Reduced virulence of Candida albicans mutants affected in multidrug resistance

Disruption of a multidrug resistance gene (CaMDR1) in Candida albicans resulted in mutant strains that colonized mouse kidneys to very high levels but were markedly reduced in their virulence. No obvious differences in several properties related to colonization and dissemination were noted among MDR+ or mdr- strains. These results suggest that specific fungal efflux pumps play a role in fungal pathogenicity.

Physiological release of excitatory amino acids

The contribution of in vivo monitoring to the study of glutamate release is reviewed. Physiological stimulation increases both glutamate and aspartate in the extracellular compartment of the brain and both amino acids show Ca(2+)-dependent K(+)-evoked release. However, the finding that only glutamate is stored in synaptic vesicles implies that glutamate is the excitatory transmitter. Released glutamate is taken up into both neurones and glia by glutamate transporters. Uptake of glutamate, in addition to clearing the synapse, has a number of additional functions. Uptake into glia leads to the release of glutamine, which is involved in the recycling of transmitter glutamate; uptake into both neurones and glia leads to the release of ascorbate; uptake into glia leads to an increase glycolysis and export of lactate, an energy substrate for neuronal metabolism. Reversal of the glutamate transporter accounts for the parallel release of glutamate and aspartate from the cytoplasmic compartment. The basal concentration of extracellular glutamate is in the micromolar range. Such levels could lead to desensitisation of both NMDA and non-NMDA receptors. The functional implications of the level of basal glutamate are difficult to assess at present in view of the existence of multiple glutamate receptor subunits with different functional properties and distributions.

Characterization of an Na(+)-dependent glutamate/aspartate transporter from cultured Bergmann glia

The properties of a transport system specific for L-glutamate and L-aspartate activity expressed in Bergmann glia cells from chick cerebellum were examined. The uptake of D-[3H]aspartate was inhibited by the endogenous substrates L-aspartate (Ki = 62 microM) and L-glutamate (Ki = 60 microM). Of the identified uptake inhibitors, L-aspartate-beta-hydroxamate (Ki = 900 microM), L-alpha-aminoadipate (Ki = 2000 microM), 4,4'diisothiocyanatos-tilbene-2,2'-disulphonic acid (DIDS) (Ki = 1000 microM) and 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS) (Ki = 100 microM) inhibited D-[3H]aspartate uptake. Northern blot analysis, revealed the expression of a chick homologue of a the rat brain L-glutamate/L-aspartate transporter (GLAST). These data suggest that GLAST may be involved in the regulation of the parallel fibre-Purkinje cell synapse ensheathed by Bergmann glia.

Down-regulation of glial glutamate transporters after glutamatergic denervation in the rat brain

Membrane-localized transporter proteins, expressed in both neurons and glial cells, are responsible for removal of extracellular glutamate in the mammalian CNS. The amounts and activities of these transporters may be under regulatory control. We demonstrate here that cortical lesions, which decrease striatal glutamate uptake in synaptosome-containing homogenates by approximately 50%, also decrease the striatal concentrations of the astrocytic glutamate transporter proteins, GLT-1 and GLAST by approximately 20-30%. Since GABA uptake activity was not decreased and glial fibrillary acidic protein was increased in the same samples, the lesion-induced losses of GLT-1 and GLAST were not caused by a general impairment of neuronal or glial function. The observed reduction in the two astrocytic glutamate transporters after corticostriatal nerve terminal degeneration indicates that their levels of expression are dependent on glutamatergic innervation.

TAP polymorphism does not influence transport of peptide variants in mice and humans

The major histocompatibility complex (MHC)-encoded transporter associated with antigen processing (TAP) delivers cytosolic peptides to the lumen of the endoplasmic reticulum (ER) for presentation by MHC class I molecules. For the rat, it has been demonstrated that TAP polymorphism results in the selection of different sets of peptides, the nature of the C terminus being of particular importance. Here, we investigated whether TAP polymorphism in mice and humans has functional consequences for transport of peptide sets variable at the C-terminal residues. Using cell lines of H-2d, H-2k, and H-2dxk haplotype and a panel of human lymphoblastoid cell lines expressing eight different TAP alleles, we detected species-specific transport patterns, but no significant influence of TAP polymorphism on peptide selection. In addition, peptides with different core sequences were translocated to the same extent by different TAP. These results suggest that a major contribution of human TAP polymorphism to disease progression and autoimmunity is not very likely.

