Differential requirements for the processing and presentation of soluble and particulate bacterial antigens by macrophages

All the small things: How virus-like particles and liposomes modulate allergic immune responses

Recent years have seen a dramatic increase in the range of applications of virus‐like nanoparticle (VNP)‐ and liposome‐based antigen delivery systems for the treatment of allergies. These platforms rely on a growing number of inert virus‐backbones or distinct lipid formulations and intend to engage the host's innate and/or adaptive immune system by virtue of their co‐delivered immunogens. Due to their particulate nature, VNP and liposomal preparations are also capable of breaking tolerance against endogenous cytokines, Igs, and their receptors, allowing for the facile induction of anti‐cytokine, anti‐IgE, or anti‐FcεR antibodies in the host. We here discuss the “pros and cons” of inducing such neutralizing autoantibodies. Moreover, we cover another major theme of the last years, i.e., the engineering of non‐anaphylactogenic particles and the elucidation of the parameters relevant for the specific trafficking and processing of such particles in vivo. Finally, we put the various technical advances in VNP‐ and liposome‐research into (pre‐)clinical context by referring and critically discussing the relevant studies performed to treat allergic diseases.

Improving the immunogenicity and protective efficacy of a whole-killed malaria blood-stage vaccine by chloroquine

A whole-killed malaria blood-stage vaccine (WKV) is promising in reducing the morbidity and mortality of malaria patients, but its efficacy needs to be improved. We found that the antimalarial drug chloroquine could augment the protective efficacy of the WKV of Plasmodium yoelii. The direct antimalarial effect of chloroquine on parasites during immunization could be excluded, as the administration of chloroquine or chloroquine plus alum every two weeks had a slight effect on parasitemia, and an immunization with NP-KLH (4-hydroxy-3-nitrophenylacetyl Keyhole Limpet Hemocyanin) plus chloroquine could significantly promote the generation of NP-specific antibodies. Additionally, alum was required for chloroquine to augment the immunogenicity of the pRBC lysate. Chloroquine did not promote the parasite-specific CD4⁺ T-cell responses, but significantly enhanced the WKV-induced germinal centre B cell reactions, class-switch recombination and secretion of functionally protective antibodies to plasmodium. The elevated parasite-specific antibodies were demonstrated to largely contribute to the chloroquine-enhanced protective immunity. Thus, we report that chloroquine could be used as an adjuvant to enhance the protective immunity of WKVs through promoting humoral responses.

Distinct Immunologic Properties of Soluble Versus Particulate Antigens

Antigens in particulate form have distinct immunologic properties relative to soluble antigens. An understanding of the mechanisms and functional consequences of the distinct immunologic pathways engaged by these different forms of antigen is particularly relevant to the design of vaccines. It is also relevant regarding the use of therapeutic human proteins in clinical medicine that have been shown to aggregate, and perhaps as a result, elicit autoantibodies.

Distinct Cellular Pathways for Induction of CD4+ T Cell-Dependent Antibody Responses to Antigen Expressed by Intact Bacteria Versus Isolated Soluble Antigen

Uptake of intact bacteria and soluble Ags by APCs is mediated by phagocytosis and endocytosis or pinocytosis, respectively. Thus, we predicted that injection of clodronate-containing liposomes (CLs), which selectively deplete cells efficient in phagocytosis, would inhibit murine CD4(+) T cell-dependent IgG responses to Ags expressed by intact bacteria but not isolated soluble Ags. Surprisingly, injection of CLs markedly inhibited protein-specific IgG responses to intact, heat-killed Streptococcus pneumoniae, as well as a soluble OVA-polysaccharide conjugate or OVA alone. IgG anti-polysaccharide responses to bacteria and conjugate were also reduced, but more modestly. In both instances, CL-mediated inhibition was associated with a significant reduction in induced germinal centers and CD4(+) germinal center T follicular helper cells. However, CL injection, which largely abrogated the proliferative response of adoptively transferred OVA peptide-specific-transgenic CD4(+) T cells in response to immunization with S. pneumoniae expressing OVA peptide, did not inhibit T cell proliferation in response to OVA-polysaccharide conjugate or OVA. In this regard, monocyte-derived cells, depleted by CLs, internalized S. pneumoniae in vivo, whereas CD11c(low) dendritic cells, unaffected by CL injection, internalized soluble OVA. Ex vivo isolation and coculture of these respective APCs from S. pneumoniae- or OVA-immunized mice with OVA-specific T cells, in the absence of exogenous Ag, demonstrated their selective ability to induce T cell activation. These data suggest that, although distinct APCs initiate CD4(+) T cell activation in response to Ag expressed by intact bacteria versus Ag in soluble form, CL-sensitive cells appear to be necessary for the subsequent IgG responses to both forms of Ag.

Autologous albumin enhances the humoral immune response to capsular polysaccharide covalently coattached to bacteria-sized latex beads

Abundant autologous proteins, like serum albumin, should be immunologically inert. However, individuals with no apparent predisposition to autoimmune disease can develop immune responses to autologous therapeutic proteins. Protein aggregation is a potential major trigger of these responses. Adsorption of proteins to particles provides macromolecular size and may generate structural changes in the protein, resembling aggregation. Using aldehyde/sulfate latex beads coated with murine serum albumin (MSA), we found that BALB/c mice mounted MSA-specific IgG responses that were dependent on CD4(+) T cells. IgGs were specific for MSA adsorbed to solid surfaces and noncross-reactive with human, bovine, or pig albumins. T cells induced in response to MSA augmented the primary and induced boosted secondary IgG and IgM responses specific for the T cell-independent antigen, capsular polysaccharide of Streptococcus pneumoniae type 14 (PPS14), when the latter was attached to the same bead. Similar to the anti-MSA IgG response, the boosted PPS14-specific IgG secondary response was CD4(+) T-cell dependent, displayed a typical carrier effect, and was enhanced by, but did not require, Toll-like receptor stimulation. These results provide a potential mechanism for the induction of responses to autoantigens unable to induce specific T-cell responses, and provide new insights into polysaccharide-specific immunity.

