A Genome-Wide Haploid Genetic Screen Identifies Heparan Sulfate-Associated Genes and the Macropinocytosis Modulator TMED10 as Factors Supporting Vaccinia Virus Infection

Vaccinia virus is a promising viral vaccine and gene delivery candidate and has historically been used as a model to study poxvirus-host cell interactions. We employed a genome-wide insertional mutagenesis approach in human haploid cells to identify host factors crucial for vaccinia virus infection. A library of mutagenized HAP1 cells was exposed to modified vaccinia virus Ankara (MVA). Deep-sequencing analysis of virus-resistant cells identified host factors involved in heparan sulfate synthesis, Golgi organization, and vesicular protein trafficking. We validated EXT1, TM9SF2, and TMED10 (TMP21/p23/p24δ) as important host factors for vaccinia virus infection. The critical roles of EXT1 in heparan sulfate synthesis and vaccinia virus infection were confirmed. TM9SF2 was validated as a player mediating heparan sulfate expression, explaining its contribution to vaccinia virus infection. In addition, TMED10 was found to be crucial for virus-induced plasma membrane blebbing and phosphatidylserine-induced macropinocytosis, presumably by regulating the cell surface expression of the TAM receptor Axl.IMPORTANCE Poxviruses are large DNA viruses that can infect a wide range of host species. A number of these viruses are clinically important to humans, including variola virus (smallpox) and vaccinia virus. Since the eradication of smallpox, zoonotic infections with monkeypox virus and cowpox virus are emerging. Additionally, poxviruses can be engineered to specifically target cancer cells and are used as a vaccine vector against tuberculosis, influenza, and coronaviruses. Poxviruses rely on host factors for most stages of their life cycle, including attachment to the cell and entry. These host factors are crucial for virus infectivity and host cell tropism. We used a genome-wide knockout library of host cells to identify host factors necessary for vaccinia virus infection. We confirm a dominant role for heparin sulfate in mediating virus attachment. Additionally, we show that TMED10, previously not implicated in virus infections, facilitates virus uptake by modulating the cellular response to phosphatidylserine.

Suppression of a Natural Killer Cell Response by Simian Immunodeficiency Virus Peptides

Natural killer (NK) cell responses in primates are regulated in part through interactions between two highly polymorphic molecules, the killer-cell immunoglobulin-like receptors (KIRs) on NK cells and their major histocompatibility complex (MHC) class I ligands on target cells. We previously reported that the binding of a common MHC class I molecule in the rhesus macaque, Mamu-A1*002, to the inhibitory receptor Mamu-KIR3DL05 is stabilized by certain simian immunodeficiency virus (SIV) peptides, but not by others. Here we investigated the functional implications of these interactions by testing SIV peptides bound by Mamu-A1*002 for the ability to modulate Mamu-KIR3DL05⁺ NK cell responses. Twenty-eight of 75 SIV peptides bound by Mamu-A1*002 suppressed the cytolytic activity of primary Mamu-KIR3DL05⁺ NK cells, including three immunodominant CD8⁺ T cell epitopes previously shown to stabilize Mamu-A1*002 tetramer binding to Mamu-KIR3DL05. Substitutions at C-terminal positions changed inhibitory peptides into disinhibitory peptides, and vice versa, without altering binding to Mamu-A1*002. The functional effects of these peptide variants on NK cell responses also corresponded to their effects on Mamu-A1*002 tetramer binding to Mamu-KIR3DL05. In assays with mixtures of inhibitory and disinhibitory peptides, low concentrations of inhibitory peptides dominated to suppress NK cell responses. Consistent with the inhibition of Mamu-KIR3DL05⁺ NK cells by viral epitopes presented by Mamu-A1*002, SIV replication was significantly higher in Mamu-A1*002⁺ CD4⁺ lymphocytes co-cultured with Mamu-KIR3DL05⁺ NK cells than with Mamu-KIR3DL05^- NK cells. These results demonstrate that viral peptides can differentially affect NK cell responses by modulating MHC class I interactions with inhibitory KIRs, and provide a mechanism by which immunodeficiency viruses may evade NK cell responses.

Natural killer (NK) cells recognize and kill infected cells without prior antigenic stimulation, and thus provide an important early defense against virus infection. NK cell responses in primates are regulated in part through interactions between two highly polymorphic molecules, the killer-cell immunoglobulin-like receptors (KIRs) on NK cells and their major histocompatibility complex (MHC) class I ligands on target cells. Inhibitory KIRs normally suppress NK cell responses through interactions with their MHC class I ligands on the surface of healthy cells. However, when these interactions are perturbed, this inhibition is lost resulting in NK cell activation and killing of the target cell. We investigated the functional implications of simian immunodeficiency virus (SIV) peptides bound by a common MHC class I molecule in the rhesus macaque that stabilize or disrupt binding to an inhibitory KIR. Whereas SIV peptides that stabilized KIR-MHC class I binding suppressed NK cell activation, peptides that disrupted this interaction did not and resulted in NK cell lysis. These findings demonstrate that viral peptides can modulate NK cell responses through KIR-MHC class I interactions, and are consistent with the possibility that human and simian immunodeficiency viruses may acquire changes in epitopes that increase the binding of MHC class I ligands to inhibitory KIRs as a mechanism to suppress NK cell responses.

