Remodeling of the lectin-EGF-like domain interface in P- and L-selectin increases adhesiveness and shear resistance under hydrodynamic force

Crystal structures of the lectin and epidermal growth factor (EGF)-like domains of P-selectin show 'bent' and 'extended' conformations. An extended conformation would be 'favored' by forces exerted on a selectin bound at one end to a ligand and at the other end to a cell experiencing hydrodynamic drag forces. To determine whether the extended conformation has higher affinity for ligand, we introduced an N-glycosylation site to 'wedge open' the interface between the lectin and EGF-like domains of P-selectin. This alteration increased the affinity of P-selectin for its ligand P-selectin glycoprotein 1 (PSGL-1) and thereby the strength of P-selectin-mediated rolling adhesion. Similarly, an asparagine-to-glycine substitution in the lectin-EGF-like domain interface of L-selectin enhanced rolling adhesion under shear flow. Our results demonstrate that force, by 'favoring' an extended selectin conformation, can strengthen selectin-ligand bonds.

Role of the C-terminal propeptide in the activity and maturation of gamma -interferon-inducible lysosomal thiol reductase (GILT)

gamma-Interferon-inducible lysosomal thiol reductase (GILT) is constitutively expressed in antigen-presenting cells. GILT facilitates unfolding of endocytosed antigens in MHC class II-containing compartments by enzymatically reducing disulfide bonds. The enzyme is synthesized as a 35-kDa precursor. Although a fraction of the precursor is secreted as a disulfide-linked dimer, the majority is directed via the mannose-6-phosphate receptor pathway to endocytic compartments where its N- and C-terminal propeptides are cleaved to generate the 30-kDa mature form. Both precursor and mature GILT reduce disulfide bonds with an acidic pH optimum. In this report, we show that the cysteine residues in the C-terminal propeptide, Cys-211 and Cys-222, serve key structural roles. Mutation of Cys-222 abolishes disulfide-linked dimerization of precursor GILT and decreases the efficiency of GILT maturation. Mutation of Cys-211 results in both impaired intracellular maturation and loss of enzymatic activity of the precursor form at an acidic pH. A similar phenotype was obtained upon mutation of Cys-200, which is retained in the mature form. Cys-200 and Cys-211 seem to form a disulfide bond that links the propeptide and the mature enzyme until reduction in the lysosome. This disulfide bridge is essential for stability of the enzyme at low pH and for its proper maturation in vivo.

Disulfide reduction in major histocompatibility complex class II-restricted antigen processing by interferon-gamma-inducible lysosomal thiol reductase

Constitutively expressed in antigen-presenting cells (APCs), interferon-gamma-inducible lysosomal thiol reductase (GILT) catalyzes disulfide bond reduction under acidic conditions. GILT contains a CXXC motif similar to the WCGH/PCK motif of proteins in the thioredoxin family. This class of enzymes catalyzes, at a neutral pH, dithiol oxidation, disulfide bond reduction, and disulfide bond isomerization. A well-established assay spectrophotometrically measures interchain disulfide bond reduction of insulin via the precipitation of aggregating free B chains. However, the insolubility of insulin at low pH limits the use of this assay. To assess the thiol reductase activity of GILT, we employed an assay that uses denatured [125I]F(ab')2 as a substrate, which is detailed in this article. In addition, we discuss approaches used to demonstrate the mechanism of action of GILT and to identify substrates.

Absence of gamma-interferon-inducible lysosomal thiol reductase in melanomas disrupts T cell recognition of select immunodominant epitopes

Long-lasting tumor immunity requires functional mobilization of CD8+ and CD4+ T lymphocytes. CD4+ T cell activation is enhanced by presentation of shed tumor antigens by professional antigen-presenting cells (APCs), coupled with display of similar antigenic epitopes by major histocompatibility complex class II on malignant cells. APCs readily processed and presented several self-antigens, yet T cell responses to these proteins were absent or reduced in the context of class II+ melanomas. T cell recognition of select exogenous and endogenous epitopes was dependent on tumor cell expression of gamma-interferon-inducible lysosomal thiol reductase (GILT). The absence of GILT in melanomas altered antigen processing and the hierarchy of immunodominant epitope presentation. Mass spectral analysis also revealed GILT's ability to reduce cysteinylated epitopes. Such disparities in the profile of antigenic epitopes displayed by tumors and bystander APCs may contribute to tumor cell survival in the face of immunological defenses.

