Tumor cells present MHC class II-restricted nuclear and mitochondrial antigens and are the predominant antigen presenting cells in vivo

MHC class II-restricted tumor Ags presented by class II(+) tumor cells identified to date are derived from proteins expressed in the cytoplasm or plasma membrane of tumor cells. It is unclear whether MHC class II(+) tumor cells present class II-restricted epitopes derived from other intracellular compartments, such as nuclei and/or mitochondria, and whether class II(+) tumor cells directly present Ag in vivo. To address these questions, a model Ag, hen egg lysozyme, was targeted to various subcellular compartments of mouse sarcoma cells, and the resulting cells were tested for presentation of three lysozyme epitopes in vitro and for presentation of nuclear Ag in vivo. In in vitro studies, Ags localized to all tested compartments (nuclei, cytoplasm, mitochondria, and endoplasmic reticulum) are presented in the absence invariant chain and H-2M. Coexpression of invariant chain and H-2M inhibit presentation of some, but not all, of the epitopes. In vivo studies demonstrate that class II(+) tumor cells, and not host-derived cells, are the predominant APC for class II-restricted nuclear Ags. Because class II(+) tumor cells are effective APC in vivo and probably present novel tumor Ag epitopes not presented by host-derived APC, their inclusion in cancer vaccines may enhance activation of tumor-reactive CD4(+) T cells.

Antigenicity and immunogenicity of an intracellular delivery system of major histocompatibility complex class I epitopes that bypasses proteasome processing

The development of a cell-free synthetic vaccine to induce an effective cytotoxic T lymphocyte response is an important challenge in T-cell--mediated immunity. Because standard vaccinations with nominal epitopes were found to be only partially effective in vivo, the authors suggest an alternative strategy: the delivery of epitopes directly to the cell cytosol in a proteasome bypass mechanism of processing. Two model peptides, the presentation level on the cell surface of which can be directly assessed, were conjugated via a cross-linker to an internalization peptide derived from an antennapedia homeobox protein. The linker was designed to undergo spontaneous hydrolysis, after which the epitope is subsequently released. The conjugates were shown to enter RMA and P815 cells, where the epitopes were released mainly in cytosol and endogenously loaded on the major histocompatibility complex class I molecules to be presented on the cell surface. Concomitant inhibition of proteasome activity by MG132 significantly increased the presentation level of both model peptides, indicating proteasome-independent processing. This phenomenon was exploited to enhance the immunogenicity of the conjugates. Conjugates were emulsified with MG132 in incomplete Freund's adjuvant and injected into mouse footpads. Analysis of the draining lymph nodes indicated an increase in the percentage of both CD4+ and CD8+ lymphocytes. In vitro cytolytic assays implied significant, albeit moderate, priming only when the proteasome inhibitor was administered with the conjugate. This approach may be useful for the development of efficient synthetic cell-free vaccines.

Archaeosomes induce long-term CD8+ cytotoxic T cell response to entrapped soluble protein by the exogenous cytosolic pathway, in the absence of CD4+ T cell help

The unique ether glycerolipids of Archaea can be formulated into vesicles (archaeosomes) with strong adjuvant activity for MHC class II presentation. Herein, we assess the ability of archaeosomes to facilitate MHC class I presentation of entrapped protein Ag. Immunization of mice with OVA entrapped in archaeosomes resulted in a potent Ag-specific CD8(+) T cell response, as measured by IFN-gamma production and cytolytic activity toward the immunodominant CTL epitope OVA(257-264). In contrast, administration of OVA with aluminum hydroxide or entrapped in conventional ester-phospholipid liposomes failed to evoke significant CTL response. The archaeosome-mediated CD8(+) T cell response was primarily perforin dependent because CTL activity was undetectable in perforin-deficient mice. Interestingly, a long-term CTL response was generated with a low Ag dose even in CD4(+) T cell deficient mice, indicating that the archaeosomes could mediate a potent T helper cell-independent CD8(+) T cell response. Macrophages incubated in vitro with OVA archaeosomes strongly stimulated cytokine production by OVA-specific CD8(+) T cells, indicating that archaeosomes efficiently delivered entrapped protein for MHC class I presentation. This processing of Ag was Brefeldin A sensitive, suggesting that the peptides were transported through the endoplasmic reticulum and presented by the cytosolic MHC class I pathway. Finally, archaeosomes induced a potent memory CTL response to OVA even 154 days after immunization. This correlated to strong Ag-specific up-regulation of CD44 on splenic CD8(+) T cells. Thus, delivery of proteins in self-adjuvanting archaeosomes represents a novel strategy for targeting exogenous Ags to the MHC class I pathway for induction of CTL response.

