin resistance, and their implications in cancer therapy.

Sec22b and Stx4 Depletion Has No Major Effect on Cross-Presentation of PLGA Microsphere-Encapsulated Antigen and a Synthetic Long Peptide In Vitro

The induction of CTL responses by vaccines is important to combat infectious diseases and cancer. Biodegradable poly(lactic-co-glycolic acid) (PLGA) microspheres and synthetic long peptides are efficiently internalized by professional APCs and prime CTL responses after cross-presentation of Ags on MHC class I molecules. Specifically, they mainly use the cytosolic pathway of cross-presentation that requires endosomal escape, proteasomal processing, and subsequent MHC class I loading of Ags in the endoplasmic reticulum (ER) and/or the endosome. The vesicle SNARE protein Sec22b has been described as important for this pathway by mediating vesical trafficking for the delivery of ER-derived proteins to the endosome. As this function has also been challenged, we investigated the role of Sec22b in cross-presentation of the PLGA microsphere-encapsulated model Ag OVA and a related synthetic long peptide. Using CRISPR/Cas9-mediated genome editing, we generated Sec22b knockouts in two murine C57BL/6-derived APC lines and found no evidence for an essential role of Sec22b. Although pending experimental evidence, the target SNARE protein syntaxin 4 (Stx4) has been suggested to promote cross-presentation by interacting with Sec22b for the fusion of ER-derived vesicles with the endosome. In the current study, we show that, similar to Sec22b, Stx4 knockout in murine APCs had very limited effects on cross-presentation under the conditions tested. This study contributes to characterizing cross-presentation of two promising Ag delivery systems and adds to the discussion about the role of Sec22b/Stx4 in related pathways. Our data point toward SNARE protein redundancy in the cytosolic pathway of cross-presentation.

Major histocompatibility complex class I assembly within endolysosomal pathways

Major histocompatibility complex class I (MHC class I) molecules facilitate subcellular immune surveillance by presenting peptides on the cell surface. MHC class I assembly with peptides generally happens in the endoplasmic reticulum (ER). Peptides are processed in the cytosol, transported into the ER, and assembled with MHC class I heavy and light chains. However, as many pathogens reside within multiple subcellular organelles, peptide sampling across non-cytosolic compartments is also important. MHC class I molecules internalize from the cell surface into endosomes and constitutively traffic between endosomes and the cell surface. Within endosomes, MHC class I molecules assemble with both exogenous and endogenous antigens processed within these compartments. Human MHC classI polymorphisms, well known to affect ER assembly modes, also influence endosomal assembly outcomes, an area of current interest to the field.

Recent progress in type 1 classical dendritic cell cross-presentation - cytosolic, vacuolar, or both?

Type 1 classical dendritic cells (cDC1s) have emerged as the major antigen-presenting cell performing cross-presentation (XP) in vivo, but the antigen-processing pathway in this cell remains obscure. Two competing models for in vivo XP of cell-associated antigens by cDC1 include a vacuolar pathway and cytosolic pathway. A vacuolar pathway relies on directing antigens captured in vesicles toward a class I major histocompatibility complex loading compartment independently of cytosolic entry. Alternate proposals invoke phagosomal rupture, either constitutive or triggered by spleen tyrosine kinase (SYK) signaling in response to C-type lectin domain family 9 member A (CLEC9A) engagement, that releases antigens into the cytosol for proteasomal degradation. The Beige and Chediak-Higashi (BEACH) protein WD repeat- and FYVE domain-containing protein 4 (WDFY4) is strictly required for XP of cell-associated antigens in vivo. However, the cellular mechanism for WDFY4 activity remains unknown and its requirement in XP in vivo is currently indifferent regarding the vacuolar versus cytosolic pathways. Here, we review the current status of these models and discuss the need for future investigation.

Perforin-2 is a pore-forming effector of endocytic escape in cross-presenting dendritic cells

During initiation of antiviral and antitumor T cell-mediated immune responses, dendritic cells (DCs) cross-present exogenous antigens on major histocompatibility complex (MHC) class I molecules. Cross-presentation relies on the unusual "leakiness" of endocytic compartments in DCs, whereby internalized proteins escape into the cytosol for proteasome-mediated generation of MHC I-binding peptides. Given that type 1 conventional DCs excel at cross-presentation, we searched for cell type-specific effectors of endocytic escape. We devised an assay suitable for genetic screening and identified a pore-forming protein, perforin-2 (Mpeg1), as a dedicated effector exclusive to cross-presenting cells. Perforin-2 was recruited to antigen-containing compartments, where it underwent maturation, releasing its pore-forming domain. Mpeg1-/- mice failed to efficiently prime CD8+ T cells to cell-associated antigens, revealing an important role for perforin-2 in cytosolic entry of antigens during cross-presentation.

From Bacterial Toxin to Therapeutic Agent: The Unexpected Fate of Mycolactone

"Recognizing a surprising fact is the first step towards discovery." This famous quote from Louis Pasteur is particularly appropriate to describe what led us to study mycolactone, a lipid toxin produced by the human pathogen Mycobacterium ulcerans. M. ulcerans is the causative agent of Buruli ulcer, a neglected tropical disease manifesting as chronic, necrotic skin lesions with a "surprising" lack of inflammation and pain. Decades after its first description, mycolactone has become much more than a mycobacterial toxin. This uniquely potent inhibitor of the mammalian translocon (Sec61) helped reveal the central importance of Sec61 activity for immune cell functions, the spread of viral particles and, unexpectedly, the viability of certain cancer cells. We report in this review the main discoveries that marked our research into mycolactone, and the medical perspectives they opened up. The story of mycolactone is not over and the applications of Sec61 inhibition may go well beyond immunomodulation, viral infections, and oncology.

Pathways of MHC I cross-presentation of exogenous antigens

Phagocytes, particularly dendritic cells (DCs), generate peptide-major histocompatibility complex (MHC) I complexes from antigens they have collected from cells in tissues and report this information to CD8 T cells in a process called cross-presentation. This process allows CD8 T cells to detect, respond and eliminate abnormal cells, such as cancers or cells infected with viruses or intracellular microbes. In some settings, cross-presentation can help tolerize CD8 T cells to self-antigens. One of the principal ways that DCs acquire tissue antigens is by ingesting this material through phagocytosis. The resulting phagosomes are key hubs in the cross-presentation (XPT) process and in fact experimentally conferring the ability to phagocytize antigens can be sufficient to allow non-professional antigen presenting cells (APCs) to cross-present. Once in phagosomes, exogenous antigens can be cross-presented (XPTed) through three distinct pathways. There is a vacuolar pathway in which peptides are generated and then bind to MHC I molecules within the confines of the vacuole. Ingested exogenous antigens can also be exported from phagosomes to the cytosol upon vesicular rupture and/or possibly transport. Once in the cytosol, the antigen is degraded by the proteasome and the resulting oligopeptides can be transported to MHC I molecule in the endoplasmic reticulum (ER) (a phagosome-to-cytosol (P2C) pathway) or in phagosomes (a phagosome-to-cytosol-to-phagosome (P2C2P) pathway). Here we review how phagosomes acquire the necessary molecular components that support these three mechanisms and the contribution of these pathways. We describe what is known as well as the gaps in our understanding of these processes.