Viral interference with antigen presentation: trapping TAP

Following primary infection, herpesviruses persist for life in their hosts, even when vigorous anti-viral immunity has been induced. Failure of the host immune system to eliminate infected cells is facilitated by highly effective immune evasion strategies acquired by these herpesviruses during millions of years of co-evolution with their hosts. Here, we review the mechanisms of action of viral gene products that lead to cytotoxic T cell evasion through interference with the function of the transporter associated with antigen processing, TAP. The viral TAP inhibitors impede transport of peptides from the cytosol into the ER lumen, thereby preventing peptide loading onto MHC class I complexes. Recent insights have revealed a pattern of functional convergent evolution. In every herpesvirus subfamily, inhibitors of TAP function have been identified that are, surprisingly, unrelated in genome location, structure, and mechanism of action. Recently, cowpox virus has also been found to encode a TAP inhibitor. Expanding our knowledge on how viruses perturb antigen presentation, in particular by targeting TAP, not only provides information on viral pathogenesis, but also reveals novel aspects of the cellular processes corrupted by these viruses, notably the translocation of peptides by the ATP-binding cassette (ABC) transporter TAP. As the various TAP inhibitors are anticipated to impede discrete conformational transitions it is expected that crystal structures of TAP-inhibitor complexes will reveal valuable structural information on the actual mechanism of peptide translocation by TAP. Viral TAP inhibitors are also used for various (clinical) applications, for example, as effective tools in antigen presentation studies and as immunomodulators in immunotherapy for cancer, heterologous vaccination, and transplant protection.

EBV lytic-phase protein BGLF5 contributes to TLR9 downregulation during productive infection

Viruses use a wide range of strategies to modulate the host immune response. The human gammaherpesvirus EBV, causative agent of infectious mononucleosis and several malignant tumors, encodes proteins that subvert immune responses, notably those mediated by T cells. Less is known about EBV interference with innate immunity, more specifically at the level of TLR-mediated pathogen recognition. The viral dsDNA sensor TLR9 is expressed on B cells, a natural target of EBV infection. Here, we show that EBV particles trigger innate immune signaling pathways through TLR9. Furthermore, using an in vitro system for productive EBV infection, it has now been possible to compare the expression of TLRs by EBV(-) and EBV(+) human B cells during the latent and lytic phases of infection. Several TLRs were found to be differentially expressed either in latently EBV-infected cells or after induction of the lytic cycle. In particular, TLR9 expression was profoundly decreased at both the RNA and protein levels during productive EBV infection. We identified the EBV lytic-phase protein BGLF5 as a protein that contributes to downregulating TLR9 levels through RNA degradation. Reducing the levels of a pattern-recognition receptor capable of sensing the presence of EBV provides a mechanism by which the virus could obstruct host innate antiviral responses.

Alternative Ii-independent antigen-processing pathway in leukemic blasts involves TAP-dependent peptide loading of HLA class II complexes

During HLA class II synthesis in antigen-presenting cells, the invariant chain (Ii) not only stabilizes HLA class II complexes in the endoplasmic reticulum, but also mediates their transport to specialized lysosomal antigen-loading compartments termed MIICs. This study explores an alternative HLA class II presentation pathway in leukemic blasts that involves proteasome and transporter associated with antigen processing (TAP)-dependent peptide loading. Although HLA-DR did associate with Ii, Ii silencing in the human class II-associated invariant chain peptide (CLIP)-negative KG-1 myeloid leukemic cell line did not affect total and plasma membrane expression levels of HLA-DR, as determined by western blotting and flow cytometry. Since HLA-DR expression does require peptide binding, we examined the role of endogenous antigen-processing machinery in HLA-DR presentation by CLIP(-) leukemic blasts. The suppression of proteasome and TAP function using various inhibitors resulted in decreased HLA-DR levels in both CLIP(-) KG-1 and ME-1 blasts. Simultaneous inhibition of TAP and Ii completely down-modulated the expression of HLA-DR, demonstrating that together these molecules form the key mediators of HLA class II antigen presentation in leukemic blasts. By the use of a proteasome- and TAP-dependent pathway for HLA class II antigen presentation, CLIP(-) leukemic blasts might be able to present a broad range of endogenous leukemia-associated peptides via HLA class II to activate leukemia-specific CD4(+) T cells.

Cowpox virus inhibits the transporter associated with antigen processing to evade T cell recognition

Cowpox virus encodes an extensive array of putative immunomodulatory proteins, likely contributing to its wide host range, which includes zoonotic infections in humans. Unlike Vaccinia virus, cowpox virus prevents stimulation of CD8(+) T cells, a block that correlated with retention of MHC class I in the endoplasmic reticulum by the cowpox virus protein CPXV203. However, deletion of CPXV203 did not restore MHC class I transport or T cell stimulation. Here, we demonstrate the contribution of an additional viral protein, CPXV12, which interferes with MHC class I/peptide complex formation by inhibiting peptide translocation by the transporter associated with antigen processing (TAP). Importantly, human and mouse MHC class I transport and T cell stimulation was restored upon deletion of both CPXV12 and CPXV203, suggesting that these unrelated proteins independently mediate T cell evasion in multiple hosts. CPXV12 is a truncated version of a putative NK cell ligand, indicating that poxviral gene fragments can encode new, unexpected functions.

