UNC93B1 interacts with the calcium sensor STIM1 for efficient antigen cross-presentation in dendritic cells

Gain-of-function human UNC93B1 variants cause systemic lupus erythematosus and chilblain lupus

UNC93B1 is a transmembrane domain protein mediating the signaling of endosomal Toll-like receptors (TLRs). We report five families harboring rare missense substitutions (I317M, G325C, L330R, R466S, and R525P) in UNC93B1 causing systemic lupus erythematosus (SLE) or chilblain lupus (CBL) as either autosomal dominant or autosomal recessive traits. As for a D34A mutation causing murine lupus, we recorded a gain of TLR7 and, to a lesser extent, TLR8 activity with the I317M (in vitro) and G325C (in vitro and ex vivo) variants in the context of SLE. Contrastingly, in three families segregating CBL, the L330R, R466S, and R525P variants were isomorphic with respect to TLR7 activity in vitro and, for R525P, ex vivo. Rather, these variants demonstrated a gain of TLR8 activity. We observed enhanced interaction of the G325C, L330R, and R466S variants with TLR8, but not the R525P substitution, indicating different disease mechanisms. Overall, these observations suggest that UNC93B1 mutations cause monogenic SLE or CBL due to differentially enhanced TLR7 and TLR8 signaling.

Direct antigen presentation is the canonical pathway of cytomegalovirus CD8 T-cell priming regulated by balanced immune evasion ensuring a strong antiviral response

CD8 T cells are important antiviral effectors in the adaptive immune response to cytomegaloviruses (CMV). Naïve CD8 T cells can be primed by professional antigen-presenting cells (pAPCs) alternatively by "direct antigen presentation" or "antigen cross-presentation". In the case of direct antigen presentation, viral proteins are expressed in infected pAPCs and enter the classical MHC class-I (MHC-I) pathway of antigen processing and presentation of antigenic peptides. In the alternative pathway of antigen cross-presentation, viral antigenic material derived from infected cells of principally any cell type is taken up by uninfected pAPCs and eventually also fed into the MHC class-I pathway. A fundamental difference, which can be used to distinguish between these two mechanisms, is the fact that viral immune evasion proteins that interfere with the cell surface trafficking of peptide-loaded MHC-I (pMHC-I) complexes are absent in cross-presenting uninfected pAPCs. Murine cytomegalovirus (mCMV) models designed to disrupt either of the two presentation pathways revealed that both are possible in principle and can substitute each other. Overall, however, the majority of evidence has led to current opinion favoring cross-presentation as the canonical pathway. To study priming in the normal host genetically competent in both antigen presentation pathways, we took the novel approach of enhancing or inhibiting direct antigen presentation by using recombinant viruses lacking or overexpressing a key mCMV immune evasion protein. Against any prediction, the strongest CD8 T-cell response was elicited under the condition of intermediate direct antigen presentation, as it exists for wild-type virus, whereas the extremes of enhanced or inhibited direct antigen presentation resulted in an identical and weaker response. Our findings are explained by direct antigen presentation combined with a negative feedback regulation exerted by the newly primed antiviral effector CD8 T cells. This insight sheds a completely new light on the acquisition of viral immune evasion genes during virus-host co-evolution.

Construction and validation of a transient receptor potential-related long noncoding RNA signature for prognosis prediction in breast cancer patients

