Protein kinase C delta stimulates antigen presentation by Class II MHC in murine dendritic cells

Peroxisome deficiency underlies failures in hepatic immune cell development and antigen presentation in a severe Zellweger disease model

Peroxisome biogenesis disorders (PBDs) represent a group of metabolic conditions that cause severe developmental defects. Peroxisomes are essential metabolic organelles, present in virtually every eukaryotic cell and mediating key processes in immunometabolism. To date, the full spectrum of PBDs remains to be identified, and the impact PBDs have on immune function is unexplored. This study presents a characterization of the hepatic immune compartment of a neonatal PBD mouse model at single-cell resolution to establish the importance and function of peroxisomes in developmental hematopoiesis. We report that hematopoietic defects are a feature in a severe PBD murine model. Finally, we identify a role for peroxisomes in the regulation of the major histocompatibility class II expression and antigen presentation to CD4+ T cells in dendritic cells. This study adds to our understanding of the mechanisms of PBDs and expands our knowledge of the role of peroxisomes in immunometabolism.

PKCδ regulates the vascular biology in diabetic atherosclerosis

Diabetes mellitus, known for its complications, especially vascular complications, is becoming a globally serious social problem. Atherosclerosis has been recognized as a common vascular complication mechanism in diabetes. The diacylglycerol (DAG)-protein kinase C (PKC) pathway plays an important role in atherosclerosis. PKCs can be divided into three subgroups: conventional PKCs (cPKCs), novel PKCs (nPKCs), and atypical PKCs (aPKCs). The aim of this review is to provide a comprehensive overview of the role of the PKCδ pathway, an isoform of nPKC, in regulating the function of endothelial cells, vascular smooth muscle cells, and macrophages in diabetic atherosclerosis. In addition, potential therapeutic targets regarding the PKCδ pathway are summarized. Video Abstract.

Immune stimulation of rainbow trout reveals divergent regulation of MH class II-associated invariant chain isoforms

Major histocompatibility complex (MHC) class II-associated invariant chain is a chaperone responsible for targeting the MHC class II dimer to the endocytic pathway, thus enabling the loading of exogenous antigens onto the MHC class II receptor. In the current study, in vivo and in vitro methods were used to investigate the regulation of the rainbow trout invariant chain proteins S25-7 and INVX, upon immune system activation. Whole rainbow trout and the macrophage/monocyte-like cell line RTS11 were treated with PMA at concentrations shown to induce IL-1β transcripts and homotypic aggregation of RTS11. S25-7 transcript levels remained unchanged in the gill, spleen, and liver and were found to be significantly decreased in head kidney beginning 24 h post-stimulation. Meanwhile, INVX transcript levels remained unchanged in all tissues studied. Both S25-7 and INVX proteins were produced in gill and spleen tissues but their expression was unaffected by immune system stimulation. Surprisingly, neither INVX nor S25-7 protein was detected in the secondary immune organ, the head kidney. Analysis of RTS11 cultures demonstrated that both INVX and S25-7 transcript levels significantly increased at 96 h and 120 h following PMA stimulation before returning to control levels at 168 h. Meanwhile, at the protein level in RTS11, S25-7 remained unchanged while INVX had a significant decrease at 168 h post-stimulation. These results indicate that neither INVX nor S25-7 is upregulated upon immune system activation; thus, teleosts have evolved a system of immune regulation that is different than that found in mammals.

Enhanced immunocompatibility of ligand-targeted liposomes by attenuating natural IgM absorption

Targeting ligands are anticipated to facilitate the precise delivery of therapeutic agents to diseased tissues; however, they may also severely affect the interaction of nanocarriers with plasma proteins. Here, we study the immunocompatibility of brain-targeted liposomes, which inversely correlates with absorbed natural IgM. Modification of long, stable positively charged peptide ligands on liposomes is inclined to absorb natural IgM, leading to rapid clearance and enhanced immunogenicity. Small peptidomimetic D8 developed by computer-aided peptide design exhibits improved immunocompatibility by attenuating natural IgM absorption. The present study highlights the effects of peptide ligands on the formed protein corona and in vivo fate of liposomes. Stable positively charged peptide ligands play double-edged roles in targeted delivery, preserving in vivo bioactivities for binding receptors and long-term unfavorable interactions with the innate immune system. The development of D8 provides insights into how to rationally design immunocompatible drug delivery systems by modulating the protein corona composition.

