Interactions between mature Der p 1 and its free prodomain indicate membership of a new family of C1 peptidases

Targeting an Initiator Allergen Provides Durable and Expansive Protection against House Dust Mite Allergy

Whereas treatment of allergic diseases such as asthma relies largely on the targeting of dysregulated effector pathways, the conceptually attractive alternative of preventing them by a pharmaceutical, at-source intervention has been stymied until now by uncertainties about suitable targets and the challenges facing drug design. House dust mites (HDMs) are globally significant triggers of allergy. Group 1 HDM allergens, exemplified by Der p 1, are cysteine proteases. Their degradome has a strong disease linkage that underlies their status as risk and initiator allergens acting directly and through bystander effects on other allergens. Our objective was to test whether target-selective inhibitors of group 1 HDM allergens might provide a viable route to novel therapies. Using structure-directed design to optimize a series of pyruvamides, we undertook the first examination of whether pharmaceutically developable inhibitors of group 1 allergens might offer protection against HDM exposure. Developability criteria included durable inhibition of clinically relevant signals after a single aerosolized dose of the drug. The compounds suppressed acute airway responses of rats and mice when challenged with an HDM extract representing the HDM allergome. Inhibitory effects operated through a miscellany of downstream pathways involving, among others, IL-33, thymic stromal lymphopoietin, chemokines, and dendritic cells. IL-13 and eosinophil recruitment, indices of Th2 pathway activation, were strongly attenuated. The surprisingly expansive benefits arising from a unique at-source intervention suggest a novel approach to multiple allergic diseases in which HDMs play prominent roles and encourage exploration of these pharmaceutically developable molecules in a clinical setting.

Novel Method for the Purification of House Dust Mite Allergen Der p 1 and Its Use in Structure-Based Chemical Design of Novel Inhibitors

House dust mites are globally significant triggers of allergic disease. Notable among their extensive repertoire of allergens are the Group 1 cysteine peptidase allergens which function as digestive enzymes in house dust mites. Compelling evidence suggests that the proteolytic activity of these molecules plays a key role in the development and maintenance of allergic diseases through the activation of innate immune mechanisms which exploit genetic predispositions to allergy. Growing interest in this area creates a requirement for high-quality purified protein, whether natural or recombinantly expressed. It has also identified these allergens as therapeutic targets for a novel approach to allergy treatment through modulation of innate immune responses. The purpose of this chapter is to describe a new method for the purification of Der p 1 and use of the protein produced in a screening assay designed for the discovery of novel inhibitors of Group 1 house dust mite allergens.

Proteolytic, lipidergic and polysaccharide molecular recognition shape innate responses to house dust mite allergens

House dust mites (HDMs) are sources of an extensive repertoire of allergens responsible for a range of allergic conditions. Technological advances have accelerated the identification of these allergens and characterized their putative roles within HDMs. Understanding their functional bioactivities is illuminating how they interact with the immune system to cause disease and how interrelations between them are essential to maximize allergic responses. Two types of allergen bioactivity, namely proteolysis and peptidolipid/lipid binding, elicit IgE and stimulate bystander responses to unrelated allergens. Much of this influence arises from Toll-like receptor (TLR) 4 or TLR2 signalling and, in the case of protease allergens, the activation of additional pleiotropic effectors with strong disease linkage. Of related interest is the interaction of HDM allergens with common components of the house dust matrix, through either their binding to allergens or their autonomous modulation of immune receptors. Herein, we provide a contemporary view of how proteolysis, lipid-binding activity and interactions with polysaccharides and polysaccharide molecular recognition systems coordinate the principal responses which underlie allergy. The power of the catalytically competent group 1 HDM protease allergen component is demonstrated by a review of disclosures surrounding the efficacy of novel inhibitors produced by structure-based design.

Cellular and Molecular Events in the Airway Epithelium Defining the Interaction Between House Dust Mite Group 1 Allergens and Innate Defences

Serodominant group 1 allergens of house dust mites (HDMs) are cysteine protease digestive enzymes. By increasing the detection of any allergen by dendritic antigen presenting cells, upregulating inflammatory signalling molecules, and activating cells crucial to the transition from innate to acquired immune responses, the proteolytic activity of these HDM allergens also underlies their behaviour as inhalant allergens. The significance of this property is underlined by the attenuation of allergic responses to HDMs by novel inhibitors in experimental models. The group 1 HDM allergens act as prothrombinases, enabling them to operate the canonical stimulation of protease activated receptors 1 and 4. This leads to the ligation of Toll-like receptor 4, which is an indispensable component in HDM allergy development, and reactive oxidant-regulated gene expression. Intermediate steps involve epidermal growth factor receptor ligation, activation of a disintegrin and metalloproteases, and the opening of pannexons. Elements of this transduction pathway are shared with downstream signalling from biosensors which bind viral RNA, suggesting a mechanistic linkage between allergens and respiratory viruses in disease exacerbations. This review describes recent progress in the characterisation of an arterial route which links innate responses to inhaled allergens to events underpinning the progression of allergy to unrelated allergens.