Breast cancer (BC) is the most commonly diagnosed malignancy in women around the world. Accumulating evidence suggests that transient receptor potential (TRP) channels play a significant role in tumor progression and immune cell infiltration. Hence, we conducted the study to investigate the correlation between TRP-associated lncRNAs and the prognosis of breast carcinoma. In the current study, 33 TRP-associated genes were selected from a review published by Amrita Samanta et al, and the TRP-related lncRNAs were identified by Pearson analysis. Based on the sum of the expression levels of 12 lncRNAs provided by the Cancer Genome Atlas (TCGA), a TRP-associated lncRNA signature was established by using Cox regression analysis. According to the median value of the risk score in the training set, BC patients were separated into high- and low-risk groups. Subsequently, functional enrichment analysis was conducted on the differential expression genes (DEGs) between different risk groups. The Estimation of Stromal and Immune Cells in Malignant Tumor Tissues Using Expression (ESTIMATE) Score was calculated by ESTIMATE, and the immune cell infiltration was evaluated by ssGSEA. Finally, the immune checkpoint gene expression levels, microsatellite instability (MSI), and immunophenoscore (IPS) were further assessed. The high-risk groups exhibited lower survival rates, while the low-risk groups showed higher survival rates. As a result, the DEGs between different risk groups were highly enriched in immune cell activation and immunoregulation. Besides, the ESTIMATE scores of patients in low-risk groups were higher than those in high-risk groups. The infiltration levels of several immune cells were remarkably elevated in low-risk groups, and various immune signatures were activated with a decreased risk score. Eventually, the TRP-associated lncRNA signature was confirmed with a highly potential ability to evaluate the immunotherapy response in breast carcinoma patients. The outcomes of the current study indicated that the 12-TRP-associated-lncRNA risk model was an independent prognostic risk factor for BC patients. This risk model could be closely related to the tumor immune microenvironment in BC. Our findings will provide new insights for future immunotherapy for BC treatment.

A Putative Role for TRPC6 in Immune-Mediated Kidney Injury

Excessive activation of the immune system is the cause of a wide variety of renal diseases. However, the pathogenic mechanisms underlying the aberrant activation of the immune system in the kidneys often remain unknown. TRPC6, a member of the Ca2+-permeant family of TRPC channels, is important in glomerular epithelial cells or podocytes for the process of glomerular filtration. In addition, TRPC6 plays a crucial role in the development of kidney injuries by inducing podocyte injury. However, an increasing number of studies suggest that TRPC6 is also responsible for tightly regulating the immune cell functions. It remains elusive whether the role of TRPC6 in the immune system and the pathogenesis of renal inflammation are intertwined. In this review, we present an overview of the current knowledge of how TRPC6 coordinates the immune cell functions and propose the hypothesis that TRPC6 might play a pivotal role in the development of kidney injury via its role in the immune system.

The role of the antigen processing machinery in the regulation and trafficking of intracellular -Toll-like receptor molecules

Intracellular Toll-like receptors (TLRs) are key components of the innate immune system. Their expression in antigen-presenting cells (APCs), and in particular dendritic cells (DCs), makes them critical in the induction of the adaptive immune response. In DCs, they interact with the chaperone UNC93B1 that mediates their trafficking from the endoplasmic reticulum (ER) to endosomes where they are cleaved by proteases and activated. All these different steps are also shared by major histocompatibility complex class-II (MHCII) molecules. Here, we will discuss the tight relationship intracellular TLRs have with the antigen processing machinery in APCs for their trafficking and activation.

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.