Immunosuppressive property within the Streptococcus pneumoniae cell wall that inhibits generation of T follicular helper, germinal center, and plasma cell response to a coimmunized heterologous protein

We previously demonstrated that intact, inactivated Streptococcus pneumoniae (unencapsulated strain R36A) inhibits IgG responses to a number of coimmunized soluble antigens (Ags). In this study, we investigated the mechanism of this inhibition and whether other extracellular bacteria exhibited similar effects. No inhibition was observed if R36A was given 24 h before or after immunization with soluble chicken ovalbumin (cOVA), indicating that R36A acts transiently during the initiation of the immune response. Using transgenic cOVA-specific CD4(+) T cells, we observed that R36A had no significant effect on T-cell activation (24 h) or generation of regulatory T cells (day 7) and only a modest effect on T-cell proliferation (48 to 96 h) in response to cOVA. However, R36A mediated a significant reduction in the formation of Ag-specific splenic germinal center T follicular helper (GC Tfh) and GC B cells and antibody-secreting cells in the spleen and bone marrow in response to cOVA or cOVA conjugated to 4-hydroxy-3-nitrophenylacetyl hapten (NP-cOVA). Of note, the inhibitory effect of intact R36A on the IgG anti-cOVA response could be reproduced using R36A-derived cell walls. In contrast to R36A, neither inactivated, unencapsulated, intact Neisseria meningitidis nor Streptococcus agalactiae inhibited the OVA-specific IgG response. These results suggest a novel immunosuppressive property within the cell wall of Streptococcus pneumoniae.

Differential idiotype utilization for the in vivo type 14 capsular polysaccharide-specific Ig responses to intact Streptococcus pneumoniae versus a pneumococcal conjugate vaccine

Murine IgG responses specific for the capsular polysaccharide (pneumococcal capsular polysaccharide serotype 14; PPS14) of Streptococcus pneumoniae type 14 (Pn14), induced in response to intact Pn14 or a PPS14-protein conjugate, are both dependent on CD4(+) T cell help but appear to use marginal zone versus follicular B cells, respectively. In this study, we identify an idiotype (44.1-Id) that dominates the PPS14-specific IgG, but not IgM, responses to intact Pn14, isolated PPS14, and Group B Streptococcus (strain COH1-11) expressing capsular polysaccharide structurally identical to PPS14. The 44.1-Id, however, is not expressed in the repertoire of natural PPS14-specific Abs. In distinct contrast, PPS14-specific IgG responses to a soluble PPS14-protein conjugate exhibit minimal usage of the 44.1-Id, although significant 44.1-Id expression is elicited in response to conjugate attached to particles. The 44.1-Id elicited in response to intact Pn14 was expressed in similar proportions among all four IgG subclasses during both the primary and secondary responses. The 44.1-Id usage was linked to the Igh(a), but not Igh(b), allotype and was associated with induction of relatively high total PPS14-specific IgG responses. In contrast to PPS14-protein conjugate, avidity maturation of the 44.1-Id-dominant PPS14-specific IgG responses was limited, even during the highly boosted T cell-dependent PPS14-specific secondary responses to COH1-11. These results indicate that different antigenic forms of the same capsular polysaccharide can recruit distinct B cell clones expressing characteristic idiotypes under genetic control and suggest that the 44.1-Id is derived from marginal zone B cells.

Intact bacteria inhibit the induction of humoral immune responses to bacterial-derived and heterologous soluble T cell-dependent antigens

During infections with extracellular bacteria, such as Streptococcus pneumoniae (Pn), the immune system likely encounters bacterial components in soluble form, as well as those associated with the intact bacterium. The potential cross-regulatory effects on humoral immunity in response to these two forms of Ag are unknown. We thus investigated the immunologic consequences of coimmunization with intact Pn and soluble conjugates of Pn-derived proteins and polysaccharides (PS) as a model. Coimmunization of mice with Pn and conjugate resulted in marked inhibition of conjugate-induced PS-specific memory, as well as primary and memory anti-protein Ig responses. Inhibition occurred with unencapsulated Pn, encapsulated Pn expressing different capsular types of PS than that present in the conjugate, and with conjugate containing protein not expressed by Pn, but not with 1-microm latex beads in adjuvant. Inhibition was long-lasting and occurred only during the early phase of the immune response, but it was not associated with tolerance. Pn inhibited the trafficking of conjugate from the splenic marginal zone to the B cell follicle and T cell area, strongly suggesting a potential mechanism for inhibition. These data suggest that during infection, bacterial-associated Ags are the preferential immunogen for antibacterial Ig responses.

