Association of the "major histocompatibility complex subregion" I-J determinant with bioactive glycosylation-inhibiting factor

Posttranslational modification of the glycosylation inhibiting factor (GIF) gene product generates bioactive GIF

Glycosylation inhibiting factor (GIF) and macrophage migration inhibitory factor (MIF) share an identical structure gene. Here we unravel two steps of posttranslational modifications in GIF/MIF molecules in human suppressor T (Ts) cell hybridomas. Peptide mapping and MS analysis of the affinity-purified GIF from the Ts cells revealed that one modification is cysteinylation at Cys-60, and the other is phosphorylation at Ser-91. Cysteinylated GIF, but not the wild-type GIF/MIF, possessed immunosuppressive effects on the in vitro IgE antibody response and had high affinity for GIF receptors on the T helper hybridoma cells. In vitro treatment of wild-type recombinant human GIF/MIF with cystine resulted in preferential cysteinylation of Cys-60 in the molecules. The cysteinylated recombinant human GIF and the Ts hybridoma-derived cysteinylated GIF were comparable both in the affinity for the receptors and in the immunosuppressive activity. Polyclonal antibodies specific for a stretch of the amino acid sequence in alpha2-helix of GIF bound bioactive cysteinylated GIF but failed to bind wild-type GIF/MIF. These results strongly suggest that cysteinylation of Cys-60 and consequent conformational changes in the GIF/MIF molecules are responsible for the generation of GIF bioactivity.

Presence of the 55 kDa glycosylation inhibiting factor in human serum

An ELISA system for the human glycosylation inhibiting factor (GIF) was established using polyclonal antibodies against highly purified 13 kDa recombinant human GIF, and the concentration of GIF in the sera of healthy donors and patients with various diseases was determined. GIF was detected in the sera of most healthy individuals and its concentration tended to increase with age. It was also found that the serum GIF levels markedly increased in some patients with rheumatoid arthritis or malignant tumors. Analysis of serum samples by SDS-PAGE and immunoblotting revealed a 55 kDa protein that has both the GIF antigenic determinant and the TCR alpha chain determinant. A 13 kDa GIF was not detected in the sera. In view of our previous findings on antigen-specific GIF from murine suppressor T cell hybridomas indicating that the 55 kDa GIF is a post-translationally formed conjugate of a TCR alpha chain with 13 kDa GIF, we suspect that the 55 kDa GIF detected in human sera is a human homologue of the murine 55 kDa GIF.

Target cells for an immunosuppressive cytokine, glycosylation-inhibiting factor

Receptors for bioactive glycosylation-inhibiting factor (GIF) were demonstrated using a bioactive mutant of recombinant human (rh) GIF, which is comparable to the suppressor T (Ts) cell-derived bioactive GIF in its affinity for the receptors on helper T (Th) hybridoma cells. Both naive T and B cells in normal mouse spleen lacked GIF receptors. However, presentation of specific antigen to naive T cells resulted in the expression of the receptors on activated T cells. Furthermore, activation of small resting B cells with F(ab')2 fragments of anti-mouse IgM plus IL-4, lipopolysaccharide (LPS) plus IL-4 or LPS plus dextran sulfate induced the expression of the receptors within 48 h of B cell stimulation. It was also found that NK T cells freshly isolated from mouse spleen, but not conventional NK cells, expressed receptors for GIF. CD4(+) and CD4(-) subpopulations of NK T cells showed a similar binding capability. Mature dendritic cells derived from bone marrow did not bear the receptors. The dissociation constant (Kd) of the interaction between the bioactive rhGIF mutant and the high-affinity receptors was 10-100 pM, whereas inactive wild-type rhGIF failed to bind to the receptors. A bioactive derivative of rhGIF suppressed both IgG1 and IgE synthesis by purified B cells activated by LPS and IL-4, indicating that the binding of bioactive GIF to its receptors on activated B cells results in suppression of their differentiation.

