The relationship between antigen concentration, antigen internalization, and antigenic complexes: modeling insights into antigen processing and presentation

Differential Regulation of Two Arms of mTORC1 Pathway Fine-Tunes Global Protein Synthesis in Resting B Lymphocytes

Protein synthesis is tightly regulated by both gene-specific and global mechanisms to match the metabolic and proliferative demands of the cell. While the regulation of global protein synthesis in response to mitogen or stress signals is relatively well understood in multiple experimental systems, how different cell types fine-tune their basal protein synthesis rate is not known. In a previous study, we showed that resting B and T lymphocytes exhibit dramatic differences in their metabolic profile, with implications for their post-activation function. Here, we show that resting B cells, despite being quiescent, exhibit increased protein synthesis in vivo as well as ex vivo. The increased protein synthesis in B cells is driven by mTORC1, which exhibits an intermediate level of activation in these cells when compared with resting T cells and activated B cells. A comparative analysis of the transcriptome and translatome of these cells indicates that the genes encoding the MHC Class II molecules and their chaperone CD74 are highly translated in B cells. These data suggest that the translatome of B cells shows enrichment for genes associated with antigen processing and presentation. Even though the B cells exhibit higher mTORC1 levels, they prevent the translational activation of TOP mRNAs, which are mostly constituted by ribosomal proteins and other translation factors, by upregulating 4EBP1 levels. This mechanism may keep the protein synthesis machinery under check while enabling higher levels of translation in B cells.

The Delivery of mRNA Vaccines for Therapeutics

mRNA vaccines have been revolutionary in combating the COVID-19 pandemic in the past two years. They have also become a versatile tool for the prevention of infectious diseases and treatment of cancers. For effective vaccination, mRNA formulation, delivery method and composition of the mRNA carrier play an important role. mRNA vaccines can be delivered using lipid nanoparticles, polymers, peptides or naked mRNA. The vaccine efficacy is influenced by the appropriate delivery materials, formulation methods and selection of a proper administration route. In addition, co-delivery of several mRNAs could also be beneficial and enhance immunity against various variants of an infectious pathogen or several pathogens altogether. Here, we review the recent progress in the delivery methods, modes of delivery and patentable mRNA vaccine technologies.

Formulation and Delivery Technologies for mRNA Vaccines

mRNA vaccines have become a versatile technology for the prevention of infectious diseases and the treatment of cancers. In the vaccination process, mRNA formulation and delivery strategies facilitate effective expression and presentation of antigens, and immune stimulation. mRNA vaccines have been delivered in various formats: encapsulation by delivery carriers, such as lipid nanoparticles, polymers, peptides, free mRNA in solution, and ex vivo through dendritic cells. Appropriate delivery materials and formulation methods often boost the vaccine efficacy which is also influenced by the selection of a proper administration route. Co-delivery of multiple mRNAs enables synergistic effects and further enhances immunity in some cases. In this chapter, we overview the recent progress and existing challenges in the formulation and delivery technologies of mRNA vaccines with perspectives for future development.

Hypochlorous acid-modified human serum albumin suppresses MHC class II - dependent antigen presentation in pro-inflammatory macrophages

Macrophages are innate immune cells that internalize and present exogenous antigens to T cells via MHC class II proteins. They operate at sites of infection in a highly inflammatory environment, generated in part by reactive oxygen species, in particular the strong oxidant hypochlorous acid (HOCl) produced in the neutrophil respiratory burst. HOCl effectively kills a broad range of pathogens but can also contribute to host tissue damage at sites of inflammation. To prevent tissue injury, HOCl is scavenged by human serum albumin (HSA) and other plasma proteins in interstitial fluids, leading to the formation of variously modified advanced oxidation products (AOPPs) with pro-inflammatory properties. Previously, we showed that HOCl-mediated N-chlorination converts HSA and other plasma proteins into efficient activators of the phagocyte respiratory burst, but the role of these AOPPs in antigen presentation by macrophages remained unclear. Here, we show that physiologically relevant amounts of N-chlorinated HSA can strongly impair the capacity of THP-1-derived macrophages to present antigens to antigen-specific T cells via MHC class II proteins at multiple stages. Initially, N-chlorinated HSA inhibits antigen internalization by converting antigens into scavenger receptor (SR) ligands and competing with the modified antigens for binding to SR CD36. Later steps of antigen presentation, such as intracellular antigen processing and MHC class II expression are negatively affected, as well. We propose that impaired processing of pathogens or exogenous antigens by immune cells at an initial stage of infection prevents antigen presentation in an environment potentially hostile to cells of the adaptive immune response, possibly shifting it towards locations removed from the actual insult, like the lymph nodes. On the flip side, excessive retardation or complete inhibition of antigen presentation by N-chlorinated plasma proteins could contribute to chronic infection and inflammation.

