Regulation of class II MHC expression in APCs: roles of types I, III, and IV class II transactivator

MHC Class II-Expressing Mucosal Mast Cells Promote Intestinal Mast Cell Hyperplasia in a Mouse Model of Food Allergy

BACKGROUND

IgE-mediated food allergy is accompanied by mucosal mast cell (MMC) hyperplasia in the intestinal mucosa. Intestinal MMC numbers correlate with the severity of food allergy symptoms. However, the mechanisms by which MMCs proliferate excessively are poorly understood. Here, we clarify the role of newly identified MHC class II (MHCII)-expressing MMCs in the effector phase of IgE-mediated food allergy.

METHODS

Mice reconstituted with MHCII-deficient or wild-type MMCs were used to generate a mouse mode of IgE-mediated food allergy. We assessed the extent of intestinal MMC hyperplasia and the severity of hypothermia in these mice. In addition, we performed in vitro antigen presentation assay using induced MHCII-expressing MMCs generated from bone marrow cells to evaluate the effect of CD4+ T cell activation on MMC proliferation.

RESULTS

In food-allergic mice, we identified the appearance of MHCII-expressing MMCs in the intestinal mucosa and showed that MMC hyperplasia was suppressed in mice with MHCII-deficient MMCs compared to mice with wild-type MMCs. In vitro assays demonstrated that MHCII-expressing MMCs incorporate food antigens directly and through the high-affinity IgE receptor FcεRI-mediated endocytosis and activate antigen-specific CD4+ T cells from food-allergic mice by antigen presentation. Activated CD4+ T cells secrete IL-4 and large amounts of IL-5, which enhance production of the mast cell growth factor IL-9 by IL-33-activated MMCs. Excess IL-9 causes excessive MMC proliferation, leading to the development of MMC hyperplasia.

CONCLUSION

Antigen presentation to CD4+ T cells by MHCII-expressing MMCs triggers intestinal MMC hyperplasia and exacerbates IgE-mediated food allergy.

ZBTB48 is a priming factor regulating B-cell-specific CIITA expression

The class-II transactivator (CIITA) is the master regulator of MHC class-II gene expression and hence the adaptive immune response. Three cell type-specific promoters (pI, pIII, and pIV) are involved in the regulation of CIITA expression, which can be induced by IFN-γ in non-immune cells. While key regulatory elements have been identified within these promoters, our understanding of the transcription factors regulating CIITA expression is incomplete. Here, we demonstrate that the telomere-binding protein and transcriptional activator ZBTB48 directly binds to both critical activating elements within the B-cell-specific promoter CIITA pIII. ZBTB48 knockout impedes the CIITA/MHC-II expression program induced in non-APC cells by IFN-γ, and loss of ZBTB48 in mice silences MHC-II expression in pro-B and immature B cells. Transcriptional regulation of CIITA by ZBTB48 is enabled by ZBTB48-dependent chromatin opening at CIITA pIII upstream of activating H3K4me3 marks. We conclude that ZBTB48 primes CIITA pIII by acting as a molecular on-off-switch for B-cell-specific CIITA expression.

CCL21 Induces Plasmacytoid Dendritic Cell Migration and Activation in a Mouse Model of Glioblastoma

Dendritic cells (DCs) are professional antigen-presenting cells that are traditionally divided into two distinct subsets: myeloid DCs (mDCs) and plasmacytoid DCs (pDCs). pDCs are known for their ability to secrete large amounts of cytokine type I interferons (IFN- α). In our previous work, we have demonstrated that pDC infiltration promotes glioblastoma (GBM) tumor immunosuppression through decreased IFN-α secretion via TLR-9 signaling and increased suppressive function of regulatory T cells (Tregs) via increased IL-10 secretion, resulting in poor overall outcomes in mouse models of GBM. Further dissecting the overall mechanism of pDC-mediated GBM immunosuppression, in this study, we identified CCL21 as highly upregulated by multiple GBM cell lines, which recruit pDCs to tumor sites via CCL21-CCR7 signaling. Furthermore, pDCs are activated by CCL21 in the GBM microenvironment through intracellular signaling of β-arrestin and CIITA. Finally, we found that CCL21-treated pDCs directly suppress CD8+ T cell proliferation without affecting regulatory T cells (Tregs) differentiation, which is considered the canonical pathway of immunotolerant regulation. Taken together, our results show that pDCs play a multifaced role in GBM immunosuppression, and CCL21 could be a novel therapeutic target in GBM to overcome pDC-mediated immunosuppression.

IFNh and IRF9 influence the transcription of MHCII mediated by IFNγ to maintain immune balance in sea perch lateolabrax japonicus

The major histocompatibility complex class II (MHCII) molecules are crucial elements of the adaptive immune system, essential for orchestrating immune responses against foreign pathogens. However, excessive expression of MHCII can disrupt normal physiological functions. Therefore, the host employs various mechanisms to regulate MHCII expression and maintain immune homeostasis. Despite this importance, limited studies have explored the negative regulation of MHCII transcription in bony fish. In this study, we found that interferon h (IFNh), a subtype of type I IFN in sea perch Lateolabrax japonicus, could inhibit the activation of IFNγ induced-MHCII expression by modulating the transcription of the class II major histocompatibility complex transactivator (CIITA). Transcriptome analysis revealed 57 up-regulated and 69 down-regulated genes in cells treated with both IFNγ and IFNh compared to those treated with IFNγ alone. To maintain cellular homeostasis, interferon regulatory factor 9 (IRF9) was up-regulated following IFNγ stimulation, thereby preventing MHCII overexpression. Mechanistically, IRF9 bound to the CIITA promoter and suppressed its expression activated by IRF1. Furthermore, IRF9 inhibited the promoter activity of both MHCII-α and MHCII-β induced by CIITA. Our findings highlight the roles of IFNh and IRF9 as suppressors regulating MHCII expression at different hierarchical levels. This study provides insights into the intricate regulation of antigen presentation and the foundation for further exploration of the interaction mechanisms between aquatic virus and fish.

Novel role of MHC class II transactivator in hepatitis B virus replication and viral counteraction

BACKGROUND/AIMS

The major histocompatibility class II (MHC II) transactivator, known as CIITA, is induced by Interferon gamma (IFN-γ) and plays a well-established role in regulating the expression of class II MHC molecules in antigen-presenting cells.

METHODS

Primary human hepatocytes (PHH) were isolated via therapeutic hepatectomy from two donors. The hepatocellular carcinoma (HCC) cell lines HepG2 and Huh7 were used for the mechanistic study, and HBV infection was performed in HepG2-NTCP cells. HBV DNA replication intermediates and secreted antigen levels were measured using Southern blotting and ELISA, respectively.

RESULTS

We identified a non-canonical function of CIITA in the inhibition of hepatitis B virus (HBV) replication in both HCC cells and patient-derived PHH. Notably, in vivo experiments demonstrated that HBV DNA and secreted antigen levels were significantly decreased in mice injected with the CIITA construct. Mechanistically, CIITA inhibited HBV transcription and replication by suppressing the activity of HBV-specific enhancers/promoters. Indeed, CIITA exerts antiviral activity in hepatocytes through ERK1/2-mediated down-regulation of the expression of hepatocyte nuclear factor 1α (HNF1α) and HNF4α, which are essential factors for virus replication. In addition, silencing of CIITA significantly abolished the IFN-γ-mediated anti-HBV activity, suggesting that CIITA mediates the anti-HBV activity of IFN-γ to some extent. HBV X protein (HBx) counteracts the antiviral activity of CIITA via direct binding and impairing its function.