Possible involvement of cyclophilin B and caspase-activated deoxyribonuclease in the induction of chromosomal DNA degradation in TCR-stimulated thymocytes

TCR engagement of immature CD4(+)CD8(+) thymocytes induces clonal maturation (positive selection) as well as clonal deletion (negative selection) in the thymus. However, the cell death execution events of thymocytes during the negative selection process remain obscure. Using a cell-free system, we identified two different DNase activities in the cytosol of in vivo anti-TCR-stimulated murine thymocytes: one that induced chromosomal DNA fragmentation, which was inhibited by an inhibitor of caspase-activated DNase, and another that induced plasmid DNA degradation, which was not inhibited by an inhibitor of caspase-activated DNase. We purified the protein to homogeneity that induced plasmid DNA degradation from the cytosol of anti-CD3-stimulated thymocytes and found that it is identical with cyclophilin B (Cyp B), which was reported to locate in endoplasmic reticulum. Ab against Cyp B specifically inhibited the DNA degradation activity in the cytosol of anti-CD3-stimulated thymocytes. Furthermore, recombinant Cyp B induced DNA degradation of naked nuclei, but did not induce internucleosomal DNA fragmentation. Finally, we demonstrated that TCR engagement of a murine T cell line (EL4) with anti-CD3/CD28 resulted in the release of Cyp B from the microsome fraction to the cytosol/nuclear fraction. Our data strongly suggest that both active caspase-activated DNase and Cyp B may participate in the induction of chromosomal DNA degradation during cell death execution of TCR-stimulated thymocytes.

Macrophage migration inhibitory factor antagonizes hydrocortisone-induced increases in cytosolic IkappaBalpha

Macrophage migration inhibitory factor (MIF) is an inflammatory cytokine secreted by several cell types, including mononuclear and pituitary cells. It has also been shown to counteract cortisol-induced inhibition of inflammatory cytokine secretion. The purpose of this study was to determine whether MIF antagonized the effect of hydrocortisone on the NF-kappaB/IkappaB signal transduction pathway in lipopolysaccharide (LPS)-stimulated human peripheral blood mononuclear cells. Physiological doses of hydrocortisone (50-200 ng/ml) diminished both the LPS-stimulated decrease in cytosolic IkappaBalpha levels and the subsequent increase in nuclear NF-kappaB DNA binding. In the presence of both LPS and hydrocortisone, 1 ng/ml of MIF antagonized the effects of hydrocortisone, resulting in decreased cytosolic IkappaBalpha levels (P < 0.05) and increased nuclear NF-kappaB DNA binding (P < 0.05). In the absence of hydrocortisone, MIF had no effect on LPS-induced decreases in IkappaBalpha. In the absence of LPS, MIF inhibited hydrocortisone-induced increases in IkappaBalpha (P = 0.03). Thus the mechanism by which MIF antagonizes the effect of hydrocortisone on the NF-kB/IkappaB signal transduction pathway is through inhibiting the ability of hydrocortisone to increase cytosolic IkappaBalpha.

