Karin Petersson

Crystal Structure of a Superantigen Bound to MHC Class II Displays Zinc and Peptide Dependence

The three‐dimensional structure of a bacterial superantigen, Staphylococcus aureus enterotoxin H (SEH), bound to human major histocompatibility complex (MHC) class II (HLA‐DR1) has been determined by X‐ray crystallography to 2.6 Å resolution (1HXY). The superantigen binds on top of HLA‐DR1 in a completely different way from earlier co‐crystallized superantigens from S.aureus. SEH interacts with high affinity through a zinc ion with the β1 chain of HLA‐DR1 and also with the peptide presented by HLA‐DR1. The structure suggests that all superantigens interacting with MHC class II in a zinc‐dependent manner present the superantigen in a common way. This suggests a new model for ternary complex formation with the T‐cell receptor (TCR), in which a contact between the TCR and the MHC class II is unlikely.

Interplay between superantigens and immunoreceptors

Superantigens (SAGs) cause a massive T‐cell proliferation by simultaneously binding to major histocompatibility complex (MHC) class II on antigen‐presenting cells and T‐cell receptors (TCRs) on T cells. These T‐cell mitogens can cause disease in host, such as food poisoning or toxic shock. The best characterized groups of SAGs are the bacterial SAGs secreted by Staphylococcus aureus and Streptococcus pyogenes. Despite a common overall three‐dimensional fold of these SAGs, they have been shown to bind to MHC class II in different ways. Recently, it has also been shown that SAGs have individual preferences in their binding to the TCRs. They can interact with various regions of the variable β‐chain of TCRs and at least one SAG seems to bind to the α‐chain of TCRs. In this review, different subclasses of SAGs are classified based upon their binding mode to MHC class II, and models of trimolecular complexes of MHC–SAG–TCR molecules are described in order to reveal and understand the complexity of SAG‐mediated T‐cell activation.

Staphylococcal enterotoxin H induces V alpha-specific expansion of T cells.

Staphylococcal enterotoxin H (SEH) is a bacterial superantigen secreted by Staphylococcus aureus. Superantigens are presented on the MHC class II and activate large amounts of T cells by cross-linking APC and T cells. In this study, RT-PCR was used to show that SEH stimulates human T cells via the Vα domain of TCR, in particular Vα10 (TRAV27), while no TCR Vβ-specific expansion was seen. This is in sharp contrast to all other studied bacterial superantigens, which are highly specific for TCR Vβ. It was further confirmed by flow cytometry that SEH stimulation does not alter the levels of certain TCR Vβ. In a functional assay addressing cross-reactivity, Vβ binding superantigens were found to form one group, whereas SEH has different properties that fit well with Vα reactivity. As SEH binds on top of MHC class II, an interaction between MHC and TCR upon SEH binding is not likely. This concludes that the specific expansion of TCR Vα is not due to contacts between MHC and TCR, instead we suggest that SEH directly interacts with the TCR Vα domain.

Crystal structure of a SEA variant in complex with MHC class II reveals the ability of SEA to crosslink MHC molecules.

Although the biological properties of staphylococcal enterotoxin A (SEA) have been well characterized, structural insights into the interaction between SEA and major histocompatibilty complex (MHC) class II have only been obtained by modeling. Here, the crystal structure of the D227A variant of SEA in complex with human MHC class II has been determined by X-ray crystallography. SEA(D227A) exclusively binds with its N-terminal domain to the alpha chain of HLA-DR1. The ability of one SEA molecule to crosslink two MHC molecules was modeled. It shows that this SEA molecule cannot interact with the T cell receptor (TCR) while a second SEA molecule interacts with MHC. Because of its relatively low toxicity, the D227A variant of SEA is used in tumor therapy.

A TCR alpha chain transgene induces maturation of CD4- CD8- alpha beta+ T cells from gamma delta T cell precursors

The proportion of CD4− CD8− double‐negative (DN) α β T cells is increased both in the thymus and in peripheral lymphoid organs of TCR α chain‐transgenic mice. In this report we have characterized this T cell population to elucidate its relationship to α β and γ δ T cells. We show that the transgenic DN cells are phenotypically similar to γ δ T cells but distinct from DN NK T cells. The precursors of DN cells have neither rearranged endogenous TCRα genes nor been negatively selected by the Mlsa antigen, suggesting that they originate from a differentiation stage before the onset of TCR α chain rearrangements and CD4/CD8 gene expression. Neither in‐frame VδDδJδ nor VγJγ rearrangements are over‐represented in this population. However, since peripheral γ δ T cells with functional TCRβ gene rearrangements have been depleted in the transgenics, we propose that the transgenic DN population, at least partially, originates from the precursors of those cells. The present data lend support to the view that maturation signals to γ δ lineage‐committed precursors can be delivered via TCR α β heterodimers.

The effects of cereal additives in low-fat sausages and meatballs. Part 2: Rye bran, oat bran and barley fibre.

