Summary
Asthma is a chronic inflammatory disease of the lung caused by a combination of environmental and genetic factors [1]. Although symptoms of mild asthma are treated with current medications, such as bronchodilators and steroids, severe asthma remains very difficult to manage. Asthma rates are constantly on the rise and there is a clear need for novel asthma therapies especially in severe asthma [2]. We have found that inhibition of Caspase-4 may present a new therapeutic approach for asthma [3]. Here, we discuss the role of Caspase-4 and its downstream target the Gasdermins in asthma and examine the potential of Caspase-4 inhibition as a novel asthma drug target. Genetic differences in Caspase-4 could be a risk factor for asthma development and severity, as has been shown with gasdermin B, which would affect early diagnosis and stimulate discussion over prophylactic treatment. The targeting of Caspase-4 and pyroptosis is therefore an intriguing prospect that could give rise to a wholly new class of drugs to treat asthma.
Asthma is a Disease with a Strong Genetic Background
It is widely accepted that asthma occurrence and severity is closely correlated with air quality [4]. The strongest risk factors for developing asthma are inhaled substances and particles that may provoke immune reactions or irritate the airways [5]. According to the European Environmental Agency (EEA) 2019 report, Ireland, together with Scandinavian countries, has the cleanest air, as evidenced by the lowest fine particulate matter, ozone and nitrogen dioxide concentrations – all of which were reported to worsen asthma, and lung function in general. The EEA report also assessed the health impact of exposure to air pollutants and demonstrated that Ireland and Scandinavian countries have the lowest overall health burden in Europe. This suggests that the incidence rates of asthma should be lower the Irish or Scandinavian populations. In fact, it is exactly the opposite [6]. Ireland has the highest values of asthma-related conditions in Europe, such as wheeze and shortness of breath [7]. This suggests that some other environmental factor is evident, or there is a genetic basis for the enhanced susceptibility to asthma in Ireland and Scandinavia.
We therefore decided to examine the genetic component of asthma, rather than environmental factors, especially given that asthma heritability has been estimated as high as 60% [8].
Asthma Susceptibility Genes are Mostly Involved in Immune Responses
Our research question focuses on the relationship between the immune system and genetics in the context of asthma. Long term goal is to understand the difference in immune response between asthmatic and healthy individuals. Focus on genes implicated in asthma may indicate new pathways for therapeutic interventions and genetic risk factors may be useful in identifying subtypes of asthma. Although asthma is not a single gene disease (such as cystic fibrosis), studies describing new genetic variations associated with asthma are very beneficial. Many physicians consider asthma as an umbrella term for many heterogeneous lung disorders [9-11]. Further genetic analysis will help to define subpopulations of asthmatic patients and support the design of personalized medicine, as well as predict the risk of asthma development.
A Genetic Variant in Gasdermin B Associated with Pyroptosis is Strongly Associated with Asthma
Several asthma susceptibility genes have been described so far and most of them are immune response genes [12-15]. Among them are innate immunity genes involved in bacterial recognition and LPS/danger signaling (NLRP3, TLR2, CD14, IL-33, IL1RL1, IL18R1) [16-22]. Exposure to certain types of bacteria and in particular LPS, plays a major role in asthma development [23,24]. Caspase-11 is a gene coding for a cysteine protease, which is a driver of pyroptosis – a highly inflammatory type of cell death involving cleavage of Gasdermins which generates pore-forming peptides that lyse the cell [25]. Caspase- 11 (mouse homologue of human Caspase-4) was recently demonstrated to be essential for LPS induced sepsis and was shown to be a novel LPS receptor [26,27]. These studies changed the existing paradigm of LPS signaling and put caspase-11/4 in the spotlight. Caspase-11 has not been studied beyond bacterial infections, which gave us an excellent opportunity to explore the role of Caspase-11/4 in asthma. We were also inspired by studies implicating a region on chromosome 17q21 in the risk of childhood onset asthma [14]. Multiple studies confirmed these results [28,29]. One of the genes located at 17q21 is Gasdermin B – an effector protein activated by caspase-11 (and Caspase-4) that has recently been demonstrated to induce pyroptosis [30,31]. This study linked strong genetic data and highlighted the potential importance of pyroptosis as a crucial process in asthma [31].