Association of deficient type II protein kinase A activity with aberrant nuclear translocation of the RII beta subunit in systemic lupus erythematosus T lymphocytes

Systemic lupus erythematosus (SLE) is an autoimmune disorder of indeterminate etiology characterized by abnormal T cell signal transduction and altered T cell effector functions. We have previously observed a profound deficiency of total protein kinase A (PKA) phosphotransferase activity in SLE T cells. Here we examined whether reduced total PKA activity in SLE T cells is in part the result of deficient type II PKA (PKA-II) isozyme activity. The mean PKA-II activity in SLE T cells was 61% of normal control T cells. The prevalence of deficient PKA-II activity in 35 SLE subjects was 37%. Deficient isozyme activity was persistent over time and was unrelated to SLE disease activity. Reduced PKA-II activity was associated with spontaneous dissociation of the cytosolic RIIbeta2C2 holoenzyme and translocation of the regulatory (RIIbeta) subunit from the cytosol to the nucleus. Confocal immunofluorescence microscopy revealed that the RIIbeta subunit was present in approximately 60% of SLE T cell nuclei compared with only 2-3% of normal and disease controls. Quantification of nuclear RIIbeta subunit protein content by immunoprecipitation and immunoblotting demonstrated a 54% increase over normal T cell nuclei. Moreover, the RIIbeta subunit was retained in SLE T cell nuclei, failed to relocate to the cytosol, and was associated with a persistent deficiency of PKA-II activity. In conclusion, we describe a novel mechanism of deficient PKA-II isozyme activity due to aberrant nuclear translocation of the RIIbeta subunit and its retention in the nucleus in SLE T cells. Deficient PKA-II activity may contribute to impaired signaling in SLE T cells.

Comparative immunoreactivity of anti-trifluoroacetyl (TFA) antibody and anti-lipoic acid antibody in primary biliary cirrhosis: searching for a mimic

Previous studies documenting the existence of cross-reactivity between the lipoated (but not unlipoated) forms of the inner lipoyl domain (E2L2) of PDC-E2 [the major autoantigen in Primary biliary cirrhosis (PBC)] and trifluoroacetylated (TFA) proteins, led us to hypothesize that PBC may be due to an initial insult with an environmental agent that cross-reacts with TFA. Therefore, we performed a comparative study of the reactivity of rabbit anti-TFA antibody and anti-lipoic acid (LA) antibody against the mitochondrial autoantigens of human PBC and various TFA and LA conjugated proteins. Whereas both anti-TFA and anti-LA reacted with PDC-E2, the wild-type lipoated form of E2L2, OGDC-E2, E3-BP and LA-KLH, neither reacted with BCOADC-E2 or the non-lipoated form of E2L2. Of interest was that while anti-TFA reacted with PDC-E2, TFA-RSA and LA-KLH, it failed to inhibit PDC-E2 enzyme function. In contrast, anti-LA demonstrated cytoplasmic and mitochondrial staining, and inhibited PDC enzyme activity. Hence, although considerable cross reactivity exists between anti-TFA and anti-LA, the molecular nature of the interaction is clearly different. One of 14 PBC sera reacted weakly with TFA-albumin, whereas four of 14 PBC sera reacted with LA-KLH. Immunohistochemically, both anti-TFA and anti-LA antibodies reacted focally with periportal hepatocytes and bile ducts in both PBC and controls. However, anti-LA produced much stronger focalized staining of the bile ducts of diseased liver. This study suggests that while anti-TFA antibody recognizes lipoic acid-linked enzymes and proteins, the epitope recognized differs from that of anti-LA antibody and PBC autoantibodies. It is unlikely that a response to TFA is the triggering event in PBC. Anti-LA antibodies share a higher degree of similarity to PBC sera providing suggestive evidence that anti-LA antibodies or anti-LA like antibodies (mimotopes) may help define the initiator of the autoimmune response.