Targeting ligands on drug carriers can trigger immune responses. Here, the authors modify liposomes with a peptidomimetic that preserves bioactivity of the nanocarrier in blood circulation and attenuates IgM absorption, thereby improving the immunocompatibility of brain-targeted liposomes.

PKCδ-Mediated Nox2 Activation Promotes Fluid-Phase Pinocytosis of Antigens by Immature Dendritic Cells

Aims

Macropinocytosis is a major endocytic pathway by which dendritic cells (DCs) internalize antigens in the periphery. Despite the importance of DCs in the initiation and control of adaptive immune responses, the signaling mechanisms mediating DC macropinocytosis of antigens remain largely unknown. The goal of the present study was to investigate whether protein kinase C (PKC) is involved in stimulation of DC macropinocytosis and, if so, to identify the specific PKC isoform(s) and downstream signaling mechanisms involved.

Methods

Various cellular, molecular and immunological techniques, pharmacological approaches and genetic knockout mice were utilized to investigate the signaling mechanisms mediating DC macropinocytosis.

Results

Confocal laser scanning microscopy confirmed that DCs internalize fluorescent antigens (ovalbumin) using macropinocytosis. Pharmacological blockade of classical and novel PKC isoforms using calphostin C abolished both phorbol ester- and hepatocyte growth factor-induced antigen macropinocytosis in DCs. The qRT-PCR experiments identified PKCδ as the dominant PKC isoform in DCs. Genetic studies demonstrated the functional role of PKCδ in DC macropinocytosis of antigens, their subsequent maturation, and secretion of various T-cell stimulatory cytokines, including IL-1α, TNF-α and IFN-β. Additional mechanistic studies identified NADPH oxidase 2 (Nox2) and intracellular superoxide anion as important players in DC macropinocytosis of antigens downstream of PKCδ activation.

Conclusion

The findings of the present study demonstrate a novel mechanism by which PKCδ activation via stimulation of Nox2 activity and downstream redox signaling promotes DC macropinocytosis of antigens. PKCδ/Nox2-mediated antigen macropinocytosis stimulates maturation of DCs and secretion of T-cell stimulatory cytokines. These findings may contribute to a better understanding of the regulatory mechanisms in DC macropinocytosis and downstream regulation of T-cell-mediated responses.

Cell Signaling Pathways That Regulate Antigen Presentation

Cell signaling pathways regulate much in the life of a cell: from shuttling cargo through intracellular compartments and onto the cell surface, how it should respond to stress, protecting itself from harm (environmental insults or infections), to ultimately, death by apoptosis. These signaling pathways are important for various aspects of the immune response as well. However, not much is known in terms of the participation of cell signaling pathways in Ag presentation, a necessary first step in the activation of innate and adaptive T cells. In this brief review, I discuss the known signaling molecules (and pathways) that regulate how Ags are presented to T cells and the mechanism(s), if identified. Studies in this area have important implications in vaccine development and new treatment paradigms against infectious diseases, autoimmunity, and cancer.

The helminth product, ES-62 modulates dendritic cell responses by inducing the selective autophagolysosomal degradation of TLR-transducers, as exemplified by PKCδ

We have previously shown that ES-62, a phosphorylcholine (PC)-containing glycoprotein secreted by the parasitic filarial nematode Acanthocheilonema viteae targets dendritic cell (DC) responses, specifically by suppressing TLR4 signalling to inhibit Th1/Th17-driven inflammation. We have now investigated the molecular mechanisms underpinning such immunomodulation and show here that ES-62-mediated downregulation of protein kinase C-δ (PKC-δ), a TLR4-associated signalling mediator required for full activation of LPS-driven pro-inflammatory responses, is associated with induction of a low level of autophagic flux, as evidenced by upregulation and trafficking of p62 and LC3 and their consequent autophagolysosomal degradation. By contrast, the classical TLR4 ligand LPS, strongly upregulates p62 and LC3 expression but under such canonical TLR4 signalling this upregulation appears to reflect a block in autophagic flux, with these elements predominantly degraded in a proteasomal manner. These data are consistent with autophagic flux acting to homeostatically suppress proinflammatory DC responses and indeed, blocking of PKC-δ degradation by the autophagolysosomal inhibitors, E64d plus pepstatin A, results in abrogation of the ES-62-mediated suppression of LPS-driven release of IL-6, IL-12p70 and TNF-α by DCs. Thus, by harnessing this homeostatic regulatory mechanism, ES-62 can protect against aberrant inflammation, either to promote parasite survival or serendipitously, exhibit therapeutic potential in inflammatory disease.