The E3 ubiquitin ligases Hrd1 and gp78 bind to and promote cholera toxin retro-translocation

To cause disease, cholera toxin (CT) is transported from the cell surface to the endoplasmic reticulum (ER) lumen where the catalytic CTA1 subunit retro-translocates to the cytosol to induce pathological water secretion. Two retro-translocon components are the Derlins and ER-associated multi-spanning E3 ubiquitin ligases including Hrd1 and gp78. We demonstrated previously that Derlin-1 facilitates CTA1 retro-translocation. However, as CTA1 is neither ubiquitinated on lysines nor at its N-terminus, the role of E3 ligases in toxin retro-translocation is unclear. Here, we show that expression of mutant Hrd1 and gp78 and a mutant E2-conjugating enzyme dedicated to retro-translocation (Ube2g2) decrease CTA1 retro-translocation. Hrd1 knockdown also attenuated toxin retro-translocation. Binding studies demonstrate that Hrd1 and gp78 interact with CT and protein disulfide isomerase, an ER chaperone that unfolds CTA1 to initiate translocation. Moreover, we find that the toxin's association with Hrd1 and gp78 is blocked by dominant-negative Derlin-1, suggesting that CT is targeted initially to Derlin-1 and then transferred to Hrd1 and gp78. These data demonstrate a role of the E3 ubiquitin ligases in CTA1 retro-translocation, implicate a sequence of events experienced by the toxin on the ER membrane, and raise the possibility that ubiquitination is involved in the transport process.

Reduced human leukocyte antigen expression in advanced-stage Ewing sarcoma: implications for immune recognition

Ewing sarcoma (EWS) is a tumour most commonly arising in bone, although on occasion in soft tissue, with a poor prognosis in patients with refractory or relapsed disease, despite multimodal therapy. Immunotherapeutic strategies based on tumour-reactive T and/or natural killer cells may improve the treatment of advanced-stage EWS. Since cellular immune recognition critically depends on human leukocyte antigen (HLA) expression, knowledge about HLA expression in EWS is crucial in the design of cellular immunotherapeutic strategies. Constitutive and IFNgamma-induced HLA class I expression was analysed in EWS cell lines (n = 6) by flow cytometry, using antibodies against both monomorphic and allele-specific antigens. Expression of antigen processing pathway components and beta-2 microglobulin (beta2m) was assessed by western blot. Expression of class II transactivator (CIITA), and its contribution to HLA class II expression, was evaluated by qRT-PCR, transduction assays, and flow cytometry. beta2m/HLA class I and class II expression was validated in EWS tumours (n = 67) by immunofluorescence. Complete or partial absence of HLA class I expression was observed in 79% of EWS tumours. Lung metastases consistently lacked HLA class I and sequential tumours demonstrated a tendency towards decreased expression upon disease progression. Together with absent or low constitutive expression levels of specific HLA class I loci and alleles, and differential induction of identical alleles by IFNgamma in different cell lines, these results may reflect the existence of an immune escape mechanism. Inducible expression of TAP-1/-2, tapasin, LMP-2/-7, and the beta2m/HLA class I complex by IFNgamma suggests that regulatory mechanisms are mainly responsible for heterogeneity in constitutive class I expression. EWSs lack IFNgamma-inducible HLA class II, due to lack of functional CIITA. The majority of EWS tumours, particularly if advanced-stage, exhibit complete or partial absence of both classes of HLA. This knowledge will be instrumental in the design of cellular immunotherapeutic strategies for advanced-stage EWS.

Ubiquitination of MHC class I heavy chains is essential for dislocation by human cytomegalovirus-encoded US2 but not US11

The human cytomegalovirus-encoded glycoproteins US2 and US11 target newly synthesized major histocompatibility complex class I heavy chains for degradation by mediating their dislocation from the endoplasmic reticulum back into the cytosol, where they are degraded by proteasomes. A functional ubiquitin system is required for US2- and US11-dependent dislocation of the class I heavy chains. It has been assumed that the class I heavy chain itself is ubiquitinated during the dislocation reaction. To test this hypothesis, all lysines within the class I heavy chain were substituted. The lysine-less class I molecules could no longer be dislocated by US2 despite the fact that the interaction between the two proteins was maintained. Interestingly, US11 was still capable of dislocating the lysine-less heavy chains into the cytosol. Ubiquitination does not necessarily require lysine residues but can also occur at the N terminus of a protein. To investigate the potential role of N-terminal ubiquitination in heavy chain dislocation, a lysine-less ubiquitin moiety was fused to the N terminus of the class I molecule. This lysine-less fusion protein was still dislocated in the presence of US11. Ubiquitination could not be detected in vitro, either for the lysine-less heavy chains or for the lysine-less ubiquitin-heavy chain fusion protein. Our data show that although dislocation of the lysineless class I heavy chains requires a functional ubiquitin system, the heavy chain itself does not serve as the ubiquitin acceptor. This finding sheds new light on the role of the ubiquitin system in the dislocation process.