Defective antigen processing in GILT-free mice

Processing of proteins for major histocompatibility complex (MHC) class II-restricted presentation to CD4-positive T lymphocytes occurs after they are internalized by antigen-presenting cells (APCs). Antigenic proteins frequently contain disulfide bonds, and their reduction in the endocytic pathway facilitates processing. In humans, a gamma interferon-inducible lysosomal thiol reductase (GILT) is constitutively present in late endocytic compartments of APCs. Here, we identified the mouse homolog of GILT and generated a GILT knockout mouse. GILT facilitated the processing and presentation to antigen-specific T cells of protein antigens containing disulfide bonds. The response to hen egg lysozyme, a model antigen with a compact structure containing four disulfide bonds, was examined in detail.

Gamma-interferon-inducible lysosomal thiol reductase (GILT). Maturation, activity, and mechanism of action

We recently identified a gamma-interferon-inducible lysosomal thiol reductase (GILT), constitutively expressed in antigen-presenting cells, that catalyzes disulfide bond reduction both in vitro and in vivo and is optimally active at acidic pH. GILT is synthesized as a 35-kDa precursor, and following delivery to major histocompatibility complex (MHC) class II-containing compartments (MIICs), is processed to the mature 30-kDa form via cleavage of N- and C-terminal propeptides. The generation of MHC class II epitopes requires both protein denaturation and reduction of intra- and inter-chain disulfide bonds prior to proteolysis. GILT may be important in disulfide bond reduction of proteins delivered to MIICs and consequently in antigen processing. In this report we show that, like its mature form, precursor GILT reduces disulfide bonds with an acidic pH optimum, suggesting that it may also be involved in disulfide bond reduction in the endocytic pathway. We also show that processing of precursor GILT can be mediated by multiple lysosomal proteases and provide evidence that the mechanism of action of GILT resembles that of other thiol oxidoreductases.

Enzymatic reduction of disulfide bonds in lysosomes: characterization of a gamma-interferon-inducible lysosomal thiol reductase (GILT)

Proteins internalized into the endocytic pathway are usually degraded. Efficient proteolysis requires denaturation, induced by acidic conditions within lysosomes, and reduction of inter- and intrachain disulfide bonds. Cytosolic reduction is mediated enzymatically by thioredoxin, but the mechanism of lysosomal reduction is unknown. We describe here a lysosomal thiol reductase optimally active at low pH and capable of catalyzing disulfide bond reduction both in vivo and in vitro. The active site, determined by mutagenesis, consists of a pair of cysteine residues separated by two amino acids, similar to other enzymes of the thioredoxin family. The enzyme is a soluble glycoprotein that is synthesized as a precursor. After delivery into the endosomal/lysosomal system by the mannose 6-phosphate receptor, N- and C-terminal prosequences are removed. The enzyme is expressed constitutively in antigen-presenting cells and induced by IFN-gamma in other cell types, suggesting a potentially important role in antigen processing.

Ribavirin polarizes human T cell responses towards a Type 1 cytokine profile

BACKGROUND/AIMS

The therapeutic benefit of ribavirin, a nucleoside analog, in the treatment of chronic HCV infection is seen even in the absence of any apparent direct antiviral effect. We surmised that ribavirin may act by eliciting altered virus-specific immune responses. Because antiviral immunity is predominantly mediated by cytotoxic T cells and antiviral cytokines, we sought to determine whether ribavirin could promote antiviral (Type 1) cytokine expression in human T cells.

METHODS

Isolated human T cells were activated in vitro with enterotoxin B or with phorbol ester plus ionomycin. Cytokine ELISAs were performed on culture supernatants, cytokine mRNA was detected following RT-polymerase chain reaction of T cell RNA, and T cell proliferation measured using MTT assay.

