Death by neglect as a deletional mechanism of peripheral tolerance

In contrast to most organs, the anatomy of the liver may allow naive CD8(+) T cells to make direct contact with liver parenchymal cells. We have previously shown, using a combination of TCR transgenic T cells specific for H-2 K(b) and hepatocytes expressing a transgenic H-2 K(b) molecule, that hepatocytes can induce antigen-specific activation and proliferation of naive CD8(+) T cells independently of CD28 co-stimulation. However, T cell activation by hepatocytes leads to premature T cell death and tolerance, both in vivo and in vitro. In this study, we investigated the mechanisms of T cell death induced by hepatocytes in vitro using primary hepatocytes to activate purified CD8(+) T cells. Neither Fas nor tumor necrosis factor receptor were involved, indicating that hepatocyte- induced death was distinct from activation-induced cell death. Before they started to divide, T cells activated by hepatocytes expressed lower levels of the bcl-x(L) survival gene and 30 times less IL-2 mRNA than CD8(+) cells activated by splenic antigen-presenting cells. Since CD28 co-stimulation increases both IL-2 and bcl-x(L) expression, this suggests that hepatocyte-activated T cells die by neglect because they fail to receive effective co-stimulatory signals. In agreement with this model, premature death promoted by hepatocytes could be prevented by cross-linking CD28. Survival after CD28 cross-linking correlated with increased IL-2 and bcl-x(L) expression, and sustained T cell proliferation, while cytotoxic T lymphocyte activity was prolonged as compared with cells stimulated without CD28 co-stimulation. This study confirms that high- affinity TCR transgenic antigen-specific CD8(+) T cells can be activated to proliferate and differentiate into cytotoxic effector cells. However, prolonged T cell survival and cytotoxicity required CD28 co-stimulation as well. To our knowledge, this is the first report suggesting that tolerance in the context of lack of CD28 co-stimulation can result from Fas-independent peripheral deletion rather than from anergy.

Measles virus infection induces terminal differentiation of human thymic epithelial cells

Measles virus infection induces a profound immunosuppression that may lead to serious secondary infections and mortality. In this report, we show that the human cortical thymic epithelial cell line is highly susceptible to measles virus infection in vitro, resulting in infectious viral particle production and syncytium formation. Measles virus inhibits thymic epithelial cell growth and induces an arrest in the G0/G1 phases of the cell cycle. Moreover, we show that measles virus induces a progressive thymic epithelial cell differentiation process: attached measles virus-infected epithelial cells correspond to an intermediate state of differentiation while floating cells, recovered from cell culture supernatants, are fully differentiated. Measles virus-induced thymic epithelial cell differentiation is characterized by morphological and phenotypic changes. Measles virus-infected attached cells present fusiform and stellate shapes followed by a loss of cell-cell contacts and a shift from low- to high-molecular-weight keratin expression. Measles virus infection induces thymic epithelial cell apoptosis in terminally differentiated cells, revealed by the condensation and degradation of DNA in measles virus-infected floating thymic epithelial cells. Because thymic epithelial cells are required for the generation of immunocompetent T lymphocytes, our results suggest that measles virus-induced terminal differentiation of thymic epithelial cells may contribute to immunosuppression, particularly in children, in whom the thymic microenvironment is of critical importance for the development and maturation of a functional immune system.

Somatostatin-dependent adenylyl cyclase activity in nonactivated and mitogen-activated human T cells: evidence for uncoupling of sst3 receptor from adenylyl cyclase

