The efficiency of antigen delivery from macrophage phagosomes into cytoplasm for MHC class I-restricted antigen presentation

The Delivery of Extracellular “Danger” Signals to Cytosolic Sensors in Phagocytes

Phagocytes, such as macrophages and dendritic cells, possess the ability to ingest large quantities of exogenous material into membrane-bound endocytic organelles such as macropinosomes and phagosomes. Typically, the ingested material, which consists of diverse macromolecules such as proteins and nucleic acids, is delivered to lysosomes where it is digested into smaller molecules like amino acids and nucleosides. These smaller molecules can then be exported out of the lysosomes by transmembrane transporters for incorporation into the cell’s metabolic pathways or for export from the cell. There are, however, exceptional instances when undigested macromolecules escape degradation and are instead delivered across the membrane of endocytic organelles into the cytosol of the phagocyte. For example, double stranded DNA, a damage associated molecular pattern shed by necrotic tumor cells, is endocytosed by phagocytes in the tumor microenvironment and delivered to the cytosol for detection by the cytosolic “danger” sensor cGAS. Other macromolecular “danger” signals including lipopolysaccharide, intact proteins, and peptidoglycans can also be actively transferred from within endocytic organelles to the cytosol. Despite the obvious biological importance of these processes, we know relatively little of how macromolecular “danger” signals are transferred across endocytic organelle membranes for detection by cytosolic sensors. Here we review the emerging evidence for the active cytosolic transfer of diverse macromolecular “danger” signals across endocytic organelle membranes. We will highlight developing trends and discuss the potential molecular mechanisms driving this emerging phenomenon.

Regulation of Antigen Export to the Cytosol During Cross-Presentation

Cross-priming refers to the induction of primary cytotoxic CD8⁺ T cell responses to antigens that are not expressed in antigen presenting cells (APCs) responsible for T cell priming. Cross-priming is achieved through cross-presentation of exogenous antigens derived from tumors, extracellular pathogens or infected neighboring cells on Major Histocompatibility Complex (MHC) class I molecules. Despite extensive research efforts to understand the intracellular pathways involved in antigen cross-presentation, certain critical steps remain elusive and controversial. Here we review recent advances on antigen cross-presentation, focusing on the mechanisms involved in antigen export to the cytosol, a crucial step of this pathway.

The Vacuolar Pathway in Macrophages Plays a Major Role in Antigen Cross-Presentation Induced by the Pore-Forming Protein Sticholysin II Encapsulated Into Liposomes

Cross-presentation is an important mechanism for the differentiation of effector cytotoxic T lymphocytes (CTL) from naïve CD8⁺ T-cells, a key response for the clearance of intracellular pathogens and tumors. The liposomal co-encapsulation of the pore-forming protein sticholysin II (StII) with ovalbumin (OVA) (Lp/OVA/StII) induces a powerful OVA-specific CTL activation and an anti-tumor response in vivo. However, the pathway through which the StII contained in this preparation is able to induce antigen cross-presentation and the type of professional antigen presenting cells (APCs) involved have not been elucidated. Here, the ability of mouse bone marrow-derived dendritic cells (BM-DCs) and macrophages (BM-MΦs) stimulated with Lp/OVA/StII to activate SIINFEKL-specific B3Z CD8⁺ T cells was evaluated in the presence of selected inhibitors. BM-MΦs, but not BM-DCs were able to induce SIINFEKL-specific B3Z CD8⁺ T cell activation upon stimulation with Lp/OVA/StII. The cross-presentation of OVA was markedly decreased by the lysosome protease inhibitors, leupeptin and cathepsin general inhibitor, while it was unaffected by the proteasome inhibitor epoxomicin. This process was also significantly reduced by phagocytosis and Golgi apparatus function inhibitors, cytochalasin D and brefeldin A, respectively. These results are consistent with the concept that BM-MΦs internalize these liposomes through a phagocytic mechanism resulting in the cross-presentation of the encapsulated OVA by the vacuolar pathway. The contribution of macrophages to the CTL response induced by Lp/OVA/StII in vivo was determined by depleting macrophages with clodronate-containing liposomes. CTL induction was almost completely abrogated in mice depleted of macrophages, demonstrating the relevance of these APCs in the antigen cross-presentation induced by this formulation.