CONCLUSION

Our findings reveal a novel antiviral mechanism of CIITA that involves the modulation of the ERK pathway to restrict HBV transcription. Additionally, our results suggest the possibility of a new immune avoidance mechanism involving HBx.

Human erythroid progenitors express antigen presentation machinery

Early-life immune exposures can profoundly impact lifelong health. However, functional mechanisms underlying fetal immune development remain incomplete. Erythrocytes are not typically considered active immune mediators, primarily because erythroid precursors discard their organelles as they mature, thus losing the ability to alter gene expression in response to stimuli. Erythroid progenitors and precursors circulate in human fetuses and neonates. Although there is limited evidence that erythroid precursors are immunomodulatory, our understanding of the underlying mechanisms remains inadequate. To define the immunobiological role of fetal and perinatal erythroid progenitors and precursors, we analyzed single cell RNA-sequencing data and found that transcriptomics support erythroid progenitors as putative immune mediators. Unexpectedly, we discovered that human erythroid progenitors constitutively express Major Histocompatibility Complex (MHC) class II antigen processing and presentation machinery, which are hallmarks of specialized antigen presenting immune cells. Furthermore, we demonstrate that erythroid progenitors internalize and cleave foreign proteins into peptide antigens. Unlike conventional antigen presenting cells, erythroid progenitors express atypical costimulatory molecules and immunoregulatory cytokines that direct the development of regulatory T cells, which are critical for establishing maternal-fetal tolerance. Expression of MHC II in definitive erythroid progenitors begins during the second trimester, coinciding with the appearance of mature T cells in the fetus, and is absent in primitive progenitors. Lastly, we demonstrate physical and molecular interaction potential of erythroid progenitors and T cells in the fetal liver. Our findings shed light on a unique orchestrator of fetal immunity and provide insight into the mechanisms by which erythroid cells contribute to host defense.

The NLR family of innate immune and cell death sensors

Nucleotide-binding oligomerization domain (NOD)-like receptors, also known as nucleotide-binding leucine-rich repeat receptors (NLRs), are a family of cytosolic pattern recognition receptors that detect a wide variety of pathogenic and sterile triggers. Activation of specific NLRs initiates pro- or anti-inflammatory signaling cascades and the formation of inflammasomes-multi-protein complexes that induce caspase-1 activation to drive inflammatory cytokine maturation and lytic cell death, pyroptosis. Certain NLRs and inflammasomes act as integral components of larger cell death complexes-PANoptosomes-driving another form of lytic cell death, PANoptosis. Here, we review the current understanding of the evolution, structure, and function of NLRs in health and disease. We discuss the concept of NLR networks and their roles in driving cell death and immunity. An improved mechanistic understanding of NLRs may provide therapeutic strategies applicable across infectious and inflammatory diseases and in cancer.

Mechanisms of Innate Immune Sensing of HTLV-1 and Viral Immune Evasion

Human T lymphotropic virus-1 (HTLV-1) was the first identified oncoretrovirus, which infects and establishes a persistent infection in approximately 10-20 million people worldwide. Although only ~5% of infected individuals develop pathologies such as adult T-cell leukemia/lymphoma (ATLL) or a neuroinflammatory disorder termed HTLV-1-asssociated myelopathy/tropical spastic paraparesis (HAM/TSP), asymptomatic carriers are more susceptible to opportunistic infections. Furthermore, ATLL patients are severely immunosuppressed and prone to other malignancies and other infections. The HTLV-1 replication cycle provides ligands, mainly nucleic acids (RNA, RNA/DNA intermediates, ssDNA intermediates, and dsDNA), that are sensed by different pattern recognition receptors (PRRs) to trigger immune responses. However, the mechanisms of innate immune detection and immune responses to HTLV-1 infection are not well understood. In this review, we highlight the functional roles of different immune sensors in recognizing HTLV-1 infection in multiple cell types and the antiviral roles of host restriction factors in limiting persistent infection of HTLV-1. We also provide a comprehensive overview of intricate strategies employed by HTLV-1 to subvert the host innate immune response that may contribute to the development of HTLV-1-associated diseases. A more detailed understanding of HTLV-1-host pathogen interactions may inform novel strategies for HTLV-1 antivirals, vaccines, and treatments for ATLL or HAM/TSP.

Is the HTLV-1 Retrovirus Targeted by Host Restriction Factors?

Human T cell leukemia virus type 1 (HTLV-1), the etiological agent of adult T cell leukemia/lymphoma (ATLL) and of HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP), was identified a few years before Human Immunodeficiency Virus (HIV). However, forty years later, our comprehension of HTLV-1 immune detection and the host immune responses to HTLV-1 is far more limited than for HIV. In addition to innate and adaptive immune responses that rely on specialized cells of the immune system, host cells may also express a range of antiviral factors that inhibit viral replication at different stages of the cycle, in a cell-autonomous manner. Multiple antiviral factors allowing such an intrinsic immunity have been primarily and extensively described in the context HIV infection. Here, we provide an overview of whether known HIV restriction factors might act on HTLV-1 replication. Interestingly, many of them do not exert any antiviral activity against HTLV-1, and we discuss viral replication cycle specificities that could account for these differences. Finally, we highlight future research directions that could help to identify antiviral factors specific to HTLV-1.

Chromatin-Remodeling Factor BRG1 Is a Negative Modulator of L. donovani in IFNγ Stimulated and Infected THP-1 Cells

Intracellular pathogens manipulate the host cell for their own survival by contributing to modifications of host epigenome, and thus, altering expression of genes involved in the pathogenesis. Both ATP-dependent chromatin remodeling complex and histone modifications has been shown to be involved in the activation of IFNγ responsive genes. Leishmania donovani is an intracellular pathogen that causes visceral leishmaniasis. The strategies employed by Leishmania donovani to modulate the host epigenome in order to overcome the host defense for their persistence has been worked out in this study. We show that L. donovani negatively affects BRG1, a catalytic subunit of mammalian SWI/SNF chromatin remodeling complex, to alter IFNγ induced host responses. We observed that L. donovani infection downregulates BRG1 expression both at transcript and protein levels in cells stimulated with IFNγ. We also observed a significant decrease in IFNγ responsive gene, Class II transactivator (CIITA), as well as its downstream genes, MHC-II (HLA-DR and HLA-DM). Also, the occupancy of BRG1 at CIITA promoters I and IV was disrupted. A reversal in CIITA expression and decreased parasite load was observed with BRG1 overexpression, thus, suggesting BRG1 is a potential negative regulator for the survival of intracellular parasites in an early phase of infection. We also observed a decrease in H3 acetylation at the promoters of CIITA, post parasite infection. Silencing of HDAC1, resulted in increased CIITA expression, and further decreased parasite load. Taken together, we suggest that intracellular parasites in an early phase of infection negatively regulates BRG1 by using host HDAC1 for its survival inside the host.

Tofacitinib enhances interferon-γ-induced expression of major histocompatibility complex class II in macrophages

Tofacitinib is the first selective Janus kinase (JAK) 1/3 inhibitor approved for the treatment of rheumatoid arthritis and has been demonstrated to exhibit its efficacy through suppression of lymphocyte activation. Although macrophages are critically involved in the pathogenesis of rheumatoid arthritis, little is known about the influence of tofacitinib on macrophage activation especially expression of major histocompatibility complex class II (MHC II) and co-stimulatory molecule CD86. In the present study, we examined the effect of tofacitinib on the expression of MHC II and CD86 in RAW264.7 murine macrophages. Interferon (IFN)-γ induces the cell surface expression of MHC II and CD86. The treatment of tofacitinib at 0.5 μM significantly upregulated IFN-γ-induced expression of MHC II, while decreased the expression of CD86. Hence the population of CD86- MHC II+ cells that induced by tofacitinib at 0.5 μM in the presence of IFN-γ were approximately three times larger than that of IFN-γ alone. Consistent with the surface expression, tofacitinib enhanced IFN-γ-induced mRNA expression of MHC II, and contrarily, decreased that of CD86. Similarly, tofacitinib increased the mRNA expression of MHC II transactivator (CIITA), especially CIITA type I, which is a key regulator of MHC II gene transcription. These findings suggested that tofacitinib enhanced IFNγ-induced MHC II expression by transcriptional regulation through induction of CIITA in macrophages and raise the possibility that a novel action of tofacitinib.