Neuropeptide somatostatin (SRIF) has been shown to modulate interleukin-2 (IL-2) secretion by mitogen-activated T cells. In this study, we further analyzed the transduction pathways underlying SRIF actions on human Jurkat T cells and compared SRIF signaling between nonactivated and mitogen-activated cells. SRIF effects on adenylyl cyclase activity in the absence and presence of mitogens were addressed by using three different analogs: SRIF14, SRIF28, and SMS 201-995. In semipurified membrane preparations obtained from nonactivated cells, all analogs inhibited adenylyl cyclase. However, in membrane preparations obtained from mitogen-activated cells, the maximal inhibition of adenylyl cyclase mediated by SRIF14 and SRIF28 equaled only one third of that measured in the absence of mitogens, whereas SMS 201-995 was completely inactive. To assess the relevant mechanisms associated with different effects of SRIF on adenylyl cyclase activity in nonactivated and mitogen-activated T cells, we performed binding assays by using iodinated SRIF as a radioligand. These experiments suggested that both the number of receptors and their affinities were almost identical in either nonactivated or activated cells. RT-PCR analysis of the pattern of SRIF receptor expression showed that nonactivated as well as activated Jurkat cells expressed only mRNA corresponding to the sst3 receptor subtype. Altogether, these data point to a functional activation-associated uncoupling of sst3 receptors from adenylyl cyclase in human T cells, indicating a T-cell activation-induced alteration in the sst3 receptor transduction pathway.

sst5 somatostatin receptor mRNA induction by mitogenic activation of human T-lymphocytes

SRIF has neuro-immunomodulatory actions on immune cells, including T-lymphocytes. Molecular mechanisms involved in these actions were studied by RT-PCR analysis of SRIF receptor expression in resting and initogen-activated human T-lymphocytes. Our results point to the mitogen-associated induction of sst5 receptor subtype. Conversely, sst3 receptor appears constitutively expressed in both activity states. Assessment of biologic actions of SRIF14 in activated T-lymphocytes indicates that, in nanomolar concentration range, this peptide moderately inhibits mitogen-induced IL-2 secretion. Nevertheless, T-lymphocyte proliferation is not inhibited in the presence of SRIF14 but is even slightly increased. Altogether these data suggest a complex mechanism of SRIF neuro-immunomodulatory actions.

Molecular modeling of hen egg lysozyme HEL[52-61] peptide binding to I-Ak MHC class II molecule

A bound conformation of the antigenic decapeptide hen egg lysozyme HEL[52-61] associated to the mouse MHC class II (MHC II) I-Ak was modeled by homology with the three-dimensional structure of hemagglutinin HA[306-318]-HLA-DR1 complex. HEL peptide Tyr53 could not be aligned with the HA peptide Tyr308 because this resulted in a buried Tyr53 side chain within the I-Ak peptide-binding groove and this conflicted with this side chain being recognized by T cells. Therefore, Asp52 of HEL was fixed as the P1 anchor and aligned on Tyr308 of HA. After molecular dynamics, the modeled complex was stable even in the absence of any constraint. The peptide backbone adopted a polyproline II-like conformation with canonical hydrogen bonding between the peptide backbone and MHC II molecule. Asp52, IIe55, Gin57 and Ser60 were predicted to be deeply buried into P1, P4, P6 and P9 MHC II pockets, and Tyr53, Leu56, Asn59 and Arg61 as TCR contacting residues. The modeling of 15 complexes associating I-Ak with peptides derived from HEL[52-61] by single amino acid substitution proved stable with conserved hydrogen bonds and side chain orientation compatible with their recognition by two T cell hybridomas. Moreover, comparison with the recently solved crystal structure of the related HEL[50-62]-I-Ak complex revealed striking similarities.

Synthesis, Biological Activity and Molecular Modelling of the 53-54 Ketomethylene Analogue ofHEL(52-61) .

Abstract:

The synthesis of the 53-54 ketomethylene isostere of HEL(52-61), an antigenic decapeptide, was realised by convergent Fmoc solid-phase peptide synthesis. In . addition to the target peptide, an unexpected hydroxysuccinyl by-product was recovered. The two ketomethylene epimers did not exhibit any stimulating or competitive activity. Molecular modelling studies of the analogues-MHC-II 1-Ak complex.suggested a departure from the canonical polyproline II conformation which could account for the absence of binding to 1- Ak molecule.