In vitro processing and presentation of a lipidated cytotoxic T-cell epitope derived from measles virus fusion protein

Lipopeptidic formulations have been described as efficient activators of cytotoxic T lymphocytes (CTL). To better understand the pathway via which lipopeptides reach the MHC class I molecules we studied the intracellular processing and presentation of a measles virus-derived CTL epitope, to which a palmitoyl moiety was added synthetically. The palmitoyl group was conjugated to the N-terminus either directly or via a spacer sequence. The use of single or double fluorescent-labeled lipopeptides allowed the visualization of intracellular processing of these antigens using confocal microscopy. Our data indicate that the spacer composition influences internalization of the conjugate into the cell, proteasomal degradation, translocation into the ER by the transporter associated with antigen processing (TAP), and the intracellular trafficking of lipopeptides.

Immune evasion by human cytomegalovirus: lessons in immunology and cell biology

The human cytomegalovirus (HCMV) has dedicated a significant part of its genome to genes encoding molecules that modulate the host immune response. Many of these genes have homologues in the host genome. Others, however, are unique in the sense that no obvious primary sequence identity is found in the available databases. The HCMV gene products interfere with the activation of MHC class I and class II restricted T cells and NK cells, modify the function of cytokines and their receptors, interact with complement factors and modulate signal transduction and transcription factor activity, in addition to interference with many other cellular functions. Investigation of these evasion strategies has not only improved our understanding of HCMV pathogenesis, but has also provided unexpected, novel insights into basic cell biological and immunological processes.

Multiple-stage tandem mass spectrometry for structural characterization of saponins

Nanoelectrospray ion trap multiple-stage tandem mass spectrometry was applied to characterize saponins present in HPLC fractions from Quil A, a commercially available bark extract. An analytical strategy was developed based on recognition of carbohydrate sequence ions as well as glycosidic ring-cross ions formed by gamma-hydrogen rearrangements and successive retro-Diels-Alder fragmentations. These ions could be used for the determination of several glycosidic linkages, ring sizes, and positions of acyl groups. The presence of an acyl group on a monosaccharide residue facilitated the determination of the substitution pattern, due to the induction of ring-cross fragmentation. Deuteriomethylation resulted in a more extended set of ring-cross ions, thus allowing determination of additional glycosidic linkages. An analysis typically consumed 200 ng of sample and a total of 1-4 h for measurement and interpretation. The applied method is attractive as a pre-NMR analysis, especially because it resulted rapidly in an overall idea of the structure even when starting from scratch. The multiple-stage tandem approach enabled structural determination of saponins to a more detailed level than achievable with current single-stage tandem mass spectrometry.

Dislocation of type I membrane proteins from the ER to the cytosol is sensitive to changes in redox potential

The human cytomegalovirus (HCMV) gene products US2 and US11 dislocate major histocompatibility class I heavy chains from the ER and target them for proteasomal degradation in the cytosol. The dislocation reaction is inhibited by agents that affect intracellular redox potential and/or free thiol status, such as diamide and N-ethylmaleimide. Subcellular fractionation experiments indicate that this inhibition occurs at the stage of discharge from the ER into the cytosol. The T cell receptor alpha (TCR alpha) chain is also degraded by a similar set of reactions, yet in a manner independent of virally encoded gene products. Diamide and N-ethylmaleimide likewise inhibit the dislocation of the full-length TCR alpha chain from the ER, as well as a truncated, mutant version of TCR alpha chain that lacks cysteine residues. Cytosolic destruction of glycosylated, ER-resident type I membrane proteins, therefore, requires maintenance of a proper redox potential for the initial step of removal of the substrate from the ER environment.

The ER-luminal domain of the HCMV glycoprotein US6 inhibits peptide translocation by TAP

Human cytomegalovirus (HCMV) inhibits MHC class I antigen presentation by a sequential multistep process involving a family of unique short (US) region-encoded glycoproteins. US3 retains class I molecules, whereas US2 and US11 mediate the cytosolic degradation of heavy chains by the proteosomes. In US6-transfected cells, however, intracellular transport of class I molecules is impaired because of defective peptide translocation by transporters associated with antigen processing (TAP). Peptide transport is restored in HCMV mutants lacking US6. In contrast to the cytosolic herpes simplex virus protein ICP47, US6 interacts with TAP inside the endoplasmic reticulum lumen, as shown by US6 derivatives lacking the transmembrane and cytoplasmic domains and by the observation that US6 does not prevent peptides from binding to TAP. Thus, HCMV targets TAP for immune escape by a molecular mechanism different from that of herpes simplex virus.