Recent developments in intracellular protein delivery

Protein therapeutics based on transcription factors, gene editing enzymes, signaling proteins and protein antigens, have the potential to provide cures for a wide number of untreatable diseases, but cannot be developed into therapeutics due to challenges in delivering them into the cytoplasm. There is therefore great interest in developing strategies that can enable proteins to enter the cytoplasm of cells. In this review article we will discuss recent progress in intracellular protein therapeutics, which are focused on the following four classes of therapeutics, Firstly, vaccine development, secondly, transcription factor therapies, thirdly, gene editing and finally, cancer therapeutics. These exciting new advances raise the prospect of developing cures for several un-treatable diseases.

Polymer blend particles with defined compositions for targeting antigen to both class I and II antigen presentation pathways

Defense against many persistent and difficult-to-treat diseases requires a combination of humoral, CD4(+) , and CD8(+) T-cell responses, which necessitates targeting antigens to both class I and II antigen presentation pathways. In this study, polymer blend particles are developed by mixing two functionally unique polymers, poly(lactide-co-glycolide) (PLGA) and a pH-responsive polymer, poly(dimethylaminoethyl methacrylate-co-propylacrylic acid-co-butyl methacrylate) (DMAEMA-co-PAA-co-BMA). Polymer blend particles are shown to enable the delivery of antigens into both class I and II antigen presentation pathways in vitro. Increasing the ratio of the pH-responsive polymer in blend particles increases the degree of class I antigen presentation, while maintaining high levels of class II antigen presentation. In a mouse model, it is demonstrated that a significantly higher and sustained level of CD4(+) and CD8(+) T-cell responses, and comparable antibody responses, are elicited with polymer blend particles than PLGA particles and a conventional vaccine, Alum. The polymer blend particles offer a potential vaccine delivery platform to generate a combination of humoral and cell-mediated immune responses that insure robust and long-lasting immunity against many infectious diseases and cancers.

Immunogenicity and protective effects of inactivated Singapore grouper iridovirus (SGIV) vaccines in orange-spotted grouper, Epinephelus coioides

Vaccination is one of the best methods against viral diseases. In this study, experimental inactivated Singapore grouper iridovirus (SGIV) vaccines were prepared, and immunogenicity and protection against virus infection of the vaccines were investigated in orange-spotted grouper, Epinephelus coioides. Two kinds of vaccines, including β-propiolactone (BPL) inactivated virus at 4°C for 12 h and formalin inactivated virus at 4°C for 12 d, was highly protective against the challenge at 30-day post-vaccination and produced relative percent of survival rates of 91.7% and 100%, respectively. These effective vaccinations induced potent innate immune responses mediated by pro-inflammatory cytokines and type I interferon (IFN)-stimulated genes (ISGs). It is noteworthy that ISGs, such as Mx and ISG15, were up-regulated only in the effective vaccine groups, which suggested that type I IFN system may be the functional basis of early anti-viral immunity. Moreover, effective vaccination also significantly up-regulated of the expression of MHC class I gene and produced substantial amount of specific serum antibody at 4 weeks post-vaccination. Taken together, our results clearly demonstrated that effective vaccination in grouper induced an early, nonspecific antiviral immunity, and later, a specific immune response involving both humoral and cell-mediated immunity.

The pathway of cross-presentation is influenced by the particle size of phagocytosed antigen

Cross-presentation is the presentation by MHC class I of antigenic peptides from exogenous proteins that have been internalized and processed by professional antigen-presenting cells, e.g. dendritic cells. We have investigated the influence of particle size and antigen load on cross-presentation following antigen delivery on microspheres (MS). Cross-presentation from small particles (0·8-μm) is sensitive to proteasome inhibition and the blockade of endoplasmic reticulum-resident MHC class I complex export, whereas cross-presentation from larger particles (aggregated clumps of 0·8-μm MS) is resistant to these antagonists. This observation may have been overlooked previously, because of the heterogeneity of particle size and MS uptake in unsorted dendritic cell populations. Larger particles carry more antigen, but we show that antigen load does not influence the cross-presentation pathway used. Whereas early endosome autoantigen 1 (EEA1) could be observed in all phagosomes, we observed endoplasmic reticulum SNARE of molecular weight 24 000 (ERS24) and cathepsin S in association with 3·0-μm and aggregated 0·8-μm MS, but not individual 0·8-μm MS. A potential mechanism underlying our observations may be the activation of β-catenin by disruption of E-cadherin-mediated adhesion. Activated β-catenin was detected in the cytoplasm of cells after phagocytosis of MS (highest levels for the largest particles). We propose that particle size can direct the use of different pathways for the cross-presentation of an identical antigen. Furthermore, these pathways have differing yields of MHC class I-peptide complexes, which is an important variable in designing vaccination strategies for maximal antigen expression and CD8(+) T-cell priming.