Enhanced MHC class II-restricted presentation of measles virus (MV) hemagglutinin in transgenic mice expressing human MV receptor CD46

This study analyzes the role of the measles virus (MV) receptor, i.e. the human CD46 molecule, in the MHC class II-restricted presentation of MV hemagglutinin (H). We generated transgenic mice ubiquitously expressing CD46, with a similar level of transgene expression on the surface of antigen-presenting cells (APC), i.e. B cells, dendritic cells (DC) and macrophages. APC isolated from transgenic mice and nontransgenic controls were tested for their ability to present MV H to H-specific CD4+ I-Ed-restricted T cell hybridomas. All three populations of APC were capable of presenting MV to T cell hybridomas, DC being the most efficient. Expression of CD46 on B lymphocytes increased MHC class II-dependent presentation of MV H up to 100-fold, while CD46-transgenic DC stimulated H-specific T cell hybridomas up to 10-fold better than nontransgenic DC. Interestingly, expression of CD46 did not change the presentation efficiency of transgenic macrophages, indicating that CD46-dependent enhancement of antigen presentation depends on the nature of the APC. Furthermore, a single injection of UV-inactivated MV particles into CD46-transgenic mice, but not nontransgenic controls, induced generation of MV-specific T lymphocytes and production of anti-H antibodies, suggesting a role for CD46 in the efficient capture of MV in vivo. These results show for the first time that one ubiquitously expressed cell surface receptor, like CD46, could function in receptor-mediated antigen presentation both in vitro and in vivo and its performance depends on the type of APC which expresses it.

Hepatocytes induce functional activation of naive CD8+ T lymphocytes but fail to promote survival

Intraperitoneal peptide injection of TCR-transgenic mice or expression of antigen in hepatocytes leads to an accumulation in the liver of specific apoptotic CD8+ T cells expressing activation markers. To determine whether liver cells are capable of directly activating naive CD8+ T cells, we have studied the ability of purified hepatocytes to activate TCR-transgenic CD8+ T cells in vitro. We show that hepatocytes which do not express CD80 and CD86 co-stimulatory molecules are able to induce activation and effective proliferation of specific naive CD8+ T cells in the absence of exogenously added cytokines, a property only shared by professional antigen-presenting cells (APC). Specific T cell proliferation induced by hepatocytes was comparable in magnitude to that seen in response to dendritic cells and was independent of CD4+ T cell help or bystander professional APC co-stimulation. During the first 3 days, the same number of divisions was observed in co-cultures of CD8+ T cells with either hepatocytes or splenocytes. Both APC populations induced expression of early T cell activation markers and specific cytotoxic T lymphocyte (CTL) activity. However, in contrast to T cells activated by splenocytes, T cells activated by hepatocytes lost their cytolytic function after 3 days of co-culture. This correlated with death of activated T cells, suggesting that despite efficient activation, proliferation and transient CTL function, T cells activated by hepatocytes did not survive. Death could be prevented by adding antigen-expressing splenocytes or exogenous IL-2 to the co-culture, indicating that hepatocytes are not involved in direct killing of CD8+ T cells but rather fail to promote survival. Dying cells acquired a CD8(low) TCR(low) B220+ phenotype similar to the one described for apoptotic intrahepatic T cells, suggesting an alternative model to account for the origin of these cells in the liver. The importance of these findings for the understanding of peripheral tolerance and the ability of liver grafts to be accepted is discussed.

An accessory peptide binding site with allosteric effect on the formation of peptide-MHC-II complexes?

MHC-II molecules bind a single peptide in their groove. Here, the authors summarise evidence that a second peptide could bind transiently to MHC-II molecules outside the groove and have an allosteric effect on peptide-MHC-II complex formation. This effect could modulate, after the antigen processing, the selection of the peptide subset presented by MHC-II molecules to the helper CD4 T cells, which regulate the specific immune response.

Measles virus modulates human T-cell somatostatin receptors and their coupling to adenylyl cyclase