Overexpression of the proteasome subunits LMP2, LMP7, and MECL-1, but not PA28 alpha/beta, enhances the presentation of an immunodominant lymphocytic choriomeningitis virus T cell epitope

The proteasome is a large protease complex that generates most of the peptide ligands of MHC class I molecules either in their final form or in the form of N-terminally extended precursors. Upon the stimulation of cells with IFN-gamma, three constitutively expressed subunits of the 20S proteasome are replaced by the inducible subunits LMP2 (low-molecular mass polypeptide 2), LMP7, and MECL-1 (multicatalytic endopeptidase complex-like-1) to form so-called immunoproteasomes. We show in this study that overexpression of these three subunits in triple transfectants led to a marked enhancement in the H-2Ld-restricted presentation of the immunodominant nonameric epitope NP118, which is derived from the nucleoprotein (NP) of lymphocytic choriomeningitis virus. Overexpression of the alpha and beta subunits of the IFN-gamma-inducible proteasome regulator PA28, in contrast, did not have a comparable effect. In vitro, immunoproteasomes as compared with constitutive proteasomes generated higher amounts of 11- and 12-mer fragments containing the NP118 epitope. These are likely to be cytosolic precursors of NP118, as a proline anchor residue in the second position of NP118 may interfere with TAP-mediated transport of the nonameric epitope itself. In conclusion, we provide evidence that up-regulation of the three inducible subunits, LMP2, LMP7, and MECL-1, can result in a marked improvement of Ag presentation and that, depending on the epitope, PA28 and immunoproteasomes may differentially affect Ag processing.

Genetically modified anthrax lethal toxin safely delivers whole HIV protein antigens into the cytosol to induce T cell immunity

Bacillus anthrax lethal toxin can be engineered to deliver foreign proteins to the cytosol for antigen presentation to CD8 T cells. Vaccination with modified toxins carrying 8-9 amino acid peptide epitopes induces protective immunity in mice. To evaluate whether large protein antigens can be used with this system, recombinant constructs encoding several HIV antigens up to 500 amino acids were produced. These candidate HIV vaccines are safe in animals and induce CD8 T cells in mice. Constructs encoding gag p24 and nef stimulate gag-specific CD4 proliferation and a secondary cytotoxic T lymphocyte response in HIV-infected donor peripheral blood mononuclear cells in vitro. These results lay the foundation for future clinical vaccine studies.

Acute ischemic stroke : polymorphonuclear leukocyte membrane fluidity and cytosolic Ca(2+) concentration at baseline and after chemotactic activation

BACKGROUND AND PURPOSE

Several reports have considered the role of systemic leukocytes in acute ischemic stroke (AIS). Initially, greater attention was focused on the leukocyte count and subsequently on their adhesiveness, aggregation, rheology, and activation. The aim of this study was the evaluation of certain polymorphonuclear leukocyte (PMN) parameters, reflecting their rheology and activation, in subjects with AIS.

METHODS

In a group of 19 subjects with AIS and in a control group of 18 subjects with asymptomatic vascular atherosclerotic disease, we evaluated the PMN membrane fluidity and cytosolic Ca(2+) concentration at baseline and after in vitro chemotactic activation with 4-phorbol 12-myristate 13-acetate (PMA) and N-formyl-methionyl-leucyl-phenylalanine (fMLP).

RESULTS

From the obtained data, it is evident that at baseline only PMN membrane fluidity distinguishes control subjects from AIS subjects. After PMN activation with PMA and fMLP, prolonged for 5 and 15 minutes, we found an increase in PMN cytosolic Ca(2+) concentration and a decrease in PMN membrane fluidity only in subjects with AIS.

CONCLUSIONS

These findings emphasize that in subjects with AIS a functional alteration of systemic PMN cells is clearly expressed during chemotactic activation, although the mechanism of this abnormality is not yet explained.