Regulated cell death is critical for the successful resolution of inflammation as well as in homeostasis [32,33]. The lung is an organ sensitive to the presence of pro-inflammatory mediators [34]. Death of lung cells and in particular inflammatory death of epithelial cells has long been suggested as one of mechanisms that can drive asthma [35], however, it was never investigated as a therapeutic target. A variety of irritants such as airborne pollutants, certain pollens with enzymatic activity and house dust mite can induce epithelial cell death [36-39]. The exceptionally strong association of a gene involved in pyroptosis inspired us to examine a role for caspase-11/4 in allergic asthma and in a series of experiments we were able to implicate Caspase-11 in disease pathogenesis in mice and identified Caspase-4 as a possible therapeutic target.
Inhibition of Caspase-4 Activity as a Novel Treatment for Asthma
Our work in a mouse model of asthma demonstrated that caspase-11 is a critical driver of asthma [3]. Caspase-11 deficient mice were fully protected from allergic asthma and did not mount an immune response to the allergen [3]. Importantly, we also found that Caspase-4 expression was enhanced in lung samples from asthmatic patients [3] and its expression correlates with asthma severity [40]. We also found that Prostaglandin E2, analogues of which are known to be efficacious in asthma, inhibited caspase-11 expression and activity and thereby prevented pyroptosis [3]. These findings support a strong rationale to develop a pharmacological inhibitor of Caspase-4. Future experiments should compare inhibition of Caspase-4 activity and pyroptosis in human cells isolated from asthmatics and healthy volunteers. Patient samples could be divided into mild and eosinophilic asthma, as well as neutrophilic asthma, steroid-resistant cohorts, and also aspiring-exacerbated asthma, since these are quite different subpopulations [40]. We would also propose to examine Caspase-4 variants among asthmatic patients. Our hypothesis is that overactive Caspase-4 causes pyroptosis of lung cells leading to an exaggerated inflammatory response and subsequently asthma. Recently our data was supported by publications providing convincing evidence implicating pyroptosis induced by genetic and environmental factors. Overexpression of Gasdermin B as well as house dust mite antigen with proteolytic activity Derf 1 drove pyroptosis in asthma [31,41].
Inhibition of Caspase-4 has Implications beyond Pyroptosi
While during bacterial or viral infection unalarming cell death can be harmful to the host [42,43], in asthma, which can be defined as an autoimmune disease, an inhibition of Caspase-4 and therefore attenuation of inflammatory cell death can be beneficial. It is likely that inhibition of Caspase-4 would also have favorable effects independent of cell death. Although, as of today, no specific Caspase-4 inhibitor exists, there is a global caspase inhibitor. When used in vivo in a mouse model of asthma Z-VAD-fmk inhibited allergic inflammation and T cell activation by a non-cell-death mediated mechanism [44]. Our work demonstrated that Caspase-11 deficient mice indeed have attenuated Th2 polarization – a hallmark of allergic asthma [3]. Other work reported that Z-VAD-fmk has been demonstrated to inhibit monocyte to macrophage differentiation [45], another relevant process in asthma [46]. Altogether, our and other data suggest that inhibition of Caspase-4 activity would have benefits beyond an inhibition of pyroptosis.
Future Perspectives
New therapeutic targets are badly needed in asthma, especially in severe cases or in steroid-resistant asthma. Our data supports pyroptosis as a key pathologic process and indicates that Caspase-4 would be an interesting target to pursue to treat asthma. Future studies should identify possible polymorphisms and posttranslational modifications of Caspase-4 among asthmatics from allergic, exercise and aspirin-induced subpopulations to shed new light on asthma etiology. This information combined with patient history might reveal novel functions for Caspase-4 activity, which can be later explored and verified in vitro. Furthermore, assessment of bronchoalveolar lavage obtained from asthmatics for danger molecules released upon inflammatory cell death, such as HMGB, ATP, IL-1β and IL-1α would paint a more comprehensive picture of involvement of Caspase-4 in asthma and possibly help to define asthma types and severity.
