Differential requirements for endosomal reduction in the presentation of two H2-E(d)-restricted epitopes from influenza hemagglutinin

Cryptic MHC-E epitope from influenza elicits a potent cytolytic T cell response

The extent to which unconventional forms of antigen presentation drive T cell immunity is unknown. By convention, CD8 T cells recognize viral peptides, or epitopes, in association with classical major histocompatibility complex (MHC) class I, or MHC-Ia, but immune surveillance can, in some cases, be directed against peptides presented by nonclassical MHC-Ib, in particular the MHC-E proteins (Qa-1 in mice and HLA-E in humans); however, the overall importance of nonclassical responses in antiviral immunity remains unclear. Similarly uncertain is the importance of 'cryptic' viral epitopes, defined as those undetectable by conventional mapping techniques. Here we used an immunopeptidomic approach to search for unconventional epitopes that drive T cell responses in mice infected with influenza virus A/Puerto Rico/8/1934. We identified a nine amino acid epitope, termed M-SL9, that drives a co-immunodominant, cytolytic CD8 T cell response that is unconventional in two major ways: first, it is presented by Qa-1, and second, it has a cryptic origin, mapping to an unannotated alternative reading frame product of the influenza matrix gene segment. Presentation and immunogenicity of M-SL9 are dependent on the second AUG codon of the positive sense matrix RNA segment, suggesting translation initiation by leaky ribosomal scanning. During influenza virus A/Puerto Rico/8/1934 infection, M-SL9-specific T cells exhibit a low level of egress from the lungs and strong differentiation into tissue-resident memory cells. Importantly, we show that M-SL9/Qa-1-specific T cells can be strongly induced by messenger RNA vaccination and that they can mediate antigen-specific cytolysis in vivo. Our results demonstrate that noncanonical translation products can account for an important fraction of the T cell repertoire and add to a growing body of evidence that MHC-E-restricted T cells could have substantial therapeutic value.

Type II alveolar cell MHCII improves respiratory viral disease outcomes while exhibiting limited antigen presentation

Type II alveolar cells (AT2s) are critical for basic respiratory homeostasis and tissue repair after lung injury. Prior studies indicate that AT2s also express major histocompatibility complex class II (MHCII) molecules, but how MHCII expression by AT2s is regulated and how it contributes to host defense remain unclear. Here we show that AT2s express high levels of MHCII independent of conventional inflammatory stimuli, and that selective loss of MHCII from AT2s in mice results in modest worsening of respiratory virus disease following influenza and Sendai virus infections. We also find that AT2s exhibit MHCII presentation capacity that is substantially limited compared to professional antigen presenting cells. The combination of constitutive MHCII expression and restrained antigen presentation may position AT2s to contribute to lung adaptive immune responses in a measured fashion, without over-amplifying damaging inflammation.

IFI30 expression predicts patient prognosis in breast cancer and dictates breast cancer cells proliferation via regulating autophagy

Introduction: Incidence and mortality rates of breast cancer are increasing in women worldwide. Immunotherapy is a relatively popular treatment modality for all malignant tumors including breast cancer in recent years. Interferon γ-inducible protein 30 (IFI30) could catalyze the reduction of disulfide bonds and enhance major histocompatibility complex (MHC) class II-restricted antigen processing. Recent studies showed that IFI30 played an important role in the immune response of malignant tumors. Methods: The Cancer Genome Atlas (TCGA) database and clinical proteomic tumor Analysis consortium (CPTAC) database were applied to predict the role of IFI30 in breast cancer and the relationship between IFI30 and prognosis of breast cancer patients. Then we detected the expression of IFI30 in clinical samples of breast cancer patients, and analyzed the relationship between IFI30 and the prognosis of breast cancer patients. We used lentivirus infection method to construct a stable IFI30 knockdown cell line, and detected the effect of IFI30 in breast cancer cells. Nude mice tumor bearing experiment was performed to investigate the effect of IFI30 on breast cancer cells in vivo. Western blot was used to verify the regulation of autophagy related protein LC3 and p62 by IFI30. Results: We found that IFI30 was highly expressed in breast cancer tissues and was associated with poor outcome of patients. The knockdown of IFI30 could inhibit the proliferation, migration and invasion of breast cancer cells and significantly inhibit tumor growth in vivo. Increased accumulation of LC3-II and p62 suggested impaired autophagy in IFI30 knockdown cells. Discussion: As a result, we suggested that IFI30 might play a key role in the initiation and progression of human breast cancer and might be a new therapeutic target in breast cancer.