The evolving biology of cross-presentation

Cross-priming was first recognized in the context of in vivo cytotoxic T lymphocyte (CTL) responses generated against minor histocompatibility antigens induced by immunization with lymphoid cells. Even though the basis for T cell antigen recognition was still largely unclear at that time, these early studies recognized the implication that such minor histocompatibility antigens were derived from the immunizing cells and were obtained exogenously by the host's antigen presenting cells (APCs) that directly prime the CTL response. As antigen recognition by the T cell receptor became understood to involve peptides derived from antigens processed by the APCs and presented by major histocompatibility molecules, the "cross-priming" phenomenon was subsequently recast as "cross-presentation" and the scope considered for examining this process gradually broadened to include many different forms of antigens, including soluble proteins, and different types of APCs that may not be involved in in vivo CTL priming. Many studies of cross-presentation have relied on in vitro cell models that were recently found to differ from in vivo APCs in particular mechanistic details. A recent trend has focused on the APCs and pathways of cross-presentation used in vivo, especially the type 1 dendritic cells. Current efforts are also being directed towards validating the in vivo role of various putative pathways and gene candidates in cross-presentation garnered from various in vitro studies and to determine the relative contributions they make to CTL responses across various forms of antigens and immunologic settings. Thus, cross-presentation appears to be carried by different pathways in various types of cells for different forms under different physiologic settings, which remain to be evaluated in an in vivo physiologic setting.

Mycolactone causes catastrophic Sec61-dependent loss of the endothelial glycocalyx and basement membrane: a new indirect mechanism driving tissue necrosis in Mycobacterium ulcerans infection

The drivers of tissue necrosis in Mycobacterium ulcerans infection (Buruli ulcer disease) have historically been ascribed solely to the directly cytotoxic action of the diffusible exotoxin, mycolactone. However, its role in the clinically-evident vascular component of disease aetiology remains poorly explained. We have now dissected mycolactone's effects on primary vascular endothelial cells in vitro and in vivo. We show that mycolactone-induced changes in endothelial morphology, adhesion, migration, and permeability are dependent on its action at the Sec61 translocon. Unbiased quantitative proteomics identified a profound effect on proteoglycans, driven by rapid loss of type II transmembrane proteins of the Golgi, including enzymes required for glycosaminoglycan (GAG) synthesis, combined with a reduction in the core proteins themselves. Loss of the glycocalyx is likely to be of particular mechanistic importance, since knockdown of galactosyltransferase II (beta-1,3-galactotransferase 6; B3Galt6), the GAG linker-building enzyme, phenocopied the permeability and phenotypic changes induced by mycolactone. Additionally, mycolactone depleted many secreted basement membrane components and microvascular basement membranes were disrupted in vivo. Remarkably, exogenous addition of laminin-511 reduced endothelial cell rounding, restored cell attachment and reversed the defective migration caused by mycolactone. Hence supplementing mycolactone-depleted extracellular matrix may be a future therapeutic avenue, to improve wound healing rates.

Kinetics of antigen cross-presentation assessed experimentally and by a model of the complete endomembrane system

Dendritic cells (DCs) have a specialized endomembrane system capable of presenting exogenous antigens in the context of MHC class I (MHC-I) molecules. This process, named cross-presentation, is crucial to activate CD8+ T lymphocytes and initiate cytotoxic immune responses. In this report, we present an Agent-Based Model in combination with Ordinary Differential Equations with enough complexity to reproduce cross-presentation. The model embraces the secretory and endocytic pathways, in connection with the plasma membrane, the endoplasmic reticulum, and the cytosol. Key molecules required for cross-presentation were included as cargoes. In the simulations, the kinetics of MHC-I uptake and recycling, and cross-presentation accurately reproduced experimental values. The model proved to be a suitable tool to elaborate hypotheses and design experiments. In particular, the model predictions and the experimental results obtained indicate that the rate-limiting step in cross-presentation of soluble ovalbumin is MHC-I loading after proteasomal processing of the antigenic protein.

Sec61 complex/translocon: The role of an atypical ER Ca2+-leak channel in health and disease

The heterotrimeric Sec61 protein complex forms the functional core of the so-called translocon that forms an aqueous channel in the endoplasmic reticulum (ER). The primary role of the Sec61 complex is to allow protein import in the ER during translation. Surprisingly, a completely different function in intracellular Ca2+ homeostasis has emerged for the Sec61 complex, and the latter is now accepted as one of the major Ca2+-leak pathways of the ER. In this review, we first discuss the structure of the Sec61 complex and focus on the pharmacology and regulation of the Sec61 complex as a Ca2+-leak channel. Subsequently, we will pay particular attention to pathologies that are linked to Sec61 mutations, such as plasma cell deficiency and congenital neutropenia. Finally, we will explore the relevance of the Sec61 complex as a Ca2+-leak channel in various pathophysiological (ER stress, apoptosis, ischemia-reperfusion) and pathological (type 2 diabetes, cancer) settings.

Endocytic membrane repair by ESCRT-III controls antigen export to the cytosol during antigen cross-presentation

Despite its crucial role in initiation of cytotoxic immune responses, the molecular pathways underlying antigen cross-presentation remain incompletely understood. The mechanism of antigen exit from endocytic compartments into the cytosol is a long-standing matter of controversy, confronting two main models: transfer through specific channels/transporters or rupture of endocytic membranes and leakage of luminal content. By monitoring the occurrence of intracellular damage in conventional dendritic cells (cDCs), we show that cross-presenting cDC1s display more frequent endomembrane injuries and increased recruitment of endosomal sorting complex required for transport (ESCRT)-III, the main repair system for intracellular membranes, relative to cDC2s. Silencing of CHMP2a or CHMP4b, two effector subunits of ESCRT-III, enhances cytosolic antigen export and cross-presentation. This phenotype is partially reversed by chemical inhibition of RIPK3, suggesting that endocytic damage is related to basal activation of the necroptosis pathway. Membrane repair therefore proves crucial in containing antigen export to the cytosol and cross-presentation in cDCs.

Vaccine adjuvants to engage the cross-presentation pathway

Adjuvants are indispensable components of vaccines for stimulating optimal immune responses to non-replicating, inactivated and subunit antigens. Eliciting balanced humoral and T cell-mediated immunity is paramount to defend against diseases caused by complex intracellular pathogens, such as tuberculosis, malaria, and AIDS. However, currently used vaccines elicit strong antibody responses, but poorly stimulate CD8 cytotoxic T lymphocyte (CTL) responses. To elicit potent CTL memory, vaccines need to engage the cross-presentation pathway, and this requirement has been a crucial bottleneck in the development of subunit vaccines that engender effective T cell immunity. In this review, we focus on recent insights into DC cross-presentation and the extent to which clinically relevant vaccine adjuvants, such as aluminum-based nanoparticles, water-in oil emulsion (MF59) adjuvants, saponin-based adjuvants, and Toll-like receptor (TLR) ligands modulate DC cross-presentation efficiency. Further, we discuss the feasibility of using carbomer-based adjuvants as next generation of adjuvant platforms to elicit balanced antibody- and T-cell based immunity. Understanding of the molecular mechanism of DC cross-presentation and the mode of action of adjuvants will pave the way for rational design of vaccines for infectious diseases and cancer that require balanced antibody- and T cell-based immunity.

Monitoring Protein Import into the Endoplasmic Reticulum in Living Cells with Proximity Labeling

The proper trafficking of eukaryotic proteins is essential to cellular function. Genetic, environmental, and other stresses can induce protein mistargeting and, in turn, threaten cellular protein homeostasis. Current methods for measuring protein mistargeting are difficult to translate to living cells, and thus the role of cellular signaling networks in stress-dependent protein mistargeting processes, such as ER pre-emptive quality control (ER pQC), is difficult to parse. Herein, we use genetically encoded peroxidases to characterize protein import into the endoplasmic reticulum (ER). We show that the ^ERHRP/^cytAPEX pair provides good selectivity and sensitivity for both multiplexed protein labeling and for identifying protein mistargeting, using the known ER pQC substrate transthyretin (TTR). Although ^ERHRP labeling induces formation of detergent-resistant TTR aggregates, this is minimized by using low ^ERHRP expression, without loss of labeling efficiency. ^cytAPEX labeling recovers TTR that is mistargeted as a consequence of Sec61 inhibition or ER stress-induced ER pQC. Furthermore, we discover that stress-free activation of the ER stress-associated transcription factor ATF6 recapitulates the TTR import deficiency of ER pQC. Hence, proximity labeling is an effective strategy for characterizing factors that influence ER protein import in living cells.

