Cloning, expression, purification and characterization of the cholera toxin B subunit and triple glutamic acid decarboxylase epitopes fusion protein in Escherichia coli

A single point mutation within the coding sequence of cholera toxin B subunit will increase its expression yield

Background: Cholera toxin B subunit (CTB) has been extensively considered as an immunogenic and adjuvant protein, but its yield of expression is not satisfactory in many studies. The aim of this study was to compare the expression of native and mutant recombinant CTB (rCTB) in pQE vector. Methods: ctxB fragment from Vibrio cholerae O₁ ATCC14035 containing the substitution of mutant ctxB for amino acid S128T was amplified by PCR and cloned in pGETM-T easy vector. It was then transformed to E. coli Top 10F' and cultured on LB agar plate containing ampicillin. Sequence analysis confirmed the mature ctxB gene sequence and the mutant one in both constructs which were further subcloned to pQE-30 vector. Both constructs were subsequently transformed to E. coli M15 (pREP4) for expression of mature and mutant rCTB. Results: SDS-PAGE analysis showed the maximum expression of rCTB in both systems at 5 hours after induction and Western-blot analysis confirmed the presence of rCTB in blotting membranes. The expression of mutant rCTB was much higher than mature rCTB, which may be the result of serine-to-threonine substitution at position 128 of mature rCTB amino acid sequence created by PCR mutagenesis. The mutant rCTB retained pentameric stability and its ability to bind to anti- cholera toxin IgG antibodies. Conclusion: Point mutation in ctxB sequence resulted in over-expression of rCTB, probably due to the increase of solubility of produced rCTB. Consequently, this expression system can be used to produce rCTB in high yield.

Cholera toxin B subunit linked to glutamic acid decarboxylase suppresses dendritic cell maturation and function

Dendritic cells are the largest population of antigen presenting cells in the body. One of their main functions is to regulate the delicate balance between immunity and tolerance responsible for maintenance of immunological homeostasis. Disruption of this delicate balance often results in chronic inflammation responsible for initiation of organ specific autoimmune diseases such as rheumatoid arthritis, multiple sclerosis and type I diabetes. The cholera toxin B subunit (CTB) is a weak mucosal adjuvant known for its ability to stimulate immunity to antigenic proteins. However, conjugation of CTB to many autoantigens can induce immunological tolerance resulting in suppression of autoimmunity. In this study, we examined whether linkage of CTB to a 5kDa C-terminal protein fragment of the major diabetes autoantigen glutamic acid decarboxylase (GAD(35)), can block dendritic cell (DC) functions such as biosynthesis of co-stimulatory factor proteins CD86, CD83, CD80 and CD40 and secretion of inflammatory cytokines. The results of human umbilical cord blood monocyte-derived DC-GAD(35) autoantigen incubation experiments showed that inoculation of immature DCs (iDCs), with CTB-GAD(35) protein dramatically suppressed levels of CD86, CD83, CD80 and CD40 co-stimulatory factor protein biosynthesis in comparison with GAD(35) alone inoculated iDCs. Surprisingly, incubation of iDCs in the presence of the CTB-autoantigen and the strong immunostimulatory molecules PMA and Ionomycin revealed that CTB-GAD(35) was capable of arresting PMA+Ionomycin induced DC maturation. Consistent with this finding, CTB-GAD(35) mediated suppression of DC maturation was accompanied by a dramatic decrease in the secretion of the pro-inflammatory cytokines IL-12/23p40 and IL-6 and a significant increase in secretion of the immunosuppressive cytokine IL-10. Taken together, our experimental data suggest that linkage of the weak adjuvant CTB to the dominant type 1 diabetes autoantigen GAD strongly inhibits DC maturation through the down regulation of major co-stimulatory factors and inflammatory cytokine biosynthesis. These results emphasize the possibility that CTB-autoantigen fusion proteins enhance DC priming of naïve Th0 cell development in the direction of immunosuppressive T lymphocytes. The immunological phenomena observed here establish a basis for improvement of adjuvant augmented multi-component subunit vaccine strategies capable of complete suppression of organ-specific autoimmune diseases in vivo.

Protection against autoimmune diabetes by silkworm-produced GFP-tagged CTB-insulin fusion protein

In animals, oral administration of the cholera toxin B (CTB) subunit conjugated to the autoantigen insulin enhances the specific immune-unresponsive state. This is called oral tolerance and is capable of suppressing autoimmune type 1 diabetes (T1D). However, the process by which the CTB-insulin (CTB-INS) protein works as a therapy for T1D in vivo remains unclear. Here, we successfully expressed a green fluorescent protein- (GFP-) tagged CTB-Ins (CTB-Ins-GFP) fusion protein in silkworms in a pentameric form that retained the native ability to activate the mechanism. Oral administration of the CTB-Ins-GFP protein induced special tolerance, delayed the development of diabetic symptoms, and suppressed T1D onset in nonobese diabetic (NOD) mice. Moreover, it increased the numbers of CD4⁺CD25⁺Foxp3⁺ T regulatory (Treg) cells in peripheral lymph tissues and affected the biological activity of spleen cells. This study demonstrated that the CTB-Ins-GFP protein produced in silkworms acted as an oral protein vaccine, inducing immunological tolerance involving CD4⁺CD25⁺Foxp3⁺ Treg cells in treating T1D.