Genetic and commensal induction of IL-18 drive intestinal epithelial MHCII via IFNγ

Major histocompatibility complex class II (MHCII) is dynamically expressed on intestinal epithelial cells (IECs) throughout the intestine, but its regulation remains poorly understood. We observed that spontaneous upregulation of IEC MHCII in locally bred Rag1^-/- mice correlated with serum Interleukin (IL)-18, was transferrable via co-housing to commercially bred immunodeficient mice and could be inhibited by both IL-12 and IL-18 blockade. Overproduction of intestinal IL-18 due to an activating Nlrc4 mutation upregulated IEC MHCII via classical inflammasome machinery independently of immunodeficiency or dysbiosis. Immunodeficient dysbiosis increased Il-18 transcription, which synergized with NLRC4 inflammasome activity to drive elevations in serum IL-18. This IL-18-MHCII axis was confirmed in several other models of intestinal and systemic inflammation. Elevated IL-18 reliably preceded MHCII upregulation, suggesting an indirect effect on IECs, and mice with IL-18 overproduction showed activation or expansion of type 1 lymphocytes. Interferon gamma (IFNg) was uniquely able to upregulate IEC MHCII in enteroid cultures and was required for MHCII upregulation in several in vivo systems. Thus, we have linked intestinal dysbiosis, systemic inflammation, and inflammasome activity to IEC MHCII upregulation via an intestinal IL-18-IFNg axis. Understanding this process may be crucial for determining the contribution of IEC MHCII to intestinal homeostasis, host defense, and tolerance.

What's in the BAGs? Intrinsic disorder angle of the multifunctionality of the members of a family of chaperone regulators

In humans, the family of Bcl-2 associated athanogene (BAG) proteins includes six members characterized by exceptional multifunctionality and engagement in the pathogenesis of various diseases. All of them are capable of interacting with a multitude of often unrelated binding partners. Such binding promiscuity and related functional and pathological multifacetedness cannot be explained or understood within the frames of the classical "one protein-one structure-one function" model, which also fails to explain the presence of multiple isoforms generated for BAG proteins by alternative splicing or alternative translation initiation and their extensive posttranslational modifications. However, all these mysteries can be solved by taking into account the intrinsic disorder phenomenon. In fact, high binding promiscuity and potential to participate in a broad spectrum of interactions with multiple binding partners, as well as a capability to be multifunctional and multipathogenic, are some of the characteristic features of intrinsically disordered proteins and intrinsically disordered protein regions. Such functional proteins or protein regions lacking unique tertiary structures constitute a cornerstone of the protein structure-function continuum concept. The aim of this paper is to provide an overview of the functional roles of human BAG proteins from the perspective of protein intrinsic disorder which will provide a means for understanding their binding promiscuity, multifunctionality, and relation to the pathogenesis of various diseases.

Spatial Organization and Recruitment of Non-Specific T Cells May Limit T Cell-Macrophage Interactions Within Mycobacterium tuberculosis Granulomas

Tuberculosis (TB) is a worldwide health problem; successful interventions such as vaccines and treatment require a 2better understanding of the immune response to infection with Mycobacterium tuberculosis (Mtb). In many infectious diseases, pathogen-specific T cells that are recruited to infection sites are highly responsive and clear infection. Yet in the case of infection with Mtb, most individuals are unable to clear infection leading to either an asymptomatically controlled latent infection (the majority) or active disease (roughly 5%-10% of infections). The hallmark of Mtb infection is the recruitment of immune cells to lungs leading to development of multiple lung granulomas. Non-human primate models of TB indicate that on average <10% of T cells within granulomas are Mtb-responsive in terms of cytokine production. The reason for this reduced responsiveness is unknown and it may be at the core of why humans typically are unable to clear Mtb infection. There are a number of hypotheses as to why this reduced responsiveness may occur, including T cell exhaustion, direct downregulation of antigen presentation by Mtb within infected macrophages, the spatial organization of the granuloma itself, and/or recruitment of non-Mtb-specific T cells to lungs. We use a systems biology approach pairing data and modeling to dissect three of these hypotheses. We find that the structural organization of granulomas as well as recruitment of non-specific T cells likely contribute to reduced responsiveness.

Interferon Regulatory Factors 1 and 2 Play Different Roles in MHC II Expression Mediated by CIITA in Grass Carp, Ctenopharyngodon idella

Expression of major histocompatibility complex class II (MHC II) molecules, which determines both the immune repertoire during development and subsequent triggering of immune responses, is always under the control of a unique (MHC class II) transactivator, CIITA. The IFN-γ-inducible MHC II expression has been extensively and thoroughly studied in humans, but not in bony fish. In this study, the characterization of CIITA was identified and its functional domains were analyzed in grass carp. The absence of GAS and E-box in the promoter region of grass carp CIITA, might imply that the cooperative interaction between STAT1 and USF1 to active the CIITA expression, found in mammals, is not present in bony fish. After the transfection of IFN-γ or IFN-γ rel, only IFN-γ could induce MHC II expression mediated by CIITA. Moreover, interferon regulatory factor (IRF) 2, which cooperates with IRF1 to active the CIITA promoter IV expression in mammals, played an antagonistic role to IRF1 in the activation of grass carp CIITA. These data suggested that grass carp, compared with mammals, has both conservative and unique mechanisms in the regulation of MHC II expression.

In Silico Validation of D7 Salivary Protein-derived B- and T-cell Epitopes of Aedes aegypti as Potential Vaccine to Prevent Transmission of Flaviviruses and Togaviruses to Humans

Mosquito (Aedes aegyptii) salivary proteins play a crucial role in facilitating viral transmission from vector-to-host due to their role in facilitating the "blood meal" of the vector. Three main proteins, D7, aegyptin and Sialokinin play a role in this process. Using in-silico programs, we identified B- and T-cell epitopes in the mosquito salivary proteins D7 long and short form. T-cell epitopes with high affinity to the most prevalent HLA MHC class-I supertypes among different population groups was chosen. It is our postulate that these epitopes could be successful in eliciting B and T cell responses, which would decrease the vector blood meal efficiency and hence protect against host infection by certain viruses. These include causative agents like Dengue viruses, Chikungunya virus, Zika and Yellow fever viruses. These viruses are of major public health importance in several countries in the Americas, Asia and Africa. Experimental evidence exists in previously published literature showing the protective effect of antibodies to certain salivary proteins in susceptible hosts. A novel approach of immunizing humans against the vector proteins to reduce transmission of viruses is now under investigation in several laboratories. We have identified the following two B cell epitopes LAALHVTAAPLWDAKDPEQF one from D7L and the other TSEYPDRQNQIEELNKLCKN from D7S. Likewise, two T cell epitopes MTSKNELDV one from D7L and the other YILCKASAF from D7S with affinity to the predominant MHC class-I supertypes were identified towards evaluation as potential vaccine.