The Delivery of Extracellular “Danger” Signals to Cytosolic Sensors in Phagocytes

Phagocytes, such as macrophages and dendritic cells, possess the ability to ingest large quantities of exogenous material into membrane-bound endocytic organelles such as macropinosomes and phagosomes. Typically, the ingested material, which consists of diverse macromolecules such as proteins and nucleic acids, is delivered to lysosomes where it is digested into smaller molecules like amino acids and nucleosides. These smaller molecules can then be exported out of the lysosomes by transmembrane transporters for incorporation into the cell’s metabolic pathways or for export from the cell. There are, however, exceptional instances when undigested macromolecules escape degradation and are instead delivered across the membrane of endocytic organelles into the cytosol of the phagocyte. For example, double stranded DNA, a damage associated molecular pattern shed by necrotic tumor cells, is endocytosed by phagocytes in the tumor microenvironment and delivered to the cytosol for detection by the cytosolic “danger” sensor cGAS. Other macromolecular “danger” signals including lipopolysaccharide, intact proteins, and peptidoglycans can also be actively transferred from within endocytic organelles to the cytosol. Despite the obvious biological importance of these processes, we know relatively little of how macromolecular “danger” signals are transferred across endocytic organelle membranes for detection by cytosolic sensors. Here we review the emerging evidence for the active cytosolic transfer of diverse macromolecular “danger” signals across endocytic organelle membranes. We will highlight developing trends and discuss the potential molecular mechanisms driving this emerging phenomenon.

Saponin-based adjuvant-induced dendritic cell cross-presentation is dependent on PERK activation

Saponin-based adjuvants (SBAs) are promising new adjuvants that stand out as they not only enforce CD4 + T cell-mediated immunity and antibody responses, but also induce an unprecedented level of antigen cross-presentation by dendritic cells (DC) and subsequent CD8 + T cell activation. We discovered that SBA’s ability to boost cross-presentation depends on the induction of lipid bodies (LBs). Moreover, the MHCII^loCD11b^hi DC subset was identified to be most responsive to SBA-induced cross-presentation. The aim is to further unravel the mechanisms behind the induction of DC cross-presentation by SBAs. Here we show that SBAs specifically induce the PKR-like Endoplasmic Reticulum kinase (PERK) pathway and that SBA-induced DC cross-presentation is dependent on activation of the PERK pathway. PERK activation and LB formation are both crucial for SBA-induced cross-presentation and PERK inhibition has little or no effect on SBA-induced LB formation. SBA’s responsiveness, LB formation and PERK activation are specific for the MHCII^loCD11b^hi DCs. These findings contribute to understanding the pathways involved in SBA-induced cross-presentation and immune activation which will ultimately lead to the development of vaccines with improved efficiency and safety.

K48- and K27-mutant ubiquitin regulates adaptive immune response by affecting cross-presentation in bone marrow precursor cells

K48-linked ubiquitination determines antigen degradation and plays vital roles in the process of cross-presentation of bone marrow precursor cell (BMPC)-derived dendritic cells (DCs). Although previous studies revealed that K48 and K27-linked ubiquitination regulates innate immunity, the exact roles of K48 and K27-linked ubiquitination in cross-presentation and BMPC-based adaptive immunity are still uncertain. In this study, we investigated the effects of K48- and K27-mutant ubiquitin (Ub) on BMPC-based adaptive immune response by observing the effects of MG132, Ub deficiency, and K48/K27-mutant Ub on cross-presentation, T cell proliferation, IFN-γ secretion, BMPC-based CTL priming, and thereby the efficiency of cytolytic capacity of BMPC-activate T cells. We demonstrated that MG132, Ub deficiency, and K48-mutant Ub impair cross-presentation, T cell proliferation, IFN-γ secretion, BMPC-based CTL priming, and the cytolytic capacity of BMPC-activated T cells. Moreover, although K27-only Ub decreases cross-presentation, the replenishment of K27-mutant Ub restores Ub deficiency impaireds the abilities of T cell proliferation, IFN-γ secretion, CTL priming, and the cytolytic capacity of BMPC-activated T cells. Thus, these data suggest that K48- and K27-linked ubiquitination contributes to BMPC-mediated adaptive immune response by affecting BMPC cross-presentation and the cytolytic capacity by up-regulating both perforin and granzyme B in BMPC-activated T cells. K48- and K27-mutant Ub might have potential clinical therapeutic function in adaptive immune response-associated diseases.

Nanovaccines with cell-derived components for cancer immunotherapy

Cancer nanovaccines as one of immunotherapeutic approaches are able to attack tumors by stimulating tumor-specific immunological responses. However, there still exist multiple challenges to be tackled for cancer nanovaccines to evoke potent antitumor immunity. Particularly, the administration of exogenous materials may cause the off-target immunotherapy responses. In recent years, biomimetic nanovaccines by using cell lysates, cell-derived nanovesicles, or extracted cell membranes as the functional components have received extensive attention. Such nanovaccines based on cell-derived components would show many unique advantages including inherent biocompatibility and the ability to trigger immune responses against a range of tumor-associated antigens. In this review article, we will introduce the recent research progresses of those cell-derived biomimetic nanovaccines for cancer immunotherapy, and discuss the perspectives and challenges associated with the future clinical translation of these emerging vaccine platforms.

Lysosomes and lysosome‐related organelles in immune responses

The catabolic, degradative capacity of the endo‐lysosome system is put to good use in mammalian immune responses as is their recently established status as signaling platforms. From the ‘creative destruction’ of antigenic and ‘self’ material for antigen presentation to T cells to the re‐purposing of lysosomes as toxic exocytosable lysosome‐related organelles (granules) in leukocytes such as CD8 T cells and eosinophils, endo‐lysosomes are key players in host defense. Signaled responses to some pathogen products initiate in endo‐lysosomes and these organelles are emerging as important in distinct ways in the unique immunobiology of dendritic cells. Potential self‐inflicted toxicity from lysosomal and granule proteases is countered by expression of serpin and cystatin family members.

The One That Got Away: How Macrophage-Derived IL-1β Escapes the Mycolactone-Dependent Sec61 Blockade in Buruli Ulcer

Buruli ulcer (BU), caused by Mycobacterium ulcerans, is a devastating necrotizing skin disease. Key to its pathogenesis is mycolactone, the exotoxin virulence factor that is both immunosuppressive and cytotoxic. The discovery that the essential Sec61 translocon is the major cellular target of mycolactone explains much of the disease pathology, including the immune blockade. Sec61 inhibition leads to a loss in production of nearly all cytokines from monocytes, macrophages, dendritic cells and T cells, as well as antigen presentation pathway proteins and costimulatory molecules. However, there has long been evidence that the immune system is not completely incapable of responding to M. ulcerans infection. In particular, IL-1β was recently shown to be present in BU lesions, and to be induced from M. ulcerans-exposed macrophages in a mycolactone-dependent manner. This has important implications for our understanding of BU, showing that mycolactone can act as the "second signal" for IL-1β production without inhibiting the pathways of unconventional secretion it uses for cellular release. In this Perspective article, we validate and discuss this recent advance, which is entirely in-line with our understanding of mycolactone's inhibition of the Sec61 translocon. However, we also show that the IL-1 receptor, which uses the conventional secretory pathway, is sensitive to mycolactone blockade at Sec61. Hence, a more complete understanding of the mechanisms regulating IL-1β function in skin tissue, including the transient intra-macrophage stage of M. ulcerans infection, is urgently needed to uncover the double-edged sword of IL-1β in BU pathogenesis, treatment and wound healing.