High-expression of the innate-immune related gene UNC93B1 predicts inferior outcomes in acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous hematological malignancy with dismal prognosis. Identification of better biomarkers remained a priority to improve established stratification and guide therapeutic decisions. Therefore, we extracted the RNA sequence data and clinical characteristics of AML from The Cancer Genome Atlas (TCGA) and Genotype-Tissue Expression database (GTEx) to identify the key factors for prognosis. We found UNC93B1 was highly expressed in AML patients and significantly linked to poor clinical features (p < 0.05). We further validated the high expression of UNC93B1 in another independent AML cohort from GEO datasets (p < 0.001) and performed quantitative PCR of patient samples to confirm the overexpression of UNC93B1 in AML (p < 0.005). Moreover, we discovered high level of UNC93B1 was an independent prognostic factor for poorer outcome both in univariate analysis and multivariate regression (p < 0.001). Then we built a nomogram model based on UNC93B1 expression, age, FAB subtype and cytogenetic risk, the concordance index of which for predicting overall survival was 0.729 (p < 0.001). Time-dependent ROC analysis for predicting survival outcome at different time points by UNC93B1 showed the cumulative 2-year survival rate was 43.7%, and 5-year survival rate was 21.9%. The differentially expressed genes (DEGs) between two groups divided by UNC93B1 expression level were enriched in innate immune signaling and metabolic process pathway. Protein-protein interaction (PPI) network indicated four hub genes (S100A9, CCR1, MRC1 and CD1C) interacted with UNC93B1, three of which were also significantly linked to inferior outcome. Furthermore, we discovered high UNC93B1 tended to be infiltrated by innate immune cells, including Macrophages, Dendritic cells, Neutrophils, Eosinophils, and NK CD56dim cells. We also found UNC93B1 had a significantly positive correlation with CD14, CD68 and almost all Toll-like receptors. Finally, we revealed negatively correlated expression of UNC93B1 and BCL2 in AML and conjectured that high-UNC93B1 monocytic AML is more resistant to venetoclax. And we found high MCL-1 expression compensated for BCL-2 loss, thus, we proposed MCL-1 inhibitor might overcome the resistance of venetoclax in AML. Altogether, our findings demonstrated the utility of UNC93B1 as a powerful poor prognostic predictor and alternative therapeutic target.

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.

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 mammalian trafficking chaperone protein UNC93B1 maintains the ER calcium sensor STIM1 in a dimeric state primed for translocation to the ER cortex

The stromal interaction molecule 1 (STIM1) is an endoplasmic reticulum (ER) Ca²⁺ sensor that regulates the activity of Orai plasma membrane Ca²⁺ channels to mediate the store-operated Ca²⁺ entry pathway essential for immunity. Uncoordinated 93 homolog B1 (UNC93B1) is a multiple membrane-spanning ER protein that acts as a trafficking chaperone by guiding nucleic-acid sensing toll-like receptors to their respective endosomal signaling compartments. We previously showed that UNC93B1 interacts with STIM1 to promote antigen cross-presentation in dendritic cells, but the STIM1 binding site(s) and activation step(s) impacted by this interaction remained unknown. In this study, we show that UNC93B1 interacts with STIM1 in the ER lumen by binding to residues in close proximity to the transmembrane domain. Cysteine crosslinking in vivo showed that UNC93B1 binding promotes the zipping of transmembrane and proximal cytosolic helices within resting STIM1 dimers, priming STIM1 for translocation. In addition, we show that UNC93B1 deficiency reduces store-operated Ca²⁺ entry and STIM1-Orai1 interactions and targets STIM1 to lighter ER domains, whereas UNC93B1 expression accelerates the recruitment of STIM1 to cortical ER domains. We conclude that UNC93B1 therefore acts as a trafficking chaperone by maintaining the pool of resting STIM1 proteins in a state primed for activation, enabling their rapid translocation in an extended conformation to cortical ER signaling compartments.

Strategy and clinical application of up-regulating cross presentation by DCs in anti-tumor therapy

The cross presentation of exogenous antigen (Ag) by dendritic cells (DCs) facilitates a diversified mode of T-cell activation, orchestrates specific humoral and cellular immunity, and contributes to an efficient anti-tumor immune response. DCs-mediated cross presentation is subject to both intrinsic and extrinsic factors, including the homing and phenotype of DCs, the spatiotemporal trafficking and degradation kinetics of Ag, and multiple microenvironmental clues, with many details largely unexplored. Here, we systemically review the current mechanistic understanding and regulation strategies of cross presentation by heterogeneous DC populations. We also provide insights into the future exploitation of DCs cross presentation for a better clinical efficacy in anti-tumor therapy.