Surface conjugation of EP67 to biodegradable nanoparticles increases the generation of long-lived mucosal and systemic memory T-cells by encapsulated protein vaccine after respiratory immunization and subsequent T-cell-mediated protection against respiratory infection

Encapsulation of protein vaccines in biodegradable nanoparticles (NP) increases T-cell expansion after mucosal immunization but requires incorporating a suitable immunostimulant to increase long-lived memory T-cells. EP67 is a clinically viable, host-derived peptide agonist of the C5a receptor that selectively activates antigen presenting cells over neutrophils. We previously found that encapsulating EP67-conjugated CTL peptide vaccines in NP increases long-lived memory subsets of CTL after respiratory immunization. Thus, we hypothesized that alternatively conjugating EP67 to the NP surface can increase long-lived mucosal and systemic memory T-cells generated by encapsulated protein vaccines. We found that respiratory immunization of naïve female C57BL/6 mice with LPS-free ovalbumin (OVA) encapsulated in PLGA 50:50 NP (∼380 nm diameter) surface-conjugated with ∼0.1 wt% EP67 through 2 kDa PEG linkers (i) increased T-cell expansion and long-lived memory subsets of OVA_323-339-specific CD4⁺ and OVA_257-264-specific CD8a⁺ T-cells in the lungs (CD44^HI/CD127/KLRG1) and spleen (CD44^HI/CD127/KLRG1/CD62L) and (ii) decreased peak CFU of OVA-expressing L. monocytogenes (LM-OVA) in the lungs, liver, and spleen after respiratory challenge vs. encapsulation in unmodified NP. Thus, conjugating EP67 to the NP surface is one approach to increase the generation of long-lived mucosal and systemic memory T-cells by encapsulated protein vaccines after respiratory immunization.

Macropinocytosis: New Insights Into Its Underappreciated Role in Innate Immune Cell Surveillance

Macropinocytosis has received increasing attention in recent years for its various roles in nutrient acquisition, immune surveillance, and virus and cancer pathologies. In most cases macropinocytosis is initiated by the sudden increase in an external stimulus such as a growth factor. This "induced" form of macropinocytosis has been the subject of much of the work addressing its mechanism and function over the years. An alternative, "constitutive" form of macropinocytosis restricted to primary innate immune cells also exists, although its mechanism has remained severely understudied. This mini-review focuses on the very recent advances that have shed new light on the initiation, formation and functional relevance of constitutive macropinocytosis in primary innate immune cells. An emphasis is placed on how this new understanding of constitutive macropinocytosis is helping to define the sentinel function of innate immune cells including polarized macrophages and dendritic cells.

Current challenges for cancer vaccine adjuvant development

INTRODUCTION

Although much progress has been made in the last decade(s) toward development of effective cancer vaccines, there are still important obstacles to therapeutic successes. New generations of cancer vaccines will benefit from a combination adjuvant approach that targets multiple branches of the immune response.

AREAS COVERED

Herein we describe how combinatorial adjuvant strategies can help overcome important obstacles to cancer vaccine development, including antigen immunogenicity and tumor immune suppression. Tumor antigens may be both tolerogenic and may utilize active mechanisms to suppress host immunity, including downregulation of MHC molecules to evade recognition and upregulation of immune inhibitory receptors, to subvert an effective immune response. The current cancer vaccine literature was surveyed to identify advancements in the understanding of the biological mechanisms underlying poor antigen immunogenicity and tumor immune evasion, as well as adjuvant strategies designed to overcome them.

EXPERT COMMENTARY

Poor immunogenicity of tumor antigens and tumor immune evasion mechanisms make the design of cancer vaccines challenging. Growing understanding of the tumor microenvironment and associated immune responses indicate the importance of augmenting not only the effector response, but also overcoming the endogenous regulatory response and tumor evasion mechanisms. Therefore, new vaccines will benefit from multi-adjuvanted approaches that simultaneously stimulate immunity while preventing inhibition.