Protein kinase C controls lysosome biogenesis independently of mTORC1

Lysosomes respond to environmental cues by controlling their own biogenesis, but the underlying mechanisms are poorly understood. Here we describe a protein kinase C (PKC)-dependent and mTORC1-independent mechanism for regulating lysosome biogenesis, which provides insights into previously reported effects of PKC on lysosomes. By identifying lysosome-inducing compounds we show that PKC couples activation of the TFEB transcription factor with inactivation of the ZKSCAN3 transcriptional repressor through two parallel signalling cascades. Activated PKC inactivates GSK3β, leading to reduced phosphorylation, nuclear translocation and activation of TFEB, while PKC activates JNK and p38 MAPK, which phosphorylate ZKSCAN3, leading to its inactivation by translocation out of the nucleus. PKC activation may therefore mediate lysosomal adaptation to many extracellular cues. PKC activators facilitate clearance of aggregated proteins and lipid droplets in cell models and ameliorate amyloid β plaque formation in APP/PS1 mouse brains. Thus, PKC activators are viable treatment options for lysosome-related disorders.

Fc receptors and their influence on efficacy of therapeutic antibodies for treatment of viral diseases

The lack of vaccines against several important viral diseases necessitates the development of therapeutics to save lives and control epidemics. In recent years, therapeutic antibodies have received considerable attention due to their good safety profiles and clinical success when used against viruses such as respiratory syncytial virus, Ebola virus and Hendra virus. The binding affinity of these antibodies can directly impact their therapeutic efficacy. However, we and others have also demonstrated that the subtype of Fc-gamma receptors (FcγRs) engaged influences the stoichiometric requirement for virus neutralization. Hence, the development of therapeutic antibodies against infectious diseases should consider the FcγRs engaged and Fc-effector functions involved. This review highlights the current state of knowledge about FcγRs and FcγR effector functions involved in virus neutralization, with emphasis on factors that can affect FcγR engagement. A better understanding of Fc-FcγR interactions during virus neutralization will allow development of therapeutic antibodies that are efficacious and can be administered with minimal side effects.

The transcription factor TFEB acts as a molecular switch that regulates exogenous antigen-presentation pathways

Dendritic cells (DCs) can initiate immune responses by presenting exogenous antigens to T cells via both major histocompatibility complex (MHC) class I pathways and MHC class II pathways. Lysosomal activity has an important role in modulating the balance between these two pathways. The transcription factor TFEB regulates lysosomal function by inducing lysosomal activation. Here we report that TFEB expression inhibited the presentation of exogenous antigen by MHC class I while enhancing presentation via MHC class II. TFEB promoted phagosomal acidification and protein degradation. Furthermore, we found that the activation of TFEB was regulated during DC maturation and that phagosomal acidification was impaired in DCs in which the gene encoding TFEB was silenced. Our data indicate that TFEB is a key participant in the differential regulation of the presentation of exogenous antigens by DCs.

The NKG2D ligand ULBP2 is specifically regulated through an invariant chain-dependent endosomal pathway

Soluble ULBP2 is a marker for poor prognosis in several types of cancer. In this study we demonstrate that both soluble and cell surface-bound ULBP2 is transported via a so far unrecognized endosomal pathway. ULBP2 surface expression, but not MICA/B, could specifically be targeted and retained by affecting endosomal/lysosomal integrity and protein kinase C activity. The invariant chain was further essential for endosomal transport of ULBP2. This novel pathway was identified through screening experiments by which methylselenic acid was found to possess notable NKG2D ligand regulatory properties. The protein kinase C inhibitor methylselenic acid induced MICA/B surface expression but dominantly blocked ULBP2 surface transport. Remarkably, by targeting this novel pathway we could specifically block the production of soluble ULBP2 from different, primary melanomas. Our findings strongly suggest that the endosomal transport pathway constitutes a novel therapeutic target for ULBP2-producing tumors.