Autoantibodies associated with acute rejection after liver transplantation for type-2 autoimmune hepatitis

Autoantibodies, markers of autoimmune diseases, can also be detected in chronic allograft rejection. However, the appearance of these autoantibodies in acute rejection after orthotopic liver transplantation has not yet been reported. Liver-kidney-microsome type-1 (LKM-1) antibodies directed against the autoantigen cytochrome CYP2D6 define a group of patients with autoimmune hepatitis type-2 (AIH-2), distinct from autoimmune hepatitis type-1 (AIH-1) in which anti-nuclear antibodies and anti-smooth muscle antibodies (SMA) with actin specificity are present in patient sera. Autoantibodies were studied by the quantitative CYP2D6 radioligand assay (RLA) that uses a radiolabeled CYP2D6 as antigen, immunoblotting using recombinant CYP2D6 protein and human liver microsomal and cytosolic fractions, and indirect immunofluorescence (IIF) using rat kidney-stomach-liver cryostat sections. In addition, the specificity of anti-SMA was detected by IIF on HEp2 cell line harvested with colchicin. This report describes the time course of CYP2D6 antibodies and the appearance of anti-SMA (without anti-actin, cytokeratin and vimentin reactivity) associated with acute rejection during a 2-year follow-up, in a patient who underwent transplantation at end-stage type 2 autoimmune hepatitis. In addition, we report a new reactivity against an unknown 40-kDa protein using a rat cytosolic fraction. The detection of autoantibodies in sequential samples may be important to better predict rejection or relapse, and to establish adequate therapy.

Tetanus toxin L chain is processed by major histocompatibility complex class I and class II pathways and recognized by CD8+ or CD4+ T lymphocytes

Tetanus toxin (TeNT) is a heterodimeric protein antigen, whose light chain (L) is translocated in the cytosol of neuronal target cells specifically to cleave its substrates, vesicle-associated membrane protein-2 (VAMP-2, or synaptobrevin) or cellubrevin. We report that the L chain behaves as a nominal antigen recognized by specific T-cell clones upon either class I- or II-restricted presentation. Three types of responses are observed: (i) a TeNT- and L-specific CD8+ T-cell response, that can be inhibited in a dose-dependent manner by the proteasome inhibitor clasto-Lactacystin beta-lactone; (ii) a CD4+ T-cell response specific for L but not TeNT, with recognition of a determinant processed in a chloroquine-sensitive and brefeldin A-resistant compartment; (iii) a CD4+ T-cell response against both L and TeNT, with processing in a brefeldin A-sensitive compartment. The L chain processing was investigated in U937 cells by internalization and localization of L chain by separation of the cell content by differential centrifugation experiments. After incubation with TeNT or L chain in the presence of H chain, the L chain was predominantly distributed in the cytosolic fraction, whereas incubation with L alone led to localization in a lysosome/membrane fraction. The distribution of the TeNT L chain in both cytosolic and endocytic compartments of the antigen-presenting cell accounted for its processing by both class I and class II pathways. Furthermore, an epitope overlapping with the zinc-binding region was recognized by CD4+ and CD8+ T cells.

Degradation of unassembled soluble Ig subunits by cytosolic proteasomes: evidence that retrotranslocation and degradation are coupled events

Many aberrant or unassembled proteins synthesized in the endoplasmic reticulum (ER) are degraded by cytosolic proteasomes. To investigate how soluble glycoproteins destined for degradation are retrotranslocated across the ER membrane, we analyzed the fate of two IgM subunits, mu and J, retained in the ER by myeloma cells that do not synthesize light chains. Degradation of mu and J is prevented by proteasome inhibitors, suggesting that both chains are retrotranslocated to be disposed of by proteasomes. Indeed, when proteasomes are inhibited, some deglycosylated J chains that no longer contain intrachain disulfide bonds accumulate in the cytosol. However, abundant glycosylated J chains are still present in the ER at time points in which degradation would have been almost complete in the absence of proteasome inhibitors, suggesting that retrotranslocation and degradation are coupled events. This was confirmed by protease protection and cell fractionation assays, which revealed that virtually all mu chains are retained in the ER lumen in a glycosylated state when proteasomes are inhibited. Association with calnexin correlated with the failure of mu chains to dislocate to the cytosol. Taken together, these results suggest that active proteasomes are required for the extraction of Ig subunits from the ER, though the requirements for retrotranslocation may differ among individual substrates.