Activation of Dendritic Cells Alters the Mechanism of MHC Class II Antigen Presentation to CD4 T Cells

Both immature and mature dendritic cells (DCs) can process and present foreign Ags to CD4 T cells; however, the mechanism by which MHC class II (MHC-II) in mature DCs acquires antigenic peptides remains unknown. To address this, we have studied Ag processing and presentation of two distinct CD4 T cell epitopes of the influenza virus hemagglutinin coat protein by both immature and mature mouse DCs. We find that immature DCs almost exclusively use newly synthesized MHC-II targeted to DM⁺ late endosomes for presentation to influenza virus-specific CD4 T cells. By contrast, mature DCs exclusively use recycling MHC-II that traffics to both early and late endosomes for antigenic peptide binding. Rab11a knockdown partially inhibits recycling of MHC-II in mature DCs and selectively inhibits presentation of an influenza virus hemagglutinin CD4 T cell epitope generated in early endosomes. These studies highlight a "division of labor" in MHC-II peptide binding, in which immature DCs preferentially present Ags acquired in Rab11a^- DM⁺ late endosomes, whereas mature DCs use recycling MHC-II to present antigenic peptides acquired in both Rab11a⁺ early endosomes and Rab11a^- endosomes for CD4 T cell activation.

Responsive Antibody Conjugates Enable Quantitative Determination of Intracellular Bond Degradation Rate

Degradable crosslinkers that respond to intracellular biological stimuli are a critical component of many drug delivery systems. With numerous stimuli-responsive drug delivery systems in development, it is important to quantitatively study their intracellular processing. Herein we report a framework for quantifying the rate of intracellular bond degradation in the endocytic pathway. Toward this end, we devised and synthesized a reduction-sensitive FRET-based crosslinker that can be readily conjugated to a variety of targeting ligands. This crosslinker was conjugated to trastuzumab, a humanized monoclonal antibody against the HER2 receptor. We developed a model based on mass-action kinetics to describe the intracellular processing of this conjugate. The kinetic model was developed in conjunction with live-cell experiments to extract the rate constant for intracellular disulfide bond degradation. This framework may be applied to other endocytosis pathways, bond types, and cell types to quantify this fundamental degradation rate parameter.

Penaeus monodon GILT enzyme restricts WSSV infectivity by reducing disulfide bonds in WSSV proteins

Gamma-interferon-inducible lysosomal thiol reductase (GILT) is involved in the adaptive immune response via its effects on major histocompatibility complex (MHC)-restricted antigen presentation. In addition to antigen presentation, GILT exerts its antiviral activity by reducing disulfide bonds in proteins involved in viral infection and assembly, thereby inhibiting viral envelope-mediated infection and viral progeny production. In black tiger shrimp, Penaeus monodon GILT (PmGILT) was cloned and characterized, and found to be involved in the shrimp innate immune response and to exert neutralizing activity against white spot syndrome virus (WSSV) infection. However, the anti-WSSV mechanism of PmGILT in the shrimp innate immune response has not been defined. To explore the anti-WSSV activity of PmGILT, a yeast 2-hybrid (Y2H) assay was performed to identify WSSV proteins targeted by PmGILT. The assay revealed 4 potential PmGILT-interacting WSSV proteins: WSSV002, WSSV164, WSSV189, and WSSV471. Three of these 4 WSSV proteins (WSSV002, WSSV164 and WSSV189) were successfully produced and confirmed to interact with PmGILT in in vitro pull-down assays. WSSV189 and WSSV471 were previously identified as structural proteins, whereas WSSV164 is an immediate-early protein which has anti-melanization activity, and WSSV002 is an unknown. Because of the thiol reductase activity of PmGILT, WSSV164 and WSSV189, both of which are cysteine-containing WSSV proteins, were chosen for disulfide bond reduction assays. PmGILT reduced intrachain disulfide bonds in both WSSV proteins, suggesting that PmGILT exerts its anti-WSSV activity via its thiol reductase activity to disrupt the WSSV protein complex and restore the melanization activity of PmproPO1 and PmproPO2.

Influence of protein fold stability on immunogenicity and its implications for vaccine design

INTRODUCTION

In modern vaccinology and immunotherapy, recombinant proteins more and more replace whole organisms to induce protective or curative immune responses. Structural stability of proteins is of crucial importance for efficient presentation of antigenic peptides on MHC, which plays a decisive role for triggering strong immune reactions. Areas covered: In this review, we discuss structural stability as a key factor for modulating the potency of recombinant vaccines and its importance for antigen proteolysis, presentation, and stimulation of B and T cells. Moreover, the impact of fold stability on downstream events determining the differentiation of T cells into effector cells is reviewed. We summarize studies investigating the impact of protein fold stability on the outcome of the immune response and provide an overview on computational methods to estimate the effects of point mutations on protein stability. Expert commentary: Based on this information, the rational design of up-to-date vaccines is discussed. A model for predicting immunogenicity of proteins based on their conformational stability at different pH values is proposed.