UNC93B1 curbs cytosolic DNA signaling by promoting STING degradation

UNC93B1 is a trafficking chaperone of endosomal Toll-like receptors (TLRs) and plays an essential role in the TLR-mediated innate signaling. However, whether it is also involved in other innate immune sensing or cellular pathways remains largely unexplored. Here we investigated the role of UNC93B1 in cytosolic DNA-triggered cGAS-STING signaling in mouse and human cell lines. We showed that while UNC93B1 deficiency blunts the signal transduction by TLR3, it augments innate immune responses to cytosolic DNA stimulation and DNA virus infection. Mechanistic study reveals a distinct action of UNC93B1 upon STING, but not other parts along the cGAS-STING-TBK1 axis, through regulating the protein level of STING at both resting and cytosolic DNA-stimulated conditions. UNC93B1 can directly interact and traffic along with STING, and the disruption of this interaction causes accumulation of STING that subsequently leads to augmented signaling responses upon its activation. These findings reveal a new function of UNC93B1 in negatively regulating STING-mediated signaling responses.

New Insights on the Role of TRP Channels in Calcium Signalling and Immunomodulation: Review of Pathways and Implications for Clinical Practice

Calcium is the most abundant mineral in the human body and is central to many physiological processes, including immune system activation and maintenance. Studies continue to reveal the intricacies of calcium signalling within the immune system. Perhaps the most well-understood mechanism of calcium influx into cells is store-operated calcium entry (SOCE), which occurs via calcium release-activated channels (CRACs). SOCE is central to the activation of immune system cells; however, more recent studies have demonstrated the crucial role of other calcium channels, including transient receptor potential (TRP) channels. In this review, we describe the expression and function of TRP channels within the immune system and outline associations with murine models of disease and human conditions. Therefore, highlighting the importance of TRP channels in disease and reviewing potential. The TRP channel family is significant, and its members have a continually growing number of cellular processes. Within the immune system, TRP channels are involved in a diverse range of functions including T and B cell receptor signalling and activation, antigen presentation by dendritic cells, neutrophil and macrophage bactericidal activity, and mast cell degranulation. Not surprisingly, these channels have been linked to many pathological conditions such as inflammatory bowel disease, chronic fatigue syndrome and myalgic encephalomyelitis, atherosclerosis, hypertension and atopy.

More Than Just Simple Interaction between STIM and Orai Proteins: CRAC Channel Function Enabled by a Network of Interactions with Regulatory Proteins

The calcium-release-activated calcium (CRAC) channel, activated by the release of Ca2+ from the endoplasmic reticulum (ER), is critical for Ca2+ homeostasis and active signal transduction in a plethora of cell types. Spurred by the long-sought decryption of the molecular nature of the CRAC channel, considerable scientific effort has been devoted to gaining insights into functional and structural mechanisms underlying this signalling cascade. Key players in CRAC channel function are the Stromal interaction molecule 1 (STIM1) and Orai1. STIM1 proteins span through the membrane of the ER, are competent in sensing luminal Ca2+ concentration, and in turn, are responsible for relaying the signal of Ca2+ store-depletion to pore-forming Orai1 proteins in the plasma membrane. A direct interaction of STIM1 and Orai1 allows for the re-entry of Ca2+ from the extracellular space. Although much is already known about the structure, function, and interaction of STIM1 and Orai1, there is growing evidence that CRAC under physiological conditions is dependent on additional proteins to function properly. Several auxiliary proteins have been shown to regulate CRAC channel activity by means of direct interactions with STIM1 and/or Orai1, promoting or hindering Ca2+ influx in a mechanistically diverse manner. Various proteins have also been identified to exert a modulatory role on the CRAC signalling cascade although inherently lacking an affinity for both STIM1 and Orai1. Apart from ubiquitously expressed representatives, a subset of such regulatory mechanisms seems to allow for a cell-type-specific control of CRAC channel function, considering the rather restricted expression patterns of the specific proteins. Given the high functional and clinical relevance of both generic and cell-type-specific interacting networks, the following review shall provide a comprehensive summary of regulators of the multilayered CRAC channel signalling cascade. It also includes proteins expressed in a narrow spectrum of cells and tissues that are often disregarded in other reviews of similar topics.