The CCR4-NOT complex contributes to repression of Major Histocompatibility Complex class II transcription

The multi-subunit CCR4 (carbon catabolite repressor 4)-NOT (Negative on TATA) complex serves as a central coordinator of all different steps of eukaryotic gene expression. Here we performed a systematic and comparative analysis of cells where the CCR4-NOT subunits CNOT1, CNOT2 or CNOT3 were individually downregulated using doxycycline-inducible shRNAs. Microarray experiments showed that downregulation of either CNOT subunit resulted in elevated expression of major histocompatibility complex class II (MHC II) genes which are found in a gene cluster on chromosome 6. Increased expression of MHC II genes after knock-down or knock-out of either CNOT subunit was seen in a variety of cell systems and also in naïve macrophages from CNOT3 conditional knock-out mice. CNOT2-mediated repression of MHC II genes occurred also in the absence of the master regulator class II transactivator (CIITA) and did not cause detectable changes of the chromatin structure at the chromosomal MHC II locus. CNOT2 downregulation resulted in an increased de novo transcription of mRNAs whereas tethering of CNOT2 to a regulatory region governing MHC II expression resulted in diminished transcription. These results expand the known repertoire of CCR4-NOT members for immune regulation and identify CNOT proteins as a novel group of corepressors restricting class II expression.

Role of PU.1 in MHC Class II Expression via CIITA Transcription in Plasmacytoid Dendritic Cells

The cofactor CIITA is a master regulator of MHC class II expression and several transcription factors regulating the cell type-specific expression of CIITA have been identified. Although the MHC class II expression in plasmacytoid dendritic cells (pDCs) is also mediated by CIITA, the transcription factors involved in the CIITA expression in pDCs are largely unknown. In the present study, we analyzed the role of a hematopoietic lineage-specific transcription factor, PU.1, in CIITA transcription in pDCs. The introduction of PU.1 siRNA into mouse pDCs and a human pDC cell line, CAL-1, reduced the mRNA levels of MHC class II and CIITA. When the binding of PU.1 to the 3rd promoter of CIITA (pIII) in CAL-1 and mouse pDCs was analyzed by a chromatin immunoprecipitation assay, a significant amount of PU.1 binding to the pIII was detected, which was definitely decreased in PU.1 siRNA-transfected cells. Reporter assays showed that PU.1 knockdown reduced the pIII promoter activity and that three Ets-motifs in the human pIII promoter were candidates of cis-enhancing elements. By electrophoretic mobility shift assays, it was confirmed that two Ets-motifs, GGAA (-181/-178) and AGAA (-114/-111), among three candidates, were directly bound with PU.1. When mouse pDCs and CAL-1 cells were stimulated by GM-CSF, mRNA levels of PU.1, pIII-driven CIITA, total CIITA, MHC class II, and the amount of PU.1 binding to pIII were significantly increased. The GM-CSF-mediated up-regulation of these mRNAs was canceled in PU.1 siRNA-introduced cells. Taking these results together, we conclude that PU.1 transactivates the pIII through direct binding to Ets-motifs in the promoter in pDCs.

Dendritic cell-derived exosomes as maintenance immunotherapy after first line chemotherapy in NSCLC

Dendritic cell-derived exosomes (Dex) are small extracellular vesicles secreted by viable dendritic cells. In the two phase-I trials that we conducted using the first generation of Dex (IFN-γ-free) in end-stage cancer, we reported that Dex exerted natural killer (NK) cell effector functions in patients. A second generation of Dex (IFN-γ-Dex) was manufactured with the aim of boosting NK and T cell immune responses. We carried out a phase II clinical trial testing the clinical benefit of IFN-γ-Dex loaded with MHC class I- and class II-restricted cancer antigens as maintenance immunotherapy after induction chemotherapy in patients bearing inoperable non-small cell lung cancer (NSCLC) without tumor progression. The primary endpoint was to observe at least 50% of patients with progression-free survival (PFS) at 4 mo after chemotherapy cessation. Twenty-two patients received IFN-γ-Dex. One patient exhibited a grade three hepatotoxicity. The median time to progression was 2.2 mo and median overall survival (OS) was 15 mo. Seven patients (32%) experienced stabilization of >4 mo. The primary endpoint was not reached. An increase in NKp30-dependent NK cell functions were evidenced in a fraction of these NSCLC patients presenting with defective NKp30 expression. Importantly, MHC class II expression levels of the final IFN-γ-Dex product correlated with expression levels of the NKp30 ligand BAG6 on Dex, and with NKp30-dependent NK functions, the latter being associated with longer progression-free survival. This phase II trial confirmed the capacity of Dex to boost the NK cell arm of antitumor immunity in patients with advanced NSCLC.

IL-33 promotes MHC class II expression in murine mast cells

Mast cells (MCs), recognized as tissue-resident cells of hematopoietic origin, are involved in cellular and pathological manifestations of allergic disorders including atopic dermatitis. IL-33, a member of the IL-1 cytokine family, activates Th2-type immune responses, and promotes the degranulation and maturation of MCs. However, it is uncertain whether IL-33 treatment induces mature mast cells to acquire the characteristics of the monocyte-dendritic cell lineage.We investigated the effect of IL-33 on the MHC class II expression and function of murine mast cells. IL-33-treated mature murine bone marrow-derived mast cells (BMMCs) were analyzed by FACS, real-time PCR, chromatin immunoprecipitation (ChIP) assay, and Western blotting. The morphology and degranulation activity of BMMCs and T-cell activation by BMMCs were also examined. BMMCs treated with IL-33 for 10 days induced cell surface expression of the MHC class II protein, whereas the expression of FcεRI and c-kit was not affected by IL-33. The expression of CIITA, driven from pIII and pIV, was up-regulated in IL-33-treated BMMCs. The amount of PU.1 mRNA and protein significantly increased in IL-33-treated BMMCs. The ChIP assay showed PU.1 binding to CIITA pIII, and enhanced histone acetylation due to IL-33 treatment. Syngeneic T cells were activated by co-culture with IL-33-treated BMMCs, although the expression of the co-stimulatory molecules, CD40, CD80, CD86, and PDL-1, was not detected. Mast cells express MHC class II after prolonged exposure to IL-33, probably due to enhanced recruitment of PU.1 to CIITA pIII, resulting in transactivation of CIITA and MHC class II. IL-33 is an important cytokine in allergic disorders. Mast cells have the ability to express MHC class II after prolonged exposure to IL-33 in a murine model. IL-33 holds a key to understanding the etiology of atopic dermatitis.

Collagen induces maturation of human monocyte-derived dendritic cells by signaling through osteoclast-associated receptor

Osteoclast-associated receptor (OSCAR) is widely expressed on human myeloid cells. Collagen types (Col)I, II, and III have been described as OSCAR ligands, and ColII peptides can induce costimulatory signaling in receptor activator for NF-κB-dependent osteoclastogenesis. In this study, we isolated collagen as an OSCAR-interacting protein from the membranes of murine osteoblasts. We have investigated a functional outcome of the OSCAR-collagen interaction in human monocyte-derived dendritic cells (DCs). OSCAR engagement by ColI/II-induced activation/maturation of DCs is characterized by upregulation of cell surface markers and secretion of cytokines. These collagen-matured DCs (Col-DCs) were efficient drivers of allogeneic and autologous naive T cell proliferation. The T cells expanded by Col-DCs secreted cytokines with no clear T cell polarization pattern. Global RNA profiling revealed that multiple proinflammatory mediators, including cytokines and cytokine receptors, components of the stable immune synapse (namely CD40, CD86, CD80, and ICAM-1), as well as components of TNF and TLR signaling, are transcriptional targets of OSCAR in DCs. Our findings indicate the existence of a novel pathway by which extracellular matrix proteins locally drive maturation of DCs during inflammatory conditions, for example, within synovial tissue of rheumatoid arthritis patients, where collagens become exposed during tissue remodeling and are thus accessible for interaction with infiltrating precursors of DCs.