Immunologic basis of vaccine vectors

Efforts to make vaccines against infectious diseases as well as immunotherapies for cancer, autoimmune diseases and allergy have utilized a variety of heterologous expression systems, including viral and bacterial vectors, as well as DNA and RNA constructs. This review explores the immunologic rationale and provides an update of insights obtained from preclinical and clinical studies of such vaccines.

Technical advance: Caspase-1 activation and IL-1β release correlate with the degree of lysosome damage, as illustrated by a novel imaging method to quantify phagolysosome damage

In addition to the lysosome's important roles in digestion, antigen processing, and microbial destruction, lysosome damage in macrophages can trigger cell death and release of the inflammatory cytokine IL-1β. To examine the relationship among endocytosis, lysosome damage, and subsequent events, such as caspase-1 activation and IL-1β secretion, we developed a method for measuring lysosome disruption inside individual living cells, which quantifies release of Fdx from lysosomes. Unperturbed, cultured BMM exhibited low levels of lysosome damage, which were not increased by stimulation of macropinocytosis. Lysosome damage following phagocytosis differed with different types of ingested particles, with negligible damage after ingestion of sRBC ghosts, intermediate damage by polystyrene (PS) beads, and high levels of damage by ground silica. Pretreatment with LPS decreased the amount of lysosome damage following phagocytosis of PS beads, silica microspheres, or ground silica. Activation of caspase-1 and subsequent release of IL-1β were proportional to lysosome damage following phagocytosis. The low level of damage following PS bead phagocytosis was insufficient to activate caspase-1 in LPS-activated macrophages. These studies indicate that lysosome damage following phagocytosis is dependent on particle composition and dose and that caspase-1 activation and IL-1β secretion correlate with the extent of lysosome damage.

Presenting exogenous antigen to T cells

Antigen processing and presentation experiments can be done with a wide variety of antigen-presenting cells (APCs). Most experiments will use one of the "professional" APC types: dendritic cells (DCs), macrophages, and B lymphocytes. Other types of cells may be used for antigen presentation in some circumstances. Each type of professional APC has an important antigen-presentation function, but the different APC types contribute to different aspects of the immune response. Therefore, selection of an APC type for study must include consideration of the stage or aspect of immune response that is to be modeled in the experiment. An important technical distinction for some types of experiments is whether the APCs are adherent or nonadherent, since this dictates the procedures that must be used to wash the cells as the medium is changed.

Sulfated glycosphingolipid as mediator of phagocytosis: SM4s enhances apoptotic cell clearance and modulates macrophage activity

Sulfoglycolipids are present on the surface of a variety of cells. The sulfatide SM4s is increased in lung, renal, and colon cancer and is associated with an adverse prognosis, possibly due to a low immunoreactivity of the tumor. As macrophages significantly contribute to the inflammatory infiltrate in malignancies, we postulated that SM4s may modulate macrophage function. We have investigated the effect of SM4s on the uptake of apoptotic tumor cells, macrophage cytokine profile, and receptor expression. Using flow cytometry and microscopic analyses, we found that coating apoptotic murine carcinoma cells from the colon and kidney with SM4s promoted their phagocytosis by murine macrophages up to 3-fold ex vivo and in vivo. This increased capacity was specifically inhibited by preincubation of macrophages with oxidized or acetylated low density lipoprotein and maleylated albumin, indicating involvement of scavenger receptors in this interaction. The uptake of SM4s-coated apoptotic cells significantly enhanced macrophage production of TGF-beta1, expression of P-selectin, and secretion of IL-6. These data suggest that SM4s within tumors may promote apoptotic cell removal and alter the phenotype of tumor-associated macrophages.