Using Antigen-Specific B Cells to Combine Antibody and T Cell-Based Cancer Immunotherapy

Cancer immunotherapy by therapeutic activation of T cells has demonstrated clinical potential. Approaches include checkpoint inhibitors and chimeric antigen receptor T cells. Here, we report the development of an alternative strategy for cellular immunotherapy that combines induction of a tumor-directed T-cell response and antibody secretion without the need for genetic engineering. CD40 ligand stimulation of murine tumor antigen-specific B cells, isolated by antigen-biotin tetramers, resulted in the development of an antigen-presenting phenotype and the induction of a tumor antigen-specific T-cell response. Differentiation of antigen-specific B cells into antibody-secreting plasma cells was achieved by stimulation with IL21, IL4, anti-CD40, and the specific antigen. Combined treatment of tumor-bearing mice with antigen-specific CD40-activated B cells and antigen-specific plasma cells induced a therapeutic antitumor immune response resulting in remission of established tumors. Human CEA or NY-ESO-1-specific B cells were detected in tumor-draining lymph nodes and were able to induce antigen-specific T-cell responses in vitro, indicating that this approach could be translated into clinical applications. Our results describe a technique for the exploitation of B-cell effector functions and provide the rationale for their use in combinatorial cancer immunotherapy. Cancer Immunol Res; 5(9); 730-43. ©2017 AACR.

Calcium-sensing receptors signal constitutive macropinocytosis and facilitate the uptake of NOD2 ligands in macrophages

Macropinocytosis can be induced in several cell types by stimulation with growth factors. In selected cell types, notably macrophages and dendritic cells, macropinocytosis occurs constitutively, supporting the uptake of antigens for subsequent presentation. Despite their different mode of initiation and contrasting physiological roles, it is tacitly assumed that both types of macropinocytosis are mechanistically identical. We report that constitutive macropinocytosis is stringently calcium dependent, while stimulus-induced macropinocytosis is not. Extracellular calcium is sensed by G-protein-coupled calcium-sensing receptors (CaSR) that signal macropinocytosis through Gα-, phosphatidylinositol 3-kinase and phospholipase C. These pathways promote the recruitment of exchange factors that stimulate Rac and/or Cdc42, driving actin-dependent formation of ruffles and macropinosomes. In addition, the heterologous expression of CaSR in HEK293 cells confers on them the ability to perform constitutive macropinocytosis. Finally, we show that CaSR-induced constitutive macropinocytosis facilitates the sentinel function of macrophages, promoting the efficient delivery of ligands to cytosolic pattern-recognition receptors.

Macropinocytosis can be induced in several cell types by growth factors to promote nutrient acquisition. Here the authors find that constitutive macropinocytosis, unique to dendritic cells and macrophages, requires the activity of calcium-sensing receptors.

On the coupling of two models of the human immune response to an antigen

The development of mathematical models of the immune response allows a better understanding of the multifaceted mechanisms of the defense system. The main purpose of this work is to present a scheme for coupling distinct models of different scales and aspects of the immune system. As an example, we propose a new model where the local tissue inflammation processes are simulated with partial differential equations (PDEs) whereas a system of ordinary differential equations (ODEs) is used as a model for the systemic response. The simulation of distinct scenarios allows the analysis of the dynamics of various immune cells in the presence of an antigen. Preliminary results of this approach with a sensitivity analysis of the coupled model are shown but further validation is still required.

Tumor-derived autophagosomes (DRibbles) induce B cell activation in a TLR2-MyD88 dependent manner

Previously, we have documented that isolated autophagosomes from tumor cells could efficiently cross-prime tumor-reactive naïve T cells and mediate tumor regression in preclinical mouse models. However, the effect of tumor-derived autophagosomes, here we refer as to DRibbles, on B cells has not been studied so far. At present study, we found that DRibbles generated from a murine hepatoma cell line Hep1-6, induced B-cell activation after intravenous injection into mice. B-cell populations were significantly expanded and the production of Hep1-6 tumor-specific antibodies was successfully induced. Moreover, in vitro studies showed that DRibbles could induce more efficient B-cell proliferation and activation, antibody production, and cytokine secretion than whole tumor cell lysates. Notably, we found that B-cell activation required proteins but not DNA in the DRibbles. We further showed that B cells could capture DRibbles and present antigens in the DRibbles to directly induce T cell activation. Furthermore, we found that B-cell activation, antibody production, cytokine secretion and antigen cross-presentation were TLR2-MyD88 pathway dependent. Taken together, the present studies demonstrated that tumor-derived autophagosomes (DRibbles) efficiently induced B cells activation, antibody production, cytokine secretion and antigen cross-presentation mainly depending on their protein component via TLR2/MyD88 dependent manner.