Macrophages pulsed with Streptococcus pneumoniae elicit a T cell-dependent antibody response upon transfer into naive mice

Macrophages are less effective than DC at priming naive CD4(+) T cells, suggesting that DC are unique in initiating T cell-dependent Ab responses. We compared the ability of DC and macrophages, pulsed in vitro with Streptococcus pneumoniae, to elicit protein- and polysaccharide-specific Ig isotype production upon adoptive transfer into naive mice. S. pneumoniae-activated DC secreted more proinflammatory and anti-inflammatory cytokines, expressed higher levels of surface MHC class II and CD40, and presented S. pneumoniae or recombinant pneumococcal surface protein A (PspA) to a PspA-specific T hybridoma more efficiently than macrophages. However, upon adoptive transfer into naive mice, S. pneumoniae-pulsed macrophages elicited an IgM or IgG anti-PspA and anti-polysaccharide response comparable in serum titers and IgG isotype distribution to that induced by DC. The IgG anti-PspA response, in contrast to the IgG anti-polysaccharide, to S. pneumoniae-pulsed macrophages was T cell-dependent. S. pneumoniae-pulsed macrophages that were paraformaldehyde-fixed before transfer or lacking expression of MHC class II or CD40 were highly defective in eliciting an anti-PspA response, although the anti-polysaccharide response was largely unaffected. To our knowledge, these data are the first to indicate that macrophages can play an active role in the induction of a T cell-dependent humoral immune response in a naive host.

Evaluation of immunological paradigms in a virus model: are dendritic cells critical for antiviral immunity and viral clearance?

We have examined the role of dendritic cells (DCs) in the antiviral immune response and viral clearance using a transgenic mouse model (CD11c-diphtheria toxin (DT) receptor GFP) that allows for their conditional ablation in vivo. DT administration systemically ablated conventional and IFN-producing plasmacytoid DCs (pDCs) in transgenic, but not nontransgenic littermates, without elimination of splenic macrophages. Unexpectedly, early (12 and 48 h postinfection) viral clearance of vesicular stomatitis virus was normal in DC-depleted mice despite markedly reduced serum titers of type I IFN. DC-depleted mice remained virus-free with the exception of a subset (approximately 30%) that developed overwhelming and fatal brain infections 6 days postinfection. However, DT treatment profoundly inhibited clonal expansion of naive CD8+ vesicular stomatitis virus-specific T cells without altering the primary Th1 and Th2 cytokine response. Optimal clonal expansion required pDCs because selective elimination of these cells in vivo with a depleting Ab also suppressed expansion of tetramer+ cells, although Th1/Th2 cytokine production remained unaltered. Collectively, these data indicate that conventional DCs and to a lesser extent pDCs are critical for proliferation of naive antiviral T cells. However, other components of the primary adaptive immune response (Th1/Th2 cytokines) are essentially normal in the absence of DCs, which may account for the efficient viral clearance seen in DC-depleted mice. Thus, sufficient redundancy exists in the immune system to sustain efficient viral clearance despite loss of an APC considered essential for induction of a primary antiviral immune response.

Recombinant replication-restricted VSV as an expression vector for murine cytokines

Vesicular stomatitis virus (VSV) is a prototypic non-segmented, negative-strand RNA virus that rapidly and efficiently shuts down the production of host cell-encoded proteins and utilizes the cell's protein production machinery to express high levels of virally encoded proteins. In an effort to take advantage of this characteristic of VSV, we have employed a reverse genetics system to create recombinant forms of VSV encoding a variety of murine cytokines. Previous studies have revealed that cells infected with recombinant VSV that lack expression of the surface glycoprotein (G protein), designated deltaG-VSV, more efficiently express and secrete recombinant proteins than do recombinant "wild-type" VSV. Therefore, murine cytokine-expressing recombinants were produced as deltaG viruses. Propagation of these deltaG viruses in cells that transiently express G protein in vitro results in G-complemented virions that can infect cells, shut down host protein synthesis, and express at high levels each virally encoded protein (including the designated cytokine). We assessed the ability of each deltaG-VSV construct to express recombinant cytokine by infecting BHK cells and then monitoring/measuring the production of the desired cytokine. When possible, the bioactivity of the cytokine products was also measured. The results presented here reveal that large quantities of bioactive cytokines can be produced rapidly and inexpensively using deltaG-VSV as a protein expression system.

IL-12-assisted immunization generates CD4+ T cell-mediated immunity to Listeria monocytogenes

Mice infected with virulent Listeria monocytogenes develop long-lived acquired immunity. We previously reported that acquired immunity to Listeria could also be elicited by immunizing mice with non-viable Listeria or listerial proteins/peptides in combination with IL-12. Here we show that this IL-12-assisted immunization strategy was effective in class I but not in class II MHC-deficient mice, suggesting that antigen-specific CD4(+) T cells are selectively generated using this adjuvant system. We have also evaluated the importance of endogenous production of IFN-gamma and IL-12 for the efficacy of IL-12-assisted immunization. IFN-gamma-deficient mice immunized with HKLM and IL-12 failed to produce effective Listeria-specific responses. In contrast, IL-12-deficient mice were able to generate protective antigen-specific T cell responses in response to immunization with HKLM and IL-12, indicating that exogenous IL-12 is sufficient to initiate a cytokine cascade that results in a potent T(H)1 response. IL-12-assisted immunization provides a model in which both the generation and effector mechanisms of anti-bacterial antigen-specific CD4(+) effector cells can be analyzed.

Adjuvanticity of an IL-12 fusion protein expressed by recombinant deltaG-vesicular stomatitis virus

The remarkable immunomodulatory and adjuvant properties of rIL-12 have been well described. Many early studies documenting the adjuvanticity of IL-12 were performed using the murine model of Listeria monocytogenes infection. In this report, we describe the construction of an attenuated recombinant vesicular stomatitis virus (VSV-deltaG) that encodes a single-chain IL-12 fusion protein (IL-12F), and the use of this virus as an expression vector to produce large quantities of IL-12F. VSV-expressed IL-12F (vIL-12F) was then co-administered to mice along with a poorly immunogenic listerial antigen preparation as a vaccine regimen and the resulting immune responses were monitored. The vIL-12F was found to have adjuvant properties similar to those observed for rIL-12. Co-administration of vIL-12F and listerial antigen elicited powerful cell-mediated immune responses that conferred long-lived protective listerial immunity. These studies demonstrated that VSVdeltaG-IL12F-infected cells secrete bioactive single-chain IL-12, and laid the foundation for studies using VSVdeltaG-IL12F as a vector for delivery of IL-12F in vivo.