Electrogenic properties of the epithelial and neuronal high affinity glutamate transporter

Active ion-coupled glutamate transport is of critical importance for excitatory synaptic transmission, normal cellular function, and epithelial amino acid metabolism. We previously reported the cloning of the rabbit intestinal high affinity glutamate transporter EAAC1 (Kanai, Y., and Hediger, M. A. (1992) Nature 360, 467-471), which is expressed in numerous tissues including intestine, kidney, liver, heart, and brain. Here, we report a detailed stoichiometric and kinetic analysis of EAAC1 expressed in Xenopus laevis oocytes. Uptake studies of 22Na+ and [14C]glutamate, in combination with measurements of intracellular pH with pH microelectrodes gave a glutamate to charge ratio of 1:1, a glutamate to Na+ ratio of 1:2, and a OH-/H+ to charge ratio of 1:1. Since transport is K+ dependent it can be concluded that EAAC1-mediated glutamate transport is coupled to the cotransport of 2 Na+ ions, the countertransport of one K+ ion and either the countertransport of one OH- ion or the cotransport of 1 H+ ion. We further demonstrate that under conditions where the electrochemical gradients for these ions are disrupted, EAAC1 runs in reverse, a transport mode which is of pathologic importance. 22Na+ uptake studies revealed that there is a low level of Na+ uptake in the absence of extracellular glutamate which appears to be analogous to the Na+ leak observed for the intestinal Na+/glucose cotransporter SGLT1. In voltage clamp studies, reducing extracellular Na+ from 100 to 10 mM strongly increased K0.5L-glutamate and decreased I(max). The data indicate that Na+ binding at the extracellular transporter surface becomes rate-limiting. Studies addressing the cooperativity of the substrate-binding sites indicate that there are two distinct Na(+)-binding sites with different affinities and that Na+ binding is modulated by extracellular glutamate. A hypothetical ordered kinetic transport model for EAAC1 is discussed.

An epitope-selective, transporter associated with antigen presentation (TAP)-1/2-independent pathway and a more general TAP-1/2-dependent antigen-processing pathway allow recognition of the HIV-1 envelope glycoprotein by CD8+ CTL

The lysis of virally infected cells by CTLs requires the recognition of processed fragments of viral proteins presented in association with class I MHC molecules on the surfaces of infected cells. Processing begins in the cytosol with the degradation of viral proteins into peptides that are then transported into the endoplasmic reticulum (ER) for association with newly synthesized class I molecules. Transport is mediated by a heterodimer of the MHC-encoded proteins, transporter associated with Ag presentation (TAP)-1 and TAP-2. Uncertainty exists over the site of processing of viral envelope (env) proteins. The extracellular domains of env proteins are not present in the cytosol, the site in which the class I-restricted Ag-processing pathway begins. Rather, the ecto-domains of env proteins are cotranslationally translocated into the ER during biosynthesis. We have analyzed the processing of the HIV-1 env protein by using a large series of env-specific human CD8+ CTL clones. These studies have led to the delineation of two distinct processing pathways. The first pathway permits a subset of class I-restricted epitopes in the ecto-domain of the env protein to be generated efficiently by a TAP-1/2-independent mechanism localized to the ER or a premedial Golgi compartment. A second, more general pathway that is capable of generating all env epitopes uses as a substrate env protein mislocalized to the cytosol and produces peptides that are transported from the cytoplasm to the ER in a TAP-1/2-dependent fashion.