The possible role of immunomodulatory peptide somatostatin (SRIF) in measles virus (MV)-induced immunopathology was addressed by analysis of SRIF receptors and their coupling to adenylyl cyclase in mitogen-stimulated Jurkat T cells and human peripheral blood mononuclear cells (PBMC). SRIF-specific receptors were assayed in semipurified membrane preparations by using SRIF14 containing iodinated tyrosine at the first position in the amino acid chain ([125I]Tyr1) as a radioligand. A determination of receptor number by saturation of radioligand binding at equilibrium showed that in Jurkat cells, MV infection led to a dramatic decrease in the total receptor number. The virus-associated disappearance of one (Ki2 = 12 +/- 4 nM [mean +/- standard error of the mean [SEM]]; n = 4) of two somatostatin binding sites identified in control Jurkat cells (Ki1 = 78 +/- 3 pM and Ki2 = 12 +/- 4 nM [mean +/- SEM]; n = 4) was also observed. Almost identical results were obtained for phytohemagglutinin-activated human PBMC. In the absence of MV infection, two somatostatin binding sites were present (Ki1 = 111 +/- 31 pM and Ki2 = 17 +/- 2 nM [mean +/- SEM]; n = 2), whereas in MV-infected cells, only the high-affinity (Ki1 = 48 +/- 15 pM [mean +/- SEM]; n = 2) binding site remained. In addition, MV infection reinforced the inhibitory effects of SRIF on adenylyl cyclase activity, since maximal inhibition at 1 microM peptide was 11% +/- 4% in control cells versus 25% +/- 3% (P < 0.05) in infected Jurkat cells. Moreover, MV infection severely impaired the capacity of adenylyl cyclase to be activated directly (by forskolin) or indirectly (via Gs protein-coupled vasoactive intestinal peptide receptor). An assessment of [methyl-3H]thymidine incorporation showed that SRIF increased proliferative responses to mitogens only in control cells, not in MV-infected cells. Altogether, our data emphasize that MV-associated alteration of SRIF transduction appears to be related to the loss of SRIF-dependent increase of mitogen-induced proliferation.

Measles virus suppresses cell-mediated immunity by interfering with the survival and functions of dendritic and T cells

Secondary infections due to a marked immunosuppression have long been recognized as a major cause of the high morbidity and mortality rate associated with acute measles. The mechanisms underlying the inhibition of cell-mediated immunity are not clearly understood but dysfunctions of monocytes as antigen-presenting cells (APC) are implicated. In this report, we demonstrate that measles virus (MV) replicates weakly in the resting dendritic cells (DC) as in lipopolysaccharide-activated monocytes, but intensively in CD40-activated DC. The interaction of MV-infected DC with T cells not only induces syncytia formation where MV undergoes massive replication, but also leads to an impairment of DC and T cell function and cell death. CD40-activated DC decrease their capacity to produce interleukin (IL) 12, and T cells are unable to proliferate in response to MV-infected DC stimulation. A massive apoptosis of both DC and T cells is observed in the MV pulsed DC-T cell cocultures. This study suggests that DC represent a major target of MV. The enhanced MV replication during DC-T cell interaction, leading to an IL-12 production decrease and the deletion of DC and T cells, may be the essential mechanism of immunosuppression induced by MV.

Measles virus nucleocapsid protein binds to FcgammaRII and inhibits human B cell antibody production

Despite the development of an efficient specific immune response during measles virus (MV) infection, an immunosuppression occurs contributing to secondary infections. To study the role of nucleocapsid protein (NP) in MV-induced immunosuppression, we produced recombinant MV NP. Purified recombinant NP exhibited biochemical, antigenic, and tridimensional structure similar to viral NP. By flow cytometry, we showed that viral or recombinant NP bound to human and murine B lymphocytes, but not to T lymphocytes. This binding was specific, independent of MHC class II expression, and dependent of the B lymphocyte activation state. The murine IIA1. 6 B cell line, deficient in the Fc receptor for IgG (FcgammaRII) expression, did not bind NP efficiently. Transfected IIA1.6 cells expressing either murine FcgammaRIIb1 or b2, or human FcgammaRIIa, b1*, or b2 isoforms efficiently bound NP. Furthermore, this binding was inhibited up to 90% by monoclonal antibodies 2.4G2 or KB61 specific for murine and human FcgammaRII, respectively. Finally, the in vitro Ig synthesis of CD40- or Ig-activated human B lymphocytes in the presence of interleukin (IL)-2 and IL-10 was reduced by 50% in the presence of recombinant NP. These data demonstrate that MV NP binds to human and murine FcgammaRII and inhibits in vitro antibody production, and therefore suggests a role for NP in MV-induced immunosuppression.