Regulation of the actin cytoskeleton by Rho kinase controls antigen presentation by CD1d

CD1d molecules are MHC class I-like molecules that present lipid Ags to NKT cells. Although we have previously shown that several different cell signaling molecules can play a role in the control of Ag presentation by CD1d, a defined mechanism by which a cell signaling pathway regulates CD1d function has been unclear. In the current study, we have found that the Rho kinases, Rho-associated, coiled-coil containing protein kinase (ROCK)1 and ROCK2, negatively regulate both human and mouse CD1d-mediated Ag presentation. Inhibition of ROCK pharmacologically, through specific ROCK1 and ROCK2 short hairpin RNA, or by using dendritic cells generated from ROCK1-deficient mice all resulted in enhanced CD1d-mediated Ag presentation compared with controls. ROCK regulates the actin cytoskeleton by phosphorylating LIM kinase, which, in turn, phosphorylates cofilin, prohibiting actin fiber depolymerization. Treatment of APCs with the actin filament depolymerizing agent, cytochalasin D, as well as knockdown of LIM kinase by short hairpin RNA, resulted in enhanced Ag presentation to NKT cells by CD1d, consistent with our ROCK inhibition data. Therefore, our overall results reveal a model whereby CD1d-mediated Ag presentation is negatively regulated by ROCK via its effects on the actin cytoskeleton.

The human biliverdin reductase-based peptide fragments and biliverdin regulate protein kinase Cδ activity: the peptides are inhibitors or substrate for the protein kinase C

PKCδ, a Ser/Thr kinase, promotes cell growth, tumorigenesis, and apoptosis. Human biliverdin reductase (hBVR), a Ser/Thr/Tyr kinase, inhibits apoptosis by reducing biliverdin-IX to antioxidant bilirubin. The enzymes are activated by similar stimuli. Reportedly, hBVR is a kinase-independent activator of PKCδ and is transactivated by the PKC (Gibbs, P. E., Miralem, T., Lerner-Marmarosh, N., Tudor, C., and Maines, M. D. (2012) J. Biol. Chem. 287, 1066-1079). Presently, we examined interactions between the two proteins in the context of regulation of their activities and defining targets of hBVR phosphorylation by PKCδ. LC-MS/MS analysis of PKCδ-activated intact hBVR identified phosphorylated serine positions 21, 33, 230, and 237, corresponding to the hBVR Src homology-2 domain motif (Ser(230) and Ser(237)), flanking the ATP-binding motif (Ser(21)) and in PHPS sequence (Ser(33)) as targets of PKCδ. Ser(21) and Ser(230) were also phosphorylated in hBVR-based peptides. The Ser(230)-containing peptide was a high affinity substrate for PKCδ in vitro and in cells; the relative affinity was PKCδ > PKCβII > PKCζ. Two overlapping peptides spanning this substrate, KRNRYLSF and SFHFKSGSL, were effective inhibitors of PKCδ kinase activity and PKCδ-supported activation of transcription factors Elk1 and NF-κB. Only SFHFKSGSL, in PKCδ-transfected phorbol 12-myristate 13-acetate-stimulated cells, caused membrane blebbing and cell loss. Biliverdin noncovalently inhibited PKCδ, whereas PKCδ potentiated hBVR reductase activity and accelerated the rate of bilirubin formation. This study, together with previous findings, reveals an unexpected regulatory interplay between PKCδ and hBVR in modulating cell death/survival in response to various activating stimuli. In addition, this study has identified novel substrates for and inhibitors of PKCδ. We suggest that hBVR-based technology may have utility to modulate PKCδ-mediated functions in the cell.

PKCδ regulates IL-12p40/p70 production by macrophages and dendritic cells, driving a type 1 healer phenotype in cutaneous leishmaniasis

The protein kinase C (PKC) family is involved in the regulation of many intracellular signalling pathways. Here, we report that the PKCδ isoform regulates IL-12p40/p70 production in macrophages and DC and that PKCδ deficiency in mice transforms the 129/Sv healer to a non-healer strain during cutaneous leishmaniasis. Leishmania major-infected PKCδ(-/-) 129/Sv mice developed a rapid increase in footpad swelling and parasite burden with disease progression, leading to necrosis and ulceration similar to non-healer BALB/c mice. Moreover, PKCδ(-/-) mice failed to develop delayed-type hypersensitivity responses against Leishmania antigen. PKCδ(-/-) macrophages were fully functional with normal MHC class II surface expression and GM-CSF production, recruitment to the draining lymph node and killing effector functions by NO production. In contrast, macrophages and DC produced significantly reduced IL-12p40 and IL-12p70 compared to the WT cells. Decreased IL-12 production resulted in diminished Th1 differentiation, as determined by a striking reduction in IFN-γ by antigen-specific stimulated CD4(+) T cells isolated from popliteal lymph nodes of L. major-infected PKCδ(-/-) mice, explaining the "non-healer" phenotype. We conclude from these data that PKCδ is a regulator of IL-12p40/p70 production by DC and macrophages, driving the healer phenotype during cutaneous leishmaniasis.