Sequence and expression analysis of cathepsin S gene in the miiuy croaker Miichthys miiuy

Cathepsin S (CTSS) is a lysosomal cysteine endopeptides of the papain family. In the present study, CTSS gene was isolated from miiuy croaker (Miichthys miiuy) by expressed sequence tag analysis. The complete miiuy croaker CTSS cDNA comprised 1,454 nucleotides, including 56 bp at the 5'-UTR and 394 bp at the 3'-UTR. The open reading frame is 1,017 bp, and it encodes 338 amino acid residues with a calculated molecular weight of 37.1 kDa. BLAST analysis revealed that miiuy croaker cathepsin S shared high similarity with other known cathepsin S, and it showed significant homology with that of Japanese flounder, the degree of conservation of the miiuy croaker CTSS nucleotide sequence in comparison with other teleost species ranged from 63.0 to 81.7%. The N-linked glycosylation sites and catalytic sites are conserved in miiuy croaker CTSS. The CTSS transcript was expressed in all tissues examined; high levels of transcripts of CTSS were detected liver, muscle, and fin. These results provide important information for further exploring the roles of cathepsin S in antigen processing.

Inhibition of B cell-mediated antigen presentation by intravenous immunoglobulins (IVIg)

Previous work from our laboratory revealed that IVIg interacted with intracellular proteins involved in antigen presentation in B cells, suggesting that IVIg might interfere with the process of antigen presentation in these cells. In the present work, we used an in vitro assay with ovalbumin as model antigen and showed that IVIg inhibited both BCR-dependent and BCR-independent antigen presentation. The inhibition could not be explained by a modulation of expression of MHC II molecules expressed on B cells and was shown to occur in an FcgammaRIIb-independent manner, suggesting that the events responsible for the inhibitory effect occur at the intracellular level. This was supported by the observation of a direct correlation between the level of spontaneous internalization of two different proteins (IVIg and HSA) and their inhibitory potential. The inhibition of B cell-mediated antigen presentation reported here may help explain some of the anti-inflammatory effects of IVIg observed in treated patients, such as a decrease in autoantibody production.

Channel catfish, Ictalurus punctatus, cysteine proteinases: cloning, characterisation and expression of cathepsin H and L

The antigen recognition by the host immune system is a complex biochemical process that requires a battery of enzymes. Cathepsins are one of the enzyme superfamilies involved in antigen degradation. We observed the up-regulation of catfish cathepsin H and L transcripts during the early stage of Edwardsiella ictaluri infection in preliminary studies, and speculated that cathepsin H and L may play roles in infection. We identified, sequenced and characterized the complete channel catfish cathepsin H and L cDNAs, which comprised 1415 and 1639 nucleotides, respectively. The open reading frames of cathepsin H appeared to encode a protein of 326-amino acid residues, which that of cathepsin L encoded a protein of 336 amino acids. The degree of conservation of the channel catfish cathepsin H and L amino acid sequences in comparison to other species ranged from 61% to 77%, and 67% to 85%, respectively. The catalytic triad and substrate binding sites are conserved in cathepsin H and L amino acid sequences. The cathepsin L transcript was expressed in all tissues examined, while the cathepsin H was expressed in restricted tissues. These results provide important information for further exploring the roles of channel catfish cathepsins in antigen processing.

Effect of multiple genetic polymorphisms on antigen presentation and susceptibility to Mycobacterium tuberculosis infection

Several molecules related to antigen presentation, including gamma interferon (IFN-gamma) and the major histocompatibility complex (MHC), are encoded by polymorphic genes. Some polymorphisms were found to affect susceptibility to tuberculosis (TB) when they were considered singly in epidemiological studies, but how multiple polymorphisms interact to determine susceptibility to TB in an individual remains an open question. We hypothesized that polymorphisms in some genes may counteract or intensify the effects of polymorphisms in other genes. For example, an increase in IFN-gamma expression may counteract the weak binding that a particular MHC variant displays for a peptide from Mycobacterium tuberculosis to establish the same T-cell response as another, more strongly binding MHC variant. To test this hypothesis, we developed a mathematical model of antigen presentation based on experimental data for the known effects of genetic polymorphisms and simulated time courses when multiple polymorphisms were present. We found that polymorphisms in different genes could affect antigen presentation to the same extent and therefore compensate for each other. Furthermore, we defined the conditions under which such relationships could exist. For example, increased IFN-gamma expression compensated for decreased peptide-MHC affinity in the model only above a certain threshold of expression. Below this threshold, changes in IFN-gamma expression were ineffectual compared to changes in peptide-MHC affinity. The finding that polymorphisms exhibit such relationships could explain discrepancies in the epidemiological literature, where some polymorphisms have been inconsistently associated with susceptibility to TB. Furthermore, the model allows polymorphisms to be ranked by effect, providing a new tool for designing association studies.