Immunogenicity of recombinant Escherichia coli expressing the omp31 gene of Brucella melitensis in BALB/c mice

BALB/c mice were immunized with recombinant Escherichia coli expressing the omp31 gene of Brucella melitensis, a gene coding for a major outer membrane protein. Immunization resulted in the production of specific antibodies to B. melitensis in the serum, the production of which was considerably increased after boosting with a dose ten times lower than the first. A significant specific proliferative response of immune spleen cells to B. melitensis was observed 5 weeks after the first immunization but this response did not persist. Despite the induction of systemic humoral and cellular immune responses by recombinant E. coli expressing the B. melitensis omp31 gene, no significant protection against a challenge with smooth B. melitensis H38S was observed in immunized mice. These results demonstrate that despite the strong antibody response induced in mice, immunization with the recombinant Omp31 of B. melitensis does not confer any protective effect against a virulent smooth B. melitensis. However, its potential protective effect for protection against rough Brucella would be worth testing.

Long-lived protective immunity to Listeria is conferred by immunization with particulate or soluble listerial antigen preparations coadministered with IL-12

The ability of IL-12 to promote the development of Th1-type immune responses, and thus promote cellular immunity, has been well documented. In a previous report, we showed that coadministration of IL-12 with heat-killed Listeria monocytogenes elicited intense antigen-specific T cell responses that conferred protective listerial immunity. Herein, we have extended those studies by demonstrating that multiple injections of heat-killed L. monocytogenes and IL-12 elicit memory responses that confer long-lived (> or = 3 months) protective immunity and that immunity can be transferred adoptively with cells from immunized mice injected into naive mice. These studies have also demonstrated that the powerful adjuvanticity of IL-12 is observed with soluble as well as particulate immunogens and is operative in mouse strains that have different MHC haplotypes. These findings suggest that IL-12 may be a useful adjuvant component of vaccines for a wide variety of pathogens in animal and human systems.

The immune response to staphylococcal antigens in mice depleted of macrophages by Cl2MDP-liposomes

To investigate the role of macrophages in the induction of the production of antibody to staphylococcal antigens, we used Cl2MDP (clodronate) liposomes as a tool for local macrophage depletion. Macrophage depletion caused in mice by intraperitoneal (i.p.) injection of Cl2MDP liposomes was associated with a reduction in the clearance of Staphylococcus aureus Cowan 1 bacteria from the tissues of infected animals and with a marked decrease in the bactericidal activity of macrophages escaping from the lethal effect of clodronate. Despite the functional defect of macrophages, the mice treated with Cl2MDP liposomes two days before the injection of alpha-toxin (toxoid) or whole heat-killed S. aureus Cowan 1 bacteria, demonstrated an enhancement in the production of anti-staphylococcal alpha-toxin IgM and anti-collagen-binding protein IgG. A similar enhancement of antistaphylococcal antibody synthesis was observed in mice after receiving phosphate buffered saline (PBS) encapsulated in liposomes.

Processing and presentation of type II collagen, a fibrillar autoantigen, by H-2q antigen-presenting cells

Native type II collagen (CII) is a high molecular-weight fibrillar molecule which induces a chronic polyarthritis in mice expressing the H-2q haplotype. The present study was initiated to analyze the processing and the presentation of this nonglobular protein by H-2q antigen-presenting cells (APC). Efficiency of presentation was assessed by the ability of antigen-pulsed APC to activate collagen-specific CD4+ T cell hybridomas. Fixation of APC or the presence of chloroquine completely blocked the reactivity of the T cell hybrids to native, denatured and cyanogen bromide (CB) degraded CII, thus indicating the requirement of intracellular processing for adequate presentation of CII peptides to T cells. In the presence of various processing inhibitors (brefeldin A, leupeptin and N-tosyl-L-phenylalanine chloromethylketone) stimulation of T hybrids by CII-pulsed APC was reduced, pointing to the need of newly synthesized class II molecules, the use of several intracellular compartments and the implications of different proteases in the generation of CII peptides. Peritoneal macrophages and, to a lesser extent, total spleen cells, presented native and denatured CII with higher efficiency than purified splenic dendritic cells, naive or even immune B cells from CII-primed mice. In contrast, these dendritic and B cells were fully competent to present intact ovalbumin to a specific T cell hybrid. The stimulation by dendritic cells and immune B cells was greater when CB peptides of CII were added instead of the native molecule. Similarly, the cleavage of CII was an absolute requirement for its presentation by epidermal cells and B cell lymphomas to the T cell hybridomas. Taken together, these findings emphasize the crucial role of intracellular processing for recognition of soluble CII, similar to most antigens. However, in contrast to ovalbumin, the size and fibrillar nature of the native CII molecule influences its capture by the APC, thus limiting the type of APC able to present this antigen.