Suppression of dendritic cell activation by diabetes autoantigens linked to the cholera toxin B subunit

Antigen presenting cells, specifically dendritic cells (DCs) are a focal point in the delicate balance between T cell tolerance and immune responses contributing to the onset of type I diabetes (T1D). Weak adjuvant proteins like the cholera toxin B subunit when linked to autoantigens may sufficiently alter the balance of this initial immune response to suppress the development of autoimmunity. To assess adjuvant enhancement of autoantigen mediated immune suppression of Type 1 diabetes, we examined the cholera toxin B subunit (CTB)-proinsulin fusion protein (CTB-INS) activation of immature dendritic cells (iDC) at the earliest detectable stage of the human immune response. In this study, Incubation of human umbilical cord blood monocyte-derived immature DCs with CTB-INS autoantigen fusion protein increased the surface membrane expression of DC Toll-like receptor (TLR-2) while no significant upregulation in TLR-4 expression was detected. Inoculation of iDCs with CTB stimulated the biosynthesis of both CD86 and CD83 co-stimulatory factors demonstrating an immunostimulatory role for CTB in both DC activation and maturation. In contrast, incubation of iDCs with proinsulin partially suppressed CD86 co-stimulatory factor mediated DC activation, while incubation of iDCs with CTB-INS fusion protein completely suppressed iDC biosynthesis of both CD86 and CD83 costimulatory factors. The incubation of iDCs with increasing amounts of insulin did not increase the level of immune suppression but rather activated DC maturation by stimulating increased biosynthesis of both CD86 and CD83 costimulatory factors. Inoculation of iDCs with CTB-INS fusion protein dramatically increased secretion of the immunosuppressive cytokine IL-10 and suppressed synthesis of the pro-inflammatory cytokine IL12/23 p40 subunit protein suggesting that linkage of CTB to insulin (INS) may play an important role in mediating DC guidance of cognate naïve Th0 cell development into immunosuppressive T lymphocytes. Taken together, the experimental data suggests Toll like receptor 2 (TLR-2) plays a dominant role in CTB mediated INS inhibition of DC induced type 1 diabetes onset in human Type 1 diabetes autoimmunity. Further, fusion of CTB to the autoantigen was found to be essential for enhancement of immune suppression as co-delivery of CTB and insulin did not significantly inhibit DC costimulatory factor biosynthesis. The experimental data presented supports the hypotheses that adjuvant enhancement of autoantigen mediated suppression of islet beta cell inflammation is dependent on CTB stimulation of dendritic cell TLR2 receptor activation and co-processing of both CTB and the autoantigen in the same dendritic cell.

Protective effects of a bacterially expressed NIF-KGF fusion protein against bleomycin-induced acute lung injury in mice

Current evidence suggests that the keratinocyte growth factor (KGF) and the polymorphonuclear leukocyte may play key roles in the development of lung fibrosis. Here we describe the construction, expression, purification, and identification of a novel NIF (neutrophil inhibitory factor)-KGF mutant fusion protein (NKM). The fusion gene was ligated via a flexible octapeptide hinge and expressed as an insoluble protein in Escherichia coli BL21 (DE3). The fusion protein retained the activities of KGF and NIF, as it inhibited both fibroblast proliferation and leukocyte adhesion. Next, the effects of NKM on bleomycin-induced lung fibrosis in mice were examined. The mice were divided into the following four groups: (i) saline group; (ii) bleomycin group (instilled with 5 mg/kg bleomycin intratracheally); (iii) bleomycin plus dexamethasone (Dex) group (Dex was given intraperitoneally (i.p.) at 1 mg/kg/day 2 days prior to bleomycin instillation and daily after bleomycin instillation until the end of the treatment); and (iv) bleomycin plus NKM group (NKM was given i.p. at 2 mg/kg/day using the same protocol as the Dex group). NKM significantly improved the survival rates of mice exposed to bleomycin. The marked morphological changes and increased hydroxyproline levels resulted from the instillation of bleomycin (on Day 17) in the lungs were significantly inhibited by NKM. These results revealed that NKM can attenuate bleomycin-induced lung fibrosis, suggesting that NKM could be used to prevent bleomycin-induced lung damage or other interstitial pulmonary fibrosis.

Mucosally induced immunological tolerance, regulatory T cells and the adjuvant effect by cholera toxin B subunit

Induction of peripheral immunological tolerance by mucosal administration of selected antigens (Ags) ('oral tolerance') is an attractive, yet medically little developed, approach to prevent or treat selected autoimmune or allergic disorders. A highly effective way to maximize oral tolerance induction for immunotherapeutic purposes is to administer the relevant Ag together with, and preferably linked to the non-toxic B subunit protein of cholera toxin (CTB). Oral, nasal or sublingual administration of such Ag/CTB conjugates or gene fusion proteins have been found to induce tolerance with superior efficiency compared with administration of Ag alone, including the suppression of various autoimmune disorders and allergies in animal models. In a proof-of-concept clinical trial in patients with Behcet's disease, this was extended with highly promising results to prevent relapse of autoimmune uveitis. Tolerization by mucosal Ag/CTB administration results in a strong increase in Ag-specific regulatory CD4(+) T cells, apparently via two separate pathways: one using B cells as APCs and leading to a strong expansion of Foxp3(+) Treg cells which can both suppress and mediate apoptotic depletion of effector T cells, and one being B cell-independent and associated with development of Foxp3(-) regulatory T cells that express membrane latency-associated peptide and transforming growth factor (TGF-beta) and/or IL-10. The ability of CTB to dramatically increase mucosal Ag uptake and presentation by different APCs through binding to GM1 ganglioside (which makes most B cells effective APCs irrespective of their Ag specificity), together with CTB-mediated stimulation of TGF-beta and IL-10 production and inhibition of IL-6 formation may explain the dramatic potentiation of oral tolerance by mucosal Ags presented with CTB.