Toward a multiscale model of antigen presentation in immunity

A functioning immune system and the process of antigen presentation in particular encompass events that occur at multiple length and time scales. Despite a wealth of information in the biological literature regarding each of these scales, no single representation synthesizing this information into a model of the overall immune response as it depends on antigen presentation is available. In this article, we outline an approach for integrating information over relevant biological and temporal scales to generate such a representation for major histocompatibility complex class II-mediated antigen presentation. In addition, we begin to address how such models can be used to answer questions about mechanisms of infection and new strategies for treatment and vaccines.

BCR targeting of biotin-α-galactosylceramide leads to enhanced presentation on CD1d and requires transport of BCR to CD1d-containing endocytic compartments

CD1d is a non-polymorphic MHC class I-related protein that binds and presents glycolipid antigens to T cell antigen receptors expressed by NK-like T (NKT) cells. CD1d-dependent immune responses play critical roles in infectious disease, autoimmunity, allergy and cancer. We tested the hypothesis that B cell antigen receptor (BCR) targeting of a biotin-modified version of the CD1d-binding antigen alpha-galactosylceramide (biotin-alpha-GalCer) results in enhanced murine CD1d-mediated presentation as compared with presentation of non-targeted biotin-alpha-GalCer. Presentation of BCR-targeted antigen to NKT cells was enhanced 100- to 1000-fold compared with non-targeted antigen. CD1d presentation of BCR-targeted antigen was observed after 4 h treatment, consistent with a requirement for endosomal trafficking. Furthermore, unlike non-targeted antigen, BCR-targeted antigen was not loaded directly onto cell-surface CD1d. Blocking BCR signaling with the Syk tyrosine kinase inhibitor piceatannol inhibited presentation of BCR-targeted antigen but not non-targeted antigen. Piceatannol blocked transport of the BCR to CD1d-containing endosomes, showing that intersection of BCR-targeted antigen with endosomes is required for enhanced mCD1d antigen presentation. Our data suggest that the BCR facilitates capture of low quantities of mCD1d antigens to enhance CD1d-dependent immune responses.

Antibodies to squalene in recipients of anthrax vaccine

We previously reported that antibodies to squalene, an experimental vaccine adjuvant, are present in persons with symptoms consistent with Gulf War Syndrome (GWS) (P. B. Asa et al., Exp. Mol. Pathol 68, 196-197, 2000). The United States Department of Defense initiated the Anthrax Vaccine Immunization Program (AVIP) in 1997 to immunize 2.4 million military personnel. Because adverse reactions in vaccinated personnel were similar to symptoms of GWS, we tested AVIP participants for anti-squalene antibodies (ASA). In a pilot study, 6 of 6 vaccine recipients with GWS-like symptoms were positive for ASA. In a larger blinded study, only 32% (8/25) of AVIP personnel compared to 15.7% (3/19) of controls were positive (P > 0.05). Further analysis revealed that ASA were associated with specific lots of vaccine. The incidence of ASA in personnel in the blinded study receiving these lots was 47% (8/17) compared to an incidence of 0% (0/8; P < 0.025) of the AVIP participants receiving other lots of vaccine. Analysis of additional personnel revealed that in all but one case (19/20; 95%), ASA were restricted to personnel immunized with lots of vaccine known to contain squalene. Except for one symptomatic individual, positive clinical findings in 17 ASA-negative personnel were restricted to 4 individuals receiving vaccine from lots containing squalene. ASA were not present prior to vaccination in preimmunization sera available from 4 AVIP personnel. Three of these individuals became ASA positive after vaccination. These results suggest that the production of ASA in GWS patients is linked to the presence of squalene in certain lots of anthrax vaccine.