Kinetics of a human glutamate transporter

Currents mediated by a glutamate transporter cloned from human motor cortex were measured in Xenopus oocytes. In the absence of glutamate, voltage jumps induced Na(+)-dependent capacitive currents that were blocked by kainate, a competitive transport antagonist. The pre-steady-state currents can be described by an ordered binding model in which a voltage-dependent Na+ binding is followed by a voltage-independent kainate binding. At -80 mV, two charges are translocated per molecule of glutamate, with a cycling time of approximately 70 ms, which is significantly slower than the predicted time course of synaptically released glutamate. The results suggest that glutamate diffusion and binding to transporters, rather than uptake, are likely to dominate the synaptic concentration decay kinetics.

Processing of major histocompatibility class I-restricted antigens in the endoplasmic reticulum

We have introduced long precursor peptides directly into the endoplasmic reticulum (ER) of a mutant cell line (T2-Db) that lacks the ability to transport peptides from the cytosol to the ER in a transporter associated with antigen processing (TAP) dependent way. This was done by expressing various influenza A-derived peptides containing the naturally processed epitope ASNENMDAM (366-374) preceded by the influenza hemagglutinin ER translocation sequence. Peptides derived from these minigenes that became associated with Db were isolated and identified by combined reversed phase liquid chromatography and detection by cytotoxic T lymphocytes. Our results establish that NH2-terminal extensions of at least 40 residues can be trimmed from peptides entering the ER, but that proteolysis of larger proteins may be limited.

Nonclassical behavior of the thymus leukemia antigen: peptide transporter-independent expression of a nonclassical class I molecule

The thymus leukemia (TL) antigen is a major histocompatibility complex-encoded nonclassical class I molecule. Here we present data demonstrating that expression of the TL antigen, unlike other class I molecules, is completely independent of the function of the transporter associated with antigen processing (TAP). The TL antigen is expressed by transfected TAP-2-deficient RMA-S cells when these cells are grown at 37 degrees C. In transfected RMA cells, the kinetics of arrival of TL antigen on the cell surface are similar to those of a classical class I molecule. The kinetics are not altered in TAP-deficient RMA-S cells, demonstrating that surface TL expression in TAP-deficient cells is not due to the stable expression of a few molecules that leak out by a TAP-independent pathway. Soluble TL molecules produced by Drosophila melanogaster cells are highly resistant to thermal denaturation, unlike peptide-free classical class I molecules synthesized by these insect cells. In addition, these soluble TL molecules are devoid of detectable bound peptides. The results demonstrate that the TL antigen is capable of reaching the surface without bound peptide, although acquisition of peptide or some other ligand through a TAP-independent pathway cannot be formally excluded. We speculate that the ability of the TL antigen to reach the cell surface, under conditions in which other class I molecules do not, may be related to a specialized function of the TL molecule in the mucosal immune system, and possibly in the stimulation of intestinal gamma delta T cells.

Heat-inactivated Sendai virus can enter multiple MHC class I processing pathways and generate cytotoxic T lymphocyte responses in vivo

We have earlier described an alternative MHC class I processing pathway for Sendai virus (SV) in H-2Kb-transfected T2 cells (T2Kb). These cells have deleted genes for transporters associated with Ag processing (TAP1/2) and proteasome subunits LMP2/7 but can still process SV for the presentation of an immunodominant nucleoprotein CTL epitope (nucleoprotein peptide 324-332, FAPGNYPAL, SV9), even in the presence of the fungal metabolite brefeldin A (BFA). Presently we have compared live and heat-inactivated SV to investigate whether infectious virus, including early events such as binding and fusion at the host cell membrane, is important for nonclassical MHC class I processing and immunogenicity. We have found that heated virus (56 degrees C, boiled or autoclaved) with no fusion and hemagglutinin-neuraminidase activities, behaves similar to live SV in T2kb cells by entering a TAP-independent and BFA-resistant pathway. In EL-4 cells, which do not express this nonclassical TAP-independent and BFA-resistant pathway, heat-treated SV is processed in a BFA-sensitive way. In T1Kb- and TAP1/2-transfected T2Kb cells, as in T2Kb cells, processing of heat-inactivated SV was completely BFA resistant. Heat-inactivated SV was also found to prime CTLs in vivo. We conclude that heat-inactivated SV can enter both BFA-sensitive and -resistant MHC class I processing pathways and that SV in this respect may be particularly efficient. What property in the SV that is important for this characteristic is presently not clear but might be useful for the deliberate generation of CTL responses in vivo.