Allergen Delivery Inhibitors: A Rationale for Targeting Sentinel Innate Immune Signaling of Group 1 House Dust Mite Allergens through Structure-Based Protease Inhibitor Design

Diverse evidence from epidemiologic surveys and investigations into the molecular basis of allergenicity have revealed that a small cadre of "initiator" allergens promote the development of allergic diseases, such as asthma, allergic rhinitis, and atopic dermatitis. Pre-eminent among these initiators are the group 1 allergens from house dust mites (HDM). In mites, group 1 allergens function as cysteine peptidase digestive enzymes to which humans are exposed by inhalation of HDM fecal pellets. Their protease nature confers the ability to activate high gain signaling mechanisms which promote innate immune responses, leading to the persistence of allergic sensitization. An important feature of this process is that the initiator drives responses both to itself and to unrelated allergens lacking these properties through a process of collateral priming. The clinical significance of group 1 HDM allergens in disease, their serodominance as allergens, and their IgE-independent bioactivities in innate immunity make these allergens interesting therapeutic targets in the design of new small-molecule interventions in allergic disease. The attraction of this new approach is that it offers a powerful, root-cause-level intervention from which beneficial effects can be anticipated by interference in a wide range of effector pathways associated with these complex diseases. This review addresses the general background to HDM allergens and the validation of group 1 as putative targets. We then discuss structure-based drug design of the first-in-class representatives of allergen delivery inhibitors aimed at neutralizing the proteolytic effects of HDM group 1 allergens, which are essential to the development and maintenance of allergic diseases.

Pathways of airway oxidant formation by house dust mite allergens and viral RNA converge through myosin motors, pannexons and Toll-like receptor 4

Introduction

Intracellular reactive oxidant species (ROS) are generated in human airway epithelial cells by the prothrombinase action of Group 1 house dust mite (HDM) allergens and by ligation of viral RNA sensor Toll‐like receptors (TLRs). We explored signaling convergence between HDM allergens and TLRs in ROS generation because epithelial cells form the primary barrier against inhaled substances and dictate host responses to allergens and viruses.

Methods

ROS formation by Calu‐3 human airway cells was studied by measuring dihydrorhodamine 123 oxidation after activation by polyinosinic:polycytidylic acid (to activate TLR3), CL097 (to activate TLR7), a natural mixture of HDM allergens, or BzATP.

Results

TLR4 activation was identified as an indispensable response element for all stimuli, operating downstream from myosin motor activation, pannexon gating for ATP release and the endogenous activation of prothrombin. Exogenous prothrombin activation by HDM allergens was prevented by SGUL 1733, a novel inhibitor of the proteolytic activity of Group 1 HDM allergens, which thus prevented TLR4 from being activated at source.

Conclusions

Our data identify for the first time that endogenously‐generated prothrombin and TLR4 form a shared effector mechanism essential to intracellular ROS generation activated by a group 1 HDM allergen (itself a prothrombinase) or by ligation of viral RNA‐sensing TLRs. These stimuli operate a confluent signaling pathway in which myosin motors, gating of pannexons, and ADAM 10 lead to prothrombin‐dependent activation of TLR4 with a recycling activation of pannexons.

International consensus (ICON) on: clinical consequences of mite hypersensitivity, a global problem

Since mite allergens are the most relevant inducers of allergic diseases worldwide, resulting in significant morbidity and increased burden on health services, the International Collaboration in Asthma, Allergy and Immunology (iCAALL), formed by the American Academy of Allergy, Asthma and Immunology (AAAAI), the American College of Allergy, Asthma and Immunology (ACAAI), the European Academy of Allergy and Clinical Immunology (EAACI), and the World Allergy Organization (WAO), has proposed to issue an International Consensus (ICON) on the clinical consequences of mite hypersensitivity. The objectives of this document are to highlight aspects of mite biology that are clinically relevant, to update the current knowledge on mite allergens, routes of sensitization, the genetics of IgE responses to mites, the epidemiologic aspects of mite hypersensitivity, the clinical pictures induced by mites, the diagnosis, specific immunotherapeutic approaches, and prevention.