Expression of SART3 tumor-rejection antigen in gastric cancers

We previously reported SART3 as a tumor-rejection antigen recognized by histocompatibility leukocyte antigen (HLA)-A24-restricted cytotoxic T lymphocytes (CTLs). In this study, we investigated the expression of the SART3 antigen in gastric cancers, as a candidate for use in specific immunotherapy. The SART3 antigen was detected in 9 of 10 (90%) gastric cancer cell lines, 35 of 52 (67.3%) gastric cancer tissues, and 0 of 20 non-tumorous gastric tissues. SART3-derived peptides corresponding to positions 109- 118 and 315-323 induced HLA-A24-restricted and tumor-specific CTLs from peripheral blood mononuclear cells (PBMCs) of gastric cancer patients. These peptide-induced CTLs recognized HLA-A24(+) SART3(+) gastric cancer cells, but not HLA-A24(+) SART3(-) or HLA-A24(-) SART3(+) gastric cancer cells. Therefore, the SART3 peptides could be useful in specific immunotherapy of gastric cancer patients.

Differential Ca2+ influx, KCa channel activity, and Ca2+ clearance distinguish Th1 and Th2 lymphocytes

In Th1 and Th2 lymphocytes, activation begins with identical stimuli but results in the production of different cytokines. The expression of some cytokine genes is differentially induced according to the amplitude and pattern of Ca2+ signaling. Using fura- 2 Ca2+ imaging of murine Th1 and Th2 clones, we observed that the Ca2+ rise elicited following store depletion with thapsigargin is significantly lower in Th2 cells than in Th1 cells. Maximal Ca2+ influx rates and whole-cell Ca2+ currents showed that both Th1 and Th2 cells express indistinguishable Ca2+-release-activated Ca2+ channels. Therefore, we investigated other mechanisms controlling the concentration of intracellular Ca2+, including K+ channels and Ca2+ clearance from the cytosol. Whole-cell recording demonstrated that there is no distinction in the amplitudes of voltage-gated K+ currents in the two cell types. Ca2+-activated K+ (KCa) currents, however, were significantly smaller in Th2 cells than in Th1 cells. Pharmacological equalization of Ca2+-activated K+ currents in the two cell types reduced but did not completely eliminate the difference between Th1 and Th2 Ca2+ responses, suggesting divergence in an additional Ca2+ regulatory mechanism. Therefore, we analyzed Ca2+ clearance from the cytosol of both cell types and found that Th2 cells extrude Ca2+ more quickly than Th1 cells. The combination of a faster Ca2+ clearance mechanism and smaller Ca2+-activated K+ currents in Th2 cells accounts for the lower Ca2+ response of Th2 cells compared with Th1 cells.

Misfolding of HLA-B27 as a result of its B pocket suggests a novel mechanism for its role in susceptibility to spondyloarthropathies

The MHC class I protein HLA-B27 is strongly associated with susceptibility to spondyloarthropathies and can cause arthritis when expressed in rats and mice, implying a direct role in disease pathogenesis. A prominent hypothesis to explain this role suggests that the unique peptide binding specificity of HLA-B27 confers an ability to present arthritogenic peptides. The B pocket, a region of the peptide binding groove that is an important determinant of allele-specific peptide binding, is thought to be critical for arthritogenicity. However, this hypothesis remains unproven. We show that in addition to its role in peptide selection, the B pocket causes a portion of the pool of assembling HLA-B27 heavy chains in the endoplasmic reticulum to misfold, resulting in their degradation in the cytosol. The misfolding phenotype is corrected by replacing the HLA-B27 B pocket with one from HLA-A2. Our results suggest an alternative to the arthritogenic peptide hypothesis. Misfolding and its consequences, rather than allele-specific peptide presentation, may underlie the strong link between the HLA-B27 B pocket and susceptibility to spondyloarthropathies.