Conformational instability governed by disulfide bonds partitions the dominant from subdominant helper T-cell responses specific for HIV-1 envelope glycoprotein gp120

Most individuals infected with human immunodeficiency virus type 1 (HIV-1) generate a CD4(+) T-cell response that is dominated by a few epitopes. Immunodominance may be counterproductive because a broad CD4(+) T-cell response is associated with reduced viral load. Previous studies indicated that antigen three-dimensional structure controls antigen processing and presentation and therefore CD4(+) T-cell epitope dominance. Dominant epitopes occur adjacent to the V1-V2, V3, and V4 loops because proteolytic antigen processing in the loops promotes presentation of adjacent sequences. In this study, three gp120 (strain JR-FL) variants were constructed, in which deletions of single outer-domain disulfide bonds were expected to introduce local conformational flexibility and promote presentation of additional CD4(+) T-cell epitopes. Following mucosal immunization of C57BL/6 mice with wild-type or variant gp120 lacking the V3-flanking disulfide bond, the typical pattern of dominant epitopes was observed, suggesting that the disulfide bond posed no barrier to antigen presentation. In mice that lacked gamma interferon-inducible lysosomal thioreductase (GILT), proliferative responses to the typically dominant epitopes of gp120 were selectively depressed, and the dominance pattern was rearranged. Deletion of the V3-flanking disulfide bond or one of the V4-flanking disulfide bonds partially restored highly proliferative responses to the typically dominant epitopes. These results reveal an acute dependence of dominant CD4(+) T-cell responses on the native gp120 conformation.

γ-Interferon-inducible lysosomal thiol reductase (GILT) maintains phagosomal proteolysis in alternatively activated macrophages

Although it is known that lysosomal cysteine cathepsins require a reducing environment for optimal activity, it is not firmly established how these enzymes are maintained in their reduced-active state in the acidic and occasionally oxidative environment within phagosomes and lysosomes. γ-Interferon-inducible lysosomal thiol reductase (GILT) has been the only enzyme described in the endosomes, lysosomes, and phagosomes with the potential to catalyze the reduction of cysteine cathepsins. Our goal in the current study was to assess the effect of GILT on major phagosomal functions with an emphasis on proteolytic efficiency in murine bone marrow-derived macrophages. Assessment of phagosomal disulfide reduction upon internalization of IgG-opsonized experimental particles confirmed a major role for GILT in phagosomal disulfide reduction in both resting and interferon-γ-activated macrophages. Furthermore we observed a decrease in early phagosomal proteolytic efficiency in GILT-deficient macrophages, specifically in the absence of an NADPH oxidase-mediated respiratory burst. This deficiency was more prominent in IL-4-activated macrophages that inherently possess lower levels of NADPH oxidase activity. Finally, we provide evidence that GILT is required for optimal activity of the lysosomal cysteine protease, cathepsin S. In summary, our results suggest a role for GILT in maintaining cysteine cathepsin proteolytic efficiency in phagosomes, particularly in the absence of high NADPH oxidase activity, which is characteristic of alternatively activated macrophages.

GILT: Shaping the MHC Class II-Restricted Peptidome and CD4(+) T Cell-Mediated Immunity

The MHC class II-restricted antigen processing pathway generates peptide:MHC complexes in the endocytic pathway for the activation of CD4(+) T cells. Gamma-interferon-inducible lysosomal thiol reductase (GILT) reduces protein disulfide bonds in the endocytic compartment, thereby exposing buried epitopes for MHC class II binding and presentation. T cell hybridoma responses and elution of MHC class II bound peptides have identified GILT-dependent epitopes, GILT-independent epitopes, and epitopes that are more efficiently presented in the absence of GILT termed GILT-prevented epitopes. GILT-mediated alteration in the MHC class II-restricted peptidome modulates T cell development in the thymus and peripheral tolerance and influences the pathogenesis of autoimmunity. Recent studies suggest an emerging role for GILT in the response to pathogens and cancer survival.