Role of newly synthesized MHC class II molecules in antigen-specific antigen presentation by B cells

Using a B lymphoma, A20-HL, bearing IgM receptors for TNP, we have shown that presentation of an Ag taken up through the receptors is highly sensitive, whereas that of an Ag taken up nonspecifically is resistant to inhibition of protein synthesis or protein transport from the endoplasmic reticulum. To analyze the difference, we have examined the effect of protein synthesis inhibition on A20-HL cells in terms of internalization and fragmentation of a specific Ag, TNP-OVA, and distribution of MHC class II molecules. Inhibition of protein synthesis in A20-HL cells with emetine, an irreversible protein synthesis inhibitor, did not decrease the surface expression of anti-TNP receptors, or the kinetics of internalization of 125I-TNP-OVA. To detect fragmentation of TNP-OVA, A20-HL cells were incubated at 37 degrees C in the presence of 125I-TNP-OVA, and the cell lysate was analyzed in SDS-PAGE. The number of detectable fragments increased with the incubation period, and inhibition of protein synthesis did not change the electrophoretic pattern. Expression of MHC class II molecules on the surface of A20-HL cells was not affected by inhibition of protein synthesis. However, intracellular MHC class II molecules markedly decreased in amount in the emetine-treated cells. Thus, presentation of an Ag taken up through Ag receptors seems to be dependent on intracellular MHC class II molecules, whereas that of an Ag taken up nonspecifically does not, suggesting that the Ag-processing pathway in B cells for a specific Ag is different from that for a nonspecific one, at least partly.

Hepatocytes can serve as accessory cells in the response of immune T lymphocytes to heat-killed Listeria monocytogenes

Previous findings in our laboratory indicated that the bulk of Listeria monocytogenes injected intravenously into mice and recovered in the liver is taken up and replicates within hepatocytes. Other investigators have shown that hepatocytes can display costimulatory adhesion molecules, express major histocompatibility complex class I and II molecules, and secrete a number of cytokines, including interleukin-1 (IL-1), IL-6, and IL-8. These data suggest that hepatocytes may serve as accessory cells in the immune response to L. monocytogenes. The accessory function and capacity of hepatocytes to present listerial antigens, however, have never been explored. We undertook a series of experiments to examine the response of Listeria-immune T lymphocytes to murine hepatocytes preincubated with heat-killed listeriae (HKL). Electron micrographs showing the organism within membrane-limiting vacuoles demonstrated the capacity of hepatocytes to internalize HKL. T cells cocultured with hepatocytes pulsed with HKL exhibited a 5- to 10-fold increase in [methyl-3H]thymidine incorporation relative to T cells cultured with either hepatocytes or HKL alone. Similarly, gamma interferon production by immune T cells was elevated significantly in cultures that contained both hepatocytes and HKL. The optimal response of T cells required lysosomal processing of HKL by hepatocytes and contact between the two cell populations. Furthermore, maximum T-cell proliferation and gamma interferon production were dependent upon the presence of CD4+ T lymphocytes and the expression of Ia antigens. Taken together, these findings demonstrate that hepatocytes pulsed with HKL can stimulate the antigen-specific response of immune T lymphocytes. These results suggest that hepatocytes can serve as accessory cells in host defenses to listerial infections of the liver.

Altered antigen-presenting capacity of human monocytes after phagocytosis of bacteria

The antigen-presenting and accessory functions of monocytes were studied after phagocytosis of bacteria. Peripheral blood monocytes isolated from mononuclear cells by counterflow elutriation were incubated with suspensions of opsonized bacteria (Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, or Salmonella enteritidis) under conditions in which at least 80% of the monocytes engulfed microorganisms. Either the cells were pulsed with antigen (purified derivative of tuberculin or tetanus toxoid) and used as antigen-presenting cells for autologous T lymphocytes or the accessory function of the cells was examined in pokeweed mitogen-activated cultures of T cells. It has been found that phagocytosis of bacteria by monocytes reduces their ability to trigger antigen- and mitogen-induced proliferation. The reduced proliferative response of T lymphocytes was not due to a change of the kinetics of the response or triggering of suppressor mechanisms. Furthermore, antigen processing was not affected much after phagocytosis of bacteria since antigen-pulsed and paraformaldehyde-fixed cells containing bacteria were comparable to control cells in their antigen-presenting capacity. This phenomenon was observed after phagocytosis of both living and dead bacteria and was not correlated to the viability of monocytes, which were more affected after phagocytosis of living bacteria than of dead ones. As a result of phagocytosis of bacteria, reduced expression of CD54, CD14, and HLA-DQ, variable reduction of HLA-DP, CD58, and CD64, and reduced viability of monocytes were observed. In conclusion, phagocytosis of bacteria by monocytes affects their antigen-presenting and accessory functions presumably because of changes in the expression of molecules essential for monocyte-T-cell interactions and reduction of their viability.

Characterization of a 58- and a 78-kD monocytic membrane protein with affinity to the acetylcholine receptor in myasthenia gravis patients

The autoimmune disease myasthenia gravis (MG), caused by the effect of specific antibodies, directed towards the nicotinic acetylcholine receptor, is triggered by autoantigen-specific T cells. In order to investigate cellular parts of the immune response in MG, the authors investigated the binding of the nicotinic acetylcholine receptor (AChR) to peripheral blood mononuclear cells (PBMC) from MG patients. AChR binding cells were identified by rosetting experiments using AChR-coated fluorescein beads. Applying this technique, a significant percentage of PBMC (21.2 +/- 7.65%) from MG patients formed rosettes with AChR-coated beads. Membrane preparations of nycodenz- or percoll-separated monocytes from MG patients or T-cell depleted monocytic subpopulations were applied to SDS-PAGE under reducing conditions. Ligand-blotting studies with biotinylated AChRs revealed two cell-membrane proteins with molecular weights of 58- and 78-kD. In parallel the same results were obtained by affinity chromatography of monocytic membrane proteins using AChR-sepharose. A possible interference of anti-AChR IgG was excluded. The 58- and the 78-kD proteins are detectable under reducing conditions by ligand blotting with AChR-biotin, while under non-reducing conditions only the 58-kD protein can be detected. Furthermore, in experiments using Endoglycosidase-H, the 58-kD protein appears to be non-glycosylated, while the 78-kD protein bears carbohydrates. These findings suggest that monocytes which bind the AChR via specific membrane proteins on their surface might act as antigen-presenting cells and may lead to an induction of the T-cell response, in the early phase of the disease.