Immunosuppressive Activities of Recombinant Glycosylation -Inhibiting Factor Mutants

We have shown previously that glycosylation-inhibiting factor (GIF) in culture supernatants of suppressor T cell (Ts) hybridomas had bioactivity, while the same cells contained a substantial quantity of inactive GIF in cytosol. Mass-spectrometric analysis of GIF in the culture supernatant and cytosol of a Ts hybridoma provided direct evidence that GIF protein was posttranslationally modified in the Ts cells, and that the GIF bioactivity is associated with the posttranslationally modified species. Assuming that conformational changes induced by the posttranslational modifications are responsible for generation of bioactivity, we constructed cysteine mutants of human rGIF (rhGIF) in which cysteine at position 57, 60, or 81 was replaced with Ala, and the mutants were expressed in Escherichia coli. Replacement of Cys57 or Cys60 with Ala resulted in generation of bioactivity, while replacement of Cys81 with Ala failed to do so. It was also found that replacement of Cys57 with Ala and carboxymethylation of a sulfhydryl group in Cys60 synergistically increased the GIF bioactivity of the GIF derivatives. A mutated GIF protein, in which Cys57 and Asn106 in the rhGIF were replaced with Ala and Ser, respectively, had immunosuppressive effects on the IgE and IgG1 Ab responses of BDF1 mice to DNP-OVA, while wild-type rhGIF did not. Evidence was obtained that the mutated GIF suppressed Ag priming of Th cells for the Ab responses and proliferative response.

Subunit of glycosylation-inhibiting factor is an abundant protein that binds to certain glycoproteins and sugars

12 kDa subunit of glycosylation-inhibiting factor (GIF) is an abundant protein that can be isolated to homogeneity from different mammalian organs by successive application of the carboxymethylcellulose cation exchanger CM52, preparative flat-bed isoelectrofocusing and repeated application of CM52-cellulose. Several isoforms of the 12 kDa GIF subunit exist in mammalian tissues. Conformational stability of two isoforms of a 12 kDa porcine GIF subunit have been studied by CD. Conformation of the protein remains stable within the range 20 degrees to 60 degrees C. Over 60 degrees C the protein undergoes irreversible denaturation. The 12 kDa GIF subunit is not stable within the pH range 2 to 3, adopts quasi-native structure within the pH range 3.5 to 5 while it remains stable between the pHs 6 to 10. The 12 kDa GIF subunit strongly binds to CM52-cellulose from which it can be eluted at concentrations of NaCl higher than 0.6 M. The GIF subunit may also be eluted from the modified cellulose using certain glycoproteins and sugars. High abundance of the 12 kDa GIF subunit in different mammalian tissues and its capacity to bind certain glycoproteins and sugars may suggest that the protein might be involved in regulatory mechanisms of glycoprotein transport (chaperone for glycoproteins) and modulation of interactions between secreted glycoproteins and the cell surface receptors.

High-affinity binding of bioactive glycosylation-inhibiting factor to antigen-primed T cells and natural killer cells

High-affinity binding was demonstrated between suppressor-T-cell-derived bioactive glycosylation-inhibiting factor (GIF) and helper T hybridomas and natural killer cell line cells. Inactive GIF present in cytosol of suppressor T cells and Escherichia coli-derived recombinant human GIF (rhGIF) failed to bind to these cells. However, affinity of rhGIF for the target cells was generated by replacement of Cys-57 in the sequence with Ala or of Asn-106 with Ser or binding of 5-thio-2-nitrobenzoic acid to Cys-60 in the molecule. Such mutations and the chemical modification of rhGIF synergistically increased the affinity of GIF molecules for the target cells. The results indicated that receptors on the target cells recognize conformational structures of bioactive GIF. Equilibrium dissociation constant (Kd) of the specific binding between bioactive rGIF derivatives and high-affinity receptors was 10-100 pM. Receptors for bioactive GIF derivatives were detected on Th1 and Th2 T helper clones and natural killer NK1.1(+) cells in normal spleen but not on naive T or B cells. Neither the inactive rGIF nor bioactive rGIF derivatives bound to macrophage and monocyte lines or induced macrophages for tumor necrosis factor alpha production.