Regulation of hepatitis B virus infection by Rab5, Rab7, and the endolysosomal compartment

Despite important progress toward deciphering human hepatitis B virus (HBV) entry into host cells, many aspects of the early steps of the life cycle remained completely obscure. Following endocytosis, HBV must travel through the complex network of the endocytic pathway to reach the cell nucleus and initiate replication. In addition to guiding the viral particles to the replication site, the endosomal vesicles may play a crucial role in infection, providing the appropriate environment for virus uncoating and nucleocapsid release. In this work, we investigated the trafficking of HBV particles internalized in permissive cells. Expression of key Rab proteins, involved in specific pathways leading to different intracellular locations, was modulated in HepaRG cells, using a stable and inducible short hairpin RNA (shRNA) expression system. The trafficking properties of the newly developed cells were demonstrated by confocal microscopy and flow cytometry using specific markers. The results showed that HBV infection strongly depends on Rab5 and Rab7 expression, indicating that HBV transport from early to mature endosomes is required for a step in the viral life cycle. This may involve reduction of disulfide bond-linked envelope proteins, as alteration of the redox potential of the endocytic pathway resulted in inhibition of infection. Subcellular fractionation of HBV-infected cells showed that viral particles are further transported to lysosomes. Intriguingly, infection was not dependent on the lysosomal activity, suggesting that trafficking to this compartment is a "dead-end" route. Together, these data add to our understanding of the HBV-host cell interactions controlling the early stages of infection.

Expanding roles for GILT in immunity

Gamma-interferon-inducible lysosomal thiol reductase (GILT), a thioredoxin-related oxidoreductase, functions in MHC class II-restricted antigen processing and MHC class I-restricted cross-presentation by reducing disulfide bonds of endocytosed proteins and facilitating their unfolding and optimal degradation. However, recent reports have greatly expanded our understanding of GILT's function. Several studies of GILT and antigen processing have shown that the influence of GILT on the peptide repertoire can alter the character of the immune response and affect central tolerance. Furthermore, a few unexpected roles for GILT have been uncovered: as a host factor for Listeria monocytogenes infection, in the maintenance of cellular glutathione (GSH) levels, and possibly outside the cell, as enzymatically active GILT is secreted by activated macrophages.

Beyond the classical: influenza virus and the elucidation of alternative MHC class II-restricted antigen processing pathways

CD4+ T cells (T(CD4+)) are activated by peptides, generally 13-17 amino acids in length, presented at the cell surface in combination with highly polymorphic MHC class II molecules. According to the classical model, these peptides are generated by endosomal digestion of internalized antigen and loaded onto MHC class II molecules in the late endosome. Historically, this "exogenous" pathway has been defined through the extensive use of purified proteins. However, the relatively recent use of clinically relevant antigens, those of influenza virus in our case, has revealed several additional pathways of peptide production, including some that are truly "endogenous", entailing synthesis of the protein within the infected cell. Indeed, some peptides appear to be created only via endogenous processing. The cell biology that underlies these alternative pathways remains poorly understood as do their relative contributions to defence against infectious agents and cancer, and the triggering of autoimmune diseases.

Cathepsin cleavage potentiates the Ebola virus glycoprotein to undergo a subsequent fusion-relevant conformational change

Cellular entry of Ebola virus (EBOV), a deadly hemorrhagic fever virus, is mediated by the viral glycoprotein (GP). The receptor-binding subunit of GP must be cleaved (by endosomal cathepsins) in order for entry and infection to proceed. Cleavage appears to proceed through 50-kDa and 20-kDa intermediates, ultimately generating a key 19-kDa core. How 19-kDa GP is subsequently triggered to bind membranes and induce fusion remains a mystery. Here we show that 50-kDa GP cannot be triggered to bind to liposomes in response to elevated temperature but that 20-kDa and 19-kDa GP can. Importantly, 19-kDa GP can be triggered at temperatures ∼10°C lower than 20-kDa GP, suggesting that it is the most fusion ready form. Triggering by heat (or urea) occurs only at pH 5, not pH 7.5, and involves the fusion loop, as a fusion loop mutant is defective in liposome binding. We further show that mild reduction (preferentially at low pH) triggers 19-kDa GP to bind to liposomes, with the wild-type protein being triggered to a greater extent than the fusion loop mutant. Moreover, mild reduction inactivates pseudovirion infection, suggesting that reduction can also trigger 19-kDa GP on virus particles. Our results support the hypothesis that priming of EBOV GP, specifically to the 19-kDa core, potentiates GP to undergo subsequent fusion-relevant conformational changes. Our findings also indicate that low pH and an additional endosomal factor (possibly reduction or possibly a process mimicked by reduction) act as fusion triggers.