The discovery of potent, selective, and reversible inhibitors of the house dust mite peptidase allergen Der p 1: an innovative approach to the treatment of allergic asthma

Blocking the bioactivity of allergens is conceptually attractive as a small-molecule therapy for allergic diseases but has not been attempted previously. Group 1 allergens of house dust mites (HDM) are meaningful targets in this quest because they are globally prevalent and clinically important triggers of allergic asthma. Group 1 HDM allergens are cysteine peptidases whose proteolytic activity triggers essential steps in the allergy cascade. Using the HDM allergen Der p 1 as an archetype for structure-based drug discovery, we have identified a series of novel, reversible inhibitors. Potency and selectivity were manipulated by optimizing drug interactions with enzyme binding pockets, while variation of terminal groups conferred the physicochemical and pharmacokinetic attributes required for inhaled delivery. Studies in animals challenged with the gamut of HDM allergens showed an attenuation of allergic responses by targeting just a single component, namely, Der p 1. Our findings suggest that these inhibitors may be used as novel therapies for allergic asthma.

Orchestration of an uncommon maturation cascade of the house dust mite protease allergen quartet

In more than 20% of the world population, sensitization to house dust mite allergens triggers typical allergic diseases such as allergic rhinitis and asthma. Amongst the 23 mite allergen groups hitherto identified, group 1 is cysteine proteases belonging to the papain-like family whereas groups 3, 6, and 9 are serine proteases displaying trypsin, chymotrypsin, and collagenolytic activities, respectively. While these proteases are more likely to be involved in the mite digestive system, they also play critical roles in the initiation and in the chronicity of the allergic response notably through the activation of innate immune pathways. All these allergenic proteases are expressed in mite as inactive precursor form. Until recently, the exact mechanisms of their maturation into active proteases remained to be fully elucidated. Recent breakthroughs in the understanding of the activation mechanisms of mite allergenic protease precursors have highlighted an uncommon and unique maturation pathway orchestrated by group 1 proteases that tightly regulates the proteolytic activities of groups 1, 3, 6, and 9 through complex intra- or inter-molecular mechanisms. This review presents and discusses the currently available knowledge of the activation mechanisms of group 1, 3, 6, and 9 allergens of Dermatophagoides pteronyssinus laying special emphasis on their localization, regulation, and interconnection.

Nonhuman targets in allergic lung conditions

Existing therapies for allergic asthma are far from perfect: the global prevalence of disease increases despite them and they are poorly effective in dealing with the exacerbations that account for hospitalization and asthma deaths. Commercially, there are pressures on these existing medicines too--a growing threat from generics and reluctance by payers to reimburse for increasingly marginal improvements in medicines with precedented mechanisms. Experience shows that attempts to devise selective small-molecule interventions directed at the myriad of downstream effector pathways has not been a fertile ground for the development of effective new medicines. An alternative strategy, exploiting breakthroughs in understanding the molecular basis of allergenicity and the key role of innate immune mechanisms in asthma, is to direct new approaches to the disease triggers themselves: allergens. This raises interesting possibilities for anti-Lipinski drug design (extracellular nonhuman targets, inhaled delivery) and creates unprecedented pharmacological opportunities in the therapeutic area.

House dust mite allergen Der p 1 effects on sinonasal epithelial tight junctions

BACKGROUND

Epithelial permeability is highly dependent upon the integrity of tight junctions, which are cell-cell adhesion complexes located at the apical aspect of the lateral membrane of polarized epithelial cells. We hypothesize that sinonasal epithelial exposure to Der p 1 house dust mite antigen decreases expression of tight junction proteins (TJPs), representing a potential mechanism for increased permeability and presentation of antigens across the sinonasal epithelial layer.

METHODS

Confluent cultured primary human sinonasal epithelial cells were exposed to recombinant Der p 1 antigen vs control, and transepithelial resistance measurements were performed over 24 hours. Antibody staining for a panel of TJPs was examined with immunofluorescence/confocal microscopy and Western blotting. Tissue for these experiments was obtained from 4 patients total.

RESULTS

Der p 1 exposed sinonasal cells showed a marked decrease in transepithelial resistance when compared to control cells. In addition, results of Western immunoblot and immunofluorescent labeling demonstrated decreased expression of TJPs claudin-1 and junction adhesion molecule-A (JAM-A) in Der p 1-exposed cultured sinonasal cells vs controls.

CONCLUSION

Der p 1 antigen exposure decreases sinonasal epithelium TJP expression, most notably seen in JAM-A and claudin-1 in these preliminary experiments. This decreased TJP expression likely contributes to increased epithelial permeability and represents a potential mechanism for transepithelial antigen exposure in allergic rhinitis.

Comparative study of mature and zymogen mite cysteine protease stability and pH unfolding

BACKGROUND

Papain-like proteases (CA1) are synthesized as inactive precursors carrying an N-terminal propeptide, which is further removed under acidic conditions to generate active enzymes.

METHODS

To have a better insight into the mechanism of activation of this protease family, we compared the pH unfolding of the zymogen and the mature form of the mite cysteine protease Der p 1.

