The quantity and quality of α-gal-specific antibodies differ in individuals with and without delayed red meat allergy

Background

IgG to galactose‐α‐1,3‐galactose (α‐gal) are highly abundant natural antibodies (Ab) in humans. α‐Gal‐specific IgE Ab cause a special form of meat allergy characterized by severe systemic reactions 3–7 h after consumption of red meat. We investigated 20 patients who experienced such reactions and characterized their α‐gal‐specific IgE and IgG responses in more detail.

Methods

α‐Gal‐specific IgE was determined by ImmunoCAP. IgE reactivity to meat extract and bovine gamma globulin (BGG) was assessed by immunoblotting and ELISA, respectively. In some experiments, sera were pre‐incubated with α‐gal or protein G to deplete IgG Ab. α‐Gal‐specific IgG_1–4 Ab in individuals with and without meat allergy were assessed by ELISA.

Results

In immunoblots, BGG was the most frequently recognized meat protein. Binding of IgE and IgG to BGG was confirmed by ELISA and completely abolished after pre‐incubation with α‐gal. Neither the depletion of autologous α‐gal‐specific IgG Ab nor the addition of α‐gal‐specific IgG Ab from nonallergic individuals changed the IgE recognition of BGG of meat‐allergic patients. Meat‐allergic patients showed significantly higher α‐gal‐specific IgG1 and IgG3 Ab than nonallergic individuals, whereas the latter showed significantly higher levels of α‐gal‐specific IgG4 Ab.

Conclusion

Patients with delayed meat allergy display IgE and IgG Ab that selectively recognize the α‐gal epitope on BGG. Their enhanced α‐gal‐specific IgE levels are accompanied by high levels of α‐gal‐specific IgG1 devoid of IgE‐blocking activity. This subclass distribution is atypical for food allergies and distinct from natural α‐gal IgG responses in nonallergic individuals.

Bet v 1--a Trojan horse for small ligands boosting allergic sensitization?

BACKGROUND

Birch pollen allergy represents the main cause of winter and spring pollinosis in the temperate climate zone of the northern hemisphere and sensitization towards Bet v 1, the major birch pollen allergen, affects over 100 million allergic patients. The major birch pollen allergen Bet v 1 has been described as promiscuous acceptor for a wide variety of hydrophobic ligands.

OBJECTIVE

In search of intrinsic properties of Bet v 1, which account responsible for the high allergenic potential of the protein, we thought to investigate the effects of ligand-binding on immunogenic as well as allergenic properties.

METHODS

As surrogate ligand of Bet v 1 sodium deoxycholate (DOC) was selected. Recombinant and natural Bet v 1 were characterised physico-chemically as well as immunologically in the presence or absence of DOC, and an animal model of allergic sensitization was established. Moreover, human IgE binding to Bet v 1 was analysed by nuclear magnetic resonance (NMR) spectroscopy.

RESULTS

Ligand-binding had an overall stabilizing effect on Bet v 1. This translated in a Th2 skewing of the immune response in a mouse model. Analyses of human IgE binding on Bet v 1 in mediator release assays revealed that ligand-bound allergen-induced degranulation at lower concentrations; however, in basophil activation tests with human basophils ligand-binding did not show this effect. For the first time, human IgE epitopes on Bet v 1 were determined using antibodies isolated from patients' sera. The IgE epitope mapping of Bet v 1 demonstrated the presence of multiple binding regions.

CONCLUSIONS AND CLINICAL RELEVANCE

Deoxycholate binding stabilizes conformational IgE epitopes on Bet v 1; however, the epitopes themselves remain unaltered. Therefore, we speculate that humans are exposed to both ligand-bound and free Bet v 1 during sensitization, disclosing the ligand-binding cavity of the allergen as key structural element.

Differential activation of dendritic cells by toll-like receptors causes diverse differentiation of naïve CD4+ T cells from allergic patients

BACKGROUND

To avert the differentiation of allergen-specific Th2 cells in atopic individuals is a major goal in the prevention and therapy of IgE-mediated allergy. We aimed to compare different toll-like receptor (TLR) agonists regarding their effects on antigen-presenting cells and the differentiation of naïve T cells from allergic patients.