Lysophosphatidylcholine-mediated functional inactivation of syndecan-4 results in decreased adhesion and motility of dendritic cells

Following antigen contact, maturation and migration of DCs into lymphatic tissues are crucial to the developing immune response or maintenance of tolerance. Lysophosphatidylcholine (LysoPC) is generated during apoptosis of cells and acts as a "find-and-eat-me" signal thought to prevent autoimmunity. Moreover, LysoPC can activate PKCδ and initiates a signaling cascade that leads to phosphorylation and inactivation of syndecan-4 (SDC4), a heparansulfate proteoglycan integrin co-receptor. In human monocyte-derived DCs, we recently demonstrated that SDC4 is upregulated during maturation thereby stimulating DC motility. Here, we investigate the effects of LysoPC on DC motility as well as on the involvement of PKCδ phosphorylation-dependent regulation of DC motility by SDC4 and PKCα. Employing a static adhesion assay and videomicroscopy, we show that LysoPC inhibits adhesion of DCs to fibronectin and motility of DCs by decreasing podosome formation. Moreover, DC podosome formation and motility, which both are regulated by SDC4 and subject to control by PKCδ-dependent phosphorylation of SDC4, were inhibited in LysoPC-matured DCs. Thus, these DC are defective in adhesion and migration. Based on our results, we hypothesize that LysoPC released during apoptosis might delay DC migration to lymphoid organs and thus prevent autoimmunity.

Genomics and proteomics of immune modulatory effects of a butanol fraction of echinacea purpurea in human dendritic cells

Background

Echinacea spp. extracts and the derived phytocompounds have been shown to induce specific immune cell activities and are popularly used as food supplements or nutraceuticals for immuno-modulatory functions. Dendritic cells (DCs), the most potent antigen presenting cells, play an important role in both innate and adaptive immunities. In this study, we investigated the specific and differential gene expression in human immature DCs (iDCs) in response to treatment with a butanol fraction containing defined bioactive phytocompounds extracted from stems and leaves of Echinacea purpurea, that we denoted [BF/S+L/Ep].

Results

Affymetrix DNA microarray results showed significant up regulation of specific genes for cytokines (IL-8, IL-1β, and IL-18) and chemokines (CXCL 2, CCL 5, and CCL 2) within 4 h after [BF/S+L/Ep] treatment of iDCs. Bioinformatics analysis of genes expressed in [BF/S+L/Ep]-treated DCs revealed a key-signaling network involving a number of immune-modulatory molecules leading to the activation of a downstream molecule, adenylate cyclase 8. Proteomic analysis showed increased expression of antioxidant and cytoskeletal proteins after treatment with [BF/S+L/Ep] and cichoric acid.

Conclusion

This study provides information on candidate target molecules and molecular signaling mechanisms for future systematic research into the immune-modulatory activities of an important traditional medicinal herb and its derived phytocompounds.

Apoptosis-induced inhibition of CD1d-mediated antigen presentation: different roles for caspases and signal transduction pathways

The stimulation of programmed cell death can either enhance or inhibit antigen presentation by classic major histocompatibility complex molecules. In the current study, we report that the induction of apoptosis by topoisomerase I inhibition or elevation of intracellular ceramide levels substantially impairs CD1d-mediated antigen presentation. In the former case, such a reduction occurred via the regulation of both the p38 mitogen-activated protein kinases and protein kinase C delta signal transduction pathways as well as the caspase cascade, whereas the latter was p38-(but not caspase)-dependent. Confocal microscopic analysis showed an altered intracellular distribution of CD1d following the inhibition topoisomerase I or by an increase in intracellular ceramide levels, that was prevented by p38 and caspase inhibitors. Thus, the induction of apoptosis in antigen presenting cells severely compromises CD1d-mediated antigen presentation by multiple mechanisms.