The Sec61 translocon is a therapeutic vulnerability in multiple myeloma

Multiple myeloma (MM) is an incurable malignancy characterized by the uncontrolled expansion of plasma cells in the bone marrow. While proteasome inhibitors like bortezomib efficiently halt MM progression, drug resistance inevitably develop, and novel therapeutic approaches are needed. Here, we used a recently discovered Sec61 inhibitor, mycolactone, to assess the interest of disrupting MM proteostasis via protein translocation blockade. In human MM cell lines, mycolactone caused rapid defects in secretion of immunoglobulins and expression of pro‐survival interleukin (IL)‐6 receptor and CD40, whose activation stimulates IL‐6 production. Mycolactone also triggered pro‐apoptotic endoplasmic reticulum stress responses synergizing with bortezomib for induction of MM cell death and overriding acquired resistance to the proteasome inhibitor. Notably, the mycolactone–bortezomib combination rapidly killed patient‐derived MM cells ex vivo, but not normal mononuclear cells. In immunodeficient mice engrafted with MM cells, it demonstrated superior therapeutic efficacy over single drug treatments, without inducing toxic side effects. Collectively, these findings establish Sec61 blockers as novel anti‐MM agents and reveal the interest of targeting both the translocon and the proteasome in proteostasis‐addicted tumors.

Biochemical and Biological Assays of Mycolactone-Mediated Inhibition of Sec61

Mycobacterium ulcerans, the causative agent of Buruli ulcer disease, is unique among human pathogens in its capacity to produce mycolactone, a diffusible macrolide with immunosuppressive and cytotoxic properties. Recent studies have shown that mycolactone operates by inhibiting the host membrane translocation complex (Sec61), with an unprecedented potency compared to previously identified Sec61 blockers. Mycolactone binding to the pore-forming subunit of Sec61 inhibits its capacity to transport nascent secretory and membrane proteins into the endoplasmic reticulum, leading to their cytosolic degradation by the ubiquitin:proteasome system. In T lymphocytes, Sec61 blockade by mycolactone manifests as a sharp decrease in the cell's ability to express homing receptors and release cytokines following activation. Sustained exposure of human cells to mycolactone typically generates proteotoxic stress responses in their cytosol and endoplasmic reticulum (ER), ultimately inducing apoptosis. Here we describe cell-free systems for studying Sec61-mediated protein translocation that allow the impact of mycolactone on the biogenesis of secretory and membrane proteins to be probed. We also describe biological assays of mycolactone-driven inhibition of Sec61 providing rapid and sensitive means to quantitatively assess the presence of the toxin in biological samples.

Mycolactone enhances the Ca2+ leak from endoplasmic reticulum by trapping Sec61 translocons in a Ca2+ permeable state

The Mycobacterium ulcerans exotoxin, mycolactone, is an inhibitor of co-translational translocation via the Sec61 complex. Mycolactone has previously been shown to bind to, and alter the structure of the major translocon subunit Sec61α, and change its interaction with ribosome nascent chain complexes. In addition to its function in protein translocation into the ER, Sec61 also plays a key role in cellular Ca²⁺ homeostasis, acting as a leak channel between the endoplasmic reticulum (ER) and cytosol. Here, we have analysed the effect of mycolactone on cytosolic and ER Ca²⁺ levels using compartment-specific sensors. We also used molecular docking analysis to explore potential interaction sites for mycolactone on translocons in various states. These results show that mycolactone enhances the leak of Ca²⁺ ions via the Sec61 translocon, resulting in a slow but substantial depletion of ER Ca²⁺. This leak was dependent on mycolactone binding to Sec61α because resistance mutations in this protein completely ablated the increase. Molecular docking supports the existence of a mycolactone-binding transient inhibited state preceding translocation and suggests mycolactone may also bind Sec61α in its idle state. We propose that delayed ribosomal release after translation termination and/or translocon ‘breathing' during rapid transitions between the idle and intermediate-inhibited states allow for transient Ca²⁺ leak, and mycolactone's stabilisation of the latter underpins the phenotype observed.

Inhibition of the SEC61 translocon by mycolactone induces a protective autophagic response controlled by EIF2S1-dependent translation that does not require ULK1 activity

The Mycobacterium ulcerans exotoxin, mycolactone, is responsible for the immunosuppression and tissue necrosis that characterizes Buruli ulcer. Mycolactone inhibits SEC61-dependent co-translational translocation of proteins into the endoplasmic reticulum and the resultant cytosolic translation triggers degradation of mislocalized proteins by the ubiquitin-proteasome system. Inhibition of SEC61 by mycolactone also activates multiple EIF2S1/eIF2α kinases in the integrated stress response (ISR). Here we show mycolactone increased canonical markers of selective macroautophagy/autophagy LC3B-II, ubiquitin and SQSTM1/p62 in diverse disease-relevant primary cells and cell lines. Increased formation of puncta positive for the early autophagy markers WIPI2, RB1CC1/FIP200 and ATG16L1 indicates increased initiation of autophagy. The mycolactone response was SEC61A1-dependent and involved a pathway that required RB1CC1 but not ULK. Deletion of Sqstm1 reduced cell survival in the presence of mycolactone, suggesting this response protects against the increased cytosolic protein burden caused by the toxin. However, reconstitution of baseline SQSTM1 expression in cells lacking all autophagy receptor proteins could not rescue viability. Translational regulation by EIF2S1 in the ISR plays a key role in the autophagic response to mycolactone. Mycolactone-dependent induction of SQSTM1 was reduced in eif2ak3^−/-/perk^−/- cells while the p-EIF2S1 antagonist ISRIB reversed the upregulation of SQSTM1 and reduced RB1CC1, WIPI2 and LC3B puncta formation. Increased SQSTM1 staining could be seen in Buruli ulcer patient skin biopsy samples, reinforcing genetic data that suggests autophagy is relevant to disease pathology. Since selective autophagy and the ISR are both implicated in neurodegeneration, cancer and inflammation, the pathway uncovered here may have a broad relevance to human disease.

Abbreviations: ATF4: activating transcription factor 4; ATG: autophagy related; BAF: bafilomycin A₁; ATG16L1: autophagy related 16 like 1; BU: Buruli ulcer; CQ: chloroquine; EIF2AK3: eukaryotic translation initiation factor 2 alpha kinase 3; CALCOCO2: calcium binding and coiled-coil domain 2; DMSO: dimethyl sulfoxide; EIF2S1: eukaryotic translation initiation factor 2 subunit alpha; ER: endoplasmic reticulum; GFP: green fluorescent protein; HDMEC: human dermal microvascular endothelial cells; HFFF: human fetal foreskin fibroblasts; ISR: integrated stress response; ISRIB: integrated stress response inhibitor; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MEF: mouse embryonic fibroblast; Myco: mycolactone; NBR1: NBR1 autophagy cargo receptor; NFE2L2: nuclear factor, erythroid 2 like 2; OPTN: optineurin; PFA: paraformaldehyde; PtdIns3P: phosphatidylinositol-3-phosphate; RB1CC1: RB1-inducible coiled coil 1; SQSTM1: sequestosome 1; TAX1BP1: Tax1 binding protein 1; ULK: unc-51 like autophagy activating kinase; UPS: ubiquitin-proteasome system; WIPI: WD repeat domain, phosphoinositide interacting; WT: wild type.