Viruses use stealth technology to escape from the host immune system

In this review, we focus on recent investigations that reveal novel mechanisms by which viruses evade detection and elimination by the host immune system. In particular, we consider the evasion mechanisms of five persistent viruses: herpes simplex virus, human cytomegalovirus, mouse cytomegalovirus, Epstein-Barr virus and adenovirus. Unravelling the strategies used by viruses to survive within the host could identify new targets for antiviral drugs and for improved vaccines. Identification of the mechanisms that underlie these strategies might also reveal new, fundamental features of biology that occur in uninfected cells and are exploited by viruses.

The HCMV gene products US11 and US2 differ in their ability to attack allelic forms of murine major histocompatibility complex (MHC) class I heavy chains

Human cytomegalovirus downregulates the expression of human class I major histocompatibility complex (MHC) molecules by accelerating destruction of newly synthesized class I heavy chains. The HCMV genome contains at least two genes, US11 and US2, each of which encode a product sufficient for causing the dislocation of newly synthesized class I heavy chains from the lumen of the endoplasmic reticulum to the cytosol. Based on a comparison of their abilities to degrade the murine class I molecules H-2Kb, Kd, Db, Dd, and Ld, the US11 and US2 gene products have non-identical specificities for class I molecules. Specifically, in human astrocytoma cells (U373-MG) transfected with the US11 gene, the Kb, Db, Dd, and Ld molecules expressed via recombinant vaccinia virus are rapidly degraded, whereas in US2-transfected cells, only Db and Dd are significantly destabilized. The diversity in HCMV-encoded functions that interfere with class I-restricted presentation likely evolved in response to the polymorphism of the MHC.

Sec61-mediated transfer of a membrane protein from the endoplasmic reticulum to the proteasome for destruction

The human cytomegalovirus genome encodes proteins that trigger destruction of newly synthesized major histocompatibility complex (MHC) class I molecules. The human cytomegalovirus gene US2 specifies a product capable of dislocating MHC class I molecules from the endoplasmic reticulum to the cytosol and delivering them to the proteasome. This process involves the Sec61 complex, in what appears to be a reversal of the reaction by which it translocates nascent chains into the endoplasmic reticulum.

Human cytomegalovirus US3 impairs transport and maturation of major histocompatibility complex class I heavy chains

The human cytomegalovirus (HCMV) early glycoprotein products of the US11 and US2 open reading frames cause increased turnover of major histocompatibility complex (MHC) class I heavy chains. Since US2 is homologous to another HCMV gene (US3), we hypothesized that the US3 gene product also may affect MHC class I expression. In cells constitutively expressing the HCMV US3 gene, MHC class I heavy chains formed a stable complex with beta 2-microglobulin. However, maturation of the N-linked glycan of MHC class I heavy chains was impaired in US3+ cells. The glycoprotein product of US3 (gpUS3) occurs mostly in a high-mannose form and coimmunoprecipitates with beta 2-microglobulin associated class I heavy chains. Mature class I molecules were detected at steady state on the surface of US3+ cells, as in control cells. Substantial perinuclear accumulation of heavy chains was observed in US3+ cells. The data suggest that gpUS3 impairs egress of MHC class I heavy chains from the endoplasmic reticulum.

The human cytomegalovirus US11 gene product dislocates MHC class I heavy chains from the endoplasmic reticulum to the cytosol

Human cytomegalovirus (HCMV) down-regulates expression of MHC class I products by selective proteolysis. A single HCMV gene, US11, which encodes an endoplasmic reticulum (ER) resident type-I transmembrane glycoprotein, is sufficient to cause this effect. In US11+cells, MHC class I molecules are core-glycosylated and therefore inserted into the ER. They are degraded with a half-time of less than 1 min. A full length breakdown intermediate that has lost the single N-linked glycan in an N-glycanase-catalyzed reaction transiently accumulates in cells exposed to the protease inhibitors LLnL, Cbz-LLL, and lactacystin, identifying the proteasome as a key protease. Subcellular fractionation experiments show this intermediate to be cytosolic. Thus, US11 dislocates newly synthesized class I molecules from the ER to the cytosol, where they are acted upon by an N-glycanase and the proteasome.

T-cell responses to outer membrane proteins of Neisseria meningitidis: comparative study of the Opa, Opc, and PorA proteins

Former studies have shown that the class 5 outer membranes proteins (Opa and Opc proteins) of Neisseria meningitidis are at least as immunogenic as meningococcal porin proteins. High antibody titers to class 5 proteins have been observed in sera obtained during convalescence after meningococcal infection. A strong increase in anti-class 5 antibodies has also been observed in vaccinees who received a meningococcal outer membrane vesicle preparation. The enhanced B-cell response to class 5 proteins may be due to the presence of immunodominant helper T-cell epitopes in these proteins. In order to investigate this hypothesis, we tested purified Opa, Opc, and class 1 proteins for recognition by human T cells. a hierarchy of T-cell immunogenicity was observed among the outer membrane proteins, the Opa protein being more immunogenic than the other proteins. In most cases, the proliferative responses elicited by Opc were higher than the responses observed for the class 1 protein. The epitopes recognized by the immune T cells were identified by using overlapping synthetic peptides spanning the protein sequences of OpaB, Opa5d, and Opc.