Disulfide reduction in the endocytic pathway: immunological functions of gamma-interferon-inducible lysosomal thiol reductase

Gamma-interferon-inducible lysosomal thiol reductase (GILT) is constitutively expressed in most antigen presenting cells and is interferon γ inducible in other cell types via signal transducer and activator of transcription 1. Normally, N- and C-terminal propeptides are cleaved in the early endosome, and the mature protein resides in late endosomes and lysosomes. Correspondingly, GILT has maximal reductase activity at an acidic pH. Monocyte differentiation via Toll-like receptor 4 triggers secretion of a disulfide-linked dimer of the enzymatically active precursor, which may contribute to inflammation. GILT facilitates major histocompatibility complex (MHC) class II-restricted processing through reduction of protein disulfide bonds in the endocytic pathway and is hypothesized to expose buried epitopes for MHC class II binding. GILT can also facilitate the transfer of disulfide-containing antigens into the cytosol, enhancing their cross-presentation by MHC class I. A variety of antigens are strongly influenced by GILT-mediated reduction, including hen egg lysozyme, melanocyte differentiation antigens, and viral envelope glycoproteins. In addition, GILT is conserved among lower eukaryotes and likely has additional functions. For example, GILT expression increases the stability of superoxide dismutase 2 and decreases reactive oxygen species, which correlates with decreased cellular proliferation. It is also a critical host factor for infection with Listeria monocytogenes.

Enhanced antigen processing of flagellin fusion proteins promotes the antigen-specific CD8+ T cell response independently of TLR5 and MyD88

Flagellin is a highly effective adjuvant for CD4(+) T cell and humoral immune responses. However, there is conflicting data in the literature regarding the ability of flagellin to promote a CD8(+) T cell response. In this article, we report that immunization of wild-type, TLR5(-/-), and MyD88(-/-) adoptive transfer recipient mice revealed the ability of flagellin fusion proteins to promote OVA-specific CD8(+) T cell proliferation independent of TLR5 or MyD88 expression by the recipient animal. Wild-type and TLR5(-/-) APCs were able to stimulate high levels of OVA-specific CD8(+) T cell proliferation in vitro in response to a flagellin fusion protein containing full-length OVA or the SIINFEKL epitope and 10 flanking amino acids (OVAe), but not to OVA and flagellin added as separate proteins. This effect was independent of the conserved regions of flagellin and occurred in response to OVAe alone. Comparison of IFN-γ production by CD8(+) effector cells revealed higher levels of SIINFEKL peptide-MHC I complexes on the surface of APCs that had been pulsed with OVAe-flagellin fusion proteins than on cells pulsed with OVA. Inhibition of the proteasome significantly reduced Ag-specific proliferation in response to OVAe fusion proteins. In summary, our data are consistent with the conclusion that flagellin-OVA fusion proteins induce an epitope-specific CD8(+) T cell response by facilitating Ag processing and not through stimulatory signaling via TLR5 and MyD88. Our findings raise the possibility that flagellin might be an efficient Ag carrier for Ags that are poorly processed in their native state.

Exosome-driven antigen transfer for MHC class II presentation facilitated by the receptor binding activity of influenza hemagglutinin

The mechanisms underlying MHC class I-restricted cross-presentation, the transfer of Ag from an infected cell to a professional APC, have been studied in great detail. Much less is known about the equivalent process for MHC class II-restricted presentation. After infection or transfection of class II-negative donor cells, we observed minimal transfer of a proteasome-dependent "class I-like" epitope within the influenza neuraminidase glycoprotein but potent transfer of a classical, H-2M-dependent epitope within the hemagglutinin (HA) glycoprotein. Additional experiments determined transfer to be exosome-mediated and substantially enhanced by the receptor binding activity of incorporated HA. Furthermore, a carrier effect was observed in that incorporated HA improved exosome-mediated transfer of a second membrane protein. This route of Ag presentation should be relevant to other enveloped viruses, may skew CD4(+) responses toward exosome-incorporated glycoproteins, and points toward novel vaccine strategies.