Presentation without proteolytic cleavage of endogenous precursors in the MHC class I antigen processing pathway

The antigen presentation pathway yields peptide-MHC class I complexes on the antigen presenting cell (APC) surface for recognition by appropriate T-cells. Expression of the peptide-MHC complex on APC surface is preceded by several steps that include the generation of peptide fragments in the cytoplasm and their assembly with MHC molecules in the endoplasmic reticulum. It is now clear that MHC binding to optimally processed peptides in the endoplasmic reticulum is obligatory for their stable expression on the cell surface. However, whether a similar obligatory relationship exists between generation of processed peptides and their expression as peptide-MHC on APC surface is not known. Here, we addressed this question by analyzing the processing of ovalbumin (aa257-264, SL8) or influenza nucleoprotein (aa366-374, AM9) analogs. We examined the generation of naturally processed peptides using precursors that did, or did not, contain residues flanking the optimal MHC-binding peptides. By characterizing the peptides generated from these precursors by T-cell stimulation assays and by high performance liquid chromatography analysis, we established that intracellular assembly of peptide-MHC complexes and their expression on the cell surface can occur with peptides that lack flanking residues. The presentation of these endogenously synthesized perfect fit peptides demonstrates that the cleavage of precursor polypeptides is an independent step in the antigen presentation pathway.

Peptide influences the folding and intracellular transport of free major histocompatibility complex class I heavy chains

Class I major histocompatibility complex molecules require both beta 2-microglobulin (beta 2m) and peptide for efficient intracellular transport. With the exception of H-2Db and Ld, class I heavy chains have not been detectable at the surface of cells lacking beta 2m. We show that properly conformed class I heavy chains can be detected in a terminally glycosylated form indicative of cell surface expression in H-2b, H-2d, and H-2s beta 2m-/- concanavalin A (Con A)-stimulated splenocytes incubated at reduced temperature. Furthermore, we demonstrate the presence of Kb molecules at the surface of beta 2m-/- cells cultured at 37 degrees C. The mode of assembly of class I molecules encompasses two major pathways: binding of peptide to preformed "empty" heterodimers, and binding of peptide to free heavy chains, followed by recruitment of beta 2m. In support of the existence of the latter pathway, we provide evidence for a role of peptide in intracellular transport of free class I heavy chains, through analysis of Con A-stimulated splenocytes from transporter associated with antigen processing 1 (TAP1)-/-, beta 2m-/-, and double-mutant TAP1/beta 2m-/- mice.

The czc operon of Alcaligenes eutrophus CH34: from resistance mechanism to the removal of heavy metals

The plasmid-borne czc operon ensures for resistance to Cd2+, Zn2+ and Co2+ ions through a tricomponent export pathway and is associated to various conjugative plasmids of A. eutrophus strains isolated from metal-contaminated industrial areas. The czc region of pMOL30 was reassessed especially for the segments located upstream and downstream the structural genes czc CBA. In cultures grown with high concentrations of heavy metals, czc-mediated efflux of cations is followed by a process of metal bioprecipitation. These observations led to the development of bioreactors designed for the removal of heavy metals from polluted effluents.

Increase in positive selection of CD8+ T cells in TAP1-mutant mice by human beta 2-microglobulin transgene

Mice harboring a deletion of the gene encoding the transporter associated with antigen presentation-1 (TAP1) are impaired in providing major histocompatibility complex (MHC) class I molecules with peptides of cytosolic origin and lack stable MHC class I cell surface expression. They consequently have a strongly reduced number of CD8+ T cells. To examine whether selection of CD8+ T cells is dependent on TAP-dependent peptides, we partially restored MHC class I cell surface expression in TAP1-deficient mice by introduction of human beta 2-microglobulin. We show that selection of functional CD8+ T cells can be augmented in vivo in the absence of TAP1-dependent peptides.