References
2. Becker AB, Abrams EM. Asthma guidelines: the Global Initiative for Asthma in relation to national guidelines. Current Opinion in Allergy and Clinical Immunology. 2017 Apr 1;17(2):99-103.
3. Zasłona Z, Flis E, Wilk MM, Carroll RG, Palsson-McDermott EM, Hughes MM, Diskin C, Banahan K, Ryan DG, Hooftman A, Misiak A. Caspase-11 promotes allergic airway inflammation. Nature Communications. 2020 Feb 26;11(1):1-1.
4. Gowers AM, Cullinan P, Ayres JG, Anderson HR, Strachan DP, Holgate ST, Mills IC, Maynard RL. Does outdoor air pollution induce new cases of asthma? Biological plausibility and evidence; a review. Respirology. 2012 Aug;17(6):887-98.
5. Kim KH, Jahan SA, Kabir E. A review on human health perspective of air pollution with respect to allergies and asthma. Environment International. 2013 Sep 1;59:41-52.
6. To T, Stanojevic S, Moores G, Gershon AS, Bateman ED, Cruz AA, Boulet LP. Global asthma prevalence in adults: findings from the cross-sectional world health survey. BMC Public Health. 2012 Dec 1;12(1):204.
7. Kabir Z, Manning PJ, Holohan J, Goodman PG, Clancy L. Prevalence of symptoms of severe asthma and allergies in Irish school children: An ISAAC protocol study, 1995–2007. International Journal of Environmental Research and Public Health. 2011 Aug;8(8):3192-201.
8. Duffy DL, Martin NG, Battistutta D, Hopper JL, Mathews JD. Genetics of Asthma and Hay Fever in Australian Twins1-3. The American Review of Respiratory Disease. 1990;142:1351-8.
9. Wenzel SE. Asthma: defining of the persistent adult phenotypes. The Lancet. 2006 Aug 26;368(9537):804-13.
10. Fajt ML, Wenzel SE. Asthma phenotypes and the use of biologic medications in asthma and allergic disease: the next steps toward personalized care. Journal of Allergy and Clinical Immunology. 2015 Feb 1;135(2):299-310.
11. Lötvall J, Akdis CA, Bacharier LB, Bjermer L, Casale TB, Custovic A, Lemanske Jr RF, Wardlaw AJ, Wenzel SE, Greenberger PA. Asthma endotypes: a new approach to classification of disease entities within the asthma syndrome. Journal of Allergy and Clinical Immunology. 2011 Feb 1;127(2):355-60.
12. Slager RE, Hawkins GA, Li X, Postma DS, Meyers DA, Bleecker ER. Genetics of asthma susceptibility and severity. Clinics in Chest Medicine. 2012 Sep 1;33(3):431-43.
13. Vercelli D. Discovering susceptibility genes for asthma and allergy. Nature Reviews Immunology. 2008 Mar;8(3):169-82.
14. Moffatt MF, Gut IG, Demenais F, Strachan DP, Bouzigon E, Heath S, Von Mutius E, Farrall M, Lathrop M, Cookson WO. A large-scale, consortium-based genomewide association study of asthma. New England Journal of Medicine. 2010 Sep 23;363(13):1211-21.
15. Torgerson DG, Ampleford EJ, Chiu GY, Gauderman WJ, Gignoux CR, Graves PE, et al. Meta-analysis of genome-wide association studies of asthma in ethnically diverse North American populations. Nature Genetics. 2011;43:887-92.
16. Daley D, Park JE, He JQ, Yan J, Akhabir L, Stefanowicz D, et al. Associations and interactions of genetic polymorphisms in innate immunity genes with early viral infections and susceptibility to asthma and asthma-related phenotypes. Journal of Allergy and Clinical Immunology. 2012 Dec 1;130(6):1284-93.