The role of endoplasmic reticulum-associated protein degradation in MHC class I antigen processing

The processing and presentation of secretory glycoprotein antigens by the MHC class I processing pathway presents an interesting topological problem. That is, how do the luminal glycoprotein antigens gain access to the class I processing machinery located in the cell cytosol? Current data indicate that the retrograde transport of glycoproteins from the endoplasmic reticulum (ER) to cytosol represents the major pathway for ER-associated protein degradation, and most likely represents a major pathway for the processing of glycoprotein antigens by MHC class I molecules as well. There is now a growing list of viral and tumor glycoprotein antigens that undergo retrograde transport from the ER to the cytosol and processing by the ubiquitin-proteasome pathway of degradation. We review here some general aspects of this "ER degradation" pathway, and how it relates to the processing and presentation of class I-associated viral and tumor antigens. In particular, we analyze the role of oligosaccharide trimming and ER molecular chaperones in this process. We would like to emphasize that the class I processing machinery has adapted a common cellular pathway for its use, and that this could lead to the identification of unique characteristics with regard to ER degradation and antigen processing.

Generating MHC class I ligands from viral gene products

MHC class I molecules function to present peptides comprised of eight to 11 residues to CD8+ T lymphocytes. Here we review the efforts of our laboratory to understand how cells generate such peptides from viral gene products. We particularly focus on the nature of substrates acted on by cytosolic proteases, the contribution of proteasomes and non-proteasomal proteases to peptide generation, the involvement of ubiquitination in peptide generation, the intracellular localization of proteasome generation of antigenic peptides, and the trimming of peptides in the endoplasmic reticulum.

Role of p38 in the priming of human neutrophils by peritoneal dialysis effluent

Peritoneal dialysis effluent (PDE) contains a low-molecular-weight substance that is able to prime human neutrophils for the release of arachidonic acid and superoxide anion. Conventional priming agents, such as tumor necrosis factor alpha (TNF-alpha), are known to signal via mitogen-activated protein (MAP) kinases; at least one possible substrate for MAP kinases is cytosolic phospholipase A(2) (cPLA(2)). Phosphorylation of this enzyme results in arachidonic acid release, and this fatty acid is a potent primer and activator of the human neutrophil NADPH oxidase. Because of the striking similarities between the priming of neutrophils with agents such as TNF-alpha and PDE, we have investigated the signalling pathways evoked by PDE and explored the possibility that cPLA(2) is a target for activated MAP kinases. Our results show that PDE treatment of human neutrophils results in the phosphorylation of the p38 kinase rather than the p42 and p44 kinases. Phosphorylation of p38 is transient with maximal activity being observed 1 min after exposure to PDE. We were unable to demonstrate that activation of p38 resulted in phosphorylation of cPLA(2); furthermore, translocation of this enzyme to a membrane-containing fraction was not enhanced in PDE-treated neutrophils. Taken together, these data suggest that, in a manner similar to that of TNF-alpha, PDE primes human neutrophils by the activation of the p38 kinase. However, unlike the cytokine, the activation of this protein does not result in phosphorylation or activation of cPLA(2).

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

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

Apoplastic and cytosolic expression of full-size antibodies and antibody fragments in Nicotiana tabacum

We compared the expression of a functional recombinant TMV-specific full-size antibody (rAb29) in both the apoplast and cytosol of tobacco plants and a single chain antibody fragment (scFv29), derived from rAb29, was expressed in the cytosol. Cloned heavy and light chain cDNAs of full-size rAb29, which binds to TMV coat protein monomers, were integrated into the plant expression vector pSS. The full-size rAb29 was expressed in the cytosol and targeted to the apoplast by including the original murine antibody leader sequences. Levels of functional full-size rAb29 expression were high in the apoplast (up to 8.5 micrograms per gram leaf tissue), whereas cytosolic expression was low or at the ELISA detection limit. Sequences of the variable domains of rAb29 light and heavy chain were used to generate the single chain antibody of scFv29, which was expressed in the periplasmic space of E. coli and showed the same binding specificity as full-size rAb29. In addition, scFv29 was functionally expressed in the cytosol of tobacco plants and plant derived scFv29 maintained same binding specificity to TMV-coat protein monomers as rAb29.