Non-specific modulation of the immune response with liposomal meningococcal lipopolysaccharide: role of different cells and cytokines

The immunomodulating action of Neisseria meningitidis lipopolysaccharide (LPS) incorporated into liposomes and the activation of different populations of immunocompetent cells or the secretion of cytokines were studied. LPS stimulated an anti-sheep red blood cell (SRBC) plaque-forming cell response in the spleen of mice after simultaneous injection of LPS and SRBC but if LPS was administered 3 days before the immunization with SRBC the response to SRBC was strongly suppressed. After the incorporation of LPS into liposomes the stimulation index was increased from 6 to 19 and the liposomal LPS did not suppress the immune response to SRBC. The incorporation of LPS into liposomes leads to enhancement of B-mitogenic properties of LPS, as liposomal LPS stimulated the proliferation of splenocytes in mice better than free LPS and has no influence on the thymocytes. The liposomal LPS induced more prolonged and significant accumulation of IgM-secreting cells in the spleen of mice in comparison with the free LPS. Liposomal LPS also induced more active accumulation of IFN-gamma in human peripheral blood mononuclear cells and less active accumulation of monokines, contributing to the realization of the toxic properties of endotoxin (IL-1 alpha, TNF-alpha, IL-6 and GM-CSF). These results demonstrated that the incorporation of N. meningitidis LPS into liposomes dramatically changed its immunomodulating activity. The data obtained are important for the construction of an adjuvant formulation for synthetic immunogens capable of inducing genetically unrestricted immune responses.

Meningococcal lipopolysaccharide (LPS)-derived oligosaccharide-protein conjugates evoke outer membrane protein- but not LPS-specific bactericidal antibodies in mice: influence of adjuvants

Meningococcal lipopolysaccharide (LPS)-derived oligosaccharides (OS) were coupled to tetanus toxoid (TT) and purified P1.7,16 outer membrane proteins (OMP). The immunogenicities of the conjugates with and without the addition of the adjuvant Quil A or the nonionic block polymer L121 were studied in mice. Immunotype L2 and L3,7,9 OS-TT conjugates induced immunoglobulin G (IgG) responses that were strongly augmented by Quil A and L121. These adjuvants not only enhanced the amount of IgG evoked but also shifted the IgG subclass distribution from mainly IgG1 toward the complement-activating subclasses IgG2a and IgG2b. The antibodies induced were directed against the OS part of meningococcal LPS. They were not bactericidal for group B meningococci. Both the L3,7,9 OS-P1.7,16 OMP conjugate and purified P1.7,16 OMP evoked a strong IgG response against the P1.7,16 OMP but not against the L3,7,9 LPS. These anti-OMP IgG responses were comparable to the IgG OMP-specific responses induced by the H44/76 or HIII-5 outer membrane vesicles but still did not lyse group B meningococcal strains. The IgG response evoked with OS-OMP or purified OMP consisted mainly of the IgG1 subclass, whereas the H44/76 or HIII-5 outer membrane vesicles induced high amounts of bactericidal IgG2a and IgG2b antibodies next to the IgG1 antibodies. The addition of the adjuvant Quil A or L121 to OS-OMP or OMP resulted in the induction of high levels of bactericidal anti-P1.7,16-specific OMP antibodies, as reflected by the presence of substantial amounts of IgG2a and IgG2b antibodies. These results indicate that (i) mouse anti-LPS antibodies evoked by LPS-derived OS-protein conjugates are not bactericidal for group B meningococci, (ii) extensive purification of P1.7,16 OMP can lead to the loss of the intrinsic adjuvant properties of outer membrane vesicle preparations, and (iii) the addition of suitable adjuvants restores the ability of these purified P1.7,16 OMP to induce bactericidal antibodies.

Interaction of pertussis toxin with human T lymphocytes

The binding of pertussis toxin (PT) to the human T-cell line Jurkat was examined by using flow cytometry. Fluorescein isothiocyanate (FITC)-labeled PT bound rapidly to the cells in a specific manner as determined by blocking experiments with unlabeled toxin, B oligomer, and the S2-S4 and S3-S4 dimers. Monoclonal antibodies against the S3 subunit of the toxin also significantly inhibited the binding of FITC-PT. Sialidase treatment of the cells resulted in decreased binding of FITC-PT, indicating that sialic acid residues are involved in the binding process. In addition, we studied the effect of PT binding on the expression of cell surface molecules. On binding of PT to the cell surface, a rapid down-regulation of the T-cell receptor (TCR)-CD3 complex was observed. The modulation of the TCR-CD3 complex was independent of the toxin's enzymatic activity, as the B oligomer and a nonenzymatic toxin mutant induced modulation comparable to that caused by the native holotoxin. Isolated dimers did not cause down-regulation. Stimulation of the TCR-CD3 complex, leading to reduced cell surface expression of this complex, provides a possible explanation for the second messenger production associated with the interaction of PT or B oligomer with T lymphocytes. We therefore conclude that PT activates T cells by divalent binding to the TCR-CD3 complex itself or by binding a structure closely associated with it.