Activation outcomes induced in naïve CD8 T-cells by macrophages primed via "phagocytic" and nonphagocytic pathways

The array of phagocytic receptors expressed by macrophages make them very efficient at pathogen clearance, and the phagocytic process links innate with adaptive immunity. Primary macrophages modulate antigen cross-presentation and T-cell activation. We assessed ex vivo the putative role of different phagocytic receptors in immune synapse formation with CD8 naïve T-cells from OT-I transgenic mice and compared this with the administration of antigen as a soluble peptide. Macrophages that have phagocytosed antigen induce T-cell microtubule-organizing center and F-actin cytoskeleton relocalization to the contact site, as well as the recruitment of proximal T-cell receptor signals such as activated Vav1 and PKC. At the same doses of loaded antigen (1 microM), "phagocytic" macrophages were more efficient than peptide-antigen-loaded macrophages at forming productive immune synapses with T-cells, as indicated by active T-cell TCR/CD3 conformation, LAT phosphorylation, IL-2 production, and T-cell proliferation. Similar T-cell proliferation efficiency was obtained when low doses of soluble peptide (3-30 nM) were loaded on macrophages. These results suggest that the pathway used for antigen uptake may modulate the antigen density presented on MHC-I, resulting in different signals induced in naïve CD8 T-cells, leading either to CD8 T-cell activation or anergy.

PHAGOCYTOSIS: At the Crossroads of Innate and Adaptive Immunity

Phagocytosis, the process by which cells engulf large particles, requires a substantial contribution of membranes. Recent studies have revealed that intracellular compartments, including endocytic organelles and the endoplasmic reticulum (ER), can engage in fusion events with the plasma membrane at the sites of nascent phagosomes. The finding that ER proteins are delivered to phagosomes, where degraded peptides are loaded onto major histocompatibility complex (MHC) class II molecules, has significantly enhanced our understanding of the immune functions associated with these organelles. Although it is well known that pathogens are killed in phagosomes, the contribution of ER proteins to phagosomes has provided a novel pathway for the loading of exogenous peptides onto MHC class I molecules, a process known as cross-presentation. Thus, phagocytosis has evolved from a nutritional function in unicellular organisms to play key roles in both innate and adaptive immunity in vertebrates.

Cross-presentation: underlying mechanisms and role in immune surveillance

It was originally thought that a cell's major histocompatibility complex (MHC) class I molecules presented peptides derived exclusively from proteins synthesized by the cell itself. However, in some circumstances, antigens from the extracellular environment can be presented on MHC class I molecules and stimulate CD8(+) T-cell immunity, a process termed cross-presentation. Cross-presentation was originally discovered as an obscure phenomenon in transplantation immunity. However, it is now clear that it is a major mechanism by which the immune system monitors tissues and phagocytes for the presence of foreign antigen. Cross-presentation is the only pathway by which the immune system can detect and respond to viral infections or mutations that exclusively occur in parenchymal cells rather than in bone marrow-derived antigen-presenting cells (APCs). Professional APCs, such as dendritic cells, are the principal cells endowed with the capacity to cross-present antigens. In this process, the APCs acquire proteins from other tissue cells through endocytic mechanisms, especially phagocytosis or macropinocytosis. The internalized antigen can then be processed through at least two different mechanisms. In one pathway, the antigen is transferred from the phagosome into the cytosol, where it is hydrolyzed by proteasomes into oligopeptides that are then transported by the transporter associated with antigen processing to MHC class I molecules in the endoplasmic reticulum or phagosomes. In a second pathway, the antigen is cleaved into peptides by endosomal proteases, particularly cathepsin S, and bound by class I molecules probably in the endocytic compartment itself. Depending on the nature of the antigen, one or both of these pathways can contribute to cross-presentation in vivo. The outcome of cross-presentation can be either tolerance or immunity. Which of these outcomes occurs is thought to depend on whether antigens are acquired by themselves alone, leading to tolerance, or with immunostimulatory signals, leading to immunity. One source of such signals is from dying cells that release immunostimulatory 'danger' signals that promote the generation of immunity to their cellular antigens. In addition to the critical role of cross-presentation in normal immune physiology, this pathway has considerable potential for being exploited for developing subunit vaccines that elicit both CD4(+) and CD8(+) T-cell immunity.