RESULTS

We showed that the presence of the propeptide does not significantly influence the pH-induced unfolding of the catalytic domain but does affect its fluorescence properties by modifying the exposure of the tryptophan 192 to the solvent. In addition, we demonstrated that the propeptide displays weaker pH stability than the protease domain confirming that the unfolding of the propeptide is the key event in the activation process of the zymogen.

GENERAL SIGNIFICANCE

Finally, we show, using thermal denaturation and enzymatic activity measurements, that whatever the pH value, the propeptide does not stabilize the structure of the catalytic domain but very interestingly, prevents its autolysis.

House dust mite allergens in asthma and allergy

IgE antibodies in house dust mite (HDM) allergy follow a predictable pattern. Half are directed against two dominant allergens and the remainder largely against four midpotency allergens. This hierarchical pattern is not changed by the titre of the IgE response or severity of disease. The structures of these allergens are known and they can be produced as authentic recombinant allergens. There is also evidence that the allergenicity is augmented by the biological activity of the key allergens, validating them as targets for vaccination. Collectively, these developments should facilitate the development of new diagnostics, improve immunotherapy and allow vaccination with defined reagents. Highly purified recombinant polypeptides representing the important mite allergens are now available so that informative and reproducible experiments can be performed with mite allergens in place of poorly defined and variable extracts.

Comparative enzymology of native and recombinant house dust mite allergen Der p 1

BACKGROUND

The cysteine peptidase activity of group 1 house dust mite allergens is important for their allergenicity and may offer new therapeutic targets for allergy treatment. Hitherto, the design of specific inhibitors has been impeded because the availability of pure, fully active allergens has limited the implementation of drug screening campaigns. Similarly, investigation of the mechanisms by which peptidase allergens promote sensitization has also been restricted. Our aim was to compare the enzymology of recombinant and native forms of Der p 1 to establish if an easily expressed recombinant form of Der p 1 could be used as a drug discovery tool.

METHODS

Enzymatic activity of natural and recombinant Der p 1 was compared fluorimetrically using a novel specific substrate (ADZ 50,059) and a novel specific active site titrant (ADZ 50,000). The effect of recombinant Der p 1 prodomain on the catalytic activity of both Der p 1 preparations was also examined.

RESULTS

Although differing substantially in molecular weight, the enzymological properties of recombinant and native Der p 1 were indistinguishable. Our data show clearly by experiment that, in contrast to some suggestions, Der p 1 is not an enzyme of bifunctional mechanism.

CONCLUSION

The catalytic activity of Der p 1 is tolerant of glycosylation differences that occur at N150 when the protein is expressed in Pichia pastoris. This suggests that this recombinant protein may be suitable for drug design studies and in the elucidation of how peptidase activity promotes sensitization to peptidase and nonpeptidase bystander allergens.

Determinants of allergenicity

The question "What makes an allergen an allergen?" has puzzled generations of researchers, and we still do not have a conclusive answer. Despite increasing knowledge about the molecular and functional characteristics of allergens that have been identified, we still do not fully understand why some proteins are clinically relevant allergens and most are not. Different approaches have been taken to identify the structural and functional features of allergens, aiming at developing methods to predict allergenicity and thus to identify allergens. However, none of these methods has allowed a reliable discrimination between allergenic and nonallergenic compounds on its own. This review sums up diverse determinants that contribute to the phenomenon of allergenicity and outlines that in addition to the structure and function of the allergen, factors derived from allergen carriers, the environment, and the susceptible individual are of importance.

Crystal structures of mite allergens Der f 1 and Der p 1 reveal differences in surface-exposed residues that may influence antibody binding

The group 1 mite allergens Der f 1 and Der p 1 are potent allergens excreted by Dermatophagoides farinae and Dermatophagoides pteronyssinus, respectively. The human immunoglobulin E antibody responses to the group 1 allergens show more cross-reactivity than the murine immunoglobulin G antibody responses, which are largely species specific. Here, we report the crystal structure of the mature form of Der f 1, which was isolated from its natural source, and a new high-resolution structure of mature recombinant Der p 1. Unlike Der p 1, Der f 1 is monomeric both in the crystalline state and in solution. Moreover, no metal binding is observed in the structure of Der f 1 despite the fact that all amino acids involved in Ca(2+) binding in Der p 1 are completely conserved in Der f 1. Although Der p 1 and Der f 1 share an extensive sequence identity, comparison of the crystal structures of both allergens revealed structural features that could explain the differences in murine IgG and human IgE antibody responses to these allergens. There are structural differences between Der f 1 and Der p 1 that are unevenly distributed on the allergens' surfaces. This uneven spatial arrangement of conserved versus altered residues could explain both the specificity and cross-reactivity of antibodies against Der f 1 and Der p 1.