Major differences in transporter associated with antigen presentation (TAP)-dependent translocation of MHC class I-presentable peptides and the effect of flanking sequences

The MHC-encoded transporter associated with Ag presentation (TAP) translocates peptides from the cytosol to the ER lumen, where association with MHC class I molecules occurs. The MHC class I/peptide complex is subsequently transported to the cell surface for presentation to CD8+T cells. We studied TAP-dependent translocation of defined MHC class I presentable murine peptides by competition for translocation of a radiolabeled model peptide, to address whether efficient peptide presentation by MHC class I molecules is preceded by equal efficient peptide translocation by TAP. Surprisingly, we observed that four immunodominant viral peptides of 16 peptides tested were very inefficiently transported by TAP. Inefficient translocation could be overcome by substitution of a proline residue present at position 3 in the peptides. Furthermore, addition of natural flanking amino acids directly surrounding a poorly transported peptide could considerably improve translocation by TAP. Our data suggest that some peptides are efficiently transported by TAP in their optimal size for MHC class I binding, whereas other peptides are transported as larger peptide fragments that need further trimming in the ER for MHC class I binding.

Ion efflux systems involved in bacterial metal resistances

Studying metal ion resistance gives us important insights into environmental processes and provides an understanding of basic living processes. This review concentrates on bacterial efflux systems for inorganic metal cations and anions, which have generally been found as resistance systems from bacteria isolated from metal-polluted environments. The protein products of the genes involved are sometimes prototypes of new families of proteins or of important new branches of known families. Sometimes, a group of related proteins (and presumedly the underlying physiological function) has still to be defined. For example, the efflux of the inorganic metal anion arsenite is mediated by a membrane protein which functions alone in Gram-positive bacteria, but which requires an additional ATPase subunit in some Gram-negative bacteria. Resistance to Cd2+ and Zn2+ in Gram-positive bacteria is the result of a P-type efflux ATPase which is related to the copper transport P-type ATPases of bacteria and humans (defective in the human hereditary diseases Menkes' syndrome and Wilson's disease). In contrast, resistance to Zn2+, Ni2+, Co2+ and Cd2+ in Gram-negative bacteria is based on the action of proton-cation antiporters, members of a newly-recognized protein family that has been implicated in diverse functions such as metal resistance/nodulation of legumes/cell division (therefore, the family is called RND). Another new protein family, named CDF for 'cation diffusion facilitator' has as prototype the protein CzcD, which is a regulatory component of a cobalt-zinc-cadmium resistance determinant in the Gram-negative bacterium Alcaligenes eutrophus. A family for the ChrA chromate resistance system in Gram-negative bacteria has still to be defined.

Binding of diverse peptides to MHC class I molecules inhibits target cell lysis by activated natural killer cells

Class I MHC expression by target cells inhibits lysis mediated by natural killer (NK) cells, often in an allele-specific fashion. It has been proposed that NK cell inhibitory receptors recognize complexes of class I molecules with specific cellular peptides that define self, displacement of which would render cells NK sensitive. By loading the mostly empty Dd class I molecules of cell lines deficient in peptide transporter molecules with synthetic or natural Dd-bound peptides, we have demonstrated specific dose-dependent inhibition of the Ly49+ subset of activated NK cells by class I-peptide complexes. Inhibition occurred with most if not all Dd-binding peptides, suggesting that Ly49+ NK cells recognize class I-peptide complexes largely independently of peptide composition. The results suggest a primary role of NK cells in the destruction of cells that have down-regulated or extinguished cell surface expression of some or all class I molecules.