RESULTS

Ribavirin enhanced a Type 1 (IL-2, IFNgamma, TNFalpha) while suppressing a Type 2 cytokine response (IL-4, IL-5 and IL-10), at the level of both protein and mRNA expression. Ribavirin mediated comparable effects on cytokine expression both following activation of specific T cell subpopulations with superantigen and following activation of a larger percentage of T cells via pharmacologic means. The in vitro effect on cytokine expression following ribavirin treatment was comparable in both CD4+ or CD8+ T cell subsets and was observed in a dose range that promoted T cell proliferation.

CONCLUSIONS

These data support the view that ribavirin promotes a Type 1 cytokine-mediated immune response, a property which may account in part for its ability to enhance the antiviral activity of interferon-alpha in the treatment of chronic HCV infection.

Oligonucleotide-mediated inhibition of CD28 expression induces human T cell hyporesponsiveness and manifests impaired contact hypersensitivity in mice

Ligation of CD28 provides a costimulatory signal essential for Ag-mediated T cell activation via the TCR. Blocking CD28 ligation can inhibit cytokine expression and elicits a state of T cell hyporesponsiveness. In this study, we examined the effect of inhibiting CD28 expression on in vitro and in vivo T cell responses. To address this, we have synthesized a series of G-rich phosphorothioate oligonucleotides that inhibited activation-induced transcription and cell surface expression of CD28 on human T cells. CD28 blockade was selective, as expression of other activation-induced receptors was unaffected by oligonucleotide treatment. Using strategic changes to base composition, we identified a minimal 12-mer sequence, containing two sets of four contiguous guanosines separated by 3 to 5 bases, which conferred activity in vitro. Furthermore, inhibition of CD28 expression mediated by one representative active oligonucleotide, GR1, resulted in a concomitant dose-dependent diminution of anti-CD3/PMA-induced cytokine (IL-2, IFN-gamma, IL-8) production. Inhibition of IL-2 synthesis was dependent on CD28 expression, as GR1 failed to abrogate activated IL-2 production in a CD28-deficient T cell line, HUT 78. The inhibitory activity of GR1 reduced T cell proliferative responses in MLR and induced Ag-specific T cell hyporesponsiveness to alloantigens. Finally, s.c. administration of GR1 impaired in vivo contact hypersensitivity responses in mice and was associated with substantially decreased CD28 and IFN-gamma mRNA expression in lymph node cells. Collectively, our studies show the tolerogenic potential of oligonucleotide-mediated CD28 inhibition on T cell activation, in vitro and in vivo.

Oligonucleotide-mediated inhibition of CD28 expression induces human T cell hyporesponsiveness and manifests impaired contact hypersensitivity in mice

Ligation of CD28 provides a costimulatory signal essential for Ag-mediated T cell activation via the TCR. Blocking CD28 ligation can inhibit cytokine expression and elicits a state of T cell hyporesponsiveness. In this study, we examined the effect of inhibiting CD28 expression on in vitro and in vivo T cell responses. To address this, we have synthesized a series of G-rich phosphorothioate oligonucleotides that inhibited activation-induced transcription and cell surface expression of CD28 on human T cells. CD28 blockade was selective, as expression of other activation-induced receptors was unaffected by oligonucleotide treatment. Using strategic changes to base composition, we identified a minimal 12-mer sequence, containing two sets of four contiguous guanosines separated by 3 to 5 bases, which conferred activity in vitro. Furthermore, inhibition of CD28 expression mediated by one representative active oligonucleotide, GR1, resulted in a concomitant dose-dependent diminution of anti-CD3/PMA-induced cytokine (IL-2, IFN-gamma, IL-8) production. Inhibition of IL-2 synthesis was dependent on CD28 expression, as GR1 failed to abrogate activated IL-2 production in a CD28-deficient T cell line, HUT 78. The inhibitory activity of GR1 reduced T cell proliferative responses in MLR and induced Ag-specific T cell hyporesponsiveness to alloantigens. Finally, s.c. administration of GR1 impaired in vivo contact hypersensitivity responses in mice and was associated with substantially decreased CD28 and IFN-gamma mRNA expression in lymph node cells. Collectively, our studies show the tolerogenic potential of oligonucleotide-mediated CD28 inhibition on T cell activation, in vitro and in vivo.