Proteins Interacting with STIM1 and Store-Operated Ca2+ Entry

The endoplasmic reticulum (ER) Ca²⁺ sensor stromal interaction molecule 1 (STIM1) interacts with ORAI Ca²⁺ channels at the plasma membrane to regulate immune and muscle cell function. The conformational changes underlying STIM1 activation, translocation, and ORAI1 trapping and gating, are stringently regulated by post-translational modifications and accessory proteins. Here, we review the recent progress in the identification and characterization of ER and cytosolic proteins interacting with STIM1 to control its activation and deactivation during store-operated Ca²⁺ entry (SOCE).

Cytoskeletal regulation of dendritic cells: An intricate balance between migration and presentation for tumor therapy

Dendritic cells (DCs) are the main players in many approaches for cancer therapy. The idea with DC tumor therapy is to promote activation of tumor infiltrating cytotoxic T cells that kill tumor cells. This requires that DCs take up tumor Ag and present peptides on MHC class I molecules in a process called cross-presentation. For this process to be efficient, DCs have to migrate to the tumor draining lymph node and there activate the machinery for cross-presentation. In this review, we will discuss recent progress in understanding the role of actin regulators for control of DC migration and Ag presentation. The potential to target actin regulators for better DC-based tumor therapy will also be discussed.

Kinesin-1 regulates antigen cross-presentation through the scission of tubulations from early endosomes in dendritic cells

Dendritic cells (DCs) constitute a specialized population of immune cells that present exogenous antigen (Ag) on major histocompatibility complex (MHC) class I molecules to initiate CD8 + T cell responses against pathogens and tumours. Although cross-presentation depends critically on the trafficking of Ag-containing intracellular vesicular compartments, the molecular machinery that regulates vesicular transport is incompletely understood. Here, we demonstrate that mice lacking Kif5b (the heavy chain of kinesin-1) in their DCs exhibit a major impairment in cross-presentation and thus a poor in vivo anti-tumour response. We find that kinesin-1 critically regulates antigen cross-presentation in DCs, by controlling Ag degradation, the endosomal pH, and MHC-I recycling. Mechanistically, kinesin-1 appears to regulate early endosome maturation by allowing the scission of endosomal tubulations. Our results highlight kinesin-1’s role as a molecular checkpoint that modulates the balance between antigen degradation and cross-presentation.

Kinesin-1 is a motor protein transporting cargo along microtubules. Here the authors show that kinesin-1 is required for antigen cross-presentation and coordinates endosome scission from early endosomes to allow sorting internalized cargoes towards the recycling endosomal or lysosomal compartments.

Twist1 promotes dendritic cell-mediated antitumor immunity

Dendritic cells (DCs) play a central role in autoimmunity, immune homeostasis, and presentation of tumor antigens to T cells in order to prime antitumor responses. The number of tumor-infiltrating DCs is associated with survival and prognosis in cancer. Twist1 is a well-known regulator of tumor initiation and promotion, but whether and how DC-derived Twist1 regulates antitumor responses remains poorly understood. Here, we generated a mouse line with Twist1 conditionally depleted in DCs and found that Twist1-deficiency in DCs did not affect the DCs and T cell homeostasis under steady-state conditions; however, in melanoma models, the proportion of conventional DCs (cDCs) in draining lymph nodes (DLNs) was significantly decreased. Accordingly, a decreased ratio and number of tumor-infiltrating cDCs were observed, which reduced the recruitment of tumor-infiltrating T cells. Furthermore, production of IFN-γ, a crucial antitumor factor, by T cells, was dramatically decreased, which can further dampen the T cell antitumor functions. Collectively, our data indicate that Twist1 in DCs regulates antitumor functions by maintain the number of tumor-infiltrating DCs and T cells, and their antitumor activity.