METHODS

Monocytes and monocyte-derived dendritic cells (mdDC) from allergic patients were stimulated with Pam3CSK4 (TLR1/2 ligand), FSL-1 (TLR2/6 ligand), monophosphoryl lipid (MPL)-A, lipopolysaccharide (LPS, both TLR4 ligands), and flagellin (TLR5 ligand). Allergen uptake and upregulation of CD40, CD80, CD83, CD86, CD58, CCR7 and PD-L1 were analyzed by flow cytometry. Functional maturation of mdDC was tested in mixed leukocyte reactions, and the synthesis of proinflammatory cytokines, IL-10 and members of the IL-12 family was assessed. TLR-ligand-activated mdDC were used to stimulate naïve CD4(+) T cells, and cytokine responses were assessed in supernatants and intracellularly.

RESULTS

All TLR ligands except flagellin enhanced allergen uptake. All TLR ligands induced functional maturation of mdDC with differential expression of surface molecules and cytokines and promoted the differentiation of IFN-γ-producing T cells. LPS-matured mdDC exclusively induced Th1-like responses, whereas mdDC stimulated with the other TLR ligands induced both Th1- and Th0-like cells. Pam3CSK4 and flagellin additionally induced Th2-like cells. Th1-like responses were associated with higher expression levels of co-stimulatory molecules, PD-L1, IL-6, TNF-α, and IL-12p70. None of the TLR-ligand-stimulated mdDC induced IL-10- or IL-17-producing T cells.

CONCLUSION

Different TLR ligands differently influence T-cell responses due to varying activation of the three signals relevant for T-cell activation, that is, antigen presentation, co-stimulation and cytokine milieu.

A hypoallergenic variant of the major birch pollen allergen shows distinct characteristics in antigen processing and T-cell activation

BACKGROUND

BM4 is a novel genetically engineered variant of the major birch pollen allergen Bet v 1 that lacks the typical Bet v 1-like fold and displays negligible IgE-binding but strong T cell-activating capacity. The aim of this study was to elucidate possible differences between BM4 and Bet v 1 in internalization, antigen processing, and presentation.

METHODS

Proliferative responses to BM4 and Bet v 1 of peripheral blood mononuclear cells and Bet v 1-specific T-cell clones were compared. Fluorescently labeled BM4 and Bet v 1 were used to study surface binding, endocytosis, and intracellular degradation by monocyte-derived DC (mdDC). Both proteins were digested by endolysosomal extracts of mdDC. BM4- and Bet v 1-pulsed mdDC were employed to assess the kinetics of activation of Bet v 1-specific T-cell clones and the polarization of naïve T cells.

RESULTS

BM4 displayed a significantly stronger T cell-activating capacity than Bet v 1. Furthermore, BM4 showed increased surface binding and internalization as well as faster endolysosomal degradation compared with Bet v 1. BM4-pulsed mdDC induced enhanced proliferative responses at earlier time-points in Bet v 1-specific T-cell clones and promoted less IL-5 production in T cells than Bet v 1-pulsed mdDC.

CONCLUSION

The loss of the Bet v 1-fold changes the protein's interaction with the human immune system at the level of antigen-presenting cells resulting in altered T-cell responses. By combining low IgE-binding with strong and modulating T cell-activating capacity, BM4 represents a highly interesting candidate for specific immunotherapy of birch pollen allergy.

Human blood basophils do not act as antigen-presenting cells for the major birch pollen allergen Bet v 1

BACKGROUND

Several studies in mice have recently shown that basophils can act as antigen-presenting cells (APC) inducing Th2-mediated immune responses against parasites or protease allergens. The aim of this study was to investigate whether human basophils function as APC for the major birch pollen allergen Bet v 1.

METHODS

Fluorescently labeled Bet v 1 was used to assess surface binding and internalization of allergen by basophils and different types of APC from birch pollen-allergic and nonallergic individuals. Sorted basophils were analyzed in terms of up-regulation of MHC class II and co-stimulatory molecules in the absence and presence of IL-3 and IFN-γ by flow cytometry. Expression of proteins crucial for antigen presentation, namely cathepsin S and invariant chain, was determined. Basophils were used as APC in co-culture experiments with Bet v 1-specific T-cell clones (TCCs).

RESULTS

Basophils from birch pollen-allergic donors very efficiently bound Bet v 1 through IgE/FcεRI complexes on their surface. In contrast to professional APC, basophils did not internalize allergen and expressed marginal levels of cathepsin S and invariant chain. HLA-DP, HLA-DQ, CD80/CD86, and CD40 were absent from purified basophils even when stimulated with IL-3 plus IFN-γ. IL-3/IFN-γ marginally up-regulated HLA-DR. Bet v 1-pulsed basophils failed to induce proliferative and cytokine responses in Bet v 1-specific, HLA-DR-restricted TCCs.