Comparative quantitative mass spectrometry analysis of MHC class II-associated peptides reveals a role of GILT in formation of self-peptide repertoire

Gamma interferon Inducible Lysosomal Thiol reductase (GILT) is a unique lysosomal reductase that reduces disulfide bonds of endocytosed proteins. Lack of GILT clearly decreases CD4 T cell-antigen specific responses against some epitopes of antigens containing disulfide bonds, but not to proteins with few or no disulfide bridges. Hence, global impact of GILT on antigen presentation is currently not well understood. We used Nano-LC-ESI-MS/MS to investigate how GILT affects diversity of self-peptides presented by MHC class II molecules. Surprisingly, the repertoire of self-peptides in the absence of GILT does not appear to be significantly different, as only few peptide species (∼2%) were found to be the unique indicators of GILT's presence or absence. In the absence of GILT about thirty peptide species (∼5%) were found either uniquely or fourteen to hundred fold more abundantly expressed than in the presence of GILT. Our data indicate that GILT has limited yet unexpected effect on self-peptide species presented by MHC class II antigens.

Increased antigen presentation efficiency by coupling antigens to MHC class I trafficking signals

Genetic modification of vaccines by linking the Ag to lysosomal or endosomal targeting signals has been used to route Ags into MHC class II processing compartments for improvement of CD4+ T cell responses. We report in this study that combining an N-terminal leader peptide with an MHC class I trafficking signal (MITD) attached to the C terminus of the Ag strongly improves the presentation of MHC class I and class II epitopes in human and murine dendritic cells (DCs). Such chimeric fusion proteins display a maturation state-dependent subcellular distribution pattern in immature and mature DCs, mimicking the dynamic trafficking properties of MHC molecules. T cell response analysis in vitro and in mice immunized with DCs transfected with Ag-encoding RNA showed that MITD fusion proteins have a profoundly higher stimulatory capacity than wild-type controls. This results in efficient expansion of Ag-specific CD8+ and CD4+ T cells and improved effector functions. We used CMVpp65 and NY-ESO-1 Ags to study preformed immune responses in CMV-seropositive individuals and cancer patients. We show that linking these Ags to the MITD trafficking signal allows simultaneous, polyepitopic expansion of CD8+ and CD4+ T cells, resulting in distinct CD8+ T cell specificities and a surprisingly broad and variable Ag-specific CD4+ repertoire in different individuals.

Target peptide sequence within infectious human immunodeficiency virus type 1 does not ensure envelope-specific T-helper cell reactivation: influences of cysteine protease and gamma interferon-induced thiol reductase activities

Recent clinical trials have shown that the presence of a robust human immunodeficiency virus type 1 (HIV-1)-specific T-cell response may not be sufficient to prevent or control HIV-1 infection. Studies of antigen processing in the context of infectious HIV-1 are therefore warranted. Envelope-specific, major histocompatibility complex class II-restricted murine T-cell hybridomas were tested for responsiveness to splenic antigen-presenting cells exposed to HIV-1-infected GHOST cells. Interleukin-2 assays showed that the presence of a peptide within HIV-1 did not ensure the reactivation of peptide-specific T cells. Further experiments defined the impact of gamma interferon-induced thiol reductase and cysteine proteases on the processing of HIV-1 peptides. The results highlight potential influences of peptide context on T-cell reactivation by HIV-1 and encourage the continued study of antigen processing as support for improved vaccine design.

Three-dimensional structure determines the pattern of CD4+ T-cell epitope dominance in influenza virus hemagglutinin

The structural context of a CD4(+) T-cell epitope is known to influence immunodominance at the level of antigen processing, but general rules have not emerged. Dominant epitopes of influenza virus hemagglutinin are found to be localized to the C-terminal flanks of conformationally stable segments identified by low crystallographic B-factors or high COREX residue stabilities. The bias toward C-terminal flanks is distinctive for antigens from the influenza virus. Dominant epitopes in antigens/allergens from other sources also localize to the flanks of stable segments but are found on either N- or C-terminal flanks. Thus, dominance arises from preferential endoproteolytic nicking between stable segments followed by loading of fragment terminal regions into antigen-presenting proteins. This mechanism probably arose in order to direct CD4(+) responses onto sequences that are conserved for structure and function. Structure-guided presentation could enhance protection against genetically drifting influenza virus variants but most likely reduces protection against new viral subtypes.

Intrinsic and cooperative antigen-presenting functions of dendritic-cell subsets in vivo

Dendritic cells (DCs) comprise several subsets, and their roles in the presentation of antigens derived from pathogens, vaccines and self tissues are now beginning to be elucidated. Differences in location, life cycle and intrinsic abilities to capture, process and present antigens on their MHC class I and class II molecules enable each DC subset to have distinct roles in immunity to infection and in the maintenance of self tolerance. Unexpected interactions among DC subsets have also been revealed. These interactions, which allow the integration of the intrinsic abilities of different DC types, enhance the ability of the DC network to respond to multiple scenarios of infection.