17. Hitomi Y, Ebisawa M, Tomikawa M, Imai T, Komata T, Hirota T, et al. Associations of functional NLRP3 polymorphisms with susceptibility to food-induced anaphylaxis and aspirin-induced asthma. Journal of Allergy and Clinical Immunology. 2009 Oct 1;124(4):779-85.
18. Eder W, Klimecki W, Yu L, Von Mutius E, Riedler J, Braun-Fahrländer C, et al. Toll-like receptor 2 as a major gene for asthma in children of European farmers. Journal of Allergy and Clinical Immunology. 2004 Mar 1;113(3):482-8.
19. Smit LA, Siroux V, Bouzigon E, Oryszczyn MP, Lathrop M, Demenais F, Kauffmann F. CD14 and toll-like receptor gene polymorphisms, country living, and asthma in adults. American Journal of Respiratory and Critical Care Medicine. 2009 Mar 1;179(5):363-8.
20. Genuneit J, Cantelmo JL, Weinmayr G, Wong GW, Cooper PJ, Riikjärv MA, et al. A multi‐centre study of candidate genes for wheeze and allergy: the International Study of Asthma and Allergies in Childhood Phase 2. Clinical & Experimental Allergy. 2009 Dec;39(12):1875-88.
21. Grotenboer NS, Ketelaar ME, Koppelman GH, Nawijn MC. Decoding asthma: translating genetic variation in IL33 and IL1RL1 into disease pathophysiology. Journal of Allergy and Clinical Immunology. 2013 Mar 1;131(3):856-65.
22. Li X, Ampleford EJ, Howard TD, Moore WC, Torgerson DG, Li H, et al. Genome-wide association studies of asthma indicate opposite immunopathogenesis direction from autoimmune diseases. Journal of Allergy and Clinical Immunology. 2012 Oct 1;130(4):861-8.
23. Liu AH. Endotoxin exposure in allergy and asthma: reconciling a paradox. Journal of Allergy and Clinical Immunology. 2002 Mar 1;109(3):379-92.
24. Kim YK, Oh SY, Jeon SG, Park HW, Lee SY, Chun EY, et al. Airway exposure levels of lipopolysaccharide determine type 1 versus type 2 experimental asthma. The Journal of Immunology. 2007 Apr 15;178(8):5375-82.
25. Chang HY, Yang X. Proteases for cell suicide: functions and regulation of caspases. Microbiology and Molecular Biology Reviews. 2000 Dec 1;64(4):821-46.
26. Kayagaki N, Warming S, Lamkanfi M, Walle LV, Louie S, Dong J, et al. Non-canonical inflammasome activation targets caspase-11. Nature. 2011 Nov;479(7371):117-21.
27. Shi J, Zhao Y, Wang Y, Gao W, Ding J, Li P, et al. Inflammatory caspases are innate immune receptors for intracellular LPS. Nature. 2014 Oct;514(7521):187-92.
28. Van der Valk RJ, Duijts L, Timpson NJ, Salam MT, Standl M, Curtin JA, et al. Fraction of exhaled nitric oxide values in childhood are associated with 17q11. 2-q12 and 17q12-q21 variants. Journal of Allergy and Clinical Immunology. 2014 Jul 1;134(1):46-55.
29. Stein MM, Thompson EE, Schoettler N, Helling BA, Magnaye KM, Stanhope C, et al. A decade of research on the 17q12-21 asthma locus: piecing together the puzzle. Journal of Allergy and Clinical Immunology. 2018 Sep 1;142(3):749-64.
30. Das S, Miller M, Beppu AK, Mueller J, McGeough MD, Vuong C, et al. GSDMB induces an asthma phenotype characterized by increased airway responsiveness and remodeling without lung inflammation. Proceedings of the National Academy of Sciences. 2016 Nov 15;113(46):13132-7.