CONCLUSION

Human basophils neither internalize, process nor present Bet v 1. Because Bet v 1 is a highly relevant allergen, we conclude that basophils play no role as APC in IgE-mediated allergy in humans.

N-glycan structure of a short-lived variant of ribophorin I expressed in the MadIA214 glycosylation-defective cell line reveals the role of a mannosidase that is not ER mannosidase I in the process of glycoprotein degradation

A soluble form of ribophorin I (RI(332)) is rapidly degraded in Hela and Chinese hamster ovary (CHO) cells by a cytosolic proteasomal pathway, and the N-linked glycan present on the protein may play an important role in this process. Specifically, it has been suggested that endoplasmic reticulum (ER) mannosidase I could trigger the targeting of improperly folded glycoproteins to degradation. We used a CHO-derived glycosylation-defective cell line, MadIA214, for investigating the role of mannosidase(s) as a signal for glycoprotein degradation. Glycoproteins in MadIA214 cells carry truncated Glc(1)Man(5)GlcNAc(2) N-glycans. This oligomannoside structure interferes with protein maturation and folding, leading to an alteration of the ER morphology and the detection of high levels of soluble oligomannoside species caused by glycoprotein degradation. An HA-epitope-tagged soluble variant of ribophorin I (RI(332)-3HA) expressed in MadIA214 cells was rapidly degraded, comparable to control cells with the complete Glc(3)Man(9)GlcNAc(2) N-glycan. ER-associated degradation (ERAD) of RI(332)-3HA was also proteasome-mediated in MadIA214 cells, as demonstrated by inhibition of RI(332)-3HA degradation with agents specifically blocking proteasomal activities. Two inhibitors of alpha1,2-mannosidase activity also stabilized RI(332)-3HA in the glycosylation-defective cell line. This is striking, because the major mannosidase activity in the ER is the one of mannosidase I, specific for a mannose alpha1,2-linkage that is absent from the truncated Man(5) structure. Interestingly, though the Man(5) derivative was present in large amounts in the total protein pool, the two major species linked to RI(332)-3HA shortly after synthesis consisted of Glc(1)Man(5 )and Man(4), being replaced by Man(4 )and Man(3) when proteasomal degradation was inhibited. In contrast, the untrimmed intermediate of RI(332)-3HA was detected in mutant cells treated with mannosidase inhibitors. Our results unambiguously demonstrate that an alpha1,2-mannosidase that is not ER mannosidase I is involved in ERAD of RI(332-)3HA in the glycosylation-defective cell line, MadIA214.

Degradation of a short-lived glycoprotein from the lumen of the endoplasmic reticulum: the role of N-linked glycans and the unfolded protein response

We are studying endoplasmic reticulum-associated degradation (ERAD) with the use of a truncated variant of the type I ER transmembrane glycoprotein ribophorin I (RI). The mutant protein, RI(332), containing only the N-terminal 332 amino acids of the luminal domain of RI, has been shown to interact with calnexin and to be a substrate for the ubiquitin-proteasome pathway. When RI(332) was expressed in HeLa cells, it was degraded with biphasic kinetics; an initial, slow phase of approximately 45 min was followed by a second phase of threefold accelerated degradation. On the other hand, the kinetics of degradation of a form of RI(332) in which the single used N-glycosylation consensus site had been removed (RI(332)-Thr) was monophasic and rapid, implying a role of the N-linked glycan in the first proteolytic phase. RI(332) degradation was enhanced when the binding of glycoproteins to calnexin was prevented. Moreover, the truncated glycoprotein interacted with calnexin preferentially during the first proteolytic phase, which strongly suggests that binding of RI(332) to the lectin-like protein may result in the slow, initial phase of degradation. Additionally, mannose trimming appears to be required for efficient proteolysis of RI(332). After treatment of cells with the inhibitor of N-glycosylation, tunicamycin, destruction of the truncated RI variants was severely inhibited; likewise, in cells preincubated with the calcium ionophore A23187, both RI(332) and RI(332)-Thr were stabilized, despite the presence or absence of the N-linked glycan. On the other hand, both drugs are known to trigger the unfolded protein response (UPR), resulting in the induction of BiP and other ER-resident proteins. Indeed, only in drug-treated cells could an interaction between BiP and RI(332) and RI(332)-Thr be detected. Induction of BiP was also evident after overexpression of murine Ire1, an ER transmembrane kinase known to play a central role in the UPR pathway; at the same time, stabilization of RI(332) was observed. Together, these results suggest that binding of the substrate proteins to UPR-induced chaperones affects their half lives.