The modified leukocytosis promoting factor (LPF)-test: a valuable supplement to the mouse weight gain (MWG)-test in toxicity control of whole cell pertussis vaccine

For the safety testing of pertussis vaccine, many in vivo assays have been developed, but none of these assays, except the Mouse Weight Gain (MWG)-test, are obligatory. Leukocytosis Promoting Factor (LPF) test, performed in mice, is one of the tests to examine the toxicity. However, due to lack of standardization, this test has not been implemented in the regular safety testing of the vaccine. Our investigations demonstrate that the LPF-test becomes more reproducible and sensitive if preparations are administered subcutaneously on day 0 and and counting of the leukocytes are done on day 6. Therefore, it is suggested to include the revised LPF-test in the quality control panel for the assessment of the toxicity of whole-cell pertussis vaccine.

Identification of T cell epitopes occurring in a meningococcal class 1 outer membrane protein using overlapping peptides assembled with simultaneous multiple peptide synthesis

The meningococcal class 1 outer membrane protein (OMP) plays an important role in the development of protective immunity against meningococcal infection, and is therefore considered to be a promising candidate antigen (Ag) for a meningococcal vaccine. The induction of an effective antibody response entirely depends upon T helper cells. To identify T cell epitopes of the OMP, we prepared 45 overlapping synthetic peptides representing the entire sequence of the class 1 protein of reference strain H44/76. Fully automated simultaneous multiple peptide synthesis (SMPS) was used to assemble the 45 twenty mer which overlapped by 12 amino acid residues on a 12 mumol scale. The peptides were tested for recognition by peripheral blood mononuclear cells (PBMC) obtained from 34 volunteers. Surprisingly, all synthetic peptides induced proliferative responses of PBMC isolated from one or more human histocompatibility leukocyte antigen (HLA)-typed immune adults. With PBMC from seven nonimmune donors, no proliferative response was observed. Immunodominant regions were found, recognized by PBMC from many volunteers, irrespective of their HLA type. Most of the immunodominant T cell epitopes are located outside the variable regions and, thus, will be conserved among different meningococcal (and gonococcal) strains. Furthermore, the overlapping peptides could be used to identify the epitopes recognized by OMP-specific T cell clones with known HLA restriction. It is interesting that the epitopes defined with the clones occur in highly conserved areas, shared by all neisserial porin proteins. In summary, this analysis of the T cell response to the meningococcal class 1 OMP constitutes a complete study of reactivity to a foreign protein, and illustrates some important features of Ag recognition by T cells. Our data demonstrate unexpected diversity in the T cell recognition of the OMP, and imply that the T cell repertoire against foreign Ag may be greater than previously assumed. This observation is supported by recent data on the interaction of peptide and major histocompatibility complex (MHC) class II, the latter being much less selective than MHC class I. Finally, a comparative analysis pointed out the limitations of algorithms predicting T cell determinants, and the importance of the empirical methodology provided by SMPS.

T cell recognition of Neisseria meningitidis class 1 outer membrane proteins. Identification of T cell epitopes with selected synthetic peptides and determination of HLA restriction elements

No vaccine is yet available against serogroup B meningococci, which are a common cause of bacterial meningitis. Some outer membrane proteins (OMP), LPS, and capsular polysaccharides have been identified as protective Ag. The amino acid sequence of the protective B cell epitopes present within the class 1 OMP has been described recently. Synthetic peptides containing OMP B cell epitopes as well as capsular polysaccharides or LPS protective B cell epitopes have to be presented to the immune system in association with T cell epitopes to achieve an optimal Ir. The use of homologous, i.e., meningococcal, T cell epitopes has many advantages. We therefore investigated recognition sites for human T cells within the meningococcal class 1 OMP. We have synthesized 16 class 1 OMP-derived peptides encompassing predicted T cell epitopes. Peptides corresponding to both surface loops and trans-membrane regions (some of which occur as amphipathic beta-sheets) of the class 1 OMP were found to be recognized by T cells. In addition, 10 of 11 peptides containing predicted amphipathic alpha-helices and four of five peptides containing T cell epitope motifs according to Rothbard and Taylor (Rothbard, J. B., and W. R. Taylor. 1988. EMBO J 7:93) were recognized by lymphocytes from one or more volunteers. Some of the T and B cell epitopes were shown to map to identical regions of the protein. At least six of the peptides that were found to contain T cell epitopes show homology to constant regions of the meningococcal class 3 OMP and the gonococcal porins PIA and PIB. Peptide-specific T cell lines and T cell clones were established to investigate peptide recognition in more detail. The use of a panel of HLA-typed APC revealed clear HLA-DR restriction patterns. It seems possible now to develop a (semi-) synthetic meningococcal vaccine with a limited number of constant T cell epitopes that cover all HLA-DR locus products.