Differential reactivity of residual CD8+ T lymphocytes in TAP1 and beta 2-microglobulin mutant mice

TAP1 -/- and beta 2-microglobulin (beta 2m) -/- mice (H-2b background) express very low levels of major histocompatibility complex (MHC) class I molecules on the cell surface. Consequently these mice have low numbers of mature CD8+ T lymphocytes. However, TAP1 -/- mice have significantly higher numbers of CD8+ T cells than beta 2m -/- mice. Alloreactive CD8+ cytotoxic T lymphocyte (CTL) responses were also stronger in TAP1 -/- mice than in beta 2m -/- mice. Alloreactive CTL generated in TAP1 -/- and beta 2m -/- mice cross-react with H-2b-expressing cells. Surprisingly, such cross-reactivity was stronger with alloreactive CTL from beta 2m -/- mice than with similar cells from TAP1 -/- mice. The beta 2m -/- mice also responded more strongly when primed with and tested against cells expressing normal levels of H-2b MHC class I molecules. Such H-2b-reactive CD8+ CTL from beta 2m -/- mice but not from TAP1 -/- mice also reacted with TAP1 -/- and TAP2-deficient RMA-S cells. In contrast, H-2b-reactive CD8+ CTL from neither beta 2m -/- mice nor TAP1 -/- mice killed beta 2m -/- cells. In line with these results, beta 2m -/- mice also responded when primed and tested against TAP1 -/- cells. We conclude that the reactivity of residual CD8+ T cells differs between TAP1 -/- and beta 2m -/- mice. The MHC class I-deficient phenotype of TAP1 -/- and beta 2m -/- mice is not equivalent: class I expression differs between the two mouse lines with regard to quality as well as quantity. We propose that the differences observed in numbers of CD8+ T cells, their ability to react with alloantigens and their cross-reactivity with normal H-2b class I are caused by differences in the expression of MHC class I ligands on selecting cells in the thymus.

Trimming of TAP-translocated peptides in the endoplasmic reticulum and in the cytosol during recycling

Cytosolic peptides are translocated to the endoplasmic reticulum (ER) lumen by the transporters associated with antigen processing (TAP), where major histocompatibility complex (MHC) class I molecules associate with peptides of about 8-10 amino acids. TAP translocates peptides of 9-13 amino acids with the highest relative affinity but also longer and shorter peptides. The fate of the peptides that fail to associate with class I molecules because of incorrect sequence or length, is unknown. Here we show that the bulk of the translocated peptides are rapidly released from the ER by a mechanism that requires adenosine triphosphate (ATP) and that could not be inhibited by GTP gamma S. TAP does not appear to be involved in this process. Whereas free peptides are slowly trimmed in the ER lumen, they are rapidly degraded in the cytosol. A fraction of the peptides released from the ER escapes complete degradation in the cytosol and recycles back to the ER in a TAP-dependent fashion. These results suggest that peptides that are too long for binding to class I molecules in the ER can be trimmed further in the ER lumen or, alternatively, can be transported back to the cytosol where a fraction of the peptides is trimmed to a size suitable for association to MHC class I molecules and recycles back to the ER.

T cell recognition of an HLA-A2-restricted epitope derived from a cleaved signal sequence

An alternative pathway for class I-restricted antigen presentation has been suggested on the basis of peptides bound to HLA-A2 molecules in cells lacking the transporter for antigen presentation (TAP). Most of these peptides were derived from signal sequences for translocation into the endoplasmic reticulum (ER). However, it is not known whether these peptides can be presented to T cells. The hydrophobic nature of an HLA-A2-restricted T cell epitope (M1 58-66) was exploited to test whether it could be presented to T cells when derived from a signal sequence. Replacing the signal sequence of the influenza virus hemagglutinin molecule H3 with an artificial sequence containing that HLA-A2-restricted T cell epitope resulted in efficient translocation of H3 molecules into the ER and transport to the cell surface. This signal sequence-derived epitope was presented to HLA-A2-restricted T cells. Involvement of cytosolic processing for this presentation is very unlikely, because (a) presentation occurred in cells lacking TAP; (b) expression of H3 molecules with the artificial signal sequence did not produce a detectable cytosolic form of H3; and (c) presentation of the same epitope expressed in cytosolic forms of antigen required TAP. Thus, a peptide derived from a signal sequence cleaved in the ER can provide an epitope for HLA-A2-restricted T cell recognition.