Ipomoeassin-F disrupts multiple aspects of secretory protein biogenesis

The Sec61 complex translocates nascent polypeptides into and across the membrane of the endoplasmic reticulum (ER), providing access to the secretory pathway. In this study, we show that Ipomoeassin-F (Ipom-F), a selective inhibitor of protein entry into the ER lumen, blocks the in vitro translocation of certain secretory proteins and ER lumenal folding factors whilst barely affecting others such as albumin. The effects of Ipom-F on protein secretion from HepG2 cells are twofold: reduced ER translocation combined, in some cases, with defective ER lumenal folding. This latter issue is most likely a consequence of Ipom-F preventing the cell from replenishing its ER lumenal chaperones. Ipom-F treatment results in two cellular stress responses: firstly, an upregulation of stress-inducible cytosolic chaperones, Hsp70 and Hsp90; secondly, an atypical unfolded protein response (UPR) linked to the Ipom-F-mediated perturbation of ER function. Hence, although levels of spliced XBP1 and CHOP mRNA and ATF4 protein increase with Ipom-F, the accompanying increase in the levels of ER lumenal BiP and GRP94 seen with tunicamycin are not observed. In short, although Ipom-F reduces the biosynthetic load of newly synthesised secretory proteins entering the ER lumen, its effects on the UPR preclude the cell restoring ER homeostasis.

Complexity and Specificity of Sec61-Channelopathies: Human Diseases Affecting Gating of the Sec61 Complex

The rough endoplasmic reticulum (ER) of nucleated human cells has crucial functions in protein biogenesis, calcium (Ca2+) homeostasis, and signal transduction. Among the roughly one hundred components, which are involved in protein import and protein folding or assembly, two components stand out: The Sec61 complex and BiP. The Sec61 complex in the ER membrane represents the major entry point for precursor polypeptides into the membrane or lumen of the ER and provides a conduit for Ca2+ ions from the ER lumen to the cytosol. The second component, the Hsp70-type molecular chaperone immunoglobulin heavy chain binding protein, short BiP, plays central roles in protein folding and assembly (hence its name), protein import, cellular Ca2+ homeostasis, and various intracellular signal transduction pathways. For the purpose of this review, we focus on these two components, their relevant allosteric effectors and on the question of how their respective functional cycles are linked in order to reconcile the apparently contradictory features of the ER membrane, selective permeability for precursor polypeptides, and impermeability for Ca2+. The key issues are that the Sec61 complex exists in two conformations: An open and a closed state that are in a dynamic equilibrium with each other, and that BiP contributes to its gating in both directions in cooperation with different co-chaperones. While the open Sec61 complex forms an aqueous polypeptide-conducting- and transiently Ca2+-permeable channel, the closed complex is impermeable even to Ca2+. Therefore, we discuss the human hereditary and tumor diseases that are linked to Sec61 channel gating, termed Sec61-channelopathies, as disturbances of selective polypeptide-impermeability and/or aberrant Ca2+-permeability.

Autophagy regulates long-term cross-presentation by murine dendritic cells

Autophagy has been reported to be involved in supporting antigen cross-presentation by dendritic cells (DCs). We have shown that DCs have the ability to store antigen for a prolonged time in endolysosomal compartments and thereby sustain MHCI antigen cross-presentation to CD8+ T cells. In the current study, we investigated the role of autophagy in long-term antigen presentation. We show that the autophagy machinery has a negative impact on storage of antigen in DCs. Atg5-/- DCs which are deficient in autophagy or DCs treated with common autophagy inhibitors showed enhanced antigen storage and antigen cross-presentation. This augmented antigen cross-presentation effect is independent of altered proteasome enzyme activity or MHCI surface expression on DCs. We visualized that the storage compartments are in close proximity to LC3 positive autophagosomes. Our results indicate that autophagosomes disrupt antigen storage in DCs and thereby regulate long-term MHCI cross-presentation.

K48-Linked Ubiquitination Contributes to Nicotine-Augmented Bone Marrow-Derived Dendritic-Cell-Mediated Adaptive Immunity

K48-linked ubiquitination determining antigen degradation and the endosomal recruitments of p97 and Sec61 plays vital roles in dendritic cell (DC) cross-presentation. Our previous studies revealed that nicotine treatment increases bone marrow-derived dendritic cell (BM-DC) cross-presentation and promotes BM-DC-based cytotoxic T lymphocyte (CTL) priming. But the effect of nicotine on K48-linked ubiquitination and the mechanism of nicotine-increased BM-DC cross-presentation are still uncertain. In this study, we first demonstrated that ex vivo nicotine administration obviously increased K48-linked ubiquitination in BM-DC. Then, we found that K48-linked ubiquitination was essential for nicotine-augmented cross-presentation, BM-DC-based CTL priming, and thereby the superior cytolytic capacity of DC-activated CTL. Importantly, K48-linked ubiquitination was verified to be necessary for nicotine-augmented endosomal recruitments of p97 and Sec61. Importantly, mannose receptor (MR), which is an important antigenic receptor for cross-presentation, was exactly catalyzed with K48-linked ubiquitination by the treatment with nicotine. Thus, these data suggested that K48-linked ubiquitination contributes to the superior adaptive immunity of nicotine-administrated BM-DC. Regulating K48-linked ubiquitination might have therapeutic potential for DC-mediated immune therapy.

Genetics in the Host-Mycobacterium ulcerans interaction

Buruli ulcer is an emerging infectious disease associated with high morbidity and unpredictable outbreaks. It is caused by Mycobacterium ulcerans, a slow-growing pathogen evolutionarily shaped by the acquisition of a plasmid involved in the production of a potent macrolide-like cytotoxin and by genome rearrangements and downsizing. These events culminated in an uncommon infection pattern, whereby M. ulcerans is both able to induce the initiation of the inflammatory cascade and the cell death of its proponents, as well as to survive within the phagosome and in the extracellular milieu. In such extreme conditions, the host is sentenced to rely on a highly orchestrated genetic landscape to be able to control the infection. We here revisit the dynamics of M. ulcerans infection, drawing parallels from other mycobacterioses and integrating the most recent knowledge on its evolution and pathogenicity in its interaction with the host immune response.

Immunity against Mycobacterium ulcerans: The subversive role of mycolactone

Mycobacterium ulcerans causes Buruli ulcer, a neglected tropical skin disease manifesting as chronic wounds that can leave victims with major, life-long deformity and disability. Differently from other mycobacterial pathogens, M ulcerans produces mycolactone, a diffusible lipid factor with unique cytotoxic and immunomodulatory properties. Both traits result from mycolactone targeting Sec61, the entry point of the secretory pathway in eukaryotic cells. By inhibiting Sec61, mycolactone prevents the host cell's production of secreted proteins, and most of its transmembrane proteins. This molecular blockade dramatically alters the functions of immune cells, thereby the generation of protective immunity. Moreover, sustained inhibition of Sec61 triggers proteotoxic stress responses leading to apoptotic cell death, which can stimulate vigorous immune responses. The dynamics of bacterial production of mycolactone and elimination by infected hosts thus critically determine the balance between its immunostimulatory and immunosuppressive effects. Following an introduction summarizing the essential information on Buruli ulcer disease, this review focuses on the current state of knowledge regarding mycolactone's regulation and biodistribution. We then detail the consequences of mycolactone-mediated Sec61 blockade on initiation and maintenance of innate and adaptive immune responses. Finally, we discuss the key questions to address in order to improve immunity to M ulcerans, and how increased knowledge of mycolactone biology may pave the way to innovative therapeutics.

Nanobiomaterial-based vaccination immunotherapy of cancer

Cancer immunotherapies including cancer vaccines, immune checkpoint blockade or chimeric antigen receptor T cells have been exploited as the attractive treatment modalities in recent years. Among these approaches, cancer vaccines that designed to deliver tumor antigens and adjuvants to activate the antigen presenting cells (APCs) and induce antitumor immune responses, have shown significant efficacy in inhibiting tumor growth, preventing tumor relapse and metastasis. Despite the potential of cancer vaccination strategies, the therapeutic outcomes in preclinical trials are failed to promote their clinical translation, which is in part due to their inefficient vaccination cascade of five critical steps: antigen identification, antigen encapsulation, antigen delivery, antigen release and antigen presentation to T cells. In recent years, it has been demonstrated that various nanobiomaterials hold great potential to enhance cancer vaccination cascade and improve their antitumor performance and reduce the off-target effect. We summarize the cutting-edge advances of nanobiomaterials-based vaccination immunotherapy of cancer in this review. The various cancer nanovaccines including antigen peptide/adjuvant-based nanovaccines, nucleic acid-based nanovaccines as well as biomimetic nanobiomaterials-based nanovaccines are discussed in detail. We also provide some challenges and perspectives associated with the clinical translation of cancer nanovaccines.