Antigen binding to GM1 ganglioside results in delayed presentation: minimal effects of GM1 on presentation of antigens internalized via other pathways

Plasma membrane rafts are sphingolipid- and cholesterol-rich patches that function as membrane trafficking and surface signalling regions. Ganglioside GM1 is an integral component of these microdomains, and Escherichia coli enterotoxin B subunit (EtxB) is a pentamer that binds with high affinity to GM1 resulting in GM1 cross-linking. We previously demonstrated that antigen coupled directly to EtxB resulted in enhanced presentation relative to antigen taken up by fluid-phase endocytosis. Here we demonstrate a new role for GM1 in antigen presentation by examining the effects of cross-linking GM1 on the kinetics of presentation and processing of antigen by the B-cell receptor (BCR), fluid-phase endocytosis and GM1-targeted antigen. EtxB bound to B cells does not augment the subsequent kinetics or magnitude of presentation of either BCR-internalized antigen or soluble antigen. Moreover, presentation of GM1-bound antigen is significantly slower than antigen presentation following BCR-mediated uptake. In contrast to the rapid internalization of BCR-bound antigen (which has a half life of 60 min), the majority of EtxB-bound antigen forms a plasma membrane depot detectable for many hours after initial incubation (and with a half life of 12 hr). We conclude that cross-linking of GM1 by EtxB minimally affects the processing and presentation of antigens internalized via other pathways. Nevertheless, binding of antigens to GM1 results in delayed presentation that has important implications for in vivo immunization using GM1-targeted adjuvants.

Modulation of TCR recognition of MHC class II/peptide by processed remote N- and C-terminal epitope extensions

N- and C-terminal extensions of naturally processed MHC class II-bound peptides may affect TCR recognition. In fact, residues immediately flanking the minimal epitope on either side can contact the MHC groove and modify the interaction with a TCR. We report now that residues much farther away from the peptide core can also modulate TCR recognition in a functional antigen presentation system. To show this, we isolated from the same donor DR5-restricted T cell clones, specific for the HIV-1 RT(248-262) sequence and differing in their ability to respond to recombinant antigens obtained by insertion of the epitope in different positions of schistosomal, human, or murine glutathione-S-transferase (GST). We found that the reactivity profile of individual clones was related to their TCR fine specificity, suggesting that processing can generate determinants focused onto the same epitope, but antigenically distinct. In addition, we analyzed the response of this panel of T-helper cell clones against GST-derived recombinant antigens in which the epitope was flanked by stretches of polyalanine or polyserine on either side. These spacers had different effects on TCR recognition suggesting that secondary structures outside the core peptide may influence MHC/epitope complex recognition over a distance of 15-30 residues from the determinant.

Delta(9)-tetrahydrocannabinol selectively increases aspartyl cathepsin D proteolytic activity and impairs lysozyme processing by macrophages

Delta(9)-tetrahydrocannabinol (THC) causes an antigen-dependent defect in the ability of macrophages to activate helper T cells, and this drug-induced impairment is mediated through the peripheral CB2 receptor. Various requirements for the processing of the antigen, lysozyme, were examined to determine where along the pathway THC exerts its influence. A THC-exposed macrophage hybridoma inefficiently stimulated interleukin-2 secretion by a helper T cell hybridoma in response to native lysozyme and its reduced form, suggesting that disulfide bond reduction was unaffected. Cell surface expression of major histocompatibility complex class II molecules was normal on THC-exposed macrophages. The drug-exposed macrophages also competently presented a lysozyme peptide to the T cells, indicating that the class II molecules were functional. The proteolytic activity of two thiol cathepsins was unaltered, but aspartyl cathepsin D activity was significantly increased in THC-exposed macrophages. Thus, selective up-regulation of aspartyl cathepsin activity accompanied the deficiency in lysozyme processing and may contribute, at least in part, to the antigen-dependent processing defect in THC-exposed macrophages.