Rye bran, oat bran and barley fibre have been compared as additives in low-fat sausages and meatballs. The water/protein ratio and starch content were constant to allow direct comparisons. Oat bran was the best alternative in low-fat sausages due to its gelling ability upon heating. These sausages exhibited low process (0.9%) and frying losses (10.9%), and high values of firmness (11.0 N) and sensory acceptance. The sausages containing barley fibre, with the highest amount of soluble β-glucan, had high losses (3.8% and 19.6%) and the lowest firmness (4.6 N). Rye bran was suitable in meatballs, probably due to its particulate nature, which is more acceptable in this type of meat product, where the gelling properties are not as important as in sausages. There was no significant difference between the firmness of meatballs containing rye bran (6.1 N) and the reference (7.5 N), after pan-frying. Meatballs with oat bran or barley fibre were less firm (3.6 N and 2.0 N).

The pTα promoter and enhancer are direct targets for transactivation by E box-binding proteins

The pre‐Tα (pTα ) gene encodes a protein that forms the pre‐TCR together with the newly expressed TCR β chain during a defined stage of early T cell development. The restricted expression of this gene has been suggested to depend on a promoter and an enhancer element containing E boxes, which are potential binding sites for basic helix‐loop‐helix (bHLH) proteins. We here report that increased expression of E47 and E12 mRNA in CD4–CD8–TCR– thymocytes correlates with the initiation of pTα  transcription. Furthermore, electrophoretic mobility shift assays suggested that E47 binds directly to both the pTα  promoter and enhancer in vitro. Ectopic expression of E47 in non‐lymphoid HeLa cells resultedin a dose‐ and template‐dependent activation of these control elements. In addition to this, ectopic expression of E47 in combination with the ets‐protein Pu.1 and / or the Pu.1‐interacting protein(Pip‐1), resulted in a synergistic effect on the activity of the pTα  control elements. Thus, we suggest that E47, Pu.1 and Pip‐1 are directly involved in the regulation of pTα expression inearly T cell development.

Impact of cell wall-degrading enzymes on water-holding capacity and solubility of dietary fibre in rye and wheat bran

BACKGROUND Rye and wheat bran were treated with several xylanases and endoglucanases, and the effects on physicochemical properties such as solubility, viscosity, water-holding capacity and particle size as well as the chemical composition of the soluble and insoluble fractions of the bran were studied. A large number of enzymes with well-defined activities were used. This enabled a comparison between enzymes of different origins and with different activities as well as a comparison between the effects of the enzymes on rye and wheat bran. RESULTS The xylanases derived from Bacillus subtilis were the most effective in solubilising dietary fibre from wheat and rye bran. There was a tendency for a higher degree of degradation of the soluble or solubilised dietary fibre in rye bran than in wheat bran when treated with most of the enzymes. CONCLUSION None of the enzymes increased the water-holding capacity of the bran or the viscosity of the aqueous phase. The content of insoluble material decreased as the dietary fibre was solubilised by the enzymes. The amount of material that may form a network to retain water in the system was thereby decreased.

Early TCR alpha beta expression promotes maturation of T cells expressing Fc epsilon RI gamma containing TCR/CD3 complexes

In a previous study we presented data indicating that the expanded population of CD4−CD8− (DN) αβ T cells in TCRα-chain-transgenic mice was partially if not entirely derived from γδ T cell lineage cells. The development of both γδ T cells and DN αβ T cells is poorly understood; therefore, we thought it would be important to identify the immediate precursors of the transgene-induced DN αβ T cells. We have in this report studied the early T cell development in these mice and we show that the transgenic TCRα-chain is expressed by precursor thymocytes already at the CD3−CD4−CD8− (triple negative, TN) CD44+CD25− stage of development. Both by using purified precursor populations in reconstitution experiments and by analyzing fetal thymocyte development, we demonstrated that early TN precursors expressing endogenous TCRβ-chains matured into DN αβ T cells at several stages of development. The genes encoding the γ-chain of the high affinity receptor for IgE (FcεRIγ) and the CD3ζ protein were found to be reciprocally expressed in TN thymocytes such that during development the FcεRIγ expression decreased whereas CD3ζ expression increased. Furthermore, in a fraction of the transgene-induced DN αβ T cells the FcεRIγ protein colocalized with the TCR/CD3 complex. These data suggest that similarly to γδ T cells and NKT cells, precursors expressing the TCR early in the common αβγδ developmental pathway may use the FcεRIγ protein as a signaling component of the TCR/CD3 complex.

The effects of cereal additives in low-fat sausages and meatballs. Part 1: Untreated and enzyme-treated rye bran.

Rye bran was added to frankfurter-type sausages and meatballs with the aim of producing low-fat products with increased dietary fibre content. The addition of untreated rye bran to sausages was detrimental, causing a substantial increase in frying loss (20% compared to 13.2%). The addition of rye bran treated with hydrolytic enzymes reduced the frying loss to 15.2-16.4%. The firmness was also improved by the treatments (12.8-14.2 N compared to 8.8 N). Enzymatic treatment of rye bran did not however improve the water-holding capacity or the texture of sausages compared to the rye bran that had only been soaked in water. The reason could be that enzymes degraded the solubilized fraction of the dietary fibre, leaving small fragments that cannot contribute to the water-holding capacity and the texture of the sausages. The benefits of treating rye bran in water were not seen in meatballs, probably due to the more particulate structure of meatballs, which is not as sensitive to additives.