The CD3 epsilon subunit of the TCR contains endocytosis signals

Ligand binding to TCR induces its internalization and cell surface down-modulation. These phenomena contribute to the extinction of activation signals. Due to the multicomponent nature of the TCR-CD3 complex, its internalization may be mediated by one or several of its subunits. Although it has been reported that CD3 gamma and CD3 delta contain endocytosis motifs involved in the internalization of the TCR-CD3 complex, other subunits could also be involved in this process. For instance, CD3 epsilon and CD zeta display amino acid sequences reminiscent of internalization motifs. To investigate whether CD3 epsilon bears endocytosis signals, we have analyzed the internalization capacity of a panel of deletion and point mutants of CD3 epsilon that were expressed on the cell surface independently of other TCR-CD3 subunits. Here we report that CD3 epsilon displays endocytosis determinants. These data indicate that CD3 epsilon could contribute to the internalization and cell surface down-regulation of TCR-CD3 complexes. Moreover, the existence of endocytosis signals in this polypeptide could serve to retrieve unassembled CD3 epsilon subunits or partial CD3 complexes from the plasma membrane, thus restricting the expression on the cell surface to fully functional TCR-CD3 complexes.

HLA-B15 peptide ligands are preferentially anchored at their C termini

Therapies to elicit protective CTL require the selection of pathogen- and tumor-derived peptide ligands for presentation by MHC class I molecules. Edman sequencing of class I peptide pools generates "motifs" that indicate that nonameric ligands bearing conserved position 2 (P2) and P9 anchors provide the optimal search parameters for selecting immunogenic epitopes. To determine how well a motif represents its individual constituents, we used a hollow-fiber peptide production scheme followed by the mapping of endogenously processed class I peptide ligands through reverse-phase HPLC and mass spectrometry. Systematically mapping and characterizing ligands from B*1508, B*1501, B*1503, and B*1510 demonstrate that the peptides bound by these B15 allotypes i) vary in length from 7 to 12 residues, and ii) are more conserved at their C termini than their N-proximal P2 anchors. Comparative peptide mapping of these B15 allotypes further pinpoints endogenously processed ligands that bind to the allotypes B*1508, B*1501, and B*1503, but not B*1510. Overlapping peptide ligands are successful in binding to B*1501, B*1503, and B*1508 because these B15 allotypes share identical C-terminal anchoring pockets whereas B*1510 is divergent in the C-terminal pocket. Therefore, endogenous peptide loading into the B15 allotypes requires that a conserved C terminus be anchored in the appropriate specificity pocket while N-proximal anchors are more flexible in their location and sequence. Queries for overlapping and allele-specific peptide ligands may thus be contingent on a conserved C-terminal anchor.

Antigen secreted from noncytosolic Listeria monocytogenes is processed by the classical MHC class I processing pathway

Intracellular bacteria can reside in a vacuolar compartment, or they can escape the vacuole and become free living in the cytoplasm. The presentation of Ag by class I MHC molecules has been defined primarily for Ag present in the cytoplasm. It was therefore thought that Ags from bacteria that remain in a vacuole would not be presented by MHC class I molecules. Although some studies have provided data to support this idea, it is not necessarily true for all intracellular bacteria. For example, we have previously demonstrated that an epitope from the p60 protein secreted by LLO- Listeria monocytogenes, which does not reside in the cytoplasm, can be presented by MHC class I molecules to a T cell clone specific for the epitope, p60217-225. We have further examined the route by which Ag secreted by LLO- L. monocytogenes is presented by MHC class I molecules. Using pharmacological inhibitors, we demonstrate that MHC class I presentation of the p60 epitope derived from by LLO- L. monocytogenes requires phagolysosome fusion and processing by the proteasome. Lysosomal cathepsins, however, are not required for processing of the p60 epitope. Similarly, processing of the AttM epitope, secreted by LLO- L. monocytogenes and presented by H2-M3, also requires phagolysosome fusion and cleavage by the proteasome. Thus, p60 and AttM secreted by LLO- L. monocytogenes are processed via the classical class I pathway for presentation by MHC class I molecules.