Comparison of cell lines deficient in antigen presentation reveals a functional role for TAP-1 alone in antigen processing

Cytotoxic T lymphocytes (CTL) recognize antigenic peptides bound to major histocompatibility complex class I antigens on the cell surface of virus-infected cells. It is believed that the majority of peptides originate from cytoplasmic degradation of proteins assumed to be mediated by the "20S" proteasome. Cytosolic peptides are then translocated, presumably by transporters associated with antigen processing (TAP-1 and -2), into the lumen of the endoplasmic reticulum (ER) where binding and formation of the ternary complex between heavy chain, beta2-microglobulin (beta 2m) and peptide occurs. In this study, we have analyzed and compared the phenotype of two mutant cell lines, the thymoma cell line RMA-S and a small lung carcinoma cell line CMT.64, in order to address the mechanism that underlies the antigen processing deficiency of CMT.64 cells. Unlike RMA-S cells, vesicular stomatitis virus (VSV)-infected CMT.64 cells are not recognized by specific CTL. Interferon gamma (IFN-gamma) treatment of CMT.64 cells restores the ability of these cells to process and present VSV in the context of Kb. We show that although CMT.64 cells express a low level of beta 2m, the recognition of VSV-specific CTL is not restored by increasing the amount of beta 2m synthesized in CMT.64 cells. In addition, we find that CMT.64 cells express moderate levels of Kb heavy chain molecules, but most of it is unstable and rapidly degraded in the absence of IFN-gamma treatment. We infer that the antigen processing deficiency does not lie at the level of beta 2m or Kb production. We find also that the mRNAs for both TAP-1 and -2 are present in RMA and RMA-S cells but are absent in uninduced CMT.64 cells. Upon IFN-gamma induction, both mRNAs are highly expressed in CMT-64 cells. In addition, we find that the low molecular mass polypeptides 2 and 7, and additional components of the proteasome are induced by IFN-gamma in CMT-64 cells. Finally, introduction of the rat TAP-1 gene in CMT.64 cells restores CTL recognition of VSV-infected cells. These results indicate that a TAP-1 homodimer may translocate peptides in the ER and explain partially the CMT.64 defect and the RMA-S phenotype. These findings link a dysfunction in the transport and/or generation of antigenic peptides to the capacity of tumor cells to evade immunosurveillance and provide a unique model system to dissect this phenomenon.

Microbial multidrug-resistance ABC transporters

Multidrug resistance in tumor cells is often caused by the increased efflux of a wide variety of drugs, mediated by P glycoprotein, a member of the superfamily of ATP-binding cassette (ABC) transporters. The genes encoding members of this superfamily have also been isolated from drug-resistant microorganisms, and the role of microbial ABC transporters in drug resistance is being investigated.

CD1 expression is not affected by human peptide transporter deficiency

Conventional major histocompatibility complex class I molecules are highly polymorphic and present peptides to cytotoxic T cells. These peptides derive from the proteolytic degradation of endogenous proteins in the cytosol and are translocated into the endoplasmic reticulum by a peptide transporter consisting of two transporter associated with antigen processing (TAP) molecules. Absence of this transporter leads to the synthesis of unstable peptide free class I molecules that are weakly expressed on the cell surface. Mouse nonconventional class I molecules (class Ib) may also present TAP-dependent peptides. In humans, CD1 antigens are nonconventional class I molecules. Recently, we characterized a human HLA class I deficiency resulting from a homozygous TAP deficiency. We show here that CD1a and -c are normally expressed on epidermal Langerhans cells of the TAP-deficient patients, as are CD1a, -b, and -c on dendritic cells differentiated in vitro from monocytes. Moreover, the CD1a antigens present on the surface of the dendritic cells are functional, since they internalize by receptor-mediated endocytosis gold-labeled F(ab')2 fragments of an anti-CD1a mAb. This suggests either that CD1 molecules are empty molecules, that they are more stable than empty conventional class I proteins, or that CD1 molecules present TAP-independent peptides.