Conversion of inactive glycosylation inhibiting factor to bioactive derivatives by modification of a SH group

Escherichia coli-derived recombinant human glycosylation inhibiting factor (rhGIF) contains three cysteine residues (Cys-57, -60, and -81). All SH groups in the cysteine residues are free, and the GIF molecule had no biologic activity. Carboxymethylation of the SH group of Cys-60 in the molecule resulted in the generation of bioactivity, although the activity of the carboxymethylated GIF was 10- to 20-fold less than that of suppressor T cell (Ts)-derived GIF. However, treatment of the inactive rhGIF with ethylmercurithiosalicylate or 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) resulted in the generation of derivatives whose bioactivity was comparable to that of the Ts-derived bioactive GIF. The activity of these derivatives was lost by treatment with DTT. Isolation and chemical analysis of the DTNB-treated GIF derivative revealed that binding the 5-thio-2-nitrobenzoic acid group with Cys-60 was responsible for the generation of the highly bioactive derivative. Inactive cytosolic GIF from mammalian cells could also be converted to bioactive derivative by treatment with the SH reagent, while Ts-derived bioactive GIF was inactivated by DTT. These results, together with an x-ray crystal structure of GIF molecules, strongly suggest that the generation of bioactivity of GIF in Ts cells is due to posttranslational modifications that result in conformational changes in the molecule.

Cellular mechanisms for the formation of a soluble form derivative of T-cell receptor alpha chain by suppressor T cells

Upon stimulation with anti-CD3, suppressor T-cell (Ts) hybridomas and homologous transfectants of T-cell receptor a (TCRalpha) cDNA in the T-cell hybridoma formed a 55-kDa TCRalpha chain derivative that bound both the monoclonal anti-TCRalpha chain and polyclonal antibodies against glycosylation inhibiting factor (GIF). The peptide is a subunit of antigen-specific suppressor T-cell factor (TsF), and is considered to be a posttranslationally-formed conjugate of TCRalpha chain with GIF peptide. The TCRalpha derivative is synthesized by the transfectant after stimulation with anti-CD3, and not derived from TCR present on the cell surface. Stimulation of the stable homologous transfectants with anti-CD3 induced translocation of the 13-kDa GIF peptide into endoplasmic reticulum (ER). When a helper Ts hybridoma or a stable transfectant of the same TCRalpha cDNA in a helper cell-derived TCRalpha- clone was stimulated with anti-CD3, translocation of GIF peptide was not detected, and these cells failed to secrete a TCRalpha derivative. However, further transfection of a chimeric cDNA encoding a procalcitonin-GIF fusion protein into the helper cell-derived stable transfectant of TCRalpha cDNA resulted in translocation of the GIF protein and formation of bioactive 55-kDa GIF. The results indicated that translocation of GIF peptide through ER is unique for Ts cells, and that this process is essential for the formation/secretion of the soluble form derivative of TCRalpha chain by T cells.

The crystal structure of human glycosylation-inhibiting factor is a trimeric barrel with three 6-stranded beta-sheets

Glycosylation-inhibiting factor (GIF) is a cytokine that is involved in the regulation of IgE synthesis. The crystal structure of recombinant human GIF was determined by the multiple isomorphous replacement method. The structure was refined to an R factor of 0.168 at 1.9 angstrom resolution. The overall structure is seen to consist of three interconnected subunits forming a barrel with three 6-stranded beta-sheets on the inside and six alpha-helices on the outside. There is a 5-angstrom-diameter "hole" through the middle of the barrel. The barrel structure of GIF in part resembles other "trefoil" cytokines such as interleukin 1 and fibroblast growth factor. Each subunit has a new class of alpha + beta sandwich structure consisting of two beta-alpha-beta motifs. These beta-alpha-beta motifs are related by a pseudo-twofold axis and resemble both interleukin 8 and the peptide binding domain of major histocompatibility complex protein, although the topology of the polypeptide chain is quite different.