The uniformity of phagosome maturation in macrophages

Many studies of endocytosis and phagocytosis presume that organelles containing a single kind of internalized particle exhibit invariant patterns of protein and phospholipid association as they mature inside cells. To test this presumption, fluorescent protein chimeras were expressed in RAW 264.7 macrophages, and time-lapse ratiometric fluorescence microscopy was used to measure the maturation dynamics of individual phagosomes containing IgG-opsonized erythrocytes. Quantitative analysis revealed consistent patterns of association for YFP chimeras of beta-actin, Rab5a, Rab7, and LAMP-1, and no association of YFP chimeras marking endoplasmic reticulum or Golgi. YFP-2xFYVE, recognizing phosphatidylinositol 3-phosphate (PI(3)P), showed two patterns of phagosome labeling. Some phagosomes increased labeling quickly after phagosome closure and then lost the label within 20 min, whereas others labeled more slowly and retained the label for several hours. The two patterns of PI(3)P on otherwise identical phagosomes indicated that organelle maturation does not necessarily follow a single path and that some features of phagosome maturation are integrated over the entire organelle.

A macromolecular delivery vehicle for protein-based vaccines: acid-degradable protein-loaded microgels

The development of protein-based vaccines remains a major challenge in the fields of immunology and drug delivery. Although numerous protein antigens have been identified that can generate immunity to infectious pathogens, the development of vaccines based on protein antigens has had limited success because of delivery issues. In this article, an acid-sensitive microgel material is synthesized for the development of protein-based vaccines. The chemical design of these microgels is such that they degrade under the mildly acidic conditions found in the phagosomes of antigen-presenting cells (APCs). The rapid cleavage of the microgels leads to phagosomal disruption through a colloid osmotic mechanism, releasing protein antigens into the APC cytoplasm for class I antigen presentation. Ovalbumin was encapsulated in microgel particles, 200-500 nm in diameter, prepared by inverse emulsion polymerization with a synthesized acid-degradable crosslinker. Ovalbumin is released from the acid-degradable microgels in a pH-dependent manner; for example, microgels containing ovalbumin release 80% of their encapsulated proteins after 5 h at pH 5.0, but release only 10% at pH 7.4. APCs that phagocytosed the acid-degradable microgels containing ovalbumin were capable of activating ovalbumin-specific cytoxic T lymphocytes. The acid-degradable microgels developed in this article should therefore find applications as delivery vehicles for vaccines targeted against viruses and tumors, where the activation of cytoxic T lymphocytes is required for the development of immunity.

Protein degradation and the generation of MHC class I-presented peptides

Over the past decade there has been considerable progress in understanding how MHC class I-presented peptides are generated. The emerging theme is that the immune system has not evolved its own specialized proteolytic mechanisms but instead utilizes the phylogenetically ancient catabolic pathways that continually turnover proteins in all cells. Three distinct proteolytic steps have now been defined in MHC class I antigen presentation. The first step is the degradation of proteins by the ubiquitin-proteasome pathway into oligopeptides that either are of the correct size for presentation or are extended on their amino-termini. In the second step, aminopeptidases trim N-extended precursors into peptides of the correct length to be presented on class I molecules. The third step involves the destruction of peptides by endo- and exopeptidases, which limits antigen presentation, but is important for preventing the accumulation of peptides and recycling them back to amino acids for protein synthesis or production of energy. The immune system has evolved several components that modify the activity of these ancient pathways in ways that enhance the generation of class I-presented peptides. These include catalytically active subunits of the proteasome, the PA28 proteasome activator, and leucine aminopeptidase, all of which are upregulated by interferon-gamma. In addition to these pathways that operate in all cells, dendritic cells and macrophages can also generate class I-presented peptides from proteins internalized from the extracellular fluids by degrading them in endocytic compartments or transferring them to the cyotosol for degradation by proteasomes.

Antigen presentation and T cell stimulation by dendritic cells

Dendritic cells take up antigens in peripheral tissues, process them into proteolytic peptides, and load these peptides onto major histocompatibility complex (MHC) class I and II molecules. Dendritic cells then migrate to secondary lymphoid organs and become competent to present antigens to T lymphocytes, thus initiating antigen-specific immune responses, or immunological tolerance. Antigen presentation in dendritic cells is finely regulated: antigen uptake, intracellular transport and degradation, and the traffic of MHC molecules are different in dendritic cells as compared to other antigen-presenting cells. These specializations account for dendritic cells' unique role in the initiation of immune responses and the induction of tolerance.