TLR7 trafficking and signaling in B cells is regulated by the MHCII-associated invariant chain

Toll-like receptor 7 (TLR7) is an endosomal receptor that recognizes single-stranded RNA from viruses. Its trafficking and activation is regulated by the endoplasmic reticulum (ER) chaperone UNC93B1 and lysosomal proteases. UNC93B1 also modulates major histocompatibility complex class II (MHCII) antigen presentation, and deficiency in MHCII protein diminishes TLR9 signaling. These results indicate a link between proteins that regulate both innate and adaptive responses. Here, we report that TLR7 resides in lysosomes and interacts with the MHCII-chaperone molecule, the invariant chain (Ii) or CD74, in B cells. In the absence of CD74, TLR7 displays both ER and lysosomal localization, leading to an increase in pro-inflammatory cytokine production. Furthermore, stimulation with TLR7 but not TLR9, is inefficient in boosting antigen presentation in Ii-deficient cells. In contrast, in B cells lacking TLR7 or mutated for UNC93B1, which are able to trigger TLR7 activation, antigen presentation is enhanced. This suggests that TLR7 signaling in B cells is controlled by the Ii chain.

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.

Molecular Mechanisms of Calcium Signaling During Phagocytosis

Calcium (Ca²⁺) is a ubiquitous second messenger involved in the regulation of numerous cellular functions including vesicular trafficking, cytoskeletal rearrangements and gene transcription. Both global as well as localized Ca²⁺ signals occur during phagocytosis, although their functional impact on the phagocytic process has been debated. After nearly 40 years of research, a consensus may now be reached that although not strictly required, Ca²⁺ signals render phagocytic ingestion and phagosome maturation more efficient, and their manipulation make an attractive avenue for therapeutic interventions. In the last decade many efforts have been made to identify the channels and regulators involved in generating and shaping phagocytic Ca²⁺ signals. While molecules involved in store-operated calcium entry (SOCE) of the STIM and ORAI family have taken center stage, members of the canonical, melastatin, mucolipin and vanilloid transient receptor potential (TRP), as well as purinergic P2X receptor families are now recognized to play significant roles. In this chapter, we review the recent literature on research that has linked specific Ca²⁺-permeable channels and regulators to phagocytic function. We highlight the fact that lipid mediators are emerging as important regulators of channel gating and that phagosomal ionic homeostasis and Ca²⁺ release also play essential parts. We predict that improved methodologies for measuring these factors will be critical for future advances in dissecting the intricate biology of this fascinating immune process.

Understanding the Role of the Unfolded Protein Response Sensor IRE1 in the Biology of Antigen Presenting Cells

The unfolded protein response (UPR) is an adaptive response that maintains the fidelity of the cellular proteome in conditions that subvert the folding capacity of the cell, such as those noticed in infection and inflammatory contexts. In immunity, the UPR sensor IRE1 (Inositol-requiring enzyme 1-alpha) has emerged as a critical regulator of the homeostasis of antigen presenting cells (APCs). In the past few years, it has become clear that IRE1 plays canonical and non-canonical roles in APCs, many of which intersect with key features of these cells, including the initiation of inflammation, antibody production, and antigen presentation. The aims of the present review are to provide recent insights on the mechanisms by which IRE1 regulates the diversity of APC functions and to highlight its relevance in the coordination of innate and adaptive immunity.

Tmem178 negatively regulates store-operated calcium entry in myeloid cells via association with STIM1

Store-operated calcium entry (SOCE) modulates cytosolic calcium in multiple cells. Endoplasmic reticulum (ER)-localized STIM1 and plasma membrane (PM)-localized ORAI1 are two main components of SOCE. STIM1:ORAI1 association requires STIM1 oligomerization, its re-distribution to ER-PM junctions, and puncta formation. However, little is known about the negative regulation of these steps to prevent calcium overload. Here, we identified Tmem178 as a negative modulator of STIM1 puncta formation in myeloid cells. Using site-directed mutagenesis, co-immunoprecipitation assays and FRET imaging, we determined that Tmem178:STIM1 association occurs via their transmembrane motifs. Mutants that increase Tmem178:STIM1 association reduce STIM1 puncta formation, SOCE activation, impair inflammatory cytokine production in macrophages and osteoclastogenesis. Mutants that reduce Tmem178:STIM1 association reverse these effects. Furthermore, exposure to plasma from arthritic patients decreases Tmem178 expression, enhances SOCE activation and cytoplasmic calcium. In conclusion, Tmem178 modulates the rate-limiting step of STIM1 puncta formation and therefore controls SOCE in inflammatory conditions.