In vivo distribution of particulate antigens and liposomes in murine spleen. A possible role in the humoral immune response

Several particulate antigens and liposomes were intravenously injected in mice in order to study their localization patterns in spleen and liver. Liposomes have been proposed as promising carriers for haptens and antigens. It was studied whether the phospholipid composition, cholesterol content and charge of the liposomes played a role in their distribution within the spleen. Different thymus-independent type 1 and type 2 and thymus-dependent particulate antigens as well as liposomes were labeled with the lipophilic fluorochrome Di-I. After labeling they were intravenously injected and spleens and livers were removed at different time intervals and prepared for light- and fluorescence-microscopy. We have observed that all particulate antigens and liposomes administered to the mice localized according to the same distribution pattern in the spleen. After 2 and 4 h particles were located in macrophages of the marginal zone and after 24 h white pulp macrophages had also ingested particulate antigens and liposomes. So we conclude that the distribution of the particulate antigens and liposomes in the spleen is independent of the immunological nature of the particles. Results are discussed with respect to the question whether or not the distribution of particulate antigens and liposome associated antigens or haptens, may be a crucial factor in determining the type of immune response to be elicited.

High-titre antibodies to a foreign epitope elicited by affinity-purified hybrid LamB proteins

A monoclonal antibody to the LamB protein, named LBS-1, was developed and characterized. It was then covalently bound to Sepharose and used to purify hybrid LamB proteins from Escherichia coli crude extracts. A peptide of the interferon-gamma (IFN-gamma) NH2-terminal region, inserted within the LamB protein, was used as a model to assess immune response in mice injected with sonicated E. coli extract or with affinity-purified hybrid LamB protein. None of the mice immunized with the whole bacterial extract produced antibodies to IFN-gamma. On the other hand, all the mice immunized with the purified protein developed high-titre anti-IFN-gamma antibodies. These results might be due to the presence of bacterial components capable of masking the LamB protein to the immune system. The use of affinity-purified LamB proteins may constitute in some instances a more effective way of generating an immune response against foreign epitopes as opposed to whole bacterial antigens.

Effect of parenteral immunization on the intestinal immune response to Salmonella typhi Ty21a

The effects of parenteral administration of a killed typhoid vaccine on the intestinal immune response to live orally administered Salmonella typhi Ty21a in human subjects was evaluated. Priming with parenteral vaccination neither enhanced nor suppressed the subsequent specific serum and intestinal immunoglobulin A (IgA) immune responses to a booster course of live oral vaccine. Neither a single oral dose of live vaccine nor a single dose of parenteral vaccine had any measurable booster effect on the observed primary intestinal IgA response to the live oral vaccine. Two booster doses of subcutaneously administered killed typhoid vaccine did result in a significant increase in the specific intestinal IgA antibody in those subjects primed with the oral live vaccine. This response was comparable in magnitude to the primary intestinal response. No evidence of this response could be found in serum IgA, although nonsignificant rises in serum IgG were evident. Previous parenteral priming had no effect on secondary immune responses to a live oral vaccine in humans. Serum immune responses were generally found to be of little value as indicators of local intestinal immunity. This study confirmed that parenteral vaccination was only able to induce an intestinal immune response following priming with live, orally administered organisms and that multiple parenteral booster doses were necessary to induce a measurable effect on intestinal immune responses.

Presentation of Leishmania donovani promastigotes occurs via a brefeldin A-sensitive pathway

For the presentation of Leishmania promastigotes to polyclonal CD4+ T cells, a processing period within activated macrophages of 3-4 h is required. Presentation can be inhibited by both chloroquine and brefeldin A (BFA), the latter implicating a requirement for newly synthesized MHC class II molecules. This inhibition is both reversible and specific, in that BFA did not inhibit mixed lymphocyte reaction stimulation by these infected macrophages. Immunogold labeling demonstrated that class II was associated with the parasite-containing phagolysosome. The level of class II was not significantly altered in BFA-treated cells in the time period studied, suggesting that antigen may exist the phagolysosome and interact with class II in another cellular compartment.

Suppression of splenic accessory cell function in mice exposed to gallium arsenide

Acute exposure of mice to a single intratracheal dose of gallium arsenide (50, 100, and 200 mg/kg) depresses the primary IgM antibody response to the T-dependent antigen sheep red blood cells (SRBC) through alterations in the function of splenic accessory cells. To determine the mechanism by which GaAs exposure influences splenic accessory cells, the cells were isolated by adherence and their functional capability investigated 24 hr following GaAs exposure in the animal. Splenic adherent cells from GaAs-exposed mice were greatly impaired in their ability to process and present the particulate antigen SRBC to a SRBC-primed T-cell population. However, GaAs exposure did not inhibit phagocytosis of fluorescent covaspheres by these cells, nor did it inhibit in vivo phagocytosis of 51Cr-labeled SRBC, indicating that the findings reported here were not due to decreased uptake of antigen by the accessory cells. Furthermore, production of IL-1 by these cells from exposed mice was not different from control and addition of exogenous IL-1 to cultures did not reverse GaAs-induced inhibition of the primary antibody response. GaAs exposure did not affect the percentage of Ia positive macrophages (F4/80 positive cells), but the amount of cell surface IAk molecules expressed was significantly decreased as measured by flow cytometry. In contrast to the SRBC response, GaAs did not suppress the ability of adherent splenocytes to process and present the antigen pigeon cytochrome c to the helper/inducer T cell clone F1.A.2 or the antigen KLH (keyhole limpet hemocyanin) to KLH-primed T cells. Therefore, GaAs exposure interferes with the capacity of splenic macrophages to process and/or present the particulate antigen SRBC, but not the soluble protein antigens pigeon cytochrome c or KLH.