Use of overlapping peptide mixtures as antigens for cytokine flow cytometry

Intracellular cytokine staining and flow cytometry can be used to measure T-cell responses to defined antigens. Although CD8+ T-cell responses to soluble proteins are inefficiently detected by this approach, peptides can be used as antigens. Using overlapping peptides spanning an entire protein sequence, CD8+ T-cell responses can be detected to multiple epitopes, regardless of HLA type. In this study, overlapping peptide mixes of various lengths were compared and 15 amino acid peptides with 11 amino acid overlaps were found to stimulate both CD4+ and CD8+ T-cell responses. Such peptide mixes stimulated CD4+ T-cell responses equivalent to those observed with whole recombinant protein, while simultaneously stimulating CD8+ T-cell responses much higher than those observed with whole protein. Although 8-12 amino acid peptides produced the highest level of CD8+ T-cell responses, 15 amino acid peptides were still very effective. Peptides that were 20 amino acids in length, however, did not stimulate strong CD8+ T-cell responses at the same peptide dose. The cytokine responses to individual epitopes added up approximately to the response to the entire mix, demonstrating that large mixes can detect responses in a quantitative fashion. Unlike whole protein antigens, peptide mixes were effective at stimulating responses in both cryopreserved PBMC and blood stored for 24 h at room temperature. Thus, overlapping 15 amino acid peptide mixes may facilitate the analysis of antigen-specific CD4+ and CD8+ T-cell responses by cytokine flow cytometry, using clinical specimens that include shipped blood or cryopreserved PBMC.

Exogenous antigens and the stimulation of MHC class I restricted cell-mediated cytotoxicity: possible strategies for fish vaccines

An MHC class I restricted cytotoxic T lymphocyte (CTL) activity assay has recently been established for rainbow trout. MHC class I restricted cytotoxicity probably plays a critical role in immunity to most viral diseases in mammals and may play a similar role in fish. Therefore, it is very important to investigate what types of vaccines can stimulate this immune response. Although logical candidates for vaccine components that can stimulate an MHC class I restricted response are live attenuated viruses and DNA vaccines, these materials are generally not allowed in fish for commercial vaccine use due to potential safety issues. In mammals, however, a number of interesting vaccination strategies based on exogenous antigens that stimulate MHC class I restricted cytotoxicity have been described. Several of these strategies are discussed in this review in the context of fish vaccination.

Characterization of hybrid CTL epitope delivery systems consisting of the Antennapedia homeodomain peptide vector formulated in liposomes

Peptide carriers, such the homeodomain of Antennapedia molecule (AntpHD), which spontaneously cross cellular membranes, have been exploited to deliver antigenic peptide Cw3 to the major histocompatibility complex (MHC) class-I presentation pathway and to prime cytotoxic T cells (CTL). However, the in vivo use of AntpHD recombinant peptide has been limited because CTLs can only be primed in the presence of sodium dodecyl sulfate (SDS) as adjuvant. In this report, we have exploited liposomes to protect the AntpHD-Cw3 from serum degradation and to facilitate the delivery of the recombinant peptide into the MHC class-I pathway of antigen-presenting cells. We have demonstrated that AntpHD recombinant peptide spontaneously associates with liposomes and this association is stable in vitro. However, exchange studies assessing the transfer of the peptide to model membranes or cells in vitro indicates that approximately 50% of the liposome-associated peptide is readily exchangeable. This is consistent with trypsin-protection assays, which have shown that approximately 40% of the liposome-associated peptide is protected from hydrolysis. Importantly, macrophages and dendritic cells are able to internalize AntpHD recombinant peptide associated with liposomes resulting in efficient delivery of the CTL peptide into the cytosol. These studies have demonstrated that dendritic cells treated with AntpHD-Cw3 in liposomes sensitize CTL clones to lyse syngeneic target cells expressing Cw3 epitope. This strategy, which combines liposomes and a peptide vector, provides a new approach for introducing molecules into the MHC class-I antigen presentation pathway of dendritic cells.

Generation of both MHC class I- and class II-restricted antigenic peptides from exogenously added ovalbumin in murine phagosomes

The phagosome fraction derived from a murine macrophage cell line (J774.1), which had internalized ovalbumin (OVA)-coated latex beads, was isolated. The peptides recovered from the phagosome fraction were separated on reverse phase HPLC and each fraction was analyzed for the content of either major histocompatibility complex (MHC) class I- or class II-restricted OVA-derived peptide. Both peptides were detected in the phagosome fraction after less than 15 min of internalization. It was also indicated that phagosomes degrade OVA protein into both MHC class I- and class II-restricted antigenic peptides by employing the same types of cathepsins. Furthermore, the results suggest that the MHC class I-restricted peptide rapidly exits from the phagosome to the cytosol. These findings illustrate a potential role for phagosomes not only in MHC class II-restricted but also in MHC class I-restricted exogenous antigen presentation pathways. Our results also point to the vital role of phagosomes in non-cytosolic antigen presentation pathway, in which further degradation of antigens by the proteasome is dispensable.