The receptor DNGR-1 signals for phagosomal rupture to promote cross-presentation of dead-cell-associated antigens

Type 1 conventional dendritic (cDC1) cells are necessary for cross-presentation of many viral and tumor antigens to CD8+ T cells. cDC1 cells can be identified in mice and humans by high expression of DNGR-1 (also known as CLEC9A), a receptor that binds dead-cell debris and facilitates XP of corpse-associated antigens. Here, we show that DNGR-1 is a dedicated XP receptor that signals upon ligand engagement to promote phagosomal rupture. This allows escape of phagosomal contents into the cytosol, where they access the endogenous major histocompatibility complex class I antigen processing pathway. The activity of DNGR-1 maps to its signaling domain, which activates SYK and NADPH oxidase to cause phagosomal damage even when spliced into a heterologous receptor and expressed in heterologous cells. Our data reveal the existence of innate immune receptors that couple ligand binding to endocytic vesicle damage to permit MHC class I antigen presentation of exogenous antigens and to regulate adaptive immunity.

Vaccine-Induced CD8+ T Cell Responses in Children: A Review of Age-Specific Molecular Determinants Contributing to Antigen Cross-Presentation

Infections are most common and most severe at the extremes of age, the young and the elderly. Vaccination can be a key approach to enhance immunogenicity and protection against pathogens in these vulnerable populations, who have a functionally distinct immune system compared to other age groups. More than 50% of the vaccine market is for pediatric use, yet to date vaccine development is often empiric and not tailored to molecular distinctions in innate and adaptive immune activation in early life. With modern vaccine development shifting from whole-cell based vaccines to subunit vaccines also comes the need for formulations that can elicit a CD8+ T cell response when needed, for example, by promoting antigen cross-presentation. While our group and others have identified many cellular and molecular determinants of successful activation of antigen-presenting cells, B cells and CD4+ T cells in early life, much less is known about the ontogeny of CD8+ T cell induction. In this review, we summarize the literature pertaining to the frequency and phenotype of newborn and infant CD8+ T cells, and any evidence of induction of CD8+ T cells by currently licensed pediatric vaccine formulations. In addition, we review the molecular determinants of antigen cross-presentation on MHC I and successful CD8+ T cell induction and discuss potential distinctions that can be made in children. Finally, we discuss recent advances in development of novel adjuvants and provide future directions for basic and translational research in this area.

RNF41 regulates the damage recognition receptor Clec9A and antigen cross-presentation in mouse dendritic cells

The dendritic cell receptor Clec9A facilitates processing of dead cell-derived antigens for cross-presentation and the induction of effective CD8⁺ T cell immune responses. Here, we show that this process is regulated by E3 ubiquitin ligase RNF41 and define a new ubiquitin-mediated mechanism for regulation of Clec9A, reflecting the unique properties of Clec9A as a receptor specialized for delivery of antigens for cross-presentation. We reveal RNF41 is a negative regulator of Clec9A and the cross-presentation of dead cell-derived antigens by mouse dendritic cells. Intriguingly, RNF41 regulates the downstream fate of Clec9A by directly binding and ubiquitinating the extracellular domains of Clec9A. At steady-state, RNF41 ubiquitination of Clec9A facilitates interactions with ER-associated proteins and degradation machinery to control Clec9A levels. However, Clec9A interactions are altered following dead cell uptake to favor antigen presentation. These findings provide important insights into antigen cross-presentation and have implications for development of approaches to modulate immune responses.

Ring Expansion Leads to a More Potent Analogue of Ipomoeassin F

Two new ring-size-varying analogues (2 and 3) of ipomoeassin F were synthesized and evaluated. Improved cytotoxicity (IC₅₀: from 1.8 nM) and in vitro protein translocation inhibition (IC₅₀: 35 nM) derived from ring expansion imply that the binding pocket of Sec61α (isoform 1) can accommodate further structural modifications, likely in the fatty acid portion. Streamlined preparation of the key diol intermediate 5 enabled gram-scale production, allowing us to establish that ipomoeassin F is biologically active in vivo (MTD: ∼3 mg/kg).

Aquaporin-3 regulates endosome-to-cytosol transfer via lipid peroxidation for cross presentation

Antigen cross presentation, whereby exogenous antigens are presented by MHC class I molecules to CD8+ T cells, is essential for generating adaptive immunity to pathogens and tumor cells. Following endocytosis, it is widely understood that protein antigens must be transferred from endosomes to the cytosol where they are subject to ubiquitination and proteasome degradation prior to being translocated into the endoplasmic reticulum (ER), or possibly endosomes, via the TAP1/TAP2 complex. Revealing how antigens egress from endocytic organelles (endosome-to-cytosol transfer, ECT), however, has proved vexing. Here, we used two independent screens to identify the hydrogen peroxide-transporting channel aquaporin-3 (AQP3) as a regulator of ECT. AQP3 overexpression increased ECT, whereas AQP3 knockout or knockdown decreased ECT. Mechanistically, AQP3 appears to be important for hydrogen peroxide entry into the endosomal lumen where it affects lipid peroxidation and subsequent antigen release. AQP3-mediated regulation of ECT was functionally significant, as AQP3 modulation had a direct impact on the efficiency of antigen cross presentation in vitro. Finally, AQP3^-/- mice exhibited a reduced ability to mount an anti-viral response and cross present exogenous extended peptide. Together, these results indicate that the AQP3-mediated transport of hydrogen peroxide can regulate endosomal lipid peroxidation and suggest that compromised membrane integrity and coordinated release of endosomal cargo is a likely mechanism for ECT.

Akt+ IKKα/β+ Rab5+ Signalosome Mediate the Endosomal Recruitment of Sec61 and Contribute to Cross-Presentation in Bone Marrow Precursor Cells

Cross-presentation in dendritic cells (DC) requires the endosomal relocations of internalized antigens and the endoplasmic reticulum protein Sec61. Despite the fact that endotoxin-containing pathogen and endotoxin-free antigen have different effects on protein kinase B (Akt) and I-kappa B Kinase α/β (IKKα/β) activation, the exact roles of Akt phosphorylation, IKKα or IKKβ activation in endotoxin-containing pathogen-derived cross-presentation are poorly understood. In this study, endotoxin-free ovalbumin supplemented with endotoxin was used as a model pathogen. We investigated the effects of endotoxin-containing pathogen and endotoxin-free antigen on Akt phosphorylation, IKKα/β activation, and explored the mechanisms that the endotoxin-containing pathogen orchestrating the endosomal recruitment of Sec61 of the cross-presentation in bone marrow precursor cells (BMPC). We demonstrated that endotoxin-containing pathogen and endotoxin-free antigen efficiently induced the phosphorylation of Akt-IKKα/β and Akt-IKKα, respectively. Endotoxin-containing pathogen derived Akt+ IKKα/β+ Rab5+ signalosome, together with augmented the recruitment of Sec61 toward endosome, lead to the increased cross-presentation in BMPC. Importantly, the endosomal recruitment of Sec61 was partly mediated by the formation of Akt+ IKKα/β+ signalosome. Thus, these data suggest that Akt+ IKKα/β+ Rab5+ signalosome contribute to endotoxin-containing pathogen-induced the endosomal recruitment of Sec61 and the superior efficacy of cross-presentation in BMPC.