T cell recognition of Neisseria meningitidis class 1 outer membrane proteins. Identification of T cell epitopes with selected synthetic peptides and determination of HLA restriction elements

No vaccine is yet available against serogroup B meningococci, which are a common cause of bacterial meningitis. Some outer membrane proteins (OMP), LPS, and capsular polysaccharides have been identified as protective Ag. The amino acid sequence of the protective B cell epitopes present within the class 1 OMP has been described recently. Synthetic peptides containing OMP B cell epitopes as well as capsular polysaccharides or LPS protective B cell epitopes have to be presented to the immune system in association with T cell epitopes to achieve an optimal Ir. The use of homologous, i.e., meningococcal, T cell epitopes has many advantages. We therefore investigated recognition sites for human T cells within the meningococcal class 1 OMP. We have synthesized 16 class 1 OMP-derived peptides encompassing predicted T cell epitopes. Peptides corresponding to both surface loops and trans-membrane regions (some of which occur as amphipathic beta-sheets) of the class 1 OMP were found to be recognized by T cells. In addition, 10 of 11 peptides containing predicted amphipathic alpha-helices and four of five peptides containing T cell epitope motifs according to Rothbard and Taylor (Rothbard, J. B., and W. R. Taylor. 1988. EMBO J 7:93) were recognized by lymphocytes from one or more volunteers. Some of the T and B cell epitopes were shown to map to identical regions of the protein. At least six of the peptides that were found to contain T cell epitopes show homology to constant regions of the meningococcal class 3 OMP and the gonococcal porins PIA and PIB. Peptide-specific T cell lines and T cell clones were established to investigate peptide recognition in more detail. The use of a panel of HLA-typed APC revealed clear HLA-DR restriction patterns. It seems possible now to develop a (semi-) synthetic meningococcal vaccine with a limited number of constant T cell epitopes that cover all HLA-DR locus products.

Acellular and whole cell pertussis vaccines protect against the lethal effects of intracerebral challenge by two different T-cell dependent humoral routes

Athymic (nu/nu) and euthymic (+/nu) BALB/c mice were immunized with a whole cell pertussis vaccine or with an acellular vaccine which contained detoxified pertussis toxin (PT) and filamentous hemagglutinin (FHA). Only the euthymic mice were protected against intracerebral challenge with virulent Bordetella pertussis which implies involvement of T-cells. As a cell transfer from mice immunized with whole cell or acellular vaccine prior to the challenge did not protect naive euthymic recipients, cellular immunity seems to be non-protective as an effector mechanism. Mice could be protected passively against a challenge by administration of immune sera. Therefore, T-cell dependent humoral immune responses to B. pertussis appear to be crucial for protection. The humoral response was further studied with athymic and euthymic mice. In euthymic mice the whole cell vaccine induced antibodies to FHA, pililipopolysaccharides (LPS) and an outer membrane protein (OMP) preparation, whereas the acellular vaccine induced antibodies to PT, FHA and OMP. Both IgM and IgG could be detected. From the nude mice only those immunized with the whole cell vaccine showed an antibody response which consisted of low titres of IgM directed to LPS. Sera from both +/nu and nu/nu mice immunized with the whole cell vaccine were bactericidal in vitro. These data demonstrate that in the mouse model protection to intracerebral challenge with B. pertussis is T-cell dependent as is the humoral response to PT, FHA, OMP and pili. The T-independent B-cell activation by the whole cell preparation is due to the presence of LPS.(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro induction of antigen specific antibody synthesis and proliferation of T lymphocytes with acellular pertussis vaccines, pertussis toxin and filamentous haemagglutinin in humans

The in vitro response of human B- and T-lymphocytes to the acellular vaccines JNIH-6 (containing pertussis toxoid and filamentous hemagglutinin), and JNIH-7 (containing pertussis toxoid), and to the purified components JNIH-4 (filamentous hemagglutinin) and JNIH-5 (pertussis toxin) was investigated. Pertussis toxoid and filamentous hemagglutinin induced specific Ig synthesis in vitro in lymphocytes obtained from convalescent pertussis patients as target cells. The antigen-dependent Ig production was demonstrated in lymphocyte culture supernatants by ELISA techniques and by a chinese hamster ovary cell toxin neutralization assay. Particularly with JNIH-4, -6 and -7, high antibody titers were obtained. At optimal antigen concentrations a marked lymphocyte blast transformation was found in lymphocyte cultures from whooping cough patients, but not in cultures of lymphocytes obtained from healthy volunteers. At high concentrations native pertussis toxin as well as the B oligomer (S2-5) of the toxin induced a strong proliferation of patient as well as control lymphocytes, indicating non-specific mitogenic activity. At lower concentrations lymphocyte blast transformation was seen in patient cultures only, which indicates an antigen-specific T-cell response. The A protomer (S1), dimer 1 (S2 + 4) and dimer 2 (S3 + 4) induced proliferation of patient lymphocytes, which demonstrates the presence of T-cell epitopes on these peptides. The in vitro B-cell response and the lymphocyte blast transformation assay are both useful tools for estimating the potency of acellular pertussis vaccines in man. Spontaneously acquired and vaccine induced immunity to Bordetella pertussis can be investigated at the level of B- and T-lymphocytes.