31. Panganiban RA, Sun M, Dahlin A, Park HR, Kan M, Himes BE, et al. A functional splice variant associated with decreased asthma risk abolishes the ability of gasdermin B to induce epithelial cell pyroptosis. Journal of Allergy and Clinical Immunology. 2018 Nov 1;142(5):1469-78.
32. Gudipaty SA, Conner CM, Rosenblatt J, Montell DJ. Unconventional ways to live and die: cell death and survival in development, homeostasis, and disease. Annual Review of Cell and Developmental Biology. 2018 Oct 6;34:311-32.
33. Galluzzi L, Vitale I, Aaronson SA, Abrams JM, Adam D, Agostinis P, et al. Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018. Cell Death & Differentiation. 2018 Mar;25(3):486-541.
34. Moldoveanu B, Otmishi P, Jani P, Walker J, Sarmiento X, Guardiola J, et al. Inflammatory mechanisms in the lung. Journal of inflammation research. 2009;2:1.
35. Holgate ST. Epithelium dysfunction in asthma. Journal of Allergy and Clinical Immunology. 2007 Dec 1;120(6):1233-44.
36. Hassim Z, Maronese SE, Kumar RK. Injury to murine airway epithelial cells by pollen enzymes. Thorax. 1998 May 1;53(5):368-71.
37. Van Cleemput J, Poelaert KC, Laval K, Impens F, Van den Broeck W, Gevaert K, et al. Pollens destroy respiratory epithelial cell anchors and drive alphaherpesvirus infection. Scientific Reports. 2019 Mar 18;9(1):1-5.
38. Winton HL, Wan H, Cannell MB, Thompson PJ, Garrod DR, Stewart GA, et al. Class specific inhibition of house dust mite proteinases which cleave cell adhesion, induce cell death and which increase the permeability of lung epithelium. British Journal of Pharmacology. 1998 Jul;124(6):1048-59.
39. Chan TK, Loh XY, Peh HY, Tan WF, Tan WD, Li N, et al. House dust mite–induced asthma causes oxidative damage and DNA double-strand breaks in the lungs. Journal of Allergy and Clinical Immunology. 2016 Jul 1;138(1):84-96.
40. Rossios C, Pavlidis S, Hoda U, Kuo CH, Wiegman C, Russell K, et al. Sputum transcriptomics reveal upregulation of IL-1 receptor family members in patients with severe asthma. Journal of Allergy and Clinical Immunology. 2018 Feb 1;141(2):560-70.
41. Tsai YM, Chiang KH, Hung JY, Chang WA, Lin HP, Shieh JM, et al. Der f1 induces pyroptosis in human bronchial epithelia via the NLRP3 inflammasome. International Journal of Molecular Medicine. 2018 Feb 1;41(2):757-64.
42. Jorgensen I, Miao EA. Pyroptotic cell death defends against intracellular pathogens. Immunological Reviews. 2015 May;265(1):130-42.
43. Upton JW, Chan FK. Staying alive: cell death in antiviral immunity. Molecular cell. 2014 Apr 24;54(2):273-80.
44. Iwata A, Nishio K, Winn RK, Chi EY, Henderson WR, Harlan JM. A broad-spectrum caspase inhibitor attenuates allergic airway inflammation in murine asthma model. The Journal of Immunology. 2003 Mar 15;170(6):3386-91.
45. Sordet O, Rébé C, Plenchette S, Zermati Y, Hermine O, Vainchenker W, et al. Specific involvement of caspases in the differentiation of monocytes into macrophages. Blood, The Journal of the American Society of Hematology. 2002 Dec 15;100(13):4446-53.
46. Zasłona Z, Przybranowski S, Wilke C, van Rooijen N, Teitz-Tennenbaum S, Osterholzer JJ, Wilkinson JE, Moore BB, Peters-Golden M. Resident alveolar macrophages suppress, whereas recruited monocytes promote, allergic lung inflammation in murine models of asthma. The Journal of Immunology. 2014 Oct 15;193(8):4245-53.