Bag6 complex contains a minimal tail-anchor-targeting module and a mock BAG domain

BCL2-associated athanogene cochaperone 6 (Bag6) plays a central role in cellular homeostasis in a diverse array of processes and is part of the heterotrimeric Bag6 complex, which also includes ubiquitin-like 4A (Ubl4A) and transmembrane domain recognition complex 35 (TRC35). This complex recently has been shown to be important in the TRC pathway, the mislocalized protein degradation pathway, and the endoplasmic reticulum-associated degradation pathway. Here we define the architecture of the Bag6 complex, demonstrating that both TRC35 and Ubl4A have distinct C-terminal binding sites on Bag6 defining a minimal Bag6 complex. A crystal structure of the Bag6-Ubl4A dimer demonstrates that Bag6-BAG is not a canonical BAG domain, and this finding is substantiated biochemically. Remarkably, the minimal Bag6 complex defined here facilitates tail-anchored substrate transfer from small glutamine-rich tetratricopeptide repeat-containing protein α to TRC40. These findings provide structural insight into the complex network of proteins coordinated by Bag6.

Hrd1-mediated BLIMP-1 ubiquitination promotes dendritic cell MHCII expression for CD4 T cell priming during inflammation

The ubiquitin pathway plays critical roles in antigen presentation. However, the ubiquitin ligases that regulate MHC gene transcription remain unidentified. We showed that the ubiquitin ligase Hrd1, expression of which is induced by Toll-like receptor (TLR) stimulation, is required for MHC-II but not MHC-I transcription in dendritic cells (DCs). Targeted Hrd1 gene deletion in DCs diminished MHC-II expression. As a consequence, Hrd1-null DCs failed to prime CD4(+) T cells without affecting the activation of CD8(+) T cells. Hrd1 catalyzed ubiquitination and degradation of the transcriptional suppressor B lymphocyte-induced maturation protein 1 (BLIMP1) to promote MHC-II expression. Genetic suppression of Hrd1 function in DCs protected mice from myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE). We identified Hrd1-mediated BLIMP1 ubiquitination as a previously unknown mechanism in programming DC for CD4(+) T cell activation during inflammation.

Common distal elements orchestrate CIITA isoform-specific expression in multiple cell types

Major histocompatibility class II (MHC-II) expression is critical for immune responses and is controlled by the MHC-II transactivator CIITA. CIITA is primarily regulated at the transcriptional level and is expressed from three main promoters with myeloid, lymphoid, and IFN-γ treated non-hematopoietic cells using promoters pI, pIII, and pIV, respectively. Recent studies in non-hematopoietic cells suggest a series of distal regulatory elements may be involved in regulating CIITA transcription. To identify distal elements in B cells, a DNase I-hypersensitivity screen was performed, revealing a series of potential novel regulatory elements. These elements were analyzed computationally and biochemically. Several regions displayed active histone modifications and/or enhanced expression of a reporter gene. Four of the elements interacted with pIII in B cells. These same four regions were also found to interact with pI in splenic dendritic cells (spDC). Intriguingly, examination of the above interactions in pI-knockout-derived spDC showed a switch to the next available promoter, pIII. Extensive DNA methylation was found at the pI region in B cells, suggesting that this promoter is not accessible in B cells. Thus, CIITA expression is likely mediated in hematopoietic cells by common elements with promoter accessibility playing a part in promoter choice.

BAG-6, a jack of all trades in health and disease

BCL2-associated athanogene 6 (BAG-6) (also Bat-3/Scythe) was discovered as a gene product of the major histocompatibility complex class III locus. The Xenopus ortholog Scythe was first identified to act as an anti-apoptotic protein. Subsequent studies unraveled that the large BAG-6 protein contributes to a number of cellular processes, including apoptosis, gene regulation, protein synthesis, protein quality control, and protein degradation. In this context, BAG-6 acts as a multifunctional chaperone, which interacts with its target proteins for shuttling to distinct destinations. Nonetheless, as anticipated from its genomic localization, BAG-6 is involved in a variety of immunological pathways such as macrophage function and TH1 response. Most recently, BAG-6 was identified on the plasma membrane of dendritic cells and malignantly transformed cells where it serves as cellular ligand for the activating natural killer (NK) cell receptor NKp30 triggering NK cell cytotoxicity. Moreover, target cells were found to secrete soluble variants of BAG-6 and release BAG-6 on the surface of exosomes, which inhibit or activate NK cell cytotoxicity, respectively. These data suggest that the BAG-6 antigen is an important target to shape a directed immune response or to overcome tumor-immune escape strategies established by soluble BAG-6. This review summarizes the currently known functions of BAG-6, a fascinating multicompetent protein, in health and disease.

Immune subversion by Mycobacterium tuberculosis through CCR5 mediated signaling: involvement of IL-10

Tuberculosis is characterized by severe immunosuppression of the host macrophages, resulting in the loss of the host protective immune responses. During Mycobacterium tuberculosis infection, the pathogen modulates C-C Chemokine Receptor 5 (CCR5) to enhance IL-10 production, indicating the possible involvement of CCR5 in regulation of the host immune response. Here, we found that Mycobacterium infection significantly increased CCR5 expression in macrophages there by facilitating the activation of its downstream signaling. These events culminated in up-regulation of the immunosuppressive cytokine IL-10 production, which was further associated with the down-regulation of macrophage MHC-II expression along with the up-regulation of CCR5 expression via engagement of STAT-3 in a positive feedback loop. Treatment of macrophages with CCR5 specific siRNA abrogated the IL-10 production and restored MHCII expression. While, in vivo CCR5 silencing was also effective for the restoration of host immune responses against tuberculosis. This study demonstrated that CCR5 played a very critical role for the immune subversion mechanism employed by the pathogen.

Characterization of the interaction between astrocytes and encephalitogenic lymphocytes during the development of experimental autoimmune encephalitomyelitis (EAE) in mice

The nature of pathogenic mechanisms associated with the development of multiple sclerosis (MS) have long been debated. However, limited research was conducted to define the interplay between infiltrating lymphocytes and resident cells of the central nervous system (CNS). Data presented in this report describe a novel role for astrocyte-mediated alterations to myelin oligodendrocyte glycoprotein (MOG)(35-55) -specific lymphocyte responses, elicited during the development of experimental autoimmune encephalitomyelitis (EAE). In-vitro studies demonstrated that astrocytes inhibited the proliferation and interferon (IFN)-γ, interleukin (IL)-4, IL-17 and transforming growth factor (TGF)-β secretion levels of MOG(35-55) -specific lymphocytes, an effect that could be ameliorated by astrocyte IL-27 neutralization. However, when astrocytes were pretreated with IFN-γ, they could promote the proliferation and secretion levels of MOG(35-55) -specific lymphocytes, coinciding with apparent expression of major histocompatibility complex (MHC)-II on astrocytes themselves. Quantitative polymerase chain reaction (qPCR) demonstrated that production of IL-27 in the spinal cord was at its highest during the initial phases. Conversely, production of IFN-γ in the spinal cord was highest during the peak phase. Quantitative analysis of MHC-II expression in the spinal cord showed that there was a positive correlation between MHC-II expression and IFN-γ production. In addition, astrocyte MHC-II expression levels correlated positively with IFN-γ production in the spinal cord. These findings suggested that astrocytes might function as both inhibitors and promoters of EAE. Astrocytes prevented MOG(35-55) -specific lymphocyte function by secreting IL-27 during the initial phases of EAE. Then, in the presence of higher IFN-γ levels in the spinal cord, astrocytes were converted into antigen-presenting cells. This conversion might promote the progression of pathological damage and result in a peak of EAE severity.