CRAC channel regulation of innate immune cells in health and disease

Calcium is a major intracellular signaling messenger in innate immune cells. Similar to other immune cell subsets, the majority of calcium entry into innate immune cells is induced by cell surface receptors that stimulate store-operated calcium entry through calcium-release activated calcium (CRAC) channels. Since the molecular description of the STIM family of calcium sensors and the ORAI family of CRAC channel proteins, the majority of studies support a dominant role for these proteins in calcium signaling in innate cells. In reviewing the literature on CRAC channel function in innate cells, several general themes emerge. All innate cells express multiple members of the STIM and ORAI family members, however the ratio and relative contribution of individual isoforms changes depending on the cell type and activation state of the cell. It is evident that study of functional roles for STIM molecules is clearly ahead of studies of specific ORAI family members in all innate cell types, and that studies of CRAC channels in innate cells are not nearly as advanced as studies in lymphocytes. However, taken together, evidence from both STIM calcium sensors and ORAI channels in innate cells indicates that deficiency of STIM and ORAI proteins tends not to affect the development of any innate cell lineage, but certainly affects their function, in particular activation of the neutrophil oxidase and mast cell activation via IgE receptors. Furthermore, there are clearly hints that therapeutic targeting of CRAC channels in innate cells offers a new approach to various inflammatory and allergic diseases.

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.

Current Concepts of Antigen Cross-Presentation

Dendritic cells have the ability to efficiently present internalized antigens on major histocompatibility complex (MHC) I molecules. This process is termed cross-presentation and is important role in the generation of an immune response against viruses and tumors, after vaccinations or in the induction of immune tolerance. The molecular mechanisms enabling cross-presentation have been topic of intense debate since many years. However, a clear view on these mechanisms remains difficult, partially due to important remaining questions, controversial results and discussions. Here, we give an overview of the current concepts of antigen cross-presentation and focus on a description of the major cross-presentation pathways, the role of retarded antigen degradation for efficient cross-presentation, the dislocation of antigens from endosomal compartment into the cytosol, the reverse transport of proteasome-derived peptides for loading on MHC I and the translocation of the cross-presentation machinery from the ER to endosomes. We try to highlight recent advances, discuss some of the controversial data and point out some of the major open questions in the field.

Evidence for the interaction of peroxiredoxin-4 with the store-operated calcium channel activator STIM1 in liver cells

Ca²⁺ entry through store-operated Ca²⁺ channels (SOCs) in the plasma membrane (PM) of hepatocytes plays a central role in the hormonal regulation of liver metabolism. SOCs are composed of Orai1, the channel pore protein, and STIM1, the activator protein, and are regulated by hormones and reactive oxygen species (ROS). In addition to Orai1, STIM1 also interacts with several other intracellular proteins. Most previous studies of the cellular functions of Orai1 and STIM1 have employed immortalised cells in culture expressing ectopic proteins tagged with a fluorescent polypeptide such as GFP. Little is known about the intracellular distributions of endogenous Orai1 and STIM1. The aims are to determine the intracellular distribution of endogenous Orai1 and STIM1 in hepatocytes and to identify novel STIM1 binding proteins. Subcellular fractions of rat liver were prepared by homogenisation and differential centrifugation. Orai1 and STIM1 were identified and quantified by western blot. Orai1 was found in the PM (0.03%), heavy (44%) and light (27%) microsomal fractions, and STIM1 in the PM (0.09%), and heavy (85%) and light (13%) microsomal fractions. Immunoprecipitation of STIM1 followed by LC/MS or western blot identified peroxiredoxin-4 (Prx-4) as a potential STIM1 binding protein. Prx-4 was found principally in the heavy microsomal fraction. Knockdown of Prx-4 using siRNA, or inhibition of Prx-4 using conoidin A, did not affect Ca²⁺ entry through SOCs but rendered SOCs susceptible to inhibition by H₂O₂. It is concluded that, in hepatocytes, a considerable proportion of endogenous Orai1 and STIM1 is located in the rough ER. In the rough ER, STIM1 interacts with Prx-4, and this interaction may contribute to the regulation by ROS of STIM1 and SOCs.