Transgenic expression of a CD46 (membrane cofactor protein) minigene: studies of xenotransplantation and measles virus infection

CD46 (membrane cofactor protein) is a human cell-surface regulator of activated complement and a receptor for the measles virus. A CD46 transgenic mouse line with an expression pattern similar to that of human tissues has been produced, to develop an animal model of (i) the control of complement activation by complement regulators in hyperacute rejection of xenografts, and (ii) measles virus infection. The mouse line was made using a CD46 minigene that includes promoter sequence and the first two introns of genomic CD46, which was coinjected into mouse ova with chicken lysozyme matrix attachment region DNA. A high level of CD46 expression in homozygotic transgenic mice was obtained with spleen cells having approximately 75% of the level found on human peripheral blood mononuclear cells. CD46 was detected in all tissues examined by immunohistochemistry, radioimmunoassay and Western blotting, showing that these mice were suitable for transplantation and measles virus infection studies. It also indicated that the transgene included the important regulatory elements of the CD46 promoter. Transgenic spleen cells were significantly protected in vitro from human complement activated by either the classical or alternative pathways and from alternative pathway rat complement. Furthermore, transgenic mouse hearts transplanted to rats regulated complement deposition in an in vivo model of antibody-dependent hyperacute xenograft rejection. Similar to human lymphocytes, transgenic lymphoblasts could be infected in vitro with measles virus; infected cells expressed viral proteins and produced infectious viral particles. The data demonstrate the suitability of this minigene for obtaining high-level CD46 expression sufficient for enhanced resistance of transgenic cells to complement attack and for obtaining wide tissue distribution of CD46, analogous to human tissues and, therefore, useful for comparative studies.

The ectodomain of measles virus envelope glycoprotein does not gain access to the cytosol and MHC class I presentation pathway following virus-cell fusion

To unravel the intracellular fate of measles virus (MV) haemagglutinin (H) following fusion of the virus envelope with the cell membrane, its presentation by MHC molecules to T cells was explored. After MV infection, murine cells expressing CD46 were lysed by MHC class I-restricted CD8 CTLs specific for the ectodomain of H. In contrast, when sensitized with UV-inactivated MV, they were not lysed by these effectors, but were recognized by H-specific and class II-restricted CD4 CTLs. Thus, after MV binding and fusion, H becomes associated with plasma membrane and its ectodomain can reach the endosomal MHC-II but not the cytosolic MHC-I antigen presentation pathway. From these data and a reappraisal of previous reports, it appears that the ectodomains of both MV haemagglutinin fusion proteins, having undergone the fusion step, are not translocated into the cytosol and end up in the endosomes.

Transgenic mice expressing human measles virus (MV) receptor CD46 provide cells exhibiting different permissivities to MV infections

We have generated transgenic mice ubiquitously expressing the human receptor for measles virus (MV), CD46 (membrane cofactor protein). Various cell types were isolated from these transgenic mice and analyzed for their ability to support MV replication in vitro. Although MV could enter into all CD46-expressing cells, differential susceptibilities to MV infection were detected depending on the cell type. Cell cultures obtained from transgenic lungs and kidneys were found to be permissive of MV infection, since RNA specific for MV genes was detected and viral particles were released, although at a low level. Similarly to human lymphocytes, activated T and B lymphocytes isolated from transgenic mice could support MV replication; virus could enter, transcribe viral RNA, and produce new infectious particles. When expressing viral proteins, lymphocytes down-regulated CD46 from the surface. Interestingly, while activated T lymphocytes from nontransgenic mice did not support MV infection, activated nontransgenic murine B lymphocytes replicated MV as well as transgenic B lymphocytes, suggesting the use of an alternative virus receptor for entry. In contrast to the previous cell types, murine peritoneal and bone marrow-derived macrophages, regardless of whether they were activated, could not support MV replication. Furthermore, although MV entered into macrophages and virus-specific RNA transcription occurred, no virus protein or infectious virus particles could be detected. These results show the importance of the particular cell-type-specific host factors for MV replication in murine cells which may be responsible for the differential permissivity of MV infection.