Contrasting effects of IFNα on MHC class II expression in professional vs. nonprofessional APCs: Role of CIITA type IV promoter

We previously demonstrated that, in ex vivo cultures, IFNα downregulates the expression of MHC class II (MHCII) genes in human non-professional APCs associated with pancreatic islets. IFNα has an opposing effect on MHCII expression in professional APCs. In this study, we found that the mechanism responsible for the IFNα-mediated MHCII's downregulation in human MHCII-positive non-professional antigen presenting human non-hematopoietic cell lines is the result of the negative feedback system that regulates cytokine signal transduction, which eventually inhibits promoters III and IV of CIITA gene. Because the CIITA-PIV isoform is mostly responsible for the constitutive expression of MHCII genes in non-professional APCs, we pursued and achieved the specific knockdown of CIITA-PIV mRNA in our in vitro system, obtaining a partial silencing of MHCII molecules similar to that obtained by IFNα. We believe that our results offer a new understanding of the potential significance of CIITA-PIV as a therapeutic target for interventional strategies that can manage autoimmune disease and allograft rejection with little interference on the function of professional APCs of the immune system.

Differential HIV-1 endocytosis and susceptibility to virus infection in human macrophages correlate with cell activation status

HIV-1 is an enveloped virus that enters target cells by fusion either directly at the plasma membrane or at the endosomal membrane. The latter mechanism follows a rapid engulfment of HIV-1 after its receptor engagement at the cell surface, and its scale depends on cellular endocytosis/degradation rates and virus fusion kinetics. HIV-1 has recently been shown to exploit a novel Pak1-dependent macropinocytosis mechanism as a way to productively infect macrophages. However, macrophages are highly heterogeneous cells that can adapt functionally to their changing environment, and their endosomal/lysosomal pathway is highly regulated upon cell activation. These changes might impact the ability of HIV-1 to exploit endocytosis as a way to productively infect macrophages. In this study, we compared HIV-1 endocytosis/degradation rates in nonactivated, M1-activated, and M2a-activated monocyte-derived macrophages (MDMs). We found that the rate of HIV-1 endocytosis was increased in M1-activated but decreased in M2a-activated MDMs. However, both M1 and M2a activations of MDMs led specifically to a greater clathrin-mediated endocytosis of HIV-1, which was independent of CD4 and CCR5 binding. Furthermore, clathrin-mediated endocytosis is unlikely to result in productive HIV-1 infection, given that it leads to increased viral degradation. Therefore, we suggest that viral fusion following endocytosis is restricted in activated macrophages.

Dysregulated recruitment of the histone methyltransferase EZH2 to the class II transactivator (CIITA) promoter IV in breast cancer cells

One mechanism frequently utilized by tumor cells to escape immune system recognition and elimination is suppression of cell surface expression of Major Histocompatibility Class II (MHC II) molecules. Expression of MHC II is regulated primarily at the level of transcription by the Class II Transactivator, CIITA, and decreased CIITA expression is observed in multiple tumor types. We investigate here contributions of epigenetic modifications to transcriptional silencing of CIITA in variants of the human breast cancer cell line MDA MB 435. Significant increases in histone H3 lysine 27 trimethylation upon IFN-γ stimulation correlate with reductions in transcription factor recruitment to the interferon-γ inducible CIITA promoter, CIITApIV, and with significantly increased CIITApIV occupancy by the histone methyltransferase enhancer of zeste homolog 2 (EZH2). Most compelling is evidence that decreased expression of EZH2 in MDA MB 435 variants results in significant increases in CIITA and HLA-DRA mRNA expression, even in the absence of interferon-γ stimulation, as well as increased cell surface expression of MHC II. Together, these data add mechanistic insight to prior observations of increased EZH2 expression and decreased CIITA expression in multiple tumor types.

Type I IFN drives a distinctive dendritic cell maturation phenotype that allows continued class II MHC synthesis and antigen processing

Microbial molecules or cytokines can stimulate dendritic cell (DC) maturation, which involves DC migration to lymph nodes and enhanced presentation of Ag to launch T cell responses. Microbial TLR agonists are the most studied inducers of DC maturation, but type I IFN (IFN-I) also promotes DC maturation. In response to TLR stimulation, DC maturation involves a burst of Ag processing with enhanced expression of peptide-class II MHC complexes and costimulator molecules. Subsequently, class II MHC (MHC-II) synthesis and expression in intracellular vacuolar compartments is inhibited, decreasing Ag processing function. This limits presentation to a cohort of Ags kinetically associated with the maturation stimulus and excludes presentation of Ags subsequently experienced by the DC. In contrast, our studies show that IFN-I enhances DC expression of MHC-II and costimulatory molecules without a concomitant inhibition of subsequent MHC-II synthesis and Ag processing. Expression of mRNA for MHC-II and the transcription factor CIITA is inhibited in DCs treated with TLR agonists but maintained in cells treated with IFN-I. After stimulation with IFN-I, MHC-II expression is increased on the plasma membrane but is also maintained in intracellular vacuolar compartments, consistent with sustained Ag processing function. These findings suggest that IFN-I drives a distinctive DC maturation program that enhances Ag presentation to T cells without a shutdown of Ag processing, allowing continued sampling of Ags for presentation.

CIITA promoter I CARD-deficient mice express functional MHC class II genes in myeloid and lymphoid compartments

Three distinct promoters control the master regulator of MHC class II expression, CIITA, in a cell type specific manner. Promoter I (pI) CIITA, expressed primarily by dendritic cells and macrophages, expresses a unique isoform that contains a caspase recruitment domain. The activity and function of this isoform is not understood but has been thought to enhance the function of CIITA in antigen presenting cells. To determine if isoform I of CIITA has specific functions, CIITA mutant mice were created in which isoform I was replaced with isoform III sequences. Mice in which pI and the CARD encoding exon were deleted were also created. No defect in the formation of CD4 T cells, the ability to respond to a model antigen, or bacterial or viral challenge was observed in mice lacking CIITA isoform I. Although CIITA and MHC-II expression was decreased in splenic DC, the pI knockout animals expressed CIITA from downstream promoters, suggesting that control of pI activity is mediated by unknown s II distal elements that could act at the pIII, the B cell promoter. Thus, no critical function is linked to the CARD domain of CIITA isoform I with respect to basic immune system development, function and challenge.

Deficient CCR7 signaling promotes TH2 polarization and B-cell activation in vivo

The chemokine receptor CCR7 has a central role in regulating homing and positioning of T cells and DCs to lymph nodes (LNs) and participates in T-cell development and activation. In this study, we addressed the role of CCR7 signaling in T(H) 2 polarization and B-cell activation. We provide evidence that the lack of CCR7 drives the capacity of naïve CD4(+) T cells to polarize toward T(H) 2 cells. This propensity contributes to a lymph node environment in CCR7-deficent mice characterized by increased expression of IL-4 and increased frequency of T(H) 2 cells. We show that elevated IL-4 levels lead to B-cell activation characterized by up-regulated expression of MHC class II, CD23 and CD86. Activated B cells are in turn highly efficient in presenting antigen to CD4(+) T cells and thus potentially contribute to the T(H) 2 microenvironment. Taken together, our results support the idea of a CCR7-dependent patterning of T(H) 2 responses, with absent CCR7 signaling favoring T(H) 2 polarization, dislocation of T helper cells into the B-cell follicles and, as a consequence, B-cell activation.