Dense bodies of human cytomegalovirus induce both humoral and cellular immune responses in the absence of viral gene expression

Infection of fibroblast cell cultures with human cytomegalovirus (HCMV) leads to the production of significant amounts of defective enveloped particles, termed dense bodies (DB). These noninfectious structures contain major antigenic determinants which are responsible for induction of both the humoral and the cellular immune response against HCMV. We tested the hypothesis that, by virtue of their unique antigenic and structural properties, DB could induce a significant immune response in the absence of infectious virus. Mice were immunized with gradient-purified DB, which were either left untreated or subjected to sequential rounds of sonication and freeze-thawing to prevent cellular entry. Titers of neutralizing antibodies induced by DB were in a range comparable to levels present in convalescent human sera. The virus-neutralizing antibody response was surprisingly durable, with neutralizing antibodies detected 12 months following primary immunization. The HCMV-specific major histocompatibility complex class I-restricted cytolytic T-cell (CTL) response was assayed using mice transgenic for the human HLA-A2 molecule. Immunization with DB led to high levels of HCMV-specific CTL in the absence of de novo viral protein synthesis. Maximal total cytolytic activity in mice immunized with DB was nearly as efficient as the cytolytic activity induced by a standard immunization with murine cytomegalovirus. Furthermore, DB induced a typical T-helper 1 (Th1)-dominated immune response in mice, as determined by cytokine and immunoglobulin G isotype analysis. Induction of humoral and cellular immune responses was achieved without the concomitant use of adjuvant. We thus propose that DB can serve as a basis for the future development of a recombinant nonreplicating vaccine against HCMV. Finally, such particles could be engineered for efficient delivery of antigens from other pathogens to the immune system.

Selective transport of internalized antigens to the cytosol for MHC class I presentation in dendritic cells

In order for cytotoxic T cells to initiate immune responses, peptides derived from internalized antigens must be presented to the cytotoxic T cells on major histocompatibility complex (MHC) class I molecules. Here we show that dendritic cells, the only antigen-presenting cells that initiate immune responses efficiently, have developed a unique membrane transport pathway linking the lumen of endocytic compartments and the cytosol. Endosome-to-cytosol transport is restricted to dendritic cells, specific to internalized antigens and selective for the size of the transported molecules. Thus, in dendritic cells, internalized antigens gain access to the cytosolic antigen-processing machinery and to the conventional MHC class I antigen-presentation pathway.

Sialoadhesin-Positive Host Macrophages Play an Essential Role in Graft-Versus-Leukemia Reactivity in Mice

We recently established an effective immune T-cell–mediated graft-versus-leukemia (GVL) murine model system in which complete tumor remissions were achievable even in advanced metastasized cancer. We now describe that this T-cell–mediated therapy is dependent on host macrophages expressing the lymphocyte adhesion molecule sialoadhesin (Sn). Depletion of Kupffer cells in tumor-bearing mice during adoptive immunotherapy (ADI) or the treatment of these animals with anti-Sn monoclonal antibodies led to complete or partial inhibition of the immune T-cell–mediated therapeutic effect. Furthermore, Sn+ host macrophages in livers formed clusters during ADI with donor CD8 T cells. To test for a possible antigen presentation function of these macrophages, we used as an in vitro model the antigen β-galactosidase for which a dominant major histocompatibility complex (MHC) class I Ld-restricted peptide epitope is known to be recognized by specific CD8 cytotoxic T lymphocytes (CTL). We demonstrate that purified Sn+ macrophages can process exogenous β-galactosidase and stimulate MHC class I peptide-restricted CTL responses. Thus, Sn+ macrophages, which are significantly increased in the liver after ADI, may process tumor-derived proteins via the MHC class I pathway as well as via the MHC class II pathway, as shown previously, and present respective peptide epitopes to CD8 as well as to CD4 immune T cells, respectively. The synergistic interactions observed before between immune CD4 and CD8 T cells during ADI could thus occur in the observed clusters with Sn+ host macrophages.