Induction of apoptosis and immune response by all-trans retinoic acid plus interferon-gamma in human malignant glioblastoma T98G and U87MG cells

Glioblastoma is the most common and highly malignant brain tumor. It is also one among the most therapy-resistant human neoplasias. Patients die within a year of diagnosis despite the use of available treatment strategies such as surgery, radiotherapy, and chemotherapy. Thus, there is a critical need to find a novel therapeutic strategy for treating this disease. Here, we have investigated the molecular mechanisms for induction of apoptosis as well as for activation of immune components in human malignant glioblastoma T98G and U87MG cells following treatment with all-trans retinoic acid (ATRA) plus interferon-gamma (IFN-gamma). Treatment of glioblastoma cells with ATRA alone prevented cell proliferation and induced astrocytic differentiation, while IFN-gamma alone induced apoptosis and modulated expression of human leukocyte antigen (HLA) class II molecules such as HLA-DRalpha, HLA-DR complex, invariant chain (Ii), HLA-DM (an important catalyst of the class II-peptide loading), and gamma interferon-inducible lysosomal thiol-reductase (GILT). Interestingly, both T98G and U87MG cells showed more increase in apoptosis with expression of the HLA class II components for an effective immune response following treatment with ATRA plus IFN-gamma than with IFN-gamma alone. Apoptotic mode of cell death was confirmed morphologically by Wright staining and biochemically by measuring an increase in caspase-3 activity. While conversion of tumor cells into HLA class II+/Ii- cells by stimulation with the helper CD4+ T cells is thought to be challenging, this study reports for the first time that treatment of glioblastoma cells with ATRA plus IFN-gamma can simultaneously enhance apoptosis and expression of the HLA class II immune components with a marked suppression of Ii expression. Taken together, this study suggests that induction of apoptosis and immune components of the HLA class II pathway by ATRA plus IFN-gamma may be a promising chemoimmunotherapeutic strategy for treatment of human malignant glioblastoma.

Inhibitory Role of IFN-γ-Inducible Lysosomal Thiol Reductase in T Cell Activation

IFN-gamma-inducible lysosomal thiol reductase (GILT) is a unique thiol reductase with optimal enzymatic activity at low pH. GILT plays a crucial role in unfolding the antigenic proteins in preparation for their proteolytic cleavage and presentation of resulting peptides by MHC class II. In this study, we demonstrate that GILT is expressed in T lymphocytes and that it has an APC-nonrelated role in the regulation of T cell activation. Surprisingly, comparison of wild-type and GILT-deficient T cell activation in vitro revealed stronger responsiveness in the absence of GILT. The effect of GILT in reducing the proliferative and cytotoxic responses was endogenous to T cells and resulted from decreased sensitivity at the individual cell level. Therefore, a molecule with primarily lysosomal localization suppresses T cell activation, a process characterized by signal transmission from plasma membrane to cytoplasm and nucleus.

Exposure of the Promonocytic Cell Line THP-1 to Escherichia coli Induces IFN-γ-Inducible Lysosomal Thiol Reductase Expression by Inflammatory Cytokines

IFN-gamma-inducible lysosomal thiol reductase (GILT), which plays a role in MHC class II-restricted processing and presentation of Ags containing disulfide bonds, can be induced in various cell types by the cytokine IFN-gamma. APCs, including circulating macrophages, constitutively express high levels of GILT, although the pathways regulating its expression in these cells have not been characterized. In this study, we used the promonocytic cell line THP-1, an established model for monocyte to macrophage differentiation, to investigate the induction of GILT upon exposure to bacteria. We show that contact with LPS or intact Escherichia coli causes THP-1 cells to undergo programmed differentiation, characterized by adhesion, cytokine secretion, and up-regulation of Ag processing and presentation components, including GILT. Unlike GILT induction in response to IFN-gamma treatment, induction by bacteria is dependent on new protein synthesis, NF-kappaB signaling, and secretion of the inflammatory cytokines TNF and IL-1beta. Furthermore, we show that both cytokines are sufficient for GILT induction in the absence of a microbial stimulus. The majority of GILT synthesized by differentiated THP-1 cells is secreted as the precursor form rather than being transported to, and maturing in, lysosomes, suggesting a novel role for GILT in cells of the macrophage lineage.