Role of PU.1 in MHC class II expression through transcriptional regulation of class II transactivator pI in dendritic cells

BACKGROUND

PU.1 is a hematopoietic cell-specific transcription factor belonging to the Ets family. We hypothesized that PU.1 is involved in MHC class II expression in dendritic cells (DCs).

OBJECTIVE

The role of PU.1 in MHC class II expression in DCs was analyzed.

METHODS

Transcriptional regulation of the DC-specific pI promoter of the class II transactivator (CIITA) gene and subsequent MHC class II expression was investigated by using PU.1 small interfering RNA (siRNA) and reporter, chromatin immunoprecipitation, and electrophoretic mobility shift assays.

RESULTS

PU.1 siRNA introduction suppressed MHC class II expression, allogeneic and syngeneic T-cell activation activities of bone marrow-derived DCs (BMDCs) with reduction of CIITA mRNA driven by the DC-specific promoter pI, and MHC class II mRNA. The chromatin immunoprecipitation assay showed constitutive binding of PU.1 to the pI region in BMDCs, whereas acetylation of histone H3 on pI was suppressed by LPS stimulation in parallel with shutdown of CIITA transcription. PU.1 transactivated the pI promoter through cis-elements at -47/-44 and -30/-27 in a reporter assay and to which PU.1 directly bound in an electrophoretic mobility shift assay. Acetylation of histones H3 and H4 on pI was reduced in PU.1 siRNA-introduced BMDCs. Knockdown of interferon regulatory factor 4 or 8, which is a heterodimer partner of PU.1, by siRNA did not affect pI-driven CIITA transcription or MHC class II expression.

CONCLUSION

PU.1 basally transactivates the CIITA pI promoter in DCs by functioning as a monomeric transcription factor and by affecting histone modification, resulting in the subsequent expression and function of MHC class II.

γ-Interferon-regulated chaperone governs human lymphocyte antigen class II expression

Antigen presentation by human lymphocyte antigen (HLA) class II peptide receptors alerts the immune system to infections. In antigen-presenting cells (APCs), HLA class II, HLA-DM, and associated invariant chain-encoding genes are exclusively regulated by the interferon γ (IFNγ)-inducible class II transactivator (CIITA). Control of CIITA expression could therefore govern expression of class II peptide receptors in the diverse group of APCs. We discovered that elevation of the HLA class III region encoded B-associated transcript 3 (BAT3) increases and depletion of BAT3 decreases expression of HLA class II, HLA-DM, and invariant chain. IFNγ strongly elevates BAT3 transcription in various tumor cell lines and in primary macrophages. BAT3 chaperones the simultaneously IFNγ-induced CIITA. Following IFNγ-treatment, both CIITA and BAT3 translocate from the cytosol to the nucleus. The nuclear import of CIITA mediated by IFNγ controls activation of HLA class II genes. BAT3 is a novel key regulator of components of the HLA class II processing pathway. We present a mechanism explaining how parallel IFNγ-mediated regulation of CIITA and of its chaperone BAT3 controls the level of components of the HLA class II processing pathway.

Human cytomegalovirus decreases constitutive transcription of MHC class II genes in mature Langerhans cells by reducing CIITA transcript levels

Human cytomegalovirus (HCMV) productively infects CD34(+) progenitor-derived, mature Langerhans-type dendritic cells (matLC) and reduces surface expression of MHC class II complexes (MHC II) by increasing intracellular retention of these molecules. To determine whether HCMV also inhibits MHC II expression by other mechanisms, we assessed mRNA levels of the class II transcriptional regulator, CIITA, and several of its target genes in infected matLC. Levels of CIITA, HLA-DRA (DRA) and DRB transcripts, and new DR protein synthesis were compared in mock-infected and HCMV-infected cells by quantitative PCR and pulse-chase immunoprecipitation analyses, respectively. CIITA mRNA levels were significantly lower in HCMV-infected matLC as compared to mock-infected cells. When assessed in the presence of Actinomycin D, the stability of CIITA transcripts was not diminished by HCMV. Analysis of promoter-specific CIITA isoforms revealed that types I, III and IV all were decreased by HCMV, a result that differs from changes after incubation of these cells with lipopolysaccharide (LPS). Exposure to UV-inactivated virus failed to reduce CIITA mRNA levels, implicating de novo viral gene expression in this effect. HCMV-infected matLC also expressed lower levels of DR transcripts and reduced DR protein synthesis rates compared to mock-infected matLC. In summary, we demonstrate that HCMV infection of a human dendritic cell subset inhibits constitutive CIITA expression, most likely at the transcriptional level, resulting in reduced MHC II biosynthesis. We suggest this represents a new mechanism of modulation of mature LC by HCMV.

Dendritic Cells and HIV-1 Trans-Infection

Dendritic cells initiate and sustain immune responses by migrating to sites of pathogenic insult, transporting antigens to lymphoid tissues and signaling immune specific activation of T cells through the formation of the immunological synapse. Dendritic cells can also transfer intact, infectious HIV-1 to CD4 T cells through an analogous structure, the infectious synapse. This replication independent mode of HIV-1 transmission, known as trans-infection, greatly increases T cell infection in vitro and is thought to contribute to viral dissemination in vivo. This review outlines the recent data defining the mechanisms of trans-infection and provides a context for the potential contribution of trans-infection in HIV-1 disease.

Regulation of antigen presentation by Mycobacterium tuberculosis: a role for Toll-like receptors

Mycobacterium tuberculosis survives in antigen-presenting cells (APCs) such as macrophages and dendritic cells. APCs present antigens in association with major histocompatibility complex (MHC) class II molecules to stimulate CD4(+) T cells, and this process is essential to contain M. tuberculosis infection. Immune evasion allows M. tuberculosis to establish persistent or latent infection in macrophages and results in Toll-like receptor 2 (TLR2)-dependent inhibition of MHC class II transactivator expression, MHC class II molecule expression and antigen presentation. This reduction of antigen presentation might reflect a general mechanism of negative-feedback regulation that prevents excessive T cell-mediated inflammation and that M. tuberculosis has subverted to create a niche for survival in infected macrophages and evasion of recognition by CD4(+) T cells.

PU.1 binds to a distal regulatory element that is necessary for B cell-specific expression of CIITA

The transcriptional coactivator CIITA regulates MHC class II genes. In the mouse, CIITA is expressed from three distinct promoters (pI, pIII, and pIV) in a developmental and cell type-specific manner with pIII being responsible for B lymphocyte-specific expression. Although the promoter proximal sequences that regulate CIITA in B cells have been described, nothing is known about additional distal elements that may regulate its expression in B cells. Sequence homology comparisons, DNase I hypersensitivity assays, and histone modification analysis revealed a potential regulatory element located 11 kb upstream of pIII. Deletion of this element, termed hypersensitive site 1 (HSS1), in a bacterial artificial chromosome encoding the entire CIITA locus and surrounding genes, resulted in a complete loss of CIITA expression from the bacterial artificial chromosome following transfection into B cells. HSS1 and pIII displayed open chromatin architecture features in B cell but not in plasma cell lines, which are silenced for CIITA expression. PU.1 was found to bind HSS1 and pIII in B cells but not in plasma cells. Depletion of PU.1 by short hairpin RNA reduced CIITA expression. Chromatin conformation capture assays showed that HSS1 interacted directly with pIII in B cells and that PU.1 was important for this interaction. These results provide evidence that HSS1 is required for B cell-specific expression of CIITA and that HSS1 functions by interacting with pIII, forming a long-distance chromatin loop that is partly mediated through PU.1.