Gallium Arsenide Modulates Proteolytic Cathepsin Activities and Antigen Processing by Macrophages

Gallium arsenide (GaAs) is a semiconductor utilized in the electronics industry. Chemical exposure of animals causes a local inflammatory reaction, but systemic immunosuppression. Mice were administered i.p. 200 mg/kg GaAs crystals or latex beads, or vehicle. Five days after exposure, splenic macrophages were defective, whereas thioglycolate-elicited peritoneal macrophages (PEC) were more efficient in processing the Ag, pigeon cytochrome c, than vehicle control macrophages. Various aspects of the MHC class II Ag-processing pathway were examined. Both macrophage populations normally presented a peptide fragment to the CD4+ T cells. Surface MHC class II expression on the PEC was up-regulated, but splenic cells had normal MHC class II expression. PEC had elevated levels of glutathione and cysteine, major physiologic reducing thiols. However, the cysteine content of splenic macrophages was diminished. Proteolytic activities of aspartyl cathepsin D, and thiol cathepsins B and L were decreased significantly in splenic macrophages. On the other hand, thiol cathepsin activities were increased selectively in PEC. Latex bead-exposed PEC were not more potent APC, and their thiol cathepsin activities were unchanged, indicating that phagocytosis and nonspecific irritation were not responsible. The phenotype of PEC directly exposed to GaAs mirrored cytokine-activated macrophages, in contrast to splenic macrophages from a distant site. Therefore, GaAs exposure differentially modulated cathepsin activities in splenic macrophages and PEC, which correlated with their Ag-processing efficiency. Perhaps such distinct alterations may contribute to the local inflammation and systemic immunotoxicity caused by chemical exposure.

Calcium response of helper T lymphocytes to antigen-presenting cells in a single-cell assay

We developed a dynamic, single-cell assay involving alternating differential interference contrast and fluorescence microscopy, together with digital imaging, for both viewing the physical interaction of live helper T lymphocytes (Th cells) with antigen-presenting cells (APCs) and monitoring the increases in the intracellular free calcium concentration of the Th cell, an early event in Th cell activation. We obtained Th-APC conjugates by allowing the Th cells to migrate toward and interact with APCs that either settled nearby or had been micromanipulated in close proximity to the Th cells. Th cell motility played an important role in initiating Th-APC contacts but not in determining the Th cell calcium response. We found that the intracellular calcium responses of individual Th cells are heterogeneous and an all-or-none phenomenon, independent of antigen concentration. However, the fraction of Th-APC conjugates involving responding Th cells is an increasing function of the antigen concentration. Finally, we measured some characteristics of the developing Th-APC contact area. We used all of these data together with previously developed mathematical models to estimate that only 1 to 20 major histocompatibility class II-antigen complexes are required in the initial Th-APC contact area to elicit a Th cell calcium response.

Treatment of leukemia with radiolabeled monoclonal antibodies

In contrast to radioimmunotherapy of solid disease, wherein the primary obstacle to success is access of radiolabeled antibody to antigen-positive cells, in the treatment of leukemia delivering a lethal absorbed dose to the isolated cell appears to be the primary obstacle. The isolated cell is defined as one that is exposed only to self-irradiation (from internalized or surface-bound radiolabeled antibody) and to irradiation from free antibody in the blood. It is isolated in the sense that the particulate (beta, electron, alpha) emissions from its nearest neighboring antigen-positive cell do not contribute to its absorbed dose. Disease in the bone marrow and other tissues, since it is confined to a smaller volume, is more easily eradicated because the absorbed dose to a given cell nucleus is enhanced by emissions from adjacent cells (a smaller fraction of the emission energy is 'wasted'). The optimization simulations presented above for the M195 antibody suggest that the optimum dose of antibody that should be administered is that required to yield a concentration within the distribution volume of the antibody that is approximately equal to the concentration of antigen sites as determined by the tumor burden. Although not specifically considered in the modeling example presented above, antibody internalization and catabolism may be expected to play an important role in radioimmunotherapy treatment planning of leukemia. Depending upon the kinetics of internalization and catabolism, the absorbed dose to the red marrow and to antigen-positive cells may be reduced considerably, since catabolism, assuming that it is followed by rapid extrusion of the radioactive label, would decrease the cells' exposure time considerably. The recently demonstrated effectiveness of radioimmunotherapy in certain cases of B-cell lymphoma and in reducing tumor burden in acute myelogenous leukemia suggests that radioimmunotherapy is beginning to fulfill the promise held when it was initially conceived. The long delay in achieving reproducible success has, in large part, been the result of the conceptual simplicity of using agents that specifically 'target' tumor cells and they may thus selectively deliver cytotoxic agents. Emboldened by this apparent simplicity, early trials of radioimmunotherapy failed to consider the many variables involved in its implementation. As has been recently demonstrated using mathematical models of antibody delivery to solid tumor, chief among these may have been the failure to select the appropriate tumor type. By significantly reducing the problems associated with antibody delivery, hematopoietic malignancies offer the optimum conditions for successful radioimmunotherapy. As evinced by the wide range of antibody and radioactivity doses administered in the B-cell lymphoma trials, the case-specific nature of radioimmunotherapy requires an understanding of the relationship between the various input parameters and patient response. The complexity and interrelationship of these parameters precludes an experimental trial-and-error approach to their optimization. A stepwise approach to radioimmunotherapy treatment planning is proposed in which a model of antibody kinetics is developed and validated.(ABSTRACT TRUNCATED AT 400 WORDS)