GRAM marks the spot for STIM. Commentary on "GRAM domain proteins specialize functionally distinct ER-PM contact sites in human cells"

GRAM domain proteins were reported as novel ER-PM tethers defining specific membrane contact sites (MCS) subdomains. GRAMD2a pre-marks the sites occupied by STIM1 at MCS and its ablation impairs STIM1 translocation, but not store-operated Ca²⁺ entry. We discuss these apparently counterintuitive findings in the context of STIM/ORAI signaling at MCS.

Immunocompetence and mechanism of the DRibble-DCs vaccine for oral squamous cell carcinoma

Background

Due to the high-quality immunogenicity of tumor-derived autophagosomes (DRibbles), we aimed to explore the antitumor ability and mechanism of DRibble-loaded dendritic cells (DRibble-DCs).

Materials and methods

DRibbles extracted from the oral squamous cell carcinoma cell line SCC7 express specific LC3-II and ubiquitination marker. Immunization of mice with the DRibble-DCs vaccine led to the proliferation and differentiation of CD3⁺CD4⁺IFN-γ⁺ and CD3⁺CD8⁺IFN-γ⁺ T cells. The expression of proteins in endoplasmic reticulum stress (ERS) pathways was determined by Western blotting. Additionally, the functional properties of the DRibble-DCs were examined in mice, and regulatory T cells were measured by flow cytometry.

Results

Excellent biocompatibility was observed in vitro when DCs were loaded with DRibbles. T cells of lymph nodes and spleens from mice immunized with DRibble-DCs had cytotoxic effects on SCC7 cells. DCs homeostasis and ERS-related proteins were affected by DRibbles. Moreover, the DRibble-DCs vaccine achieved significantly better antitumor efficacy than DRibbles and tumor cell lysate-loaded DCs.

Conclusion

The results validated the antitumor immune responses to the DRibble-DCs vaccine in vivo and in vitro. The ERS pathway can be affected by DRibbles.

STIM1 promotes migration, phagosomal maturation and antigen cross-presentation in dendritic cells

Antigen cross-presentation by dendritic cells (DC) stimulates cytotoxic T cell activation to promote immunity to intracellular pathogens, viruses and cancer. Phagocytosed antigens generate potent T cell responses, but the signalling and trafficking pathways regulating their cross-presentation are unclear. Here, we show that ablation of the store-operated-Ca²⁺-entry regulator STIM1 in mouse myeloid cells impairs cross-presentation and DC migration in vivo and in vitro. Stim1 ablation reduces Ca²⁺ signals, cross-presentation, and chemotaxis in mouse bone-marrow-derived DCs without altering cell differentiation, maturation or phagocytic capacity. Phagosomal pH homoeostasis and ROS production are unaffected by STIM1 deficiency, but phagosomal proteolysis and leucyl aminopeptidase activity, IRAP recruitment, as well as fusion of phagosomes with endosomes and lysosomes are all impaired. These data suggest that STIM1-dependent Ca²⁺ signalling promotes the delivery of endolysosomal enzymes to phagosomes to enable efficient cross-presentation.

STIM proteins sense Ca²⁺ depletion in the ER and activate store-operated Ca²⁺-entry (SOCE) in response, a process associated with dendritic cell functions. Here the authors show STIM1 is the major isoform controlling SOCE in mouse dendritic cells and provide a mechanism for its requirement in antigen cross-presentation.