Differential activation of T cells by antibody-modulated processing of the flanking sequences of class II-restricted peptides

Despite poor presentation of measles virus (MV) nucleoprotein (NP) by MHC class II of infected cells, NP-specific antibodies are one of the hallmarks of the early immune response against this virus. To study the influence of antibodies on processing and presentation of NP to three different T cell hybridomas, mAb recognizing distinctive llnear NP epitopes were developed. Two T cell hybridomas TNP408B and TNP408 reacted with the same core epitope of NP (amino acids 383-391), but differed in their sensitivity to the flanking sequences of peptides containing this epitope. TNP408B reacted with minimal concentrations of NP when this was complexed with mAb BNP146. NP alone or saturating concentrations of other mAb did not activate this T cell. Both T cells, TNP408 and TNP408B, were similar in their sensitivity to NP in the presence of saturating concentrations of BNP146 or of appropriate peptide (NP379). TNP408 did not differ from another T cell hybridoma (TNP79) in its sensitivity to different mAb, suggesting a specificity-dependent and a specificity-independent effect of mAb. Antibody-mediated activation was attributed to FcR-mediated uptake independent of the fine specificity of the mAb. In the case of TNP408B, this effect was further enhanced by a specific effect of BNP146. While all NP-specific mAb were sufficient to enhance presentation to TNP408 and TNP79 of their respective peptides derived from processed NP, BNP146 was necessary to generate the peptides with the proper flanking sequences required by TNP408B. Since the binding site of BNP146 coincides with the T cell epitope of TNP408B (and TNP408) it is suggested that binding of this mAb modulates processing of the flanking sequences of the peptides corresponding to this epitope. This study shows that antibodies can influence the T cell response to an antigenic protein quantitatively and qualitatively by taking advantage of the sensitivity of T cells to flanking sequences of class II-restricted peptides.

Type III bare lymphocyte syndrome: lack of HLA class II gene expression and reduction in HLA class I gene expression

The bare lymphocyte syndrome (BLS) consists of an association between a combined immunodeficiency disease and a significantly reduced expression of either human histocompatibility leukocyte antigens (HLA) class I (HLA-A, -B, -C) or HLA class II (HLA-DP, -DQ, -DR) at the cell surface. BLS type III, the more frequent form of this syndrome, is characterized by impaired expression of both class I and class II antigens on patients' cells, in particular on leukocytes. We describe herein the demonstration that expression of HLA class I molecules was reduced by approximately half on Epstein-Barr virus-transformed B cells (LCL) derived from type III BLS patients. HLA class I mRNA level was also decreased to the same extent. Expression of HLA class I molecules was also very significantly reduced at the surface of these fibroblasts as was mRNA specific for HLA class I. Simultaneously, the expression of HLA-DR molecules on LCL was even more greatly decreased, and the expression of HLA-DQ antigens was virtually abolished. Molecular analysis demonstrated an absence of mRNA for the alpha- and beta-chains of HLA-DQ and HLA-DR in the patients' lymphocytes. In general, such patients present with an association of an absence of expression of HLA class II antigens and a significantly reduced expression of HLA class I antigens. The mechanism of this association is still uncertain.

Substitution of peptide bond 53-54 of HEL(52-61) with an ethylene bond rather than reduced peptide bond is tolerated by an MHC-II restricted T cell

To probe the interactions between major histocompatibility class-II molecules and the amide bonds of the antigenic peptide main chain, we synthesized ethylenic and reduced analogues of HEL(52-61), an immunogenic peptide for murine major histocompatibility class-II IA k restricted T-cell clones. The synthesis of the corresponding ethylenic analogue of HEL(52-61) in position 53-54 was performed by coupling the Fmoc-protected tripeptide Asp-Tyr-psi [E, CH = CH]Gly with HEL(55-61). Biological tests showed that the ethylenic peptide was presented by major histocompatibility class-II IA kappa molecule and recognized by HEL(52-61)-specific T-cell clones. The corresponding reduced peptide of HEL(52-61) at position 53-54 neither stimulated T-cell clones nor competed with the natural peptide. These results show that, while reduced pseudopeptides might not be appropriate, ethylenic pseudopeptides may be used as probes to dissect the role of hydrogen bonding between the peptide main chain and MHC residues and also help in the design of more stable immunogenic peptides.