Cultured human Langerhans cells process and present intact protein antigens

Epidermal Langerhans cells (LC) undergo profound phenotypic and functional alterations when cultured for 2 to 3 d. To determine whether the in vitro culture of human LC modulates their capacity to process and present intact protein antigens, we compared the ability of freshly isolated LC (fLC) and cultured LC (cLC) to stimulate in vitro T-cell proliferative responses to recall antigens. We found that human fLC and cLC were able to process and present recall antigens to primed T cells, inducing significant proliferative responses. For tetanus toxoid and Candida albicans extract, T-cell proliferative responses at 6 d to antigen-pulsed fLC were slightly greater than responses to antigen-pulsed cLC. For live influenza A virus, the T-cell responses induced by antigen-pulsed cLC were comparable or slightly greater compared with fLC. Allogeneic T-cell proliferation for both LC preparations were also comparable. The exogenous pathway of antigen processing was demonstrated by chloroquine inhibition.

Internalization of CD4 molecules in human T-cells demonstrated by immuno-electron microscopy

Internalization of CD4 molecules on human CD4-enriched T-cells was demonstrated by immunocytochemical electron microscopy. CD4+ T-cell subclones were obtained from normal human peripheral blood, followed by one-way MLC screening and co-culturing with IL-2. Fixed and non-fixed T-cell samples were indirectly immunolabeled with mouse anti-human CD4 monoclonal antibody and goat anti-mouse IgG conjugated with peroxidase. Unfixed T-cells were immunolabeled at 4 degrees C and then re-incubated for 5-45 min at 37 degrees C. The selected CD4+ T-cell subclones showed strong CD4 binding on the cell surface after IL-2 incubation. However, fresh T-cells, monocytes, bone marrow cells and CD8+ T-cells all stained negative for CD4. The distribution of CD4 molecules on the fixed cell surface showed a homogeneous pattern. Capping and internalization of CD4-antibody-peroxidase complexes from the cell surfaces were observed follow a pathway of receptor-mediated endocytosis in unfixed T cells. Endocytotic vesicles, vacuoles of diverse sizes and shapes near the cell membrane or deep in the cell center were found to contain CD4 molecules. Negatively stained Golgi saccules were observed up to 45 min after re-incubation. These results suggest that increased CD4 molecules can be induced on the surface of normal human T-cells in vitro. Internalization and accumulation of CD4 molecules occurred in CD4-enriched T-cells with IL-2 pretreatment.

Antigen processing and presentation: how can a foreign antigen be recognized in a sea of self proteins?

A mathematical model describing the time-dependent events of antigen processing and presentation is utilized to quantitatively analyze the importance of newly synthesized Ia molecules as well as Ia molecules internalized from the cell surface in the formation of Ia-antigen complexes, the T cell receptor ligand. It has recently been shown that antigen presenting cells are not selective for the proteins they process and present. Therefore, we also investigate the ability of macrophages and B cells to process and present antigen in the presence of competing proteins often present in the extracellular environment. A set of criteria is formulated based upon experimental data to determine the validity of two model variations. We draw two major conclusions from our simulations. First, we determine that macrophages and B cells can present between 1-3 Ia-antigen complexes micron-2 for antigen concentrations in the range of 4-7 microM while in the presence of approximately 0-10 microM competing proteins or peptides. Second, we find it likely that antigen presenting cells, both B cells and macrophages, need to internalize Ia molecules from the cell surface in order for a sufficient number of Ia-antigen complexes to be presented. Binding of antigen to newly synthesized Ia alone does not, given experimentally reported values for Ia synthesis, allow sufficient Ia-antigen complex formation.

Molecules that modify antigen recognition

MHC class II molecules assemble in the presence of invariant chains. These probably serve not only to protect the peptide-binding site on MHC class II molecules from endogenous peptides, but also to sort MHC class II molecules from the Golgi complex to endosomes and there to retain the class II polypeptides to allow binding of peptides generated from exogenous antigens.