Localization of major histocompatibility complex class II molecules in phagolysosomes of murine macrophages infected with Leishmania amazonensis

Leishmania-infected macrophages are potential antigen-presenting cells for CD4+ T lymphocytes, which recognize parasite antigens bound to major histocompatibility complex class II molecules (Ia). However, the intracellular sites where Ia and antigens may interact are far from clear, since parasites grow within the modified lysosomal compartment of the host cell, whereas Ia molecules seem to be targeted to endosomes. To address this question, the expression and fate of Ia molecules were studied by immunocytochemistry in Leishmania amazonensis-infected murine macrophages stimulated with gamma interferon. In uninfected macrophages, Ia molecules were localized on the plasma membrane and in perinuclear vesicles, but they underwent a dramatic redistribution after infection, since most of the intracellular staining was then associated with the periphery of the parasitophorous vacuoles (p.v.) and quite often polarized towards amastigote-binding sites. The Ii invariant chain, which is transiently associated with Ia during their intracellular transport, although well expressed in infected macrophages, apparently did not reach the p.v. Similar findings were observed with macrophages from mice either resistant or highly susceptible to Leishmania infection. In order to determine the origin of p.v.-associated Ia, the fate of plasma membrane, endosomal, and lysosomal markers, detected with specific antibodies, was determined after infection. At 48 h after infection, p.v. was found to exhibit a membrane composition typical of mature lysosomes. Overall, these data suggest that (i) Ia located in p.v. originate from secondary lysosomes involved in the biogenesis of this compartment or circulate in several endocytic organelles, including lysosomes and (ii) p.v. could play a role in antigen processing and presentation. Alternatively, the presence of high amounts of Ia in p.v. could be due to a Leishmania-induced mechanism by means of which this organism may evade the immune response.

Sarcoidosis: in search of Kveim reactivity in vitro

Granuloma formation in patients with sarcoidosis may be evoked by the intradermal injection of homogenised sarcoid tissue (the Kveim reaction). Attempts to demonstrate an in vitro counterpart of the reaction have been unsuccessful. The cytokine interleukin-2 (IL-2) enhances immune responses in vivo and in vitro. We report here an attempt to amplify the Kveim reaction by the addition of IL-2. We studied the effect of Kveim reagent on the proliferative responses of peripheral blood mononuclear cells (PBMC) in the presence or absence of exogenous IL-2. Twenty-eight patients were studied and 14 healthy subjects served as controls. PBMC were cultured, in vitro, in the presence of Kveim reagent. Recombinant IL-2 or both of these combined. Proliferative responses were measured by [3H]-thymidine incorporation. The response of patients' PBMC in the presence of Kveim reagent at a dilution of 1:40 was significantly below the unstimulated response (P less than 0.01). Kveim reagent at a dilution of 1:40 also inhibited the proliferative response of patients PBMC to IL-2 (P less than 0.005); greater dilutions (1:100 and 1:1000) of Kveim reagent were not inhibitory. Responses of PBMC from control subjects (both unstimulated and IL-2 generated) were reduced in the presence of Kveim reagent, however, these reductions were not statistically significant.

Antigen processing and presentation in vivo: the microenvironment as a crucial factor

Antigen processing and presentation in vitro is an increasingly well understood phenomenon. However, in vivo, a large number of variables conspire to obscure and confuse. In this article, Nico van Rooijen attempts to bring order to events that occur in the spleen after antigenic challenge: starting with the large body of reliable in vitro data he incorporates information on splenic anatomy, cell trafficking and the cellular microenvironment to arrive at a physiological model for antigen handling in vivo.

Elimination of spleen and of lymph node macrophages and its difference in the effect on the immune response to particulate antigens

To study the role of macrophages in the in situ immune response to particulate antigens in spleen and popliteal lymph nodes (PLN), mice were injected with dichloromethylene diphosphonate (Cl2MDP)-containing liposomes to eliminate macrophages, followed by immunization with trinitrophenylated sheep red blood cells (TNP-SRBC). Depletion of macrophages in the spleen caused a strong decrease in the number of antibody-forming cells (AFC), which develop after intravenous (i.v.) injection of the antigen. These results strongly suggested the involvement of splenic macrophages in the processing of TNP-SRBC. In particular, the populations of marginal zone macrophages may be involved in the inductive phase of an antibody response to particulate antigens. These macrophages are strategically positioned at the end of the white pulp capillaries in the marginal zone of the spleen and they have their cell processes between the marginal zone-B cells. Elimination of macrophages in PLN had no effect on the number of AFC, which develop after subcutaneous (s.c.) injection of the antigen in the hind footpads. This indicates that the macrophages are not essential for the induction of a local immune response to the particulate antigen TNP-SRBC. After depletion of lymph node macrophages, the number of AFC developing in the spleen after s.c. footpad injection of the antigen increased and the anti-TNP serum titers were elevated. This may well be caused by the fact that more of the antigen reaches the circulation and subsequently stimulates the spleen.