Quantification of measles virus by a virus receptor-dependent and haemagglutinin-specific T cell stimulation assay

The human measles virus receptor CD46 plays a major role in the uptake of measles virus (MV) for antigen presentation by major histocompatibility complex class II molecules to T cells. On this basis, a new bioassay has been set up to quantify measles virus in a cell free tissue culture supernatant. A stable mouse B cell transfectant expressing CD46 was used as the antigen presenting cell for presentation of measles virus to a haemagglutinin-specific and class II-restricted mouse T cell hybridoma. The measles virus haemagglutinin was quantified by its ability to stimulate IL-2 secretion by the T cells. A good correlation was found between the amount of haemagglutinin measured in supernatants from infected cells using the CD46-dependent T cell stimulation assay and the number of infectious viral particles as determined in a plaque assay. When MV was purified on a discontinuous sucrose gradient, most of the infectious virus and the haemagglutinin antigen were recovered in the same fraction. These data indicate that the CD46-dependent haemagglutinin-specific T cell assay could be used to measure the production of measles virus in the supernatant of infected cells. The assay required only 48 h, was sensitive, highly specific, and did not rely on the replication of the virus. This new bioassay would be applicable for the detection of any other virus provided that antigen presenting cells expressing the corresponding virus receptor and virus envelope glycoprotein-specific T cells are available. Moreover, it would be an interesting tool to monitor the receptor binding properties of attenuated vaccine virus and envelope glycoprotein subunit vaccines.

Cell-to-cell contact via measles virus haemagglutinin-CD46 interaction triggers CD46 downregulation

CD46 downregulation by measles virus (MV) occurs after expression of virus haemagglutinin (H) protein on the surface of the infected cell and is a consequence of CD46-H interaction on the cell membrane. To assess whether CD46 downregulation also occurs after CD46-H interaction when these two molecules are expressed on distinct cells, we used human T cell line Jurkat (expressing CD46) and transfected murine fibroblast line L stably expressing MV-H protein (L.H). FACS analysis shows that cell-to-cell contact of 1 h at 37 degrees C triggers a reduction of CD46 cell surface labelling as detected by MCI20.6, GB24 and J4-48 monoclonal antibodies. This reduction is similar to that observed after MV infection or after infection with recombinant vaccinia virus encoding MV-H protein. By contrast, MV-H protein was downregulated only when CD46-H interaction occurred on the same cell membrane. CD46 downregulation is specific for CD46-H interaction because it was not observed after coincubation of Jurkat cells with either L cells expressing MV nucleoprotein (L.NP) or L cells. Moreover, this downregulation could be blocked by either anti-CD46 or anti-H antibodies. The H-mediated CD46 downregulation is reversible and restricted to CD46 since expression of other surface markers (CD3, CD14, CD47 and CD63) is unaffected. It is apparently not mediated in a protein kinase (PK) A- or PKC-dependent manner. Altogether, our results provide an unequivocal demonstration that interaction between the extracellular domains of CD46 and MV-H is sufficient to trigger CD46 downregulation.

Formaldehyde inactivation of measles virus abolishes CD46-dependent presentation of nucleoprotein to murine class I-restricted CTLs but not to class II-restricted helper T cells

To induce an MHC-restricted specific CTL or Th response, an antigen must be delivered into the appropriate cellular compartment. We explored the role of CD46 in the presentation of measles virus (MV) nucleoprotein (NP) to murine NP-specific and MHC Class I-restricted polyclonal CTLs and the effect of inactivating MV by uv or formaldehyde. CD46(-)- and CD46(+)-transfected murine cells were used as target cells. After MV infection, only the targets which expressed CD46 were lysed by NP-specific class I-restricted CTLs. When MV was uv-inactivated, NP presentation by MHC class I molecules was retained but could be blocked by fusion inhibitors which block virus cell entry. When MV was inactivated with formaldehyde, NP was no longer presented by MHC class I molecules, although it was still presented by MHC class II molecules to a NP-specific class II-restricted T cell hybridoma. These data show that MV binding to the CD46 molecule is a prerequisite for virus-to-cell fusion and that cytosolic delivery of NP is necessary for presentation by class I molecules. Moreover, formaldehyde inactivation of virus induces the loss of class I-restricted presentation of NP due to selective abrogation of fusion and cytosolic delivery of NP.