Small Molecule Enhancers of Endosome-to-Cytosol Import Augment Anti-tumor Immunity

Cross-presentation of antigens by dendritic cells (DCs) is critical for initiation of anti-tumor immune responses. Yet, key steps involved in trafficking of antigens taken up by DCs remain incompletely understood. Here, we screen 700 US Food and Drug Administration (FDA)-approved drugs and identify 37 enhancers of antigen import from endolysosomes into the cytosol. To reveal their mechanism of action, we generate proteomic organellar maps of control and drug-treated DCs (focusing on two compounds, prazosin and tamoxifen). By combining organellar mapping, quantitative proteomics, and microscopy, we conclude that import enhancers undergo lysosomal trapping leading to membrane permeation and antigen release. Enhancing antigen import facilitates cross-presentation of soluble and cell-associated antigens. Systemic administration of prazosin leads to reduced growth of MC38 tumors and to a synergistic effect with checkpoint immunotherapy in a melanoma model. Thus, inefficient antigen import into the cytosol limits antigen cross-presentation, restraining the potency of anti-tumor immune responses and efficacy of checkpoint blockers.

Unraveling the regulatory role of endoplasmic-reticulum-associated degradation in tumor immunity

During malignant transformation and cancer progression, tumor cells face both intrinsic and extrinsic stress, endoplasmic reticulum (ER) stress in particular. To survive and proliferate, tumor cells use multiple stress response pathways to mitigate ER stress, promoting disease aggression and treatment resistance. Among the stress response pathways is ER-associated degradation (ERAD), which consists of multiple components and steps working together to ensure protein quality and quantity. In addition to its established role in stress responses and tumor cell survival, ERAD has recently been shown to regulate tumor immunity. Here we summarize current knowledge on how ERAD promotes protein degradation, regulates immune cell development and function, participates in antigen presentation, exerts paradoxical roles on tumorigenesis and immunity, and thus impacts current cancer therapy. Collectively, ERAD is a critical protein homeostasis pathway intertwined with cancer development and tumor immunity. Of particular importance is the need to further unveil ERAD's enigmatic roles in tumor immunity to develop effective targeted and combination therapy for successful treatment of cancer.

Protein translocation and retro-translocation across the endoplasmic reticulum are crucial to inflammatory effector CD4+ T cell function

Effector CD4⁺ T cells can be classified by the cytokines they secrete, with T helper 1 (Th1) cells generating interferon (IFN)γ and Th17 cells secreting interleukin (IL)-17. Both Th1 and Th17 cells are strongly implicated in the initiation and chronicity of autoimmune diseases such as multiple sclerosis. The endoplasmic reticulum (ER) has been implicated as a potentially crucial site in regulating CD4⁺ T cell function. Secretory and transmembrane proteins are shuttled into the ER via the Sec61 translocon, where they undergo appropriate folding; misfolded proteins are retro-translocated from the ER in a p97-dependent manner. Here, we provide evidence that both processes are crucial to the secretion of inflammatory cytokines from effector CD4⁺ T cells. The pan-ER inhibitor eeeyarestatin-1 (ESI), which interferes with both Sec61 translocation and p97 retro-translocation, inhibited secretion of interferon (IFN)γ, interleukin (IL)-2 and tumor necrosis factor (TNF)α from Th1 cells in a dose-dependent manner. Selective inhibition of Sec61 by Apratoxin A (ApraA) revealed that ER translocation is crucial for Th1 cytokine secretion, while inhibition of p97 by NMS-873 also inhibited Th1 function, albeit to a lesser degree. By contrast, none of ESI, ApraA or NMS-873 could significantly reduce IL-17 secretion from Th17 cells. ApraA, but not NMS-873, reduced phosphorylation of Stat1 in Th1 cells, indicating the involvement of ER translocation in Th1 differentiation pathways. ApraA had modest effects on activation of the Th17 transcription factor Stat3, while NMS-873 had no effect. Interestingly, NMS-873 was able to reduce disease severity in CD4⁺ T cell-driven experimental autoimmune encephalomyelitis (EAE). Together, our data indicate that CD4⁺ T cell function, and Th1 cell function in particular, is dependent on protein translocation and dislocation across the ER.

Genomic programming of IRF4-expressing human Langerhans cells

Langerhans cells (LC) can prime tolerogenic as well as immunogenic responses in skin, but the genomic states and transcription factors (TF) regulating these context-specific responses are unclear. Bulk and single-cell transcriptional profiling demonstrates that human migratory LCs are robustly programmed for MHC-I and MHC-II antigen presentation. Chromatin analysis reveals enrichment of ETS-IRF and AP1-IRF composite regulatory elements in antigen-presentation genes, coinciding with expression of the TFs, PU.1, IRF4 and BATF3 but not IRF8. Migration of LCs from the epidermis is accompanied by upregulation of IRF4, antigen processing components and co-stimulatory molecules. TNF stimulation augments LC cross-presentation while attenuating IRF4 expression. CRISPR-mediated editing reveals IRF4 to positively regulate the LC activation programme, but repress NF2EL2 and NF-kB pathway genes that promote responsiveness to oxidative stress and inflammatory cytokines. Thus, IRF4-dependent genomic programming of human migratory LCs appears to enable LC maturation while attenuating excessive inflammatory and immunogenic responses in the epidermis.

Langerhans cells (LC) can prime tolerogenic as well as immunogenic responses in the skin. Here the authors show, by transcriptomic, epigenetic and CRISPR editing analyses, that during LC migration and maturation the transcription factor IRF4 regulates expression of antigen presentation and co-stimulatory gene modules while attenuating inflammatory response genes.

Cross-presentation of exogenous antigens on MHC I molecules

In order to get recognized by CD8 T cells, most cells present peptides from endogenously expressed self or foreign proteins on MHC class I molecules. However, specialized antigen-presenting cells, such as DCs and macrophages, can present exogenous antigen on MHC-I in a process called cross-presentation. This pathway plays key roles in antimicrobial and antitumor immunity, and also immune tolerance. Recent advances have broadened our understanding of the underlying mechanisms of cross-presentation. Here, we review some of these recent advances, including the distinct pathways that result in the cross-priming of CD8 T cells and the source of the class I molecules presenting exogenous peptides.

Distinct Subcellular Compartments of Dendritic Cells Used for Cross-Presentation

Dendritic cells (DCs) present exogenous protein-derived peptides on major histocompatibility complex class I molecules to prime naïve CD8⁺ T cells. This DC specific ability, called cross-presentation (CP), is important for the activation of cell-mediated immunity and the induction of self-tolerance. Recent research revealed that endoplasmic reticulum-associated degradation (ERAD), which was first identified as a part of the unfolded protein response—a quality control system in the ER—plays a pivotal role in the processing of exogenous proteins in CP. Moreover, DCs express a variety of immuno-modulatory molecules and cytokines to regulate T cell activation in response to the environment. Although both CP and immuno-modulation are indispensable, contrasting ER conditions are required for their correct activity. Since ERAD substrates are unfolded proteins, their accumulation may result in ER stress, impaired cell homeostasis, and eventually apoptosis. In contrast, activation of the unfolded protein response should be inhibited for DCs to express immuno-modulatory molecules and cytokines. Here, we review recent advances on antigen CP, focusing on intracellular transport routes for exogenous antigens and distinctive subcellular compartments involved in ERAD.

Antigen processing and presentation

Dendritic cells are at the center of immune responses. They are defined by their ability to sense the environment, take up and process antigen, migrate to secondary lymphoid organs, where they present antigens to the adaptive immune system. In particular, they present lipids and proteins from pathogens, which they encountered in peripheral tissues, to T cells in order to induce a specific effector immune response. These complex antigens need to be broken down into peptides of a certain length in association with Major Histocompatibility Complex (MHC) molecules. Presentation of MHC/antigen complexes alongside costimulatory molecules and secretion of proinflammatory cytokines will induce an appropriate immune response. This interaction between dendritic cells and T cells takes place at defined locations within secondary lymphoid organs. In this review, we discuss the current knowledge and recent advances on the cellular and molecular mechanisms that underlie antigen processing and the subsequent presentation to T lymphocytes.