Intracellular hemolysin-producing Listeria monocytogenes strains inhibit macrophage-mediated antigen processing

We found that virulent hemolysin-producing (Hly+) Listeria monocytogenes strains inhibit antigen processing and presentation when added to macrophages in vitro. A virulent Hly- bacteria caused little or no inhibition. Live Hly+ bacteria inhibited presentation of both heat-killed L. monocytogenes and ovalbumin. Several observations indicate that hemolysin produced by intracellular bacteria was responsible for the inhibition. First, inhibition was observed even when extracellular bacteria were removed after a brief 10-min bacterial uptake period. Second, inhibition was not prevented by the addition of cholesterol, a substance which inactivates soluble hemolysin. Third, only very high concentrations of soluble hemolysin were inhibitory. Under conditions which inhibit antigen presentation (10(5) per well), macrophages retained normal levels of Ia, maintained normal morphology, and were not permeable when assayed by chromium release. The uptake and catabolism of 35S-labeled live bacteria by macrophages were similar for both Hyl+ and Hly- bacteria. Only a small decrease in uptake and catabolism of surface-iodinated heat-killed L. monocytogenes by macrophages pretreated with inhibitory numbers of live Hly+ bacteria was observed. Additionally, macrophages pretreated with live Hly+ bacteria and fixed 1.5 h later were able to effectively present an ovalbumin peptide (amino acids 323 to 339) to the T-cell hybridoma DO11.10. Hemolysin-producing bacteria inhibited the presentation of antigens that need processing better than they did of antigens that do not require a processing event. Thus, we have demonstrated inhibition of an intracellular antigen processing pathway by hemolysin-producing L. monocytogenes, which may contribute to the virulence of this pathogen.

Cloning and characterization of T-cell-reactive protein antigens from Listeria monocytogenes

To explore the molecular basis of the T-cell-mediated immune response to Listeria monocytogenes, we cloned and expressed listerial antigens in Escherichia coli using the lambda-ZAP bacteriophage and Bluescript plasmid vectors. A two-stage screening strategy was implemented to identify T-cell-reactive antigens; the first stage involved antibodies or oligonucleotide probes and the second stage was based on assays for T-cell activation. A library of genomic DNA from L. monocytogenes was generated in lambda-ZAP, and then antigens, were detected in infected cells with a polyclonal rabbit anti-L. monocytogenes antiserum and an L. monocytogenes-specific monoclonal antibody. Also, synthetic oligonucleotide probes corresponding to the structural gene for listeriolysin O (LLO) were used to screen the recombinant DNA library. In each case, positive isolates were evaluated for T-cell antigenicity by measuring antigen-induced interleukin-2 production by polyclonal T cells taken from L. monocytogenes-immune mice. Phage clones were subcloned and expressed in the Bluescript plasmid and tested further for antigenic activity and LLO expression. Using this screening strategy, we successfully identified bacterial clones producing recombinant listerial antigens which activate L. monocytogenes-immune T cells in vitro. Antigens operative in the T-cell response during infection with L. monocytogenes include LLO, 62- and 39-kilodalton proteins, and other poorly defined bacterial surface components. We also found that high concentrations of recombinant LLO inhibited macrophage-mediated antigen presentation. These results are discussed in terms of the multiple functions of LLO as a virulence factor, inhibitor of antigen presentation, and potent antigen in the T-cell response to L. monocytogenes. These studies represent the first step toward a genetic definition of the antigens recognized in immune defense to L. monocytogenes.

Characterization of a Listeria monocytogenes-specific protein capable of inducing delayed hypersensitivity in Listeria-immune mice

Recovery of the host after infection by the intracellular pathogen Listeria monocytogenes is dependent on cell-mediated immunity. Little is known of the nature of listerial antigens that induce cell-mediated responses in the infected host. In this study we report on the identification and cloning of an Escherichia coli recombinant encoding a listerial antigen, designated ImaA, capable of eliciting a specific delayed-type hypersensitivity response in Listeria-immune mice. Nucleotide sequencing of the Listeria DNA insert in plasmid pLM10 showed that the ImaA gene product consisted of 170 amino acids with a molecular weight of 17,994. The predicted amino acid sequence suggests that the protein is localized to the bacterial plasma membrane or cell wall. The ImaA gene was unique to the pathogenic species L. monocytogenes and Listeria ivanovii; it was not present in any other species of the genus Listeria.

Antibody response to a foreign epitope expressed at the surface of recombinant bacteria: importance of the route of immunization

A genetic procedure has been previously established to expose a foreign epitope at the surface of Escherichia coli by using the outer membrane LamB protein as a carrier. A portion of the pre-S2 region of hepatitis B virus, residues 132-145, has been inserted at amino acid position 153 of the LamB protein, in a cell surface exposed loop. In the present study, we have analysed the antibody responses induced by these recombinant bacteria (live, heat-killed or sonicated) depending upon the route of immunization. The intravenous (i.v.) or intraperitoneal (i.p.) administration of the live recombinant bacteria to mice induced the synthesis of antibodies against both the inserted peptide and the native LamB protein. The antibodies raised recognized HBsAg particles. These mice also had high titres of antibodies against E. coli antigens (as determined using a crude bacterial sonicate). In contrast, mice immunized subcutaneously (s.c.) did not develop antibodies against the pre-S2 peptide nor against the HBsAg particles. Their anti-LamB responses were low compared with the response of mice immunized by the parenteral route. Interestingly, s.c. or i.v. immunizations induced comparable levels of anti-E. coli antibodies. Thus, the antibody response to the inserted peptide generally parallels the response to the LamB protein (and not to the bulk of E. coli antigens). However, this treatment corresponding to a 'pre-processing' of the recombinant bacteria was not sufficient to obtain an anti-peptide response following s.c. immunization.