Sialoadhesin-Positive Host Macrophages Play an Essential Role in Graft-Versus-Leukemia Reactivity in Mice

We recently established an effective immune T-cell–mediated graft-versus-leukemia (GVL) murine model system in which complete tumor remissions were achievable even in advanced metastasized cancer. We now describe that this T-cell–mediated therapy is dependent on host macrophages expressing the lymphocyte adhesion molecule sialoadhesin (Sn). Depletion of Kupffer cells in tumor-bearing mice during adoptive immunotherapy (ADI) or the treatment of these animals with anti-Sn monoclonal antibodies led to complete or partial inhibition of the immune T-cell–mediated therapeutic effect. Furthermore, Sn+ host macrophages in livers formed clusters during ADI with donor CD8 T cells. To test for a possible antigen presentation function of these macrophages, we used as an in vitro model the antigen β-galactosidase for which a dominant major histocompatibility complex (MHC) class I Ld-restricted peptide epitope is known to be recognized by specific CD8 cytotoxic T lymphocytes (CTL). We demonstrate that purified Sn+ macrophages can process exogenous β-galactosidase and stimulate MHC class I peptide-restricted CTL responses. Thus, Sn+ macrophages, which are significantly increased in the liver after ADI, may process tumor-derived proteins via the MHC class I pathway as well as via the MHC class II pathway, as shown previously, and present respective peptide epitopes to CD8 as well as to CD4 immune T cells, respectively. The synergistic interactions observed before between immune CD4 and CD8 T cells during ADI could thus occur in the observed clusters with Sn+ host macrophages.

T Cell Responses to Gram-Negative Intracellular Bacterial Pathogens: A Role for CD8+ T Cells in Immunity to Salmonella Infection and the Involvement of MHC Class Ib Molecules

Despite being a major group of intracellular pathogens, the role of class I-restricted T cells in the clearance of Gram-negative bacteria is not resolved. Using a murine typhoid model, a role for class I-restricted T cells in the immune response to the Gram-negative pathogen Salmonella typhimurium is revealed. Class I-deficient β2-microglobulin−/− mice show increased susceptibility to infection with S. typhimurium. Following infection, CD8+ CTLs specific for Salmonella-infected targets can be readily detected. The Salmonella-specific CTLs recognize infected H-2-mismatched targets, suggesting the involvement of shared class Ib molecules. Studies using transfectants expressing defined class Ia and class Ib molecules indicate the involvement of the class Ib molecule, Qa-1. Ab-blocking studies and the measurement of bacteria-specific CTL frequencies identified Qa-1 as a dominant restricting element. The Qa-1-restricted CTL recognition depends on TAP and proteasome functions. Surprisingly, Qa-1-restricted CTLs recognized cells infected with other closely related Gram-negative bacteria. Taken together, these observations indicate that Salmonella-specific CTLs recognize a cross-reactive epitope presented by Qa-1 molecules and, as such, may be novel targets for vaccine development.

Delivery of protein to the cytosol of macrophages using Escherichia coli K-12

Listeriolysin O (LLO) is an essential determinant of pathogenicity whose natural biological role is to mediate lysis of Listeria monocytogenes containing phagosomes. In this study, we report that Escherichia coli expressing cytoplasmic recombinant LLO can efficiently deliver co-expressed proteins to the cytosol of macrophages. We propose a model in which subsequent or concomitant to phagocytosis the E. coli are killed and degraded within phagosomes causing the release of LLO and target proteins from the bacteria. LLO acts by forming large pores in the phagosomal membrane, thus releasing the target protein into the cytosol. Delivery was shown to be rapid, within minutes after phagocytosis. Using this method, a large enzymatically active protein was delivered to the cytosol. Furthermore, we demonstrated that the E. coli/LLO system is very efficient for delivery of ovalbumin (OVA) to the major histocompatibility (MHC) class I pathway for antigen processing and presentation, greater than 4 logs compared with E. coli expressing OVA alone. Moreover, the time required for processing and presentation of an OVA-derived peptide was similar to that previously reported when purified OVA was introduced directly into the cytosol by other methods. Using this system, potentially large amounts of any protein that can be expressed in E. coli can be delivered to the cytosol without protein purification. The potential use of this system for the delivery of antigenic protein in vivo and the delivery of DNA are discussed.