Oxidizing potential of endosomes and lysosomes limits intracellular cleavage of disulfide-based antibody-drug conjugates

Antibody-drug conjugate therapy entails targeted killing of cancer cells with cytotoxic compounds covalently linked to tumor-specific antibodies and shows promise in the treatment of several human cancers. Current antibody-drug conjugate designs that incorporate a disulfide linker between the antibody and cytotoxic drug are inspired by indirect evidence suggesting that the redox potential within the endosomal system is reducing. It is presumed that antigen-dependent endocytosis leads to disulfide linker reduction and intracellular release of free drug, but direct demonstration of such a mechanism is lacking. To determine whether the disulfide N-succinimidyl 4-(2-pyridyldithio)pentanoate (SPP) linker would be reduced during endocytic recycling of the anti-HER2 antibody trastuzumab (Herceptin, Genentech), we synthesized a trastuzumab-SPP-Rhodamine red conjugate and developed a linker cleavage assay by using the self-quenching property of this fluorophore. In breast carcinoma SKBr3 cells, no SPP linker cleavage was observed, as detected by fluorescence dequenching upon internalization. By contrast, the conjugate did display fluorescence dequenching when diverted to the lysosomal pathway by geldanamycin, an effect partly due to proteolytic degradation rather than disulfide reduction. To understand why linker reduction was inefficient, we measured redox potentials of endocytic compartments by expressing a redox-sensitive variant of GFP fused to various endocytic proteins. Unexpectedly, we found that recycling endosomes, late endosomes, and lysosomes are not reducing, but oxidizing and comparable with conditions in the endoplasmic reticulum. These results suggest that intracellular reduction is unlikely to account for the potency of disulfide-linked antibody-drug conjugates.

Compartmentalization of class II antigen presentation: contribution of cytoplasmic and endosomal processing

During antigen processing, peptides are generated and displayed in the context of major histocompatibility complex (MHC) class II molecules on the surface of antigen-presenting cells (APCs) to modulate immune responses to foreign antigens and guide self-tolerance. Exogenous and cytoplasmic antigens are processed by distinct routes within APCs to yield class II ligands. Exogenous antigens are internalized, processed, and bound to class II molecules within endosomal and lysosomal compartments of APCs. Studies reviewed here demonstrate the importance of reduction in regulating exogenous antigen presentation. The differential expression of a gamma-interferon-inducible lysosomal thiol reductase in professional APCs and melanomas is discussed in the context of tumor immune evasion. Cytoplasmic autoantigens, by contrast, are degraded by the proteasome and other enzymes in the cytosol, with the resulting peptides translocating to endosomal and lysosomal compartments for intersection with class II molecules. Processing and editing of these antigenic peptides within endosomes and lysosomes may be critical in regulating their display via class II proteins. Multiple pathways may regulate the transit of cytosolic peptides to class II molecules. The role of lysosome-associated membrane protein-2a and heat-shock cognate protein 70 in promoting cytoplasmic peptide presentation by MHC class II molecules is discussed.

MHC immunoevasins: protecting the pathogen reservoir in infection

Alteration of antigen recognition by T cells as result of insufficient major histocompatibility complex (MHC)-dependent antigen-presenting function has been observed in many cases of infections, particularly in in vitro systems. To hide themselves from an efficient immune response, pathogens may act on MHC-related functions at three levels: (i) by limiting the number of potential antigens that can be presented to naïve T cells; (ii) by synthesizing proteins which directly affect MHC cell-surface expression; and (iii) by altering the normal intracellular pathway of peptide loading on MHC. Here, we review examples of pathogens' action on each single step of MHC function and we suggest that the result of these often synergistic actions is both a limitation of the priming of naïve T cells and, more importantly, a protection of the pathogen's reservoir from the attack of primed T cells. The above mechanisms may also generate a skewing effect on immune effector mechanisms, which helps preserving the reservoir of infection from sterilization by the immune system.

A cytosolic pathway for MHC class II–restricted antigen processing that is proteasome and TAP dependent

By convention, presentation of major histocompatibility complex (MHC) class I-restricted epitopes involves processing by cytosolic proteasomes, whereas MHC class II-restricted epitopes are generated by endosomal proteases. Here, we show that two MHC class II-restricted epitopes within influenza virus were generated by a proteasome- and TAP-dependent pathway that was accessed by exogenous virus in dendritic cells (DCs) but not cell types with less permeable endosomes. Both epitopes were presented by recycling MHC class II molecules. Challenging mice with influenza or vaccinia viruses demonstrated that a substantial portion of the MHC class II-restricted response was directed against proteasome-dependent epitopes. By complementing endosomal activities, this pathway broadens the array of MHC class II-restricted epitopes available for CD4(+) T cell activation.