Cognate interactions between mast cells and helper T lymphocytes

Mast cells are key effectors in allergy and inflammation. Endowed with a large panel of surface receptors and a huge arsenal of bioactive mediators, they readily communicate with various cellular partners during innate and adaptive immune responses. Recent lines of evidence show that mast cells are also able to establish cognate interactions with helper T lymphocytes for antigen presentation and bidirectional cell-cell cooperation. In this short review we focus on the role of mast cells as unconventional antigen presenting cells for helper T lymphocytes. We discuss how looking at mast cell biology from this new angle can help to better understand their pleiotropic role in health and disease.

Chlamydophila pneumoniae downregulates MHC-class II expression by two cell type-specific mechanisms

Chlamydophila pneumoniae was shown to prevent IFN gamma-inducible upregulation of MHC-class II molecules by secreting chlamydial protease-like activity factor (CPAF) into the cytosol of those host cells which support the complete bacterial replication cycle. CPAF acts by degrading upstream stimulatory factor 1 (USF-1). However, in cells like bone marrow-derived macrophages (BMM), which restrict chlamydial replication, we show that CPAF expression is barely detectable and the expression of USF-1 is induced upon infection with C. pneumoniae. Nevertheless, the infection still reduced base line and prevented IFN gamma-inducible MHC-class II expression. Similar results were obtained with heat-inactivated C. pneumoniae. In contrast, reduction of MHC-class II molecules was not observed in MyD88-deficient BMM. Reduction of IFN gamma-inducible MHC-class II expression by C. pneumoniae in BMM was mediated in part by the MAP-kinase p38. Infection of murine embryonic fibroblasts (MEF) with C. pneumoniae, which allow chlamydial replication, induced the expression of CPAF and decreased USF-1 and MHC-class II expression. Treatment of these cells with heat-inactivated C. pneumoniae reduced USF-1 and MHC-class II expression to a much lower extent. In summary, C. pneumoniae downregulates MHC-class II expression by two cell type-specific mechanisms which are either CPAF-independent and MyD88-dependent like in BMM or CPAF-dependent like in MEFs.

CCAAT/enhancer-binding protein beta and delta binding to CIITA promoters is associated with the inhibition of CIITA expression in response to Mycobacterium tuberculosis 19-kDa lipoprotein

TLR2 signaling by Mycobacterium tuberculosis 19-kDa lipoprotein (LpqH) inhibits IFN-gamma-induced expression of CIITA by macrophages. Microarray analysis, quantitative RT-PCR, and Western blots showed that LpqH induced C/EBPbeta and C/EBPdelta in kinetic correlation with inhibition of CIITA expression. Of the C/EBPbeta isoforms, liver inhibitory protein (LIP) was notably induced and liver-activating protein was increased by LpqH. Putative C/EBP binding sites were identified in CIITA promoters I and IV (pI and pIV). LpqH induced binding of C/EBPbeta (LIP and liver-activating protein) to biotinylated oligodeoxynucleotide containing the pI or pIV binding sites, and chromatin immunoprecipitation showed that LpqH induced binding of C/EBPbeta and C/EBPdelta to endogenous CIITA pI and pIV. Constitutive expression of C/EBPbeta LIP inhibited IFN-gamma-induced CIITA expression in transfected cells. In summary, LpqH induced expression of C/EBPbeta and C/EBPdelta, and their binding to CIITA pI and pIV, in correlation with inhibition of IFN-gamma-induced expression of CIITA in macrophages, suggesting a role for C/EBP as a novel regulator of CIITA expression.

Control of in vitro immune responses by regulatory oligodeoxynucleotides through inhibition of pIII promoter directed expression of MHC class II transactivator in human primary monocytes

Ag presentation is a key step in the initiation of adaptive immune responses that depends on the expression of MHC Ags and costimulatory molecules. Immune-enhancing CpG and non-CPG oligodeoxynucleotides (ODNs) stimulate Ag presentation by stimulating the expression of these molecules and by promoting dendritic cell maturation. In this report, we identify immunoregulatory orthophosphorothioate non-CpG molecules, referred to as regulatory ODNs (rODNs), by their ability to inhibit allogeneic monocyte-stimulated T cell responses and down-regulate HLA-DR in human primary monocytes. The rODNs promoted the survival of macrophages and were able to activate IL-8 secretion through a chloroquine-resistant pathway. Messenger RNAs for HLA-DR alpha and beta and the MHC CIITA were reduced by rODNs but not by stimulatory CpG ODN2006 and non-CpG ODN2006a. CIITA transcription in monocytes was controlled primarily by promoter III and not by promoter I or IV. rODNs blocked promoter III-directed transcription of CIITA in these cells. Under conditions that induced dendritic cell differentiation, rODNs also reduced HLA-DR expression. The activity of rODNs is phosphorothioate chemistry and G stretch dependent but TLR9 independent. G tetrads were detected by circular dichroism in active rODNs and associated with high m.w. multimers on nondenaturing gels. Heat treatment of rODNs disrupted G tetrads, the high m.w. aggregates, and the HLA-DR inhibitory activity of the ODNs. The inhibition of immune responses by regulatory oligodeoxynucleotides may be useful for the treatment of immune-mediated disorders including autoimmune diseases and graft rejection.

Stat2-dependent regulation of MHC class II expression

MHC type II (MHC II) expression is tightly regulated in macrophages and potently induced by IFN-gamma (type II IFN). In contrast, type I IFNs (IFN-Is), which are far more widely expressed, fail to induce MHC II expression, even though both classes of IFNs direct target gene expression through Stat1. The unexpected finding that IFN-Is effectively induce MHC II expression in Stat2(-/-) macrophages provided an opportunity to explore this conundrum. The ensuing studies revealed that deletion of Stat2, which uniquely transduces signals for IFN-Is, leads to a loss in the IFN-I-dependent induction of suppressor of cytokine signaling-1. Impairment in the expression of this important negative regulator led to a striking prolongation in IFN-I-dependent Stat1 activation, as well as enhanced expression of the target gene, IFN-regulatory factor-1. The prolonged activity of these two transcription factors synergized to drive the transcription of CIITA, the master regulator of MHC II expression, analogous to the pattern observed in IFN-gamma-treated macrophages. Thus, IFN-I-dependent suppressor of cytokine signaling-1 expression plays an important role in distinguishing the biological response between type I and II IFNs in macrophages.

Regulation of MHC Class II Expression and Antigen Processing in Murine and Human Mesenchymal Stromal Cells by IFN-γ, TGF-β, and Cell Density

Mesenchymal stromal cells (MSC) possess immunosuppressive properties, yet when treated with IFN-gamma they acquire APC functions. To gain insight into MSC immune plasticity, we explored signaling pathways induced by IFN-gamma required for MHC class II (MHC II)-dependent Ag presentation. IFN-gamma-induced MHC II expression in mouse MSC was enhanced by high cell density or serum deprivation and suppressed by TGF-beta. This process was regulated by the activity of the type IV CIITA promoter independently of STAT1 activation and the induction of the IFN regulatory factor 1-dependent B7H1/PD-L1 encoding gene. The absence of direct correlation with the cell cycle suggested that cellular connectivity modulates IFN-gamma responsiveness for MHC II expression in mouse MSC. TGF-beta signaling in mouse MSC involved ALK5 and ALK1 TGF-beta RI, leading to the phosphorylation of Smad2/Smad3 and Smad1/Smad5/Smad8. An opposite effect was observed in human MSC where IFN-gamma-induced MHC II expression occurred at the highest levels in low-density cultures; however, TGF-beta reduced IFN-gamma-induced MHC II expression and its signaling was similar as in mouse MSC. This suggests that the IFN-gamma-induced APC features of MSC can be modulated by TGF-beta, serum factors, and cell density in vitro, although not in the same way in mouse and human MSC, via their convergent effects on CIITA expression.