Ipomoeassin F Binds Sec61α to Inhibit Protein Translocation

Ipomoeassin F is a potent natural cytotoxin that inhibits growth of many tumor cell lines with single-digit nanomolar potency. However, its biological and pharmacological properties have remained largely unexplored. Building upon our earlier achievements in total synthesis and medicinal chemistry, we used chemical proteomics to identify Sec61α (protein transport protein Sec61 subunit alpha isoform 1), the pore-forming subunit of the Sec61 protein translocon, as a direct binding partner of ipomoeassin F in living cells. The interaction is specific and strong enough to survive lysis conditions, enabling a biotin analogue of ipomoeassin F to pull down Sec61α from live cells, yet it is also reversible, as judged by several experiments including fluorescent streptavidin staining, delayed competition in affinity pulldown, and inhibition of TNF biogenesis after washout. Sec61α forms the central subunit of the ER protein translocation complex, and the binding of ipomoeassin F results in a substantial, yet selective, inhibition of protein translocation in vitro and a broad ranging inhibition of protein secretion in live cells. Lastly, the unique resistance profile demonstrated by specific amino acid single-point mutations in Sec61α provides compelling evidence that Sec61α is the primary molecular target of ipomoeassin F and strongly suggests that the binding of this natural product to Sec61α is distinctive. Therefore, ipomoeassin F represents the first plant-derived, carbohydrate-based member of a novel structural class that offers new opportunities to explore Sec61α function and to further investigate its potential as a therapeutic target for drug discovery.

Toxins Utilize the Endoplasmic Reticulum-Associated Protein Degradation Pathway in Their Intoxication Process

Several bacterial and plant AB-toxins are delivered by retrograde vesicular transport to the endoplasmic reticulum (ER), where the enzymatically active A subunit is disassembled from the holotoxin and transported to the cytosol. In this process, toxins subvert the ER-associated degradation (ERAD) pathway. ERAD is an important part of cellular regulatory mechanism that targets misfolded proteins to the ER channels, prior to their retrotranslocation to the cytosol, ubiquitination and subsequent degradation by a protein-degrading complex, the proteasome. In this article, we present an overview of current understanding of the ERAD-dependent transport of AB-toxins to the cytosol. We describe important components of ERAD and discuss their significance for toxin transport. Toxin recognition and disassembly in the ER, transport through ER translocons and finally cytosolic events that instead of overall proteasomal degradation provide proper folding and cytotoxic activity of AB-toxins are discussed as well. We also comment on recent reports presenting medical applications for toxin transport through the ER channels.

Mycobacterium ulcerans mycolactones-fungi crosstalking

The opportunistic pathogen Mycobacterium ulcerans, which is responsible for Buruli ulcer, synthesizes a series of plasmid-encoded macrolide exotoxins termed mycolactones. These toxins destabilize cell membranes and induce apoptosis-associated pleiotropic effects including tissue destruction, analgesic and anti-inflammatory effects. Despite its medical interest, M. ulcerans is primarily an environmental mycobacterium and the primary functions of mycolactones in the natural ecosystems are unknown. High throughput biochemical profiling findings suggested that M. ulcerans may interact with fungi. Here, we report that semi-purified and purified mycolactones significantly enhance spore germination of Scedosporium apiospermum, Fusarium equiseti and Mucor circinelloides; and that M. ulcerans mycolactones significantly attract colonies of M. circinelloides whereas no significant effect was observed on S. apiospermum and F. equiseti. These experimental results suggest that mycolactones exhibit a chemoattractant activity independent of their cytotoxicity. In natural ecosystems, M. ulcerans mycolactones may act as spore germination inducers and chemoattractants for some fungi, suggesting a novel role for this unique class of mycobacterial toxins in natural ecosystems.

Spontaneous healing of Mycobacterium ulcerans disease in Australian patients

Background

Mycobacterium ulcerans causes necrotising infections of skin and soft tissue mediated by the polyketide exotoxin mycolactone that causes cell apoptosis and immune suppression. It has been postulated that infection can be eradicated before the development of clinical lesions but spontaneous resolution of clinical lesions has been rarely described.

Methodology/Principal findings

We report a case series of five Australian patients who achieved healing of small M. ulcerans lesions without antibiotics or surgery. The median age of patients was 47 years (IQR 30–68 years) and all patients had small ulcerative lesions (median size 144mm², IQR 121-324mm²). The median duration of symptoms prior to diagnosis was 90 days (IQR 90–100 days) and the median time to heal from diagnosis without treatment was 68 days (IQR 63–105 days). No patients recurred after a median follow-up of 16.6 months (IQR 16.6–17.9 months) from the development of symptoms and no patients suffered long-term disability from the disease.

Conclusions

We have shown that healing without specific treatment can occur for small ulcerated M. ulcerans lesions suggesting that in selected cases a robust immune response alone can cure lesions. Further research is required to determine what lesion and host factors are associated with spontaneous healing, and whether observation alone is an effective and safe form of management for selected small M. ulcerans lesions.

Mycobacterium ulcerans causes a destructive infection of skin and soft tissue known as Buruli ulcer that when severe can lead to serious long-term deformity and disability. It is currently not well documented whether people with Mycobacterium ulcerans disease can cure themselves without treatment. In our study we describe five people with small ulcers who cured their disease without specific medical or surgical treatment. This suggests that a proportion of people can develop an immune response sufficient enough to eradicate the disease without the help of medical intervention. This is an important step, as recognition of this possibility provides important further insights into the human immune response against the disease. It also opens the possibility to further studies that may determine characteristics of the organism and hosts that favour spontaneous healing of lesions. This knowledge may in turn improve efforts to prevent and control the disease which are currently lacking.

Regulation of Antigen Export to the Cytosol During Cross-Presentation

Cross-priming refers to the induction of primary cytotoxic CD8⁺ T cell responses to antigens that are not expressed in antigen presenting cells (APCs) responsible for T cell priming. Cross-priming is achieved through cross-presentation of exogenous antigens derived from tumors, extracellular pathogens or infected neighboring cells on Major Histocompatibility Complex (MHC) class I molecules. Despite extensive research efforts to understand the intracellular pathways involved in antigen cross-presentation, certain critical steps remain elusive and controversial. Here we review recent advances on antigen cross-presentation, focusing on the mechanisms involved in antigen export to the cytosol, a crucial step of this pathway.

Adjuvants Enhancing Cross-Presentation by Dendritic Cells: The Key to More Effective Vaccines?

Over the last decades, vaccine development has advanced significantly in pursuing higher safety with less side effects. However, this is often accompanied by a reduction in vaccine immunogenicity and an increased dependency on adjuvants to enhance vaccine potency. Especially for diseases like cancer, it is important that therapeutic vaccines contain adjuvants that promote strong T cell responses. An important mode of action for such adjuvants is to prolong antigen exposure to dendritic cells (DCs) and to induce their maturation. These mature DCs are extremely effective in the activation of antigen-specific T cells, which is a pre-requisite for induction of potent and long-lasting cellular immunity. For the activation of CD8⁺ cytotoxic T cell responses, however, the exogenous vaccine antigens need to gain access to the endogenous MHCI presentation pathway of DCs, a process referred to as antigen cross-presentation. In this review, we will focus on recent insights in clinically relevant vaccine adjuvants that impact DC cross-presentation efficiency, including aluminum-based nanoparticles, saponin-based adjuvants, and Toll-like receptor ligands. Furthermore, we will discuss the importance of adjuvant combinations and highlight new developments in cancer vaccines. Understanding the mode of action of adjuvants in general and on antigen cross-presentation in DCs in particular will be important for the design of novel adjuvants as part of vaccines able to induce strong cellular immunity.